Built an f1 some time ago despite nelsons and zen mod best efforts amplifier did not work.Have tried again this year results look more promising but still have a problem with dc offset on the right channel despite following nelsons Instruction in the service manual circuit diagram on building and matching all parts including the mosfets.Read all i could find on the forum with regards to the f1 and any problems that may occur .Will now put down measurements.
L Drain R Drain
q1-13.85 q1-13.83
q2-13.87 q2-13.65
q3-13.86 q3-13.85
q4-13.89 q4-13.67
q7-2.048 q7-2.068
L-Gate R-Gate
q1-7.45 q1-7.44
q2-7.45 q2-7.31
L Dc Offset R Dc Offset
+-10mv +-218.4mv
- -19mv - -205.6
These measurements were after one hour the amplifier is hot cannot be held more than about 15- 20 seconds.Have tried a 10ohm resistor in parallel with r27 r28 with no effect .Thus the offset on the right channel will not come down. as seen by the numbers there is no problem on the left channel.Anyone with any ides or suggestions.Sorry for the long post one may ask why iam perservering with the f1 lowther are my only speakers owning three pairs utilised in am medallion horn speakers having built and use a f2j iam enamoured with the sound of transconductance amplifiers giving me the best sound i have ever heard Determined to get the f1 working thus can hopefully have a different flavour of these wonderful amplifiers.Also have an f3 to build and some old quad II to rebuild as well as a couple of ladyday 300b amplifiers that require fettling.Many thanks for any and all help spades.
L Drain R Drain
q1-13.85 q1-13.83
q2-13.87 q2-13.65
q3-13.86 q3-13.85
q4-13.89 q4-13.67
q7-2.048 q7-2.068
L-Gate R-Gate
q1-7.45 q1-7.44
q2-7.45 q2-7.31
L Dc Offset R Dc Offset
+-10mv +-218.4mv
- -19mv - -205.6
These measurements were after one hour the amplifier is hot cannot be held more than about 15- 20 seconds.Have tried a 10ohm resistor in parallel with r27 r28 with no effect .Thus the offset on the right channel will not come down. as seen by the numbers there is no problem on the left channel.Anyone with any ides or suggestions.Sorry for the long post one may ask why iam perservering with the f1 lowther are my only speakers owning three pairs utilised in am medallion horn speakers having built and use a f2j iam enamoured with the sound of transconductance amplifiers giving me the best sound i have ever heard Determined to get the f1 working thus can hopefully have a different flavour of these wonderful amplifiers.Also have an f3 to build and some old quad II to rebuild as well as a couple of ladyday 300b amplifiers that require fettling.Many thanks for any and all help spades.
Did you add resistance in parallel to both R27 and R28?
Just guessing here, but I would imagine in this case you would want to to just one
side and see how and if the offset changes.
Hopefully others will chime in.
Cheers,
Dennis
Just guessing here, but I would imagine in this case you would want to to just one
side and see how and if the offset changes.
Hopefully others will chime in.
Cheers,
Dennis
No parts were substituted 10ohm was added to r27 then r28 individualy tested one at a time there were no changes in the offset;running out of ideas ps is it possible to add more than 10ohm to lower the offset.thanks Spades.
Have you measured the current through Q7 , Q3 and Q4 and then compare this with the good chanel and are you 100% sure the source resistors are the right value?
Seems the culprit is r28 paralleled with 67ohm resistor from my box of spares offset on L channel now 45mv will buy some better resistors next week.Thanks for the help and suggestions.ps i hope the amount of resistance will not affect the sound .will try out tomorrow.
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