Review: Threshold 400a Amplifier

Category: Amplifiers

This is the amp that started high end as we now know it. Volumes upon volumes of reviews, and lavish praise have been heaped upon this classic Threshold 400A amp,delivering 100 watts rms per side in Class A output.

This is the amp that forever established not only Threshold but Nelson Pass as well, as one of the most gifted of audio designers. A true visionary product from the mind of Nelson Pass.

To say that the 400A caught the audio world by complete surprise is a major league understatement. Solid state prior to this time was not held in to high regard, except the power amps from CM Labs. But here for once was a major technical break through in solid state running in Class A and at that time an unheard of 100 Watts RMS per side. The sonics were to die for and the control and musicality this amp put on speakers were nothing less than a tour de force, that one does not often experience in life in any medium. I remember sitting for hours in a dealers show room listening to the 400A and playing damn near every musical genre I could find, jazz,classical,rock and just sat mesmerized by what I was hearing. Just one jaw dropping experience after another!

I was already 20 years into this hobby when the 400A came out and was becoming rather jaded at the time and about to abandon the hobby when the 400A made its debut. Without question for me this is the product that totally renewed my interest in hi-fi. Plus this amp has opened more eyes and ears to the possibilty of high end like no other before it and to this day remains one of the most sought after amps in the secondary market place, a true timeless classic.

Since then have had many power amps in and out of various systems over the years, most of them Threshold or Forte power amps with some others as well. But for me, have for the most part remained firmly entrenched in the Nelson Pass camp. I just like the way his products perform and the sonics, plus the build quality is way above repproach and set a standard the industry had not seen before and continues to this day with his Pass Labs gear.

Recenly had the opportunity to acquire a very pristine example of a 400A complete with box,owners manual and the supporting documentation. This had apparently been put away for sometime as it does not look used at all, in fact it looks like new out of the box. Just splendid condition as is rarely seen today.

Once home installed into my system and powered the 400A and let it have about 90 minutes of warm up time before auditioning the 400A. The 400A will remained powered 24/7 except for when I am away for long periods of time. Class A amps do need to be powered 24/7 for best possible sonic siganture and thermal stability. Key word is thermal stability as switching Class A amps on/off puts additional strain on the amp as opposed to leaving it on. A good surge protector or power conditioner is strongly recommended for any Class A amp.

Would it have the same magic for me as it did in 1978? or has time and technology pushed it aside? For me as soon as I dropped the tone arm on the LP, it still retained that same magic I had experienced so long ago, at once I was transported back to a time when the music really mattered and I could not wait to get home to indulge my senses in a pure musical experience each day. A time when I enjoyed visiting the record store and finding gems of music to be played by the 400A. This was like lost innocence revisited and a time when above all the music mattered the most, as it should. After all this endeavour is about the music and the reproduction thereof.

The 400A retains its ability to handle speakers with aplomb and reproduce music with a solid verve, not often found today at any price, a musical amplifier that totally involves one with the music, that one can easily get lost in and forget about equipment. It is very easy to get lost in the sonics of the 400A.

So as far as I am concerned here is a classic Nelson Pass product that has truly withstood the test of time and continues to deliver the promise and has done so since its inception. Very few products can lay claim some 27 years down the road. While in some aspects perhaps time and technology may have surpassed the venerable 400A, but for me and most likely for a vast majority of us the 400A will remain a benchmark that others will be judged against. And with updates available from Jon Soderberg at Vintage Amp,I can clearly see the 400A delivering the promise well into the 21st century. In my opinion this offering from the then fledgling Threshold Corp has in years since become somewhat of an icon in power amps. Prices continue to rise on pristine examples of this amp and now command about an
average of $800.00. Each year the prices continue to rise on the 400A as more audiophiles aquaint themselves with this legend.

Lastly the 400A is not everybodys cup of tea, although in your search for a power amp, one can do seriously worse than the 400A and few if any will every have the outright longevity of this timeless classic.

As I contemplate plans to relocate to Costa Rica next year,I will no doubt bring along the 400A. For me it is that good.

Yes I do have more modern amplfiers that the 400A, but the 400 A for me is just magical and I just don't grow tired of listening to it. For me the magic of long ago is back.

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Threshold,Forte,Spectral,CM Labs,Classe,Marsh,Levinson,Coda, in 47 years name a amp,probably have had it at one time or another.
How did your upgrade go? I am in process of restoring one myself. The big caps are available from Mouser
part num 539-cgs100v10000), exact fit and values and are a must change, one of mine smoked after three hour, and I tested/regrew the oxides first. I also had to replace two power transistors, one emitter resistor, and had to re-grease every transistor and heat sink, as there wasn't much left, none on heat sinks in fact. I have a Sencore LC-102 capacitor analyzer and found both 470uF/16V caps nears inputs broke down, leakage, at rated voltage. The caps in the bias stack 47uf/50Vwere OK, in fact had lower ESR than the ones I bought to replace them, so I left them in. All else was fine, but make sure you get the re- greasing done, as it did not heat evenly until I did it. Right now I am closing in on getting each channel to run at 125degF which is Nelson Passes recommendation. He sold Threshold years ago, but he will answer technical questions on the Pass forum at I have been tube only but wanted to see what a good transistor amp could do and it certainly has me riveted to the listening room. Good luck. Kirby
Great review, except for some of the placebic statements made, class A amps don't need to be "on all the time" this is audio hyperbole...without getting into too much EE detail...Output devices thermal stabilize after a period of time, and don't get any better, so leaving it on all the time will only get you a higher electricity bill, not a more stable or better sounding class a short, the measured performance after thermal stabilization doesn't get any better whether you turn it on or off or leave it on...
Sound bites is quite correct in regards to thermal stablization, that usually occurs in around to 60 to 90 minutes of operation. However with that being said Class A amps for the most part are convection cooled. The cycling of a Class A Amp through on/off cycles causes premature wear and tear on the amplifier. Lets face it if Nelson Pass wanted these units turned off frequently he would not have placed the power switch on the Aleph amplifiers where it is difficult to get to.

I would say for further enlightenment contact Nelson Pass, he will talk with you, no worries there.

I remember in high school some 50 years ago. Our science teacher was running a test with an ordinary light bulb to see how long it would last. When I was in his class the bulb had been on for three straight years. However the the bulbs that had been tuned on and off he had gone through 7 light bulbs in that period of time. And when I left high school a few years later the constant on bulb was still going strong. So based upon this it appears there is some solid fact rendering regarding continued on/off cycling.
Just saw this review and really enjoyed it. I owned a 400A myself MANY moons ago (ca 1985 or so...) and it still sticks in my mind as one of the best I've ever owned; wish I still had it! If I'm ever able to afford a "super amp", there's a good chance it will be a Pass design.

I trained to be a tech and have a degree in electronics technology... The worst activity that any electrical circuit can perform is turning "on" and "off" (NOT the same os on & off in a digital or switching circuit, I'm talking about the complete change in the "powered" state). These both drastically change the quiescent state of a circuit and ultimately cause failures. The two worst componenbts in almost any elctrical system are connection points and power switches!

Hope the 400A is still pumping out the music for you!
GAS Ampzilla was released in 1974. It predates the 400A slightly and was and is a highly regarded solid state game changer.