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50k pot on Mini3?

post #1 of 15
12/10/09 at 2:36am
Thread Starter 
I'm wondering if I can substitute a 50k pot for the 10k one specified in my Mini3 build so I can get away with lower cap values for my diyMod.

From what I read, a higher impedance pot can increase noise, but that an increase to just 50K is unlikely to do this. On the other hand, I've also read that the Mini3 doesn't respond well to tinkering.. so is this doable, or just a bad idea? If I were to swap the pots, I understand I would need to swap R2 to ~500K, but is that the only change? I'm only starting to get into DIY, so I'm afraid I don't understand everything in the amp's layout.
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post #2 of 15
12/10/09 at 3:10am
the noise isnt the number 1 issue, it's the input bias.

With 10k, you get smaller impedance changes seen by +IN and -IN. But with 50k, you see larger changes. AD8397 is a bipolar input OPAMP so the dc offset range gets higher if you use 50k pot.

Recalculation of values is necessary.
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post #3 of 15
12/10/09 at 3:16am
Are you sure you can find a 50K pot with switch to fit on the mini3 board???
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post #4 of 15
12/10/09 at 12:33pm
50K is available in a single gang (mono) unit. 20K is the highest you'll get in a stereo/dual gang (if you can find them in stock somewhere).

RK097/RK098 Series｜Products Line
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post #5 of 15
12/10/09 at 2:38pm
AMB said i could use the blue velvet from the M3/ckIII and be fine without altering other values. Worked out pretty good since mine wasnt portable. I did this because alot of the usb dacs dont like 10k inputs with their wee little output caps.
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post #6 of 15
12/10/09 at 2:40pm
[EDIT]

Quote:
Originally Posted by nightanole View Post
AMB said i could use the blue velvet from the M3/ckIII and be fine without altering other values. Worked out pretty good since mine wasnt portable. I did this because alot of the usb dacs dont like 10k inputs with their wee little output caps.
Really? I'm surprised. I recall reading that the 10k value was pretty critical. Must be thinking of something else.
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post #7 of 15
12/10/09 at 3:02pm
Strait from the source...

Quote:
Originally Posted by amb
No problem with using a 50K pot. Just make sure it's audio (log) taper.

-Ti

Quote:
Originally Posted by nightanole
Hello im working on my gamma 2 and also a mini 3. Im putting the mini3 in a large case with the gamma so size doesnt matter. I was wondering if i use 50k pots like the M3 in this case instead of the standard 10k pot? Or would the increase in resistance increase noise to the point its not worth the upgrade?
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post #8 of 15
12/10/09 at 11:14pm
just make sure you test the range of the volume pot and check the DC offset range to be safe.
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post #9 of 15
12/11/09 at 4:55am
I use Tangent's DC-offset calculator in the examples below. When going from a 10 k pot to a 50 k pot you should also increase R2 (in the mini3 schematic) to 470k.

When the "typical" values (offset voltage, offset current, bias current) from the data sheet are used the DC-offset goes up from 18 mV to 62 mV by replacing the 10 k pot with a 50 k pot.

If the max values are used the DC-offset goes up from 70 mV to 276 mV.

If you use low impedance phones such a large DC-offset might be harmful.
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post #10 of 15
12/11/09 at 12:28pm
Quote:
Originally Posted by nightanole View Post
AMB said i could use the blue velvet from the M3/ckIII and be fine without altering other values. Worked out pretty good since mine wasnt portable. I did this because alot of the usb dacs dont like 10k inputs with their wee little output caps.
how does the output caps affect things?
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post #11 of 15
12/11/09 at 12:33pm
At the most simple level...... the lower the input impedance of an amp, the higher the capacitance you need on the output of the source. Otherwise you start cutting off the bass.
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post #12 of 15
12/11/09 at 12:44pm
Quote:
Originally Posted by Beefy View Post
At the most simple level...... the lower the input impedance of an amp, the higher the capacitance you need on the output of the source. Otherwise you start cutting off the bass.
ah something to do with the cut off frequency of RC filter...
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post #13 of 15
12/11/09 at 12:46pm
Quote:
Originally Posted by chesterqw View Post
ah something to do with the cut off frequency of RC filter...
Yep, that's it
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post #14 of 15
12/11/09 at 12:46pm
thank you for the help
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post #15 of 15
12/12/09 at 2:03pm
Thread Starter 
Quote:
Originally Posted by diditmyself View Post
I use Tangent's DC-offset calculator in the examples below. When going from a 10 k pot to a 50 k pot you should also increase R2 (in the mini3 schematic) to 470k.
This is what I thought, but I'm not sure how the calculations come about for the DC offset.

Fred brings up a good point, though, I hadn't thought about the size. I think I'll skip the Mini3 altogether, since I have no use for it as a desktop amp, or as a portable as it is.

Thanks for the help, everyone!
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