Musical fidelity A120 A200, how to DIY increase bias current (more CLASS A)

[ygg-it]

Hello,
please find attached A200 schematic.

Musical Fidelity A200 - Audiomaniacy

I own Musical Fidelily A120 which has more or less same schamatic as A200.

As you may know Class A is limited to a few watts beyond which (A1, A100, A120) switch to class AB.

Which component should I have to change and trim to SAFELY increase the bias for a wider class A range?

Actually is shown 300mV on 0.47 ohm, that is 0.64A only!
Definetively not a 40+40W Class A as declared.

 

[analog_sa]

These amps like dying from a heatstroke with the factory bias

Unless you rebuild it using a proper PS and proper heatsinks you should leave the bias alone

 

[duncan2]

"Strongly biased into class A" isn't pure class A .

A hint is the bias you measured in the link I provided the version quoted is biased at 700manot much different from your measurements .

It therefore reverts to to class AB at about (his version) 8watts.

Musical Fidelity A1 › Technical

 

[A 8]

These amps like dying from a heatstroke with the factory bias

Unless you rebuild it using a proper PS and proper heatsinks you should leave the bias alone

Very true!

 

[ygg-it]

Thanks, I know.

But can you please indicate which is the resistor that sets the bias?

 

[NickKUK]

Just looking at the face of my A220 "Class A", manual states Class A too.

Looking at the schema - the MF A2/A220 is a chain of two differential amps with a Push Pull with 2 devices each side for each channel.

It looks class AB1 to use a tube analogy.

 

[ygg-it]

So, I need an analog expert that kindly tells me which are the resistors that trim the bias. Anybody here?

 

[as_audio]

In this kind of circuit no resistive bias trim.

 

[analog_sa]

You cannot do it with a single resistor. The drivers emitter resistors are one option.

 

[analog_sa]

Another option is to copy the B1 setup, as the topology is nearly identical

 

[ygg-it]

Thanks, so in my linked scheme of A200, bias should depend on resistor R30/R31, R36/37, R61/62, problable the last four, correct?