I need t set the bias on a Genesis mini block, the blue trim pot appears to be just turning, I think it's a 25 turn pot, i believe you should be able to feel when it's close to being wound fully back as the screw becomes a little tighter?
I there any way of testing the trim pot is ok before I order a new one.
Also do I set the bias by reading the voltage in Mv across the output emitter resistors with no signal or load to the amp?
Thanks in advance
I there any way of testing the trim pot is ok before I order a new one.
Also do I set the bias by reading the voltage in Mv across the output emitter resistors with no signal or load to the amp?
Thanks in advance
with the power off, you can measure the resistance of the adj pot with a meter. Turn the screw one way then the other, the value should change. I dont know the brand, it could be 10 or 20 turn. I can usually hear a faint click when I go past either limit. Check the pot with the meter and see if there is an open spot as you go from one extreme to the other.
I checked the pot resistance and with 11 turns clockwise it was down to 0.01 ohms
I assumed this was as close to zero as I could get.
So I powered up the amp and it blew an irf540, I've replaced the irf540 and turned the bias pot counter clockwise 10 turns and the amp powers up o.k again.
I was checking the voltage across the emitter resistors with the meter set to mV and the voltage measured zero no matter which way I turned the pot, when I had turned close to the 0.01 ohm point (11 turns clockwise)- that's when the irf540 blew?
Any ideas what's causing the output failure, I would have thought the 0.01 ohm point would be the same as turning the gain pot fully down?
I assumed this was as close to zero as I could get.
So I powered up the amp and it blew an irf540, I've replaced the irf540 and turned the bias pot counter clockwise 10 turns and the amp powers up o.k again.
I was checking the voltage across the emitter resistors with the meter set to mV and the voltage measured zero no matter which way I turned the pot, when I had turned close to the 0.01 ohm point (11 turns clockwise)- that's when the irf540 blew?
Any ideas what's causing the output failure, I would have thought the 0.01 ohm point would be the same as turning the gain pot fully down?
Can you read a or adjust the bias voltage on the working channel?
On a typical classAB amplifier the bias adj pot is between the base and emitter of the bias transistor. If the pot goes open, the transistor will remove bias to the power transistors to protect them. Do you have a schematic that you can post here?
On a typical classAB amplifier the bias adj pot is between the base and emitter of the bias transistor. If the pot goes open, the transistor will remove bias to the power transistors to protect them. Do you have a schematic that you can post here?
You set the pot to minimum and turned off the bias transistor setting the bias to max. Pot should be at max value if it is between base and emitter.
Doesn't sound like you are using a light bulb tester or a current limited bench supply to test your amplifier.
Doesn't sound like you are using a light bulb tester or a current limited bench supply to test your amplifier.
best when you measure voltage across output emitter resistors with no signal (grounded inputs), output left without speakers - open
set it first time, play music and than measure again when heatsink is in working temperature
set it first time, play music and than measure again when heatsink is in working temperature
The amp is a mono block, I don't have a schematic.
I didn't use a current limiter as the amp had been working fine, it was just getting a little hot after 10 mins of use.
I didn't use a current limiter as the amp had been working fine, it was just getting a little hot after 10 mins of use.
I'd recommend monitoring the current draw from the 14v supply, set the bias so the draw is 2,4 amps. this is optimum.
rrae - with the Power Supply set to 14v the amp draws 1.5 amp, adjusting the bias pot made no difference to the current draw.
Thanks Perry I'll give Genesis a go
Thanks Perry I'll give Genesis a go
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