Phono 3 input capacitance

[A.N. Beal]

Dear All,

I have just checked the input signal cable currently supplied by World Designs with my multimeter and it measures about 145pF/m, which is quite high, so the capacitance of the internal connections between the input sockets and the circuit board is about 20pF. Therefore if my calculated Miller effect capacitance of the amplifier with ECC83 valves of 115pF is correct, the total input capacitance would be 135pF.

Unfortunately my multimeter refuses to give a sensible capacitance reading if I connect it across the input terminals to check if my calculation is right.

With 75pF tonearm leads the total combined capacitance would be 210pF and with 180pF leads it would be 315pF. These figures compare with 150-200pF recommended for my Goldring G900IGC, or a G1042, 100-200pF recommended for Audio Technicas and 150-300pF recommended for the Ortofon Black.

If the input capacitance is lower than recommended, this is easily rectified with small parallel capacitors but unfortunately there is no easy remedy if it is too high, which is the reason why I would like to know a definitive answer the the question of the input capacitance of the Phono 3 with either ECC83 or 5751 valves. Can anyone help?

Regards,

Alasdair

[bob orbell]

Alasdair, give John Casswell a PM or email, he may know and be able to help. Bob

[Phil Y]

Hello Alasdair,

I have not measured the cable value myself although I suppose I could if I could be bothered!
The 180pF value was given to me upon inquiry (not stated in instructions) to the arm maker, Audionote UK.
I'm obviously also interested to know the answer to this one.
My Phono is home built (not with the WD kit) using the older Phono II PCB but I don't see that making any significant difference.

Regards,Phil.

[John Caswell]

Hi all,
Idly sitting around doing nothing I found I had an assembled phono2 PCB so got to measuring with a capacitance meter.
Phono 2 without 47K grid resistor ≈9pF + cold ECC83 (various makes) ≈13pF + phono socket ≈ 19pF + a short cable say 6" twisted pair ≈29pF so all in all ≈29pF
If the gain is 50 and the valve is an ECC83 the a-g capacitance is ≈3pF inc stray capacitance so with Miller effect = 51 X 3pF = 153pF plus all the other bits and pieces.

Hope that helps

John

[A.N. Beal]

Dear John,
Many thanks for the information. Silly question: is the gain 50 on the ECC83 in this circuit or is there a chance ot might be lower, reducing the input capacitance? Also how much would the input capacitance be reduced if a 5751 valve is substituted (due to its lower gain)?
Sorry to 'murder you with questions' but for those who use moving magnet cartridges it would be nice to get to the bottom of this.
Thanks in advance,
Alasdair

[bob orbell]

Looking at 5751 valve data it would be slightly lower due to the smaller gain, but I think insignificant. Franks Valve Data sheets are your friend here . Bob

[John Caswell]

Alisdair wrote
"is the gain 50 on the ECC83 in this circuit".
I don't know - what I will do is haul my phono3 out and measure it, as opposed to calculating it, with an ECC83 and a 5751, might even have a 12AY7 as well.
Will let everyone know here in a day or so.

John

[John Caswell]

Phono 3 measurements March 2021

These are measurements made on my own Phono 3 which will generally relate to phono 2 as well. They are not absolute measurements for phono 2/3 as various readings will vary slightly depending on capacitors, resistors, valves and voltages.
That having been said my phono 3 consists selected Mullard ECC83 valves, bulk foil resistors in first stage anode and grid circuits, 1% metal film resistors elsewhere, battery bias, and relay switched MM/MC inputs and the 3.18µS “mod”
So……

HT + 250V dc heaters 6.3V dc.
Unless specified all figures are “A” weighted.
Overall gain @ 1kHz MM input to output 5mV in = 1400mV out = Gain 280 ≈ 49dB
First stage gain ECC83 5mV in 250mV out = Gain 50 ≈ 33dB
First stage gain 5751 5mV in 200mV out = Gain 40 ≈ 32dB

Residual Hum and Noise ECC83 “A” weighted wrt 775mV
Input o/c -64dB. Input s/c with 75R -74 dB
Unweighted ie wideband o/c -54 dB s/c 75R – 60dB.

H &N figures will depend very much on valves, resistors, PSU etc. but the figures are achievable without a fancy PSU and the like.
Using selected 5751 valves I would expect H&N to be similar but overall gain to be down 2-4db as several of us have found.
To give an idea of the effect of H&N, on my system the volume control needs to be way above listening level before both become intrusive.
Don’t ask me how it sounds “beauty is in the eye of the beholder”.
That should stir things up nicely!

John

[A.N. Beal]

Dear John,

Many thanks for these measurements, which do indeed 'put the cat among the pigeons'.

Firstly, the current Phono 3 instruction manual has HT on the input valves (pin 6) as 170v and mine measures close to that. Secondly, when HiFi World tested the Phono 3 in June 2007, Noel Keywood measured the MM overall gain as x182. Have I misunderstood your measurements, or has your hot-rodding increased the gain of your Phono 3?

Regards,

Alasdair

[John Caswell]

Hi all,
That figure could be a accounted for by differences in valves, resistors and the like, it equates to a difference of 4dB overall.
The figure that you give is not the HT it is the voltage on the anode of the first stage ECC83, my figure is the HT on the input to the PCB.

John