Monoblock design - left and right bank of output devices

lae2 · 5th March 2010, 05:32 AM

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.

AndrewT · 5th March 2010, 10:40 AM

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%.

PMA · 5th March 2010, 12:11 PM

Beware of inductances.

lae2 · 5th March 2010, 01:19 PM

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.

5th March 2010, 05:32 AM	#1
lae2 diyAudio Member Join Date: Jan 2010 Location: Hershey, PA	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.

5th March 2010, 12:11 PM	#3
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Beware of inductances. __________________ Pavel Macura http://www.linkedin.com/pub/pavel-macura/4/783/637 http://web.telecom.cz/macura/audiopage.html

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5th March 2010, 10:40 AM	#2
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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%.

5th March 2010, 01:19 PM	#4
lae2 diyAudio Member Join Date: Jan 2010 Location: Hershey, PA	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.