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.
ferrari

Showing 7 responses by ferrari

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.
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.
For those of you that have the talent to work on these amps check out.

http://www.diyaudio.com/

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.
From NP on longevity of his amps:

(quote)

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.
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.
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.