Ian Thompson-Bell wrote:
Patrick Turner wrote:
Ian Thompson-Bell wrote:
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.
Yes, but what you are paying per 100 uF isn't too high a price for a
blame free PSU.
Using large 40uF motor start caps would be more expensive, but the C
values are way too low.
Is the price of a 100uF cap 1/5 of the price of a 470uF?
Roughly.
I buy my electros from spare parts suppliers like wescomponents.com
who supply to the repair trades, and never from local retail electronic
places such as Maplin; Jaycar and Dick Smith Electronics are the Oz
equivalent,
and they charge twice the trade price.
Interesting. Not sure what the equivalents are in the UK. Don't they
have rather large minimum order quantities or values??
Serch for them.
You won't know about mimimium orders until you ask them.
There is RS and Farmells, but they try to cater for engineers with deep
pockets.
Most trade repair supply companies deal with repair ppl trying to fix TV
sets
for a low price to maximise earnings from pplo who hate having to have
anything repaired. The repair guy might only want ONE cap today,
but in a year will use 10, so he buys 10 to begin with, and so do I;
maybe I by 50 pcs.
I also buy from a mob here who buys job lots of parts left over from
mainstream manufacture runs, rockby.com.au
Sometimes thay have 470uF x 400V caps for $2 each.....
All these caps and things are made in asian factories by thye million,
and the price ex the factory is maybe 20c.
Guess I need to investigate the Pommie equivalents.
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.
50mV at the CF cathode is bleedin awful!!!!
The reason the ripple at the cathode is 20 times lower at the cathode is
that the
series voltage NFB acting in the CF reduces any noise applied to the
anode,
or applied from an ac heater. Cathode Rout = 500 ohms, anode Rout or its
Ra lokking into the
the anode circuit is say 10k + so the hum at the anode is reduced by
what is effectivly
an active resistance divider with a gain tube and lots of NFB.
So if the filter gave only 0.2mV of ripple at the CF anode, noise at the
cathode
would 10uV, and no more than other parts of the tube will manufacture.
You need far more filtering.
Indeed I do. That is the main lesson I have just learned.
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.
The step up tranny and 6AU6 in triode should be fine if the mic is low
impedance.
It is.
In the old days of good recordings, there were no j-fets....
But now a 2SK369 j-fet costing $1.10 from wescomponents.com is cheaper
than a transformer.
A pair used for balanced input also work fine for lower distortion.
How true, but I have designed semiconductor audio stuff for years
(actually decades). Now I want to design/build some (pure) tube gear.
Filtering is just as important in SS circuits....
Patrick Turner.
Ian
Patrick Turner.
Many Thanks
Ian