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NFB Question

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Will C

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The specifications for my pp EL34 amplifier state that there are 26 db of feedback around the amplifier and 38 db total feedback including that inherent in the voltage amplifier and phase splitter stages.
That amount of feedback is with a 1.2K resistor in the feedback loop.
Is there a way to calculate the amount of feedback that exists now that I have changed the feedback resistor from 1.2K to 10K?
As a matter of curiosity, first impressions are favourable. Piano and cymbals seem to be more realistic, and although the bass is ever so slightly less tight it sounds great.
The higher feedback did produce an amplifier with no audible hum or noise even with an ear to the speaker, now a bit of hum and noise can be heard 5 or 6 inches from the speaker.
Bill

Follow Ups:

Re: NFB Question - Meister 05:05:41 09/19/04 (1)
First order calculation of global feedback would indicate a reduction of 20 x xlg(10/1,2)= 18,4dB. Total feedback now around 26-18,4= 7,6dB. This would only produce a damping factor of 2,4 in pentode mode! A little bit critical for most speakers.
- Re: NFB Question - Will C 05:44:42 09/19/04 (0)
  Thanks Meister,
  I didn't know how to make that calculation.
  Bill
NFB Question Thanks everyone! - Will C 04:00:01 09/19/04 (0)
Thanks Steven, Tim and Serus,
Very helpful comments.
The designer did say one of his goals was to produce an amplifier with feedback as high as possible consistent with good stability! Perhaps that just reflects the thinking in the 1960's.
I guess I will try different feedback values from time to time just for fun.
Luckily, this isn't my main amplifier so I can try different things without worrying about down time.
Bill
It all depends on your starting point - serus 22:25:31 09/18/04 (2)
If your amp has a relatively high output impedance or the circuit is inherently non-linear (i.e. distorting) then you MUST USE NFB to correct these faults.
The concept of Hafler et al in the 50's was that we can use the crappiest components but build a lot of gain into the circuit, then add a hefty amount of NFB and fix all these problems with no sweat.
The problem with these assumptions is the simplistic feedback theory they rely on. Over the years we gained some knowledge regarding the bahavior of feedback under a large loop delay. While some readers may feel lost at this point, the idea is quite easy to understand. If you return signal from the output and subtract it from the input, you essentialy negate the added products, i.e you reduce distortion. If the delay of the returned signal is a significant portion of the input waveform (like 0.1 mSec for a 1 KHz signal), the subtraction process is not that "clean". We can show mathematically that some NEW PRODUCTS appear at the output, not directly related to the input!
That scenario is your sonic nightmare. The ear can relate 2 KHz and 3 KHz harmonics to the fundamental signal at 1 KHz, but it would have no "explanation" to an ARBITRARY PRODUCT like 1.1 KHz!
From this little explanation one can derive a few scenarios here feedback might work well. For example, in an amp with a very small delay from input to output.
It would not be a good thing to have global feedback in a complex amp with more than say 3 stages made of inherently slow devices.
Let's look now on the arguments from a purist point of view with some analogy. If you want to build a wheel for your race car, you can take a standard rim from a production car and examine it. If it's not balanced, you add weights in strategic points. Have you seen serious race car rims balanced that way? Probably not.
This method is good enough for a road car. The racing mechanic may opt to machine the rim for a significantly better balance tolerances.
This solution is the IDEAL SOLUTION for a street car as well, but the practical and COST EFFECTIVE solution is adding weights!
If you are in the business of selling mass production amps, you'll go with a cost-effective method that Hafler et al have chosen and apply feedback to a reasonably cheap implementation.
If you want a BETTER RESULT you will start from a more naturaly "cleaner" design and tweak it AS LITTLE AS POSSIBLE. Just common sense.
A good amplifier would use circuits and components that are inherently not distorting. There is no perfection in the real world, so you might add SOME LEVEL OF FEEDBACK to that design. You use feedback as the last resort, not as a magic fix (which it's not!)
That's the theory.
When it comes to an actual amplifier, you should first determine how good it is without feedback. If it's all triode (inherently linear) and inherently maintains a stable operating point, then you don't need much feedback. In fact, you would prefer not to add feedback, so you don't introduce delay-induced distortion products!
If your amp has a high-gain pentode gain stage and tetrodes in the output stage, you don't really have a choice but to use LOTS of feedback... OUCH...
- Very good post ... Heard the Hurricanes? - KIS 08:06:08 09/19/04 (1)
  I have heard the Hurrcanes which has no NFB and they do work without NFB. In triode and also in Ultralinear.
  From my experience, Ambience details gets worse when Feedback increases.
  Of course bass always gets better with more feedback - which is why most prefer NFB.
  For good bass, the OPT must be very good. Some OPT just can't handle 0 NFB. Most ordinary OPT can't produce anything above 15k without Feedback.
  Anyway, the Hurricanes sounds and measure very well without Feedback.
  Cheers
  - Excellent observations! - serus 08:29:01 09/19/04 (0)
    Detail level is the first to go with excess feedback. Instruments become "smooth" and clinical and ambience details more diffused.
    Bandwidth increases with feedback, if it's not limited by a component "brick wall" limit.
    Bass level sometimes depends on the output impedance, which drops when you increase the feedback level. In some cases I prefered the zero feedback bass, due to its precise tone and "definition". That was the case with a Dyna MkIII amp which I modified to zero-feedback with triode front-end and 6550 finals in triode mode. In that case the front-end circuit (not the final stage) was measued at 80 KHz bandwidth and the full amp extended beyond 20 KHz - without feedback!
    I applied my simple rule, if it ain't broke I ain't gonna fixit!
Re: NFB Question - g 19:58:21 09/18/04 (1)
From WZ Johnson, David Hafler, Herb Keroes, George Fischer (DUMBLE!!!!) ad infinitum-- Negative feedback is NEVER bad unless it causes amplifier instability/oscilation OR is used to camoflage engineering mistakes. I have been around numerous experimenting with reducing NFB in a fixed circuit-- I never thought it improved anything. The tweaks doing the NFB reduction were universally ignorant of what they were really doing; some didn't even realize the perceived "improvement" was due to the fact that reducing NFB increases circuit gain-- at the "usual" preamp setting, the amp was now louder, and thus seemed to "fill the room with an expansive soundstage" Feh.
- Re: NFB Question - Meister 05:13:41 09/19/04 (0)
  I am a lover of well done (!) feedback but I can't agree. Most tube amp's sound suffer from wrong done feedback. In most feedback amps distortions are lowered but delayed. The ear could detect delayed distortions much more easily than in time distortions because of the changes of the zero crossing of the signal.
I wuz gunna say that 'Ut depends, maaate', but - Timbo in Oz 17:42:09 09/18/04 (1)
Stephen Oda said it all, and more specikifaly like Ie wiv a few examples.
;-)!

