From: spam@spam.com
On Tue, 13 Sep 2011 11:41:11 -0500, John Byrns
wrote:
>In article <4e68e238.222722@news.eternal-september.org>,
> spam@spam.com (Don Pearce) wrote:
>
>> On Thu, 08 Sep 2011 09:31:41 -0500, John Byrns
>> wrote:
>>
>> >In article <4e664bf7.8680753@news.eternal-september.org>,
>> > spam@spam.com (Don Pearce) wrote:
>> >
>> >> On Tue, 6 Sep 2011 09:03:54 -0700 (PDT), Engineer
>> >> wrote:
>> >>
>> >> >On Sep 5, 1:46 am, s...@spam.com (Don Pearce) wrote:
>> >> >> On Sun, 4 Sep 2011 22:05:12 +0000, John L Stewart
>> >> >>
>> >> >> wrote:
>> >> >>
>> >> >> >The drive requirements of the OP stage are not as critical as
>> >> >> >mentioned
>> >> >> >in earlier posts. The KT88, EL34 & 6550 all have G about double that
>> >> >> >found in the original Mullard circuits where we find 6L6, 5881, Etc.
>> >> >>
>> >> >> >A single dominant pole is a safe way to go in this cct. There are a
>> >> >> >couple of worked samples in RDH4 worth looking at.
>> >> >>
>> >> >> >Also strongly recommended by none other then Norman Crowhurst. You
can
>> >> >> >read about how to implement all in a couple of his articles.
>> >> >>
>> >> >> >I like the cct proposed by Alex Pogossov recently. The attachment
>> >> >> >shows
>> >> >> >how it is connected in an SE Amp. The frequency selective network is
>> >> >> >in
>> >> >> >the cathode of the first stage rather than between stages.
>> >> >>
>> >> >> >My opinion, anyway, having done it.
>> >> >>
>> >> >> >Cheers, John Stewart
>> >> >>
>> >> >> >PS- The bicycle went more than 1000 KM in August (622 miles).
>> >> >> >Pedalling
>> >> >> >again today. Hey Patrick, hows it going in OZ? J
>> >> >>
>> >> >> Why would you want a circuit that drops the gain by about 16dB at
>> >> >> 100Hz? What is its actual purpose?
>> >> >
>> >> >Ans: To get the loop gain (forward gain times NFB gain) well below
>> >> >unity at the low frequency where the phase lead becomes 180 degrees
>> >> >(due to two RC couplings and one OPT.) Very low frequency oscillation
>> >> >in the range 0.5 to 2 Hz can result if there in no low-frequency shelf
>> >> >inside the loop.
>> >> >Cheers,
>> >> >Roger
>> >>
>> >> So it is a horribly bodged solution to a problem that has no business
>> >> being there in the first place? The idea of implementing negative
>> >> feedback and then defeating it by killing the open loop gain it
>> >> mediates is beyond ludicrous.
>> >
>> >On Sunday you suggested "a single pole that doesn't let up until beyond the
>> >unity gain frequency", isn't that equally ludicrous as it also "kills the
>> >open
>> >loop gain"?
>>
>> Not at all. It operates as a local feedback loop within the overall
>> loop. At lower frequencies, where it really matters to have
>> particularly the output stage included, the loop stretches from
>> beginning to end. As the frequency rises the loop transfers steadily
>> to just the voltage amplifier. This does result in a slight rise in
>> harmonic distortion above about 10kHz, but frankly by then who cares?
>>
>> And this pole is going to have to be present in some or other form
>> anyway because high frequency instability needs to be tamed - so why
>> not make it simple, predictable and controllable?
>
>So if I follow you, you are not talking about a single overall negative
feedback
>loop containing a dominant pole within, but instead you are talking about
nested
>feedback loops where the dominant pole within the overall feedback loop is
>created by a second shorter internal feedback loop that rolls off the high
>frequencies?
That is exactly what a dominant pole is - a nested feedback system in
which the voltage amplifier is controlled by a single capacitive
feedback element. The overall feedback which surrounds it is flat, and
composed of two resistors, the ratio of which sets the gain of the
amplifier.
d
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