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Old 10th April 2013, 04:33 PM   #1
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Join Date: Aug 2005
Location: Aveyron
Default Change the pot or what?

About 11 years ago a friend on my old forum offered to build a shunt attenuator for me using a Seiden switch and Welwyn RC55Y resistors.

Not long after the idea of 'shunting the pot' was put forward by the mod on that forum. various resistors were tried - Tants/Riken/Kiwames and cheap metal films from Maplins, I decided to try Vishay Z foils.

Everyone who tried this mod agreed that there was no comparison between a straight pot and shunting it - simply because whatever pot was used it got shunted out of the way.

It controlled the volume to a class A valve amp. I now use a Bada PH32 integrated hybrid amp which with only a couple of cap changes sounds way better than the class A valve amp.

Of course I shunted the 50K Alps Blue that came with the Bada using 47K Z foils. This amp supplies class A for the first few watts then class AB, I want the amp to use only class A as I intend to build very efficient OB speakers in the future.

My problem is that I have to wind up the volume due to the -5/6dB that happens shunting the pot and I felt that I was losing some dynamics.

So I removed the Z foils. I hav'nt listened to an amp using just a pot for about 11 years now and the sound is awful, I doubt if it would be much better using another pot regardless of price.

Question - the 'received wisdom' is to use a resistor about 90% of the pots value - why can I not use a 50K resistor and what effect if any will this have?

Or - if I use a lower value pot shunted of course will that give me that little bit fuller natural oomph that I miss?

My phono stage is a Talk Audio MC3 with easily adjustable load and gain settings (it's also a cracker). I don't want to use too much gain if I can help it. I also use an old 63KI CDP but in the future intend to digitize my music so it will have input from a DAC only.

I don't mind if I cannot use as much of the pot as normal. I like to listen to music which fills the room and I don't now and will not in the future have to mind about upsetting neighbours as we will have none really close.

If you have a powerful amp that can handle the 6dB drop you really should try shunting the pot with Z foils or Caddocks if you have experience of them - it really is a no brainer. The volume control has such an influence on the sound of your whole system one wonders why more manufacturers don't use this in the design and build of their amps. A couple of Z foils are around $12 a piece + a simple pot of course I'm sure that a Penny & Giles may well be even better but by how much and at what a price and size disadvantage.

I really welcome responses but only from those who have practical empirical experience.
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Old 21st April 2013, 04:19 PM   #2
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Location: West USA
I listen to the system at the same level all the time. Therefore, I just soldered in a voltage divider in place of the pot using resistors of choice. If you require more than one setting, use a good quality 2-6 position switch with your choice of stepping attenuator (series,shunt, etc). You may get even better results by getting rid of the pot completely and cheaper than a Penny & Giles!
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Old 28th April 2013, 09:26 AM   #3
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optimationman,
thanks for the reply, I could'nt reply earlier as I was in hospital - out and feeling fine now.

A good suggestion and I'm sure it works for you but is anathema to those who think they need a zillion positions. Because music is recorded at different levels I feel there is a need for at least 3 settings.

Shunting the pot means literally that - it shunted out of the way.

Thanks to a Canadian on another thread I now know about mil. spec pots produced in Ontario that (a) will fit in the same restricted space as an Alps Blue and have a wide range of specs. to choose from.

As the Alps and most available pots go from 10K to 50K they are a no-no. These mil. spec pots are available from Digikey but not with all the options I would like so I shall phone them this Monday to see if I can buy a 35K pot directly - vamos a ver.
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Old 3rd May 2013, 09:39 AM   #4
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optimationman,
the more I think about your reply, the more I like it - definately thinking out of the box.

Since no other replies are forthcoming I tried using a different value of shunt but of course it did'nt work because the pot is still 50K. So before I can put your idea into practice I need to establish exactly what resistor values float my boat.

So I shall buy a cheap Alps blue of 20-25K and experiment with resistor values from around 38K or less. When I am satisfied I will then purchase a quality 6 position switch (probably Seiden) and use 0.01% Vishay Z foils.

Last night I played a few diverse LPs - Jan Gaberak - Making Music/ Jackon Browne - Late For The Sky and Jacintha - Tribute to Ben Webster. The last is a direct cut from the analogue tape and was not only sonically superb but played much louder even than the ECM recording - so yes 3 maybe 4 resistor pairs, thanks again for the positive idea - it's what the forum should always be about.
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