Crosstalk in Passive Summing Mixer

[dyamakuchi]

"Switched" connectors:

https://www.mouser.com/ProductDetai...MI8Jju3dHJ8AIVARtlCh1iPwXeEAQYCyABEgJ83_D_BwE
or the one I like for PCBs:

https://www.neutrik.com/en/product/nys216
Crosstalk is traditionally caused by capacitive coupling and should increase with increasing frequency. Usually @ 6dB per octave. It should also be 60~80~100dB down even in a poor design. What you have is probably a design error. A very high impedance node _could_ couple rather strongly through stray capacitance but, again, the coupling will _increase_ with freq...if it's typical "crosstalk."

If it doesn't go away more or less completely at LF what you have might be more accurately called a "fixed pan."

 

[cpsmusic]

"Switched" connectors:

https://www.mouser.com/ProductDetai...MI8Jju3dHJ8AIVARtlCh1iPwXeEAQYCyABEgJ83_D_BwE
or the one I like for PCBs:

https://www.neutrik.com/en/product/nys216
Crosstalk is traditionally caused by capacitive coupling and should increase with increasing frequency. Usually @ 6dB per octave. It should also be 60~80~100dB down even in a poor design. What you have is probably a design error. A very high impedance node _could_ couple rather strongly through stray capacitance but, again, the coupling will _increase_ with freq...if it's typical "crosstalk."

If it doesn't go away more or less completely at LF what you have might be more accurately called a "fixed pan."

Thanks for the explanation.

 

[cpsmusic]

So, I've disconnected the four mono inputs, and the crosstalk is gone - that's good!

Obviously I've wired them wrong - I wired them as follows:

T -> T Left via 15k, T Right via 15k
R -> R Left via 15k, R Right via 15k
Ground is common to all sockets

Can anyone see what's wrong with this?

Cheers

Chris

 

[ruffrecords]

That looks OK to me. If you short tip and ring that should give no crosstalk. Try checking your 300 ohms across the bus - you haven't got from left to right instead of cold to hot?

Cheers

Ian

 

[abbey road d enfer]

Crosstalk is traditionally caused by capacitive coupling and should increase with increasing frequency.

In this particular case, no. X-talk is caused by the topology, that allows mixed signals to be reinjected into the sources. Depending on the actual impedance of said sources, the effect is more or less pronounced. Zero-ohm sources suffer no x-talk, when open circuit is tehemost prone.

 

[cpsmusic]

That looks OK to me. If you short tip and ring that should give no crosstalk. Try checking your 300 ohms across the bus - you haven't got from left to right instead of cold to hot?

Cheers

Ian

Pretty sure the 300 ohms are ok as each one is wired up across two terminals of the the two output sockets.

 

[cpsmusic]

After disconnecting the four mono sockets I tried reconnecting one and the problem reappeared.

One thing I noticed - the four mono sockets are different to the L/R sockets. The mono ones have four connectors while the L/R ones only have three. They were old ones that came from a large amount of electronics stuff that I inherited a few years back. Could they be different somehow?

 

[abbey road d enfer]

A "stereo" switching jack socket has 5 or 6 lugs. Yours does not obey the definition.
E.G. Neutrik makes plastic jack sockets with 3 blades for active contacts TRS and 3 switching blades TS-RS-SS

 

[ruffrecords]

After disconnecting the four mono sockets I tried reconnecting one and the problem reappeared.

To be clear, when you reconnected, the problem was there with all the mono inputs fitted with plugs that short tip to ring?

One thing I noticed - the four mono sockets are different to the L/R sockets. The mono ones have four connectors while the L/R ones only have three. They were old ones that came from a large amount of electronics stuff that I inherited a few years back. Could they be different somehow?

Sre you sure they are TRS?

Cheers

Ian

 

[ruffrecords]

In the pic you posted I do not see any 300 ohm terminating resistors.

Cheers

ian

 

[cpsmusic]

To be clear, when you reconnected, the problem was there with all the mono inputs fitted with plugs that short tip to ring?

Sre you sure they are TRS?

Cheers

Ian

Not exactly, I disconnected all four mono sockets, then reconnected one. The mono socket worked fine, however the problem reappeared on the L/R sockets. I'm thinking now that they are a different type of socket.

 

[cpsmusic]

In the pic you posted I do not see any 300 ohm terminating resistors.

Cheers

ian

The 300 ohm resistors are the ones on the far right of the picture, just near the switch.

 

[cpsmusic]

I'm going to try wiring up one of the L/R sockets as a mono socket and see what happens.

Cheers!

 

[abbey road d enfer]

I'm going to try wiring up one of the L/R sockets as a mono socket and see what happens.

Your earlier description of these jack sockets clearly indicates they are not what you need. Don't waste time in hopeless experiments, buy the right jacks.

 

[cpsmusic]

Your earlier description of these jack sockets clearly indicates they are not what you need. Don't waste time in hopeless experiments, buy the right jacks.

If the L/R socket works then I'll know that the problem is with the mono sockets and not how I've wired them. Strangely enough I don't consider this to be a waste of time as I find I learn more when things don't work out the way I expect!

 

[cpsmusic]

Had another chance to look at the mixer today and I think I've found the problem. To test the mixer I've been using a 1kHz sine wave with the unit in position near the rack where it's going (that's the only space I've got to test it!). By chance, I rotated the mixer by 90 degrees and I noticed the hum disappear. Initially I though that there must be a bad connection somewhere. Out of interest, I put a spectrum analyzer on the output, and there it was, 50Hz hum! Looks like the unit is picking up hum from some gear nearby. The reason I didn't think of this is because beneath the mixer are two patchbays. What I forgot was that they're not very deep, especially in comparison with what's underneath - an audio interface and a Neve-style mic preamp.

So it looks like I'm going to need some sort of shielding - any suggestions?

Cheers!

 

[abbey road d enfer]

Seems to me it's the first time you mention noise. So far it's been about x-talk...?
Anyway it should not be like this. I believe all teh jacks are non insulated. That may be cause for a ground loop. Have you tried isolating the unit from the rack?

 

[cpsmusic]

Seems to me it's the first time you mention noise. So far it's been about x-talk...?
Anyway it should not be like this. I believe all teh jacks are non insulated. That may be cause for a ground loop. Have you tried isolating the unit from the rack?

I thought it was crosstalk - turns out I was wrong! I'll try testing the unit with longer leads so that it's further from the rack.

 

[cpsmusic]

A quick question about grounding my summing mixer - what I did was connect all the earths of the sockets together, including the L/R outputs. Is that the best way to do it? Doesn't this mean that there are two separate paths to ground (via the Left and Right outputs)?

 

[abbey road d enfer]

A quick question about grounding my summing mixer - what I did was connect all the earths of the sockets together, including the L/R outputs. Is that the best way to do it? Doesn't this mean that there are two separate paths to ground (via the Left and Right outputs)?

Well, anyway, being non-insulated jacks, they are all interconnected via the front panel. That is a dead certain way of creating ground loops. You should use insulated jack sockets.