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Old 26th July 2002, 01:06 AM   #1
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Default The Pearl-X

I started building the Pearl phono stage and decided to go with a balanced output stage. Because of all the information that now exists on the forum for the X topology I thought I might try one in this application. My goal was to improve the perfomance of the pearl output stage but still stay close to the original design and keep it as simple as possible. The resistor values for the cross-coupling are a guess taken from the P1.7 preamp. All of Nelsons comments regarding unbalanced operation of the X topology I hopefully have implemented correctly! As far as the feedback is concerned I have no clue at this point. It may not have enough excess gain to give any away in my particular application. I left the source resistor out so the J-FETs run at IDss, about 10-12ma.
I hope Nelson and the rest of you will please comment on the attached schematic. This is my first attempt at audio design in recent history so try not to be too tough on me.
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Old 26th July 2002, 08:00 PM   #2
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I would drop the 3.9K's to ground for this
application, which is single-ended input.

Probably you will end up with the pot at 0 ohms,
but that's OK.

Oh yeah, and add some output series resistance for
HF stability, 100 to 220 ohms.
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Old 26th July 2002, 08:10 PM   #3
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I've always wondered why more phono stages aren't balanced INPUT, seeing as a cartridge is an inherently balanced device. Dealing with such low voltage signals, I'd think that a balanced input would reduce noise. I built a phono stage based on the INA103 instrumentation amp, which is balanced input. It's very quite, especially once I had the phono cable sheild properly grounded

How could we convert this Pearl-X, or the original Pearl, or even the Ono to balanced input? I don't care so much about balanced output, the signal is pretty robust at that point.

RonS
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Old 26th July 2002, 08:25 PM   #4
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Very tough to reject noise on balanced input phono
without transformer.
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Old 26th July 2002, 08:28 PM   #5
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Default Design advice

5K and 10K votage divider should be changed to 15K and 10K.

500 ohms too small for collector resistor, Very low open loop gain.

What is the closed loop gain that you want? Do not design until you know.

DO NOT RUN JFETS AT Idss. 3 to 5 mA in each Jfet is probaly enough if this circuit is to drive >= 50K loads.

SK170s should be biased less than Idss each half of SK389.

Gate resistors on SK389.

You need a negative voltage supply also.
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Old 26th July 2002, 08:29 PM   #6
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Default Balanced Phono Input

Well, I'm sure you are correct, but how do the studio guys do it with mic's? Essentially they are the same, a low level balanced signal, going into either an active balanced stage like the INA103, or a transformer. I understand that the CMRR degrades for an active stage if the source impedance isn't equal in both legs, but it should still be worth something, no? Good transformers, like the Jensen's, are expensive.

Any thoughts?

RonS
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Old 26th July 2002, 09:05 PM   #7
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With a moving magnet cartridge, the signal is high
enough, but the load impedance is also high, so you
get a lot of common mode pickup.

With a moving coil, the impedance is low, but so is the
output, so the signal to noise range is lower.

Your typical microphone is high output but low impedance,
so it is easier to make work with active balanced input.
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Old 26th July 2002, 09:25 PM   #8
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Default Balanced MC stage

Quote:
With a moving coil, the impedance is low, but so is the
output, so the signal to noise range is lower.
Yes, that's exactly what I'm refering too, low signal level from MC cartridge. Is there nothing that can be done to improve the CMRR in a phono stage? Or is it a moot point?

RonS
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Old 27th July 2002, 12:12 AM   #9
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Nelson and Harry thanks for the help. I'll be driving a 22K balanced load (P1.7 Input) with the phono section in the same box. I'd like to obtain a gain of about 24dB-30dB out of this circuit.
As far as power supplies go I'd like to stick with a single supply to make this mod as painless as possible since the basis is the Pass labs Pearl PCB.
As soon as I get further along I will post an updated schematic.
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Old 27th July 2002, 01:39 PM   #10
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Smile The Pearl-X Revision 1

This version incorporates Nelson's and Harry's comments implemented correctly I Hope! The changes are:

1. Deleted 3.9k's dangling to ground(Nelson is this correct)
2. Added 200 ohms at the outputs and gates of diiff pair (HF
stability)
3. Rebiased to less than Idss (10ma) and added another dual J-
FET to the Diff pair (more gain)
4. Increased collector resistor on cascodes(more gain)
5. Added feedback, gain is about 21X I think, which is enough for the second stage since my High output MC puts out about 3mv.
Thanks again and any additional comments are greatly appreciated.
Vince
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