Moderators.

[bob orbell]

What has happened to our moderators ? it appears only Colin is making regular visits and why has no one posted a response to my finding of the 3.18 correct resistor value for Phono II III and IIIs ? are there no others who can do maths ? . Bob

[John Caswell]

Well because the moderators are not often actively on the forum does not mean that "Big Brother" is not watching what is going on.
Perhaps people do or don't think that the 3.18µS mod is worth worrying about. I for one would not bother as, due to wearing hearing aids, I doubt if I could tell the difference. I would have to rely on memory, and that can play tricks. All I do is set my phono stage up using a square wave signal and an inverse RIAA network for a good square wave response and trust that it will sound correct, which according to my lady wife it does. Also I haven't heard the lady wife complain that it sounds poor, and she can easily tell the difference between an s/s amp and valve one as well as FM/DAB and vinyl/CD.

John

[bob orbell]

Fair comment John, but oscilloscopes dont lie . Bob

[John Caswell]

I don't quite understand this comment "but oscilloscopes dont lie".
Have you seen my workshop? How do you think I set things up??????

John

[bob orbell]

John, for many years I have said that the resistor value for the Phono II and III is wrong, myself and many others have stated that the high frequencies are too bright and sibilance is present with R21-22 fitted with either 14454 ohms (as stated in the FAQ) or the supplied 14K3, myself and others just fit a wire link, this removes the 3.18 time constant and the brightness and the sibilance are removed. A few weeks ago I was discussing this with an old school chum who was and still is very good at maths, he offered to do some calculations with my help in suppling data and equations, he came up with 2177.812 ohms, I fitted 2K2 in my Phono and liked what I heard, a few weeks after I found my original build sheets from Peter C on the Phono III and in one of the pictures of the PCB at R22 is a 2k2 resistor. Now I am convinced the 14K3 is wrong, and the 2177 is correct. As I said, oscilloscopes dont lie. Bob

[Richard]

Hi Bob, all,
I/we visit daily, there is no need to log on to read the bb and I only log on if I need to post or see an attachment.
As to the phono 3.18 mod I did the maths 15 years ago when building mine. They should still be on here somewhere. They are a little ponderous but not difficult. There are several things to note though. They will change depending on your valves (I used 5751 instead of ECC83). There was a change in the original schematics to leave off the cathode bypass caps. This will change the response as it changes the valve op impedance. Mine was 5751 with bypass caps fitted and I then checked the result with a reverse riaa filter and scope.
If folk don't want to do that I can concur with John that the published circuits are very good and there is very little difference despite the small changes in corner frequencies and top end roll off.
I am pleased though that you have improved yours and if others wish to comment or experiment I'm sure they will