ALEPH30 QUESTIONS

I am about to build the Aleph30 from Volksamp. The circuit is slightly modified as compared to Pass Labs Aleph3. Could someone throw some light on the values of R1 and R8. In the Volksamp schematic, R1 is connected between the Source of Input MOSFET Pair and Drain of Q2/Negative Power Rail. R8 is connected from Gate of Q2 to Input pair Source and another R8 is in shunt with C11 in -IN line.

Any ideas on replacing Z3 9v1, with a transistor and forming a current source in conjunction with Q3 in order to reduce zener noise flowing into the input pair?

My simulations with Electronics WorkBench 5.0 showed that the square wave response is better with C8 10pf cap removed altogether and a capacitor about 47pf or higher connected in shunt with Z1 and Z2. Any comments?

Thanks everybody.
 

djk

R.I.P
2001-02-04 4:23 am
USA
Garbage in garbage out.The zener diodes only make noise when they conduct.They will never conduct under normal audio use.Simulation programs are worthless for analog.Build it and then adjust the frequency compensation cap.If you're not going to use the balanced input and you don't like seeing that cap then change it to the Aleph 3 feedback scheme.The extra resistor in shunt with the feedback cap reduces DC off-set.Again if you don't like it change it to be like the Aleph 3.
 
djk, thanks for the reply. However, I wasn't referring to the back-to-back connected zeners which protect the input mosfet pair. My reference was to the zener connected to the positive rail which together with the relevant mosfet forms the current source. Hence, my question about changing the topology of this current source for a 'better one' still stands to reason and ideas/comments from anyone.
 

djk

R.I.P
2001-02-04 4:23 am
USA
I see part of the problem.On the Aleph 30 schematic page 16 Z3 is conneced to the diff pair Q1,2 and Z5 is connected to Q3.If you don't want to use a zener for Z5 you could replace it with a 2.7K resistor and change R13 to a J-FET with the IDSS trimmed to 3.3mA .That would give the same voltage and current in the circuit as it is running at now.Pass did something similar to this in the A40 .