And noise is gone with good waveform again.

getting the dc working points right is just the 1st step in designing a good amp.

next, you may wish to explore its ac stability - it shouldn't be a problem given that the amp does not have a global feedback loop.

after that, you make sure that it is thermally stable.

and then figure out a way to explore its ac performance: manage its thd composition, response time / slew rates, over-driven behaviors, resistance to power supply ripples, or outside interferance, etc.

all those fun things.

 

The bandwith is 100 MHZ.
Protek 6510.
Dual trace.

The reason for all this other info, because I needed more words, in this post to be able to post it.

When you were experiencing the apparent RF interference, did you turn up your scope's sweep speed and try to zoom in on a single cycle of the output of your amplifier? You could determine the dominant frequency of the interference that way. Maybe there is a radio or TV station near you causing the unwanted pickup.

 

So Millwood, what configuration would you suggest for a audio amplifier? CC, CE, or CB?

 

A power amplifier has a complementary (NPN and PNP) emitter-followers (CC) output operated in class-AB.

 

When you were experiencing the apparent RF interference, did you turn up your scope's sweep speed and try to zoom in on a single cycle of the output of your amplifier? You could determine the dominant frequency of the interference that way. Maybe there is a radio or TV station near you causing the unwanted pickup.

No, I never thought about checking the freq. of the interfering noise.

But as far as rf interference, lots of times,before,I would build a quick 3 stage CE amp. and by the time I get to the input stage I would have the thing so sensitive that I would many times pick up radio broadcast on the output speaker, so mostly I would just throw a disc cap right at the base of the input stage and that took care of it.

Then I would get the acoustical feedback between speakers. input and output. (I liked that) That's the sensitivity I would get on these home brew circuits.

 

So Millwood, what configuration would you suggest for a audio amplifier? CC, CE, or CB?

it depends on what kind of audio amps you are designing.

in a typical Linn 3-stage design, you have a differential pair upfront - a form of common emitter amplification.

the voltage amplification stage, if cascoded, utilizes common base amplification.

the output is mostly common collector, but can be a combination of emitter follower / common emitter (JLH1969 for example, SRPP, or those quasi complimentary topologies), or common emitter / source - transformer coupled output stages for example.

and that's only for one form of analog amps.

so depends on what you are trying to do, you can have many combinations and it is impossible to say which one is the best to use.

As to amp classes, it can be any of classes.

 

No, I never thought about checking the freq. of the interfering noise.

But as far as rf interference, lots of times,before,I would build a quick 3 stage CE amp. and by the time I get to the input stage I would have the thing so sensitive that I would many times pick up radio broadcast on the output speaker,

that's typically due to excessively high input impedance. Unless I have to, I usually set my amp's input impedance between 47k, and typically in the 10k - 22k range to minimize stray interference.

input impedance higher than that does you no good.

 

getting the dc working points right is just the 1st step in designing a good amp.

next, you may wish to explore its ac stability - it shouldn't be a problem given that the amp does not have a global feedback loop.

after that, you make sure that it is thermally stable.

and then figure out a way to explore its ac performance: manage its thd composition, response time / slew rates, over-driven behaviors, resistance to power supply ripples, or outside interferance, etc.

all those fun things.

Some time I'm going to try to make a half decent audio amp, using power trans. needed to drive the output, and try to design it so that trans. bilaterall effect (output affecting the input) is not as prominent, see what I can come up with.


Like I said before these other circuits, were just homebrew stuff thrown together based on BIAS points, only, but after reading all these posts you written has made be well aware that there is a lot more to it then just getting the bias right.

Thanks again

 

that's typically due to excessively high input impedance. Unless I have to, I usually set my amp's input impedance between 47k, and typically in the 10k - 22k range to minimize stray interference.

input impedance higher than that does you no good.

That's good to know, thanks,
here I was always assuming, I need to get that input up as high as possible, no wonder I'm running into so much rf sensitivity, by the time I get to the input stage.

 

with high impedance, the pick up wires, the pcb traces will act like antenna and the b-e junctions as a diode so you may pick up broadcast signal that way.

sometimes a simple remedy in cases like this is to ground the input with a small resistor, anything in the 10k - 47k range will do.

 

Like I said before these other circuits, were just homebrew stuff thrown together based on BIAS points,

and getting the bias points right is a good starting point. many "experts" don't even know how to do that, .

now, you just need to take it to the next levels.