Measurements and the Correlation of What We Hear

[MylesBAstor]

From Manley Labs:

What's so cool about variable Feedback?

Global negative feedback takes a small portion of the output signal and re-injects it back into the input stage which greatly improves linearity and distortion performance of an amplifier. A tasteful amount of negative feedback is the key: too much and you're into poor transient response and phase shift problems. We generally use somewhere between 4 to 14 dB's of feedback in our line preamplifiers and within that window is a definite point of maximized stability with optimum sonic performance. So we give you the choice of where you want to be in several of our products. Changing the amount of feedback alters the overall gain of the amplifier as well as the slew rate (speed).

In the context of a hifi amplifier or preamplifier with the Variable Feedback feature, such as our 50 Watt Monoblock or Neo-Classic SE/PP 300B amplifiers, EveAnna uses variable feedback to suit different CD's and recording styles when she's listening to music at home. "For the most part I keep the variable feedback controls around the middle but sometimes when I'm playing a CD which is kind of harsh and aggressive sounding or I just want to mellow out, I just turn those control down and apply more feedback to get a smoother, richer texture. But when I'm the one in the aggressive mood, I crank 'em up for a faster, punchier sound. The bass gets tighter and the top end sizzles. Vocal placement; is more forward. And, oh yeah, it gets louder too!"

The "GAIN" switch on our pro studio Mono and Dual Mono Micpreamps, as well in the Micpre section of the Voxbox is actually a variable feedback switch offering a range of 20dBs of global negative feedback in five settings of five dB steps to alter how much variable feedback is applied around the tube circuit. This GAIN switch can be used in the higher 55 and 60dB settings (in conjunction with turning up the Input Attenuator) to achieve maximum amplification for quiet singers or lower output ribbon mics. With most "normal" condenser mics and applications, the GAIN switch setting can be chosen at will for sonic variety for a "more tube-y" sound in the lower 40 or 45 dB settings, "straight forward" sounding middle positions, or "more agressive" "punchy" sounds in the higher gain positions. There is no right or wrong with this switch. Have fun playing around with it and we hope it brings you some interesting results.

http://www.aikenamps.com/NegativeFeedback.htm

http://milbert.com/articles/future_without_feedback

http://www.aikenamps.com/NegativeFeedback.htm

 

[mep]

What answer? Everything I told you was verfied by the articles. None of them mention including feedback from power supplies with voltage regulation. They all are talking about the feedback from the output of the circuit to the input of the circuit as I told you before Tom. So what are you talking about??

 

[mep]

Here is what I said after a remark about high feedback in an earlier post:

Well now, that feedback thing, now lets not forget that almost all tube pre-amps used (use) 100% feedback circuits. Feedback is your friend too.


And the 710 amp in above post most likely also uses 100% feedback circuits too. I did find it curious that they did not show an IM plot in their technical specifications....given superb conventional specs or as MEP seems to be harping on, measurments ( specs are results of measurments in my book and always have been IMO).

Tom
To be specific, I said 100% feedback circuits...not the entire thing uses 100% global feedback. All those EEs out there know of a circuit that uses 100% feedback and is common in tube pre-amps.

I am enjoying the discussion or should I say debate we have going Mark! Perhaps another myth can be busted. And IMO, this particular circuit does NOT sound good, and I have meausured what it does to the signal as far as harmonics, but thats my little secret I guess.

Tom

Again I ask you, what circuit are you talking about that has 100% feedback and you have measured it? And I will tell you again, having 100% feedback in a tube preamp circuit is not true. You still haven't proved one that does. And stay out of the power supply because that doesn't count. You aren't putting myths to bed, you are creating myths.

 

[Phelonious Ponk]

Just when think you can't amaze me anymore, you amaze me some more. I guess we are now full circle with you from specs are the same as measurements, to the "duh" statement that of course people know that specs are full of shenanigans and half-truths back to specs are measurements. Which is it Tim? You can't have it both ways. It's one or the other. I will say it one more time: SPECS AREN'T MEASUREMENTS. They are words and numbers written on a piece of paper unless and until they are verfied through measurements.

Here is my assumption, Mark, and it may be wrong: I'm assuming that if a reputable manufacturer - even one who might pick and choose the specifications that put his product in the best light - "specs" his product at .006% THD, he has measured it at some point and it has fallen at or below that number. Perhaps I'm being naive. Perhaps manufacturers just make up numbers and put them down on a sheet, then actually develop their products totally by ear without ever measuring anything.

It could happen.

Tim

 

[Gregadd]

... now lets not forget that almost all tube pre-amps used (use) 100% feedback circuits...

Tom

Tom "...Hold on, I said most (many) tube units use 100% feedback circuits. Not all..." Post #374.

