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Surfy 10th May 2011 10:39 AM

Threshold SA-2 front end updates?
Hello DIY'ers,

I am the new and proud (but 3000 euro lighter) owner of a pair of SA-2's with optical bias :D
After reading the thread by BrianEno I noticed that Mr Pass stated there was a more perfect front end designed, but "management" at the time did not go with it...
Would it be possible to modify the front end of these optical bias version SA-2's to that / a more perfect version?:confused:
The seller also owned a pair of SA-11M thresholds which, according to him sounded better on voices; he also stated they were specificly designed for the german market...
My amplifiers seem to work fine (they are almost spot on as far as heatsink temperature is concerned), an sound great except indeed for the highs, which could be a bit sweeter, would this be because of the front end used?:confused:
One amplifier does give a bigger "thud" on the woofer when switching on and off, the second only when switching on. I read here Mr Pass recommends replacing electrolytic capacitors. Could I start with just those small ones in the front end, anyone recommendations for specific good sounding brands?
As the amplifiers are working fine (except for the thud, and a little treble harshness) I would not like to take unnecessary risks, I am not an engineer, just a hifi enthousiast with some solduring skills and a multimeter :) but I am willing to try with some help of you guys!

Surfy 11th May 2011 07:37 AM

This should be the same front end board as in the SA-1 optical bias version... Nobody any ideas or suggestions for upgrades, or experience with these front end boards?
thanks in advance!

Zen Mod 11th May 2011 08:27 AM

Papa isn't all the time on board

give him some time for answer

Surfy 23rd May 2011 09:31 PM

I noticed an SA/2e for sale recently, there was a sticker on it, it was upgraded to the e series. Can anyone, or maybe Mr Pass, please tell me what to do to upgrade my SA2 too? It would be interesting to know if it is just some components I have to change (and by what I have to replace them with) or if it is a totally different front end board alltogether.
Thanks in advance for help!

Nelson Pass 23rd May 2011 10:12 PM

2 Attachment(s)
Attached are the last sets of front end schematics. FE89 saw production,
FE90 represents some improvements to that. As to how you alter your
existing circuits to implement this is a DIY project.


Surfy 24th May 2011 07:28 AM

Dear mr Pass, thank you for the fast reply and schemes!Would you happen to have this scheme also for my production line which uses the front end board with single ended input only and optical bias, I suppose 1987 production run? thank you!

RobertS61 24th May 2011 10:58 AM

I would leave it as it is.
You have already the last SA/2 Series 'optical bias'.
The only thing you could consider is an update of the electrolytics.
I would not tamper with a 'Nelson Pass' Original.

The DC problem that you have is probably a failed transistor you have to isolate.
I had one in my SA/1's and I discoverd it because it had a much higher biasvoltage then the other ones. I replaced the faulty one, don't worry for matching because that's not necessary with all the parallel devices and the emmitterresistors that will balance the load.
You need some tools of cause like a Variac, DMM, dummyload and preferably a Scope.
These guys here can help you out I'm for sure.

You said you bought it for 3000 Euro's and that's a lot of money for a not working set of SA/2's
The seller should refund you the money if you did not already knew before that they had problems.
I sold mine for only a thousand more and they were perfect, had all the electrolytics updated and were SA/1's.
The seller should solve your problem for that amount you payed!

SL10User 29th July 2011 11:17 PM

unknown frontend
1 Attachment(s)
Hello all,

recently got a S/500. It had DC offset 3 V in one channel and distortions at higher levels.

Found defective device 2N5566. Have ordered replacement already, hope it will work...

I noticed that these both previously posted schematics (Thanks, Nelson!) doesn't match with the PCB in my S/500. A matching schematic would have been very helpful. So I had to find the defective device by swapping with the good channel...

Does anybody (Nelson?) have knowledge of this specific version? Seems to be an early version (or stripped down)?

Here is a picture:


SL10User 29th July 2011 11:54 PM

1 Attachment(s)
Second picture.

There is a # on the board - 540305b.

Nelson Pass 30th July 2011 05:07 PM

1 Attachment(s)
Well, there's this:

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