Op-Amp RIAA Preamp with Tone Controls - Help!

[cathoderay57]

Thanks for all of the advice. It wasn't the neon lamp and yes, it did work OK on batteries. Today I have relocated the power supply into a separate enclosure and replaced the 220uF smoothers with 470uF. Still not much better, unfortunately. George is I think on the money - although just disconnecting the coax braid doesn't fix the problem, there is definitely some sort of issue concerning the earthing of the input and output phono sockets. I have tried earthing each pair of sockets to a separate solder tag connected to the metal back plate of the box, which in turn is earthed. That was probably a bad idea - introducing additional earth loops. Maybe it's better to connect the socket shells directly to one earth star point. I think unfortunately my layout is basically crap and so I will have to redo it. If I can solve the hum issue I will probably replace the power supply using Top Cap's circuit with regulators. If not then maybe I'll bite the bullet and just build Top Cap's design - it is certainly well documented! Cheers, Jerry

[buggies]

It was reading the last paragraph on the first page of TopCap's description that might match the problem:-
"It is most important to use fully isolated RCA jacks on the input as those, with screens grounded to the turntable ground, may introduce a hum loop. "
Maybe unscrew them from the chassis?

[cathoderay57]

They are cheapo jacks mounted on a small plastic panel so isolating them is easy. Also Craig's point is very pertinent - I just put the scope on the PSU rails and there is unacceptable ripple so that definitely won't be helping things. I've just ordered LM7815 & LM7915 regulators. I'm still not sure why the hum is worse on one NE5534 than the other - maybe just differences in layout and proximity of power and signal rails.

[Radio Wrangler]

Is there any linkage at the turntable between the signal cable braids and the turntable metalwork?

Is the turntable earthed to its mains plug, or is it fitted with a 2-core mains cable?

Does the turntable have a pair of Y capacitors in some filtering arrangement?

Is there a ground wire from the turntable to the amplifier?

David

[cathoderay57]

Hi David, it's a Thorens TD150 Mkii.
Is there any linkage at the turntable between the signal cable braids and the turntable metalwork? Yes
Is the turntable earthed to its mains plug? No
is it fitted with a 2-core mains cable? Yes
Does the turntable have a pair of Y capacitors in some filtering arrangement?
Maybe, see attached image. But I think the caps are across live/neutral and on-off switch, not between line and earth.
Is there a ground wire from the turntable to the amplifier? No, earthing is via the coax braid only.
BTW I have rejigged the PSU - in a previous post on the pre-amp schematic I was using a single diode bridge taking the 15v rails from the + and - with earth return to the link between the two 9v windings (CT). I have now separated the windings and I am using 2 diode bridges as shown. Effect? Now I have 100Hz ripple instead of 50Hz. Odd, as I thought the previous arrangement was also full-wave rectification. Maybe I need to go back to power supply school.....I will be replacing the transformer with a 20-0-20v unit and adding LM7815/7915 regulators. Cheers, Jerry

[Radio Wrangler]

Difficult to be sure of where the centre points of those capacitors goes to. A stab with an Ohmmeter might clarify.

TD150s are good decks and lot's of people have used them without problems, but me being me I'd rather see the cold ends of the cartridge windings go back to the preamp with nothing getting in on the act... to be honest I'd rather have the whole thing differential, but that's too wild for the market in general.

Jumping back, does the hum swap channels if the input phonos are swapped?

Does it swap channels if the outputs are swapped?

Are any ground tracks on the PCB shared with anything cattying 50Hz or ripple current?

THere are lots of places to look too many for blind guessing, but try simplifying things.

Fit a couple of 220 ohm resistors mounted in phono plugs in place of the turntable and have ther turntable earth wire off. Any hum?

Now test continuity of that earth wire back to arm metalwork, if OK, connect it to preamp. Any hum?

Are cartridge ground/braid connections intact. Measure cartridge resistence rounf the loop by probing each phone plug.

You basically have to muck around hoping to stumble on a clue. Never admit it to anyone, of course...

David

[cathoderay57]

Thanks David - appreciate the support. I'll pause this job for the time being, while I wait for the regulators to arrive. Starting fault finding again after I have connected up a good stable ripple-free PSU seems sensible. Also I have another job on, as the Power Mac G4 with the dicky Acbel SMPSU has just been returned for Phase 2 diagnostics - you were helping on that thread as well, so expect more updates there soon. Cheers, Jerry

[Radio Wrangler]

It's keeping me busy and stopping the gears in my skull seizing up.

Just remember... IT knows what's wrong with it. no-one else does, but it has a bit of a problem communicating. You have to work out the right questions for a sort of yes/no interlude (now that's going back in British TV... Open the box!)

David