I know I shouldn't feed the troll but..
Only when you re-define "feedback" to support your prejudices.
Go and re-read Crowhurst 1957 "Some defects in Amplifier Performance not covered by Standard Specifications " JAES 5(4).
It uses a different definition of feedback: Please do debunk it.
Some of us fools have been using that definition for nearly seventy odd years.
Only when you re-define "feedback" to support your prejudices.
Go and re-read Crowhurst 1957 "Some defects in Amplifier Performance not covered by Standard Specifications " JAES 5(4).
It uses a different definition of feedback: Please do debunk it.
Some of us fools have been using that definition for nearly seventy odd years.
I've been reading that NFB destroys the stereo imaging in a valve amp ?
Taking 2 x Valve mono-blocks with high NFB will have a poor stereo image ?
I've never heard of this with SS amps and they use NFB.
Taking 2 x Valve mono-blocks with high NFB will have a poor stereo image ?
I've never heard of this with SS amps and they use NFB.
Ignoring the idiots for a minute (which I really should try to do more often) NFB is a wonderful tool for getting a low THD number out of a crap circuit with lots of gain.
Crowhurst demolished THD as a figure of merit more than half a century ago. And that was before we discovered TIM in transistor amplifiers.
My final dissertation was an example of taking NFB a bridge too far - too much non-linear gain and non-linear capacitance made my "amplifer" into a lovely voltage controlled oscillator. Regardless of what the small signal models said.
Global NFB also has the nasty side effect of including your load and source impedances in the numbers, ensuring that claims of stability are almost inevitably "conditional".
NFB also affects the distortion spectrum of your circuit and psychoacoustics tells us that not all distortion is equal.
NFB is just another tool in the armory. It needs to be understood. And if your open loop circuit is linear enough GNFB is unnecessary.
Here's a quote from someone whom you might classify as an "idiot."
From "Valve Amplifiers," 4th edition.
Morgan Jones said:
From "Valve Amplifiers," 4th edition.
It is that very 'blandness' which confirms that the feedback is doing exactly what it was engineered to do! Blandness is good, for hi-fi.Shoog said:
NFB is an even more wonderful tool for improving a good circuit e.g. by reducing output impedance.thoglette said:
THD is a figure of merit, but it contains less information than some (allegedly) claim and much more information than others claim. Curiously, THD is most often mentioned nowadays by those who are dissing it.
Slapping feedback around a parametric oscillator is unlikely to turn it into a nice amplifier. This is not a valid argument against feedback.
No, not almost inevitably. You can design a bad circuit using many different techniques.
Yes. People use NFB because they want to improve the distortion spectrum by having less of it.
Yes. Sadly, it is frequently misunderstood both by those who like it and those who don't like it.
True (in fact this is a truism), unless you want low output impedance too. Everything is unnecessary if the thing which calls for it is not present.
One comment about concertina splitter described at pag 6-7. The usual conclusion about different output impedances is wrong. When you load both anode and cathode output resistance is the same for both (thanks to local feedback!).
What has happened to the Williamson amp; pointless auguments about feedback; insults , of other members; how about a few modern circuits : Of Today's Version of The Williamson Amp please.
phil
If there's sufficient interest, I could do this as my next project and document it, once I'm through with the phono stage I'm currently working on, and a speaker pair that I've been itching about. Probably early spring (Northern Hemisphere). Same basic topology, but using a modern approach to controlling voltages and currents, as well as multiple internal feedback loops and circuitry immune to blocking distortion.
Not at all - I'm very much aligned with that statement (and I think the general thrust of your line of argument) in that I'm a fan of obtaining linear open circuit behaviour before adding global feedback.
Now local negative feedback (in the sense that the text books define it) is a very valuable tool in obtaining open loop linearity.
What I have a problem with is unconditional dogmatic statements. As your quote notes, if the circuit is marginal to begin with adding GNFB has risks which need to be addressed.
Yes, this is a recognition of basic control theory, well known to just about every practicing engineer.
This is the article that I read....Welcome to the most complete do it yourself guide on the D. T. N. Williamson tube amplifier.
In the introduction he states.."Is this amp still any good? Is it worth to build one? Are more recent circuits not much better? Williamson designed his amp for mono reproduction with a fairly high amount of negative feedback. That is ideal for mono. All designs in that time were mono-amps. Later, when stereo was invented, they just build two monoamps on the same chassis and claimed that it was a stereo amp. Unfortunately stereo amps must sound roomy and it is just not good enough to copy mono amps for stereo use. In most cases however decreasing the amount of feedback makes an amp better for stereo use. "
In the introduction he states.."Is this amp still any good? Is it worth to build one? Are more recent circuits not much better? Williamson designed his amp for mono reproduction with a fairly high amount of negative feedback. That is ideal for mono. All designs in that time were mono-amps. Later, when stereo was invented, they just build two monoamps on the same chassis and claimed that it was a stereo amp. Unfortunately stereo amps must sound roomy and it is just not good enough to copy mono amps for stereo use. In most cases however decreasing the amount of feedback makes an amp better for stereo use. "
SY, correct me if I'm wrong, but what you're offering to do, if there's enough interest is the following;
SY will take the original 1947 Williamson schematic and make only the following mods;
1) use a modern approach to controlling the voltages
2) use a modern approach to controlling the currents
3) use multiple internal feedback loops
4) use circuitry that is immune to blocking distortion
And you would also Document it and publish the finalized schematic.
...is that right?
There may be one or two other tricks, and it will not use KT66 at the output, but basically, that's it. It's a great topology. People who are looking for effects boxes will not care for it. People who want a tube amplifier that merely makes the input signal bigger will. That was the whole point of the original circuit, a low distortion, low output impedance amplifier having high fidelity, in the true sense of that phrase.
Sy, I'm still trying to find the article, someone has already done some of the work for you, undoubtedly you can still add a bit of your magic to it too.
He gets much wrong in his analysis. I'll just start with Williamson and go from there.
Could be an interesting project/design study, I'm part way through updating a pair of Williamson monos that I've had for years.
So far have replaced original front end with LTP phase splitter w/CCS in tail, and fixed bias in output stage. Next step is likely to be adding MOSFET source followers DC coupled to output grids. And tidying up the power supply.
Since no NFB applied currently, the gain is a little high, but works nicely with passive attenuators in lieu of an active pre.
So far have replaced original front end with LTP phase splitter w/CCS in tail, and fixed bias in output stage. Next step is likely to be adding MOSFET source followers DC coupled to output grids. And tidying up the power supply.
Since no NFB applied currently, the gain is a little high, but works nicely with passive attenuators in lieu of an active pre.
count me in as one interested.....the one thing is dislike about the williamson is the number of coupling caps that the signal had to traverse thru...
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