From stereo to monoblocks

[EC8010]

Originally posted by Guydot
I had a "fantasy" about transforming the input stage into a differential amp stage with an SE output so it can receive a balanced signal feed (i might have to connect it using pretty lengthy cables), but since i don't feel comfortable with the needed knowledge and experience there's a good chance i'll drop that ambition

A differential input stage feeding the ECC82 (I won't comment) differentially would be a very good idea. The design's form of phase-splitter ideally needs a high-mu valve, which the ECC82 isn't.

[Guydot]

hi all,

Fred thanks for your help mate!

And EC8010 - ok so i understand you object to the use of ECC82's as phase splitters

Can the ECC82's be fed from a differential output? I thought the design permitted only SE feed to the phase splitter - interesting

That would make the circuit almost fully balanced, wouldn't it? How very cool

I wish i had the knowledge to modify the input stage, b/c i'd do it in a second

Cheers,
Guy.

[EC8010]

There's no problem in replacing the input stage and making the whole amplifier balanced.

The problem comes when you want to retain the feedback from the output transformer. You see, feedback networks are determined by trial and error, and match a specific output transformer to a driver. I started drawing a circuit for you, but realised that it was more likely to oscillate than not. Perhaps you could try later on, when you've gained more experience.

[Guydot]

Hi,

Thanks for the input.

If i understand you correctly you are refering to the amount of feedback on the overall feedback circuit.

The truth is i'd love to try it out. Since i'm replacing the OPT's in any case i'd have to integrate them into the circuit and i will be
more than willing to play with the feedback untill the circuit is stable and secure.

So if you do fee like giving me a drawing of the input stage as a bal. diff. amplifier i will be MORE than appreciative - I don't have a problem trying out different values on the feedback network to find the right amount, it's the design of the diff. amp that i am not confident about

I really want this amp fully balanced for some reason

Cheers,
Guy.

[EC8010]

OK. This is the most stable configuration I could think of. If it works, if should give a sensitivity of around 600mV for full power. The capacitors across the 1M5 feedback resistors need to be about 1pF - which is very small, and getting towards the value of the capacitance between the end caps of the 1M5 resistors, so they're probably not needed. Both the valves shown are ECC82.

Try it, by all means, but don't spend significant money on it, as it may not be possible to get it to work properly. If it can be made to work without oscillating, there are all manner of things that could be done to make it sound really nice.

[Guydot]

Hi,

Sorry for not replying sooner, had a busy couple of days.

Wow - thank you very much for your help!

3 questions that arise:

1)What are those 1Mohm resistors you used in series in the input stage?

2) Am i mistaken or did you use DC coupling between the first and second stage?

3) Why did you chose to use ECC82's in the input stage instead of 2 6au6's like in the original desing? I'm not complaining or anything (lol), just curious as to why you think it's better to do it with a double triode instead of 2 pentodes? (I like the idea though - maybe i can can be addapted further and try out a 6SN7 instead of the ECC82 ).

Cheers,
Guy.

[EC8010]

The 1M resistors are grid-leaks to hold the grids of the input valves at 0V. The two stages are DC coupled. Triodes (even ECC82) produce less distortion, plus a double triode is cheaper and requires less metalwork than a pair of pentodes.

If the thing works without oscillating, I would far rather use 7N7 or 6SN7 than ECC82 (much lower distortion). But see if it works with ECC82 first...

[Guydot]

Many thanks mate - you are of enormous help - i'll report once i have a breadboard model - if it works i'm definitely moving to 6SN7's

Cheers,
Guy.

[PTSOUNDLAB]

Hello Boys,

I have build a KIT88 (without PCB). It is very difficult to find good 6AU6, so I try to replace 6AU6 by 6SN7. Does somebody have try this ? replacing 6AU6 ?

Thank you very much

Pascal.

[PTSOUNDLAB]

Hello Guydot,

I work on this amplifier since a long time and I have made some modifications on it. I will be please to exchange experiences with you. Can you contact me by mail (ptsoundlab@aol.com) ?

I hope read you soon.

"Cordialement".

Pascal.