George,
The 32 (and partly) 52 amp topology is as follows:
A 4 OPAMP instrumentation amplifier is placed directly after the input selector.
It is used by both the XLR and RCA inputs, where the RCA ground is a bit floating (not hard grounded, to get an unbalanced to balanced conversion.
This input stage is where the gain is adjusted. (o, 6 12 and 18dB)
This stage feeds the floating volume control board. A 16 bit R2R ladder network with a al the SMD resistors and fet switches into an AD797 I to V conversion. (this partly SMD board in the 32 is reused in the 320 and 326s)
The output of the balanced volume control directly feeds two identical partly discrete output buffers.
It also feeds in parallel an opamp that feeds the RCA output.
The output stage of this buffer uses a matched pnp and npn transistors (LS3250A and LS3550A) in a metal TO71 can. (you can see them close to the output connectors on a picture where the input relay board is removed)
These transistors are the class A output transistors but to keep them cool they applied a (genius) cascode stage based on a current-feedback video opamp ADEL2020.
The beauty of this configuration is that the miller capacity (base to collector) is almost removed and the power dissipation is moved to the ADEL2020 (only 0,3W).
The DC offset of this stage is removed by an integrator dual opamp in the middle.
So these two buffers sit between the output XLR/RCA connectors and the floating volume board over the complete width of the mono board (left and right) so about 1/3 of the total board space.
It is a bit technical sorry, but this is how it works in the 32.
Peter
|