I am challenging myself at understanding my integrated amp's MM/MC phono stage; The amp is a Rega Elicit mk. I. It is a somewhat complex design and as a hobbyist I'm out of my league. The circuit looks a bit like an op-amp built out of discrete parts, so I have taken the typical op-amp block diagram as an inspiration and color-coded the sub-circuits I think I recognize but I'm not certain of anything except the constant-current sources and sinks as I recognize their LED voltage references.
Sorry for the poor image quality, the amp is more than 30 years old and all I have are those scans of the schematics (They're not even of the right board revision but are close enough). Image taken out of Hi-Fi Engine Library : https://www.hifiengine.com/hfe_downloads/index.php?rega/rega_elicit_schematics.pdf Below are the block outline legend:
- Blue: Darlington pair emitter follower current buffers
- Red: Constant current sources and sinks
- Brown: Cascaded emitter follower/common base differential input stage
- Green: Level shifter
- Cyan: Class AB output stages
- Orange: Long-tail pair diffential amplifiers with constant current sources and current mirror active loads performing single-ended to push-pull conversion
- Gray: Unknown configuration...
I would appreciate any help in correctly identifying the sections and type of the building blocks so I can study each sub-circuit in depth. I am in the process of porting the schematic into LTSpice but I see no point in trying to analyse a simulation if I don't understand the basics first.
I have also a couple unresolved interrogations:
- Why use multiple transistors in parallel for each half-phase of the input? I would guess to lower the output impedance and possibly "swamp" any Hfe variations?
- How is the RIAA equalization performed?
Many thanks in advance.