Global Feedback - A huge benefit for audio

[Bonsai]

In my GainWire mk2 (pre amp with phone and line output, not power amp) it is possible to switch from high GNFB(CFA) to no GNFB, just to move one jumper. Some DIYers like GNFB and some like more non GNFB.
I could not decide wath to prefar more, could be old ears reason for that.

Very good dadod.

its the same for me on my e-Amp. I cannot decide which sounds best with the type of feedback.

🙂

 

[wahab]

under biased outputs have dead zones - feedback simply doesn't work when there's no forward gain

That s specificaly right for BJTs, with lateral fets for instance very few bias is required if there s enough NFB around the devices, wich is eased by their relatively high FT.

Personaly i shoot for very low bias, hence my preference of high NFB ratios and overall it s not worse than high bias designs..

 

[atmasphere]

Maybe, but it's not a requirement to have low Z is it. We could accept high Z output as a perfectly valid solution. It may require different speaker cross-over network design compared with 'voltage drive'.

What I was saying, many pages back on this thread. Such speakers already exist BTW.

 

[AKSA]

In my experience, very high DFs (more than 50 or so) don't sound good; bass is too controlled, over damped. A bit of overshoot give an impression of better bass - just subjectives, but critical to make a sale when the customers is sitting in front and auditioning his favorite tracks.

 

[jcx]

so put a R in series inside the loudspeaker box

or include a R selector switch on a near 0 Ohm output amp

then do the blind controlled listening test

simple frequency response shaping, setting output Z are known techniques that don't necessarily say anything about global feedback in amp designs

null testing, impedance, frequency response tweaking lead to:

Carver Challenge Details...

--------------------------------------------------------------------------------

For those who might be interested, I dug up the Carver Challenge article in Stereophile. The brief background is Bob Carver (the man behind Phase Linear, Carver and now Sunfire) made it known to the press he would put his inexpensive amplifiers up against ANY amplifier in a blind test if he had a few days to adjust the transfer function of his amp to match the challenger's amp. Stereophile took him up on the challenge and here are some quotes from the article written by J. Gordon Holt:

"We knew that Carver couldn't possibly pull this off, at least not to the point where none of us would be able to distinguish between his modified 1.0 and our reference amp. After all, some of the most highly trained audio ears in the world would be listening for the differences."

"...the reference unit is a high-powered, very expensive stereo unit with a strong and unique sonic "personality" and a penchant for being very finicky about the loudspeakers it works with. It was, we were gleefully confident, likely to be very dissimilar in sound from Carver's own designs."

"Not surprisingly, the reference amplifier sounded very different [from the Carver] and, in our opinion (shared, in most respects, by Bob), much better."

"Bob didn't have to concern himself about quality capacitors, minimal internal wiring, gold connectors, or any of those things; all he needed to do was duplicate, at the output of his amplifier, the sum of their effects at the output of the reference amp. Once he had obtained the necessarily deep null between those amplifiers, it was his belief that ears were not going to pick up on what was left."

"After the second day of listening to his final design, we threw in the towel and conceded Bob the bout."

"We had thrown some of the most revealing tests that we know of at both amps, and they came through identically. Even on the subliminal level--the level at which you gradually get the feeling that one amplifier is more "comfortable" than another--we failed to sense a difference between the two amps."

"We wanted Bob to fail. We wanted to hear a difference. Among other things, it would have reassured us that our ears really are among the best in the business." (italics emphasis in original article)

"According to the rules of the game, Bob had won."

"The implications of all this are disquieting, to say the least. If, after only four days of work, it is possible for someone--design genius or not--to make a $700 amplifier sound exactly like a state-of-the-art amplifier costing many times as much, what does that say for the cost-effectiveness of the latter?"

The amplifier used was a Carver M1.0 selling for $699. Bob used null difference testing to tweak the M1.0 until he obtained a deep null with the (unnamed) Stereophile tube reference amp. They did not reveal the reference amp because they felt it would be unfair to that manufacture who might ask: "why us?".

It was later revealed the most significant modification Bob made was to simply put some series resistance into the output of the M1.0 to better approximate the much higher output impedance of the tube amp. The other tweaks were supposedly limited to a small R-C network in the feedback loop.

It should be noted that J. Gorden Holt was the Editor-At-Large and Chief Tester at that time. Larry Archibald, the Publisher, and John Atkinson, the International Editor and a frequent reviewer, also participated in the listening sessions.

The challenge showed two things IMHO:

1 - It validates null difference testing with "some of the most highly trained ears in the world". Bob simply nulled his amp to the reference and JGH, JA and LA at Stereophile could not tell them apart.

2 - It shows that you don't need expensive components or exotic techniques to make a very modest amplifier with mainstream parts sound like a much more esoteric amp.

