Class A amplifier, full Dual Mono

Hi,
ad. 1. Potentiometer R9 on the power amplifier board,
ad. 2. I set the offset so that "cold" has about -100mV, after heating it has +20mV,
ad. 3. Loaded with 6 ohm speakers, after an hour of moderate play,
ad. 4 and 5. The given values were calculated (with a margin for drops in the bridge, pulsations, etc.), then checked in practice (several watts more),
ad. 6 and 7. I was playing it safe - the transformers are very close to the transistors of the voltage amplifier, and since JFETs work there, I was afraid of hum. Steel sheet, because ferromagnetic, brass on top to improve the visual experience (flatter the ego),
ad. 8. About 35dB - the value of the feedback resistors of the preamplifier and power stage was changed relative to the diagram,
ad. 9 and 10. The amplifier with the connected generator was loaded with resistors made of resistance wire (4 and 8 ohms), the voltage value was measured at the output (power control), and the shape of the voltage was checked on the emitter resistors (indirectly, the current). I assumed the shape of the emitter current as the limit value, which, at the peak of the sinusoid, did not drop to zero yet, but was already visibly deformed (cross distortion) - this occurred when the emitter current decreased to about 40mA.

Ps. Thanks again my friend aaantek for helping me half a year ago (of course thanks also to my friend jozgo)

Maciek_C wrote:

ad. 3. Loaded with 6 ohm speakers, after an hour of moderate play,

I was also concerned about the ambient temperature. And since it's class A, there should be no difference with and without load. As one of my colleagues mentioned earlier.

Maciek_C wrote:

ad. 4 and 5. The given values were calculated (with a margin for drops in the post, pulsations, etc.), then checked in practice (several watts more),

Practically it's like: - for the whole band, for 1kHz on what load(s)?

Maciek_C wrote:

the transformers are very close to the transistors of the voltage amplifier, and since JFETs work there, I was afraid of hum. Steel sheet, because ferromagnetic

Have you thought about the magnetic permeability of steel at 50 Hz?

Maciek_C wrote:

and the shape of the voltage (indirectly the current) was checked on the emitter resistors. I assumed the shape of the emitter current as the limit value, which, at the peak of the sinusoid, did not drop to zero yet, but was already visibly deformed (cross distortion) - this occurred when the emitter current decreased to about 40mA.

Wait a minute, but how does this relate to going from A to AB? Cross distortions concern class B - no quiescent current or its insufficient value.
Did you use the transistors in the diagram, including the TIP3055/TIP2955 power transistors?

aaanteka wrote:

Maciek_C wrote:

ad. 3. Loaded with 6 ohm speakers, after an hour of moderate play,

I was also concerned about the ambient temperature. And since it's class A, there should be no difference with and without load. As one of my colleagues mentioned earlier.

Room, approx. 23 degrees.

aaanteka wrote:

Maciek_C wrote:

ad. 4 and 5. The given values were calculated (with a margin for drops in the post, pulsations, etc.), then checked in practice (several watts more),

Practically it's like: - for the whole band, for 1kHz on what load(s)?

1kHz, load as in point 9 and 10.

aaanteka wrote:

Maciek_C wrote:

the transformers are very close to the transistors of the voltage amplifier, and since JFETs work there, I was afraid of hum. Steel sheet, because ferromagnetic

Have you thought about the magnetic permeability of steel at 50Hz?

NO.

aaanteka wrote:

Maciek_C wrote:

and the shape of the voltage (indirectly the current) was checked on the emitter resistors. I assumed the shape of the emitter current as the limit value, which, at the peak of the sinusoid, did not drop to zero yet, but was already visibly deformed (cross distortion) - this occurred when the emitter current decreased to about 40mA.

Wait a minute, but how does this relate to going from A to AB? Cross distortions concern class B - no quiescent current or its insufficient value.

Maybe I named them wrong. My point is that near the transistor plugging area, the voltage across the emitter resistor was starting to deform noticeably.

aaanteka wrote:

Did you use the transistors in the diagram, including the TIP3055/TIP2955 power transistors?

Yes.

In general, the moment of transition to class B can be detected by measuring the current drawn by the output stage from the power supply. As long as it works in class A, the current consumed practically does not depend on the drive, after switching to class B it starts to increase with the drive.

Great tip, hadn't thought of that, thank you