Randy Slone: Opti-MOS

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I'm going to build an amp from Randy Slone called the Opti-MOS.

Go check this out: http://www.sealelectronics.com/

It's a traditional amp with incredible specs. Has anyone else built or looked at these amps?

I called and talked to Randy Slone. He's an incredibly nice guy. He didn't know me, but we talked Audio Philosophy for a half hour. The main theme of the conversation was amplifier transparency. Which I agree with, wholeheartedly. That's one reason I bought the book, "High Power Audio Amplifier Construction Manual" by none other than Randy Slone, of course.

I was looking for a high power transparent amplifier. It can be made to be 80-200W. The worst case distortion is about .04 according to Mr. Slone. That's not the 1kHz 0dB number which is 0.005. That's the worst case anywhere between 0-infinite dB and 20-20kHz. That's awesome!

Don't get too besotted with distortion specifications. They mean virtually nothing. Tube amps typically have distortion specs an order of magnitude higher (or more) and just as routinely trounce solid state amps for sound quality. (Note that there's no spec for 'transparency,' although it's a commonly accepted term to describe the sound of audio equipment.)
If it's power that you want then fine, but there are lots of amps out there (viz. 'pro' amps for PA or what have you) with vanishingly low distortion specs...but most don't sound all that good.
Phrased another way: Negative feedback is a cheap, easy fix.
Tube amps generally have NFB on the order of 6 to 12 dB. Solid state amps...well, the sky's the limit, and the specs get better and better all along the road. But, funny thing, the amps sound worse and worse. Dry and lifeless. Sure, the music comes through, but does it have that magic?
The disclaimer here is that I've never heard the amp you're describing, although I looked at the website once upon a time. It may be just wonderful. I hope so. We can use all the great sounding gear we can get (whether tube or solid state).
Good luck.


You bought a very good book. I think you have already read the chapter two (misinformation in audio) so that you have a consistent basis now to understand why these standardsare important. I'm sure you will make a very good amp.

Tell us the results, please.


I think you need to pick up this book and read Chapter 2.
There is no magic in power amplification. If I want to add
that "magic", I can buy a tube pre-amp to add the 2nd order
distortion you love.

Personally, I own a McIntosh that I love. It's distortion meassurements are the same as the Opti-MOS. I won't say it sounds wonderful. I would say it has no sound. It doesn't change what the artist put on the recording. That is the ultimate goal here!

I've held these views for years. I bought the book by
Mr. Slone because he agrees with me. When we talked, he
stated an even more convincing argument. If you add
harmonics to an already harmonically rich recording like
piano or guitar, it will sound muddy. I personally have
experienced this. I had an amp that when it came to
Mozart's piano concerto #21, it sounded like the
piano was under a heavy blanket. With my McIntosh the piano sounds like... a Piano!

That's the best counter argument I can come up with at the
moment. Any more takers on this Grey's side? I love a good
debate on Audio Philosophy. There can be no winner! It's
a matter of personal taste. If Grey wants to watch a TV
adjustmented so the faces are greeen. Then that's his
perrogative. Personally, I wouldn't buy a TV that is
factory standard that way though. If I did, I wouldn't
be able to watch the TV in real colors. I would buy a real
TV and add a Blue-Red filter to the input.

