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ACOUSTIC - 450 Lead / Bass Amplifier - 1970's Vintage Amp
ACOUSTIC - 450 Lead / Bass Amplifier Head - A 1970's Vintage Monster

The Acoustic 450 Series of Lead/Bass Amplifiers (Heads) were produced between 1973 and 1977 by Acoustic (the shortened version of Acoustic Control Corporation External Link). This was arguably the height of the monster stage-amp days. Though Acoustic Control Corporation wound up their business in the 1980's, the company did come back to life in 2007 under the name Acoustic Amplification (and is presently reusing the Acoustic Control Corp External Link monicker).

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Why Even Own or Get An OLD Amp?

To keep production cost down and still provide a magic box that includes features like Digital Signal Processing, Amp Modelling, Impulse Responses and almost endless tonal control ... everything has had to be miniaturised to a minimal electronic footprint and size / quality specification.

The use of SMD (Surface Mount Devices) technology, those tiny electronic components, brings with it the benefit of functionality at the cost of longevity and serviceability. Most SMD based circuit boards are replaced, not repaired when they fail ... and they will fail, because they are NOT intended to last.

Amps like the Acoustic 450 however will function for decades, possibly centuries, as long as the electrolytic caps are replaced every 25 years or so. This incredibly rugged and robust nature is the real benefit to owning well made transistor and/or valve amps. That's not to say that all old amps were well made and deserve maintaining.

Try to find an Acoustic 450

Acoustic 450 amps have always been somewhat rare outside of the USA, but you could get lucky if you remain persistent.
I have even seen them show up locally here in southern Tasmania ... as complete rigs, with cabs!

About This Article

When I first created this page some years ago ... I was surprised at how difficult it was to find information on the Acoustic 450 Lead/Bass Head. There was info available about this old classic on the Internet, but it was scattered and generally incomplete.

Various 'unofficial' web sites had enticing, but often broken links, that are now also long gone. Any meaningful research was somewhat tedious at best. Even the now resurrected and official Acoustic Control Corp web site offers zero information.

A few unsatisfying YouTube vids remain, including a slightly useful one on an Acoustic 360 head repair.

This article and review is an attempt to redress that situation.

While the 80's might be remembered as the era of the 'big hair' and shoulder pads, the 70's was surely the era of The BIG Amp. By the mid 70's the Acoustic Control Corporation (established 1967) was becoming a serious contender in the world of large-stage sound reinforcement.

Their range of stage gear included Mixers, PA's, Foldback, Bass and Guitar rigs (that you could just about hear from orbit ... and yep, that's a metaphor).

Many notable musicians, mostly bass players have used (and in some cases still use) Acoustic amps.

Suggested Speaker Combos for use with the Acoustic 450 Lead/Bass Head:

If by chance you purchased or intend to purchase an Acoustic 450 head without the original speaker bins (as I did), here are some suggested speaker combos ...

For Guitarists

  • A 12" Quad Box (or two if they are rated close to or under 100W RMS each). A 10" Quad Box combo may sound too bright (tinny and/or brittle).

  • For many years, I ran the Acoustic 450 through a non-ported bin that houses a 15", Full Range 8Ω 150W Nessel Audio driver plus a Motorola Piezo Horn and Tweeter (for a total of 250W RMS capacity). I originally designed these bins to be used as a pair, but due to the very high efficiency of the speakers used, one bin is more than adequate for those not suffering from industrial deafness and/or playing stadiums.

  • For what it's worth ... I would not go out of my way to match this amp up with something like the Acoustic 201 bin (Horn + 2 x 15" drivers). The 201 horn loaded bin tends to sound somewhat harsh. Particularly when closer than 3m (10ft).

For Bass Guitarists

  • A folded W bin (or two) for large venue use only.

  • A couple of good front facing 15" drivers in a suitable vented cabinet for smaller venues.

  • A combination of the above two options.

  • A ported version of the above single 15" guitar bin that I built for use with guitar (but without the piezo horns) will work well for really cosy spaces.

    A Hartke single 15" box can work, but may not handle full power.

Note that the W bins tend to really project the bass, but it can be almost inaudible at close proximity. It will however give you a good tingle in the gonads when you're up close.

Please bear in mind that this amp really does deliver and can toast speakers. Make sure that any speakers you plug in to this amp can safely manage the output.

Suggested Modifications For The
Acoustic 450 Lead / Bass Amplifier:

Fortunately the circuit board design / layout of the Acoustic 450 is conveniently modular and access to the electronics is relatively easy. This makes most mods and repairs simple to effect.
  • Most of the electrolytic capacitors in any 'original condition' 450 will be a decade or three past their expected life span. Replacement is recommended. See the documents section for a parts list. This is one case where 'if it ain't broke don't, don't fix it', does not apply.

