[andrew]

Hi Jim,

  Way back when I bought and built  my Cornet I got some hum, this was eliminated by connecting the ground plane to the earth post and using isolated RCAs - a simple enough error easily fixed.

  I thought I could hear diode buzz when I put my ear to the back of the unit - but nothing came through the speakers so no I had no real concerns.

  Recently I invested in a MC cart and a pair of TX103 step ups. The buzz was back with avengence. John Chapman suggested I earth the RCA grounds on the TX103s and this reduced the hum. However, what I had created, though nearly silent, was an earth loop. This bugged me!

  So, my next step was to install a earth lift on the Cornet between the PCB ground -  the hole middle of the far end of the PCB - and earth. I used a 100R bypassed with a 10nF.

  The earth loop was gone, at least I can't hear it, because the diode buzz now comes through the speakers. I changed the diodes, wondering if I had a faulty set, but the new set is just as bad, if not worse.

  My question is...do the diodes share the same ground plane as the signal elements, if so could the diode switching noise be getting dumped in the ground plane and causing a dirty signal ground, that I can hear as it's now got nowhere to go?

      many thanks,

[hagtech]

The diode return loop is localized.  It connects to the ground plane at only one point.  There is a slight residual 120Hz hum injected into the ground plane (way way down), but not any buzz (high frequencies).

I would try to get the grounds and cans of the TX103 to use the same ground as the RCA jacks.  Same as TT ground.  The you won't need any lift elsewhere.

The TX103, even though well shielded, can easily pick up a stray magnetic field.  Even if you don't have ground loops.  Might have to play a little with proximity and orientation with other components.

jh

[andrew]

The diode return loop is localized.  It connects to the ground plane at only one point.  There is a slight residual 120Hz hum injected into the ground plane (way way down), but not any buzz (high frequencies).

I would try to get the grounds and cans of the TX103 to use the same ground as the RCA jacks.  Same as TT ground.  The you won't need any lift elsewhere.

jh

When I say buzz, it is definitely 100Hz. UK mains...

Currently, the lowest noise setup I have found, is to have the earth post on the 103s tied to the signal ground on the 103s. This is then all tied back to the earth post on the Cornet. Cornet is wired as standard, I have left the lift in but bypassed it with a wire. I would have thought this results in a ground loop?

  many thanks,

[hagtech]

I think you might be ok.  Not sure why bypassing the 100R creates a loop.  The TT floats, so does the tonearm and cartridge wires.  The ac mains Earth (if you have one in UK, goes to Cornet chassis.  This then goes to PCB, then to RCA, to TX103, to TT.  Not sure where it gets looped back to Earth or anywhere else.

jh

[andrew]

Hi Jim,

Sorry, my explanation needs to be better.

Situation a) with the Cornet grounded as per your design. I get no discernable hum and as long as the TX103 earth post is cross connected to the RCA common on the TX103. As you suggest the TX103 earth is also connected  to the TT earth lead and to the earth post on the Cornet. If I remove the extra TX103 cross connection a slight hum appears.

Situation b) the earth lift is installed on the Cornet, that is a 100R bypased with a 10nF, between the earth post and the PCB earth point. Now I hear the diode buzz through the speakers. I'm assuming the diode switching noise is getting dumped into the ground plane, as it was befoe, but the earth lift is sufficiently high enough resistance to make the switching noise flow down the interconnects into the power amp rather than out through earth on the Cornet.

So, situation a) bugs me because I definitely have an earth loop as signal ground and earth are connected at both TX103 and Cornet.

In situation b) I have no idea if the original hum is gone because the switching noise seems to get to the power amp.

Now, in situation a) the sound is more or less hum free but I shouldn't need the cross connection between the TX103 earth post and RCA common.

   many thanks for the advice,

[hagtech]

The Cornet PCB ground needs a reference.  So you can connect it through the RCA shield to the Earth ground at TX103 can.  But I doubt there is any power cord connection to TX103, so no Earth reference there.

Somewhere, you have to connect the power cord green wire (Earth) to the system.  The chassis of Cornet must have that connection.  The PCB ground only connects in one place.  It does not connect via transformer.  Secondary is isolated.

I would recommend connecting the power cord green wire to chassis.  Then make sure Cornet ground lug connects to chassis.  Connect it to TX103 case ground.  Then connect TX103 case ground to TX103 signal ground and RCA shields and TT ground.  Now the Cornet PCB gets it's reference from the RCA shield.  No loops.  The "single point" is effectively the crossconnect at TX103.  All things tie to that point.  Perhaps this is close to situation A?

jh

[andrew]

An update.

Attempting to explain my problem in plain text was doing my head in, so I sent Jim a wiring diagram by e-mail (I couldn't get the BB to upload my image.)

His promtly reply was.
 

Andrew, Great pic.  I can see your "crossconnect" now. Do you need to connect the RCA shield on TX103 to Earth on TX103?

Yes, that's the one that's bugging me too! If I remove it then the hum comes back.

I have tried the following, this morning, in reverse order, worst first.

1) Removed connection between Cornet chassis earch and Cornet PCB earth - very loud hum.

2) Removed connection between 103 earth and Cornet chassis earth - hum.

3) Removed connection between 103 RCA ground and 103 earth (this is as per John C's original design) - low hum.

4) And best of all, the diagram I sent with a cross connection between the 103 RCA signal ground and 103 earth - almost silent, even at full volume, and way below LP surface noise.

Which is counter intuitive to the received wisdom about earth loops - since that is exactly what I've got!

I've half a mind to pop the lids of the 103's and hardwire a switched cross connect between the RCA ground and earth. That way if I solve the problem I can just pop the lids and flip the switch.

Thanks for the advice so far, all ideas gratefully received as this is now starting to drive me up the wall.

  cheers,

[hagtech]

One thing I had forgotten was that each TX103 has it's own can.  That adds a bit of mystery here.  Ground wires everywhere.  Three chassis.

Anyway, sounds like you have a working solution.  That's the good part.  If it works, keep it.  

jh

[andrew]

Jim,

  Been staring at the manual, then I noticed something...

  Now I'm confused, which way around do the diodes D1 and D3 go? The silk screen on the board appears to be different from the PDF manual. Could it be I have D1 and D3 the wrong way around? Mine are all oriented the same way as depicted on the board's silk screen. The PDF has D1 and D3 cathodes in the opposite orientation to D2 and D4.

  Thinking about this, if I had these diodes the wrong away around then the heater circuit wouldn't work. What voltage should I probe for after the diodes?

  cheers,

[hagtech]

Well it might depend on which Cornet board you have.  I re-did the heater circuit to reduce hum and I added the constant current sources when I ordered the second batch of production units.  

The new board has diodes in opposite directions.  The silk screen and PDF files are exactly the same thing.  However, maybe I had the diodes different in the first version.  Don't have any old boards to confirm or not.

Whatever, the silkscreen on the board will be correct.

jh

[andrew]

Thanks Jim,

That's what I thought - I must have the older version as mine are all the same way around. It was just a wild guess at what was causing my hum - albiet it is now much reduced.

  cheers,