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transducer 15th October 2002 03:09 AM

Passive Preamp Hum
 
2 Attachment(s)
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 :D but is probably no longer available :bawling: it's from my stash of junk parts.

Thanks,
Ron

pinkmouse 15th October 2002 02:23 PM

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 15th October 2002 03:10 PM

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 15th October 2002 03:28 PM

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 15th October 2002 04:05 PM

Sounds like a ground loop issue.

transducer 15th October 2002 04:34 PM

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 :xeye:

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 16th October 2002 08:07 AM

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 16th October 2002 08:26 AM

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 16th October 2002 08:49 AM

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 16th October 2002 12:05 PM

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


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