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Old 4th September 2002, 02:20 AM   #1
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Default :: mm pre phono ::

Dear all,

I've built this phono preamp [sch attached] ... but why thats unstable ... if i turn on ... the left side is okay ... but the right one is ready 1 minute later ...

Can you help me? ... Any comment was appreciate ...

Regards:
ragil.hastomo
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Old 4th September 2002, 02:27 AM   #2
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This the inside of my preamp

Left to Right: prephono board - line level preamp board - the psu ...
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Old 18th September 2002, 07:48 AM   #3
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Knock .. knock .. knock .. heloooouwww .. anybody help me???

And the following picture is my turntable ... not a good turntable but its 'TURNTABLE' ... I use Audio Technica AT-71 MM-cartridge ...

Regards: ragil.hastomo
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Old 18th September 2002, 10:39 AM   #4
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i can see a highly stressed turntable with all those cds around...
you may want to include individual gate stopper resistors on your input fets.

peter
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Old 18th September 2002, 10:45 AM   #5
artnyos is offline artnyos  United States
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Default Gate stoppers

On ALL your fets. Try about 200 to 300 ohms.

Art
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Old 18th September 2002, 11:58 AM   #6
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First of all: Decoupling caps! Try to put 10-100 nF, ceramic or polyester (small 63 V are good!) in parallel with your big mamas. You may also have too high ground impedance, small and thin copper traces.

I think also that your parallel connection of the output stage is unwise. Try to insert source resistors so you have a controlled current sharing. Make the value 1/10 or 1/5 of the total value. The result is attenuation of 0.9 or 0.8.

I suggest also that you increase the current of the output buffer to as much as the fet's can stand. Otherwise you will get a rather high output impedance (loss of high frequencies).

R1 is a little bit high I think. I would have chosen 100-390 ohms. In fact it's better to have lower R and bigger C's. If you have a "stable" power supply voltage you will probely have better chances of getting stability.

Have you considered the output impedance of the first stage which feeds the RIAA network? I think you will get an error here due to unsufficient drive capability.

Hope this can be to any help.
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Old 19th September 2002, 02:10 AM   #7
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Gate damping resistors!!!!!!!!!! I don't see any, do you, Arty?

Jocko
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Old 19th September 2002, 02:35 AM   #8
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Default RIAA network

Ah.....where did you get the resisor and capacitor values from?? You might want to check them. I do not believe that they are correct.

Jocko
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Old 19th September 2002, 07:42 AM   #9
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Default The input stage

The input stage has very little gain if I'm not mistaken, approx. < 5. The sources of the input JFET's must be decoupled if you want higher gain. The caps must be big, > 1000 µF at each fet. Haven't done any calculating so the value is rough.

The DC gain (or at 50 Hz) must be 300 to 1000 depending on pickup.

As Jocko Homo notice, the RIAA network don't look right...but I haven't done a calculations here either.

I suspect that you want to design this RIAA amp by yourself but if you want to take a shortcut, take a peek at Nelson Pass' (or Mr Wayne Colburn's) Pearl phono amp. The basic idea is the same.

http://www.passdiy.com/projects/pearlono1.htm
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Old 19th September 2002, 08:25 AM   #10
Werner is offline Werner  Europe
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Drop the whole front-end and replace it with a single 2SK170BL. Run it without feedback or just a small source resistor (10-20R), and load it with 1-2k or so.

Recalculate the RIAA.

Indeed, look at the Pearl or the Pacific for inspiration.
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