OPA1632 schematic for bridging LM3875

[Mrpong]

Hi, I drafted out a general schematic for OPA1632 based on its datasheet. I will use it for two LM3875 in bridged mode. My concern is that I'm not sure whether this schematic is proper in paticular for bridging purpose. According to the datasheet, C8 and C9 are Tantalum type. C3 and C4 are Poly or ceramic type. C6 and C7 are poly type. Is it OK? Is there anything I should adjust or add in?

[juma]

Hi Mrpong,
your schematic is pretty much OK, but you should leave out C6 and C7 (they create low pass filter and the OPA1632 is very stable - you really don't need them). Also, you should add a buffer in front of the OPA1632 (1K is a hard load for the previous stage).
It is very important that R1, R2, R3 and R4 are very precisely matched. The same apllies to gain setting resistors for LM3875. Equal branch gain in bridged amplifier is of essence.
If you can't find 0.1% resistors, buy a bunch of 1% and match them by hand.

Here is the circuit I successfully used to bridge two LM3886s:
more on: http://jrexton.ecv.ms/GC3.htm

[Mrpong]

Hi Juma..Thank a lot for your comment

Regarding buffer, I have some OPA627 and plan to try it. Can I just replace AD797 with OPA627 in your schematic?

[banana]

Shouldn't V_OCM pin be connected to ground through a capacitor?

[juma]

Originally posted by Mrpong
Can I just replace AD797 with OPA627 in your schematic?

Yes, it will work nice, but this is not an optimal implementation for OPA627. Search the forum for that chip. I used AD797 here as a buffer, you can use any other buffer that you like (and that can drive 1K load). You'll find many buffer circuits in this forum.
For OPA627, buffers, etc... have a look here:
http://myweb.tiscali.co.uk/nuukspot/...ainclone2.html

Originally posted by banana
Shouldn't V_OCM pin be connected to ground through a capacitor?

Not in this application.

[Mrpong]

OK I will check it out.

Actually I'm also interested in this discrete buffer. Has anyone tried it espectially for chipamp?

http://www.diamondstar.de/dDB_overview.html

[juma]

Hi Mrpong,
that diamond buffer is a bit of a overkill (it drives loads down to 8 Ohms and it was intended for headphone amp.), but you are free to try. Though, take care of DC offset (use coupling cap).

Also, I can recommend you another discrete buffer circuit (devised by Greg Ball, also known as amplifierguru) that drives happily 1K load and sounds great (also, no coupling cap). Here it is:

[Mrpong]

Hi Juma..thank a lot for another nice buffer. What's its input impedance? I may have to use a coupling cap to block short high-voltage surge from my tube preamp.

[juma]

Originally posted by Mrpong
What's its input impedance? I may have to use a coupling cap to block short high-voltage surge from my tube preamp.

Input impedance of this buffer is practically the value of the pot (from wiper to ground) paralleled with JFET's input capacitance (JFET's input resistance is in the range of GOhms). Since you have a tube preamp you must use the coupling cap on preamp's output, but not on the output of the buffer.
I supose you will put volume control pot on the tube preamp's input.
If that's the case, instead of P1 pot in the schematic put 47K-100K resistor (I suppose it won't overload your tube preamp - it depends on output impedance of tube preamp).
To avoid creating low pass filter with JFET's input capacitance use 2SK246 instead of 2SK170.

[Mrpong]

OK.got it