Yet Another Amplifier Project - Borbely-derived Hybrid Design

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Ray P
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#1 Yet Another Amplifier Project - Borbely-derived Hybrid Design

Post by Ray P »

I mentioned I have been having a bit of a rummage/organise of my 'Stuff' and I came across some PCBs and varous parts for a hybrid-design amplifier that was derived from an Erno Borbely Hybrid Headphone amplifier design from way back. Here's a schematic for the hybrid amplifier and a link to info about the HPA (note that my hybrid amp PCBs only have one pair of output devices);
Capture.JPG
https://audioxpress.com/article/a-hybri ... -amplifier

IIRC the reason I parked this some years ago was because of the pretty scathing assessment of the hybrid amp by Broskie in his TCJ, here's a link to the first page;

http://www.tubecad.com/march2001/page19.html

however, on rereading this it seems his main beef is the operating point used for the 6922/ECC88 valve as these are not true low voltage devices - the implication is that distortion of operating these valves at +/-35V compromoses the sound quality too much and this seems to be supported by comments about distortion differences with different valves in the article about the headphone amplifier above.

Anyway I prefer not to summarily bin stuff, especially as I have most of the parts in the box, so thought it might be worth experimenting with but, instead of going down the Broskie 'solution' route, I'm thinking it might be simpler to get a better result with the hybrid amplifier by using a valve that is designed to operate at low voltages, such as the ECC86 or 6GM8. My thinking is to modify the design driver stage to use the HPA's ECC86 arrangement, powered with dedicated 24V regulated supplies, with just the output MosFETs using the +/-35v supplies.

It'll cost me little to try it but I thought I would seek the opinons of our learned panel before doing anything with it (which, anyway, will be some months yet given the other stuff to finish first!).
Sorry, I couldn't resist!
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Nick
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#2 Re: Yet Another Amplifier Project - Borbely-derived Hybrid Design

Post by Nick »

Would using 24v for the driver and 35v for the output mean you were very limited by the output swing you could manage?
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Ray P
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#3 Re: Yet Another Amplifier Project - Borbely-derived Hybrid Design

Post by Ray P »

Hmmm, more to think about - I risk getting distracted from the MoFo so I think this box will have to go on the shelf for a while more...
Sorry, I couldn't resist!
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