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Old 9th January 2004, 07:54 PM   #1
Aiace is offline Aiace  Italy
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Lightbulb phono disign

What do you think about my phono disign?
Any mistakes ?
Do you have same advice for me?
Thank you, my friends
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Old 9th January 2004, 08:45 PM   #2
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Bf 245 are old fets with relatively big noise - try 2 SK 170 from Toshiba. Why are you using symetrical output ?
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Old 9th January 2004, 09:00 PM   #3
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Hello Aiace,
I'm just a bit curious...
How precise is your RIAA correction???
It seems "to simple"
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Old 9th January 2004, 09:26 PM   #4
Aiace is offline Aiace  Italy
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At the moment this design is only on the paper, so I can't say how good is riaa response. During simulation it should be into +/- 0.6 dB range. But simulation don't say the always the truth.

I wont to use a pseudo differention beacause my goal is to minimise common mode noise.

Now I'm workin into pcb.

Thanks
Aiace
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Old 10th January 2004, 02:05 PM   #5
dimitri is offline dimitri  United States
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Dear Aiace
before working on PCB please simulate two things:
- dc (working) points/operation conditions, you have too much dc gain, think about servoloop (you can eliminate C4 and C...)
- output resistance 10kOhm loaded on 100pF cable - too high
Ciao
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Old 10th January 2004, 08:26 PM   #6
Aiace is offline Aiace  Italy
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I thank you everybody for your help. Here are my answers and new questions.
1) In the schematic there is a mistake: I'm going to use BF244 insted BF245. What do you think about BF244 ? I know it's used in several famous disign like "volks preplifier" and many other I've found on the web

2) Yes Dimitri 10k on 80 pF probably is too high. I forgot to think at the cable capaciatace. But....I'm going to use a special cable ( I invented it ). It has 300 ohm and so it should to have 10-15 pF. I think it sholud to be to help. But probably...I'll low the output impedence. Thanks.

3) Why Giacomo do you think that if you connect R9, R8 to the ground you'll have less distortion ?

4) I forgot to say that I won't connect the ground of phono stage to the ground of Line preamplifier, but I'm going to gonnect one output (i.e. R12 ) to the ground of line amplifier and the other ( R13) to the input. So the pseudo-differetial became unbilanced.

5) Everybady told me "It's too simply!". Yes I realy wont to do it simply, 'couse more simply it is, less complonets it has and more it sound better. ( I think it has too many components for the disign that I usally I do with Tubes, but transistor needs transistrs...sigh!). What do you think about that ? And do you say it is simply couse do think it misses something?

6) Giocomo, in my schematic The RIAA EQ is between the two section.

Thank you my friends....
Aiace
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Old 10th January 2004, 10:44 PM   #7
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Hi Aiace, " full pasive " RIAA correction give good sound results, but have big noise. Better is three amp design, with correction tau 1 + tau 2 connected between first and second amp and tau 3 between second and third amp. It have noise like feedback correction and the best sound results.
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Old 11th January 2004, 05:01 PM   #8
dimitri is offline dimitri  United States
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Dear Aiace
Let me warn you to think again about dc (working) points/operation conditions. You have about 50 dB of gain at dc, and several milliVolts mismatch between input JFETs will results that the outputs of your output stage will be in saturation, not in the state of active amplification. And I bet you couldn’t match JFET even with milliVolts precision. Use your brain at first not the simulator.
Ciao
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Old 11th January 2004, 09:14 PM   #9
Aiace is offline Aiace  Italy
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I thank you Dimitri for your advice. I think to use a pot (R1 in this schematic) to adjust the difference between the two fets.
Do you think it's enaught? If no, can you tell me what you'll do about that.

Here a new schematic, more realistic then the previus.

Bye, Aiace
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Old 12th January 2004, 05:52 AM   #10
Werner is offline Werner  Europe
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Why do you use these two noise generators in there (TL431)?
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