I hope this is OK to post in Pass Labs... Talking about the old Adcom 555, but it is a Pass design!
I've been selling replacement boards for the GFA-555 for some time now, and I am about to design new versions with some subtle improvements.
I have a question about thermal tracking. I read somewhere you were describing the thermal design of the 555, and what I gathered, was that Q617 and Q619 provide some measure of tracking with temperature... But I wonder if this is one of those cases where the physical layout doesn't match the designer's intentions. Should these transistors be mounted on the output module heatsinks, and not on the input board?? The 555 MK1—as manufactured—actually has quite a pronounced positive thermal coefficient. At idle, each output runs about 50mA, but when hot, this can rise to as much as 150mA, and it takes the amp longer to cool down because of this.
Was the thermistor TH601 in your original design? Maybe it's not even necessary if Q617 and Q619 are mounted to the heatsink?
If this is the case, I can design new output module boards to include these transistors.
Thanks for your consideration, and the excellent designs that I continue to learn new things from.
-Chris
I've been selling replacement boards for the GFA-555 for some time now, and I am about to design new versions with some subtle improvements.
I have a question about thermal tracking. I read somewhere you were describing the thermal design of the 555, and what I gathered, was that Q617 and Q619 provide some measure of tracking with temperature... But I wonder if this is one of those cases where the physical layout doesn't match the designer's intentions. Should these transistors be mounted on the output module heatsinks, and not on the input board?? The 555 MK1—as manufactured—actually has quite a pronounced positive thermal coefficient. At idle, each output runs about 50mA, but when hot, this can rise to as much as 150mA, and it takes the amp longer to cool down because of this.
Was the thermistor TH601 in your original design? Maybe it's not even necessary if Q617 and Q619 are mounted to the heatsink?
If this is the case, I can design new output module boards to include these transistors.
Thanks for your consideration, and the excellent designs that I continue to learn new things from.
-Chris
I would have to go look, but I don't recall this being the original circuit. However the techs at Adcom were competent to make changes as they saw the need, and them doing so was never a problem.