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Old 25th February 2010, 10:32 AM   #21
Beftus is offline Beftus  Netherlands
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Using a resistor divider is a better method. The use of a dedicated rail splitter (TLE2426) is the best way to prevent differences in rail voltages.

There's a lot of information (with schematics) on this subject here:
Virtual Ground Circuits
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Old 25th February 2010, 10:38 AM   #22
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Thanks for the explanation, although I'm a bit confused by it! Here's another photo (with the LED lead pulled out of the way, so you can see what's happening with the leads to the capacitors):

Click the image to open in full size.

You're telling me to connect a resistor in parallel across the capacitors, and to alter the connections to the switch, presumably so it looks something more like the supply to this standard CMOY circuit?

Click the image to open in full size.
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Old 25th February 2010, 11:33 AM   #23
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Quote:
Originally Posted by Beftus View Post
Using a resistor divider is a better method. The use of a dedicated rail splitter (TLE2426) is the best way to prevent differences in rail voltages.

There's a lot of information (with schematics) on this subject here:
Virtual Ground Circuits
Thanks for the link, Beftus, and from what I've read there the sound will be distorted long before the voltage drops sufficiently to cause a DC offset problem. That said, I do fall asleep sometimes using my headphones...
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Old 25th February 2010, 11:35 AM   #24
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Yep - the way I described the switch is different, but if you work through my post with those diagrams for reference, you'll figure it out.

I was suggesting that the least-effort method was to switch the connection between the batteries, whereas that version switches between the battery and the board - which is the way I'd do it if I was building from scratch.
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Old 25th February 2010, 11:38 AM   #25
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Incidentally, that board design is utterly horrible - take a quick look over Tangent's tutorial, you could do a better job on protoboard.

How to Build the CMoy Pocket Amplifier
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Old 25th February 2010, 06:50 PM   #26
Beftus is offline Beftus  Netherlands
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BTW, the use of 0.1F input caps in a bog standard CMoy is not recommended by Tangent. A 1.0F input cap is a better choice. Read this: Input Capacitors for Headphone Amps
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Old 25th February 2010, 08:14 PM   #27
Zigis is offline Zigis  Latvia
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I see TheSeekerr good describe practical way to fix this amp.

Johnny Blue, here is my step by step tutorial:
1. Remove two wires from switch to board completely
2. Desolder from switch two wires, coming from both battery, connect both wires together and isolate
3. Desolder from board one wire, coming from battery + terminal (red)
4. Solder in this place spare red wire
5. Solder switch between this wire and desoldered battery + wire
6. Solder two 4.7K - 10k resistors paralell to el caps. You can use any resistance in this range, important is - both must be matched at least 1%. This determine both voltages equal and low DC offset.

Now measure voltages between ground and +, ground and -, both must be equal.
Now measure DC offset in both channels, if it is good, you are ready to connect phones.

However this board is very strange, both el. caps are in surprising right place, close to chip, near the chip's power pins (almost like in by boards). This increase sound quality, however not many CMoys builder use this technique, common in Gainclone.
To utilize this better, solder caps deeper in board, with as short as possible legs.
Maybe, if your enclosure allow it, you can solder both rail splitter resistor on boards other side, right to caps pads.
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Old 25th February 2010, 08:34 PM   #28
Zigis is offline Zigis  Latvia
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Beftus,
there is two ways to increase low frequency, if input caps are used:
1. Increase cap from 0.1 to 1.0-2.0 mf
1. increase input to ground resistors from 100K to 1M

I use 1Mohm input resistor and 0.22mf, good quality polypropylene with great success.

B.T.W. If someone is interested, I can offer CMoy PCBs and kits with high quality parts. Boards size is 4 x 5 cm, designed for Hammond 1593PBK enclosure.
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Old 25th February 2010, 08:55 PM   #29
Beftus is offline Beftus  Netherlands
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Quote:
Originally Posted by Zigis View Post
increase input to ground resistors from 100K to 1M
This would also necessitate the use of a 100K volume pot instead of a 10K pot.
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Old 25th February 2010, 09:03 PM   #30
Zigis is offline Zigis  Latvia
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Quote:
Originally Posted by Beftus View Post
This would also necessitate the use of a 100K volume pot instead of a 10K pot.
I use lile different schematic - 10K pot right after input jack, then 0.22uF cap, then chips + input pin with 1M to ground.
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