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-   -   Symasym PCB (http://www.diyaudio.com/forums/solid-state/134408-symasym-pcb.html)

Khron 6th December 2008 10:17 PM

Symasym PCB
 
2 Attachment(s)
I put together an "alternative" PCB for Mike Bittner's Symasym 5.3, with 2 pairs of output trannies. I do hope he won't mind me "messing" with his amp :smash:

That's about as close as i could get to having a star ground on-board. I implemented John's (MJL21193) idea of 22r//diodes for the small-signal ground "decoupler.

If anyone has ideas for improving this, don't be afraid to step forward and speak your mind :)

anatech 6th December 2008 10:37 PM

Hi Khron,
I have not examined the PCB closely, but there are a couple things that come to mind.

Move the capacitors near your fuse holders towards the front of the PCB. This makes more room for emitter resistors and may allow the use of the "plate" types.

Use normal fuse clips for the "GMA" size fuses. The ones you have there don't look that common and may be very difficult for anyone else to source.

Allow for a single turn 300 type trimmer control. 10 turn types have small wipers and are limited in current, never mind that the single turn types are easy enough to use. I would find a 10 turn annoying as heck! I think there is a resistor in series with the control, can't remember that clearly enough to be sure.

Lastly, possibly allow for smaller input coupling capacitors. Just lay a trace in line and provide extra holes.

-Chris

ostripper 6th December 2008 10:39 PM

It's already being done on "frugalamp2" thread.. much
higher power "symasym" type amp with MUCH better
performance (200-300W) with 6-8 op device scalability...

First "real" amp will be done for all to see next wed./ thurs.
on the "frugal amp" thread.
OS

lineup 6th December 2008 10:40 PM

hello Khron

looks nice to me :)
but I am no expert on pcb

anatech 6th December 2008 10:47 PM

Hi ostripper,
Quote:

It's already being done on "frugalamp2" thread.. much higher power "symasym" type amp with MUCH better performance (200-300W) with 6-8 op device scalability...
Sure, but what's wrong with the original design? I think this one might fill a nice gap in a slightly higher power bracket that also allows low impedance loads.

I think this might fill a nice gap between a 50 watt amp and the 200 + watt amp.

-Chris

Khron 6th December 2008 10:49 PM

Thanks for all the quick replies / comments :)


Anatech,

I used that type of fuse holders because i've got a bunch of them scavenged from some unused medical PSU's, just for a few of these boards. Same goes for the trimmers: i've got a few 1K or 2K's somewhere at home. Yes, there is a resistor in series with the pot. And besides, is the current going through the trimmer really "too big" for a 10-turn? And i guess a few extra pads for the input cap won't hurt :) (I've gathered a bunch of Roederstein MKT1822 caps, 1,5u-6,8u with that footprint)


OS,

I've been watching your FA2 thread these last few days, and it looks like a really good amp :) It's just those japanese transistors that kinda "scare" me :D ie. not that common, and where i might find them, they're not exactly "for free" :) One of the reasons i chose the Symasym is because of the "usual" transistors used (and getting real good performance from them :smash: )

anatech 6th December 2008 10:57 PM

Hi Khron,
It was found that 500 ohm trimmers worked better. I can verify that as I have some built.

Quote:

I used that type of fuse holders because i've got a bunch of them scavenged from some unused medical PSU's, just for a few of these boards.
That's what I figured when I saw your PCB. Can you simply add trace to allow the clips to be used? I'm just thinking that the boards are more universal then.

Quote:

It's just those japanese transistors that kinda "scare" me
I think those Japanese signal transistors are the best there are. Having said that, the new On-Semi power transistors are really good too. I built my first amps with the (now discontinued :bawling: ) MJW0281A and MJW0302A. I haven't seen any transistors so far that were as good as these are. They didn't have the highest ratings, but they were all very close in parameters to each other. I would have very happily standardized these for outputs except in the much higher power ratings.

The Japanese signal transistors are not that expensive. Buy them in 50s or 100s.

-Chris

Khron 6th December 2008 10:58 PM

2 Attachment(s)
Rev 1, i guess :)


Anatech (i'm Chris too :D )
I tried to follow (a few of) your tips. Here's the result this far:

EDIT: There's a LOT of stuff i'd buy in 50s or 100s, but i'm (still) a mere student, so... Sometimes, that's not an option and / or i have to make do with what i can get my hands on at little or no cost :) I'm sure you've "been there, done that", some time ago :D

anatech 6th December 2008 11:05 PM

Hi Chris,
Much better, thank you.

I guess the board needs to be checked in detail now.

Quote:

Sometimes, that's not an option and / or i have to make do with what i can get my hands on at little or no cost I'm sure you've "been there, done that", some time ago
Some time ago?? That's a permanent condition.

Or you can overspend and end up with a horrible bill like I did.

-Chris

ostripper 6th December 2008 11:09 PM

I meticulously simulated the original "symasym" and have
knowledge of it's "big brother" , Ampslab's Bi120.

My answer to this is the FA2, while saluting mike B's contribution
to DIY, and not infringing on Ampslab's IP, offers a very
superior Alternative to both. (prototyped and simulated symasym and found stability problems which were only balanced
by "gross compensation" at OP stages):(

I know you all like your symasym's and I would not want to "bastardize" a good design, but I think there is MUCH room
for improvement ,as some have bet their business on it.
(M. Chua - Ampslab)
OS


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