Negative feedback

A somewhat confusing issue.

I've been reading over on the Transcendent forum that a more "tubey" sound can be obtained by decreasing negative feedback. The message there, as elsewhere, is that NFB reduces distortion. Reducing NFB, goes the accompanying warning, accomplishes a pleasant (to some) tubey sound at the expense of increased distortion and, in the case of the Transcendent preamp, a compromise in the speedy responsiveness of the amp.

Yet elsewhere (e.g., UHF magazine's guide to hi fi, as well as this website: http://homepage.mac.com/tlinespeakers/vaughn/downloads/Damping-Factor.pdf )

there is a different perspective, which holds, in a nutshell, that NFB lowers distortion under conditions of a steady signal -- so that the amp performs well on specification tests, but that under conditions where the sound is not so steady but always changing (e.g., music), NFB actually has an undesirable impact on the music. The web article cited refers to "smearing" the sound, homogenizing it and generally sucking the soul out of it.

I'm a bit perplexed, and would be eager to hear some ... 'positive feedback' from the knowledgeable folks on this board.

I note that the ST70 uses 13 db of NFB. In the sphere of things is this a lot of negative feedback? Is it possible to improve the "sound" of these amps by reducing NFB at the expense of an increase in inaudible Harmonic distortion? (ok that last question has two parts).

Thanks for your thoughts.

Brenton

In addition to the advantages that Bob notes, negative feedback also lowers noise. Too much negative feedback for a given design usually results in ultrasonic oscillation that can muddy the sound and destroy tweeters. I usually test for too much negative feedback by injecting a 20 Hz sine wave and looking for bursts of oscillation on the waveform in an oscilloscope as I raise the level. Some amplifiers had so much negative feedback that any component changes due to age push the amp over the edge into instability.

The VTA design uses a safe amount of negative feedback that allows it to sound clean while keeping away from stability problems.

All of the respected classic tube amplifiers that turned people onto the tube sound used plenty of negative feedback. Over the years, many of these amplifiers have developed various problems due to aged parts. I wonder if some listeners have come to associate the sound of improperly working amplifiers with tube sound.

In my view, a properly working amplifier should not change the sound it is reproducing very much. If a listener demands a strong characteristic sound from a tube amp, adding frequency response anomalies, distortion, and even noise is a way to achieve this. After our ears hear a difference, it is easy to convince ourselves to prefer that difference.

Bob and Danf,

Thanks for these helpful explanations that contextualize the use of negative feedback for me. As I read your replies, it's a judicious use of NFB that is actually essential to these amplifiers, while overly increasing or overly decreasing NFB creates some real problems!

You're right about accurate sound being the goal. In the web article I noted, the idea that NFB (in any amount) can have audibly beneficial effects is summarily dismissed.

Brenton

100% agreement with Bob and DanF comments, the amount of NFB in the VTA70 design was carefully chosen, it is MUCH lower than with 60s era tube amps, but is high enough that the driver is
100% stable even with various component modifications and under all operating and test conditions.
Roy www.tubes4hifi.com

Hi Tube Nube

Long time no speak! (Job pressure on my end)

On this subject (and most other design differences between SET and push-pull), here is a very well-written piece, the author makes many points very clear:
http://homepage.mac.com/tlinespeakers/vaughn/downloads/SE-v-PP-Part1.pdf

And by the way, I do own a SET amp as well... this so-called "push-pull smeared sound" is absolute rubbish IMHO. The two amps (ST-70 and SET) does sound different, in large orhestral works (I'm a Mahler fanboy) I prefer the ST-70 - it absolutely buries my SET amp in its ability to resolve instruments in complex passages. Fair enough, the SET amp is not a 300B or 845 - based design (instead based on the "lowly" 6L6GC) but I very much doubt that it would have made any difference.

The SET and ST-70 are pretty well-matched when listening to Jazz music at low volume levels, but for almost anything else I prefer the ST-70. Detail-wise both seem to me to have the same ability to resolve detail, but at loud volumes (neighbours-calling-the-cops level) the ST-70 runs rings around the SET.

