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Old 6th June 2005, 12:21 PM   #1
AndyUK is offline AndyUK  United Kingdom
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Default Les Sage amp circuit here for analysis

I came across an old tread in the archive about the encapsulated amps made by Les Sage in the UK around 1990. They were supposed to be Class A in terms of Sliding bias (non-switching) rather than the pointless but easy 50% bias most people use. Anyway, I unpotted one of his SUPERMOS amps and here is the circuit. It's not very impressive. Les is no longer alive so he can't answer for his claims...pity. See attached pic. regards Andy.
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Old 6th June 2005, 02:00 PM   #2
JohnW is offline JohnW  Hong Kong
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Hi Andy,

Thanks for posting the circuit - I guess this was a SuperMOS 1 module - not SuperMos II, do you have details of the SuperMOS II?

I had a pair off SuperMOS II modules - however Sage Audio mounted the 3 pairs of output devices DIRECTLY to the heatsink module WITHOUT insulators. The anodising broke down in a short time and both modules failed...

So much for the claim of "No Capacitor" sound!

John
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Old 6th June 2005, 03:24 PM   #3
AndyUK is offline AndyUK  United Kingdom
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The Superamp had the '1' added when the bigger version was released.
I never bought a SuperMOS II

I've attached the datasheet to clarify the claims made for this design. (2x tif files in a ZIP. JPGs were too big if readable)

> So much for the claim of "No Capacitor" sound!
Also, so much for...
"Complex multiple nested compensation loops"
"low overall negative feedback"
"non-switching" ... etc

regards
Andy
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Old 6th June 2005, 03:49 PM   #4
JohnW is offline JohnW  Hong Kong
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Andy,

Can you see ANY nested FB loops?

I thought it had no "Single Pole Compensation" - Er so whats the function of the 12pF Cap?

ARE you SURE this is the correct Schematic of SuperMOS - Sage Audio claimed 2 pairs of output devices - or did you not add them for clarity...

If true - it would seem that all the claims are false... ???

Does and body have a SuperMOS II module - I would be interested in buying and "De-Potting" the module...

John
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Old 6th June 2005, 04:23 PM   #5
AndyUK is offline AndyUK  United Kingdom
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Yes it's the right circuit. only 2 O/P devices. I bought them new and put them in a 3U rack box with separate 625VA toroidals (225VA would have been fine!) One module overheated and went BANG at a party, 4R with no fan :-( , no thermal protection. So I disected them both. It took me 2 weeks to fully un-pot them using dichloromethane. I may even have the bits at home, will post a pic if I do. Pity I don't have one left or I could measure it on my R&S UPL.
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Old 6th June 2005, 06:18 PM   #6
JohnW is offline JohnW  Hong Kong
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You have the R&S UPL - there great little machines - I have a couple of UPD's - fully loaded - these R&S are so much better then the AP's for R&D work - I've only used my AP2 C++ twice since new!!

John
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Old 6th June 2005, 06:33 PM   #7
JohnW is offline JohnW  Hong Kong
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In the Sage Audio info. you posted above, it claims they applied for a patent on the "8 transistor constant current / constant VCE superlinear gain block" - however I could find no record of a patent application by Les Sage or Sage Audio - can anybody shed any more light on this patent claim?

John
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Old 6th June 2005, 06:45 PM   #8
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Hi Andy
is it really never switching off or only provided no phase shift between current and voltage?
I wish I understood how it actually works and which part is doing what. It will take me some time I'm affraid ... complicated
regards
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Old 6th June 2005, 06:46 PM   #9
JohnW is offline JohnW  Hong Kong
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Andy,

Are you sure that the 1nF Cap show connected between the output and the potential divider chain shound not have been connected to gound?

I don't understand this part of the circuit- it appears to be part of the output overcurrent protection (BC237B / BC307 / 4V7 Zeners + IN4148 etc) - or the "sliding Class A bias"?

John
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Old 7th June 2005, 09:38 AM   #10
AndyUK is offline AndyUK  United Kingdom
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> Are you sure that the 1nF Cap show connected between
> the output and the potential divider chain shound not
> have been connected to gound?

Very sure.
I now remember I threw away the bits 2 years ago :-(

Correct me if I'm wrong but I see it as:-

When output is s/c to 0V:-
When upper FET Vgs exceeds 4V7 ( the lower zener) plus 0v7 (Vbe of upper BC237B) the BC237B switches on and clamps the drive to the FET. Say around 5V4 Vgs

When output is NOT s/c to 0V:-
The 1N4148 adds a current drain path to a mid-rail pot-down. The 1K from the lower zener will need to supply more current before if can get to Vbe of the upper 327. The V(1K) adds to the existing 5V4 limit, so more than 5V4 Vgs allowed if output is NOT s/c.

At HF, the 1n will pull the mid-rail pot dynamically to follow the output, so reducing its function of leaking current away from the Vgs limiter. So Vgs MAX at HF may revert to 5V4 max even if output is UNLOADED.
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