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nattawa 28th December 2012 09:02 AM

Circuit and PCB layout review will be appreciated
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I decided to build myself an amp based on Cordell's circuit and have just completed a PCB design (on Christmas day!).

I tried to make a layout that's compact in size yet would offer a few interesting options. The circuit includes a TMC compensation, a baker clamp, a diamond buffer triple output stage employing up to 5 pairs of On-semi ThermalTrak devices, an op-amp based bias control, a solid state speaker relay, an output DC detection circuit that operates that solid state speaker relay, and an output over-current protection. I use through hole and SMT mixed design and managed to squeeze things into a double sided 230mm x 66mm area. Components are mostly on top side of the board, the output transistors and the Baker clamp components are the only parts installed on the bottom side.

The schematic and layout may appear to be a bit confusing as there seem to be a lot of redundant parts, such as paralleled zener diodes, doubled-up base stopper resistors, and doubled-up 0.1u bypass caps. These are for the SMT/through-hole part option purposes, just in case.

Not all component values have been calculated/simulated (I'll need to learn the LTspice, not even a newbie I am to simulation right now), but I supposed they wouldn't be too far off in terms of size of package/footprint which matters to the layout.

Before I order the PCB I'd very much appreciate the help of extra pairs of eyes just in case there are show-stoppers that I never would realized by myself until the boards come back. Thank your for any input.

Wish all a happy new year!

rsavas 28th December 2012 05:54 PM


There is a lot to review in this design. I actually received Bob's book from Santa & have not thoroughly read it, let alone absorb all the information.
I would suggest to perform the LTSPICE simulations, to perform virtual testing and analysis= sanity check.
I see that you have the RC Zobel R34/C86. L1 is to have a R in parallel? I guess that you will wrap L1 around a resistor in the layout, so that is why it is not shown on the schematic.
R47/48 are shown as 0? not sure why they are there, would think that they are optional.
Layout looks good, I do not see anything obvious = good job.

Good Luck & thanks for sharing

DUG 28th December 2012 07:34 PM

...LED 0805...have fun finding A from C


1206 is bad enough.

Looks like there are enough options to keep you busy for a while.

Looks marvelous.

nattawa 28th December 2012 08:04 PM

Thanks, Rick. You must have done something good and Santa knew it well.

I Santa sent me two tubes of transistors 25x NJL3281 and 25x NJL1302 :D

You're right about the simulation. I put off learning the LTspice for too long I should pick it up ASAP.

The missing paralell resistor on L1 was exactly what you thought. Good point. The layout software would not like a part in a schematic that does not have a place in the layout. I'll put a resistor into the schematic as a non electrical part, so that it does not upset the software while allowing the resistor to show up in the BOM.

R47/R48, along with R49/R50 and the 3k3 Rs off the 0.33R resistors, has to do with the behavior of the over current protection. I haven't go through the calculation so I put 0R for now.

Dug, obviously I'll go out and get a good helping-hand with a big glass for those LEDs:D. Thanks for you kind words.

Vostro 28th December 2012 08:08 PM

Diode scale on DMM and finding A K is ok , they light slightly. but fine tweezer work sucks.

TANDBERGEREN 28th December 2012 08:18 PM

LED A-C tester is easy, dug. And a tiny little dot from a pencil and the Cathode is marked good enough for larer mounting, don't You think?

nattawa, You got a Santa far better than mine. I just got a bag full of BC-capacitors.

R47/48 shorts the protectioncirquit BE on Q31 and Q32
They aren't to be mounted unless You want to run this one without protection, are You?

nattawa 28th December 2012 08:35 PM

@ TANDBERGEREN. R47/R48 are 0R temporarily until I have gone through the calculation.

TANDBERGEREN 28th December 2012 08:58 PM

Ok nattawa.
As I guessed then.
Well, otherwise the most looks good to me.
0.33Ohms emitter-resistors seems to be a little too much anyway.
Would let it go close to 4Amps /transistor before the protection should go in and do anything. This would be close to 0.15Ohm.
But as You say, you should do the math first.

Nice build this.

DUG 29th December 2012 03:37 AM

[QUOTE=TANDBERGEREN;3302276]LED A-C tester is easy, dug. And a tiny little dot from a pencil and the Cathode is marked good enough for larer mounting, don't You think?

They are usually already marked. Just very small.

Hence the :)

gmphadte 31st December 2012 06:06 AM

May I ask, how many watts is this amp.

Gajanan Phadte

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