Phono 3 input capacitance

[A.N. Beal]

Does anyone know what the capacitance is across the phono inputs of the Phono 3? I haven't found it quoted anywhere.

[bob orbell]

Why ? Bob

[A.N. Beal]

I'm going to be using a Phono 3 with a Goldring G900IGC, which is very fussy (to my ears) about the capacitance is loaded with, so it would be very helpful to know what the input capacitance of the Phono 3 is.
Regards,
Alasdair

[Phil Y]

Hi Alasdair

I'm afraid I can't off the top of my head provide the evidence but I did look into this at one point. I think my research suggested that the input capcitance of a typical ECC83 stage is something like 50pF. My arm cable (cartridge tags to RCAs) is 180pF according to the manufacturer. This makes 230pF.
I use a Goldring G1022gx which the manual says requires 150-200pF so I use it with no extra capacitors fitted at the input and for what it is worth it sounds fine to me ☺

Regards, Phil.

[A.N. Beal]

Dear Phil,
Thanks, that's very helpful.
Regards,
Alasdair

[A.N. Beal]

Dear All,

I have researched this a little further following initial listening tests on my newly completed Phono 3. I am using a Goldring G900IGC cartridge. I found that this sounded 'right' with my previous phono preamplifier (a Graham Slee Genera) if I provided an input capacitance of 115pF, which combined with my tonearm leads (about 75pF) to give a total input capacitance of 190pF. Therefore if the Phono 3 input capacitance was 50pF I would expect it to sound a bit dull with a 'spitty' top end. However the sound wasn't dull and my only complaint was a slightly excessive sharpness at the extreme top. This was rectified by inserting shorting wires across resistors R21 and R22 to remove the 3.18 'enhanced RIAA' equalisation which the Phono 3 applies as standard, leaving standard RIAA equalisation and it now sounds fine.

Therefore I investigated the Phono 3 input capacitance a bit further. On the internet I found a calculation of Miller effect input capacitance on the website of 'The Valve Wizard' (Merlin Blencowe) (http://www.valvewizard.co.uk/gridstopper.html). He states the formula as Cin = Cgk + CGa(1+A), where CGa is capacitance between grid and anode, CGk is between grid and cathode and A is voltage gain. For an ECC83 datasheets give Cgk = CGa = 1.6pF but he says the capacitance between the pins of the valve base socket and associated wiring (typically 1.5pF) should be added to the capacitance inside the valve, making the formula Cin = 3.1 + 3.1(1+A).

So the input capacitance depends on the gain of the input stage. The website gives a link to the chapter on 'Fundamentals of valve amplification' from one of Blencowe's books and the following calculation is based on my (novice) reading of this. Based on the circuit diagram for the Phono 3, published ECC83 valve data and the formula in Blencowe's book I calculated the gain 'A' of the Phono 3 input stage as 35. If this is correct, the above formula for input capacitance gives the Phono 3 Miller effect capacitance as 115pF. If we add (say) 10pF for the signal leads between the input sockets and the circuit board the total input capacitance would be 125pF. (A calculation based solely on the valve Cgk and CGa of 1.6pF gives total capacitance 68pF.)

It would be useful if someone more knowledgeable than me could check whether my calculations are correct (or not!). If the input capacitance of the Phono 3 with ECC83 valves is 125pF this would explain why it sounds OK with the Goldring G900IGC cartridge and standard RIAA equalisation.

If all of this is true, then why does Phil's G920 sounds good with the 3.18 'enhanced RIAA' equalisation and 180pF leads, which would give a total input capacitance of 300pF? One possible explanation would be that the high frequency rolloff caused by the high input capacitance of 300pF is being balanced by the rising top end response of the 3.18 'enhanced RIAA' equalisation.

Comments on any or all of the above from knowledgeable experts would be welcome.