Troubleshooting Hum

maLx

Member
2018-02-21 7:02 pm
Hi, Everyone.

I don't expect magic, but maybe the community here can give me some troubleshooting steps to fix a hum. It's going to be hard because both of these were built custom.

I have a Solid State Phono Preamp going to a Solid State Amp, When I turn the gain all of the way up, I get a low hum. It's no where near at the listening level and you have to have your ear against the speaker to hear it, but since I did wire these myself, I'd like to see if I can find the source and correct here. Here are the scenarios.

1: With the Amp ON, and no input connected to the RCA input, there is no hum.

2: With the Amp ON and the Phono Amp ON, and the input connected to the RCA in, from the Phono Amp out, with NO phono cable plugged into the input on the Phono Amp, there is a very low hum.

3: With the Amp ON and the Phono Amp ON, and the input connected to the RCA in, from the Phono Amp out, with one more many phono cables plugged into the input and properly grounded, there is a very low hum. Same as #2

4: With the Amp ON, and the input connected to the RCA in from the Phono Preampliifer out, with the Phono Preamplifer OFF, there is a very loud hum.

Let me know if anyone has any thoughts.

Thanks,
Joey
 
Hi Joey,

The easiest is 4). When you turn the Phono Preamplifier OFF, the Phono circuit becomes high impedance and the Amp picks up hum through the input. That is a trivial effect. Try to put 47K between each of the Amp RCA center terminals and signal ground (RCA outer terminal). Then, you maximum have 47K of impedance.

Let's see what comes up of suggestions for 2) and 3). Hum from net cables will always leave the risk of some coupling to sensitive circuits in the amplifier. It may eventually enter on the signal ground wire. Just noticeable is quite common.
 
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Any phono preamp can create low level noise.
With the phono input shorted, a proper preamp should only produce a very low hiss if the amp volume is turned up all the way.
That is normal, because we don't listen to records at maxmum volume, unless we're deaf or a 5 year old child.
Any noticable "hum" is the result of a bad preamp power supply, crappy cables, and no solid grounding.
And this thing of "putting your ear to the speaker" is also nonsense, because we don't listen to music that way.
If you can't hear the trivial noise from your normal seating position, then relax.


Take a pair of shorting RCA plugs and put them on the phono inputs - the system should only make a tiny hiss at max amp vol.
 

maLx

Member
2018-02-21 7:02 pm
After a ton of digging..

Changing phono cables, cartridges, headshells, interconnects, power cables, and outlines, still the same hum. At full volume, you stop hearing the hum about 1foot back from the loudspeaker. What do you guys think? Normal? I've tested the frequency it's a 60hz hum about 10db louder than my room. My room ambient noise is about 50db and this is about 60 if you take it a few inches from the driver.;