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Potentiometer - Volume Pot

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Posted on January 4, 2017 at 12:09:40
sony6060
Audiophile

Posts: 613
Location: SE MI
Joined: August 8, 2014
I am building a stereo amplifier. Instead of using a balance pot, I am using a dual concentric audio taper potentiometer having independent volume control for each channel.

Only pot I could find is dual 250K, but circuit calls for 100K. Is it ok to shunt the pots with a 180K carbon comp resistor for approximately 100K? I understand carbon film resistors can distract from good sonics, so I propose using carbon comp resistors.

 

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RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 12:30:55
gusser
Audiophile

Posts: 2322
Location: So. California
Joined: September 6, 2006
You can but consider the line impedance will actually be much lower from your source component. In fact a commodity CD player may be as low as 100 ohms.

Furthermore you will not have any measurable (read audible) differences between carbon and metal film in this application.

I would go ahead an put the 100K resistors in anyway.

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 15:55:07
Eli Duttman
Audiophile

Posts: 9316
Location: Monroe Township, NJ
Joined: March 31, 2000
Whenever current flows through carbon comp. resistors, the issue of noise arises. Carbon film parts don't have the noise issue and have good sonics. You've been paying attention to malarkey or misinterpreting that which was stated.

Placing a resistor in parallel with the control will adversely affect the requisite logarithmic taper. People have combined resistors with linear taper controls to fake a logarithmic taper.

Please post the schematic for your project. The 250 Kohm control MAY be suitable.

Eli D.

 

TKD makes stereo 100K volume pots., posted on January 4, 2017 at 15:58:13
Cougar
Audiophile

Posts: 2390
Joined: June 25, 2001
They are not cheap but are very nice. These are probably the best volume pots next to stepped attenuators. I use them in my projects.

 

RE: TKD makes stereo 100K volume pots., posted on January 4, 2017 at 16:01:36
sony6060
Audiophile

Posts: 613
Location: SE MI
Joined: August 8, 2014
I visited the TKD site. No dual concentric pots.

 

Go to Percy Audio..., posted on January 4, 2017 at 16:05:38
Cougar
Audiophile

Posts: 2390
Joined: June 25, 2001
That's where I bought mine from. Hopefully he still has them. Also Partsconnexion sells them too.

 

You are now entering guitar part world, posted on January 4, 2017 at 16:22:52
Chip647
Audiophile

Posts: 1608
Location: The South
Joined: December 24, 2012
That old-timey vibe

 

RE: You are now entering guitar part world, posted on January 4, 2017 at 16:59:28
sony6060
Audiophile

Posts: 613
Location: SE MI
Joined: August 8, 2014
No dual concentric pots at Percy Audio or Parts Connextion.

Thanks Eli, will shunt with carbon film.

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 17:59:51
gusser
Audiophile

Posts: 2322
Location: So. California
Joined: September 6, 2006
But if his source impedance is low - like even as high as 10K ohms, putting a 100K across a 250K pot is irrelevant.

If he'e coming out of a plate coupled tube preamp, then there might be some issues

Edit: I am of course assuming the volume pot is directly across the input jack. If it's between stages within the amp's design, then disregard my
prior statements. In that case we do need to see the schematic.

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 18:27:27
sony6060
Audiophile

Posts: 613
Location: SE MI
Joined: August 8, 2014
Pot is directly across the input jack.

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 18:35:03
Eli Duttman
Audiophile

Posts: 9316
Location: Monroe Township, NJ
Joined: March 31, 2000
What tube is in the 1st stage? If it's a pentode, like the EF86 or 6SJ7, 250 Kohms is perfectly fine. OTOH, a 12AX7 section will be in trouble. Consider the low pass filter formed by the 1st tube's CMiller and the I/P resistance. Good phase shift performance considerations suggest that the LPF "corner" above 100 KHz.

Eli D.

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 18:50:28
gusser
Audiophile

Posts: 2322
Location: So. California
Joined: September 6, 2006
But again if he's looking back into a CD player with a output impedance of 100ohms via an OPAMP, the 250K input pot with a 100K shunt is irrelevant.

How much shunt capacitance do you need to attenuate 20khz by even a tenth of a db at 100 ohms?

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 18:52:10
sony6060
Audiophile

Posts: 613
Location: SE MI
Joined: August 8, 2014
Exactly Eli. I am wanting 100K pot feeding a single section of a 6CG7. BTW- I have a 50K pot. Is 50K better for a 6CG7?

