Patrick Turner
Guest
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 Posted:
Sun Sep 11, 2005 11:25 am Post subject:
Re: Potentiometer question |
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Chris Hornbeck wrote:
| Quote: | On Fri, 09 Sep 2005 09:33:43 GMT, "Ian Iveson"
IanIveson.home@blueyonder.co.uk> wrote:
Why not a third stage follower? Is it such a crime?
Not at all (not in the UK, anyway AFAIK, yet...could slip in with
all this anti-terrorist stuff...caught in possession, third stage
follower, five years solitary without charge).
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,
| Quote: | Things are much worse in the US. You really *don't*
want to know.
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So, tube crafting is done clandestinely, in closits,
and well outa sight of the goons from
uncle dubbya's departments.
| Quote: |
Anyone up to showing how to calculate the four time constants in the
circuit?
Quick and easy to simulate, anyway.
The conflicting pairs (500Hz and 2122Hz) and (C1-R8 and
C2-R11) are the interesting bits.
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You can calculate.
Looking around the net its obvious nobody agrees on
what are the values for RIAA filters.
But I have an inverse filter, and when you connect that up in series with
a sig gene
with a square wave, the preamp wave should also have a square wave
output,
although the rise verticals will have a slower time.
But at least the horizontals should be horizontal
at between 500 and 8kHz; any peaks, droops, ripples indicate a wrong
RIAA preamp filter.
Trimming the values of the preamp filter to geta good square wave
on the CRO is the best way to KNOW that you have got the darn phono amp
working right.
The square wave response should be flat when sweeoing the whole band.
Amplifier couplings and miller etc should give -3dB at 10Hz and 50 kHz.
| Quote: |
Many strategies have been worked out over the years to
rule-of thumb the 500/2122 pair, but IMO the only good
approach to the subsonic pole pair (not even counting
the power supply's non-zero impedance) is to make a
single dominant pole.
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You mean the pole for the roll off below 20Hz?
| Quote: |
In this case, maybe move the loop feedback to the
upstream side of C2 and choose a value of C1 the makes
a pole at some ridiculously high number, maybe 30 Hz.
Then C2 can be large enough to couple to modern
loads.
And while I'm up on this soapbox, let me just preach
for a moment on the value of a decent load for the poor
unbuffered second stage. Can I get an Amen? Even
Dynaco, heartless and Godless though they may have
been, provided 100K. Can we do less? 'S all i'm sayin'.
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I told Jon if he used the spare tube socket he now has since he
wrenched out the tone control tubes that he could wire up a new
phono stage such as the one at
http://www.turneraudio.com.au/htmlwebpgs02/schem10tubepreamp.htm
The second gain tube is SRPP, but this can further improved if
the top triode's grid is biased through 470k from the bottom of the
existing
1k5, then 5k6 added between the bottom of the 1k5 to the bottom triode
anode, and
the top tube grid can then be cap coupled to the bottom tube anode with
0.47uF.
This then converts the SRPP to a µ-follower, with purer even order triode
harmonics
AND makes the Rout of this stage at about 1.5k, much lower than SRPP,
and able to power the loads of balance and gain pots when connected in
series,
something also shown at the above schematic.
The linearity of the µ-follower is excellent with
not more than around 0.25% thd at 10Vrms output voltage.
The schematic at my site is an example of passive RIAA eq,
and there is no feedback eq, and thus there is no FB applied
of say a typical 10dB even at 20Hz as in many RIAA FB eq systems.
So with a normal MM cart with 5mV of signal, the
phono stage VO is higher than the FB amp for a given amount of tube gain.
Patrick Turner.
| Quote: |
Arf. Thanks as always, and No, nobody wants to ASCII in
the complete calculations. It's the computer age, Mon.
You just look it up on the Internet!
"Trailing clouds of Glory, I'm down" -Firesign Theater
Chris Hornbeck |
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