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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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?


Dr. Joe:

Yes the Threshold SA 3.9e would certainly fill that requirement. However the SA 3 if you can find one is sonically better. I have heard both and the SA 3 just
seems more musical that the SA 3.9e. But also consider
the newer Pass Labs Aleph 3 or Aleph 30 for speakers of
that efficency in a room of moderate size. And as always
consider a late model 400A, identified with IEC plug in connector as opposed to the ones with fixed power cord. There were additional updates done to the later models as well. Just remember to leave powered on 24/7 and try to avoid use of tube preamps with Threshold or Pass Labs gear as you will have to turn on/off the Threshold and or Pass Labs gear to use tube preamps, as leaving tube preamps on 24/7 shortens tube life and switching the amp on/off shortens the life of the amp.
Found some technical data of the Threshold 400A for the tech minded among us. Not the usual specs one sees from a manufacturer. But then again Threshold under Nelson Pass was not the usual manufacturer. What really mattered was the music and this amp plays music with a convincing authority one has to hear.

Description: Two Channel audio power amplifier employing patented dynamic bias circuits that maintain the amplifier in Class A operation to beyond 500 watt transient output levels per channel.

Rated Power: 100 watts per channel, both channels driven into 8 ohms, 20 Hz through 20 kHz at 120 volt line source.

Harmonic and Intermodulation Distortion: No greater than 1 percent total harmonic or intermodulation distortion at rated power, decreasing monotonically with lower power. Distortion virtually unmeasureable at small signal levels. Harmonic distortion components are dominated by 2nd and 3rd harmonics. Intermodulation sidebands are primairly 1st and 2nd order. Distortion components of higher orders are negligible below 40 kHz.

Transient Intermodulation Distortion: Transient intermodulation distortion sidebands are at least -75dB below a 10 watt ouput signal consisting of a 1.5 kHz sine wave linearly mixed with a -20 dB 80 kHz sine wave. the sidebands are symmetric about the central frequency decreasing to approximately -90 dB by the seventh order.

Rise Time: Leading and trailing edge 1 microsecond.

Slew Capabilities: Maximum voltage slewing rate each channel 50 volts per microsecond. Maximum current slewing rate each channel 25 amps per microsecond.

Loop Propagation Time: Propagation delay appearing at the bases of the input differential transistors 20 microseconds.

Current Output Capability: Maximum instantaneous output current capability 20 amps per channel. Maximum continous output current capability 5 amps per channel.

Voltage Output Capability: Maximum voltage swing -+ 50 volts per channel.

Frequency Bandwidth: +0,-3 dB 1 Hz through 250 kHz at small signal levels. -+0 dB 20 Hz through 20 kHz at 200 watts per channel.

Input Impedance: 75 kohms

Damping Factor: Constant 100 from dc through 20 kHz measured at output terminals

Hum and Noise: .5 millivolts at the outputs.

Displays: Individual peak vs average output level readings for each channel.

Rather amazing specifications for a amp produced and built from 1977 to 1981.
I've been using a Threshold 4000 (the follow up to the 400A) to drive a pair of Beveridge Model 3 electrostats for the last 25 years. I've actually bi-amped with two 4000's upgraded by Jon Soderberg for the last two years. They continue to drive the Bev's tough load without issue and have plenty of power to overcome their relatively low efficency. As you point out, Nelson's known how to do it right for a long time.
Not many amps will drive those speakers past or present amps for that matter.

Yep next on the agenda is a Threshold 4000 and have Jon do his number on one of those for me. Really don't need that kind of power, but always want a 4000 anyway. It is way overkill for my present application though.