Pixie, blmn, et. al.:
Trust me, I'm well aware of all the arguments about specifications and tube vs. solid state. I've been at this for a long, long time. I've worked in stereo shops, both mid-fi and high end. I've listened to many, many systems, ranging from beginning to ultra high end.
Here's what I found:
When I began, I, like many others, wanted to make the best choices I could when plunking down my hard-earned money. I thought that the numbers on a piece of paper correlated to the quality of the sound coming from the equipment. I bought accordingly, to the tune of tens of thousands of dollars. (That 's' is not a typo...I spent a lot of money--enough to buy a very nice car.) I was one of the 'cool' people. I bought politically correct equipment...look, the specs prove I'm right! The techno freak in me was a very happy critter. Hell, I still get a kick out of hardware for hardware's sake, it's all part of the hobby. But...but...after I'd had all these various pieces of hardware for eight or ten years or so, I came to realize something, and trust me, this was damned hard for me to admit out loud at the time, for all their technological prowess, there were systems that were simpler and cheaper that sounded better. There were aspects to the sound that I had always blown off as hyperbole at best, and possibly downright lies, that were not only true, but actually correlated to what I heard when I heard music live. Things like image, for instance. I thought I had image. I was only partially right. What I had was a lateral spread, from left to right, so that I could point at, say, a guitarist. I thought that was all there was. I was wrong. With a system that can image, you can hear *deep* into the stage; it becomes three dimentional. Spooky, the first time you hear it. But I couldn't hear that on my system--just the lateral spread. But my prized low-distortion equipment! How could it *not* do everything right? Isn't getting the waveform through intact enough? The answer is: No. What's going on, here? Frankly, nobody knows what the whole story is. Phase shift plays a role in it. I'm reasonably certain of that. I'm also 100% certain that it's not going to be the whole story, either. I have an analogy with medicine. There are diseases out there that can kill you. That's a given. Medical science has made great strides in treatment of some of them. We have a vaccine for smallpox, for instance. Cheap, effective. Good stuff. But just because you have in hand an effective treatment for smallpox and pneumonia and can control some forms of heart disease does not mean that cancer does not exist. Cancer exists. Back to audio. Distortion, to me, represents an audio disease that has been cured, or at least managed. But to pat one's self on the back over having slain one beast whilst others are still on the loose stikes me as foolish. Let's be honest, audio is a niche market. The amount of money that the audio industry can bring to bear on studying pathologies of the sound is miniscule, compared to what's spent in the medical industry in a single day. Will some clever young lad (or lassie) uncover another pathology of sound? Yes. When? I dunno. In the meantime, you and I have to muddle on with what we have. I'm not saying that distortion specs aren't useful--they are--down to about 1% or so. After that, it's time to go looking for other problems. Yes, yes, yes...I'm fully aware of all the 2nd harmonic arguments, but there's nothing like a stereo shop on a slow afternoon, with five or ten really good (spec-wise and/or sound quality wise) amps/preamps/what-have-you to teach you that specs are virtually meaningless in the real world. And I had many slow afternoons in the trade. Even before that I can tell you that the day I bought my Linn Sondek LP12 (yes, I go back that far) was the first time I did something *right* in my audio system. I could hear more detail and the beginnings of a rudimentary image. And this from a turntable (turntables were commonly regarded as having no effect on the sound) that had lousy specs as far as rumble, etc. However, Ivor (head guy at Linn Sondek) had this idea he called Loss of Information Theory. In other words, he'd identified a pathology, and set out to cure it. Others later went far beyond his work, and didn't even bother formulating theories to cover what they were doing, but turntables got better because Ivor had the essential breakthrough idea. Thanks, Ivor. Mind you, I'm not aware that anyone has ever bothered to come up with a method to measure LOI in a turntable. All they did was tighten up the tolerances on the bearings and make all the mechanical parts fit together tightly. But just because no one bothered to make up a spec doesn't make the concept invalid, nor does it mean that you can't hear the difference.
Regarding hearing differences: It has been my experience that there is a breakover point, a threshold below which improvements to an audio system are inaudible. Let's take a gross example. Put the finest, most elaborate turntable/CD player into a table radio (we're assuming phono/line in jacks). Ya can't hear it. How good does a system have to be before you can hear a difference? That's hard to say. It depends on what you're trying to hear. Image, for instance, is particularly fragile. I've never heard a mid-fi system image. Only in rare cases have I heard anything like an image from a (forgive the hodge-podge) lower high-end system. There's also the hurdle that you have to educate your hearing. Huh? You know how to hear, don't you? Well, yes, and no. When people are stricken blind, they report that their hearing acuity increases. Your hearing can be 'taught.' The trick is to heighten your hearing, while retaining your eyesight. Try this. You know your home, right? Try navigating from room to room purely by your hearing, eyes closed (or at night). Listen for the walls. They're there, and you can hear them in subtle ways by the sound that reflects off of them. The ticking of clocks, the shuffle of your feet. Listen! You lose points every time you slam into a wall. Give it time. You're learning a new talent here. You're learning imaging. Then go back to your measurements and tell me where what you're learning about imaging fits in. It doesn't. Neither does transparency, which I happen to think is a very important descriptive term. I can hear transparency, but I'll be damned if I can think of a way to measure it, bottle it, and put it on the shelf along with harmonic, IM, and TIM distortions, S/N ratios, etc. Pixie, if you can figure out a way to quantify transparency, my hat's off to you, tell us all how to fix it, and we will. In the meantime, speaking as one who spent many, many dollars chasing spec sheets and ended up with a pretty flat sounding system, I'll keep going to hear live music (not rock concerts--no longer--too loud and too processed) to keep my hearing tuned up and going for sound quality instead of specs.
Oh, and as a footnote, I own both tube and solid state (Would you believe Nelson Pass era Threshold S-500's? Putting a couple of film caps across the electrolytics in the power supply does wonders for the high end.) equipment, and like both...for what each does well. I have just reconfigured my system for quad-amp (tweeter/mid/woofer/sub) using solid state for the lower frequencies, tubes for the mids, back to solid state for the highs. The dust hasn't settled yet. I have projects in the works which may be of interest to some of you. I'll post when I have results to report. (I seem to remember that there was a post a few months ago about an active crossover question-> Would an emitter/source follower work with a Sallen-Key topology? Yes, it does. If anyone is interested in active crossovers, and if I can figure out a way to get the schematics up, I'll be glad to share. I plan (pax, Pixie) to try a tube version and compare sound-wise, as soon as I can clear out a few other things, like the amp I'm building.)
Jason: Sorry about the length of the post.

