Hum in Passive Volume Control

[craigmj]

passive volume hum

Hi all,

I'm trying to build a passive volume control between my CD player and studio monitors, but as soon as I dial down the volume, I get hum. I've detailed exactly what I'm doing at **broken link removed**.

Any help greatly appreciated,
All the best,
Craig

 

[umery2k75]

volume control hum

May be potentiometer is fine, but there's some other thing which might be responsible for producing the hum.You have 250K Ohm potentiometer, the half value to it is 125K Ohm and you do get hum at 250K,as you have mention the hum fades when reaching 0 Ohm.If you just replace the potentiometer with the simple resistance of 250K, do you still get the hum?
Tell us about this.

 

[nandhu015]

hum at half the volume

Most of time the hum is caused by the powersupply.

at low volume you can clearly hear the hum, at higher volumes the hum is within the audio which you cannot distinguish.

check out your power source first.


Regards
Nandhu

 

[sashijoseph]

Looking at the picture in your website,it seems you haven't connected the rca ground points(You've used the chassis as the connector between the 3 rca gnd points).
I would suggest first to insulate all 3 rca's from the chassis then connect their individual ground points to the vol. control ground pin.Then connect the mains earth wire to the chassis at some point and from there run a wire to the common point on the vol control.
For a more elaborate connection layout see the attached figure.
And the value of the pot is too high.....I would suggest max 100k,with a 15k for log taper.

 

[craigmj]

Hi all

Many thanks for the assistance. I've tried replacing the potentiometer with resistors in series, and the hum arrives as soon as the resistors are in the circuit - so perhaps it's not the pot.

I ordered 75k Ohm resistors, but their colour coding is Brown, Red, Black, Green, and a thin brown. I think that makes them 2MOhm, if I'm reading it right, at 1% tolerance, but at any rate, I'm not sure that they really are 75k Ohms, although my multimeter seems to think they are (assuming I'm reading that right, as well ;-)

So my next step, I think, will be to move to a 110k pot with a 25k (or 15k) resistor for log taper. If that still introduces the hum, I'll try for the full circuit so kindly suggested by sashijoseph. If that still doesn't work, I'll post again. But since I'll have to buy the parts, it'll only happen in the new year.

Many thanks to all, and a merry Christmas season,

All the best,
Craig

 

[craigmj]

Hi again,

Ok, I've rebuilt with a 100k potentiometer, and a 30k 5W resistor from the potentiometer output to ground. The hum is still there. As soon as I remove the potentiometer from the circuit the hum goes, so I don't think it's a problem with my wiring. On the other hand, if I put a resistor in place of the pot, the hum is still there.

So I'm wondering if there are special resistors one should use for audio applications, ones that don't generate hum? Mine are 1% tolerance, but otherwise off-the-shelf.

All the best,
Craig

 

[btbass]

You seem to have your volume control backwards, your input to the volume control should go to the point you have marked as 'out', and your output to the amps should come from the pot wiper on which you have your input switch.
At the moment, when you have your volume at zero, the cd is driving into a short circuit. That wont do it any good!
You need to know the input impedance of the power amplifiers. This will typically be 47K or 22K or it might be as low as 10K, you can get this from the spec sheets of the speakers. If you have a long lead from the attenuater to the speakers, it will pick up hum.
Unless it is driven by a low impedance source.
you need to find out the output impedance of the cd to see what minimum load it can drive.
A rule of thumb is you need a factor of ten.
So if the speakers have 22K input impedance, the maximum value of the pot should be 2K2.
Then the output impedance from the cd should be less than 220 ohms. Check cd spec sheet, so it can drive the pot ok. It should be ok, most have very low output impedance.
The ideal solution is to buffer the output from the pot with an op-amp.

 

[XNOX_Rambo]

As btbass says you have your pot wired backwards - and your resistor too. :wink:

https://www.elexp.com/t_resist.htm

 

[craigmj]

Ok, finally got it done, changed the circuit entirely. My monitors are 30k Ohm input impedance, and I got hum for everything except 1k Ohm pots. Now it's working well, with almost no noise. It works fantastically for my CD, but I need to turn the volume up high when using my computer for output... hmm, more to think about.

Many thanks for all the assistance.
All the best,
Craig