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Old 18th April 2005, 03:36 PM   #1
findog is offline findog  Australia
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Default Suitable dual gang pot for NIGC LM3875?

Hi guys,

Excuse my lack of electronics knowledge, this is all quite new!

I'm building the standard Premium LM3875 non inverting GC kit (Brian's PCBs+components). I am not using a preamp, so I need a dual gang pot to attenuate the input.

I [i]think[/] i need a dual gang 10k linear cermet, is this on target?
Also where can I source a good quality one, preferably from an Australian supplier but will order international if needed.

Any suggestions will be greatly appreciated!
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Old 18th April 2005, 03:52 PM   #2
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If you are looking for Australian supplier get this one: "Silver-Carbon" Pot ex Tawan - Photo - & ignore my ugly Pinky!
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Old 18th April 2005, 04:48 PM   #3
findog is offline findog  Australia
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Cheers heaps Peter. Tis a relief to see a AUS$7 pot literally a short drive from my place seems to be one of the best options.

I'm interested in the law faked config using a linear pot, and I will be running 91db 4ohm speakers on these. Would you recommend 10k or 50k, with the 22k shunt resistor? At this price I might by both and experiment anyway.

Also a description of exactly how to wire this correctly into the circuit would be greatly appreciated! I'm hoping to receive the kit in the mail any day now, can't wait to hear how it sounds.
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Old 18th April 2005, 05:24 PM   #4
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Quote:
Originally posted by findog
I'm interested in the law faked config using a linear pot, and I will be running 91db 4ohm speakers on these. Would you recommend 10k or 50k, with the 22k shunt resistor? At this price I might by both and experiment anyway.

Also a description of exactly how to wire this correctly into the circuit would be greatly appreciated! I'm hoping to receive the kit in the mail any day now, can't wait to hear how it sounds.
Both pot values would work fine, best is to experiment. You might have more control with 50k, but 10k value may sound cleaner. Send me an e-mail and I will send you connection diagram.
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Old 18th April 2005, 06:03 PM   #5
homer09 is offline homer09  Canada
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i think this thread may also be helpful to you:

*technical elec question* DC offset/input impedance relation

there is a spreadsheet in the last post to help you choose pot value.

i will have to go against peter on this one (im thinking he didnt realise it) but you can't effectively use a linear pot value smaller than the shunt resistor you wont be fake lawing at all, volume control will be linear.

although, a shunt value larger than pot works well if your pot is log taper, and im pretty sure thats what peter uses.

with a 22k shunt, i decided to use a 250K linear pot.
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Old 18th April 2005, 06:13 PM   #6
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I was discussing log pots only. I don't have any experience with law fake setups, and personally I'm not really into it as those require high pot values, which seem not much appealling.

Besides, from some posts we've seen here, linear pots can have much more tracking error (between decks) than well made log pots.
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Old 18th April 2005, 06:14 PM   #7
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Linear taped pot for volume regulation ? Who recomend here this nonsense ?
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Old 18th April 2005, 06:23 PM   #8
homer09 is offline homer09  Canada
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Thats what i was thinking peter, but findog mentioned he was looking for a fake-lawing solution, i was just giving him info for what he needed.

yes you are right, fake-lawing always requires larger pot values, but i think you could still maintain the advantage of lower pot values (lower-noise) by placing yet another resistor in parallel with pot (connected across the two ends, not to the wiper). this will keep your input impedence much more constant and low, eliminating the drawback of a large pot value.

a 250k linear pot + 22k fake law + 22k 'consistency' resistor, will provide a log attenuation, and an input impedence equal to a 20k pot at min volume and an input impedence equal to a 10k pot at max value.
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Old 18th April 2005, 06:25 PM   #9
homer09 is offline homer09  Canada
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Quote:
Originally posted by Upupa Epops
Linear taped pot for volume regulation ? Who recomend here this nonsense ?

why don't you ask for the motivations/reasons for the recommendation instead of calling it nonesense? now its clear why many are hostile to your comments on this forum.
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Old 18th April 2005, 06:43 PM   #10
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I think, that question was clear : pot for volume control. What do you mean by motivation ? Linear pot you can use only for " fine " input level tuning of two different amps, which are used commonly in active boxes. As volume regulator is this pot insufficient, which know every little child.
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