  • Add a signal bypass switch for the Graphic EQ.

  • Replace single wire signal carrying cables between the Pre Amp, EQ and Power Amp boards with better quality shielded cable.

  • Replace signal carrying cables to the Inputs and rear panel Line Out and Accessory jacks with shielded cable.

  • Add a front panel switch for Auxiliary (Master) Volume. Normally this is only available via the foot switch (or via use of a permanent jumper-plug in the rear jack).

  • Hard wire a permanent bypass to the GND reverse switch (so that it can't accidentally be used).

Aside from the last two mods, the rest should all improve the overall signal to noise performance of this amplifier.

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A quick Summary of the Acoustic 450 Amplifier Head:

An Acoustic 450 head was the central component of my guitar rig for around years. Given that the 450 is still in 100% working condition (now years old), that's some fairly serious durability.

It is funny looking back now to when I bought this amp as used gear, around the end of the 70's. Back then Acoustic amps were not that common in Australia and I never imagined this amp would gain the interest and considerable following that it has today.

Further Details...
A short note on the Acoustic 270 Series amps / speakers

At a solid 170 Watts RMS the Acoustic 450 amp is really quite a brute. However the designers at Acoustic understood the 70's lead guitarist mentality only too well. So they also produced the 270 series amp and speaker bins.

A friend I used to jam and occasionally gig with happened to own an Acoustic 271 rig. This comprised of the 270 head, that could put out a staggering 275 Watts RMS (albeit into a hazardous 2Ω), combined with the matching 201 bin, comprised of Horn Loaded cabinet with 2 x 15" Altec 418's. Acoustic 271 Guitar Rig

To this day, it's the loudest single amp and bin combo that I've ever heard ... a true 70's lead guitarist's wet dream. Of course ... it's also largely useless ... unless you regularly gig in a stadium ;-)

A quick Summary of the Acoustic 270 head:
  • Manufactured: 1971 to 1976
  • Style: Piggyback Guitar Amp
  • Rated Output: 275 Watts RMS
  • Hi/Lo inputs
  • Bright switch
  • Volume / Treble / Mid / Bass / Accessory Vol. (Master Vol.)
  • Symmetrical Distortion / Spring Reverb / Tremolo
  • 5-band Graphic EQ
  • Booster Out (a crude onboard D.I. circuit)
  • Direct Out (Line Out)
  • Accessory In / Out Jack (Line Level Effects Send / Return)
  • Footswitch (Master Volume, Distortion, Reverb, Tremolo).

Acoustic 450 Lead/Bass Head

Everything about the Acoustic 450 Lead/Bass head suggests rugged durability. The 20mm (3/4") thick vinyl covered plywood (not particle board) shell has steel corner caps, chunky rubber feet and a large inset cast aluminium handle. I replaced the original rubber feet with some lower profile ones. It just looks better when you are not using a massive speaker bin. Also note the (particularly useful) hard rubber feet on the end opposite from the handle. I often store this amp standing up.

The outer casing holds in place a lightweight but solid aluminium chassis that supports the internal electronics. To access the electronics requires the removal of only 8 Phillips head screws. Four screws holding only the handle in place, then another four screws from underneath. The chassis can then slide out of the case.

The flat aluminium outer rear panel also serves as the heatsink for the six power transistors. Personally I would prefer to see a (real) heat sink with fins. The molded plastic front fascia is also mounted onto an aluminium panel. This allowed Acoustic to be a little more creative with the appearance of the head. Personally, I've always rather liked the look of this amp - Classic Seventies.

What's important is that this is truly a 'Made To Travel' piece of gear that can readily survive being trucked around without any need for delicate handling (one of the big and obvious advantages of solid state electronics over valve amps).

Worth noting is that the electronics of the Acoustic 450 amp head are all discrete electronic components. At the time this amp was built, they had only just invented the LED. So not surprisingly there is not a single IC (Integrated Circuit / Op Amp) chip to be seen.

All internal electronic components are readily accessible. Due to the relatively low component density on the circuit boards, components are also generally easy to replace. This of course assumes you can acquire or make-up the correct component values.

There are individual circuit boards for the Preamp, Equalizer and Power Amp stages, which also simplify making repairs or mods.

With original handwritten values:
Acoustic 450 circuit diagram/schematics (PDF 3.2MB - Rev 1.5 Dec. 2013).

Redrawn, revised and clarified version - Please watch out for typos. While I have done my best, even the original schematic had errors ;-)
Acoustic 450 circuit diagram/schematics (PDF 3.5MB - Rev 1.7 Oct. 2022).