Something I forgot to put into my previous posting that I find to be food for thought. In my experience, there have been many, many people who have gone from believing that specs count for something to actually listening and comparing what they hear to real music. But not a single person have I ever known, nor have I ever even *heard* of, who 'saw the light' and went from listening to believing the specs. The uni-directional flow is instructive.
There are people who get disgusted with the pricing of good gear and quit the hobby. There are people who hit hard times and have to accept cheaper gear because they no longer have the resources to buy something better (but don't get in their way when better times come along, because they're headed for the nearest audio shop). There are people who fall to the Spousal Acceptance Factor and have to either drop back or get divorced. There are those who get irritated over the hocus-pocus factor put out like a fog around some manufacturers' product (some of the product sounds good, and some doesn't--absolute dreck--but the hand-waving is highly annoying). And there are those of us who turn to DIY.
But never have I known someone who went back to specs...

You supported my arguement. The turntable is the place to add the "imaging" or "warm sound". Even the pre-amp.
But, the amp will have no effect on that sound if it accurately reproduces the input wave. There is no way adding more distortion in the amp phase is the answer.

If we can illiminate the amp as a sound shaping element,
we can add shaping to any input. Which means we could have the nice 60's warm valve sound from the turntable, but have nice perfect sound from the DVD player.

Personally, I want that option. I don't want a separate set of amps for each medium.

If it's price you're worried about. The Opti-MOS is $162 plus the cost of the power supply. That's incredibly cheap. It would cost less I'm sure if you make your own boards and price shopped for the parts. I'm buying the kit for simplicity. I went through the whole process of etching my own boards and buying the parts for the Zen. I've had that experience, I don't really need to do it again. But, the Zen didn't sound as good as my McIntosh. So, I'm going for quality this time, not an illusive sound.

I absolutely do not want the amp to shape the sound for me.
As Randy Slone puts it,
"The holy grail of amplifiers would be a wire with gain."
Or from Nelson Pass,
"Areas of criticism of the design(Zen) all relate to the objective, measurable performance, but in addressing them, I found that the subjective performance improved with the measurements."

That's the end of the arguement from me. This has been a facinating debate of philosophy, but like I said there can be no winner. We are the extremes of a age old debate. If the hundreds before us haven't come to a conclusion. Then neither will we.

Thank you for the stimulation in my otherwise dull day.

