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I've not heard this yet. I bought a clean looking C4/M4 set on ebay and have been rebuilding it. I chose the C4/M4 after scouring through schematics on full discrete units with accurate, well designed RIAA sections as I was looking to upgrade my current preamp, which was one I made back in the 80s and has served me very well over the years.
The C4 stopped me in my tracks. It has the most unusual RIAA/Phone Eq circuit I've ever seen. A really nice MC stage of parallel transistors on input to keep noise down, then a really unusual feedback system in the RIAA stage that reamplifies the output with a reverse RIAA equalization and feeds it back into the input via an array of resistors. I had to get my hands on one.
I've not heard it yet, but I'm really looking forward to finishing. I've recapped throughout and hand picked the crucial caps and resistors, especially the RIAA time constant components. Initially I just matched their values, but on redoing the maths, the 75us time constant is bang on, but they compromised on the 3180us (and therefore the 318 time constant). The 3180us is easy to sort, but affects the 318us as they are interellated, so I'll come back to it later.
What is really interesting is the reverse RIAA amp that in addition to -ve feedback in the phono eq amp section, has a separate feedback loop to the input resistors. I'd be interested in anyone that happens to have looked at this as well. My thinking is that the reverse RIAA amp is there to create a virtual input resistance. Rather than have switchable input resistors of 100/33k/47k/68k&100k, (the actual resistances are around 10 times these values), they feedback the -ve output signal back to the input via the reverse RIAA amp so that the input 'sees' these input resistances (voltage divider across a resistance 10 times larger across a signal -10 times the value looks like the correct input resistance) but without the Johnston noise. So around a 3db decrease in noise.
I'm really looking forward to hearing this once all sorted.
Rating:  [4 of 5 Stars!] |
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