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Old 13th April 2004, 02:15 PM   #1
edpgc is offline edpgc  United Kingdom
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Default LM3875 Amp/ Headphone Amp and USB DAC

Hi All

My first post, and my first real audio project! Right I've probably choosen something a little complex to start off with, so please be gentle!

I want to make a USB box which acts as a headphone amp and a power-amp also, I've based the design around the DAC PCM2702, and the OPA627/BUF634 combo. With LM3875's driving the speakers.

Perhaps some of you guys can have a look at my schematic and give me some feedback, I haven't really worked out a lot of component values yet, but as a proof of concept, perhaps let me know if I am on the right track!

I'm a little confuesd over how to configure the opa627/buf634s for headphone driving vs driving the LM's

Here is the schematic
http://www.edpgc.pwp.blueyonder.co.u...es/ampRev3.gif

Cheers
Ed
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Old 13th April 2004, 02:41 PM   #2
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Just one point from having a quick look, is that you don't seem to have any sort of comprehensive volume control included in the circuit (apart from the two position level switch). If you are intending to use the volume contol on your PC (the software one) I would advise against this, as all it does is chop of some of the information in the signal, so that the output is only using some of the output range of your DAC (and hence giving less dynamic range and signal quality).

So, I would sujest putting in a 'propper' analouge volume control before your headphone stage so that you can leave the digital volume control at mak all of the time to acheive the best sound quality you can get from the source.
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Old 13th April 2004, 02:52 PM   #3
edpgc is offline edpgc  United Kingdom
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Ah cheers for the tip, I didn't realise it worked like that! I'd better work out a volume control.
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Old 13th April 2004, 02:59 PM   #4
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Hi, you should be bale to do this quite easily by replacing the three resistrs you have on each secton of the level switch you have there for a potentiometer (a dual one would be best so both are simply tracked together as normal).

Also, one more thing I just noticed is that at the minute you have the gain on your LM3875 set to about 17 which seems quite low to me, and I thik it would be more normal to find a gain in the 25-30 range (but I'm not sure of the output level from your DAC chip so this could be fine).

Other than that, the analouge section looks good, but I can't realy coment on the DAC bit as I haven't tried anything like this myself (yet) and this should turn out to be a very intresting project.
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Old 13th April 2004, 03:07 PM   #5
edpgc is offline edpgc  United Kingdom
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Yeah I need to think a bit more about those component values, I'll prob start designing the board once, I get the whole concept sorted, swapping resistor values is no big deal once I've got something to play with.
Ed
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Old 13th April 2004, 09:17 PM   #6
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Having a quick look at your schematic a couple of things "sticks out".

*I think you need more gain in your headphone section. (you don' t have values on al components, so this may be OK)

* You should alos put a small resistor at the output of your BUF634.

*Why does you put your power amp after the headphone sektion ??

I have a headphone amplifier build around the OPA627/BUF634T. It is VERY good so you should have something to look forward to.

I thing the gain of 17 for your power amp is OK. 25-30 seems to high for me.......................

bigparsnip is right about the analog volume control

Have fun

THomas
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Old 13th April 2004, 09:22 PM   #7
edpgc is offline edpgc  United Kingdom
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I got the impression the LM3875 performs better being driven from the opa627/buf634 combo, ie more punchiness. I'm not really sure though, how would you suggest configuration?

I'm working on the next revision with control for switching off the power amp when headphones are connected, and stuff like that, so thanks for your other tips.

Ed
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Old 13th April 2004, 09:34 PM   #8
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"isolate" your volume control and then take the signal to the power amp and the headphone amp from there.

Thomas
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Old 13th April 2004, 10:23 PM   #9
Nisbeth is offline Nisbeth  Denmark
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Thomas has a point about the gain in the headphone section. Which headphones do you have?


/U.
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Old 13th April 2004, 10:37 PM   #10
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the unity gain buffer is good for driving your LM1875 but i wouldn't take the signal for your phones from the buffer, use the LM1875 output instead with a resistor in serie with it.
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