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batdan 12th December 2010 03:43 AM

Audiosector LM3875 - Volume Control / Preamp options
This was posted on the chip amps forum, however it was suggested I post here also.

Hi all,

I have built the LM3875 Dual Mono Premium Kit successfully and have used an ALPS 50k log pot on the input side, however the music sounds "restrained" on playback.

But, if I use the preamp section of my receiver (Yamaha RXV1900) and turn the pot all the way up on my LM3875's I get the sound I am wanting. My source is a Gigawork DAC.

So, what is the best way to move forward? Will changing the ALPS out for something else be OK, or do I need a preamp? Is my ALPS at 50K to high? How do I calculate the correct value? If the preamp route is the way to go, can anyone point me in the direction of a suitable kit?


Peter Daniel 12th December 2010 05:39 AM

Regarding schematic posted here: the 47k series resistors seem a bit high, I would rather not go over 10k. How far you turn the pot at normal listening?

batdan 12th December 2010 06:15 AM


Originally Posted by Peter Daniel (
How far you turn the pot at normal listening?

Hi Peter,

One quarter turn for normal listening.


batdan 14th December 2010 06:44 PM

So is it worth trying a 10K pot instead of my 50K? Can anyone point me to somewhere that shows how best to calculate what value to use, or is it a matter of trial and error?


Peter Daniel 15th December 2010 01:09 AM

If you are using fixed series resistors shunted by a pot, it doesn't matter what value is a pot, for a given required attenuation the pot will be always set to a same shunt resistance value, no matter if it's 50k or 10k.

However, it's still worthwhile to try smaller value series resistors, like 10k suggested previously. And it looks like you need a lot of attenuation, so if 10k series resistors work better, most likely you will need to go with 10k pot indeed, to get more control over attenuation range.

batdan 2nd January 2011 05:59 AM


I bought a 10K "21 Step Attenuator Potentiometer" from Ebay seller Gigawork for $10 and replaced the 50K ALPS. The sound now is perfect, exactly what I am after. Thanks for the great kits.


charlesbahr 20th April 2011 02:54 PM

iPhone/headphone or line-out and potentiometer

I have two questions about what potentiometer to add to my Lm3875 kit. I was originally planning to take the headphone-out of my iPhone, send it directly into the LM3875, and put the output into my old Stax electrostatic headphones. I was going to use the volume slider on the iPhone to control the sound level.

But then I started thinking about a pot for a more flexible design of the chipamp... and then as I was trying to find out about the output voltage and impedance of the iPhone jack, I thought - maybe I should be using the line-out?

I think I should be able to take a headphone-out into the chipamp, but I would like advice from the experts. And then - what is an acceptable pot to get. I've looked at the recommendations -- ALPS RK27? 20K ohms? The resistance just limits the "off" level in the way it's wired in the diagram above, right?

Peter Daniel 24th April 2011 05:00 PM

I would suspect that line out from the iPhone should be superior to headphone output.

20K or similar value pot should be fine.

iPod and iPhone Line Out Cable with USB

charlesbahr 25th April 2011 11:50 AM

Many thanks! I may get that adapter you referred to.

syklab 5th June 2011 10:21 AM

Hi Peter,
I have a Elma 24 step ladder type 100K volume contrl from my old tube pre-amp, will that be suitable for use on your LM3875 GC kit?


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