Jon does terriffic work on the vintage Threshold gear, in my opinion no one better.
I purchased a 400a about a year ago from a local hi-fi resale shop on their recommendation and after a quick in home test (I guess its the 1st generation since the power cord is permanently wired). Initially I was running an Eico hf85 preamp into it and driving Kilpsch Heresy II's.. I hated the sound really.. it was too fatiguing for me and lacked seriously in bass response. I added a powered subwoofer but could never get it to "fit in"... so after talking with my local shop they recommended a pair of Vandersteen 2ce's (non-signature).. All I did was replace the Klipsch with these babies and now my system is musical and easy to listen to.. The 400a thru these speakers anyway sounds just wonderful; its seems very fast and doesn't seem to fall on its butt when things get musically busy like other amps did I have tried before.. especially tube amps. And the bass is HUGE! This amp can really move and shake from top to bottom of the listening spectrum! I wish it were a little more airy on the top end.. but I bet if I upgraded my 2ce's to the signature series that might take care of it.. or maybe this ol'e baby has some limitations! But who cares? Try and find a better amp used in this power range for $800.. and especially one that is known for its tank-like qualities.. Ok;it did break under my 90 day warranty (great reseller!).. and both of the huge caps were replaced..(I might have had something to do with it.. I was messing around quite a bit with interconnects.. and well...) and now my 400a is back on its feet again.. I do not leave it on 24/7 as some have suggested.. and I am not sure why I couldn't even with a tube preamp as some have suggested... I have tried to listen for the difference cold vs. hot and I cannot tell the difference.. also have bi-wired my vandy's with no appreciable difference.. I have noticed that stuff comes loose inside the 400a from time to time.. so open her up and check all the nuts on the xformer and banana plug every so often.. Anyway.. I guess I am into my system for about $1800 (Denon DCD-100 cd player) now but am pretty happy.. (bought the Eico @ garage sale for $2.00.. what can I say? got lucky)... remove the 2nd @ in my email if you'd like to contact me.. I might be interested in a 2nd 400a if someone has one cheap.. I'd like to try bi-amping my Vandersteens...

Thanks to all who took the time to review this really fun amp.
I just got done rebuilding a later version 400a and yes, overall is a well done amp. A couple of people I have talked with complain about it blowing fuses when really pushed. I have my idea(s) as to what it could be but will refrain from speculation here. I'm currently rebuilding an early version and uncovered the problem of the lack of bass which looks to be easily corrected. Not sure why the original design falls short on this. hotrodding the amp is fun and not too terribly difficult and I'm "modding" it a bit for driving stats without blowing fuses. as usual, the sky is the limit with what can be done,..and plenty can be done. The early version can even be updated to a full fledged cascode if desired and there's plenty of room on the circuit board to get as imaginative as you want. In my mind, it's one of the few amps with all the goods to begin with to even warrant a rebuild.

After installing better parts and thermal tracking, I was told it warmed up in an hour instead of the usual 4-8 hours.
For those of you that have the talent to work on these amps check out.

A wealth of information there, plus from time to time Nelson Pass contributes to those threads and certainly the 400A is one of his favorites.

In my opinion the early Threshold amps and preamps are true genius from the mind of Nelson Pass and readily accept upgrades. The 400A as well as the 4000 and Stasis 1,2 and 3 were cutting edge,over the top designs, that have withstood the test of time, and continue to deliver the promise some 25 plus years since new.

Nelson Pass was trained as a Physicist, as opposed to an Electrical Engineer and I believe this is one of the reasons his designs and products have that certain sonic signature a vast majority of us seek. Plus his designs have been ripped off more times than I can remember.But remember a forged Picasso will never be a true Picasso. So why even bother with a ripoff.

Although Threshold under Nelson Pass continued to deliver one break through design after another and continues to this day with Pass Labs. The main drive has always been the music and accurate reproduction thereof. And to this day Nelson Pass has remained totally dedicated to two channel audio.

