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Bender Comments Page

I just thought I'd write down some comments, from when people ask me questions, so if you pose some good ones, the answer may appear here.

For the past several years, I've gone beyond building other peoples' designs, so I get asked about either the sound, distortion or frequency response of my Rebuild Amplifiers.

Well here are some comments from others, and also some of my own:

First, some comments that other people wrote, which appear in my mailbox from time to time... mostly talk about amplifiers, reel to reel tape decks, bench testing for wow and flutter, or distortion specs, and sonic fidelity...

Comments PAGE 1 / About Amplifiers #1

"In practice, I pretty much agree with Chris. Looked at in terms of measurable distortion, 100% fidelity has never been achieved. New equipment is continually available, and measures ever-cleaner on the bench, but often ends up sounding either horrible or boring.

The gear most preferred in pro recording, whether tube or solid-state, is very likely to have a 'sound' (which includes it's residual distortion) with a tendency to help create an *impression* of greater 'fidelity' (for want of a better word). Yes, (Kurt) it goes against the grain of textbook engineering but don't you (if only half-knowingly!) often benefit from such benevolent imperfection?!!...

An analog: Impressionist painters saw the finite nature of Realism as a technique, and decided a quantum leap was necessary they purposely adopted a 'distorting' method that could produce 'larger than life' results (more 'real' in some ways than the new-fangled photograph!)"

-Dave H.

Reply to Comment #1

It must be that you just happen to like the sound of distortion. If I record my singing voice with a recording chain that makes me sound better than real (distortion), this is good. But it seems that the allure of some tube gear goes beyond just the distortion characteristics. Perhaps there is solid state gear that can do the same thing. Some of the pre-amps that I use seem to create a much more realistic space.

The way I hear it is that they make mono sound like stereo, much more three dimensional. I have no way of determining if this is distortion, but if I have a choice between a recording that sounds 3-d and one that sounds flat, I choose the 3-d regardless of what you call it. - Jim Kollens

********** And Bender Sez: **********

Both of these writers, have brought up two very interesting points... The first that no amplifier is yet perfect; the second and more important point being about the dimensionality of the sound. I take that to mean the clarity of sonics between each note, which defines the overall attack or lack of slur in the attack of the individual sounds, for want of a better phrase... Yes, this is primo-important, very important.

These things would seem to be best described as the "detail" in and among, the inherent perceived discontinuities defining the sounds. Such clarity, can be perceived by the ear both at lower levels and also at high peak levels, perhaps as an overall matter of clarity... Like the plucking of a guitar against the cacaphony background sonic of all the other instruments.

To be very exact here, is difficult, due to limitations of the language, but it is an experience that we all can hear, one not of frequency response, but of: cleaner / dirtier, and perhaps the lack of ringing, which would be an extension of a stopped signal within the time domain, which being in the realm of discontinuities ( a foreground versus background signal - so to speak ) may have more to do with some form of signal-to-noise ratio, than with distortion properties.

While I think that is stretching the analogy a bit, even if it did have to do with that, it might be a peak signal to noise, or even some inter-signal to not-signal form of signal to noise ratio.

If you've ever used a cell phone, then you know what strange between the voice sounds come out through on such a system, a weird non-voice, and also non-musical content that gives it an errie out-of-this-world phasing sonic, and an often nasty quality due to aliasing aspects and compression schemes resulting in a highly unnatural and often equally unpleasant sound eminating between, as well as along with the voice content.

In analog systems such "between the voice content" lets call it, should be: absolute silence, but often I suspect it is not, and perhaps that defines for me, what any given amplifier is doing to the music. Or, rather, between the music.

Problem: Since neither distortion measurements, or frequency response graphs use non-discontinuous input, in the absence of a non-alike input, the use of a continuous sine wave signal, can never impart any aspect of discontinuous effects present, or resulting from an amplifier's amplifiying. Which is why advocates of simply decreasing THD Distortion to Zero to make the amplifier perfect ( such as Douglas Self, and G. Randy Slone ) are truly deluding themselves with all but meaningless measurements.

So, I've never much been a fan of either distortion measurements, or frequency response graphs, as my own interaction with both seemed to never instill in me, a direct link or correlation to how the amplifier sounds with high quality music source inputs ( the resulting sound ).

Therefore to post such graphs is nearly meaningless, and should the graphs even be identical, what would that mean ? Would it mean some finite limitation of the test equipment has been reached, or that whatever differences of sonics that can be heard are aspects which simply cannot be distinguished by mere THD distortion measurements, or frequency response graphs ? I'm quite sure it is the latter, otherwise one must assume that all amplifiers simply sound the same, which is laughable :-).