Acoustic 450 User Manual with Specifications (PDF 1MB - Rev 1.1 Feb. 2014).

And just for the truly faithful, a completely redone parts manifest. This now Includes a shopping list of items to replace (like electrolytic capacitors).
Acoustic 450 Parts Manifest (PDF 1MB - Rev 1 Jan 2023).

5 Band - 70Hz / 140Hz / 280Hz / 1100Hz / 2200Hz.
Boost/Cut range 32dB.
Discussed in detail further down the page.


The Power Amp stage of the Acoustic 450 Lead/Bass head delivers a thumping 170 Watts RMS (@ less than 5% THD) or a momentary peak power of around 450 Watts into 4Ω (@ 5% THD), reduced to 400 Watts peak into a 2Ω load (@ 5% THD) - according to the manual (I have seen the peak power stated as 275 Watts elsewhere, though I have no way to know how they got that figure).

Acoustic claimed, and I quote from a 1976 brochure: "The power stage is revolutionary. The 170 watts RMS output isn't load sensitive. This means that the amp will put out the same power no matter what the load from the speaker is. The effect of this feature is increased low bass and high treble reproduction that is lacking in most other amp tops." An interesting claim to say the least.

The reality as stated in the Acoustic 450 manual (page 14) is that, for best overall performance, the ideal speaker load is 4Ω. In practice, the Acoustic 450 will comfortably drive speaker combinations / loads anywhere from 16Ω down to 2Ω. However, the manual states/warns that the 450 may run hot when driving a 2Ω load. My recommendation is, don't go there. Stick to 8Ω and 4Ω speaker combinations.

Unlike some manufacturers, Acoustic were renowned for being conservative with their specifications. This means that you would really have to push this puppy very hard to exceed its capabilities.

Further Details...
Rugged   Does Not Equal   Indestructible (... Just in case you were wondering)

Having just raved about how rugged this amp is ... I do know that the former owner of my 450 head (a bass player), did manage to blow a set of output transistors by dumping everything the amp could deliver (and then some) into a 2Ω load for over an hour. Being a hot summer's night, he might have got away with it, had he used a fan to cool the rear panel/heat sink.

After replacing the blown power transistors, that Acoustic 450 amp has continued to work flawlessly for another years! (javascript used to calculate) Though admittedly it's had a relatively easy life for the last few decades.

There are plenty of musicians out there who have and still do successfully use the 450 (and other Acoustic amps) running into 2Ω loads. Just be aware that all things tend to wear out more quickly when you run them hot. Also - Please note that Not All Acoustic amps were designed/intended to work with a 2Ω load. Check the amp's specifications before you try it.

The easiest way to generally describe the electronics, is to simply move from left to right across the front panel. It should be noted however that the actual circuit stages do NOT always follow eachother in this order.

Acoustic 450 Lead/Bass Head Controls

There are two inputs provided, in what has become the fairly standard High (47KΩ) and Low (22kΩ) impedance input options. The difference between the two inputs is referred to as sensitivity in Acoustic's brochures, but in actual fact it's also impedance. The High input being suited to your average electric guitar or bass (nominal signal level: 15mV RMS), while the Low input is more suited to a stronger signal like that from a Keyboard or Multi-Effects unit (nominal signal level: 45mV RMS). This is not however a line level input.

Normal / Bright Switch:
Important: The effect of the Bright switch (BRT on the front panel) is Volume Level Dependant.

With the primary Volume control set to 100% the Bright switch will have NO effect. However, as the primary Volume control is turned down from 100%, a slight boost to higher frequencies will start to occur, becoming more pronounced as the volume decreases.

Because the Bright switch is volume level dependant, it's not really surprising that guitar and bass players alike, who consistently have their primary volume level set at or above 70%, have mistakenly believed that this switch 'doesn't do much'. There have even been times when I thought it was 'broken', because (until I saw the circuit schematic) I had no understanding of how it was meant to work.

According to the manual, the boost provided with the Bright Switch set to On is 6dB per octave from 200Hz to 5KHz.

In reality ... because this is a simple passive High-Pass filter rather than a Band-Pass filter, that +6dB/octave boost will apply to all frequencies above 200Hz. So suggesting that there is an upper cut-off at 5KHz is somewhat misleading. Aside from that, I think the Bright switch and it's implementation is actually an excellent idea.

Further Details...
Normal / Bright (NORM / BRT) Switch - Functional Diagram and Explanation

The Bright switch on Acoustic amps is similar in concept to a Loudness switch on Hi-Fi amplifiers. The main functional difference being that the Bright switch only boosts the treble content of the signal, whereas a Loudness switch boosts both treble and bass as the volume is turned down. The Loudness control on Hi-Fi amps is designed to compensate for the human hearing's natural decrease in sensitivity to treble and bass at lower volumes (usually based on the Fletcher-Munson Curves).

