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Phono stage tweak

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Hi
I have an entry-level stand along Phono stage and plan to do some mod /tweak on it.
It comes with 2 stages for MC/MM cart. I��m using MM cart only so will just leave the MC section alone. I studied the circuit and made a schematic at
http://www.geocities.com/lkho4926/riaa.gif
Question:
1. C1 and C6 at input and output: Are they simply coupling Caps or they play part of the RIAA game? Can I replace them with smaller value higher quality caps? Can I just remove C1? The stock caps are electrilicity. I tried to replace C1 with a ERO 1mfd and C6 with Solen 6mfd. But could not feel much difference. (not so significant like Auricap Vs. Solen in my Foreplay)
2. Note the + of C6 is connected to the output and the + of C7 is connected to Ground. How come they are reversed direction and different from other schematics I��ve seen?
3. What I have in mind is to upgrade C1 C6 R1 R4. Any other suggestion? How about better OPAmp?
Thanks in advance for any comments
Robin

Follow Ups:

you can go much much much better - Steve 17:29:46 03/11/02 (0)
You could get much much better. You might consider going tube and passive RIAA. Can be extremely quiet. Hope this helps.
steve
Re: Phono stage tweak - eagle1 07:35:14 03/10/02 (0)
Hello Robin,
Changing the op-amp for a OPA627 or OPA227 will surely help. R2, C1, C2, C5 should be remove. R3 change for a 47K. C7, C6 change for a good film/foil. R4 should be lowered to 47 ohms and R5 should be a 100K. It seems that the pole at 50KHz is not implemented, so you'll need to add a resistor in the feedback. Everything in the feedback loop need to be matched at 1% at least. Running it on batteries will help too.
If you didn't heard a difference between caps, your circuit is way to much coloured. For the orientation, the DC offset created by the NE5534 is potentially negative.
Hope it helps,
Daniel Trudeau
You might want to consider... - plexus 19:52:16 03/09/02 (0)
sell it and getting a parasound pph-100. your preamp doesn't have that great a design, running negative feedback through the RIAA network and only having one gain stage.
the PPH-100 uses a better circuit design and is well suited to modifications. you can get them on ebay for under $100
plexus

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