Randy Slone is a very nice guy indeed. However, if you read his web site, you will find that he is also a jesus freak, a creationist that, when it comes to his spiritual beliefs, refutes the scientific evidence to the contrary. He completely turns around and takes the objectivist approach as his audio philosophy. Here only the hard experimental data matters. Therefore, an amp with low measured distorsion will always sound more transparent. As a result of this conflicting views it hard for me to believe the man. I guess that at the end of the day he was able to sell you an amp and I hope you are happy with it.
There is a review of his book on the Jan issue of Audio Express that I recommend you read. The reviewer points out some of the more objectionable and bad-sounding choices for example the back2back tantalum decoupling capacitors.
As far as I am concerned I take an empirical approach: change both the circuit topology and the components until you get the best possible sound to your ears.
Just for fun, I checked out the Slone book from the library. And, for the record, I agree with perhaps 30% of the things he says in Chapter 2. As for the rest...well, there are some real howlers in there.
One thing which I find distressingly common is the setting up of a straw man, i.e. intentionally (or, to be charitable, unintentionally) wrongly stating a case in order to make it fallacious, hence easily knocked down. Case in point: The "Trade Secret" at the bottom of page 19, where he states,"Monoblock construction always [sic] provides audible sonic improvement." Sometimes, perhaps. Definitely a good idea if you're running a smallish power supply because of inability to get, say, a 20A transformer, but can get two 10's and use one for each channel. (Something DIYers come up against frequently.) But that 'always' makes it very easy to take the moral high ground and 'prove' that you're right. It's all in how you phrase it.
His insistence that high end amps are of necessity complex flies in the face of reality, witness the Zen amp Pixie built. In fact, it is generally the simpler ones that sound better.
He mentions the recording (aka 'pro') industry as being well supplied with 'high-end' gear, and holds it up as an 'example' in the sense of someting to emulate. Whew! That's a good one. In reality, much studio gear is not even up to the standards of an ordinary mid-fi system. When he said,"Their livelihood [recording enginneers] depends on the highest quality sound obtainable at any price," I nearly fell out of my chair. In fact, the recording industry is, like everyone else, obsessed with the bottom line, and even in the cases where the folks in the studio want better quality gear, the brass won't let them have the budget to buy it. The product they're putting out is selling just dandy, so why spend more money to improve it? That's on page 30, by the way. Hysterical.
P 17, (yes, I know I'm jumping around, page-wise) he says,"If elusive audio subtleties were actually degraded by solid-state processing, as many subjectivists claim, how could they even exist on the sound medium?" Thanks, guys, this guy's a card. I laughed so hard I nearly wet myself. I guess by Slone's 'logic' if one tiny spck of dirt gets on my window pane, then no light will come though at all, right? By definition, degraded means that the original quality has deteriorated...not stopped entirely. Yet, Slone seems to feel that it's an all or nothing proposition; a brick wall.
The word sophistry comes to mind.
I could go on and on and on. There's hardly a page in Chapter 2 that doesn't have at least one seriously flawed statement. My problem with this is that (aside from the fact that the distortion figures don't correlate with reality--if they did two amps from different manufacturers with the same specs would sound the same, and they most assuredly do not) for a philosophical stance that claims to be based on logic...it isn't. The 'logic' is seriously flawed, and it's all right out there in public. The story The Emperor's New Clothes comes to mind.
When the Hafler apply-the-input-to-the-output cancellation strategy came out, I was pleased. Finally, I thought, we'll get to the bottom of some of these things. So simple. So elegant. The only problem is that amplifiers did *not* take the expected quantum leap forward.
So what gives?
I'd love to know, myself. I hear things that no one has (yet...I'm still hopeful) tracked down in measurements. Does that mean they don't exist? Not by a long shot. Maybe we're hitting transient problems. (N.B: *Not* TIM; I'm not impressed by slew rates. I'd love to figure out a way to look at real music transients in slo-mo, compared to the input. Steady state specs don't cut it when we're dealing with a moving target.) Back to the lab, guys, we're going to have to figure this one out ourselves, because it's a cinch that we're not going to get a grant from Washington. But we won't get anywhere by claiming that the differences/problems with sound reproduction aren't there.
I listen, then try to figure out why I'm hearing what I'm hearing. Sometimes I succeed, sometimes I don't, but that doesn't mean that I retroactively do an Orwellian doublethink and blot out the fact that I heard something. Other folks seem to prefer to tell their ears what to hear, based on what they've read, whether specs or soi disant logical arguments. I'd like to note also that there's an element of self-fulfilling prophecy here--people make up their minds that high end is bunk, so of course they're not going to 'waste' all that money on something that costs as much as good gear costs. Reasonable, on the face of it. (I'll be the first to admit that the pricing of high end gear is horrendous these days.) So they end up with a system that doesn't have sufficient resolution to let them hear the difference between, say, capacitors or speaker cables, then they say (Suprise! Suprise!) that there's no difference to be heard. Moral of the story: Don't buy a Walmart microscope and expect to see paramecium--you'll be lucky to see your fingerprint.
So...what to do? Not an easy question to answer. The quick and easy answer *should* be to visit a high end shop and listen for yourself...something 'objectivists' (Slone's word--I prefer to reserve that term for another use.) are reluctant to do. Why? They've already made up their minds there's nothing to hear...another self-fufilling prophecy. And frequently the sales staff are over-egoed and under-trained, sometimes downright greedy...they want that commission, or they're facing a sales quota (ugh--not a bright thing to saddle a fella with when he needs to take time with a customer). It's not uncommon to find the systems in disarray. Sigh. The problem isn't a new one.
I'm more than willing to have people listen to my system, but I don't think any of you are in my area. As I said above, my system is in transition at the moment, but I can probably demonstrate the reality of a few things that 'objectivists' love to pick on. Then we can put our heads together and try to figure out why such reasonable-seeming things as why the Hafler test don't show what they ought to show. Man, I had high hopes for that one! Trust me, I want to be able to measure what I'm hearing, so as to figure out ways make it even better.
Oh, got the Douglas Self book, too. Will try to at least skim both over the next few days. Yes, I saw that Self has a chapter on objective/subjective. Haven't read it yet. Only so many hours in a day and I've got three audio projects in motion, stories to write, and a full time job.
Jason: I know we're not killing trees, here, but do you have a limit on post lengths? (Assuming that we stay more-or-less on the topic.)

