John Byrns wrote:
>
> Hi Ian, I hope you don't mind my jumping into your
> discussion.
Not at all, I always hope that's what discussions will be
for.
>> Why did you decide not to direct-couple the voltage amp
>> to
>> the concertina, and to share the valve halves by channel
>> rather than by function?
>
> Conversely why would one want to direct couple voltage amp
> and the
> concertina stage? By avoiding direct coupling the design
> of each stage
> can be optimized. The only down side is that somewhat
> greater care must
> be taken in the choice of the three low frequency time
> constants.
Well, quite. Some folks do it one way and some another. Some
might go to the extreme of direct coupling and use a -ve
supply for the first stage, so that both stages can still be
optimised, presumably.
In general, it is a bit odd that, after so many years, no
particular valve amp design has become ubiquitous, as I am
told it has become in the SS world for a large swathe of the
market.
>> What's going on with the connection of the concertina's
>> cathode load to the cathode of the voltage amp, rather
>> than
>> to ground? How well is the splitter balanced, given the
>> different total loads?
>
> Flipper explained that it is positive feedback used to
> increase the open
> loop gain of the amplifier. This is common in phono
> preamp stages, and
> not unheard of in power amps.
OK, sorry flipper. But why, exactly? Is there some general
consideration that leads a designer, in this or that
situation, to nest a lesser positive loop inside a greater,
negative one? I imagine it may be possible to get more gain
out of a relatively linear section of the amp, so that can
be used to counter the distortion produced by a relatively
non-linear section somewhere else in the global loop. Or
perhaps I am missing some advantage in the frequency domain?
> As far as precise balance of the
> concertina loads goes, aren't you being a bit anal, they
> are close
> enough.
Some folk go to extraordinary lengths to try and AC balance
concertinas precisely at all frequencies. I don't know where
fidelity stops and anal begins for everybody, and it's
interesting to know where other folk think that point is.
>> I can't see how the output valves' bias is adjusted.
>
> I assumed it isn't adjusted, why should an adjustment be
> provided?
I'd want to mess about with it :)
It could be that old valves would be better off with a bit
less current. This is quite an issue, to my mind, especially
as it is relatively simple these days to add some
intelligence to the bias circuit. What is the best way to
keep valves optimally biased as they age?
>> Why do Zobels go to anode supply, rather than to screens?
>> I
>> have read quite a lot of waffle around this subject but
>> nothing very convincing. What, exactly, are those Zobels
>> for?
>
> When I use Zobels I connect them to the anode supply as
> flipper did, why
> would you want to connect them to the screens?
Just asking. Not my amp so I can't know where I would put
them. But there is a more general issue here, about exactly
what such Zobels are designed to achieve. I see a logical
argument for Zobels between anodes and screens but not
between anodes and ground or anode supply. The logic may
well be misplaced...
So what, exactly, are these Zobels for? I had assumed they
are to deal with some problematic local relation****p, and
that must be between anode and screen...perhaps because the
anode circuit contains the leakage inductance and winding
capacitance, whereas the screen circuit doesn't. I further
surmised that the most direct approach to damping would be
the best. Maybe there's not much difference.
>> Using a resistor in series with headphones isn't ideal,
>> to
>> my mind. My opinion is that a head amp should be have an
>> output impedance of no more than 5 ohms, or one tenth of
>> the
>> 'phones' impedance.
>
> I'm not into headphones and hence don't understand how to
> properly
> interface them. That said I was surprised that flipper
> didn't find some
> attenuation necessary beyond that provided by a series 51
> Ohm resistor?
> I would think that a different circuit could be found that
> would meet
> your 5-Ohm impedance criteria, perhaps by simply tapping
> the headphone
> output doen on the 8 Ohm load resistors and eliminating
> the 51 Ohm
> series resistor. Not understanding headphone circuits I
> would be
> interested in learning what flipper's criteria were for
> the design of
> this part of his amplifier?
Yes. It could be that the 'phones output was an
afterthought, as they have tended to be over the years. Or
it may be just me, being anal about headphones...
Headphone amps are much more im****tant now, and hi-fidelity
examples will be a big thing in future. Increasingly, music
is designed for heads rather than rooms.
>> Nice pictures. Speakers are beautifully done. Amp looks
>> functional. Valves look a bit gloomy. Transformers look
>> cheerful but very small. How's your bass?
>
> The transformers do look "cheerful", but I don't
> understand why you say
> the "Valves look a bit gloomy", they look quite bright to
> me?
They look a bit drab to me, and the glass looks smoked in
the big photo. Not a serious complaint...some folk are fond
of industrial chic.
Cheers, Ian
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