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Old 30th December 2009, 01:02 AM   #1
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Default PCB Mounted Trimpots/Potentiometers

I find PCB mounted (though-hole) trimpots very useful in almost every one of my amplifier designs.

My latest amplifier - a valve amp - used 10 Ohm and 20 Ohm trimpots to pass ~40mA. I wanted single-turn pots to make things easy, but DigiKey only had Bourns 0.25W multiturn pots. Even though the power dissipation is small (40mA), I would have liked 1W pots if possible.

Anyone have trimpot/potentiometers recommendations for hifi applications?
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Old 30th December 2009, 02:15 AM   #2
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Quote:
Originally Posted by Kashmire View Post
I find PCB mounted (though-hole) trimpots very useful in almost every one of my amplifier designs.

My latest amplifier - a valve amp - used 10 Ohm and 20 Ohm trimpots to pass ~40mA. I wanted single-turn pots to make things easy, but DigiKey only had Bourns 0.25W multiturn pots. Even though the power dissipation is small (40mA), I would have liked 1W pots if possible.

Anyone have trimpot/potentiometers recommendations for hifi applications?
Better to be safe in this case with higher wattage ?
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Old 30th December 2009, 04:10 AM   #3
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Quote:
Originally Posted by Kashmire View Post
Anyone have trimpot/potentiometers recommendations for hifi applications?
For 1 Watt single turn check out the 3345 series. Doesn't look like digikey has the 20 ohm size thou. I was going to try them (20k) in my PP EL34 amp. The stock pots are crap.

Jeff
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Old 30th December 2009, 02:07 PM   #4
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The stock pots are crap.
Ugh. That's why I started this thread. I checked out the Bournes web page, and there are only a few pots that fit requirements - but nobody stocks them. What do you solid state guys use for trimpots? I assume you use trimpots for DC balance and setting bias currents. My Pass Zen and other amps use the standard Bournes pots from Digikey.
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Old 30th December 2009, 02:24 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
is that the trimpot between the cathodes?
How about trying a cathode resistor about 110% of required value and add a trimpot across it that has about 10times this value? Now the trimpot passes ~10% of the cathode current and references the transformer to the trim position.

Does that still operate the same way?

If not, then split the cathode resistor and connect the junction to the wiper of the trimpot.

You can add a protection resistor to either end of the trimpot to limit current if one inadvertently moves the trimmer out of range.
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Old 30th December 2009, 04:33 PM   #6
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The trimpot balances between the two output tubes, so it has to be in-between.

I'm not asking just for my tube amp, I seem to use a lot of trimpots and are looking if there is a better alternative - either by rearranging the topology (as you were suggesting) or a different product.
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Old 30th December 2009, 06:31 PM   #7
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This works for me in my mosfet amp: I let the resistors you labeled "2" do the primary work (they're connected directly to the 100 ohm/220uF pair). I then parallel a high ohm value pot with the pair of "2" (straddle them).
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