Troubleshooting Hum

[maLx]

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

 

[rayma]

What equipment do you have? If home brew, there are many possible reasons.

 

[maLx]

What equipment do you have?

It's a custom build amp. I'm thinking it's a cable problem with maybe a long run RCA, I'm going to swap out the RCA cables tonight to see if that persists. The phono cables are Analysis Plus DIN cables but like I said, they don't change the outcome at all.

 

[rayma]

If only one channel of the source is plugged in, is there more or less hum?
Do you have a ground loop break 10R resistor from each input jack common to audio ground?

 

[maLx]

If only one channel of the source is plugged in, is there more or less hum?

Good question! Let me try that.

When you mean source, do you mean to unplug the DIN Phono cable from the phono preamp.

 

[FauxFrench]

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.

 

[rayma]

Good question! Let me try that.

When you mean source, do you mean to unplug the DIN Phono cable from the phono preamp.

If there is a stereo DIN from the source, you can't unplug one channel.

 

[maLx]

So last night I unplugged one of the RCA inputs and the hum didn’t change. I’m going to try some different RCA interconnect cables to see if it’s picking up hum.

 

[rayma]

So last night I unplugged one of the RCA inputs and the hum didn’t change. I’m going to try some different RCA interconnect cables to see if it’s picking up hum.

Probably a ground loop in the circuit. Post some photos of the circuitry, and a schematic.

 

[scottjoplin]

When I turn the gain all of the way up, I get a low hum.

At what point does it go?

 

[maLx]

At what point does it go?

What do you mean at what point? When I mean all the way up, I mean up to the listening level. I can run a test with pink noise, see how high the pink noise DB has to be in order to hear it 1m back. Will do that tonight as well.

Cheers,
Joey

 

[scottjoplin]

I presumed you meant all the way up, ie all the way.......up

 

[wiseoldtech]

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]

Ok, so I've troubleshot and tried different interconnects, power cables, and I've isolated it to either the phono cable, tonearm, or amp. Doing more digging and will report back.

 

[maLx]

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.;

 

[scottjoplin]

That's a very good result, even more so for a DIY setup, phono amps are high gain, you are worrying unduly.

 

[Mark Tillotson]

At normal listening levels, at the normal listening position its completely inaudible? Not an issue. Its nice to reduce it further, but unlikely to be achievable without moving everything outside away from house wiring!