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 19:14:35
Tre'
Industry Professional

Posts: 11850
Location: So. Cal.
Joined: February 9, 2002
It's not the output impedance of what's feeding the pot that matters, it's the output impedance of the pot and that will be determined by the position of the pot.

At the -6db point (equal resistance either way from the wiper) a 250k ohm pot will have an output impedance of 62.5k ohms and that assumes the source impedance feeding the pot is zero ohms. (a 100 ohm source feeding the pot will increase that number but only by a tiny fraction)


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 19:17:35
Tre'
Industry Professional

Posts: 11850
Location: So. Cal.
Joined: February 9, 2002
As long as the source feeding the pot is happy with 50k.

The lower you can go (without causing problems for your source) the happier the 6cg7 will be. (lower impedance feeding the Miller capacitance of the 6cg7 = "better" highs)

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 19:19:52
gusser
Audiophile

Posts: 2322
Location: So. California
Joined: September 6, 2006
That's correct. My point is that with an OPAMP drive on the source, the input impedance of the tube power amp first stage is not that important.

Now if he has a Dynaco PAS-3, then we might have a problem!

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 19:21:32
sony6060
Audiophile

Posts: 613
Location: SE MI
Joined: August 8, 2014
An op-amp would destroy the sound of the amp.

 

RE: Potentiometer - Volume Pot, posted on January 4, 2017 at 19:25:18
Tre'
Industry Professional

Posts: 11850
Location: So. Cal.
Joined: February 9, 2002
Now I get what you're saying.

You're saying he could use a 10k pot and the source would still be happy and the Miller of the first tube in the amp would be more than satisfied.

Sorry I mis-read you.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Source component?, posted on January 4, 2017 at 19:28:58
gusser
Audiophile

Posts: 2322
Location: So. California
Joined: September 6, 2006
What is your source? it's it's a typical CD player, you most likely have an OPAMP driving your new amp like it or not!

Now if you are using a turntable with a tube preamp, different story altogether.

And seriously, modern OPAMPs designed for audio are practically transparent with noise floors below any source material you can find.

Tubes are fun but let's not discount our vast technology advances since then. But then there are plenty of people here who like to implement tube I/V stages in their CD players and outboard DACs. So if you really hate OPAMPs, you can still have tubes with your CDs!

 

RE: Potentiometer - Volume Pot - Slagle Autoformer, posted on January 4, 2017 at 21:44:27
Iain42
Audiophile

Posts: 774
Location: Arcansaw
Joined: February 10, 2004
I use Slagle Autoformers and will be contacting him soon to get a few more for an amplifier project in the works.








High sensitivity, wide dynamic range, low distortion, and smooth frequency response. Pwk

http://www.itishifi.com

 

RE: Potentiometer - Volume Pot, posted on January 5, 2017 at 05:24:32
Laudanum
Audiophile

Posts: 1017
Joined: March 1, 2003
Are you on a budget?

This probably wont be popular here but ... The Alps conductive plastic pots are readily available. You may have to shorten the shaft and they have pins for thru hole mounting but you can buy or build a board to make wiring easier, or just wire direct. They are only 15 bucks (or 22 bucks at Angela).

http://www.mouser.com/ProductDetail/ALPS/RK27112A00AK/?qs=sGAEpiMZZMtC25l1F4XBUyOcKw5CnqLBt7NWCgOxelA%3d

http://www.angela.com/alpspotentiometerbluevelvet100kstereo.aspx

There is also PEC for about 30 bucks. http://www.digikey.com/product-detail/en/precision-electronics-corporation/KKA1041S28/KKA1041S28-ND/770961

And the TKD for 100 bucks. http://www.partsconnexion.com/tkd_64187.html

Or maybe a Goldpoint stepped attenuator. Already built or build it yourself with one of their switches. http://www.goldpt.com/index.html

 

RE: Potentiometer - Volume Pot, posted on January 5, 2017 at 07:08:24
krankkall
Audiophile

Posts: 236
Location: New Mexico
Joined: April 5, 2014
Keep searching flea-bay, the dual concentric 100K audio log pots DO show up on occasion.
I have one in an old DIY 6AQ5A SE integrated amplifier.

Steve

 

You should ask yourself if you really need "stereo.", posted on January 5, 2017 at 08:19:25
Chip647
Audiophile

Posts: 1608
Location: The South
Joined: December 24, 2012
Balance controls fix problems that are better fixed elsewhere. If you have a listening setup where one speaker close to you and another far away and want to balance the sound, give mono a try. You can build something much better for the same money if you are not going for a "sweet spot" imaging. One amp channel, one speaker. If you have not tried real, well done mono, you will be surprised.

 

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