Monoblock design - left and right bank of output devices - diyAudio
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Old 5th March 2010, 05:32 AM   #1
lae2 is offline lae2  United States
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Default Monoblock design - left and right bank of output devices

Two heatsinks make up the left and right walls of a chassis I hope to use for a monoblock amplifier. How do I combine the output devices from each side? For example, I may use two complimentary pairs per side for a total of 8 output devices. Do I merely feed the drive signal to each bank and then tie the outputs together? Do I then derive a feedback path from the terminal of the combined outputs? Or instead, should there be parallel feedback signals from each bank of output devices? Is it all right that each bank has its own bias adjustments. I can reason how to do this from a looking at a schematic but I am interested in knowing how it is actually done in construction. Thanks.
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Old 5th March 2010, 10:40 AM   #2
AndrewT is online now AndrewT  Scotland
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one bias adjuster should bias the current through all 8 output devices.
The 4 NPN should be matched for Vbe at the actual bias current you intend using. At the same Vbe the Ic match should be <+-5%
Similarly the 4 PNP should be matched for Ic @ Vbe.

If you can also select similar hFE maybe +-10%.
regards Andrew T.
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Old 5th March 2010, 12:11 PM   #3
PMA is offline PMA  Europe
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Beware of inductances.
Pavel Macura
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Old 5th March 2010, 01:19 PM   #4
lae2 is offline lae2  United States
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Thanks for responding. I would like for this construction to be as modular as possible. Can each bank of output devices have its own VAS + Output and then connect the output from each bank to the same (there is no other) "+" speaker terminal with the "-" speaker terminal referenced to signal ground?

Alternatively, is a phase splitter used to invert the signal at one bank and then connect non-inverted bank (output) to the "+" speaker terminal and inverted bank (output) to the "-" speaker terminal?

Thank you for the comment about the single bias adjustment. I can see why that should be.

I am not certain of the inductance problem unless I put all devices on a 24" long heatsink and then cut the heatsink in half along with the wires. I could bridge the wires across across the chassis but that, although electrically sound at DC, does not seem to be a great idea.

There must be a traditional solution considering that many commercial monoblocks have two banks of heat sinks. Thanks again.
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