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Old 4th April 2012, 07:04 PM   #4481
BFNY is offline BFNY  United States
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Quote:
Originally Posted by udailey View Post
The stepped way you are talking about I heartily agree with. Should work well but they still have to be moderately matched because when temp changes and they change resistance you want them to change in the same direction/rate. Also I wondered if people would put up with soldering 24 3 legged pots that cost about 84 cents each for each LDR. You might get away with 24 for each channel
Agree on still matching, to some level. Then only add pots as needed, to one side (L or R) as needed to get good balance.

I thought about a board, with mostly blank spaces for adjustment pots, which are in series with the normal shunt/series resistors. Also would have a jumper wire in parallel to the adjustment pot position to start. The jumper is cut when the pot is added.
Or use the alternate way suggested with parallel pots.

Probably excellent balance is only needed at louder volumes. Does anyone care about 2 dB mismatch for background/very low levels?
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Old 4th April 2012, 07:25 PM   #4482
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Originally Posted by AndrewT View Post
fit fixed resistors.
Then trim (by adding a parallel resistor) one or other of the pair to bring the LDRs to similar attenuation.

The trimming could be by using a variable resistor (1M0 or 2M2) and replace with a fixed once measured.

The big advantage of switched & fixed resistors, is the ability to choose the attenuation to suit the listening conditions.
You could have coarse steps from -1dB to -100dB or finer steps from -1dB to -30dB or again finer steps from -20dB to -50dB, or finally from -40dB to -80dB. Just get a ball park attenuation range that suit your equipment and range of listening levels. Then choose the fixed resistors to suit.

You could use a 3step attenuator:
background music / answering the doorbell.
listening attentively
party !
Good idea, that would work great.

I also thought about resistor selection based on LED/LDR measurements.

If you designed a program/spreadsheet, you could enter the LED/LDR measurements for resistance vs. current for series and shunt positions, and than have the program tell you what resistors to load for a perfect match on your stepped attenuator.

This way you'd know before you had to buy them, so you didn't need every possible value on hand.
You would need decent resolution on the measurements, at least 10-12 points perhaps, to get a good curve fit.

Last weekend I made such a 10 step attenuator to measure devices on hand, 3 at a time. It measures them from 5ma to 1ua, and works great. Figuring out the temp way to reliably hold the LED/LDRs was the hardest part. Those leads are pretty non-standard.
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Old 4th April 2012, 07:25 PM   #4483
udailey is offline udailey  United States
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The jumper wire is a good idea. Maybe jumper trace with no solder mask on it so you can just scratch it out.
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Old 4th April 2012, 07:57 PM   #4484
udailey is offline udailey  United States
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And try to get jumper traces all in a row at the bottom of the board or a side so that its easy to access them once components are populated.
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Old 4th April 2012, 08:11 PM   #4485
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My initial thoughts were indeed a 24 pole rotary switch with 48 (oh my god...) trimmers customized to the specific LDRs. However this seemed too much of a chore and would create a huge mess on the pcb. Now I am working on something different Saying no more... for now...
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Old 4th April 2012, 08:33 PM   #4486
udailey is offline udailey  United States
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Yeah its to much of a chore and to expensive. To do just stereo you would need nearly 100usd of trimmers.
PV36Y102C01B00 Murata Trimmer Resistors - Through Hole
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Old 4th April 2012, 08:39 PM   #4487
udailey is offline udailey  United States
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This is a shunt attenuator. LDRs in series, Dales in shunt. I get zero volume at min, customized curve, great channel matching, wonderful sound and there are zero contacts in the signal. Channel matching is best with matched resistors though, so still didnt get away from that part. THD is lower than LDR/LDR situation and channels are better matched. I can have infinite channels as well.
https://picasaweb.google.com/lh/phot...eat=directlink
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Old 4th April 2012, 08:43 PM   #4488
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By the way, the pot you included on the kit is not log right?
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Old 4th April 2012, 08:45 PM   #4489
udailey is offline udailey  United States
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If you want to talk kits we have to go to email, but its a log taper.
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Old 4th April 2012, 09:12 PM   #4490
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Hello Urriah,
I'm interested in your shunt attenuator. LDRs in series, can you give me more information?
Do you use a resistor replacer with a normal (only half) switched pot?
Thanks a lot.
Guglielmo
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