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Old 28th January 2004, 04:41 PM   #1
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Default Strange vol. pot behaviour ?!

Hi!


I'm in the final stages of putting together my DIY "boomblaster", consisting of 4 GC circuits (with MFB topology filters), 2 bass drivers, 2 coax mid-high-drivers, and a subwoofer equalizer (Thomesen).

Upon connecting the signal cables to the (cheap) 100k linear pot and testing (this being the second tests, inside the final enclosure, on workbench everything worked before), the sound was not as expected.

First of all, sound is OK (without volume regulation, that is) when I connect the cd player directly to the amps.

But when using the pot, sound becomes very quiet, and lacks any highs. Upon slowly turning up the volume, at about 80% max volume, the sound level "jumps" up to what it should be like, and the highs and everything else is there, like it should.

Is maybe the pot broken (too long soldering or something)? I exchanged it for another 100k pot (old, used, could also be broken), and it was the same.

I am going nuts over this! Any suggestions?

Thanks,

Arndt
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Old 28th January 2004, 04:50 PM   #2
SY is offline SY  United States
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Why are you using linear taper?
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Old 28th January 2004, 04:56 PM   #3
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Because that's what most people used in their GCs...

On the other hand, I do not have any resistors from input to ground (other than the connection through the pot, that is)

But I always thought that using a linear one instead of a log pot would not result in the sound "jumping" up at one special position of the pot?


Thanks so far. I will take a look if I have a log pot lying around (I doubt it).

Otherwise, would a 15k resistor to ground after the pot help?

Bye,

Arndt
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Old 28th January 2004, 05:09 PM   #4
Kermit is offline Kermit  Norway
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If the pot is wired corectly I can't see how it would make the sound jump, regardless off liner/log ore resistor or not after the pot.
Are you sure you have wired it correctly? loking at the pot from the front you have gnd(left)-out(middel)-in(right)? If you swaped in and out I supose somthing like this could happen.
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Old 28th January 2004, 05:10 PM   #5
mskeete is offline mskeete  United Kingdom
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Yes. A 15k - 25K resistor will help
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Old 28th January 2004, 05:12 PM   #6
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Hi!


If I'd swapped (mirrored) (I did that one before) cables, it should still function normal, but with anti-clowise rotation for making the volume louder.

It is connected the way you described it...

I haven't found time to do the reistor to ground, in two hours or so...


Bye,

Arndt
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Old 28th January 2004, 05:26 PM   #7
mskeete is offline mskeete  United Kingdom
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Do you have a buffer after the volume control ?
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Old 28th January 2004, 05:30 PM   #8
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Quote:
Originally posted by mskeete
Do you have a buffer after the volume control ?

Nope, directly connected to high power OP-Amps (OPA541 and OPA548), with MFB-circuits in order to implement high/low-pass filters, gain 22,...


... like I always did when implementing gainclones

But I never used a vol pot before (all my other DIY-amps are directly connected to DSP with digital volume control)...

Bye,

Arndt
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Old 28th January 2004, 05:38 PM   #9
mskeete is offline mskeete  United Kingdom
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I think you'll find that your filter response will vary depending on the volume control setting unless the resistor values in your filters are really high compared to the value of the pot
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Old 28th January 2004, 05:44 PM   #10
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What you are experiencing is high frequency oscillation due to the wide bandwidth of the IC.

Install the volume control external to the amplifier and enjoy the sound.
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