LM4780 Parallel/Bridge - Prelim Schematic - diyAudio
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Old 15th July 2006, 11:42 PM   #1
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Default LM4780 Parallel/Bridge - Prelim Schematic

Hey guys.

After reading hundreds of posts in here and starting other very interesting threads i put together my first chip amp design. It is based on the LM4780 datasheet and the BPA-200 AN.

There are a couple of things that i'm not sure about.

1. Both parts of the bridge are non-inverting since the job is done by the DRV134? Is this correct? I'm really not sure. (sorry about my ignorance, this hasn't been by field Yet!)

2. Can the OPA134 be used as buffer as on the schematic? And is this buffer configuration correct?

3. The 2.7R in series with the 0.1u cap on the output of the amps, is this supposed to be followed on the p/b connection? On the BPA200 there is no such thing.

The Voltage is calculated for a 6ohm load (most likely to be used).

Thank you very much for all your help til now, if you can answer these questions and tell me what you think about the schematic it would be great. Thank you!
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Old 16th July 2006, 06:34 PM   #2
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Anyone ? :-)
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Old 16th July 2006, 07:35 PM   #3
juma is offline juma  Germany
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Default Re: LM4780 Parallel/Bridge - Prelim Schematic

Quote:
Originally posted by otherside

There are a couple of things that i'm not sure about.

Everything is OK with your schematics. It will work all right. It's tried out many times.
You can leave out C2, C3, C12 and C13 since you have C1 and C20. It will improve the sound.
Put 10uF caps in parallel with R25 and R27 - it will significantly improve ripple rejection. Calculate correct values for R25-R28 (with all of them being 10K output voltage will be 2,5 V - not 15V). LM337 doesn't have the same pinout as LM317 (317's output is on pin 2, 337's is on pin 3).
Make sure to absolutely match feedback (R2,R6,R16,R18) and gain setting (R1,R5,R13,R17) resistors. It's vital for proper functioning of the amp.
Also, to make it sound great (not just good) use OPA1632 or THS4131 instead of DRV134 as single to balanced signal convertor and some other buffer instead of OPA134 (Nuuk's or Pedja's buffers are very good). I like this one (proposed by John Curl and Erno Borbely) - it's extremely simple to build, works in class A, drives 1k with ease and sounds great (and if you match JFETs it won't have DC offset on output) :

Cheers !
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Old 17th July 2006, 09:15 AM   #4
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Juma thanks a lot for the reply.

I know about the regulators. I just put them on the design as a reference, these will even go to a different PCB. I'm aware of the calculations, the caps and protection diodes, but thanks anyway :-).

Resistors will all be 0.1% some excellent quality vishay parts.

About OPA1632 and THS4131, i had good look on the datasheet but i'm not very sure how exactly i can use them, could you point me to a schematic for similar use? Thank you

Quote:
and some other buffer instead of OPA134 (Nuuk's or Pedja's buffers are very good). I like this one (proposed by John Curl and Erno Borbely) - it's extremely simple to build, works in class A, drives 1k with ease and sounds great (and if you match JFETs it won't have DC offset on output) :
Could you give me some links please? I've been searching the forum but the information volume is huge :-)
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Old 17th July 2006, 09:48 AM   #5
juma is offline juma  Germany
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Hi,

Pedja's buffer can be found here:
http://www.pedjarogic.com/gc/index.html

and Nuuk's buffer is here:
http://myweb.tiscali.co.uk/nuukspot/....html#discrete

You'll have to ask them if these buffers can drive 1K load.

There is a great buffer here (it can drive a load down to 32 ohm), but it's a bit more complex:
http://www.diamondstar.de/dDB_design.html

This is the way I use OPA1632 (or THS4131 - they are pin-to-pin compatible and almost completely identical inside) :

Cheers !
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Old 17th July 2006, 11:04 AM   #6
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May i ask, where did you get the K170BL/J74BL, they don't look to be that popular or available anywhere ?


Thank you very much for your help.

Eyxaristo poli, na sai kala. (same in greek)
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Old 17th July 2006, 11:38 AM   #7
juma is offline juma  Germany
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Quote:
Originally posted by otherside
where did you get the K170BL/J74BL

I bought a bunch in a local electronic shop an year ago, but you can find them here:
http://www.schuro.de/preisl-jap-trans.htm
or here:
http://www.reichelt.de/inhalt.html?S...OVID=0;TITEL=0
under Japan Transistoren link

Cheers !
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Old 17th July 2006, 12:57 PM   #8
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Is the difference between using these buffer circuits and a simple op-amp huge in quality?

Is it worth the effort in contrast with a good quality op-amp?
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Old 17th July 2006, 01:07 PM   #9
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For the buffer, stick with the OPA134 for now, you can always use sockets if you want to try other op amps later.

As for the phase splitter, (yes, I know it's the wrong term, but if the tube guys can get away with it, so can I ), I would stick with the DRV134. To exceed it's performance you will have to spend a lot of time matching resistors, as accurately as you possibly can, as any imbalance will really aggravate distortion. Again you could just stick a socket in it's place, then you could build daughter boards with whatever you want to try later.
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Old 17th July 2006, 06:13 PM   #10
juma is offline juma  Germany
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Quote:
Originally posted by otherside
Is the difference between using these buffer circuits and a simple op-amp huge in quality?
No it's not. It is a difference between very good and excellent.

Quote:
Originally posted by otherside
Is it worth the effort in contrast with a good quality op-amp?
Depends what your goal is - very good or excellent amplifier.

BTW, DRV134 has something "industrial" in it's sound, something that you meet in professional PA systems. If you love that, go for it.

Cheers !
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