BTW, most well-informed engineers consider that a SET amp will be more prone to "smearing" sound in complex & loud music passages - simply because the nature of the SET beast is such that it usually does not have the headroom to deal with it.


-- JunkyJan
Ps. how did your X-country go?

Hi Jan,

Thanks for this update, and link. I'll read it with interest.

I'll tell you why I got worried is because I've yet to make a "final" (always a relative term) speaker choice, but would very much like to try building a horn speaker. I'd planned to build the well reviewed Zigmahornets, though not really a horn speaker, I learn. I also learn from the DIY sites that they maybe aren't as good as the 6moons web article portrays.

So, I found another design, bigger, but looks to be something of a giant killer. Single, full-range driver, about 6 ft high, 15 in deep, 8 in wide.

But, then I read something about how NFB amps might not sound good with these types of speakers and read a few disparaging things on this from SET-maniacs.

I'm much reassured by what reads, to me, as more balanced information from Bob, Roy, Danf, and now you! It's especially good that you're an experienced consumer of both types of amp.

Oh, one more thing is I read a really good review of the speaker I want to build by a guy who was specifically using an ST70. So I think I'm well reassured now.

Working hard, hey?! Well, I'm glad to know we both still have a job!

I'll tell ya, JJ, I've had 10, maybe more cross countries cancelled. The first one I cancelled cause I wasnt' ready. Just as well, cause that was back in January, I think. Or thereabouts. And I've taken every opportunity to book in another, and everyone has been lost to weather. So, had I gone, way back when, I'd still be waitin to do the solo XC, and I don't think I'd have much enjoyed the long delay there.

Mean time, I've worked up a flight planning spreadsheet that does all the calculations for me, except for looking up and interpolating TAS, %BHP, and fuelflow based on selected RPM -- but I'm workin on that last part.

Just have to enter wind direction, speed, temp, and altimeter setting data for aerodrome, and at altitude, and the spreadsheet does the rest. To heck with the E6-whatchamacallit!

I'm sure that attitude will go over well on the flight test, ey?!

Bob Latino wrote:Brenton,

Reduction in feedback can give a more "tubey" sound but > Do you really want a tubey sound or do you want an accurate musical sound ?

A push/pull amplifier like the ST-70 needs some feedback not only to give lower distortion and give a wider bandwidth but also to keep the amp stable under a wide variety of speaker loads. Without some feedback certain push/pull designs can be come unstable. Those audiophiles that are into SET (single ended triode) designs generally knock push/pull amplifier designs because an SET amp generally doesn't need any feedback while a push/pull amp does. SET designs, however, have their own problems. They sound (IMHO) overly "tubey", need larger output transformers, have much lower power output, have higher amounts of distortion and restricted frequency response at the top and bottom of the frequency range. You are limited to very high efficiency speakers because of the power output restrictions imposed by SET designs.



Bob

Bob,

Since when is "needs larger output transformers" a problem? SET amps also do not have restricted frequency response, as long as quality iron is used. I also do not consider 90db to be "very high efficiency", and that is the starting point for a SET amp.

As to a SET amps higher amounts of distortion, yes, it will have higher 2nd order harmonic distortion, but, the human ear actually finds 2nd order pleasing. THD is nothing more than a marketing sales tool. It should never be used to judge the sound quality of any type of amp.

There are Class A PP amp designs that do not use any NFB. So to say that some PP amps need NFB, is a bit misleading.

I have one of each an ST-70 and a SET amp (using a 2A3). I prefer the SET amp. Now part of that preference is that the SET amp is connected to some very high efficiency horns, that pit out a very crisp sound. There is no muddyness.

Say, ST70-J,

How do those horns sound with the ST 70?

