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Old 10th July 2014, 11:34 PM   #1
greg7 is offline greg7  United States
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Default LM1875 potentiometer value question

I'm building my first gainclone, the Chipamp.com LM1875 kit. What value potentiometer should I use -- 100K, 50K, etc. ??? I can't seem to find this information anywhere

Unrelated question: any reason why I can't use an EI transformer rather than a toroid? Or a pair for dual mono? My only concern is hum ...
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Old 10th July 2014, 11:52 PM   #2
esgigt is offline esgigt  Netherlands
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Depends on the preamp... I guess 20K to 50K will do. 100K might add too much noise.
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Old 11th July 2014, 12:42 AM   #3
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Quote:
Originally Posted by greg7 View Post

Unrelated question: any reason why I can't use an EI transformer rather than a toroid? Or a pair for dual mono? My only concern is hum ...
So long as you keep the transformers away from audio wiring an EI should be fine. Toroids are better at keeping the magnetic field in the core.
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Old 11th July 2014, 01:26 AM   #4
drahcir is offline drahcir  Philippines
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Quote:
Originally Posted by greg7 View Post
I'm building my first gainclone, the Chipamp.com LM1875 kit. What value potentiometer should I use -- 100K, 50K, etc. ??? I can't seem to find this information anywhere

Unrelated question: any reason why I can't use an EI transformer rather than a toroid? Or a pair for dual mono? My only concern is hum ...

my first gaincloe is lm1875 powered by 15-0-15 EI transformer using 20k log potentiometer and is dead silent no hum or hiss.
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Old 11th July 2014, 02:48 AM   #5
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Hi Greg,

I built mine from Peter's kit (which spare from my friend) 2 weeks ago. A great amp and now temporary using cheap 50k v.control, mic cable as interconnect, even small gauge power cables and works really fine with proper gain and volume level. Very quiet back ground. Volume to 2 o'clock and no hum and hiss even ear near to tweeter and speaker. (initially humming but earth the v.control to chassis and that all solve the problem). My EI tranny was custom made and just inches from the entire wiring.

Hook up simple as labtop- spdif converter - passive out dac - gainclone (passive pre) - speaker.
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Old 11th July 2014, 09:54 PM   #6
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The value of toroid transformers depends on who you talk to. If you're talking to somebody who swells a bit when they say, "I run a toroid," then the value is high. If you're talking to somebody like me, who knows there's a rectifying-filtering circuit downstream from the transformer, then the value is zero.

The potentiometer you're talking about is an input volume control for the LM1875 amp, yes? To take the place of R1 (1 meg) on the National schematic?

Short answer: Your sound source is going to be a player of some sort? You'd actually be fine with any of the values other posters have mentioned. I personally would vote for 20k because it approximately matches the 22k value of R2 on the National schematic. The two resistances in parallel give an effective input impedance of approximately 10k, which is fine. Input impedence varies with frequency, so there's no point to being nitpicky.

The output from your player connects to one of the outside terminals on the potentiometer, the other outside terminal connects to ground. The potentiometer's center terminal (the wiper) connects to the left-hand side of C2 on the National schematic.

That's all there is to it, except that as alfred wan said, running a ground wire to the body of the potentiometer is often needed. This is usually done not by soldering a wire to the potentiometer body (although that would be fine), but by squeezing a wire between the chassis and the potentiometer with the mounting nut. But if the pot body is not connected to the shaft, then of course you'll have to solder. Hint: use flux. Regardless of using flux core solder, use flux anyway.

Somewhat longer answer: If you've heard of "impedance matching," it's a concept that doesn't apply to chip amps, or little black chips in general. The rule is low impedance feeds high impedance, which luckily happens pretty much automatically.

All players have more or less low impedance outputs, and all amplifier chips have more or less high impedance inputs. You're all set, so what you have to do is avoid screwing the works. A potentiometer in the 20-100k range will keep you safe.

In a sidebar, the Natonal schematic shows a 1 meg value for R1. In audio terms this is very high impedance, and noise problems can multply. Obviously the engineers at National know a lot more than I do, but I can't figure out why they put in that 1 meg resistor.

Hope this might be of some help.
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Old 12th July 2014, 03:56 AM   #7
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Greg. Simple for u. 50 to 60k suity for passive and anything above use for active pre then ho la....
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