"Warm Sounding" Solid State Amplifiers

Thanks again folks. The Threshold amps appear to show up at a lower price points on the used markets. Seems worth while to check out! If I'm lucky I might find a seller in Southern Ontario. There's one for sale in Alberta but freighting heavy and expensive things across Canada is somewhat of a costly hassle.

As for Luxman amps I tend to see a lot of the more vintage models for sale.

Csontos, thank-you for your reasoned reply. I thought you might like to hear a number: 600, as in 600V/uS, the risetime of the output section of our amps. Tubes don't have to be slow nor dull any leading edge transients.

I agree with Mapman- there is something that is neutral, neither bright nor dull. The thing is, when audiophiles talk about bright or dull it is rarely about actual frequency response errors. The brain translates distortion into tonality. This is why single-ended circuits (tube or solid state) tend to sound 'warm'; its not a frequency response error, its the brain translating a 2nd ordered harmonic into tonality.

Our ears use the higher orders, 5th and above, as loudness cues- the brain uses these harmonics to calculate how loud a sound is and does not rely on the fundamental tone for that. As a result, our ears are more sensitive to these harmonics than the best test equipment- we can easily hear these harmonics as distortion where its difficult to measure.

This is why two amps on the bench might have the same bandwidth, but one might be bright (has a trace of higher ordered harmonic distortion) while the other does not. In addition, the ear/brain system while translating higher ordered harmonics into brightness, also translates the same into harshness.

Its my stipulation that it does not matter whether the amp is solid state or tube so long as these harmonics are not added by that amplifier. IMO/IME the best systems have a sense of ease at any volume and there is never the quality of 'loud' no matter how loud its actually playing. This BTW is a property of real music.

Sunn made some guitar amplifiers that were solid state back in the 1970s. Now if anyone here plays guitar and uses an amplifier, they know that most guitar amplifiers are vacuum tube, on account of if you want to overdrive (distort) them, you need tubes to have them still sound like music. But Sunn made solid state amps that at the time and to this day were recognized for having a 'warm' sound to them. When you look at the schematic its obvious why: the preamp section employed FETs in a single-ended topology with zero feedback (the FETs being very linear did not need feedback; the primary distortion product being the 2nd harmonic), and the power amp section was also single-ended until converted to push-pull by a driver transformer. So the power amp was rich in 2nd orders as well.

Now I am of the opinion that the 2nd order that is traditionally associated with tubes does not have to be there. That is why we make our amps fully differential and balanced; even orders are canceled not just in the output section but at every stage inside the amp. So while they sound smooth and are detailed and fast, they don't have the warmth associated with a lot of tube amps. The take-away is that topology plays a bigger role.

The Sunn amplifiers are considered obsolete designs, but if you really want warmth as a sonic attribute finding an amplifier of that type (or building one) might be a consideration.

Ralph, can you describe live unamplified without reference to amplified? Where does 'warm' come from in reference to unamplified? To me, the 'holy grail' is 0 distortion. With that comes perfect neutrality vis-a-vis, 'live', no? Ime, non detectible IMD/TIM results in the intended goal when the FR is truly flat. I truly believe this is what we're all looking for whether we know it or not. It's the jargon that gets in the way of arriving at a real understanding of what constitutes live sound in an amp.

What are the loudness cues our brains use for natural, unamplified sound, or a tube amp? Without higher order harmonics, what do we use?

When you describe "bright" sounding amps, are you comparing to other amps, ie: tube, fet, etc.? Or are you using live unamplified sound as your reference? More often than not the comparisons I read are tube vs. ss.

I think live as an established reference for both tube and ss reveals the only truly accurate yard stick in ranking the performance of an amp.

Both fall short, some deliberately(usually tubed),some not(usually ss).

What you appear to be making very clear is that distortion is the arbiter in judging neutrality. So other than utility, tubes are not necessarily required to achieve the desired goal. And I don't want to beat you up on this but if I may digress, you did use the term 'warm' in reference to live unamplified sound.

Having said all that, I suppose my next question would be, 'typically, how flat is the FR of one of your amps? I mean if you stretched the tape out to 5 feet.

The most notable difference I've experienced among the amps I've owned and still own, is that character you touched on regarding volume. I have only two amps, both ss, that have that quality of high volume without fatigue. This is mine, and I believe everyone else's natural yardstick to recognizing life-like sound and coincidentally, noticeable distortion.