So while either distortion measurements, or frequency response graphs, are insufficent to the task, I do use such testing, and such test equipment -to find if something is obviously wrong. If something is grossly distorted, that is an aspect which test equipment can do well. One does need to know the baseline distortion or frequency response for a given design, even though, neither measurement will tell you how it will sound, good or bad.

Unless you have developed a new method of attaching big test load resistors on both sides of your head, and managing by some method to be able to use them to listen... in which case, I would suspect the load resistors are "singing" too much, and that "they" are defective. So I do trust the ears more than the meters, and the combination of the amp and the speakers is truly the only thing the "reality of the ears" really cares about.

Others have written to me with inquiries about the Citation 12 PASS Modification, and I like some many others have found the exact parts used, are simply no longer available, and the same for the revised parts. So for most, it simply is no longer an option to build that. While it might be interesting to give it a listen, along with other amplifiers, indeed, I might try and conduct such a series of listening tests at some point soon.

All in good time, I hope to be able to get some people over here to do some listening sessions, comparing many amps... including, but not limited to: my Dynaco Stereo-120.ULT/Level 3a, same with the newest Options 1/2/3; my new Bender-Citation 12.ULT; against the B&K ST-140, the Hafler DH-120, as well as the PASS modified Citation 12. and possibly tube amps too. I hope that I or some others will be able to write some reviews about these various listening sessions too.

-Steven L. Bender

comment #2

Background: In February, 2002, I bid, and won an amp described as the "Nelson Pass Mod of Citation 12" this was on eBay. When it arrived, I questioned the seller about it, since it had what I noted were some significant differences ( circuit omissions mostly ) from what was a well defined set of specific changes described and published in the Nelson Pass article that appeared in Audio Amateur magazine in the second issue back in 1981.

The most notable differernce in there being the Hitachi Vertical Mosfets on the heatsinks, instead of the higher power dissipation capable International Rectifier Enchancement-Mode Mosfets...

Ebay Sellers Response: wrote:

hi wasn't my intention to be misleading, as far as i knew, it WAS a Nelson Pass Mod (that's what i paid for) please bear in mind, this was the version of the mod that existed in 1982. I haven't been to the PASS site, so, i can't say how it compares to the current version (if any differences at all). I didn't do the mod myself, i had a friend do it some number of years ago......i really didn't want to sell the amps, I was forced to for monitary reasons.

As far as the holes, these amps were a quantity purchase by Stereo cost Cutters, from a military installation, and several of the amps were somehow linked together, therefor the use of holes for connections.

I never really considered the citation 12 to be much of a cosmetic piece anyway, matter of fact much of the slick Press's criticisms lead to the demise of the venerable 12, to be replaced by the flashy bells/lights/whistles of the 16a. Still, on a perfomance scale, the Nelson/ cit 12 equals at least $800.00 of todays cheaper dollars worth of amplifier.

Yours is the second unit in a series that i have sold, the other buyer was very pleased, and paid close to the same bargain price you did. That's the function of the bidding process, for i " as in me, i" cannot raise or lower said price, it's a function of what one is willing to pay. sorry you are dissappointed, if i ever obtain enough money to buy more than beans/ and rice, i'll offer to buy the unit back from you.

And he included these added comments by the actual modifier:

I don't know what the buyer is complaining about, all the parts came from audio amatuer and the output FETs were what Nelson Pass himself, on the phone, said to use in place of the outputs that came with the kit. The original FETs either didn't work at all or stopped after only a few minutes of service. We got replacement FETs from AA and they did the same thing in spite of being careful for static discharge problems.

Those amps worked for at least 13 years with no problems and were asked to drive a good many different speakers to very high levels. As for the holes, the amps came that way, but the dust and dirt is what happens when you never need to take the amp apart.

********** And Bender Sez: **********

There was only one article which appeared in early 1981, which also appears verbatim on the Nelson Pass web site. Its highly likely your amp builder blew up those original MOSFETS because...

1) Your friend didn't use any of the proper original small signal transistors, he used general NTE substitute-types, which probably were/are different in their charactoristics.

2) Both Spike Reduction / Reverse Conduction diodes D1, D2 were simply omitted from both channels.

3) The power supply was not modified as specified in the original article. Both bypass capacitors and noise/spike reducing capacitors were simply omitted by the builder. These which are part of the Nelson Pass Modification - are needed to prevent the modified amplifier from self-destructing. ( As he admits, he apparently found that out, not once, but twice, right ? But still he left those parts out... ).

