buzziness in passive preamp

[markwilliams]

Hello Grainger and everyone,

I will post here because I know Grainger has mentioned to me in the past he uses the cheapie Radio Shack $3 Alps volume pot, and so do I.  This is a 100K, stereo pot.

I'm working on a simple passive preamp, in a wooden enclosure.    I've built lots of them before and they sounds great other than I get this high end tizzy stuff that is dependent on volume control level.

I've tried at all kinds of grounding schemes - separating the input grounds from the output grounds, NOT grounding to the wooden case at all (well, other than the ground tab of the RCA's), attempting to ground the case of the volume pot itself.      And of course minding the placement of the input/output wiring.   

I've experimented with larger boxes, smaller boxes.  Nothing seems to rid me of this high end tizz.   

No, no, no.......not my amp (I've tried lots of them), or my interconnects (I've tried lots of those too).  Some interconnects have been very short, and some have been 1M.  If I use my D.I.Y. Nelson Pass B1 buffered preamp I don't get this high end tizz.

I'm sure folks will pounce on me about the wooden box, BUT, I just read yesterday about some British company using an all plastic box for their passives.  So, it must be possible to get a quiet passive and use a wooden box, right?    The before mentioned Pass preamp is built in a wooden box.  No metal at all.

Is there some ultimate grounding scheme that I might need to try?

Or, is this just the nature of this inexpensive Alps pot?

Ideas?

Thanks!
Mark

 

[Grainger49]

What is the setup, source, amp and speakers?

Tizz should have nothing to do with the volume control, and I wouldn't think it had anything to do with the wooden box (lack of shielding).  But your saying that it varies with the volume makes me think it is from the source.  Otherwise it would be constant.  

Have you tried measuring the AC on the chassis of the source and amp.  This measurement is made with them plugged in, turned on but not connected to anything else (speakers are ok in the case of the amp but no input).

Power transformers induce some voltage into metal a chassis.  It has no power capability but causes noise in systems.  The voltage "seeks" ground.  A well grounded chassis keeps the voltage from creeping through the interconnects to another component.

If you have two prong plugs try the measurement with the power cord inserted both ways.  I.E. rotate the power cord 180 degrees.

As for it being the nature of the Alps pots, I don't hear it on the ones I have used but then my high frequencies are going.

 

[Paul Joppa]

100K is a pretty high impedance for a passive; they are usually around 10K. The high impedance exposes you more to electric-field interference, which is reduced by shielding - so the possibility of it being related to the box remains. Careful use of shielded wires and grounding the pot shell can be nearly as good a a shielded box.

 

[markwilliams]

Hi Grainger and Paul,

Well, I should have mentioned the amp and CD player.  But I didn't see the point.  There is zero noise when I run them with my DIY Nelson Pass buffered preamp (uses a 25K pot and is in a totally wooden box incidentally).

That said, I am currently running a Bada CD player and a DIY Darling Amp.  As I said, no noise with the Pass buffered preamp, but some with my Quickie and more with the passives.

I need to find some flexible shielded wire, which so so far has been difficult to obtain.

No Grainger, I haven't measured anything yet. 

I should also say I have been playing with two different R.S. $4 Alps pots.

Mark

 

[markwilliams]

Guys,

I do have one.  Will give it a shot.

It's nothing real special either.  Just from Parts Express I think.

Mark

 

[markwilliams]

Well boys, they don't call Paul "Brainiac" for nothing!

The 10K volume pot I had works just great!  No more noise!

So, was this an impedance mis-match?

Thanks to both of you fellows!    I was about to go mad!
Mark

 

[Grainger49]

I remember the warnings about high value volume controls on the old board.  I'm guessing this is one of the cases where the higher value pot allows noise.

I have a 100k Creek OBH-10 (a remote volume control, passive, because I'm old and lazy) between my FP 2 and Paramours.  It has an ungrounded metal case.  There is no noise.

And once I moved the hum pots to the top plate of my Paramours so I could adjust them easily the system is deadly quiet.

 

[Paul Joppa]

... It has an ungrounded metal case.  There is no noise. ...

I won't go into detail - whole books have been written about it! - but grounding is not shielding, and shielding is not grounding. We use the terms in contracted form so much that sometimes we forget we are talking in shorthand.

 

[Grainger49]

Paul,

The OBH-10 is powered by a wall wart.  That is why I referred to it as ungrounded.  I was thinking of how Bottlehead safety grounds every chassis.  The incoming and outgoing shielded cables might have the RCA shell tied to the case.  In that case the metal case is grounded to the FP 2 and Paramours.

I'm aware that the metal case is a noise shield.

 

[Paul Joppa]

Haha! Yes I am confident Grainger knows what I'm saying - I sometimes say widely-known things just so it exists in a thread for anyone who doesn't know.