In the diagram below the Bright switch (at the bottom) is set in the Off/Normal position. So lets see what happens to the signal ...

Bright Switch Set To NORM (Off)
The guitar signal goes into the preamp stages and is amplified to Line Level (around 0.7V RMS). The signal comes out of the preamp and passes through C2 (a decoupling capacitor) which prevents any unwanted DC voltages from being amplified by subsequent stages. The signal then passes through the volume control and goes on to the next stage (Tone Controls in this case).

Bright Switch Set To BRT (On)
As before, the guitar signal is amplified by the preamp stages and decoupled by C2. With the Bright switch ON however, the signal now has two paths. One path is through the Volume control. The other path is through resistor R1 and capacitor C1 (collectively called an RC network), which in practice is a simple passive tone control, in this case acting as a High Pass filter (allowing higher frequencies to pass through).

An electrical signal, like water, will always take the path(s) of least resistance. So when the Volume control is all the way up (minimum resistance), all the signal goes through the Volume control to the next stage and the RC network has no effect. As the Volume control gets turned down however (and the resistance increases), more and more signal will pass through the RC network which gradually increases the amount of high frequency (treble) present in the signal which is being sent to the next stage. At the same time the central wiper of the volume control moves closer to ground (resistance to ground decreases), and the signal gets quieter and is eventually turned off.

In case you are wondering ... The 1meg ohm resistor (R2) across the Bright switch is there to prevent a capacitive charge build up on either side of the switch which might cause an audible 'pop' when switching from Normal to Bright. Due to the high resistance value of R2, there is no other appreciable effect on the circuit.

Normal / Bright Switch - Functional Diagram

Volume and Tone Controls:
These are fairly predictable when compared to modern equipment. It's worth noting however that, back when these amps were first released, most similar amps had only Treble and Bass controls. So an amp with Treble, Midrange and Bass tone controls was in itself a standout, let alone one with a built-in 5 Band Graphic Equalizer.

The functional range of the tone controls is:

Bass: +15dB Boost / -15dB cut @ 50Hz
Mid-Range: +7dB Boost / -30dB cut @ 400Hz
This centre frequency fits between Graphic EQ controls 3 & 4
Treble: +15dB Boost / -25dB cut @ 10KHz

Important! With the Tone Controls set with the notch straight up (12 o'clock position), output is flat.

In the amp's circuitry, the primary Volume control sits at the end of the input preamp stage. From here the signal is passed via a (footswitch controlled) switching FET to the Master Volume and final line level amplification section. This then feeds into the Tone Control section, followed by the Graphic EQ module, which finally drives the Acoustic 450's Power Amp.

Distortion Switch / Control:
The provided Distortion control includes a switch so that it can be switched in/out without requiring a foot switch. Distortion is created by symmetrical diode clipping and is not particularly spectacular when compared to similar functions in modern amplifiers. Put simply, it sounds rather harsh and (not surprisingly) very 'solid-state'. It lacks the warmth of the even harmonic distortion which is the hallmark of Tube Amps. That said though, with all of the amazing stomp boxes around these days, it's probably somewhat redundant anyway. Used sparingly, it can apply a bit of extra dirt, punch and level to a guitar signal without sounding too awful.

Auxiliary Volume (Master Volume):
The Aux Volume control is only effective when used in conjunction with a footswitch (unless you do a mod as i did). The Auxiliary Volume essentially allows you to set the 450 Lead/Bass amp up with independent volume levels for both Lead and Rhythm. You can then select either option on the fly simply by pressing a latching footswitch.

Distortion and Aux Volume Foot Switch:
It's quite easy to make your own footswitch controller for the Distortion and Aux Volume controls. A simple matter of completing a circuit to ground in each case. The wiring requires two stereo 6mm (1/4") jacks connected by a 3 conductor cable. Speaker cable is recommended over audio or guitar leads because it is less likely to get glitchy from being repeatedly trodden on. Shielded cable is not required. Light duty 3 conductor electrical cable (e.g. mains power lighting grade or light-duty extension cable) will do admirably. I would suggest a minimum length of at least 3m to 5m (10' to 16').

A pair of chunky (stomp-box grade) latching (SPST) switches in a sturdy metal or plastic box will do nicely to house the switches. You could also combine each of these switches with a momentary push-to-make, non-latching switch. These will only activate the desired function while the switch is depressed, so you can have a momentary change in Volume level and/or distortion without having to double-stomp.