Instead a 1000 ohm resistor was placed across one of each pair of power supply capacitors in each channel - for some unknown reason. Although I suspect ithese are responsible for causing the set to have a nasty loud pop as its power is turned off. Given the wide bandwidth capable in this set, the original design needed items 1, 2, and 3, or, doh! - The set would self-destruct.

4) The 2SK135 Hitachi Vertical MOSFETs currently residing in the amp in question really should have blown up at any time, simply because of the unrestricted Drain to Source voltages present in the circuit. Since said circuit was designed around Enhancement Mode MOSFETs, which these parts are NOT!

Vertical MOSFETS do need the added voltage limiting protection between their Drain pin and their and Source pin as is found in the circuits of the mid-1980's Hitachi, Hafler, B&K and all other power amplifiers which utilize the 2SJ50 and 2SK235 Vertical MOSFETs as the output devices in their designs.

5) Lastly, the Zobel Network was also changed, from a .068uF to a 0.1uF cap, representing an almost 50% change, and also the specified carbon composition resistor was implemented with a metal-oxide resistor, a combination of factors, which will certainly alter the overall frequency response, by decreasing the audio bandwidth and introducing changes in the final frequency response of the amplifier and its feedback stability.

Frankly, I don't believe Nelson Pass himself would have instructed anyone to do such a set of substitutions and blatent omissions without taking the mitigating factors into account. If he had, I'm sure he would have in doing so, modified the circuit to add in protections. I've never seen or heard of any addendum or instruction about such a set of changes as detailed in the five items above, which doesn't make much sense.

That your friend states the set worked for 13 years... Well, so what ? The rest of his response was most probably made up, simply because I called this bastardization - a bastardization...

You can call it whatever you want, and you can sell it on ebay, but it is still a bit of a stretch and still misleading to call this set, "The Nelson Pass mod of Citation 12", when so many areas of the Pass Design were either simply omitted, or changed without thought or consequence. Such actions make it an unfortunate bastardization of the original design as put forth in the 1981 article.

I design Rebuild Kits for The Dynaco Stereo-120, and now for the Harmon-Kardon Citation 12 using a special type of modern proprietary semiconductor that I have made up specially for the task. Since people still email me regularly, asking about comparisons to the Audio Amateur / Pass Modification, I spent my money to get this set, for the purposes of having a baseline "Pass Modification Amp" to answer those questions. This set is pretty much useless for that purpose.

As the original IR MOSFET Parts are no longer available twenty years later, it is unlikely I can redo your set to be exactly like the PASS Mod, so it is less useful to me than I had hoped. I have six other H/K Citation 12's, my design prototypes, representing more than half a dozen variations of my own Rebuild Designs, which I've spent the last two years and a half years perfecting, as I finalize this project.

While the current amp may work, I have not had time available to test or fully check it out yet. It is unlikely to either have the same frequency response, distortion, signal to noise ratio, or sonic charactoristics as the actual Nelson Pass Modification for the Citation 12 would. I don't have the time or inclination to rebuild someone elses' set at this point in time, just because they couldn't do it right the first time...

Lastly, omitting the fact that there were extra large holes made in the sides and extra small holes in the sides and bottom of the chassis, could have been self-serving. It is unlikely I, or anyone else, would have bid so high, on a set where the chassis was so chopped up with extra holes. Such lies-by-omission do not indicate that you were being truthful about either the amp or its cosmetic condition.

Maybe you didn't know much about the Pass Mod, but you certainly saw those extra big holes with the sharp edges... and neglected to mention them.

-Steven L. Bender

Comment #3

Background: In May, 2003, a thread on the Dynaco Stereo 120 went on for a few days... on Newsgroup:

Subject: Re: Dynaco ST120

"Arny Krueger" wrote "bigdogdude" wrote

I replaced both caps on each channel - and then had the main PS regulator fail, not a biggie. I was so impressed with the sound of the amp, that I had to, uh, make sure it kept working for a while ;) This amp has an unenviable reputation for sounding bad. I suspect that field failures had something to do with that reputation.

The amp's most notable sonic feature IMO is the fact that it in fact has no output inductors, due to the fact that the ones that were put into the design didn't electrically materialize due to bad implementation (the shorted turn effect I mentioned earlier). This means that the amp doesn't have the usual high frequency dip when driving most speakers - it might actually sound a tad brighter than other amps with output inductors that work.

********** And Bender Sez: **********

I'd say lets get some historical perspective here, as I disagree with much of what is being said here about the Dynaco Stereo 120...

First lets go back to 1962, there were not a lot of silicon amplfiers out in the world. There were tubes, Fisher 500C's, Stereo 70's, Dynaco MKIII's, and there were Germanium sets - like the Harman-Kardon SR-900's or the Citation A and B...

