hi guys,
I'm in the middle of building 4 leach amps, to be run in btl to drive a pair of 400Wrms 8ohm 15" speakers... infortunately, the leach amps are giving me problems. The one I'v got on the bench atm seems to work alright, but it dissipates FAR too much quiescently. It fraws 3.4A per rail when run at 25V. My heatsink gets really hot really quickly.
Anyone know what might be wrong? I've already had to deal with counterfeit transistors from an extremely reputable realer here in south africa.
btw, does it matter that the driver transistors (MJE15030 and MJE15031) have mismatched gains? The 031 has an hfe of 200 or so, and the 030 has an hfe of like 60.
anyway, any help would be greatly appreciated.
Bosium
I'm in the middle of building 4 leach amps, to be run in btl to drive a pair of 400Wrms 8ohm 15" speakers... infortunately, the leach amps are giving me problems. The one I'v got on the bench atm seems to work alright, but it dissipates FAR too much quiescently. It fraws 3.4A per rail when run at 25V. My heatsink gets really hot really quickly.
Anyone know what might be wrong? I've already had to deal with counterfeit transistors from an extremely reputable realer here in south africa.
btw, does it matter that the driver transistors (MJE15030 and MJE15031) have mismatched gains? The 031 has an hfe of 200 or so, and the 030 has an hfe of like 60.
anyway, any help would be greatly appreciated.
Bosium
this can happen
bacause of the different values of the transistors (often related to the year that they were made)
and other components that decide idle currents
that is why most amplifiers have a possibility to adjust
the current - a potentiometer
the difference for NPN and PNP MJE15031
has no effect in this matter
besides it is one of the best TO220 transistors there is
also important that the heat/current sensor transistor
is in good contact with heatsink
I would imagine this transistor to be between 031 and 030
if they are the driver transistors for the outputstage
bacause of the different values of the transistors (often related to the year that they were made)
and other components that decide idle currents
that is why most amplifiers have a possibility to adjust
the current - a potentiometer
the difference for NPN and PNP MJE15031
has no effect in this matter
besides it is one of the best TO220 transistors there is
also important that the heat/current sensor transistor
is in good contact with heatsink
I would imagine this transistor to be between 031 and 030
if they are the driver transistors for the outputstage
Have you attempted to set the bias current? Maybe there is problem with that circuit - i.e. a backwards diode, bad pot, etc. If the current is to high, the amp will get very hot.
Is the amp oscilliating? Are the wires from the driver transistors to the output stage short? They need to be. Do you have the output inductor (L1) wound on R49? You need it, especially if the leads to the output transistors are not real short.
As holojoy states, if the thermal diodes are not in good contact with the heatsinks, you can get thermal runaway. Use silicon grease to ensure good thermal contact.
The gain mismatch would not cause this problem.
Good luck!
Jeff R.
Is the amp oscilliating? Are the wires from the driver transistors to the output stage short? They need to be. Do you have the output inductor (L1) wound on R49? You need it, especially if the leads to the output transistors are not real short.
As holojoy states, if the thermal diodes are not in good contact with the heatsinks, you can get thermal runaway. Use silicon grease to ensure good thermal contact.
The gain mismatch would not cause this problem.
Good luck!
Jeff R.
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