5 Band Graphic Equalizer:
The 5 band graphic EQ is situated on its own PCB between the Preamp and Power Amp sections after the Tone Controls. It provides a Boost/Cut range of 32dB (±16dB) for each frequency, starting at 70Hz and going up in roughly one octave intervals to 2.2KHz. While the faceplate for the Graphic Equaliser and the User Manual both state the Graphic EQ centre frequencies as 70Hz / 140Hz / 280Hz / 1100Hz and 2200Hz, the circuit diagram specifies the last two frequencies as 900Hz and 1800Hz ... An interesting anomaly.

Worth noting here ... The Mid-Range Control provides +7dB Boost and -30dB cut @ 400Hz. This centre frequency is designed to operate between the third and fourth Graphic EQ controls.
Acoustic 450 Five Band Graphic EQ
Sadly however, the Graphic EQ (in my opinion at least) is the Acoustic 450 Lead/Bass amp's Achilles' Heel. When the EQ controls are set to any substantial level of Boost, the Graphic EQ is simply one noisy piece of electronic crap.

The more boost (particularly the higher frequencies 1100Hz and 2200Hz) the more noticeable the noise gets. Notice that I specifically mentioned the boost setting. When the sliders are kept close to, or below the centre line, noise performance is generally 'acceptable'.

Hint: Create your EQ Curve by using cut instead of boost wherever possible.

The best option is to insert a Bypass Switch that completely removes the Acoustic 450's Graphic EQ from the audio circuit path. Thanks to the modular design and layout of the circuit boards, this is an inexpensive and relatively easy modification to make. The result is much improved overall noise performance when the Graphic EQ is not in use.

In fairness, most analogue Graphic Equalizers are noisy devices. This is largely due to the high levels of gain provided by these filters at their respective centre frequencies (a hefty ±16dB in the case of the Acoustic 450). Since the noise produced from each stage is additive overall, five EQ stages potentially equals five times the noise (depending of course on the level of boost applied). Even with all sliders in the Flat position there is still noticeable noise when compared to switching the Graphic EQ completely out of the circuit (requires a circuit/amp mod).

The LEAD / BASS Switch is simply a switch in series with a 3.0mH inductor, which is switched into the circuit for the BASS mode and bypassed when in LEAD mode. The inductor acts as a passive Low Pass speaker cross-over circuit. One would therefore expect a gentle filter slope of roughly -3dB/octave from the stated cross-over frequency of 1200Hz upwards. Essentially this progressively reduces the higher harmonics.

This cross-over circuit on the speaker output really only becomes significant in the context of using the Treble Tone Control and the highest of the Graphic EQ sliders (@ 2200Hz), it should have little or no effect on the other frequencies covered by the Graphic EQ.

For example: There is little point to boosting the Treble (centre frequency nominally around 10 KHz) and then switching the amp to BASS mode, which would essentially cut these very same frequencies (In BASS mode: @ 2.4KHz = -3dB, @ 4.8KHz = -6dB, @ 10KHz = -9dB).

Further Details...
LEAD BASS Switch - Functional Diagram

LEAD / BASS Switch - Functional Diagram

The Lead/Bass switch is located right before the output to the speaker terminals and is followed only by a somewhat massive 2000μf capacitor (almost the size of a beer can). It is the charging up of this capacitor that produces the very audible hum then thump (with accompanying and readily observable speaker cone movement), when the amp is first turned on. Just to be clear, that thump you hear at turn on is completely normal and not a fault.

Important!: This switch does NOT reverse signal and signal ground (like a Phase Switch). It swaps over the Active and Neutral AC connections in the power supply with respect to the amplifier's ground.

At a time when electrical standards were, shall we say, a little more chaotic around the world … this may have been a good idea by allowing you to connect the amp to other equipment built with questionable wiring standards. These days, it's either completely redundant or just plain dangerous. If you think you need to use this switch, you probably shouldn't be using the equipment you're connecting to (i.e. it's probably a health hazard). I would recommend hard-wired bypass surgery.

Well, that's it for the front panel … now for the connections on the back.

Acoustic 450 Lead/Bass Head
Acoustic 450 Lead/Bass Head

Acoustic 450 Rear Panel:

Being able to securely stow a generous 4m (13') mains power cord, is one of the things I've always liked about the Acoustic 450 Lead/Bass head rear panel layout (the original power lead was replaced by a longer one when the amp was serviced). Just beside the power cord (far left) is an unswitched power-through socket and next to that the primary fuse. Unfortunately the power-through socket is a US standard socket, so it's not of much use to the rest of the world.

