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Old 25th November 2007, 11:01 PM   #1
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Default Potentiometer Puzzle A20k, B50k, T-amp?

Hi, I've a Sonic T-amp that's been modded simply and reboxed.

Well, it sounds great, and its powerful, but I'm stumped with the pot.

I have an A50k (audio taper) currently. This replacement is better sounding, but exibits the same weird problem as the factory pot. Here's the problem:

The amp doesn't produce useful power util the last 1/3rd of the volume pot's travel. In the first 2/3rds of the pot, its not outputting much beyond a headphone amp.

The Audio taper effect seems too severe on the several I've tried.

The very last bit of travel "almost all the way up" contains the only "sweet" selection, wheras "all the way up" isn't good, even when the input gain is low enough to avoid clipping.

Also, I was thinking about the mostly inneffective volume control, and that A50k probably isn't right for the little T-amp.
The one little sweet spot deal isn't good.

Do I need an A20k (audio 20k) or a B50k (linear 50k)?
IE: Eliminate the sweet spot ordeal with the linear taper, or give a more effective ground at "all the way up" with the A20k (possibly increase/decrease audio power output)?

There's my guesses, but I just don't know what I'm doing. Help?
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Old 26th November 2007, 01:32 PM   #2
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50k Log is the common choice I think..
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Old 26th November 2007, 06:34 PM   #3
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Sounds to me like you've got the pot wired wrong.

Double check that, and post a photo or diagram of what you did, if you don't cure it.

50K A is indeed the common choice. 20K will also work just fine. I've even used 50K lin with no problems.
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Old 26th November 2007, 07:51 PM   #4
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Default Its like this one

The pot is wired just like this one
http://www.michael.mardis.com/sonic/...nput-sonic.jpg
however, I have the replacement caps in their original locations.

The thing is, its like this as I turn the knob: quiet, quiet, quiet, quiet, quiet, quiet, quiet, quiet, loud, too much.

So, maybe I need the linear?

I did happen to notice that the factory pot had the ground lead wired only to one connection on the pot, not both; but, I wired them to both, just like in the drawing. Does this make a difference?

Thanks for helping me out when I'm way over my head!

P.S. Have you seen these?:
$10 T-amp, Ebay# 230195633403 (Sure)
$25 Stepped pot, Ebay# 290186015168 (Dale)
I don't have either of those, but I'm curious.
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Old 27th November 2007, 04:46 AM   #5
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I still think you have the pot wired wrong, or a bad connection.

Can you post a photo of your work?
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Old 27th November 2007, 06:40 AM   #6
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Quote:
Originally posted by panomaniac
I still think you have the pot wired wrong, or a bad connection.

Can you post a photo of your work?
No, I'm sorry that I don't have the wherewithall to make that photo.
Anyway, it has the outer two wires of the ribbon cable cut off, because the 1/8" input jack is not in use.
It has the two remaining "hot" wires going to the center of the pot.

Following is "as facing the knob, with the pins straight up". . .

The center connection of the ribbon cable, and all audio grounds are connected to both of the right side pins.
(However, the factory pot had the ground connected only to 1 of the grounds on the pot, not both as illustrated in usual instructions for connecting a pot.)

The audio inputs are connected to the left side pins.

The ground cable, after the pot, is very long. It is wrapped around the cat5 pair that goes to the center of an on/on dpdt switch.
The ground doesn't connect there, but continues to (and braided with), now 2 pair of cat5 pair onto 2 pair of RCA jacks, where the ground connects to all 4 RCA jacks. The 2 pair of cat5 leads connect to the center of the RCA jacks.
This is input 1 and input 2 (music or TV).

There is no hum, hiss, or noise at all.

The amp sounds very nice, but its volume control is mostly innefective, until suddenly. . . there's the sweet spot (almost all the way up). . . then anything past that is compressed.

Dynamics are so sweet! However, orchestra hits and any dance beat always comes in at concert levels. . . or very quiet.

Maybe I need to forget about audio taper pots with small amps?

Thanks again!
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Old 27th November 2007, 08:33 AM   #7
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Check your connections against attatched picture..
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File Type: jpg sonic t amp basic mod.jpg (98.5 KB, 174 views)
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Old 27th November 2007, 11:08 AM   #8
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Default B100k?

The new a50k pot is wired just like that photo.

The factory pot also exibited the same deal. . . just one sweet spot; however the factory pot was not wired like in the photo, and the amp wasn't as "compressed".

Mine is the newer version that has C3 and C4 on top.

The factory pot used only one of the ground connections on the pot, while one was not connected to anything. I think that the amp had larger dynamics that way.

I think it wants "more hot, less ground" so that the volume increases earlier in the pot's travel and that the dynamics return.
Would that actually be a B100K pot?
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