[Patrick Turner]

Ian Bell wrote:

Patrick Turner wrote:

Ian Bell wrote:
Patrick Turner wrote:

snip,

What is the maximum voltage gain at say 1kHz?


24dB

Gee, that's only about 15x no?

What is the noise at the amp output with maximum gain and with input
grid directly shunted to 0V close to the input?


Not possible to measure accurately at the moment as the LF blips whack
the meter needle all over the place one you try to see noise below 1mV.
That said, looking at it on a scope you can see the broadband noise
underneath the LF blips and I would estimate the noise at the output
with the input shorted as about 50uV rms.

If you have 50uV of noise at the output and gain is 15x, and input is
grounded, then you could have a total of 2uV grid input noise if the
input tube is a real good one. 2uV gets amplified to make about 30uV at
the output, and some of that is LF noise. where does the rest of the
noise come from? By observation you should be able to see where the
noise is being generated and how, and find ways of stopping it without
much complexity and cost. In my MC phono amp without any GNFB with RIAA
correction, but using a passive RIAA, the LF gain at 20Hz is MUCH
greater than your 24dB yet the LF noise at the output is minimal, and
the result using 1 fet in cascode with 1 triode, then 2 triode
µ-follower gain stage produces an outcome equal or better than most
other phono amps I have tried including SS with opamps.



What happens if you temporally connect a spare 1,000uF or more to be in
parallel to the last 100uF cap in the filter line up, ie, the filter cap
giving the B+ supply to stage 1 of the mic amp?


Not tried that yet, I have a spare 470uF or two so I'll try that.

Noise should fall a lot with the extra C added where it'll do the most
good.

Think big, use enormous C values if you cannot bring yourself to make
what might be a very simple shunt regulator in your preamp.


Yes and no. I was using just a couple of RC stages using 470uF but then
I realised the five RC stage of 100Uf each would perform better. I
suppose I could go bananas and replace all the 100uF caps with 470uF ones.

The cost now of generic 470uF caps rated at 350Vdc is not huge, and far
cheaper than the 100uF caps were in real terms back in say 1960 when a
100uF cap was seen as a frivolous extravagance by bean conters in charge
of design teams at major manufacturers. Keen diyers will *NEVER* try to
emulate the pausity of design by accountants amoung yesterday's people.

Beware using simple zener diode based shunt regs close to mic input
stages though. The LF noise of the zener will find its way into signal
paths.

Agreed. I have been looking at the Maida regulator as a means of
eliminating the LF noise *prior* to the normal RC string.

Using a regulator right after the resevoir C is OK and you can then make
RC filters after that to all stages without risk of LF motorboating.
And such LF oscillations may not be obvious at first. A PS and amp can
be right on the brink of oscillation at LW and the slightest noise
will become amplified by a the peak in the response if there is one
below 1Hz.

Zeners placed across the second cap in an CRCRCRC filter can reduce LF
content and any noise the zeners generate is less than the noise which
is shunted, and following RC stages filter the noise of all types.
Zeners have higher noise at lowish currents. So if you have a +375V B+
rail and held by 5 x 5watt x 75Vdc rated zeners, heatsink the zeners
with a wrap around strip of Al aor Cu and bolt to a chassis or sink and
allow the pda to be a safe 0.75Watts each, which means you'd have Izener
= 10mA at least. And and in a preamp, the simplest shunt reg that isn't
a simple zener string is to have the string feed a base of an npn bjt
with emitter to 0V and a current limiting R between collector and B+
rail being shunt regged. This shunt eg has much lower output resistance
than a plain zener string, and a much "sharper" threshold of turn on, as
the current in the zeners gets amplified by the bjt. The bjt needs a
high Vce rating, and such bjts have low hfe and a darlington pair is the
best solution, and with a limiting series base resistance and filter cap
at the base to 0V to filter out the zener noise. Such a shunt reg works
only at LF and simply keeps the Vdc stable while your large value
electros do the job on higher F. Shunt regs are good for low current
preamp supplies and screen voltage supplies in power amps and have the
advantage that in the case where the output becomes shorted or over
currented, then the regulator doesn't have any current and survives
while it is the low cost series R in RC section that cops the heat and
fails.

I have used such shunt regs in power amps with choke input supplies, so
that the shunt reg shunts enough anode supply dc current right after
turn on to stop the B+ soaring. As the input stages and output stages
turn on the "bleeder" current of the shunt reg reduces to a low level
enough to reg the B+ to stage 1. So thus the high current in a
permanently connected bleeder resistance is avoided.

Patrick Turner.



Cheers

Ian

Patrick Turner.