Warmest
Timbo in Oz
The Skyptical Mensurer and Audio Scrounger
'Still not saluting.'
Read about and view system at:
http://www.theanalogdept.com/tim_bailey.htm
- more thoughts, - Timbo in Oz 17:50:21 09/18/04 (0)
  "we" rebuilt (butchered I think KK calls it)
  a pr of St20 Leak stereo PP EL84 amps with much gruntier low-gain tubes in the gain and phase splitter stages. Ipso facto much lower NFB than before, and we used b'all anyway, a CCS sounds better. SO we put a LOT around the output stage, output Z just sank and power bandwidth -ened, and then we eased back a bit, until we still liked it, and there we were.
  Overall total is down and it sounds great. on the 4 ohm tap where the NFB is a lot less 'in the circuit', I think it sounds OK, but mine are real 8 ohm loads. and ONLY 91db/w! ;-)
  
  Warmest
  Timbo in Oz
  The Skyptical Mensurer and Audio Scrounger
  'Still not saluting.'
  Read about and view system at:
  http://www.theanalogdept.com/tim_bailey.htm
Re: NFB Question - steven oda 15:40:58 09/18/04 (0)
Will,
I've found the EMPIRICAL test the best. Buy several values of carbon comp resistor and bypass caps (if they exist in the NFB).

If you are using fairly linear tubes---I find best to go with mucho NFB to keep the tone pure.
If you like "dirt" (like us guitar freaks) no NFB and EL84 or 6V6GT are muy sabroso. As little NFB (highest resistance) for 6L6GC and/or 5881 for not-so-brittle over-the-top sound. Keep NFB high for KT-88 and 6550 to keep tight bass and clean top end.
Experiment, until you get the tone you are seeking.

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