I started with minimal bias (Idle_Pot set to max: 520 Ohm).
Voltage between driver's bases is 8.4V
At normal bias (40mV idle current) it's supposed to be 1V - 2V higher than that.
Now I'm trying the same in the sim (low bias), and it looks like with minimal bias (Idle_Pot 520 Ohm) - it is oscillating in LT Spice.
I'm assuming this - as the ltspice takes forever to finish simulation (been running for 5 min already), which usually means that something bad is happening.
Will play a little with sim, and I guess with try with higher/normal bias. Obviously, it sims OK with normal bias.
So far I've been testing without input, and without output load.
Voltage between driver's bases is 8.4V
At normal bias (40mV idle current) it's supposed to be 1V - 2V higher than that.
Now I'm trying the same in the sim (low bias), and it looks like with minimal bias (Idle_Pot 520 Ohm) - it is oscillating in LT Spice.
I'm assuming this - as the ltspice takes forever to finish simulation (been running for 5 min already), which usually means that something bad is happening.
Will play a little with sim, and I guess with try with higher/normal bias. Obviously, it sims OK with normal bias.
So far I've been testing without input, and without output load.
Last edited:
If you increase the capacity of c10, you can catch trouble. the resistor is better to start with 5-10.
With higher bias is oscillating/behaving the same way. So that's not the issue.
So now looking at R20, C7, C10, C2, C8.
When you say "Turn off the limiter diodes temporarily" you mean
something like "remove R4" ?
The sim doesn't work with R4 removed completely.
I guess you mean removed diodes only.
R4/C7 is needed for compensation.
But I think D3/D4 shouldn't matter too much, they just add several pF of capacity to C7, that's all (as far as problem of oscillations goes).
So this amp has 3 RC compensation networks:
C2/R30, R4/C7, R9/C8
and possibly 4th one: R20/C10
So now looking at R20, C7, C10, C2, C8.
When you say "Turn off the limiter diodes temporarily" you mean
something like "remove R4" ?
The sim doesn't work with R4 removed completely.
I guess you mean removed diodes only.
R4/C7 is needed for compensation.
But I think D3/D4 shouldn't matter too much, they just add several pF of capacity to C7, that's all (as far as problem of oscillations goes).
So this amp has 3 RC compensation networks:
C2/R30, R4/C7, R9/C8
and possibly 4th one: R20/C10
Question - this is 1st time I'm testing amp with NFB returning to the input signal (op-amp (+) pin).
R1 + R26 (resistors at the input) will determine the gain.
Shouldn't I connect input to the ground when testing?
If nothing is connected to the input of the amp, what's gonna happen to the gain??
If R1+R26 as not grounded by the input signal (pre-amp) the gain will be 0 ?
I wonder why we don't see this solution more often, in other amps? It allows to save on one big, bipolar capacitor
usually connected from NFB to the ground via R, so it should be used more widely.
Perhaps this solution has some other shortcomings?
R1 + R26 (resistors at the input) will determine the gain.
Shouldn't I connect input to the ground when testing?
If nothing is connected to the input of the amp, what's gonna happen to the gain??
If R1+R26 as not grounded by the input signal (pre-amp) the gain will be 0 ?
I wonder why we don't see this solution more often, in other amps? It allows to save on one big, bipolar capacitor
usually connected from NFB to the ground via R, so it should be used more widely.
Perhaps this solution has some other shortcomings?
Last edited:
