Aleph 5 mods

[tonistefanovski]

Little help,
Yesterday I have rechecked the reference voltage regarding Aleph 5 service manual. All voltage are OK, DC offset below 10mv on both chanels, but I'm realy concerned about source resistors voltage. They are between 580mV and 625mV. Temperature on the heatsinks are 35C and near fets about 42-43C.

I wolud like to increase the bias by removing R19, but as I said, I'm little concerned about differences on source resistors voltage, and about any damage to resistors it self (MILS MRA-05 5W/1%-wirewound) because they are allready hot.
Thank you at advance,
Regards,
Toni

[Zen Mod]

Originally posted by tonistefanovski
Little help,
Yesterday I have rechecked the reference voltage regarding Aleph 5 service manual. All voltage are OK, DC offset below 10mv on both chanels, but I'm realy concerned about source resistors voltage. They are between 580mV and 625mV. Temperature on the heatsinks are 35C and near fets about 42-43C.

I wolud like to increase the bias by removing R19, but as I said, I'm little concerned about differences on source resistors voltage, and about any damage to resistors it self (MILS MRA-05 5W/1%-wirewound) because they are allready hot.
Thank you at advance,
Regards,
Toni

measure these resistors in cold (amp off) ;
maybe they aren't 1%

how tight your output mosfets are matched ?

and - last - what's temperature of these resistors ?

[GRollins]

580mV to 625mV doesn't sound outrageous to me. Even well-matched MOSFETs will show some variance once they're in the circuit, particularly if they're matched at a current different from the one they see in the amplifier.
I'd say you're cleared for takeoff, just watch the temperature on the heatsinks if you do decide to increase the bias.

Grey

[tonistefanovski]

Choky,
1. I've mesured resistors, they are all 1Ohm
2. FET's are matched for about 5 min with VGS in range of 40mV between them.
3. Temperature of the resistors is about 45C.

Grey,
Regarding heatsinks a think that will be OK. They are big enough (450mm x 300mm), but I'm worried about source resistors. They are not metal film but wirewound MILLS MRA05-5W/1%!
Thank you both,
Regards,

[Zen Mod]

Originally posted by tonistefanovski
Choky,
1. I've mesured resistors, they are all 1Ohm
2. FET's are matched for about 5 min with VGS in range of 40mV between them.
3. Temperature of the resistors is about 45C.

Grey,
Regarding heatsinks a think that will be OK. They are big enough (450mm x 300mm), but I'm worried about source resistors. They are not metal film but wirewound MILLS MRA05-5W/1%!
Thank you both,
Regards,

it's probably ( no - fer sure! ) as GR wrote to you - just crank i t and observe overal htsnk temp.

in last time - since I started messing with these small number legged creatures , I use to match them ( both small signal and biggies) exactly on same current on which they'll be used in circ .

like that - you can expect smaller differences and better behavior , current sharing and DC offset wise