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.

Associated gear
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Similar products
Threshold,Forte,Spectral,CM Labs,Classe,Marsh,Levinson,Coda, in 47 years name a amp,probably have had it at one time or another.
I own the Threshold 4000 the 200watt/channel bigger brother of the 400a. I bought it after I spoke with Nelson Pass over 25 years ago as it was one of the few high current amps that could drive my modifed 1 ohm Dayton Wright XG 8's. It never failed to play music at any load. I'm going to have it brought up to power on a variac and will try it with my 1.6 maggies. I'll update as soon as I can. I expect that better binding posts and capacitors as well as rewinding the transformer would have these amps challenging most of todays SOTA pretenders. Has anyone reviewed a modified 400a or 4000?
That is a great review - thank you. It seems like there are many "bang-for-the-buck" Nelson Pass designs out there. I would love it if people would chime in on this and other Pass amps from the past that are still very worthy of recognition and constitute a good buy. Recently, the folks at Von Schweikert recommended to me the Adcom 555 (original) as an excellent buy Pass design. Costs around $300 and can be upgraded with great effect.
I've had a love/hate relationship with the Threshold 400A
Cascode. I heard it on a friends' Snell Type A's and it
sounded excellent. I later purchased the amp and tried it
on my Magnepan MG-IIIa speakers and never could get it to
sound decent. I even used it in a biamp setup. It worked
OK as the bass amp but when I tried it on the midrange/tweeter
it sounded cold and steely. My Dynaco 416
actually did a better job on the top end. I sold it to
a friend and he enjoyed it with his system. Just goes to
show that everything is system-dependent.
Thanks to Ferrari for another nice review!

I have a 400a that I bought used for $200. As Ferrari pointed out, one should preferably leave it on 24/7. I didn't, and after a few weeks it made a loud buzzing sound after power up. I will work on it this winter to bring it back to shape so I have a decent amp to power my Quads. An extremely valuable resource is the Pass Labs forum at where a lot of knowlegdeable people contribute - including Nelson Pass himself.


- Harald
Many thanks for the review, Ferrari.

I'm in the process of downsizing and am looking for a modestly powered but high current solid state amp to power fairly sensitive (94 dB) speakers that don't drop below 5 ohms.

Would the early 1990s 60 wpc SA/3.9e have many of the same virtues as the 400a?