Are "0 Feedback" (Solid State) Amps Better?
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ferninando
Lunatic Member
o boy. this will be good.
M Jarve
Audio Geek and NGE Freak
From what I understand, the less NFB used, the lower slew-rate, TIM, and IIM distortion is. Matti Otala, designer of h/k's über-low NFB gear wrote some papers on the negative effects of negative feedback, I believe.
Nelson Pass's PassDIY site may also have some interesting articles about the evils of NFB. Many of his designs use little or no global NFB (i.e. going from the final amplification stage back to the first stage), though they usually use some form of local NFB (i.e. going from the end of one stage back to the beginning of that stage).
There is the opinion that amps (solid-state and tube) that use little NFB can more accurately track very complex and fast-changing signals, as whatever transient distortion is introduced by the NFB circuit is inconsequential to the output. Or, if you have no NFB, there is nothing to consider. Use of negative feedback also reduces the gain of a circuit compared to its open-loop gain (sometimes by half).
Many amps that claim not to use negative feedback do indeed use it. They simply do not used global NFB, but still use some local NFB.
Nelson Pass's PassDIY site may also have some interesting articles about the evils of NFB. Many of his designs use little or no global NFB (i.e. going from the final amplification stage back to the first stage), though they usually use some form of local NFB (i.e. going from the end of one stage back to the beginning of that stage).
There is the opinion that amps (solid-state and tube) that use little NFB can more accurately track very complex and fast-changing signals, as whatever transient distortion is introduced by the NFB circuit is inconsequential to the output. Or, if you have no NFB, there is nothing to consider. Use of negative feedback also reduces the gain of a circuit compared to its open-loop gain (sometimes by half).
Many amps that claim not to use negative feedback do indeed use it. They simply do not used global NFB, but still use some local NFB.
Douglas Self, addressing audio pseudoscience:
Read the whole article: http://www.dself.dsl.pipex.com/ampins/pseudo/subjectv.htm
TIM distortion, as applied to reasonably well-made audio gear, is pretty much a myth IMHO, and is applicable only to gear that enthusiasts such as ourselves would not likely be interested in anyway. Otala has some points, but they simply do not apply here.
IIM distortion (as in, interface intermodulation distortion) is another myth, even shakier than Otala's TIM. Read here: http://www.cordellaudio.com/papers/interface_intermodulation_distortion.shtml
Or here, if you want to shell out the $20: http://www.aes.org/e-lib/browse.cfm?elib=11887
A poorly designed amp is pretty likely to sound like a poorly designed amp, but the factors that make it so are unlikely to be traced back to the amount of global feedback used.
I always love this argument, which suggests that a sine-wave is too simple to be an accurate test for the performance of an amplifier. Here's what Doug has to say about that:
Bold & italics mine. Lesson here is that what appears to be complex to us is just another signal to an amp. No biggie.
Read the whole article: http://www.dself.dsl.pipex.com/ampins/pseudo/subjectv.htm
TIM distortion, as applied to reasonably well-made audio gear, is pretty much a myth IMHO, and is applicable only to gear that enthusiasts such as ourselves would not likely be interested in anyway. Otala has some points, but they simply do not apply here.
IIM distortion (as in, interface intermodulation distortion) is another myth, even shakier than Otala's TIM. Read here: http://www.cordellaudio.com/papers/interface_intermodulation_distortion.shtml
Or here, if you want to shell out the $20: http://www.aes.org/e-lib/browse.cfm?elib=11887
A poorly designed amp is pretty likely to sound like a poorly designed amp, but the factors that make it so are unlikely to be traced back to the amount of global feedback used.
ducati_EL34
Addicted Member
Well why does Doug say a 0 NFB amp is near impossible to build? There are plenty of 0 NFB amps around. The K&K driver board for the ST-70 does not use NFB.
f. langford-smith in radiotron even says the highest form of fidelity is a PP triode amp, without feedback. Or is he only talking about SS amps?
Most supposed 'Zero Feedback' circuits simply mean no global feedback. Circuit design without local feedback is pretty much impossible with SS gear, and difficult with tube gear. And places wayyy too much emphasis on tube parameters. Feedback stablizes these parameters and in addition to lowering distortion and increasing bandwidth (among a dozen other things), it allows for tube rolling without changing the sound or gain so radically that only one type of tube is suitable.
And if F. Langford-Smith says it's true*, then I guess we should all throw away out SS gear or SE & OTL tube amps. But maybe I'll hold off and decide for myself (which is pretty much what everyone should do).
*maybe it was true in 1953, when his books were written. But a thing or two has happened since then.
