V-I limiter problems

[James UK]

Hi,

I've been trying to add a V-I limiter to an amp design but I can't get it to work properly in the simulation. It's intended to limit the output current to 8A and it is doing that on the positive half-cycle. However, there is no effect on the negative part. I can't figure out what the problem is. Any ideas?

Thanks,
James

 

[stanislav1957]

This protection affects the sound of the amplifier. Why are analogs of TFR not used in defense ? An example of such a latch on VT6/VT7 and VT8/VT9

 

[MarcelvdG]

Just a hunch: the current that Q7 can draw during clipping is large, while Q8's current has a well-defined upper limit. Maybe Q15 loses from Q7.

 

[ximikas]

Q8 is current source , while Q7 has not limited output current. This is drawback of asymmetric design.

 

[James UK]

Thanks for spotting that! I'll add a current limiter to the VAS, hopefully this sorts it. I've seen a lot of circuits that use VI limiting but omit any VAS current limiting. Presumably they encounter the same problem.

Thanks,
James

 

[MarcelvdG]

It could also blow up Q7 and Q15 (R13, D4) during current limiting, and Q6 during clipping.

Any emitter resistor you add to Q7 will aggravate the right-half-plane zero that destabilizes the feedback loop. I don't know if that is bad enough to be an issue. If it is, you could connect a resistor with equal value in series with the Miller capacitor or put a relatively small decoupling capacitor across the emitter resistor.

 

[James UK]

I've added some current limiting to the VAS. The VAS emitter resistor had to be increased to 33R to limit the current on the negative swing to 8A.