More Discrete Opamp

[analag]

This is a schematic of one of my old battle horses. Good for driving 600:600 trannies.

analag

 

[Samuel Groner]

It looks to me as if Q8 needed an emitter resistor (preferably bypassed with a 10 nF for best phase margin) or another current limiting mechanism if the short-circuit protection is to work.

Nonetheless Q8 might die with a short. To solve this you'd need to increase the value of R7.

Adding the emitter resistor should improve slew symmetry as well.

Samuel

 

[Svart]

looks like a melcor with current sinks added?

 

[Samuel Groner]

I'd vote for a 990 "upside down". The Melcor has a resistive load for the first stage and a balanced second stage which makes things somewhat different.

Samuel

 

[analag]

http://www.dself.dsl.pipex.com/ampins/discrete/discrop.htm. I have never used an emitter resistor in the VAS stage. It does nothing, really. Decreasing the current through the CCS cuts the output drive capability. I like at least 6ma standing current flowing through there.

 

[tk@halmi]

[quote author="analag"]I have never used an emitter resistor in the VAS stage. It does nothing, really.[/quote]

Of course it does. It provides local feedback that has many known benefits. It does not rob the gain much as the composite gain of Q7 and Q8 is 10000+ and needs to be reined in with AC feedback through C1. You can always adjust the current back to where you want it with R7. But why mess with if it works?
I like that your amp uses easily obtainable parts. Do you put it on a carrier board or you make it part of something larger?

 

[analag]

[quote author="tk@halmi"]I like that your amp uses easily obtainable parts. Do you put it on a carrier board or you make it part of something larger?[/quote]

Both.
Once again I'm making (2), to use as the output amp in a Urie 546 type clone I'm currently sourcing parts for. I also have the idea of replacing the input electronic balancing with transformer balancing.

analag