Re: Value of coupling cap's

<r.laury@xxxxxxxxxxx> wrote

> Deep Subject

Much deeper than it used to be!

Things have changed since Patrick's day. Driving some of today's
excellent but awkward speakers down to their depths poses a real
challenge now. I'm talking myself into a low-turns-ratio-or-bust
mentality.

> I've always been taught that bandwidth (LF point partly determined
> by the coupling caps) should get wider
> as you travel through the amp toward the output. Taking a 2 stage
> amp, with the same identical bandwidths in
> each stage can be a problem. When you hit the -3dB point of the
> first stage, you'll be -6dB down at the second
> stage output. Now you have some really funny phase shift stuff
> happining (multiple poles) an octave or so before the first -3dB
> point. And this is worse for a 3 stage amp! I always try limit the
> bandwidth of the first stage to a reasonable point.
> And then an octave wider for the next stage and ect. With a
> reasonable amount of negative feedback you will end up with a
> stable amp with a bandwith wider than the first stage anyway.

Yes, but combination and permutation are important for different
reasons AFAIK.

Total function is a product of stage functions, so sequence doesn't
matter with a linear system, and wouldn't matter anyway if poles and
zeros were the only considerations.

Ignoring the sequence, there are two ways of choosing poles and
zeros. You can distance them all, frequency-wise, carefully from
each other to avoid conspiracy, or you can stuff them as high or low
as you can at the expense of bunching, and then slug one of them.
Slugging involves moving that pole or zero so that it is dominant,
and far enough from the others to make them insignificant.

There is a rule I've been looking for concerning slugging but I
can't find it. It expresses how the amount of feedback is related to
the number of octaves required between the slugged pole or zero and
the bunch. Perhaps an octave per 6dB? Should be easy to visualise
but I'm not very focussed at present.

Power supply and linearity issues determine the order, I assume. No
point in amplifying a frequency only to attenuate it later. Better
efficiency and lower IMD if you start narrow and get wider.

>> Depending on the circuit, a (too) large coupling cap may cause
>> motorboating and/or blocking of the power tubes.

What's blocking?

Motorboating is often a power supply thing, and is best dealt with
by improving PSRR of gain stages, not by slugging. Received wisdom
suggests a strategy of distributing poles and zeros through the PS
is just too hard to accomplish.

Voltage regulation for gain stages kills all PS related stability
issues.

cheers, Ian


.

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