Phono Stage PCB Layout

[simonra]

The two traces on your scope images are for the right and the left channel simultaneously in one image, right ?

That's right.

What can't be seen is the DC voltage, so whether the Servo is working or not is impossible to see.

Correct, I measured the offset separately with a DMM.

But do I understand correctly that in all settings with servo, DC output is ca. 1mV ?

Yes, the servo appears to be working now.

You mentioned to have some other Riaa amp, try to do the same measurement with the TT connected, and see if there is a difference.

Good plan... I'll try that this evening.

And last but not least, is your amp encapsulated in a metal box, properly grounded to main earth and what about connecting your TT to main earth ?

What's probably not clear is that I've got two amps made up at the moment, one with a servo and one without. Neither were boxed up for these measurements, I connected the TT ground to the central ground point on the PS. I have made measurements with one in a grounded aluminium box but it appeared to make no difference to the results.

Thanks for the help,
Simon.

 

[planet IX]

Image below shows what I suggest, including some Farnell part numbers.
On top of that, you have used Omron relays the G6SK series.
From TE Connectivity there is the IM series. Their footprint is much smaller, cost is almost 50% lower and they are very reliable.


Hans

Sorry to be off-topic…

Hans, I just read that you suggest Panasonic PPS caps for the RIAA network.
Do you have any experience in soldering them to a board? The reason I ask:
I made a phono amplifier some years ago with these caps (Panasonic ECHU) and
I made the experience that as soon as I touch the caps with the tip of my soldering iron,
they change their capacity irretrievable.
Since I haven’t got a reflow soldering machine I ended up replacing them with THT caps…
Regards, Boris

 

[simonra]

I tested an old RIAA amp using the same set up, discovered it was oscillating and wasted 1/2 an hour messing around with that one. Then I remembered what I was supposed to be doing and boxed up the new amp to hook up to the TT and try to work out where the interference is coming from. So I had the volume right up and my head close to the speakers when my wife plugged in her hair straighteners in the next room . Once I picked myself up off the floor I started thinking maybe the interference is coming from the mains.

Hopefully that's good news and means I can stop torturing the PCB desoldering and re-soldering components. I get the interference whether the TT is plugged in to the mains or not so I guess it's being picked up by the cart... not sure what I can do about that...

 

[Hans Polak]

Sorry to be off-topic…

Hans, I just read that you suggest Panasonic PPS caps for the RIAA network.
Do you have any experience in soldering them to a board? The reason I ask:
I made a phono amplifier some years ago with these caps (Panasonic ECHU) and
I made the experience that as soon as I touch the caps with the tip of my soldering iron,
they change their capacity irretrievable.
Since I haven’t got a reflow soldering machine I ended up replacing them with THT caps…
Regards, Boris

Hi Boris.
What an unfortunate experience.
I have never come across this problem, but I'm warned, thank you for that.


Hans

 

[simonra]

Just to close this one out I tracked down the source of the problem to a dodgy interconnect (the same one I've always used with this TT). Anyway the amp sounds great and I'm gradually working my way through my vinyl hearing extra bits of detail here and there. The only problem with improving the playback system is it makes crappy production less and less bearable however the great recordings just sound even better.

Hans - many thanks for your help, it's been a great learning experience. I'm still determined to understand what was going on with the Servo's so I'll use my spare boards to experiment and hopefully get to the bottom of it.