I've now read the subjective/objective portion of Self's book, and am pleased to report that he, for the most part, resists painting himself into logical corners quite as baldly as Slone does. I also enjoy his slightly harrumph-harrumph British tone of writing. That said, yes, he makes some of the same logical errors, albeit more subtly. I find it interesting that he, on page 22, seems to see no problem with a current-limiting amp, i.e. one which is incapable of doubling its power when going from an 8 ohm load to a 4 ohm load. I would think it a trivial observation even to someone who puts their faith in specifications that, given that speakers rarely (yes, there are exceptions) have flat impedance over their entire operating range, an amplifier which could not deliver sufficient current to meet demands would not be the proverbial 'straight wire with gain' that we all desire.
This is, I find, a common failing. Components measure one way on the bench, but differently when faced with the real world. Putting a capacitor on the output while on the bench is, perhaps, a start, but since real-world speakers are more complex RCL networks, I would think that it would be obvious that someone should try more global measurements involving entire systems, just as an illustration of the sorts of things that could go wrong. Yes, I realize it would only show the interactions between that one set of components--this is not to propose some global table of every extant component where one could run a finger across a column and see how preamp A would behave with amp B, running into speaker C in terms of frequency response at rated power. But a limited subset might prove instructive in the sense of revealing interactive errors capable of objective measurement.
In the case above, presumably subtractive ones where, as an example, amp that delivered a rated 100W at 1KHz, and possibly at 200Hz, but only two-thirds of the power required at woofer resonance, or at a crossover point where the impedance dipped.
Pixie, would you not agree that this would be a measurable problem...one with real world consequences? If so, then we have a starting point...no 'magic' involved, just pure math, meters, and oscilloscopes.
Simple enough to cure, of course, bigger power supply, for one, possibly bigger output stage. All this means money. Compromises would be in order, either in performance, or in price. Or both. But it would provide us with a philospohical starting point for dealing with real honest-to-goodness objective measurements of hitherto 'subjective' issues.

You still don't see the other point of view. Your mind is completely closed. So, I'm not going to continue to talk to this wall.

You can slander the guy's personal views or call him plainly wrong. But, you can't prove to me that shaping the sound in the amp phase is a good thing.

Input * Gain = Output. period.

I've been an audiophile for nearly 15 years now. I've been near many high-end audio shops for years. I've listened to dozens and dozens of amplifiers. I simply hate the sound of tubes. They sound muffled. Even the $10,000 tube amps sound terrible. I've listened to just about every type of amp from $300-$10,000. Both tube and solid state. I have come to love the deep image and clarity of the mid-high end solid state amps. $1000-2000. That's why I bought my McIntosh and why I'm going to build one that claims to be just as good if not better.

