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  #1  
Old 31st July 2021, 08:55 AM
MrKettle MrKettle is offline
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Location: Warwickshire
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Default Single gain stage Pre3?

I have just got a Pre3 back up and running in my second system. The power amps are the KMTech gainclone boards from ebay set for gain of x22 so as you would expect even on the lower x6 wiring for the outlut transformers the gain is far too much.

Which got me thinking. From my back of a packet workings it looks to me like you could wire up a single ECC82 as per the second stage of the normal Pre3 circuit and give about unity gain. Is this something that anyone has tried, with this or another valve (given the space constraints it would probably need to be a noval base)?

I am aware of the CF mod, but like the concept of transformer coupling so would prefer to keep it that way, if only to allow balanced output if required in the future.
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Old 31st July 2021, 04:42 PM
John Caswell John Caswell is offline
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Default Re: Single gain stage Pre3?

Hi,
Been there and done that - removed the coupling cap between first and second stage and fed signal into grid of second stage. I can only say it sounded a bit flat,
Pre 4 has a preset volume control between first and second stage of 10K with the coupling cap being up to 10F which then allows infinite gain adjustment.
The other thought is that as the OPTX has 4 secondaries, these could be wired as 1, 2 3, or 4 in series/parallel to give you 4 different levels of single ended output or could be wired to give you 2 different levels of balanced output.
You would need to discover which are the start and finish of the windings to get the polarity correct so would need at least a scope and signal generator to sort them out.
I have this as a job to do and will post info in the FAQ as and when.

John
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Old 31st July 2021, 05:33 PM
MrKettle MrKettle is offline
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Default Re: Single gain stage Pre3?

If it makes a difference my Pre3 is from around 2008. Has there been any changes in transformer spec since then?
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Old 31st July 2021, 06:28 PM
John Caswell John Caswell is offline
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Default Re: Single gain stage Pre3?

No

John
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