Patrick Turner wrote:
snip
But these problems dissapear if you use LARGE value capacitors
which are now quite cheap, such as 470uF x 450V rated.
These are used now abuntanly in SMPS where they work hard, but in your
preamp
they'll be working real easy, and be very effective.
Excellent idea. To date I had assumed the 100uF 450V electrolytics I
could get from suppliers like Maplin were about all that was available.
It seems both RadioSpares and Farnell do 470uF 450V caps but they are
quite expensive - the cheapest I found was over 4GBP before tax in 10
off quantities. However, Digi-key also seems to do them at 3.40GBP at 10
off quantities but by the time you add in shipping and handling they are
over 5GBP each. So either way it works out at about 5 pounds a pop.
If you have 330 ohms plus 470uF, the attenuation factor is the same as
using 3,300 ohms plus 47 uF, or 33k and 4.7 uF.
At 100Hz, 470uF has Z = 3.4 ohms, and with 330R the attenuation of
rectifier hum
is around 1/100, and 3 such cascaded RC filters will reduce say 0.2Vrms
of ripple
at C1 by 1/1,000,000 to less than 1.0uV. If Idc was 10mA, the V drop is
only 10Vdc across
3 x 330 ohm R.
Idc is a little under 10mA but the basic ripple across the 47uF
reservoir cap measures 1.2V pp which must be about 0.4V rms (can't
remember the crest factor of a sawtooth). This directly feeds the CF
anode and the ripple at its cathode is just 50mV pp so the CF reduces
the ripple by about 20 times which is interesting.
Let the moths out of your purse and buy some decent sized capacitors.
Yes I shall. I think I'll go for 470uF reservoir then two RC stages for
the CF and the gain stage respectively each with 470uF caps.
The current design ends up with 191V (measured) on the CF cathode which
is a bit high so I can afford to drop the CF HT quite a bit. The preamp
is supplied via 22K and 47Uf at present which gives it a 255V (measured)
supply and a 117V (measured) anode voltage.
snip
In a mike preamp, bass isn't favoured like in a phono amp. But you don't
want to have a preamp
with its 0V signal wobbling up and down like the crude amps made in 1960
all used to to do.
Tube preamps for microphones are damned noisy where the mic signal is
below say 5mV.
So for good SNR, the old practice was to use a step up microphone
transformer.
The mic was very low Z, and its noise much lower than a tube.
When the mic signal was transformed up 10 times, the noise was still
lower than a tube.
That's still current practice too. I am using Sowter transformers (10:1)
at the input.
And the tube at the input should be a triode; 6AU6 is fine, but betst
srtapped in triode,
or used with the screen as the anode, and anode taken to 0V.
See RDH4 about such antiquated details.
Yes it is strapped as a triode for the usual reasons. I am using screen
and suppressor strapped to anode as it appears to give slightly better
gm values with the 6AU6 than the screen as anode connection. Heater
induced hum is not an issue as I plan to use a dc heater supply.
Of course if you used a j-fet for the input you'd not need a
transformer,
and following stage after the fet can be almost any triode.
I am trying to avoid semiconductors wherever possible.
A cascode circuit is also very good, see http://www.vacuumstate.com and
Allen Wright's
hybrid stages for phono. They can be used for microphone, without the
RIAA filters.
There are some clues on PSU design at my site,
http://www.turneraudio.com.au
Patrick Turner.
I have printed this email and stuck it in my day book.
Many Thanks
Ian