Feel free to modify your statement. Almost all would be like ninety percent. Most would be more than fifty percent. I am not sure about many, but it's probably more than one.

 

[mep]

Tim-take a stroll through past editions of Stereophile or any other mag that actually measures the gear they review and see how many products don't meet their specifications in one or sometimes many more areas. It is not uncommon. You can choose between faith or facts. I will choose facts backed by measurements any day.

 

[microstrip]

The CJ unit you link to has a fully regulated power supply...that is 100% feedback to name just one example. And for those not so technical about all this audio mumbo jumbo we have all surely heard the the phrase that the amplifier modulates the power supply...in other words, the power comes from the power supply, the audio signal comes from the power supply.

Tom

Tom,
You were unlucky with this example - although must power supplies are regulated using negative supply, cj is one of the very few manufacturers to use feedback free power supply regulation - a simple zener based voltage reference biased with a current source followed by a darlington configuration emitter follower.

 

[FrantzM]

True, I just went off what their website said, but it still stands, thats still 100% feedback, its still an emitter follower if I follow your description correctly.

Perhaps we have drifted enough into feedback theory in a measurments thread anyway!!

Cheers,

Tom

May have to go with Tom on that ... An emitter follower .., is 100% feedback ..back to measurements ...

 

[Phelonious Ponk]

Tim-take a stroll through past editions of Stereophile or any other mag that actually measures the gear they review and see how many products don't meet their specifications in one or sometimes many more areas. It is not uncommon. You can choose between faith or facts. I will choose facts backed by measurements any day.

I will too, Mark. And I've never bought a piece of gear based solely on specs. Or independent measurements. What were we talking about?

Tim

 

[fas42]

Dear oh dear, this has been a ding dong battle, and about what: semantics. Specifications are limits or averages of what the manufacturer, if of reasonable honesty, expects his equipment to generate if subject to a set of Measurements. Conscientious manufacturers, especially of their premium lines, may actually perform such measurements for each and every unit they produce, and include this unit-specific information with the product. Then what they are supplying are Measurements. But in every other case all they will offer are Specifications, in one sense a best guess of how that particular unit will measure.

And this does not even bring in the whole messy area of defining precisely how a particular characteristic was measured, or if only specification figure, how it SHOULD be measured, the precise environment in which to do the measuring. That alone will guarantee a possibly great divergence between what a manufacturer specifies, and what a magazine review will measure.

As regards feedback, what's best is to discard the concept of the adjective 100% with feedback. Every circuit, no matter what the manufacturer says, has elements of feedback, obvious or otherwise, within its working, to have a decent chance of reasonable fidelity. One can use the term of 100% feedback when referring to particular, very standard and common combinations of electronics parts in areas of an overall circuit, but there is nothing special about these situations: they are just standard building blocks in electronic circuits, there is no magic in the phrase, 100%.

Frank

 

[Phelonious Ponk]

My god...I agree with Frank......

Tim

 

[mep]

I asked a friend of mine who is the smartest EE (he actually has a master’s degree in EE) I know that if a tube preamp uses a cathode follower, does that mean it has 100% feedback. Here is his answer:

“Not really. A follower of any type (cathode, emitter, source) is just a (near) unity gain buffer, and so there's really no feedback mechanism that achieves the unity gain; it's just a property of that topology. This is very different than say a unity gain configuration of an op-amp where the open-loop amplifier has very high gain but the feedback reduces it to 1. Degenerative (i.e. something that reduces gain) feedback usually refers to something that takes the output back to the input, although a non-bypassed cathode resistor also used to be referred to as "current feedback" since it reduced the gain of a common-cathode amplifier in a similar manner. So maybe it's just semantics, but if I don't have to check a Bode plot for the effect of the "feedback" (notably stability), I'd consider it to be an intrinsic property of the topology instead of a true feedback mechanism.”

I didn’t bother to ask him about the statement that was made that if you have a power supply with voltage regulation that means you have 100% feedback in the preamp or power amp. I knew that was nonsense.

 

[mep]

Tom-I give up. Uncle. Somebody please put this thread out of its misery because I can't take half-truths and nonsense posing as engineering anymore. I don't even know how this smokescreen about feedback ever got started.

 

[RogerD]

Tom-I give up. Uncle. Somebody please put this thread out of its misery because I can't take half-truths and nonsense posing as engineering anymore. I don't even know how this smokescreen about feedback ever got started.

Lol!

 

[andy_c]

I asked a friend of mine who is the smartest EE (he actually has a master’s degree in EE) I know that if a tube preamp uses a cathode follower, does that mean it has 100% feedback. Here is his answer:

(...beginning snipped...) “So maybe it's just semantics, but if I don't have to check a Bode plot for the effect of the "feedback" (notably stability), I'd consider it to be an intrinsic property of the topology instead of a true feedback mechanism.”

