On Sat, 07 Jun 2008 16:30:34 GMT, Paul <paulguy@[EMAIL PROTECTED]
> wrote:
>On Fri, 06 Jun 2008 22:41:08 -0500, flipper <flipper@[EMAIL PROTECTED]
> wrote:
>
>....snip.....
>>
>>> That's probably because I have quite a bit of overall
>>>negative feedback,
>>
>>How much now, and before the PFB increase?
>>
>
> The global negative feedback is set by "beta = B", at mid-band its
>set by the resistor (10K) that goes from the first cathode to the
>output terminal, and another resistor (470 ohms) that goes from the
>1st cathode to ground. That sets B=0.0448 . This doesn't change with +
>feedback.
Right, if gain is high enough. I usually end up doing some fine tuning
but perhaps I'm just being a bit persnickety.
> The open loop gain before +feedback was about 152. The open loop
>gain with +feedback is approximately 267.
> Using the closed loop gain formula (G=A/(1+BA)). I calculate a
>closed loop gain of 20. My measured closed loop gain is 22.
> BA, the "loopgain" (B=beta-the feedback network ratio, and A=open
>loop gain) is 6.8 with no +feedback, and about 12 with +feedback. This
>is the number that reduces all the extraneous junk that is produced
>inside the feedback loop.
Yes, minus the amount PFB increased the gain stage's distortion by but
that's usually inconsequential compared to the overall in a power amp.
Worth noting, though, because it might not help across internal stages
as it depends on the PFB stage being significantly better than what
you wrap the NFB loop around.
>>Also, I'm curious. What triodes are you using and how much gain are
>>you getting now?
>>
> Before +feedback, the 1st stage (5751) is about 39. After +feedback
>it's about 65. 2nd stage (6SN7) shows a gain of 14, with or without +
>feedback.
In my modest experience that seems reasonable and, depending on the
triode, I usually end up between 2x and 3x vs what you'd get with a
straight bypassed Rk.
It's possible to get more but I like to make sure it's stable open
(global) loop.
>>> and the phase ****fts from the zero's (coupling
>>>capacitors) were made worse by lowering the one stage's coupling
>>>capacitor. If I'm not mistaken, the output transformers primary
>>>inductance is effectively a "zero" (an "s" on the bottom of the
>>>transfer function ?, or inductors in parallel act like caps in
>>>series). So you'd have 3 zero's in the feedback loop, even if you
>>>discount the power supply. That works out to almost 270 degrees of
>>>phase ****ft... if you have any degree of gain at those low
>>>frequencies, the thing is going to oscillate.
>>> By decreasing a coupling capacitor, the phase ****ft will get worse
>>>at a given low frequency, and make stability worse (for the same
>>>amount of feedback).
>>
>>Well, except it isn't passing the signal through, unless you have so
>>much PFB it's still got gain down there.
>>
>>> So by decreasing that capacitor in the positive feedback loop, the
>>>local loop became more stable, but the global loop became more
>>>UNstable!
>>> Since the transformer is pretty well a constant, overall low
>>>frequency stability would improve by increasing the values of coupling
>>>capacitors, until the transformer "dominates" the "zero's".
>>
>>Hmm. Well, I'm no math whiz when it comes to poles but having the amp
>>trying to drive the OPT lower than it can pass seems backwards to me.
>>
>>That will cause large voltage swings in the amp, as it tries to
>>'force' things through, and it often causes HF bursts. Speaking of
>>which, the HF bursts can cause motor boating too although I presume
>>you're not having those or else you'd see them.
>>
>>Where is the LF it's 'amplifying' coming from? Strikes me that it
>>might still be B+ filtering and it's just more sensitive now that gain
>>is higher.
>>
> At the upper end of the amplifier's response you have to compensate
>the feedback loop by adding a capacitor in parallel across the
>resistor (between cathode and output).
Word of caution, that doesn't always 'work' because you're increasing
feedback at the frequency the phase has become positive. Might seem
stable but they can sometimes burst into oscillation during some upset
condition (like clipping or OPT core saturation when trying to push
through LF it can't pass) or if the output pole ****fts, like a
different speaker.
> At high frequencies you have
>added an extra "zero" in the B(Beta) network to compensate for the
>poles in the amplifier (A). In an AC amp we suffer from "zeros" at
>low frequencies. So..... we could add a pole in the compensation (B)
>network, similiar to what we did at the high frequencies.
> I think you are correct about the B+ being the problem.... I think
>the supply is adding "zero" behaviour at the low frequencies, and the
>larger A (open loop gain) is making things worse. BA must never have a
>value more than 1 when its phase is 180 degrees, so I have a number of
>options...
> 1- regulate the B+ (eliminates power supply "zero" behaviour,
>reduces phase ****ft) .... a zener maybe?
> 2- lower the amp gain (A) - no local +FB - keeps BA from getting
>close to 1, but increases distortion
> 3- put a pole in the feedback network, thus ****fting the offending
>phases the other way.
I don't know what your filtering looks like but I'd think a few more
uFs, or another RC, should be fine.
> If you were brought up using RDH4, you would be using attenuation'
>skirts (multiples of 6db/octave) instead of the more abstract poles
>and zeros. They work out much the
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