CHANGSTAR: Audiophile Headphone Reviews and Early 90s Style BBS
Lobby => Vinyl Nutjob World => Topic started by: OJneg on August 31, 2015, 08:18:47 PM
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(http://i.imgur.com/77zK5Gx.jpg)
(http://i.imgur.com/PXAKWEe.jpg)
(http://www.avsforum.com/photopost/data/2326804/7/78/78ef4a67_TCC-TC750-Phono-Preamp-Schematic-1200.png)
More to come
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Replace C2 with higher quality cap. It's an electrolytic. It probably doesn't need to be 10uF.
Replace C8 with a bigger and higher quality cap.
Bring in power to right after R16. No need for D1, R16, C10, C9. (Note both C10 and C9 are marked incorrectly. Boom! if they were actually installed like that.)
Play with C1 and/or R2 depending on your cart.
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Very interested in how this ends up. What's the goal with the mods?
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Very interested in how this ends up. What's the goal with the mods?
Do silly audiophile things. See if they make a difference
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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
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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
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
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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.
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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.
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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.
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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.
Yes. I removed it on mine.
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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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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 ?
Ok this makes the most sense. Keep bias point stable with extra cap and filter any HF noise at that point
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It looks like the Q1-Q2 thing is a convenient way of biasing up the input transistor's base without injecting power supply noise into the input. That also creates a DC feedback loop: Q1's collector sets Q2's emitter voltage, in turn defining its current by Ve = Ic * (5.6k + 3.4k). Similarly, Q2's emitter sets Q1's base voltage through the R7/R8 divider. (Vb - Vbe) / 10k = Ic, and 12V - Ic * 270k = Vc for Q1. I wonder how sensitive the bias is to scratches and stuff.
The 2SC1815s are 60V parts, so I'd be curious to see if the circuit could be linearized with a higher supply rail. Could be fun to replace R10 with a JFET connected as a current source, and perhaps increase R11 to 1M.
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It looks like the Q1-Q2 thing is a convenient way of biasing up the input transistor's base without injecting power supply noise into the input. That also creates a DC feedback loop: Q1's collector sets Q2's emitter voltage, in turn defining its current by Ve = Ic * (5.6k + 3.4k). Similarly, Q2's emitter sets Q1's base voltage through the R7/R8 divider. (Vb - Vbe) / 10k = Ic, and 12V - Ic * 270k = Vc for Q1. I wonder how sensitive the bias is to scratches and stuff.
The 2SC1815s are 60V parts, so I'd be curious to see if the circuit could be linearized with a higher supply rail. Could be fun to replace R10 with a JFET connected as a current source, and perhaps increase R11 to 1M.
I was thinking replace R12 and R14 with active current sources. Higher voltage rail might be interesting too. Current source would be necessary to keep same bias points
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The idea of the increased rails would have been to run the devices a bit hotter, too. :) I'm not sure if I can attach it, but I have a Spice sim set up that makes it easier to figure out what's going on.
Replacing Q1's load with a CCS is a great idea-- if you have a hard time getting the current low enough without the CCS pinching off, you can shunt Q1 with a large (500k or so) resistor to ground. It will dominate the load impedance and leak some current, making the adjustment a bit less finicky.
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Just so you guys know I'm finding some slight discrepancies between that schematic and my board. Minor ones albeit
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Merv, BTW, you should run the 750 through the QA400 test suite. Surprising results