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anatech 3rd October 2006 03:12 PM

Symasym - going further (development)
This is a thread intended for advancing and modifying the Symasym amplifier designed by Mike and Pavel. Those discussions tend to get lost in a general thread.

The original thread is here.


MikeB 3rd October 2006 04:14 PM

2 Attachment(s)
Okay, i'll start with a conceptual schematic, containing any possible cascoding i could think off. Overkill ? Which cascodes do make sense ?


MikeB 3rd October 2006 04:21 PM

2 Attachment(s)
This would be estimated psrr from above circuit (green/red = pos/neg)


-_nando-_ 3rd October 2006 04:35 PM

I need this amp with 150W @ 8 Ohms @ THD > 1% :D So, 180W at full power @ 8 Ohms ?

I don't know how to modfy it, I'm reading the BIG Symasym topic, any help will be very well acepted ! :)

Kind Regards,


anatech 3rd October 2006 04:42 PM

Hi Mike,
I am so impressed by the thought of cascoding the drivers. This will linearize the current stage more. :cool: Whether it's truly helpful or not is hard to say. We really have to try this.

This is not really a Symasym in concept, but very worthwhile to try. The cascodes can be bypassed so a direct comparison can be made stage by stage. Therefore it's a fantastic proof of concept, or study amplifier. If all cascodes end up being used it may be a great higher power amplifier (that everyone is asking for). Actually, even if only the front end ones and Vas are used we now have a higher power amplifier.

I'd say do a board this way, study it and make a final determination at the end. It will be a great learning tool I think. make room for more output pairs dude! ;)


pinkmouse 3rd October 2006 04:49 PM


Quick question, relevant to all the Symasyms really, but why are you taking the feedback off after the Zobel? Most references seem to suggest taking it off before.

As others are working on the 5.3 board, I'll volunteer to give this one a go.

anatech 3rd October 2006 04:56 PM

Hi Al,
Me too Me too Me too :D

It's going where I wanted to go, and further.


mlloyd1 3rd October 2006 05:03 PM

my votes:

keep the cascodes on the input jfets.
trash the cascode current sink for the diff amp - i'd just stick with a simple bipolar transistor and use a red LED for reference voltage.
keep the cascode on the second stage diff amp. there's probably some clever sharing to be done between current sources and voltage references.
keep the cascode on the second stage's current mirror.
trash the cascode on the output drivers.


MikeB 3rd October 2006 05:05 PM

Hi Al, the feedback is taken before zobel on the board, only on schematic its taken after. (in sims the same analog node anyway)

Chris, according to sims the cascoded drivers heavily reduce compression at high power levels, as the hfe of the drivers does no longer change with swing. At low power levels they only increase psrr ~50db at 10khz. :clown:

Fernando, there is one problem with increasing power for symasym, especially up to 150w into 8ohms. To get 150w/8r you need supply rails of at least ~55v. Also bias in vas needs to be increased to keep up with the increased current demand from outputstage. This will result in increased dissipation levels of ~2.5, toasting the to92 devices used in the vas. With 55v supply, the mpsa18 must be replaced by higher voltage types. bc546 should do it.
Also, at least 2 outputpairs are necessary, 3 pairs if you still want to drive 4ohms. Are you sure 60watts are not enough ?

Al, Chris, the 1.87v in the vas represent a green LED, maybe simply feed it with a 22k res. Don't forget basestoppers, or the cascodes might oscillate.


PMA 3rd October 2006 05:07 PM

Hey guys with good advices,

I have one, too. Try (built) it and compare with original before you start suggesting ....

Take it easy :D,


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