My reason for my initial posting was that I was having a hard time reconciling discrepant information I was reading from other sources, and it left me with the impression that full-range single driver horn speakers (which I'd like to try) won't work with amps using NFB. (specifically because NFB tends to go hand in hand with higher damping factor, and the higher DF will overcontrol the driver in the application noted, iirc)

Tube Nube wrote:Say, ST70-J,

How do those horns sound with the ST 70?

My reason for my initial posting was that I was having a hard time reconciling discrepant information I was reading from other sources, and it left me with the impression that full-range single driver horn speakers (which I'd like to try) won't work with amps using NFB. (specifically because NFB tends to go hand in hand with higher damping factor, and the higher DF will overcontrol driver in the application noted, iirc)

That is a good question, for which, I do not have an answer. I have just been so pleased withe the SET and the horns, that I have not had any urge to try out the ST-70. I have the horns and the SET amp downstairs and the Dynaco stuff upstairs in the den. But since you asked, I will move the dynaco stuff downstairs later today and give a listen.

As for single drivers and NFB, I have never read that, and all the people that I know of that are big into single drivers, all use SET amps.

I appreciate you're willing to perform the experiment, and I'll be most interested to hear about what you find.

-Brenton

Tube Nube wrote:I appreciate you're willing to perform the experiment, and I'll be most interested to hear about what you find.

-Brenton

Brenton,


I got the amp and preamp moved down and connected to the Klipsch LaScalas. I must say the ST-70 sounds great driving them. I listened to a variety of music, Rock, Country, Blues, Jazz, and heavy metal. FYI, my ST-70 currently has the stock driver board. I am running one of Curcio's power supply boards, that replaces the can capacitor. It also replaces the vacuum rectifier. The PAS preamp has the stock circuit boards, but, they are new replacements from dynakit.com. The tone controls, blend switch, balance, as well as all filter switches are no longer in the circuit. As far as tone controls go, I do not like boomy bass.

Comparing the 70 to my 2A3 SET amp, I will say that the 70 does not shame the SET amp. The SET amp has more than enough power to drive the LaScala speakers. There were certain songs where I felt the vocals came out clearer on the SET amp. On the 70, the singer sounded like he was standing in the back of the room.

The bad thing is the sound of the ST-70 on the LaScalas is sooo much better than the B&O speakers, but, I knew that would be the case. I hate to have to hook them up to the B&O speakers.

I guess the next experiment is to change the ST-70 to run triode mode.

Robert

Hi Robert,

Thanks for the update on your experiment of the ST 70 into the horns.

I'm encouraged by your findings.

I also found a speaker review written by a fellow using an ST 70 into a pair of Tybone speakers, which are a commercially produced version of a DIY single driver horn speaker called a Saburo that I read about here:

http://www.frugal-horn.com/spawn.html

I've committed myself, now, to finishing my DIY Zigmahornets, but user/builder reviews lead me to believe I may not be completely satisfied. If not, I'll try the this Saburo, or the smaller brother.

The PAS preamp has the stock circuit boards, but, they are new replacements from dynakit.com. Robert[/quote]


Are you referring to Dynakitparts.com? I thought they only had circuit boards for Dynaco power amps, not preamps. Have I missed something on their website?

Charlie

bnc31 wrote:The PAS preamp has the stock circuit boards, but, they are new replacements from dynakit.com. Robert

Are you referring to Dynakitparts.com? I thought they only had circuit boards for Dynaco power amps, not preamps. Have I missed something on their website?

Charlie[/quote]

I apologize for the bad sentence structure. What I have from Dynakitparts.com is the driver board for the ST-70. The PAS replacement boards were purchased through ebay.They could be Roy Mottram's boards, I do not remember.

Robert,

Thanks for your reply and no apology is necessary. I just wanted to be sure I hadn't missed something. Actually, I "scratch" built my PAS preamp back in the 90's from a NOS chassis/cover/transformer, purchased from Sound Values and circuit boards/selector switch/Noble pots, purchased from Frank Van Alstine. Still works great, especially with the new ST-35 amplifier I just built, primarily from Dynakitparts except for Triode Electronics transformers. BTW: I replaced a SET (2A3) amplifier with the ST-35. I am driving a pair of "one way" folded horn loudspeakers, am very pleased with the results, and it is unlikely that I will return the SET to service.

