2. member-reviews
  3. 1128468228

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.
ferrari
ferrari OP
617 posts
 
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.
bmcleod-afc4
1 posts
 
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.
ferrari OP
617 posts
 
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.
joe-558f
1 posts
 
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.
audio_ken
6 posts
 
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.
More to discover