Speaker Outputs
In the centre of the rear panel are two standard 6mm (1/4") mono phone jacks for the speaker outputs. These are wired in parallel. The Acoustic 450 is designed to accommodate speaker combinations running at 8Ω, 4Ω and 2Ω. The most likely/common speaker combinations will be either a single 8Ω speaker bin or two 8Ω bins in parallel (resulting in a 4Ω load). A 4Ω load is recommended in the User Manual.

The lower the load resistance, the hotter the amp will run. Experience suggests that the 2Ω option is somewhat higher risk and should best be avoided.

To the right of the speaker connections are a further three 6mm (1/4") phone jacks for the Booster Output, Line Output and Auxiliary Output.

At the far right is the Distortion and Aux Volume Foot Switch jack (stereo 6mm (1/4") phone jack). The functionality of which is described earlier in this review.

Acoustic 450 Lead/Bass Amplifier Performance:

When it comes to raw power, the Acoustic 450 is one very loud brute of an amplifier. This is a true stage amplifier made for use at larger venues rather than intimate nightclubs. Particularly when used with the matching speaker bins. I can't recall ever having used the 450 in public with the volume above 60%.

When used without the distortion and Graphic EQ, the output is frighteningly clean. In it's day this was seen by some as a liability, particularly those used to the warm distortion of valve amps. Personally, I happen to like the clean sound because you can add whatever guitar and stomp box combinations you like and still get predictable results at any volume level.

That said however, this amp really shines when you crank it up. Just make sure you hook it up to speakers that can comfortably handle (at minimum) the rated 170W RMS output … or you will almost certainly end up replacing fried speaker coils.

This amp is a vintage classic from a time when 'On Stage Grunt' still mattered. Alternate uses for this amp and a good set of speakers include shaking the dust off the walls (... of your place AND the house next door ;-)

When used with efficient speakers this amplifier is easily capable of producing sound pressure levels that can permanently damage your hearing.

Thanks to Julien (from France) who provided me with a scan of the original Acoustic 450 User Manual. I'd still like to get hold of a Service Manual though. If anyone would care to send me a copy of the Service Manual in PDF form (send to ), it would be greatly appreciated.

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Your Comments:

231123 - (Excellent) I had one of these amps in South Australia in the mid 70s. One of the best amps I've owned.

230808 - (Excellent) Thank you very much for this detailed page!
I have a 450 that has been my main rig for over 15 years now and still running strong.

Editor's Note: You're welcome

230517 - (Excellent) I own 2 of these beasts, and go back and read this page over and over. Thank You!!

Editor's Note: OMG! - I have a follower !!

230206 - (Excellent) Excellent info thanks! I've been using one for almost 30 years (15 of those were on the road in Punk and loud alternate rock bands). Never let me down and never needed to crank it. Down low I was still usually the loudest on stage. Brilliant amp.

Editor's Note: Always nice to get positive feedback from youngsters

220916 - (Excellent) Phenomenal write up, one million thanks for the level of detail provided. I've had this head for 10 years and it's never done me wrong - fantastic piece of equipment that now, thanks to you, I know so much more about. Well done, and thanks again.

Editor's Note: Thanks and best wishes

220814 - (Excellent) Awesome write up.

Editor's Note:

221027 - (Excellent) I've owned one of these since the '90s.  Mainly used with guitar.  I see I need to bug my friend to help me with the upgrades you mention, as well as checking on the caps and cleaning the pots.
I will never get rid of it.

Editor's Note: Ahh! A true devotee

210602 - (Excellent) great clean sounding amp  versatile sound ... I used the 2 channel model 470 version in dance clubs ... great amp and 2 separate channels a great plus.

210427 - (Excellent) thank you.

Editor's Note:

210427 - (Excellent) I still have my original 450 from 1975 that I bought new because I thought it would be cool to have the loudest amp in the band. In reality, I probably never turned the volume up above 3 because we played clubs, not arenas. Sadly, it has been on a shelf in my garage for decades, though I did cart it around to gigs as a backup in case our PA amp died. That actually happened once, and everyone thought I was a hero to have such a beefy amp as backup. This article is terrific and brought me back to how excited I was to have this amp in those early years. Makes me want to get a cabinet with two 12's (so I can carry it) and the 450 can perform again. If a 65 year old geezer like me can still get out to do a gig once in a while, a 46 year old amp ought to be able to do so as well ...

Editor's Note: Thanks for the show of appreciation - I too don't seem to be able to find a use for my 450 - or - let it go. And I remember both the quad boxes (4 x 12) and the 201s (2 x 15 + horn). They were all a two man job to move, unless you had suicidal tendancies. I totally agree with the 'old geezer' remark. Thanks to a series of NDEs, my philosophical perspective of life has been reduced / contracted to ... 'If I wake up in the morning - it's a good day!'