The build quality of the early Threshold products set a bench mark that to this day has not been surpassed. These are true lifetime products of the highest caliber. No doubt, this is why these have survived all these years and will continue to do so. Plus the craftmanship of these products are a tour de force seldom found.
Ferrari, ... you sure you're not N.P. in (JK!!!) Yes, I agree that the early amps are truly a breakthrough, monster heatsinks, power supply and all. And they no doubt can be updated with new parts and tweaked to not play second fiddle to any amp. Someone mentioned a Dynaco 416 earlier and did you know that this amp has the same "stacked" output stage as the 400a and 4000 and also has fewer components to muck up the sound? Also, there are circuit similarities too, which sort of makes me wonder if some ideas were not taken from the Dynaco to make the Thresholds. I'm no patent expert or anything, but I can't help but make the observation and have to speak up for the Dynaco and say it's not nearly as bad as some who have slammed it before would have you believe. As far as comparing the amps to each other, it's pretty much a joke as they all have cheap parts which limit the sound. Hotrod each amp and the differences become less I believe. Too bad it's a fact of life that good parts are not used in manufacturing most amps as they are so incredibly vital to good sound. But it is true the Thresholds took it all to another level and made us realize that we need amps like this to get the job done. I got my 400a and 4000 and feel complete with not much need to look further. I look at most other amp's power supplies and heatsinks and just chuckle! Amps like this are a rare breed indeed.
From NP on longevity of his amps:


In fifteen years the electrolytic power supply capacitors will get old. Depending on
usage, you will begin to have semiconductor and other failures between 10 and 50 years after
date of manufacture. Later, the sun will cool to a white dwarf, and after that the universe will
experience heat death.(end)

In my experience with Threshold amps and preamps, it is about at the 25 year mark, where service should be done.But it all depends on usage and how used as to when service should be done.

Just had Jon Soderberg go through the 400A and NS10 I have and should be good to near heat death - LOL.

After 47 years in this hobby and other than Threshold gear, there is precious little else that warrants service,or modding. And where else can you get help and advice from the man that started it all.
Not a 400a response but a tilt of the hat to Jon Sonderberg for his amazing mod skills. He just moded my Threshold s450e amp and Forte 44F preamp. A truely nice update and they should last me the rest of my natural life too.

Oh yeah, I am the orginal owner of the s450e (1990) and Jon stated when he removed the mail caps one of them had a little leak happening (material damage to cap but no where else). Even though I was just doing some preventive maintenance/life extention by asking him to replace them in the first place, I'm even happier after I learned what he found. A leaking main cap is not a good thing.

I have the big brother to this amp the 800A. It has been, and continues to be a pleasure to own. What can you tell me about it?
Az Ed
From what I remember on this unit. It is 200 per side Class A output and the only Threshold amp made with a cooling fan in the back of the unit. Very short production run. I believe only 168 of these were produced. This was the first power amp from Threshold, produced from 1975 to 1977,under the leadership of Nelson Pass. Very impressive with its large VU meters and would drive the hardest of speakers loads with ease.

If it were mine I would ship off to Jon Soderberg at vintage amp, for a complete check up and upgrade of the Caps. This is by far to rare of a Threshold piece to let something happen to it. If it hasn't been serviced recently,it would be very wise to do so, before it needs a major rebuild.

Got a great one, enjoy it.
Absolutely fantastic review from a guy that fnally was able to purchase a 400A 20 year after introduction. With depreciatin of the Amp itself and 20 years of increased dollar value all contributing to putting an "extremely" high end esoteric class A amp into the hands of a pioneer reciever guy like the reviewer.
Unfortunately high end amplifiers are not like 250 GTO Ferrari's in that the car was impossible to buy 20 years ago and now thirty years later is "beyond" possibility for ownership combiming the resourses of the reviewer and "everyone" he knows or has ever been aquainted with " to consider purchasing used now!
It's really sad when thresholds become low-fi equipment and attract reviews from the integrated sanyo japanese audio crowd.
Best leave the reviews to audiophiles who purchased and "used" these Amps "decades " ago rather than reviewers who consider themselves on the cusp of high end because the amp came "boxed with instructions!
It's impossible to live in 1980 no mattter how hard you try!!!!!!
Regards Robert Lock Threshold-Ampzilla- Bryston
Hi Rlock
I didn't quite understand what you meant by your post- who were you criticizing?
first off, where does it say the reviewer has a Japanese receiver? second, who cares if he does? so you think audio is only for the rich and a middle class person who enjoys the best sound he can afford (pioneer receiver) is inferior to you? screw you!