I just received an e-mail from someone probably scared to post it here. He just built the Opti-MOS and loves it. "Great clarity and details", according to him.

So, I encourage anyone on my side of this great wall to join me and build a high quality solid-state amp. And I encourage everyone on the other side to be happy with the soft glow of their tubes.

And leave Jesus out of this. Some/most of us probably don't believe in him, but that's no reason to reject Mr. Slone. Can't everyone just accept people for who they are? A fanatical devotion to Jesus has NOTHING to do with audio technology. The bible says nothing about thou shalt listen to electronis embedded in small glass containers.

Gently now, Pixie.
You accuse me of not understanding--you forget that I used to feel the same way you do now.
You make a blanket statement that tube amps 'shape' the sound, but by implication ignore any shaping that solid state amps do to the sound. We all want a straight wire with gain. That's unattainable in the real world. How close can we come? There you get into tradeoffs. Don't forget that I have both solid state and tube equipment, trying to balance the strengths of each.
I did not 'slander' Slone. I made no statements regarding his personal integrity, beliefs, etc. I am amused by his illogic...that is not slander.
Regarding things objective vs. subjective, I have found a place in Audio Power Amplifier Design Handbook (Douglas Self) where he grumbles a bit (having positioned himself squarely in the if-you-can't-measure-it, it-doesn't-exist camp) before admitting that well, harrumph, harrumph, yes, it has been proven that absolute phase is audible. Since for years and years, the measurements crowd claimed that it wasn't...and ridiculed high end listeners who said they could hear it...I found his more-or-less graceful retraction gratifying.
N.B.: This is an edit from here on--I hit the submit button by mistake--hope Jason's edit function works.
In light of the fact that measurements people claim that there's no such thing as image depth, I'd like to propose an experiment. Perhaps we can put image on a 'scientific' footing.
Take a decent system and a well recorded record of a small acoustic ensemble recorded in a natural environment. Ask the listener/test subject (optionally blindfolded, but don't cover the ears) to shine a flashlight on the floor where they perceive the sound of that performer's instrument or voice to originate. Mark the floor in that spot with a piece of masking tape. (You can write the test subject's name on the tape if you think it's relevant.) Then--and this is what will make the scientific crowd happy--cover the floor around the speakers with blankets so other listeners are not influenced by the sight of someone else's tape mark. Repeat as often as necessary to get a large statistical sample. If the marks cluster in a smallish area, then we have an image. If they're all over the place, we have no image.
Anyone game?


P.S.: Jason, can we have a Preview button in the edit function, please? It might also be nice to have previous postings handy, like in the reply function. Thanks

[Edited by GRollins on 03-01-2001 at 12:44 PM]
Welcome Imcmaju,
Congratulations on completion of your project! Many people who begin a circuit never finish. You've joined the few, the proud, the DIY... (Okay, enough of that--turn down the soundtrack.)
As I said above, I have no opinion on the Opti-MOS circuit, which I have not heard. My point is that specifications are virtually worthless as a predictor of the sound of an amplifier...and they all have a sound, no matter how vehemently someone insists that *their* amp (be it tube or solid state), operating class (A/AB/B etc.), or circuit topology has no sound at all; that it's all those other poor, deluded fools' amplifiers that are imperfect. I've heard far, far too many amplifers that spec'ed the same, but sounded different, spec'ed flat from DC to light with nine zero's distortion that sounded like sandpaper, or...had mediocre specifications, but sounded just wonderful.
I haven't looked yet, but I am assuming that the Opti-MOS circuit is in Slone's book. If I've got sufficient parts on hand (and can find time), I may try to build one, just to see how it sounds.

I can't believe I read all of that .... I really am having a quiet afternoon ;-)

Herein this should officially be renamed the "Thread of Overly Long Posts" !!

Pixie, please go ahead and build the Opti_MOS amp, looking at the design etc it will likely work quite well. I note numerous similarities in the circuit topology to Leach's amps and having built a good number of these I suspect it will serve you well. (suspicious similarities actually)

While the Leach amp was quite good (I built these in my early teens!!) I was stunned by the difference in performance of a 20W SE Class A amp I built for a friend of my father. This was published in Audio Mag in 1977 ..... designer N.Pass. Yep, I've been DIYin' as long as the best of 'em ;-)

The difference really was quite dramatic. Alas, in my youth I could not afford to build anything like this so lived with a "good" class AB amp for many years.