I don't know if the Dynaco Stereo 120 was actually the first power amplifier to use all silicon transistors, including the outputs, but when it came out in the 1962/63/64 era, it was pretty much all alone in that respect. Silicon was "new" and there wasn't a whole lot of it. In terms of high ampere silicon output transistors, you basically had 2N3055's, and 2N3055's, and other parts, which were house numbered 2N3055's.

All Humor Aside, the very minimalist design of the Stereo 120 was rather revealing and sonically pretty decent for the 1960's.

Yes, the output devices were used at or beyond their limits, when pushed hard, as the heat sinking was very minimal and they certainly did thermally cycle. But they were also run CLASS B. CLASS B with an NPN/NPN output stage. And yet the sound was tops, or at least Stereophile had the Stereo 120 listed as a Class A Component, among all components available in that mid-1960's time-frame...

Yes, it its circuit is a weird design, with some transistor parameter interactions that were never quite disclosed or set down in writing by Dave Hafler and his designers, whoever they are.

Yes, there is that quintessential output inductor, which being a physically large output coil, using a large diameter and thick, heavy wire ( as opposed to the small ten turns of thin #26 wire on a resistor type found on most other amps ) so the thick wire coil is really a smaller influence, and is also within the main feedback loop, so I doubt it is nearly as nasty as some people would have us believe.

The protection was in the form of a highly unusual regulated power supply, with current foldback limiting, so the output transistors would not attempt to sink infinite current into a short and self-destruct. Probably came in useful sometimes, without any sonic detriment when it wasn't in foldback mode.

Let us also not ignore, or forget... that even if one manages to connect heavy Gauge 00 speaker wires on the output jacks, or thin speaker wire, all that total 60 Watts of output power still must get through the just larger than a hair-thin Emitter Lead present inside each of the output transistors. A quite sobering thought for most end-users, as well as for manufacturers of wires and speaker cables as thick as your wrist that empty the wallets of so many eager audiophiles!

Also interesting is that in all cases of TO-3 power transistor failure that I have been able to evaluate, the actual silicon die is blasted and exploded apart, while the hair-thin wire remains intact.

Yes, the Stereo 120 was made available continuously for about 20+ years, not many other amps ever were. Production probably exceeded 100,000 or 110,000 units, I doubt any other set was ever so popular. As for the many field failures, they can be placed into three general categories:

1) The silicon being new, was being pushed beyond safe limits, but designers had no history of working with them, so designs were not at all "conservative".

2) Amps were built to a price point, and a rather cheap price at that. So parts were inconsistent, there were some kits that had carbon comp resistors, some had carbon film, some had a mixture, some had good caps, some had cheap caps.

3) What other amp had like 75,000 "kits" soldered and built by total amateur teenagers in the 60's and 70's... So what would you expect ? Probably Total Novices at soldering, most of them. So most Dynaco Stereo 120 amps would eventually blow up immediately, or some 10, 20, or 30 years later, most not from the faults of the design, but from: bad soldering; bad soldering; and bad soldering...

Yes, compared to the Stereo 120 modern day amps do sound different, and oftimes I think modern day amps sound WORSE. 3m3 

In the usual, and extreme case one now finds half a dozen or more complementary/symmetry stages of NPN/PNP devices cascaded so that the particular non-linear aspects of those "improved" designs get compounded and snowballed as the NPN and PNP devices are hardly EVER quite complementary and are not truly symmetrical...

For instance - equal ft's (no); equal beta's (no); equal time delays or group delays (no); same carrier charges (no); equal ( INSERT ALMOST ANY OTHER OPERATIONAL PARAMETER HERE! ) and the answer is NO! Now all these impurity defined problems of NPN / PNP device pairs are then under the influence of massive amounts of global feedback... to make it work. So is it any wonder modern amps today are often sterile sounding ( also sometimes blamed on precision metal film resistors ) ?

So when 35 years later = modern ( improved small signal and power transistors ) are properly ( note - I said properly! ) inserted into the original Dynaco Stereo 120 circuit, along with other more modern caps and resistors; one has the highest possible: clarity, transparency, and signal to noise ratio. One also has a very musical amplifier which comes closer to the sound of the finest tube amplifiers - made using solid state devices.

Last, but not least, my own Bender.ULT Rebuild Kits do that (no, not shameless advertising, just a simple statement of sonic fact).

-Steven L. Bender

This page is under construction ( Like software, the job is never done!....)
Last Update - February 6th, 2004 9:00 PM.

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