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.

[simon]

This has been on a back-burner whilst the humming amp has been with me but I took it to Nick's this weekend and had a good chat about it and the earthing "challenges". I learnt a lot!

I made a few amendments to the wiring and placement of the transformers. Two boards would probably be best but the easy solution was pushing the trafos to the outside of the existing board. The wires from the shunt reg were moved to the Aikido from the C3g. My thinking had been that as the signal is smallest here the shunt reg would have the greatest effect with the sense and force attaced close by. But Nick explained that as the signal is larger at the Aikido the negative effect would be smaller. I think. I haven't explained that very well.

Anyway, powering on and there was 6mV on the outputs which was promising. Hooking up the stepups and powering on there was a little buzzy hum but better than I remembered.

I tried inserting a steel plate between the shunt regs and audio section, and brushing the shunt reg force and sense wiring the amp went very quiet.

I switched on the fluorescent ring lamp I use and instantly the noise increased significantly, so I switched it off sharpish. But the noise was unchanged. Odd.

I had a careful poke around with a meter probe, moving wires, but nothing changed. But the phono leads from the step ups to the phono stage seemed to be causing the problem - pressing them down affected it. Changing the leads stopped the noise! Well pretty much most of it. The volume has to be all the was up before it's noticeable.

I'm guessing the last bit of noise is due to physical layout of the trafos, being on an open board, and some remaining less than ideal earthing. It feels like a journey that started around 2006 is coming to an end.

So how does it sound? Well really rather good I reckon. I've been listening to HD streaming for quite a while and come to the conclusion it's really pretty good. But vinyl is a step up again. Andrew really did do a top job on this design.
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[simon]

There was some mechanical buzzing though. It seemed to be coming from the audio section/valves but I didn't want to get my head too close tbh. I used a probe to see if I could feel any vibration anywhere but couldn't. I think it might be coming from one or both C3gs. It doesn't feel like a good thing, any ideas what it might be?

[Nick]

Well done. Not sure about the buzzing. Does pressing on the valve case alter it? Is it 50Hz, or something else?

[simon]

No, pressing the cans doesn't make any difference.

But this time I approached the amp from the other side of the amp and the buzz seems to be coming from the one of the transformers. This would make a lot more sense. Doubting myself I listened from the other side again and sure enough it sounded like it was coming from the audio section. Perhaps I need my ears cleaning out!

Fingers crossed it is a buzzy trafo. Not sure if I've got any suitable alternatives though, I'll have to have a look tomorrow.

[Mike H]

Congrats.

[simon]

Thanks Mike

[izzy wizzy]

Weel done Simon . No mean feat getting these things quiet.

[simon]

Thanks Steven, you're not wrong. Expert advice from Nick was required!

[Nick]

Hardly expert. The trick (as I have said before) with noise/hum I find is getting used to thinking in terms of current as being the driving aspect and voltage being a resultant. A lot of electronics (especially valve) gets us used to thinking in terms of voltage only. Bipolar solid state is about current, but you can get away (mostly) thinking in voltage terms (though I would argue that you never actually understand it if you do). It goes back (IMHO) to the fact that you are taught to think in either Norton or Thevenin but actually you need(sometimes have) to think in a superposition of them both.

[simon]

Good points, I do struggle to think in terms of current. Took me long enough with voltage! Any suggested reading? I probably need something at the basic end to start...