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TC Phono-Pre Reverse-Engineering/Mods/Tweaks
OJneg:
--- Quote from: Solderdude on September 01, 2015, 04:58:45 AM ---On top of Merv's recommendations may I add the following suggestion.
(Although that may change the actual RIAA correction)
I would change point where the RIAA feedback comes from, from B Q3 to E Q3.
Reason: the output R of Q2 is rather high (and is why Q3 is there).
For low frequencies the load of the RIAA is no problem but for the highest frequencies the load drops to around 3k which results in a different gain for Q2.
On E Q3 the output R is MUCH lower and the difference in load is moot.
Should the RIAA be designed to take this into consideration it may be off in that case.
A simple matter of measuring the response
--- End quote ---
I'd wager they accounted for that but would be interesting to try.
Solderdude, what do you think of the path going from E Q2 to B Q1. Obviously it's being used to bias Q1 but I can't figure the purpose of R9 and C7. Is that another part of the RIAA curve? TC =.263s -> 3.8Hz
ultrabike:
Take this with a grain of salt OJ, cuz I have not done a lot of work on discrete amplification. But it seems that's negative feedback (perhaps due to lots of gain). It could be frequency dependent giving perhaps more feedback at low frequencies (< 100 Hz) where the C7, R8 and R9 network tend to R8, than at high frequencies (> 100 Hz) where the network tends to R8||R9.
Solderdude:
The only possible reason I can think of is that it may be of 'help' during power up because of the charging of R3,C3 and C7 of Q2 ?
It is indeed for DC-biassing the circuit only, there is no AC gain determined in that circuit.
The frequency range where it is 'active' is even below turntable rumble frequencies and too small in gain 'lift' to be of influence the audible range.
Normally, gain for the lowest frequencies needs to be reduced (when following RIAA) but this is done here by R3, C3
The input and output caps are designed to introduce no phase shifts or roll-off.
Chris F:
Is C1 the capacitance loading? I don't think I have seen a modern MM cart that didn't prefer less capacitance so you can probably get rid of it entirely or reduce it to as little as possible.
Marvey:
--- Quote from: Chris F on September 01, 2015, 01:54:48 PM ---Is C1 the capacitance loading? I don't think I have seen a modern MM cart that didn't prefer less capacitance so you can probably get rid of it entirely or reduce it to as little as possible.
--- End quote ---
Yes. I removed it on mine.
--
May as well start from scratch with a passive RIAA than mod the circuit. It works, sounds good. Just replace a few key parts with better quality components. Even then, the biggest improvement will be real changes like replacing the tiny SMPS with a good power supply.
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