Charlie

ST70_Junkie wrote: FYI, my ST-70 currently has the stock driver board.

Both the tubes4hifi board (12au7 model) and K&K Audio boards run, or can be modified to run, without global NFB. And both would still sound different from each other. Just as you rightly point out that low THD is exploited in the marketplace, so too is the superstition about NFB "sucking the soul" out of an amp.

The stock ST70 has gobs of global NFB and is possibly the most wooly, veiled, congested, lazy thing I've ever heard. And the K&K driver, which has none, is the least "tubey" sounding of any I've auditioned. My point is that driver design can make or break a topology regardless of class of operation, output tube choice and configuration or the use of global NFB.

A driver design which inarguably failed to utilize the strengths of a given designer output transformer in a SET amp would be rejected without a second thought. That fact that it is permissible in the case of a PP amplifier such as an ST-70 boggles my mind.

bnc31 wrote:Robert,

BTW: I replaced a SET (2A3) amplifier with the ST-35. I am driving a pair of "one way" folded horn loudspeakers, am very pleased with the results, and it is unlikely that I will return the SET to service.

Charlie

Charlie,

I too enjoyed the sound of the ST-70 on the LaScalas, very much. I also enjoy the sound of the SET amp driving them. I guess I get to enjoy both. Oh well, it is a burden I will have to bare...

Robert

Brinkman wrote:
Both the tubes4hifi board (12au7 model) and K&K Audio boards run, or can be modified to run, without global NFB. And both would still sound different from each other. Just as you rightly point out that low THD is exploited in the marketplace, so too is the superstition about NFB "sucking the soul" out of an amp.

Well I have not heard that description of NFB. I just like the "minimalist" approach. I am also the same way about motorcycles, when it comes to pure enjoyment of riding them.

What would be involved in running the ST-70 without NFB, and it still remain a stable running amp?


Robert

ST70_Junkie wrote:
What would be involved in running the ST-70 without NFB, and it still remain a stable running amp?

A great person to ask would be Kegger at Audiokarma. He emailed me directions on how to modify the VTA driver to run open loop (no global NFB). It is his preferred mode of operation. You can always decrease the NFB by simply replacing the NFB connection from the 16-ohm tap to the 8-ohn tap. When I was using the VTA driver, I never attempted to remove NFB completely.

Shannon at diytube forums designed a driver for the ST70. In the instruction manual to it, open-loop measurements are included so I'm sure he could explain how to modify his driver for that mode of operation.

The K&K Audio driver doesn't use NFB in its stock configuration. You can ask any questions at the K&K Forum at Audio Asylum. This is the board I'm currently running and yes, it is stable.

However, a great sounding driver that uses NFB isn't going to necessarily sound better without it. There's lots of topologies out there and if any "one way" was the best that's how all amplifiers would be designed. NFB helps simplify the design of a wide-bandwidth driver circuit but is not a prerequisite for it.

I know the story's been told a million times, but the original ST-70 circuit was designed simply to be inexpensive and functional. The power supply was minimal. The rectifier was being pushed to it's limits. The irony of all this is that it allowed A LOT of ST-70 amps to be sold back in the day. Had it not been a success, we wouldn't be here discussing all the different options of how to go about modifying it.
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I wanted to add that in my last post I asserted my opinion in regards to the ST-70's stock driver. Later in the evening I realized that I may have disrespected Kevin at dynakitparts.com by speaking unfavorably about a product he sells. Kevin stands behind the products he sells, is a pleasure to do business with, and is more than generous with his time when it comes to troubleshooting. He sells detailed reproductions of original dynakits and his transformers are top-notch. It was not my intention to offend.