Passive Preamp Hum

[transducer]

I just put together a passive preamp based on Harry Haller's schematic. I'm using a Radio Shack Alps 100k pot and a couple of generic film resistors. Originally tried 10k Holco's, but didn't have enough range of volume, it was either off or blasting, so I increased it to 26k resistors (where HH shows 11k). At zero volume I get no hum, but as I turn the volume up I get a hum, not really noticable when music is playing, but definitely there when there is no music.

This is anoying, and I'd like to know why it's happening. Any ideas? The case is plastic, but I don't think it's RFI otherwise I should get it at all volumes.

By the way, this pot has excellent tracking but is probably no longer available it's from my stash of junk parts.

Thanks,
Ron

 

[pinkmouse]

As the hum is modulated by the volume control it suggests to me that it is coming in on the signal lines. Try swapping out your cables, or even running it with no input at all, to see if this makes a difference

 

[transducer]

I'll try that tonight, but I'm using the exact same cables with an active preamp with no problems. Only thing I can think of is that the output cable is very long, 3 meters. I don't need something that long though, so I'll try something shorter.

Thanks,
Ron

 

[pinkmouse]

The active pre probably has it's own earth though, and that may be grounding out any noise before it gets to the amp stages.

 

[cyclotronguy]

Sounds like a ground loop issue.

 

[transducer]

Pinkmouse, tonight I'll try different cables, and no cables, and maybe even shorted inputs!

Cyclotron guy, can you elaborate? You've seen the attenuator schematic. The D/A that feeds it is grounded through the ground pin. The amp does not use a ground pin (Rotel). I have no ground issues with an active pre (grounded through the ground pin), or if I plug the D/A straight into the amp. This is not the first time I've had difficulty with a fixed series/shunt pot setup.

If it is a ground issue, how can I correct this in a passive attenuator? I'm building this unit for a cousin who wants to hook up his DVD straight into his Rotel amp. I'll probably get this thing quiet, only to have hum in his setup

I've just ordered a 5k pot from Radio Shack, hopefully it is stereo. This should work well for an attenuator provided the DVD player can drive into such a low load. If I could find a 10k pot cheap that would be best, but I can't.

Thanks all,
RonS

 

[mozfet]

Passive Responce

Hi,

Is the circuit you have drawn accurate? if so it wont work, what you need to do is tie the wiper and one end of the track to ground the other end goes to the output side of the fixed resistor, as the action of the attenuator is different you should be connecting the track end that would normally be grounded, to the fixed resistor, when building a passive preamp I usually ground the chassis to Earth rather than using the signal grounds. Do you know the output impedance of the circuit that is driving the passive?

Mozfet

 

[mozfet]

OOPS!

OK so I've just woke up! the circuit as drawn will of course work, however I dont think its ideal leaving one end of the potentiometer floating, so please try connecting as my previous post.

Mozfet

 

[yeti]

Volume control

In that setup your input impedance varies from 10 to 110kohms, with 10k at zero volume and 110k at max.
Because the source impedance of interferences is usually high, it may help decreasing the input resistance by a 10 to 20k shunt resistor across the inputs.

regards

Arne

 

[transducer]

Mozfet, source impedence of d/a feeding attenuator is 75 ohms. I had also thought of tying the "loose" end of the pot to the wiper.

Arne, you are correct regarding the input impedence, that's what I've measured. So what you're saying is that I'm getting interference because of the high input impedence. Should the shunt resistor go across the resistive element in the pot or directly across the input jack from hot to ground? Would this not make the input as low as 5k with a 10k resistor? I'd be be a little concerned about the source being able to drive such a low impedence.

Thanks,
Ron

 

[yeti]

pot

What I meant was from input hot to ground, this will pull down the interferences.
Usually the output of your source device is in the range of a few ohms to aprox. 100 - 470 ohms, so the voltage loss will not be too much.

Arne

 

[transducer]

Success!

Arne,

Well, it only took me a month to get around to soldering in a couple of resistors, but I finally put an 18k (it's what I had) resistor across each input to ground tonight, and it worked The passive preamp is absolutely silent! Even at maximum volume, with my ear against the speaker, I don't hear anything. Thanks very much for the helpful tip.

Now my only problem is that this pre was built for my cousin, who is coming tomorrow to collect it. But it sounds more transparent than my DIY active pre based on the BB OPA627BP opamp And I'm using cheap metal film resistors and a Radio Shack volume control.

For anyone wanting to build a passive preamp, I highly recommend HH's design with Arne's mod of the input resistor across ground. Resistor values are not critical, I used a 100k pot, an 18k resistor across input, and somewhere around 20k for the series resistor.

Cheers,
Rons

 

[imkhawx]

hum problem on my passive pre.

Hi guys...

I am new to diy and i just built a passive pre and i am having the same trouble as transducer. I have read the posting and I don't understand what transducer and yeti means.

do i have to add in a resistor from the rica grounding tabs to the chassis ground?
I had a common copper wire from all the RCA grounding tabs of the inputs to the chassis.

so should I add a resistor across from each input grounding tab to ground or will one common resistor do?

thanks in advance.

imkhawx