One of the problems with discussions of feedback is that, when it comes down to brass tacks, feedback's definition is mathematical in nature. So discussions about it get bogged down due to deficiencies in the ability of natural language to describe it.

One thing of note is that emitter followers can oscillate. That is, they can be unstable. One manufacturer of "no-feedback" amplifiers had to modify his circuit before releasing it because the emitter follower output stage was unstable into reactive loads. The traditional way of analyzing stability of a feedback amplifier is, as your friend says, Bode plots. These are usually plots of what's called the "loop gain" of the feedback amplifier. But with emitter followers, cathode followers and source followers, how does one analyze the loop gain? It appears impossible at first. But there is an answer, and it was found by the late R.D. Middlebrook of Caltech. He calls it the General Feedback Theorem. The PDF describing it is here. Using this technique, through circuit analysis, one can find the Bode plot of the loop gain for such a follower. The method has been further developed for the freeware circuit simulator LTspice by Frank Wiedmann as described here.

 

[DonH50]

Not sure I have ever used percentages in feedback; what does "100% feedback" mean? 100% with respect to what? Or does it mean every stage has feedback, and/or in addition to global feedback? I am confused.

While any real circuit has feedback at the micro level, followers (cathode, emitter, or source) are considered to be open-loop (no feedback) circuits. They improve performance by providing impedance transformation, providing a high impedance to the driving circuit ("unloading" it), and a low impedance at the output (for higher drive capability). It is essentially a buffer.

Middlebrook developed a more complete method of analyzing feedback circuits and his method works very well. It is necessary to take into account circuit impedances to perform a proper analysis, which his method does. There are other approaches (e.g. driving point impedance) but his scheme is straightforward to use, if tedious without SPICE or some other mathematical analysis program (Mathcad, Matlab, etc.)

 

[mep]

Don-If you read back a couple of pages, Tom started this off by saying that if you have a regulated power supply, you have 100% feedback in your power supply. And since the kneebone is connected to the leg bone, once you have 100% feedback in your power supply, you now have 100% feedback in your signal circuit. That discussion then morphed into cathode followers cause your preamp circuit to have 100% feedback. Good luck trying to explain your 100% feedback theory Tom. I will bring you a ladder in case the hole you dig gets too deep to climb out of.

 

[andy_c]

Middlebrook developed a more complete method of analyzing feedback circuits and his method works very well. It is necessary to take into account circuit impedances to perform a proper analysis, which his method does. There are other approaches (e.g. driving point impedance) but his scheme is straightforward to use, if tedious without SPICE or some other mathematical analysis program (Mathcad, Matlab, etc.)

Your description here leads me to believe you're referring to Middlebrook's technique from 1975, normally implemented with a loop gain probe in SPICE. But if you try that technique with an emitter follower, it does not work to find the loop gain. To properly analyze the loop gain of an emitter follower, it's necessary to use Middlebrook's GFT, which is a much more recent development. It's not a well-known technique, and when I first tried to figure it out, it took me a while. I'd call the loop gain probe technique straightforward, but I'd also argue that the GFT is anything but. That's because there are different "GFT templates" required for different circuit configurations, and one must figure out the appropriate one to use for a given circuit. (Edit) With the GFT, there are also restrictions as to where the probe can be placed, while with the 1975 loop gain probe, anywhere inside the loop is fine.

100% feedback is a very common term. It's where the ratio of the signal fed back to the signal sampled is 1, such as a unity-gain op-amp circuit. Or an emitter follower (considering Vbe as the error voltage) .

 

[mep]

Most people (non-engineers) think about feedback in the classic sense where you are taking the output signal and feeding it back into the input (global feedback). A cathode follower is not doing that and it is not a unity gain device. This whole discussion about feedback has nothing to do with the intent of this thread, it detracts from the original point, and the feedback theories that Tom has come up with in order to say that all tube preamplifiers have 100% feedback only serves to confuse people and they are incorrect. And Don and Andy, I appreciate you jumping in here and trying to add a little clarity to the confusion.

Tom has asked me to revise this post because he said he didn't say "all" tube preamps have 100% feedback. In Tom's own words, this is what he said:

Well now, that feedback thing, now lets not forget that almost all tube pre-amps used (use) 100% feedback circuits. Feedback is your friend too.

 

[fas42]

Yes, it would be rather nice to talk again about the fact that the numbers offered by manufacturers, and reviewers, whether specifications or measurement, are of little value in determining whether one piece of kit will make your system sound "better" than another piece of kit ...

Frank