And this is the crux of the situation as they say. It now becomes quite obvious that for the most part, you by default, are correct in that very few ss amps with that characteristic exist. All those apparently 'warm' ss amps are going to hurt just as much as any other low distortion ss amp when cranked.

However, it didn't take $100,000.00 for me to acquire that quality. But it did take time/research. Interestingly enough, my favorite LSR&D amps are also obsolete. I think it serves to point out that these amps are the late Dr. Marshall Leach's famed LOW TIM design. They can still be had as diy projects. Also, this design has been incorporated into some other brands such as Heathkit(vintage), some Crest(sound reinforcement), HK 990, to name a few. The other is my Boothroyd Stewart Meridian 105 monos. The Brits have a pretty good handle on what we're talking about here imo.

Also, I think the famous Bob Carver Challenge has a place in this discussion. Still not sure if Mr. Carver is in a league by himself. On the one hand ostracized for having the ability to mimic the sound of any amp, ss or tubed without having to copy the circuit, and on the other, celebrated for his sonic achievements. He's clearly shown that neutrality is a matter of circuit design and that the question of tubes or ss does not enter into it.

I still wish I would have scooped that S-30 before it was too late, though:)

I currently own the NAD M2 and as much as I really like it, I would never consider it warm, technical would be a better way of describing it. My front end is digital so it meets my needs just fine. If I was using a TT, this is an amp I wouldn't buy.

Many years ago I owned a Bedini 25/25 and that is an amp I regret selling. It was what I considered warm in sound but hot to touch. Class A power meant a lot of heat!

When I was thinking of selling my M2, my thought was to go towards NAIM for that warm sound. In the end I thought there was nothing really wrong with the M2 and it sounds great with my Harbeth 30.1.

My favorite thing about the amp is I don't need a separate DAC. My system looks nice and clean, very minimalist.

Good luck with your search.

Csontos, it does not matter if it is live or reproduced, our ears listen for those harmonics (which are always there) regardless. We can't change that!

Quite simply, the application of physics to design equipment to honor our human hearing/perceptual rules will result in better sounding equipment:

What we *can* is change our approach to how we are going to playback recordings; i.e. design the equipment with intention to simply not make those distortions to which our ears are the most sensitive. These distortions are IM (which might also be termed a special form of 'inharmonic distortion') and the higher orders of harmonic distortion, the 5th and above.

(Our amps (Mk3.2) are full power out to about 300KHz since you asked.)

Those designers like Nelson Pass and Charlie Hanson (Ayre) that have sorted out that feedback can be dispensed with are also demonstrating that such leads to a more musical approach. Norman Crowhurst is required reading for anyone designing audio circuits. About 60 years ago he wrote about how the application of loop negative feedback in an amplifier that does not exhibit higher ordered harmonic distortion (like an SET which might have the 2nd, 3rd and 4th harmonics) might well reduce the lower orders to vanishingly low levels, but in the process higher orders (starting with the 5th harmonic) will be added going clear up to the 81st harmonic! In addition, intermodulations can be introduced at the feedback node in the amplifier.

The result is that the noise floor is fundamentally altered. In an amplifier that has no feedback, the noise floor is hiss, not unlike that of the wind and the sound of water moving. Not by coincidence, our ears are adapted such that they can hear into such a noise floor, some say as much as 20 db but to be safe 10 db for sure; this is the *one* exception to the human ear's masking rule. This allow us to hear detail that exists below the noise floor of the amplifier and if you think about it, essential to our survival.

When loop negative feedback is applied (per Crowhurst) this noise floor is altered and while it might sound the same, the peculiarity is that our ear's making principle is in full force- we cannot penetrate that noise floor, so the detail below that point is lost.

This is why amplifiers that employ loop feedback seem to loose low level detail in which room ambiance and imaging detail in the rear of the soundstage resides. To hear this occur, you must start with a recording that has plenty of depth, then you can audition that difference between the two approaches.

Since the application of loop feedback also adds harmonics, this is why any amplifier using it can sound brighter as well, since our ears sense those added loudness cues.

Nelson Pass has a wonderful article on distortion on his website: https://www.passdiy.com/project/articles/audio-distortion-and-feedback; Nelson is one of the leading designers worldwide.