Hi all,
I have just replaced an old scratchy 100K Alps RK27 pot with a 15K wire wound pot in my preamp. I had read about the possibility of using WW pots for volume controls on these boards but had been apprehensive about the potential inductive qualities. Still I thought I had little to lose by a little experiment so I dropped it in.
Amazing a real transformation in clarity across the whole frequency range, all fears of inductive roll off were obviously unfounded. The only obvious pain is the linear tapper which I can do little about. I put a 8K2 resistor on top of the pot to improve the adjustment range and that helped a lot.
I am currently dragging the parts together for a balanced input transformer coupled 5687 preamp with Output transformer. This is hopefully going to be a zero capacitor job and I expect it to be very transparent so I believe I will go ahead and use a wire wound pot for the volume control. The only issue I have is tracking down a quad gang wire wound pot - preferably multiturn.
Shoog
I have just replaced an old scratchy 100K Alps RK27 pot with a 15K wire wound pot in my preamp. I had read about the possibility of using WW pots for volume controls on these boards but had been apprehensive about the potential inductive qualities. Still I thought I had little to lose by a little experiment so I dropped it in.
Amazing a real transformation in clarity across the whole frequency range, all fears of inductive roll off were obviously unfounded. The only obvious pain is the linear tapper which I can do little about. I put a 8K2 resistor on top of the pot to improve the adjustment range and that helped a lot.
I am currently dragging the parts together for a balanced input transformer coupled 5687 preamp with Output transformer. This is hopefully going to be a zero capacitor job and I expect it to be very transparent so I believe I will go ahead and use a wire wound pot for the volume control. The only issue I have is tracking down a quad gang wire wound pot - preferably multiturn.
Shoog
Nice one Shoog.
I am a newb so I know little but I was going to ask if you could explain the above problem? Is it that you have to wind up the dial more to get volume or less?
The reason I chimed in here is because a mate has some crazy ceramic ww pots in his junk pile. A single gang 10inch which weighs about 10kg and a smaller 4inch dual gang which I thought (Newb Thoughts) could possibly be used in a passive pre amp?
The problem with a linear taper is that the adjustment range is to small at low volumes -but by placing a resistor before the pot of about equal value to the pot, the effective adjustment range is much improved - but at the cost of losing half of your signal. In my case this is not a problem since I never got the volume control past half way previously and so half of the available signal voltage was never utilized anyway.
As to your friends pots - the dual gang sounds interesting especially if it is a multiturn pot. Another board member reported good results from a 10turn pot in a passive preamp.
Shoog
As to your friends pots - the dual gang sounds interesting especially if it is a multiturn pot. Another board member reported good results from a 10turn pot in a passive preamp.
Shoog
In a balanced line amp, I've read in various places (including M. Jones) that you don't want to use a ganged pot for adjusting volume, because any imbalance between resistive elements will knock your balanced signal out of balance. Instead, it's recommended to use two series resistors and a single pot with the resistive element between the + and - signals. Like in this picture:
It's discussed in MJ's "Valve Amplifiers" Fig. 7.10, page 512-513.
Is there a reason not to use this setup?
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BTW, it seems wirewound resistors are more useful for audio than once thought. Now I'm dying to try a wirewound pot as a vol ctrl...
An externally hosted image should be here but it was not working when we last tested it.
It's discussed in MJ's "Valve Amplifiers" Fig. 7.10, page 512-513.
Is there a reason not to use this setup?
--
BTW, it seems wirewound resistors are more useful for audio than once thought. Now I'm dying to try a wirewound pot as a vol ctrl...
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The precision between gangs is truly amazing at about 0.05% so I am not concerned about introducing imbalance. I am basing my design on the Raven preamp and the pot loads the input transformer. In this case its important to have a predictable load on the secondary which your suggestion cant provide.
I scored this one;
http://www.ebay.ie/itm/320872019277?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Cheers
Shoog
I scored this one;
http://www.ebay.ie/itm/320872019277?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Cheers
Shoog
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