no one is better than anyone else. and money and toys are certainly not a criteria to judge people. we all die and you can't take your possessions with you!

besides, what makes you so superior? you own nice gear? so what. you didn't design or build it. you just flipped your fricken credit card at the dealer. oh, that take lots of skill!
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...
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.
the more recent models still sound great especially paired w/ a Pass Labs pre-amp.
Happy Listening!
Thanks ferrari for a great review.
It puts even more life and certainty of the Threshold 400A.
I bought mine in March 1981, almost 39 years ago.
I went into the shop Best Sound in Stockholm, listened to its sound through other components. Finally I bought it. It was there in the closet for 2 years before I bought more parts in the stereo system. I was a student and couldn't afford more after I bought the Threshold. I never regretted that purchase ))
It still delivers, it still lives, it still performs magic sound.
It will last surely through 40 years.
Many other components failed throughout the years; preamp, cassette deck, CD-player etc. Still the Threshold amp stands tall.

I have a pristine re-capped 400A, a later model with an ICE input, that is part of my collection of Threshold gear.

A good friend who loves tube amps, builds his own and their are quite good, burrowed the 400A just for fun to see how it would sound in his system.  He was some what shocked how good it was!  He said as far as timbres/colors it sounded just like his tube gear and had a lot more bass power/control on his speakers then his EL-34 based amplifier.  Now he is only in his early thirties, therefore he knew about these legendary Threshold pieces, but had never heard one before.  He chuckled that an amp that's older then him, particularly a SS design could sound this good now.  

I have also a pristine SA-4e and a 550e that compete with most amps on today's market and actually out perform many of them.  Yes, the 400A is not quite as transparent as the later Threshold amps, but it still has magical timbres and tonality and overall musicality even today.  Nelson was and still is a genius at building musical devices.  For me the wonderful XA-25, which I reviewed professionally and bought the review amp, is a modern version of the Threshold 400A. 
I am an original owner of the 4000, bought new in the early 80s....I sent it off the Jon Soderberg around 2000, and continue to use it, w/o problems, to this day....I have had several preamps over the years, currently using LTA MicroZotl and Herron Phono stage, and things sound real nice.  I suspect my low end has gradually lost a little, but as it is gradual, I cant be sure.  Am getting ready to send it back to Soderberg, but wonder how it  sounds compared to current Pass Labs amps?  Anyone have any experience?  If the change is substantive, I would consider a switch and selling my beloved 4000.(any interest?)......

I will answer my own question, posted just above.  My Threshold 4000 was sold to a wonderful new home( it was actually emotional to let it go after all these years of wonderful sound).  I replaced it w Pass Labs X250.8. Let me say, briefly, the improvement is quite noticeable In the bass, midrange tone, air, soundstage depth and separation.  In a word, amazing!
Kudos to Nelson Pass!

Old thread, but I recently purchased a later model 400A re-capped, upgraded and including XLRs in place of the RCAs. My local highend shop convinced me that I would not be disappointed and boy were they right! My daily driver is a Mark Levinson 585 integrated, and it sounds a little cold and sterile in comparison, which I didn't really think until the Threshold was put in place. I have had many Amps over my 50+ yrs and I can't remember one being this 'listenable'. It is not the last word in resolution and blackness of background, but it is soooo smooth and vocals are as good as I think I've heard (in my system). It is also not the most neutral Amp especially by todays standards, but is as musical as one can reasonably ask for from a $1500 investment. Now I'm wondering what a Stasis model would sound like in comparison?.....