Sadly, many of the amps and other bits of tech being manufactured today will probably be junked (or hopefully recycled) within the next ten years. The era of building stuff to last, is well and truly behind us. It's almost reassuring to own gear that you know will outlive you ;-))

210321 - (Excellent) Well done! Very thorough.

190812 - (Excellent) Very Usefull, now I finally got the schematics to service my 450. Thx !!!

190807 - (Excellent) Great review and documentary for this great amp.

190614 - (Excellent) This is all the info I was looking for, and all in one place.

190427 - (Excellent) Thanks!
Question: Do the [Line Out] and [Accessory In/Out] work as an effects [send] and [return] respectively?

Editor's Note: The short answer is 'yes', but it is actually a little more complicated. The [Accessory In/Out] is actually for use with a stereo jack, where the tip [return] is connected to the Power Amp and the sleeve ring [send] is connected to the Pre-amp Output, as is the [Line Out]. This configuration allows effects send and return to be achieved with either a single stereo jack or two mono jacks. See excerpt from schematic below:

Acoustic 450 Lead/Bass Head Controls

181221 - (Excellent) Thanks!
Question: The Booster Output, can this be linked to a Power amp booster, such as a Peavey Festival Booster? Is this what that particular output is for? Or is this specifically for the Line Out?

Editor's Note: The Acoustic 450's Booster Output is absolutely NOT a Line Output (Line Level is from the Pre Amp Output, Booster Output is derived from the Speaker Output), though the two signal levels will be similar.

Booster Output is derived from the Speaker Output via a 33Kohm resistor. The Ground for the Booster Output is derived from Supply GND (not Speaker GND) via a 680ohm resistor. The result (from a voltage perspective) is a roughly 50 to 1 reduction of Speaker Output voltage. For all intents and purposes, it functions like a very simple DI (Direct Inject) box output (but without the usual transformer isolation). If an amplifier provides a dedicated Booster Input port, then I'm guessing that should be a match ;-)

181102 - (Excellent) One thing you missed. The bright func isn't to be taken in isolation. Just as frequency response of pickups interact with each other part of the chain in complex math ways, the bright likely has resonance at 5khz and then is heavily damped by preceding or subsequent circuits. The Beauty of analog.

Editor's Note: As you mention, this is an analogue system, so everything does indeed interact "in complex math ways" as you put it.
However for the Bright switch to have resaonance, it would need to be switching in/out a bandpass filter, wthin a circuit with a Q-factor, providing a notch or peak with depth/gain (where Q is the width/range of the notch or peak). In this case however, the Bright switch is a very (very) simple passive high pass filter that is NOT within a gain loop (a look at the provided circuit diagram will confirm). The Graphic EQ section however, is an example of a series of bandpass filters with a fixed Q and of course gain.

180506 - (Excellent) Thanks for that. I use my 450 w/ a 1-15" cab or a 2-10". I play upright with 2 Kay&Kay pickups mixed thru a berringer euro into the 450. I get a very warm, woody sound. I've also used the 450 as just a pre-amp for bass guitar gigs. Even though it's kinda bulky just for a pre-amp, the tone is worth it. Thanks again, Bill in Boise.

170120 - (Excellent) I have a 450 with an acoustic cabinet that has 4 x 12's and 2 horns. I bought it used in 1978. I used it for 5 or 6 years, then put it in a storage room in my mom's basement years ago. I pulled it out last year and it sounds just the same after sitting for yrs. Great old amp.

170101 - (Excellent) I owned a 470. Gave women orgasm if turned up more than 30%. Rattled window a mile away. Sold when I was piss poor broke.

Editor's Note: ;-)) Hmm ... The birth of an urban legend perhaps? In fairness though, I have had my balls tingle when in front of a 450 powered W Bin ... And a Happy New Year to all (or at least, hopefully a better one).

161113 - (Excellent) I have this amp and it still performs as when it was purchased. I also purchased the lifetime warranty for $25 and wonder if it is still honored. Thanks for this article, enjoyed reading it.

Editor's Note: The current Acoustic Amplification company does mention and trade off its long history (going back to the original Acoustic Control Corporation ), even using more or less the same logo. I’m reasonably confident though, that there is no financial or legal connection between the two corporate entities (old and new). Therefore the current business has no responsibilities for warrantees issued before it actually existed. Having looked at their web site, they won’t even sell you parts ;-) Hopefully your beast will continue to serve you well. And fortunately, they’re generally not that hard to fix (subject to parts availability, which is usually not too difficult).

