WAD Phono2 PCB Confusion

[NickG]

No, I think it was ok to start with, then they tried without the cathode caps, and decided it was bettre, and in the process some values were changes and others were left as they are.

So, I think they were both right.

Give me 10 minutes, and I will run a spice sim and you can see what the differences are.

[NickG]

Ok. this image shows what spice says about the two circuits. The solid blue trace is the original with bipassed cathodes, the lower green is with unbipassed cathode resistors.

http://home.lurcher.org/nick/images/...comparison.gif

You can see the 8dB or so difference, and the slight effect of the different Ra on the lower bass response. The unbipassed case has a slight peak in the bass response.

However I suspect that in listening tests the circuit was prefered so thats what they went with.

Removing the 20k difference caused by the Ra, so making the 316k resistor 296k removes the peak, and makes the bass performance of the two circuits about the same. The unbipassed circuit has slightly less loss above 1k, so I would guess the unbipassed circuit with 180k in the second stage instead of 274k would have a more open top end. You could even try a 165k resistor there to lift the top end a little more. Depends on you preference I guess.

[NickW]

Hi Nick

Brilliant stuff. My first stage anode has 100K, and the second stage anose has 330K.

If I understand you correctly, I could (should?) reduce the second stage anode resistor from 330K to more like 180K ? - all assuming no cathode decoupling.

regards, Nick

[NickG]

No, I have not explained that correctly. The anode resistor stays at 330k, the resistor I am talking about is the resistor part of the RIAA, that 180k one.

I would suggest you just build it as WAD suggested, you can try all these things later. But first build to the circuit you have would be my advice.