And if F. Langford-Smith says it's true*, then I guess we should all throw away out SS gear or SE & OTL tube amps. But maybe I'll hold off and decide for myself (which is pretty much what everyone should do).
*maybe it was true in 1953, when his books were written. But a thing or two has happened since then.
Scuzzer
Fixed Bias
Triode tubes have internal feedback, its just part of the way the operate. I'd say it would be a form of very local feedback. Whether or not this counts depends on your definition of feedback.
Byrdsmaniac
Active Member
I completely agree EchoWars on this. He and Douglas Self know what they are talking about. Very few audio reviewers do. Many are completely clueless technically. Your Denon is an excellent amp. Enjoy it.
ducati_EL34
Addicted Member
Well "difficult" is still a long way from impossible. He should have clarified his statement.
And if F. Langford-Smith says it's true*, then I guess we should all throw away out SS gear or SE & OTL tube amps. But maybe I'll hold off and decide for myself (which is pretty much what everyone should do).
*maybe it was true in 1953, when his books were written. But a thing or two has happened since then.
What has happened? Have triode tube characteristics changed since 1938 (not 1953)? A PP amp still cancels out 2nd order harmonic distortion and triodes do not produce much in the way of higher order harmonic distortion.
Vacuum triodes are about the lowest distortion amplifier components there are... but distortion isn't the only parameter that matters (and transformers create distortion and phase shfit too...). Not all speakers are efficient enough or tolerant of the high output impedance of zero-feedback amps.
MOSFETs and bipolars aren't linear enough to use without feedback but voltage gain is REALLY cheap, so feedback can linearize the circuit easily. A simple example is the cathode / emitter / source follower - 100% local feedback, very low distortion.
MOSFETs and bipolars aren't linear enough to use without feedback but voltage gain is REALLY cheap, so feedback can linearize the circuit easily. A simple example is the cathode / emitter / source follower - 100% local feedback, very low distortion.
Write him. Maybe he'll change the article to make you happy. Or not. D. Self is a SS designer of some note, and undoubtedly 'loves' tubes about as much as I do, which is why his statement ignores devices with 10% THD or more. (OK, I'll apologize for that one...I was baiting you
)
This inconsequential thing called Solid State. Tough to make a blanket statement about the 'best sounding amp ever' and ignore sixty years of electronic design.
I've yet to hear a zero feedback tube amp that I didn't think sounded like the south end of a northbound mule. I'm sure someone has built one, but I haven't heard it.
The OP said nothing about tubes, but did mention his Denon, which is solid state. If you want to discuss tubes specifically, start a new thread in the tube or DIY forum. I'm simply the wrong guy to discuss tube design with...mainly because I just have no real love of tubes. (That's simply a polite way of saying that...oh, nevermind.)
Carry on Gents. :yes:
zenith2134
Lunatic Member
First off, it is impossible to define fidelity as it pertains to the listening audience as a whole. Way too subjective.
If I was looking for a new (to me) amplifier, whether integrated or seperate, I'd try to ideally find a unit which incorporates no global feedback in its operating parameters.
If I was looking for a new (to me) amplifier, whether integrated or seperate, I'd try to ideally find a unit which incorporates no global feedback in its operating parameters.
ducati_EL34
Addicted Member
Yes solid state is great for television (whether TV is any good or not, is another discussion
) and computers, but, as for audio amplification, that is open for debate...
Personally, I think tubes are great for microwave ovens, but as for audio amplification, that is open for debate. (Touché)Yes solid state is great for television (whether TV is any good or not, is another discussion
OK...back on topic. The OP is asking about feedback, not the old tube vs. SS debate, which is done to death and will likely get this thread closed. If you have something to offer in this vein, please feel free to opine.
djv8ga
Super Member
:yes:
I shouldn't have started this thread. I didn't know it was one of those subjects that starts trouble. I was just curious because of my amp and very limited knowledge of electronics.
"This style"? What style? SS amps can't really be designed with zero feedback, but you also have to define what kind of feedback, local or global. SS amps can be put together with no global feedback (or low global feedback), but I don't know of any methods to avoid using local feedback.
This paranoia about the use of negative feedback is baloney, IMHO. It is possible to design a crappy SS amp with and without global feedback, so I'd not get caught up in the hype one way or another. The acid test is what you think of it when you listen, no what someone else tells you is good or bad...especially if they're hyping a zero feedback design, as I know there are SS amps out there being marketed and sold solely on the basis of having "Zero Feedback", which is misleading and certainly not what I'd base a purchase on.
Nothing wrong with your post or asking questions. No one has all the answers (and certainly I do not), but for those who would proffer their opinion, I'd urge some serious technical reading first to get the pseudoscience out of the way before posting about things they have little understanding of. It's on this point that I have a low tolerance.