BPA300 & PA150 setup on the bench

[AndrewT]

the 0r2 resistor separates the two chipamps and separates the load from the chipamps.
Are you using the 0r2 as the LOAD?

[Mark Allen]

Since we solved the overshoot problem my amps have been running flawlessly, no measurable drift to panic about on the output when bridged. You only see the difference of the two. They also sound very, very nice. I've toyed with the idea of trying different resistors in the .2 ohm position because I noticed that Jeff Rowland uses alightly higher value resistors @ .5 ohm on the big LM-3886 power amp. I believe this value would cause less interaction between chips. I'm still thinking of an added servo as being the right way out completing this project.

[toufu]

Pardon my technical ignorance, I built the BPA150, are you saying that we are suppose to short out the trimmers to get it right? Thanks.

[Ted205]

well i've managed to source the problem to the zobel which i have removed for now.


thanks for the help people.

[pacificblue]

'The' Zobel? PA150 should have three Zobel networks, one at each output, before the load sharing resistors.

[Ted205]

the schematic can be found here - DIY BPA300 6x LM3886 300W audio Amplifier

only one zobel network is used

[shmulik]

Mark,
I think these resistors influence the output impedance of the amplifier. the single chip LM3886 amp has a very small (less than 0.01Ohm) impedance meaning very high damping factor (may be governed by the wires resistance). adding resistors enlarge the output impedance. I made a rough measurement of my modules' impedance by comparing the output voltage without and with a load resistor and reach 0.15Ohm figure.

[AndrewT]

Hi,
damping factor with Rout=0r01, cable R=0r08, speaker 8ohm DF~89
damping factor with Rout=0r01, resistor 0r22, cable R=0r08 speaker 8ohm, DF~26
3parallel Rout=0r01, resistor=0r22, cable R=0r08, speaker 8ohm DF~51

The damping factor of a typical single chipamp could be around 70 to 100 with 8ohm. A 3parallel with large balancing resistor will have a reduced DF of around 35 to 55.

I doubt you will hear much difference. Some speaker simulators can plot this change of low bass response with added output impedance. Try it. The results may surprise you.

[shmulik]

I can hardly relate measurements to sound and have no sense for correlating between the magnitude of an effect plotted to how it distorts the sound, so I suspect the simulation graphs will be wasted on me. However, if you have a link at hand please share.
The predecessor of the BPA300 had a DF of 20 and it did not bothered me a bit, though I never confronted it with difficult speakers.