C3g Aikido Phono

[Nick]

Do you still use screened wire, screen connected to ground at the pcb end but to nothing at the phono socket?

Yes, screened, screen connected at both the PCB and phono socket. The phono socket is floating WRT chassis ground.

Actual ground point is grounding post? One 100R per phono socket? Screened wire, screen only connected at one end?

Again, screen connected at both ends, phono floating.

That's a fat piece of wire!

Its to meet USA ground current tests regs and the CE (at a lower current) equiv. And its also nice and low resistance.

Isn't ground also connected via the HT PS?

No the power supplies float and are connected to the ground plane. Directly in the case of the HT supply and via a potential divider and caps in the case of the heater supply. The whole thing is inside the earthed metal casework.

It may be you have more ground connections that are ideal. I reference to ground at the output, as that's the point where another grounded device will connect and where the reference to the signal to ground will be "measured" by the following amplifier/preamp.

[simon]

Right I think I get this now (famous last words...). On reflection when I said my phonos are grounded that might be incorrect - they're floating.

Essentially then, Nick's has the input and output phonos connected to the audio circuit and the phonos are floating but the outer is connected to the screen. The HT PS is floating and connected to the audio circuit. The audio circuit is grounded via a 100R resistor from the output phono outer to the grounding point. There are also 10nF caps from the input phonos' outer to the grounding point.

I ground mine by grounding the second raw PS cap to the star earth (after a ground lift R). The input and output phonos are completely floating with no C or R. I think I still only have one connection to ground and there shouldn't be an earth loop.

I'll play with Nick's arrangement too. Thanks for prompting this discussion Stephen, I've learnt quite a bit.

[simon]

Well, I'm not sure if this is two steps forward and one back, or one forward and two back.

My memory really isn't good at the moment - the HT PS was floating and grounded from the pcb to the star earth. I've rejigged that and grounded via the 2no. 100R resistors on the output phonos' outer, and added the 10nF to the input phonos.

Overall it's quieter now with the 100R phono plugs in the input phonos .

Interesting no. 1:
With the output phonos connected to the outside world there's all manner of oddness at the output phonos on the scope, but it's not heard when played through the amp and speakers.

Interesting no. 2:
Plugging the cartridge and step ups in to the input phonos instead of the 100R test phonos resulted in a lot of noise through the speakers which wasn't there before. To put it in to perspective if 12 o'clock is zero volume I can first detect hum at 6 o'clock with the input phonos grounded, but 2 o'clock with the step ups attached.

So now I'm suspecting there's something going on before the phono. Possibly a grounding issue, but I can't think what might have changed to cause this.

Time for some lunch and a think.

[IslandPink]

I haven't read everything carefully, but are your step-up cans grounded ?

[simon]

I think so. It's a long time ago now, 15 years???, but I think I had to ground them to stop hum.

I think I tried to run balanced maybe 10 years ago and couldn't stop hum so reverted to grounded.

They're probably worth revisiting, but I can't work out what's changed to make them an issue now.