Now the Leach amp measures well but falls a long way second to my last DIY amp (a Pass Aleph4).

Remember when you look at the Zen/SonOfZen etc these are more simple circuits than the commercial offerings, with significantly higher distortion specs .... for what they are worth (settle Grey!). What NP was doing in those circuits was trying to get the "best" performace he could from a single gain stage, *before* using the design tools available to improve THD etc. Thus, you will find NP's commercial amps do use multiple gain stages and negative feedback, just not very much! (similar philosophy to the *good* tube amps .... yes, they are crappy one's of these too)

While we are here, distortion specs are not "worth nothing" (Grey didn't actually say that) but they are not everything.
No-one is trying to say that an amp with 3-5% 2nd-H distortion will sound good just 'cause its a tube ... nice "bottle". But similarly, not all SS amps sound good just because they have really low THD.

I'll give you an easy example. Build a 100W Class AB amp, biased at 400mA/Ch. Give it a 1A power supply rail. Measure THD on a fixed signal at 1W and all will be fine. Play a dynamic piece of music and it will sound like ****!

Personally, I find the hardest thing is to find good quality recordings. Most of the music today is so "processed" that any pretence that it will sound "live" is just laughable. The vocalist singing 4-part harmony behind himself simply cannot sound "live" ...... if he does, then something artificial is happening somewhere, because the recording never contained a "live performance".

Exactly how do we know what a particular recording *should* sound like ...... we enter a land of much subjectivity, sigh!

On a fun note, I live 5' down the road from Duntech. Took my amp etc down there on Wednesday and spent the afternoon listening to my favourite CD's across the range ... damn good fun (thanks James!)

Hey Mark,
Mea culpa on the length.
I have no quarrel with your restating of my position. Sounds as if we have compatible views. You are correct in saying that I am not against distortion specifications per se, simply that undue reliance on them is ill-advised. I view them as a useful tool in the design phase, but as I stated above, once you get to, say, 1% or below it's time to turn your attention to other matters. (I believe Douglas Self said something to that effect in his book, but without mentioning any numbers. I imagine he would prefer to set a lower number, though.) N.B.: As I read it, Pixie seems to feel (no specific numbers mentioned) that tube amps sui generis have high distortion. Compared to many (not all, by any means!) solid state amps, they do. However, I seem to recall running across some pretty decent numbers here and there, which leads me to suspect that there is at least the possibility that Pixie simply has a hard line position against tubes, regardless. No problem...there are lots of good SS amps out there. Whether the Slone circuit is one of them, I can't say.
I skipped through the book, but Slone doesn't list any of the schematics as Opti-MOS; just Figure 11.4, etc. Which, if any, of these things is the Opti-MOS circuit?
Re: '77 Pass circuit...man, I nearly soiled my underwear the day that issue of Audio showed up in my mailbox! I wanted to build that amplifier so badly, but...alas, none of the parts were available in my area, and I never got around to mail-ordering them. I'm half tempted to build the thing now just as a nostalgia trip.
Re: recordings...you like jazz?

Something I forgot to mention: Douglas Self, in spite of being hard line on specs, has a graph of distortion due to an electrolytic capacitor in the circuit. Since, like absolute phase, image, etc., audibility of passive components has long been a high end tenet, I am again gratified that the nabobs of numbers have deigned to notice that we were right. No credit given to us...natch.
One of these days these folks might, just might, slip up and listen with the wrong resistor in the NFB loop, hence discovering that low distortion specs aren't the Holy Grail they'd been thought to be.
(We probably won't get any credit for that, either...)
Speaking of credit where credit is due, a tip of the hat to Richard Marsh for his work on capacitors; measuring what we knew we were hearing, but had no way to describe in a technical manner. (Also Audio magazine, ca. '70's--you know sometimes those guys could come through in the clinches.)

I think one of the key issues that gets lost in this whole subjective/objective debate is that both schools of thought have merit. Anyone who chooses an amp without hearing it is making a big mistake -- my old Sony ES receiver had great TDH numbers, but sounded pretty bad.

