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
Click to view my Virtual System

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.
Thanks for review. I was trying to remember the specs on Threshold 400 and 4000. I worked at a store where they were sold, and a good very friend of mine bought a 4000, and another guy I know had an 800. I've also always wondered what Nelson meant by "dynamic bias Class A." How does that work, and what kind of power is dissipated at idle, and how much at peak? Interesting that now Nelson, who pioneered a non-traditional bias system now goes for pure and true Class A. Even if you leave your Threshold on all the time you don't have total temperature stability because the amps are not true Class A. The power dissipation depends on power delivered to some degree.

BTW, at the store where I worked, the amplifier the staff really liked best was a cheap Nikko Alpha III, a cheap and simple Class AB Mosfet amp. Of course we didn't tell anyone because we didn't sell them. But we had one in the back room. We also liked tube amps. I'm not saying we were right, but that's what we thought back then.

I just bought a used Class A amp myself, a Krell FPB-300 which looks, works, and sounds beautiful, and I need that kind of power for my Acoustat 1+1's which suck gobs of current that shut down my old 125/200 watt amplifier. The Krell has a "plateau" bias system with 7 plateaus IIRC. I hooked it up temporarily to a kill-o-watt meter to see how the bias system works. The idle is at 300 watts which is already like a moderate power Class A amp. Give it a little kick and it moves up to 500 watts. The highest plateau I measured (didn't want to break my meter) was 1200 watts.

Krell is quite specific about not recommending you keep the amp idling all the time. They recommend you put into standby mode when not in use, which draws 60 watts and keeps the rail regulators (regulated supply for outputs!) charged. I think this is better than leaving amp fully powered as it reduces heat (which shortens component lifetime) and saves energy. BTW, I use 100% wind power through Windtricity and Native Energy.

Now certainly this doesn't fit Nelson's current thinking, which emphasizes temperature stability. Unfortunately, temperature stability costs a lot if you want 300 watts (or actually 1200 watts into 2 ohms!) pure class a with no dynamic bias or any other kind of bias trick. Such an amp would draw about 3000 watts continuously! Nelson's biggest XA design does burn indeed quite a lot of power constantly, though a bit less than that IIRC because it doesn't have quite the 2 ohm power of the Krell. I decided, given my need for power, that temperature stability was something I'd have to sacrifice. I still get most of the benefits of class a. Though I wonder how important these benefits actually are, and it seems that good old class AB designs nowadays may do equally well. The Bryston 4BSST has distortion even lower than Krell, though without the Krell's 2 ohm power. And the lowest distortion amp of all, Halcro, DM80, with parts per billion distorion, is also class AB. It may be that high bias Class AB like the Bryston is all you really need. (I am not able to do the comparison...the Bryston would have cost me just as much or more than the Krell, and I don't live near any high end dealer. I just came across an exceptionally good deal on the Krell and decided to go for it, always wanted one anyway, kinda like having a Ferrari.)

And, in fact, ignoring the "dynamic Class A" stuff, what you have with your early Threshold amps is something like high bias Class AB, and not really "pure" Class A like his newer designs.

Often you Threshold guys seem to forget there were other designers too. For example, James Bongiorno pioneered some important concepts, and designed some really hot amps back before the Threshold, and later, and is now back again with some new hot amps. (His fans also seem to forget there were other amplifier designers...) And a lot of amps like the Krell, Aragon, and Mark Levinson have fairly anonymous designers and/or teams of designers. IIRC, my Krell was actually designed by Jon Snyder. And also John Curl, who designed some of the earliest Levinson electronics (although Levinson's first preamp was designed by Levinson himself...), and now has award winning Parasound amps like the JC-1. According to James Bongiorno, one of the first good transistor amps was designed by his mentor Sid Smith, the Marantz Model 15. At the time of his death a few years ago, Sid was still using Model 15's. He was also responsible (along with Dick Sequerra) for some of the most highly regarded tube amps, the Marantz models 8 and 9, and the 10B tuner. Some wondered why Sid had not gone back to the tube designs, given their recent popularity, but he always liked his Model 15 best. And on and on. But just look at all the hundreds of highly regarded amplifiers of the last 30 years, and behind each of them is one or more incredibly talented designers, even if no one has been as successful as Nelson Pass with so many different highly regarded models over so many years under his own name. But if it was perfect, he would have only needed to design one.
Thanks for the great dialogue on the Threshold 400A. I used to enjoy one in my college days, and was just able to acquire the exact one by a stroke of good fortune. Unfortunately, it came DOA -- a rail fuse would blow upon powering it on.
I know that everyone is raving about Vintage Amp, but I happened to find Threshold-Audio, Corp. in Houston, and just sent it off to them for service and possible upgrade. Anyone ever use them? They stated that they have upgrades for the power supply and FEB (amp front end). Would you recommend them?
Thanks, and look forward to sharing my thoughts as we go...
Zachary (Linn LP12/ARC SP-3a1/Dahlquest DQ-10 fan! Loved those 1980's!)
The 400A was certainly a groundbreaking product for its time, and one of the better SS amps available. I ran one for nearly 20 years with only a few predictable repairs required while logging A LOT of playing hours at decent, sometimes indecent, playback levels. I did not leave it on 24/7, which seems awfully wasteful.

I think the display was bit of unnecessary (and probably expensive) audio bling designed to get people's attention away from Asian products that featured pointlessly complex features and garish designs. I can almost see Mr Pass cackling, "You want lights...fine! I'll give you pretty effin lights!".

But the amp itself was a solid performer. There were others in that price range I liked more at the time, such as GAS Son of Ampzilla, but all three of those that I owned self-destructed within weeks and almost destroyed my beautiful new B&W DM6 speakers (I owned one of the first 50 pairs sold in the US) in the process. The dealer (remember them?) apologetically offered the 400A as a trade-up. The 400A could be a little hard on top, but, oh, what control and detail over the entire spectrum!

Well, after 20 years of excellent service, I realized amplifier design had definitely advanced and the 400A was sounding a bit rough compared to new stuff. It's still a decent used amp, but many recent designs run rings around it.

I was in awe of Nelson Pass at the time, and I still think he is a pretty cool dude who deserves our respect and admiration for his dedication to quality audio and his generosity to the DIY community. However, I wish he would turn his considerable talents and lifelong focus on Class A amplification toward more efficient designs. I'm not an eco nut, but I don't intend to ever use Class A amplifiers again, no matter how good they sound. I would consider small tube amps, but otherwise I'm after efficiency as much as sonic perfection these days. I think running something as inefficient as a Class A amp is wasteful; leaving it powered 24/7, even when not in use, is just plain stupid.
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...