Bob literally bought the components used to modify the stock M1.0 at Radio Shack and worked out of his hotel room in Sante Fe (home of Stereophile). He made a 20 pound (9kg) mass production solid state power amp full of cheap parts (with a rail switching class-G power supply no less) sound so close to a very expensive heavy monster tube design that some of the mightiest GoldenEars couldn't tell them apart.

Considering that Stereophile is mainly filled with ads from high-end vendors hurt by the outcome of the challenge (versus just one advertiser--Carver helped by it), and that everything would point to the editors not wanting to admit a $700 amp can sound the same as a five figure one, I have to assume they wrote an accurate article and were not paying Bob any special favors

 

[bcarso]

And not too long after, Carver made an amp with ~one ohm power resistors in series with one set of the outputs, calling them iirc "current source" outputs.

 

[atmasphere]

Yeah that was pretty funny...

 

[Bigun]

we're all here trying to explain why our amps are better than somebody else's because they sound different and Bob showed everyone we're on a hiding to nothing !

(Of course the output resistor should be 2 ohms not 1....)

 

[Bonsai]

Putting a 1 ohm resistor in series with the output seems to 'loosen' things up a bit.

Might be just a psychological thing, but it sounds quite pleasant - try it.

 

[Charles Darwin]

Putting a 1 ohm resistor in series with the output seems to 'loosen' things up a bit.

Might be just a psychological thing, but it sounds quite pleasant - try it.

It does loosen things up and can be pleasant at times but drum kits end up sounding like timpani and timpani sound like 'mush'.

 

[Bonsai]

Exactly. Pleasant and tube like.

😀

 

[DIYBras]

Putting a 1 ohm resistor in series with the output seems to 'loosen' things up a bit.

Might be just a psychological thing, but it sounds quite pleasant - try it.

Really pleasant - for me at least, and most of speakers I have, the "low" DF always sounds more natural and less electronic in high range at least. For midrange most of time I like the low DF, but for bass is very variable: normally bad results for bass reflex, excellent results for sealed, mixed results for TL and good results for TQWT.
Most of time I use low DF amps, unless the loudspeaker have some big trouble with it.
Active crossover can overcome some problems with difficult speakers and we can choose the most adequate DF for each driver.

 

[Bonsai]

no offence meant - it was tongue in cheek from my side.

🙂

The 1 ohm resistor definitely changes the sound - nice for acoustic music.

 

[DIYBras]

No problems... maybe is my tastes or... luck with some speakers, or both 😉 !
Even if this method is not the most technically correct for most conventional crossover systems, at least this approach does not make me run out with headache five minutes after start the audition ...

 

[billshurv]

It fulfills the brief for an effects box that can be bypassed therefore is not harmful in any way 🙂

 

[vacuphile]

It can lower driver distortion somewhat, feeding it through a resistor.

 

[billshurv]

true, but it can also barf up your Frequency response.

 

[DIYBras]

hmmm... For some speakers sometimes the high DF is the "effects box" approach for harshness (electronic unnatural high-pitched sounds if compared with natural sounds; aka false detail) and drivers HD/IMD, so complete set of measurements is advised.
Of course, for most speakers this approach can be the effects box for frequency response. But (un)surprisingly some is little affected by that.
But hey, bring me some effects box if this relaxes me... no problem with it.

 

[DIYBras]

hmmm... For some speakers sometimes the high DF is the "effects box" approach for harshness (electronic unnatural high-pitched sounds if compared with natural sounds; aka false detail) and drivers HD/IMD, so complete set of measurements is advised.
Of course, for most speakers this (LOW DF) approach can be the effects box for frequency response. But (un)surprisingly some is little affected by that.
But hey, bring me some effects box if this relaxes me... no problem with it.

Ooops, term missing...

 

[atmasphere]

we're all here trying to explain why our amps are better than somebody else's because they sound different and Bob showed everyone we're on a hiding to nothing !

(Of course the output resistor should be 2 ohms not 1....)

🙂 I've never met anyone who was truly objectivist in their approach to audio. Never heard of anyone either. IME anyone who says they are is fooling themselves.

It does loosen things up and can be pleasant at times but drum kits end up sounding like timpani and timpani sound like 'mush'.

?? never experienced that...

I find that the recording has more to do with it, and as a string bass player I can't get the bass to sound right if the damping factor is too high.

true, but it can also barf up your Frequency response.

It can but does not have to.

Not to make too fine a point of it Bill, but I did present a known method for dealing with that. You went out of your way to attempt to take me to task on it as well. Remember?

Anyone who thinks they have flat frequency response is also fooling themselves. Just take a look at driver response and you will see what I mean. Amplifiers aren't the problem- speakers are. They are all over the place for FR. So if you like any particular given speaker because of something it does, the chances are better than not that it has some interaction with your room that you find pleasant. One thing is certain: its not flat FR. Run a pink noise test sometime...