I'm just trying to work out this circuit of this JBL 6260...
I notice the VAS seems to run 'hot' in temp and also the drivers Q30,Q19, Q31,Q21 are also hot. These all have heat sinks but I'm surprised how hot these are running, 60 degrees approx. Why do they need to be so hot? All of these are the MJE340/50's I pulled out the drivers and measured the hfe. The NPN's it was very low around 20 but the PNPs were over 100. I put in MJE15030/31's instead which had higher gain.
I take it the outputs are in an emitter follower?
The current/voltage protection seems to be a conventional VI limiter transistors. DC protection seems to involve an opamp that will trigger relay over a determined voltage. There is also delayed startup to avoid thumps.
The interesting thing is these amps have quite a bit of hiss on output even at 0 gain.
Distortion was respectable at 0.0040 @ 1Khz and 0.0051 at 20Khz at mid power.
Bias set at 540mV between base and emitter on the outputs.
I believe this amp was marketed as a 'low feedback' design at a time where TIM was a thing.
I notice the VAS seems to run 'hot' in temp and also the drivers Q30,Q19, Q31,Q21 are also hot. These all have heat sinks but I'm surprised how hot these are running, 60 degrees approx. Why do they need to be so hot? All of these are the MJE340/50's I pulled out the drivers and measured the hfe. The NPN's it was very low around 20 but the PNPs were over 100. I put in MJE15030/31's instead which had higher gain.
I take it the outputs are in an emitter follower?
The current/voltage protection seems to be a conventional VI limiter transistors. DC protection seems to involve an opamp that will trigger relay over a determined voltage. There is also delayed startup to avoid thumps.
The interesting thing is these amps have quite a bit of hiss on output even at 0 gain.
Distortion was respectable at 0.0040 @ 1Khz and 0.0051 at 20Khz at mid power.
Bias set at 540mV between base and emitter on the outputs.
I believe this amp was marketed as a 'low feedback' design at a time where TIM was a thing.
The supply seems to be 67V and the VAS transistors have 22 ohm emitter resistors against a diode-drop, so a very large VAS current of about 30mA seems to be used instead of a more reasonable 5 to 10mA range. That means 2W per VAS transistor.... so yes they will run very hot. The standing current in the differential pairs will reduce that a bit, maybe 7mA or so down to 23mA, but still its a lot at 67V
With decent driver and output device current gains, you only need ~1mA of VAS drive for 10A of output change, and VAS currents can be low and run cooler. I suspect the high VAS current is a way to compensate for poor driver/output device current gain performance under load - for instance the 2SA1170's (PNP outputs) have a current gain of only 20 at 8A. Compared to a modern device like a NJW1302 that's pretty low.
With decent driver and output device current gains, you only need ~1mA of VAS drive for 10A of output change, and VAS currents can be low and run cooler. I suspect the high VAS current is a way to compensate for poor driver/output device current gain performance under load - for instance the 2SA1170's (PNP outputs) have a current gain of only 20 at 8A. Compared to a modern device like a NJW1302 that's pretty low.
Hi Mark, thanks for your input. That did cross my mind that they were running VAS with high current. All I can think of is the designer was trying to make the amplifier as linear as possible before feedback.
Those MJE340/50's were definitely being driven very hard.
Those MJE340/50's were definitely being driven very hard.
I don't think extra current would help with linearity - more linear modern output devices would (aka sustained beta devices), as well as take the strain off the VAS devices by having dependable beta of around 100 upto high current. Its more likely a ploy to ensure the units can drive 4 ohms without premature clipping due to VAS overload?