However, to extend Grey's medical analogy, someone who is cancer free but still has smallpox is hardly healthy. By the same note, an amplifier that doesn't have imaging problems but has 2-3% THD is hardly a good amplifier, in my opinion.

It's been shown THD, TIM, etc. have serious and noticeable effects on sound quality. I wouldn't consider purchasing an amplifier that didn't have respectable performance numbers. However, of the many amps with nearly equal good specifications, only a listening test will tell you which one you prefer.

Self, in his book, mentioned this as his criterion for a "blameless" amplifier. He does not claim that all of audio is completely understood, so thus there is no perfect amplifier. However, the designer should go through every effort to minimize the problems that are well known before tackling these more mysterious ones.

To create a new analogy, all cars sold in the U.S. have to meet minimum standards for safety, efficiency, and performance. Even if you could buy a car that didn't meet these specifications, why would you want to? However, just because all cars on the market have these same similar specs does not mean that a BWM is going to drive the same as a Volvo.

Also like cars (but unlike diseases), personal preference means a lot. I like sports cars; many of my friends prefer SUVs. I would personally rather have a Yugo than one of their trucks, but there obviously must be some appeal to them.

Audio is much the same way (probably worse). I've had a lot of people recommend tubes, gushing about how "musical" and "transparent" they are. So when looking for a new amp, I auditioned several tube amps, both SETs and more conventional Push/Pull designs. Personally, I'm with Pixie on this one -- I thought they sounded like crap: veiled, muddy, and lacking in detail. There is no doubt a tube sound and whether that is 2nd harmonic distortion, lower feedback, magical fairies tweaking the electrons or what, it doesn't matter, I just know I don't like it very much.

I know (and believe it or not, still respect) several people who like the way tubes sound. I don't. They probably think my Plinius sounds like crap. That's fine. That's why it's a good thing there are so many choices.

The first big mistake in audio is to believe that objective measures are the ultimate truth. The second one is to believe that subjective measures are anything more than that: subjective. Where tube lovers hear music, I hear distortion and rolled off highs. Where I hear it, they may only hear SS grain and flat imaging.

Ain't it great that the world is big enough for both points of view.

Hear, hear...so to speak,
I am scrupulous about withholding opinion on circuits I haven't heard. Haven't heard the Plinius, but would like to, as I've seen some decent reviews from people whose ears I respect. However, no one within shouting distance has one, so I guess I'll have to wait.
However, I do have one counterpoint, Jon...ever hear the old Electrocompaniet 50W Class A amp? I don't recall that it had a name or model, just the company and wattage. It was solid state, yet sounded more like tubes than tubes. An amp like that will blow all your preconceived prejudices right out the window.
At this point, you fall back on the thing about making your final choice of amp by ear. Okay, I have no problem with that. I do the same thing. But...to make blanket assumptions about "tube sound" vs. "soild state" sound is foolish. (I'm thinking of an Audio Research [don't recall the model at the moment] that, while tubed, sounded more 'like solid state.') I'd give a shiny penny to catch you and Pixie in a blind listening test and watch you try to ID those two amps against one another. (Both would pass reasonable muster on specs.) I'm not saying I could, mind you! It's just that it's damned foolish to get wrapped up in the gain device 'us vs. them' game when there are clear examples to the contrary.
For this, I am grateful for having worked in the industry and had the chance to hear so many pieces of equipment up close and personal--some of which I sold, some were manufacturers' demo pieces, some belonged to customers, and the ones I've owned. Not to mention the ones I've built...

A couple of my made Kiwi mates are big Plinius fans ..... not just because they are "all black", either!

Have heard both their integrated amps and the big ClassA monster. All in all they are a quality product and I would not hesitate to recommend one (especially the later .... I'm getting a little biased in my old age).

Comments re needing to hear before you buy are beyond dispute, however I do feel for pixie, I can remember a time when cash was short and building was the *only* option. A little hard to hear before you build on the most part!

My comments re the circuit are based on the premise pixie would be unlucky if this were a really bad sounding amp .... but you are absolutely correct, this is no guarantee.

Re jazz ... just this morning bought a great dave brubeck CD. Probably some of the "cleanest" recordings I have heard are simple piano/drums/bass.

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