Diy Audio mixer grounding help.

Hi all.
I recently built a 5 channel audio mixer. Inputs are all unbalanced. It has Left & right output jacks on the rear.
All jack sockets are steel Switchcraft types.
Unit has an internal transformer power supply with mains safety earth tied to the chassis.
My question is, will it be OK to connect the input &output socket grounds directly to the chassis or will I need to isolate all the sockets from the steel chassis?
To be honest, I have already mounted them directly without any audible issues. Im just curious about the proper way of doing this and the theory behind it. The output of the mixer is connected to my audio power amp and speakers. Any help appreciated!
As a side note, I have noticed that many commercial audio mixers on the market, have all the input jack sockets (line inputs) grounded via their mounting nuts etc.


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Paid Member
2003-06-12 7:04 pm
Maine USA
_I_ would try bonding all input jacks to chassis and to each other (because nuts come loose). Your output jacks are plastic. For line-impedance load I would run their shells to the input jack bus.

Power supply grounding is usually far more important. I don't see any obvious problem.

That is a HECK of a big transformer for two opamps. I'm surprised that isn't throwing hum into everything. Even toroid. I've had to put a doughnut outside the case to reduce hum.
Somebody here posted a chart of cabling &noise that alleged that best noise on single wire single shield coax was to connect the shield to earth. The metal shell of an appliance is bonded to earth as part of the regulations of the safety agencies. Many preamps & mixers follow that practice of grounding the RCA jack ring to case metal and then earth ground. I found on my RA-88a mixer, with 6 to 12' cables going in 4 directions (turntable, CD player, radio, power amp) I got less hum by insulating the RCA jack rings from case with o-rings, and connecting the rings to analog ground, middle of the op amp power supplies.
Your results may vary. In diy, we can experiment for best result.
BTW I use elastic stop nuts on projects to keep them from coming loose.
I recently discovered that inserting another mixer between RCA jack disco mixer, and the 12' cable run to the power amp, it decreased hum & noise even more. The PV8 mixer has TRS 1/4 phone plugs for the outputs, allowing me to run twisted pair cable to the power amp with TRS jacks. Insert 4 each 4565 op amps in the sound path, the result is quieter! Go figure. 12' run on RCA coax cable is that big a problem I suppose.
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Line inputs are a lot more forgiving than mike or phono, as far as ground problems go.

There is probably no need to put up with mounting nuts coming loose. There is a broad range of locking compounds manufactured to solve that very problem. Ask for 'threadlockers' at your local hardware store. Permatex is one brand; they make a whole range of products according to what the two metals in contact are, and how frequently disassembly is expected.

Your lament "12' run on RCA coax .." is familiar. It would be affected by the output impedance and bandwidth of the driving circuit -- those LM4565 have a very strong output stage -- 130/160mA. That could be part of the difference.

But there's certainly no beating balanced/differential cabling for immunity to noise and interference.


P.S. That's a really nice looking build! ;)
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