| edpgc |
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....ges/ampRev3.gif
Cheers
Ed |
|
|
| bigparsnip |
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. |
|
|
| edpgc |
| Ah cheers for the tip, I didn't realise it worked like that! I'd better work out a volume control. |
|
|
| bigparsnip |
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. |
|
|
| edpgc |
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 |
|
|
| tlmadsen |
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 |
|
|
| edpgc |
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 |
|
|
| tlmadsen |
"isolate" your volume control and then take the signal to the power amp and the headphone amp from there.
Thomas |
|
|
| Nisbeth |
Thomas has a point about the gain in the headphone section. Which headphones do you have?
/U. |
|
|
| SpaBlauw |
| 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. |
|
|
| edpgc |
Nisbeth: Some pretty unexciting Senn HD495's at the moment
SpaBlauw: I thought the opa627/buf634 was the ideal solution for driving headphones? So you are saying I can run the headphones directly from the outputs of the LM3875's, with some resistors?
hmmm
Well I'm designing a PCB at the moment, I think I've gone a bit nuts with the number of caps!! |
|
|
| edpgc |
What are other peoples opinions on this, how best is it for me to configure my opamp/buffer combo and poweramp, and where is best for driving the headphones from? I've been trying to work this out, I'm a little concerned about running headphones directly from the LM3875s, I'm presumably quite likely to blow them up if Im not careful!
Cheers for all the input guys.
Ed |
|
|
| paulb |
| quote: | Originally posted by SpaBlauw
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. |
I disagree. The OP/BUF combo will be much better to drive the headphones from. Unless you have 8-ohm headphones, which you don't.
LM3875 is acceptable to drive headphones but noisier than the OP/BUF. Did you get the OP/BUF concept from Headwize? |
|
|
| edpgc |
Umm somewhere on these forums, there is a link to a very nice looking smd headphone amp, its based on that, I'll dig up the link if you want.
Ed |
|
|
| edpgc |
Hmm thats a cool looking headphone amp.
Another question, I was planning to use SMT OPA and BUF, however I'm worried a bit about heat now, should I go with the TO220 so it can dissipate more heat?
Ed |
|
|
| paulb |
| Not sure if you need to, but I would. Although SMD parts can dissipate a surprising amount of power. |
|
|
| SpaBlauw |
| quote: | Originally posted by paulb
I disagree. The OP/BUF combo will be much better to drive the headphones from. Unless you have 8-ohm headphones, which you don't.
LM3875 is acceptable to drive headphones but noisier than the OP/BUF. Did you get the OP/BUF concept from Headwize? |
you're right, but the opa/buf circuit lacks off gain in his schematic. The resistor in serie with the +output takes care of the ohm difference.
If I were him I build 2 opa/buf circuits, one for the phones with gain of 4-5 and the 1 gain circuit for driving the LM1875. |
|
|
| jwb |
| I like the concept. You might consider using the PCM2902 USB DAC instead. Neither the 2700 nor 2900 series are very good DACs, but the 2902 has a S/PDIF output, so you could use the 2902 with, e.g., the CS8414 receiver and CS43122 DAC to create a significantly better source. You could bus-power the 2902 and not need a 3.3V rail. |
|
|
| edpgc |
JWM: Thats an interesting idea, but a little complex for me at the moment! Might be worth it if I was getting a 24bit source from the computer, but the PCM2902 is only 16/48
SpaBlauw: I could do that, but do I really need to? Can easily switch the gain when the headphones are connected and at the same time power off the LM3875's, so you reckon a gain of 4 for the headphones?
When I go into headphone mode, is changing the gain enough, or will I need to also chane the attenuation on the inputs to give me sensible levels?
Cheers
Ed |
|
|
|
