6 channel DIY amp help

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Hello, I am designing a 6-channel class A/B amp to power my DIY active loudspeaker and was hoping to get some help on the design.

I believe I'm going to use this case from the diyaudiostore

Dissipante 5U – diyAudio Store (BETA)

I want to use this schematic as the basis for the circuit I want to use.


A couple questions I have about the schematic are, the 100ohm 1 watt resistors just to the left of the output stage, what is their purpose? They seem unneccesary since the differential pair has a current mirror and current sink.

Also, if I'm not mistaken, the 1k potentiometer below Q4 is for biasing the circuit so that the idle current is just right, and that is measured by measuring the voltage drop across either of the .22ohm resistors tied to the drains of the output transistors.

I mostly just want to understand some of the components of this circuit before I use it, and then make some mods to my taste and use that.

Also I don't expect to use all of the power from all 6 channels at once so I'm planning on making the amp "4-ohm usable" which should give 200watts peak output with the ~50 volt rails. Then I want to use a 1000VA transformer, which technically would saturate with all channels driven to peak, but to my belief this is a non-issue except in extreme cases where all channels peak all at once. Also will the heatsinks on that case be able to handle this dissipation? I don't expect to have much sustained power output compared to the 200WPC rating, I just want that for transients so my assumption is that the heatsinks should hold up just fine in all but extreme cases.
Also how necessary are bypass capacitors? I've done some research into it that certain capacitor types have significant advantages, but also that bypass capacitors with much smaller values won't really make a noticeable difference. What is the purpose of bypass capacitors if there is one?
The 100ohm resistors will have a voltage drop across them. This will decrease the supply voltage to the voltage amplifier stage. It means that the VAS will clip before the output stage.

Bypass capacitors are always a good idea. They keep out unwanted noise from the PSU etc.

The small capacitor in parallel with the large one is there for the reason that the big one has inductance that is cancelled out by the small one.
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The 100R resistors are a form of RC decoupling with the 47uF capacitors.

TBH correct bias for mosfets is a nightmare *, and simply put your channels
will not swing rail to rail due to the high drive voltages needed for mosfets.

As drive voltage is very related to output current your amplifier will
automatically reduce its voltage swing when faced with 4 ohm loads.

rgds, sreten.

* There is no optimum setting, just run it as warm as you feel comfortable.
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