Hello, I have a trouble in the VAS stage of an amplifier.
The amplifier is powered by +10V _ GND _ -10V. The voltage at the Q5 base should be about 9.2V as simulator said, but it's only about 1.3V.
Measuring under bench power supply, R2 seems to be shorted. And also C1 seems to be shorted, but only in a direction: from the Q5 base to output stage.
The transistor are KSC1845 (Q5) and KSC3503 (Q6) (EDIT: Q6 is KSA1381, KSC3503 is the complementary NPN). Measuring with the amplifier off, there is no continuity between the pins of the same transistor, so I imagine they are healthy, especially because they have had the same effect since the first power on.
Should the scheme work? Does this configuration have any error?
The amplifier is powered by +10V _ GND _ -10V. The voltage at the Q5 base should be about 9.2V as simulator said, but it's only about 1.3V.
Measuring under bench power supply, R2 seems to be shorted. And also C1 seems to be shorted, but only in a direction: from the Q5 base to output stage.
The transistor are KSC1845 (Q5) and KSC3503 (Q6) (EDIT: Q6 is KSA1381, KSC3503 is the complementary NPN). Measuring with the amplifier off, there is no continuity between the pins of the same transistor, so I imagine they are healthy, especially because they have had the same effect since the first power on.
Should the scheme work? Does this configuration have any error?
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Total power consumption is 0.08A per power rail (total 0.16A for the two power rails).
Current limit for testing is 0.2A, so it's no limited.
Total power consumption with class-B bias must be about 0.01A.
Yes, IN node is the amplifier input, before that there is a resistor connected to the ground and before that a decoupling capacitor.
FEEDBACK is connected with two resistor to the amplfiier output, classic feedback network for input pair.
The topology seems ok, maybe someone else can do a full simulation.
Did you try higher power supply voltages than 10VDC?
Normally there would be a large capacitor across the Vbe multiplier from C to E,
but that should not cause the problem here.
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Yes, tried +30/0/-30V. In the simulation is all ok, but in real world the amplifier does not really turn on because the power supplies go into protection. (1.39A absorbed, and not by the output stage, but by the VAS).
So the VAS transistor failed when powered up, both Q191 and Q192?
Did the Vbe multiplier Q188 fail also, or the drivers Q190, Q193?
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Drivers are ok, Vbe Is ok too. Replaced transistor, same problem. Probably, it's only a problem of PCB design.
There was over 1A in the Vbe, but it's ok? Q188 must have saturated, otherwise there would have been too much Pd,
and it would have failed. Check the Vbe pinout and related resistor values. Were the R197 and R203 swapped?
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I say that Vbe Is ok because bypassing it I got the same results. The 0.7k ohm resistor is a potentiometer, varying it drivers and output transistor being polarized, but this does not affect the current absorbed by the VAS.
I know, it's strange, that's because I asked
I got PCBs from JLCPCB, with route width of 0.8mm, probably too close to the pins. I had already imagined that the problem could be that. However before taking others with smaller and better isolated routes, I wanted to be sure that the topology was correct.
Did you breadboard the circuit before making a pcb, or just do simulation? I've always benefited from prototyping first.
When laying out the pcb, did you use a net list, or route manually? Errors can happen with manual routing.
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You might try some resistance in the Emitters of Q193 and Q194. Two transistors with the same part number will simulate 'exactly balanced', which is almost never the case when you build it.
With CCS loads throughout, the intrinsic V-gain is sky high. Any small imbalance and it will slam as far as it can go since there's minimal resistive load to hold it back.
Glad you're building .. and not just simulating
Rick
With CCS loads throughout, the intrinsic V-gain is sky high. Any small imbalance and it will slam as far as it can go since there's minimal resistive load to hold it back.
Glad you're building .. and not just simulating
Rick
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