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Old 16th November 2014, 04:35 AM   #1
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Default First time building CMOY - Need some help

Hi guys,

I been lurking around for sometime looking for something to build and i saw alot of people recommending for first time builder to build a CMOY.

So i went and brought this cmoy kit off ebay to build.
DIY Classic Cmoy HI FI Stereo Headphone Amplifier TI OP LM833 Dual Audio AMP | eBay

The problem i am having is that it is giving off some terrible distortion when ever i try to play anything thru it and i have no idea why.
Ive checked all my soldering and they all look fine.

I have read many posts and the official cmoy page and started troubleshooting.
cmoy says the output should be no higher than 20mv between it and the ground, but for some reason i get 0.3v.

The diagram below was taken from another project but i have edited it with what is on the board and what i can see.

any help on this would be appreciated thank you

EDIT
btw this is using the LM833 chip and not the OPA2132 chip as shown in the diagram
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File Type: png cmoy_altoids_schematic.png (73.9 KB, 279 views)

Last edited by pirogoth; 16th November 2014 at 05:59 AM. Reason: More infomation added
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Old 16th November 2014, 05:26 AM   #2
bimo is offline bimo  Indonesia
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Quote:
Originally Posted by pirogoth View Post
Hi guys,

I been lurking around for sometime looking for something to build and i saw alot of people recommending for first time builder to build a CMOY.

So i went and brought this cmoy kit off ebay to build.
DIY Classic Cmoy HI FI Stereo Headphone Amplifier TI OP LM833 Dual Audio AMP | eBay

The problem i am having is that it is giving off some terrible distortion when ever i try to play anything thru it and i have no idea why.
Ive checked all my soldering and they all look fine.

I have read many posts and the official cmoy page and started troubleshooting.
cmoy says the output should be no higher than 20mv between it and the ground, but for some reason i get 0.3v.

The diagram below was taken from another project but i have edited it with what is on the board and what i can see.

any help on this would be appreciated thank you
0,3V at output is to high for op-amp. May be some resistors is not connected / unsoldered. Please check all input inverting (2 and 6) and non inverting (3 and 5). It should be only several mV.
If you can made operating point right, than you can connect it to high impedance headphone (> 600 Ohm) to get reasonable distortion.
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Old 16th November 2014, 05:35 AM   #3
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Quote:
Originally Posted by bimo View Post
0,3V at output is to high for op-amp. May be some resistors is not connected / unsoldered. Please check all input inverting (2 and 6) and non inverting (3 and 5). It should be only several mV.
If you can made operating point right, than you can connect it to high impedance headphone (> 600 Ohm) to get reasonable distortion.
2 show 34mv. while 3 shows 28mv
3 show 33mv and 5 show 27mv

Ill take a another look at the solder and try reheating them to see if they sit better
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Old 16th November 2014, 06:33 AM   #4
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So i have now resolder and checked all the points but still no luck. still getting the same voltage reading.

Click the image to open in full size.
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Old 16th November 2014, 07:19 AM   #5
Mooly is offline Mooly  United Kingdom
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The LM833 is a device with high (relatively speaking) DC input bias currents, typical of many "bjt" devices.

To minimise DC offset, the bias currents that flows out of the two input pins need to be identical, and that is achieved by making sure each sees the same DC "impedance". The 100k and 2k are hopelessly mismatched, and what is happening is that the opamp is amplifying its own offset error. The OPA opamps are FET devices and have no such issue.

(I wouldn't like to say at this stage whether you have other problems or not such as instability, but that circuit is not suitable for an LM833 as it stands)
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Old 16th November 2014, 07:25 AM   #6
Mooly is offline Mooly  United Kingdom
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Lets put some numbers in. DC gain of your circuit is (20000+2000)/2000 which is 11. The LM833 has a worst case bias current of 1000na (nano). So 1000na flowing through 100k develops 0.1 volt at the input. Amplify that by 11 and you get a DC offset (worst case) of 1.1 volts DC.
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Old 16th November 2014, 08:38 AM   #7
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Originally Posted by pirogoth View Post
So i went and brought this cmoy kit off ebay to build.
DIY Classic Cmoy HI FI Stereo Headphone Amplifier TI OP LM833 Dual Audio AMP | eBay

btw this is using the LM833 chip and not the OPA2132 chip as shown in the diagram
I'm afrayed you've been Ebayed. OPA2132 costs around $15US in small quantity and even OPA2134 is around $5US. LM833 or its cousin, NE5532, cost a mere $1 or so. Mooly has given you the technical reason and I'm sure you get the picture now, why cheap = bummer when it simply means skimping on specified quality parts. As the kit specifies LM833, you have no redress, unfortunately.
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Old 16th November 2014, 08:49 AM   #8
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i kinda get what your saying but the whole impedance thing goes over my head sorry.

i was thinking of lowering the gain by swapping out the 20k resistor for a 10k so ill get a gain of 6.
hopefully that will help fix it if not would it be wise to change the chipset to an OPA2132 like what most people have?
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Old 16th November 2014, 09:01 AM   #9
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Originally Posted by Ian Finch View Post
I'm afrayed you've been Ebayed. OPA2132 costs around $15US in small quantity and even OPA2134 is around $5US. LM833 or its cousin, NE5532, cost a mere $1 or so. Mooly has given you the technical reason and I'm sure you get the picture now, why cheap = bummer when it simply means skimping on specified quality parts. As the kit specifies LM833, you have no redress, unfortunately.
haha yeah. well first time build these so its a learning curve.
so it would be better to get the OPA2134 to replace it then?
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Old 16th November 2014, 09:01 AM   #10
Mooly is offline Mooly  United Kingdom
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First thing... don't mess your board up... its far easier just to fit a FET device.

If you snip the 2k's and add a series 10 or 22uf cap to them then you AC couple the feed back and greatly improve the offset situation. If you do that then check the cap polarity by measuring the voltage across it even though its very small.
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