Iddle current setting
I finished building the M250 but I'm having some problems to set the iddle current. The amp is working fine but I have 1.92 mA flowing trough the collector of T3. When I try to set it to 1.8 mA as in the original schematic the amp starts consuming a lot of power ( I have a 100W lightbulb in series for test). When I move P1 the iddle current doesn't change for almost all the possible setting and the suddenly there is a drop of current to 1.8 mA but then the amp drows a lot of power from the line.
I'm using 5000 uF caps in the PSU and noticed that at higher volumes the voltage drops too much ( from +/- 52V iddling to +/- 40 volts) . Are the caps not big enough ?
Any help with this is welcome.
I finished building the M250 but I'm having some problems to set the iddle current. The amp is working fine but I have 1.92 mA flowing trough the collector of T3. When I try to set it to 1.8 mA as in the original schematic the amp starts consuming a lot of power ( I have a 100W lightbulb in series for test). When I move P1 the iddle current doesn't change for almost all the possible setting and the suddenly there is a drop of current to 1.8 mA but then the amp drows a lot of power from the line.
I'm using 5000 uF caps in the PSU and noticed that at higher volumes the voltage drops too much ( from +/- 52V iddling to +/- 40 volts) . Are the caps not big enough ?
Any help with this is welcome.
Did you design your own PCB or did you use Marcus's ? What transistors did you use?
This sounds like oscillation to me.
This sounds like oscillation to me.
Hi Jaycee thanks for the reply. I'm using Chemalec schematics with a couple of modifications , I used the original transistors but I'm not sure about they being REALLY originals and not fakes, and just 3 power transistors per rail instead of the 4 suggested in the schematics. I built this board for an old Peavey Bass Guitar amp and used the original heatsink, thats why I used just 3 power transistors. I'm using also the transformer from the same equipment and thats why I'm working with +/- 43 Volts rails. So far it seems to work fine, I set the quiscient current to 10 mA now trough each output transistor, I'm going to try to hook it up to my computer running a sine wave generator software to check the crossover and watch the output with my osciloscope to see if there is any change tweaking the bias pot. Another question: there is not suposed to be a resistor in series with P1 just to avoid burning the output devices ? Someone might turn the amp on with P1 the wrong way the first time and end up with blue smoke. Some reason not to put a resistor in series ?
The amp is not oscilating, actually is working pretty good, the only thing is that for me the current trough T3 is more like 1.9 mA. Someone has an idea of what is the right voltage across each .33 resistor ?
Congratulations to Marcus, very easy to build and very simple design, I have a gig tonight and I'll try it with my bass guitar cabinet to see how it performs.
The amp is not oscilating, actually is working pretty good, the only thing is that for me the current trough T3 is more like 1.9 mA. Someone has an idea of what is the right voltage across each .33 resistor ?
Congratulations to Marcus, very easy to build and very simple design, I have a gig tonight and I'll try it with my bass guitar cabinet to see how it performs.
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