161101 - (Excellent) Okay be jealous. Picked one up in a thrift store bin for $20 when looking for a computer mouse. Was dirty but turned on. Needed a bass head for my Stingray. Washed it off, plugged it in and a real Wow. All good. That great bass sound right off. Tried my guitar and as a tube man, really liked the even but musical tones. Even the distortion generated cool feedback singing from the guitar. Definitely use it for guitar in many situations. But Bass - yeah I could get what I want out of it. Found a 15" cab for $40 online. Tightened up the cabinet by knocking on sides and back and damped with various pieces of rigid fiberglass, rigid packing foam and weather stripping around back panel. Think speaker has a poor recone as can't get rid of weird buzz, but reduced it. Amp does everything! I use my bass for every type of sound. Like the bright switch On for whacking the strings. Aaaah - I always use a compressor to control that hot bass. Thank you so much for the superlative article. BTW did you add up the total cost?

Re: Previous, I used a product called LPS 1 to clean all pots and contacts. Was recommended by a trusted source. Don't use too much, however.

Editor's Note: Well, it's good to hear that you got a bargain. Even better to hear that you are pleased with it. Thanks for the note about the LPs 1. and thanks for writing ;-)

160609 - (Excellent) Very informative.   Editor's Note: Thanks ;-)

160522 - (Excellent) I bought 2 of these beasts in the last 6 months because of this web page. One off CL and 1 from eBay, neither of them working. One has been rebuilt with all new caps and output transistors and it kicks ass. The other is a work in progress. I owned one back in 74 and played it through a Sound City 18 in reflex cab. Great web page.

Editor's Note: The big electrolytic capacitors on these old amps are basically the weakest link in the electronics. Being electro-chemical, they will all degenerate with age and ultimately fail. Whether the amp is used or not! This is something anyone looking buy 'any' old amp should be aware of. Old amps, in many cases, are a 'project'. But these old Acoustic Control Corp amps really are worth fixing, particularly if you like loud. Thanks for writing ;-)

160125 - (Excellent) This article is fantastic, I've had a 450 head for a few years now but just recently noticed distortion when the volume is set at half-way and above. the cab is a 4x10 rated at 350 watts at 4 Ohms - I didn't seem to notice this distortion when I used the head with the original 401 cab, it was a lot louder too! Any advice? Also what should the Rev/Gnd switch be set to? Thanks

Editor's Note: The first thing to determine is whether it is the head or the speakers that are distorting. Plug the head into another quad box and see what happens. It might just be a problem with one or more of the 10" drivers (e.g. check that all are mounted securely). If it's the amp/head, it would need to be bench tested to determine if there is actually a problem and what is causing the distortion.

A general rule, always check for hardware problems first, for example: Speaker leads and plugs and/or things that might have become loose, damaged or worn over time (if in doubt, ask a roadie).

As for the Ground Reverse switch - leave it in the un-reversed position (pressed in at the top of the rocker switch ;-)   Hope that helps.

151028 - (Excellent) Specs, schematics, manufacturer instructions ... what else could you want?

150819 - (Excellent) Thank you for posting this. I love these old guys. They run forever with a little TLC.

150730 - (Excellent) Just purchased in AUS an older 450 Head now about 42 years old. Had the whole thing checked over by an Amp Tech, she has had some work over the years, but still sounds great.

150504 - (Excellent) I have the 2 channel 470. Yes nice amp, built to last and timeless styling!

Editor's Note: I agree, it is a good looking old beast (but then, that's what I say when I look in the mirror each morning. Hell, someone has to ;-)

150418 - (Excellent) From a 470 man.

150320 - (Excellent) Great review of a mighty fine pedigree. With the homeunix site gone now, this kind of information on ACC equipment is a rare treasure. Thanks!

Editor's Note: So sad to hear that the old acc.homeunix.net site is down. They kept a lot of useful info on all manner of Acoustic Control Corporation gear, right across the range of products. That will be sorely missed by many! You can find what little is left of the old unix site in these web archive pages. Unfortunately most of the documents (manuals/schematics/images) are missing.

150223 - (Excellent) I have this amp. Bought it a year ago. Thank you very much for this information.

150112 - (Excellent) I had a 470 for years.  Sadly, after divorce someone caught wind of the situation and talked me out of it for 100 bucks.

Editor's Note: That really bites! ;-)

141103 - (Excellent) Great informative summary!

141014 - (Excellent) I have the 2 channel 470 model, yes a nice reliable and clean amp.

140610 - (Excellent) I have one of these 450's, I bought it 20 years ago with two JBL bins for $750.
Still use it now and again. Nicest solid state I played through ...

131024 - (Excellent) Very good, thank you. I still have the one I bought in 1975 and it meets all the points discussed. Love it.

Incept Date: Wizard - 130929
Last Updated: Wizard - 240529


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