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Old 2nd August 2002, 10:00 AM   #1
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Question diy MC-phonostage

Hi there,
I'm new to this place, but hopefully in the right place...
As you might guess, I'm mostly into vinyl reproduced music.
I'm currently trying to build a mc-stage for my diy-ed phonostage 'le pacific', a two-stage jfet Design using the KISS-approach developed by a french audiophile.

It basically works; but while trying out several jfets, there is some strange behavior. It might be a kind of oscillating, I don't know. It produces a frequent and stable popping. Playing with load resistor,, it is possible to 'silence it', but the signal is still modulated with it.

The stage consists basically of a 4K Load-Resistor, a 2SK170 JFET (tried 3,5 and 6,3 mA IDSS), tried 0-68R Sourceresistor, and 0,65uF Cap at the output. Driven with 24V batterie, and this stage is etrxa de-coupled with 10.000uF Elna electrolyte.
Used with 2SK369 the stage works (with and without electrolytes), but lacks a bit of bass and tightness.

Where can I start to track the problem down? I'm more or less experienced with existing designs, but new to try some of my own.

greets,
Rudiger
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Old 2nd August 2002, 10:12 AM   #2
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Default 2nd...

I might add, it's a breadboard design, and the MM-part works fine even without housing or shielding.
The MM-part is as follows:
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File Type: jpg pacific_riaa_pre.jpg (40.3 KB, 2754 views)
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Old 3rd August 2002, 03:21 PM   #3
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you may need to decouple the stages from one another. circuit are known to oscillate without decoupling. normally go with a RC to form a lowpass filter. I don't have the proper formally for the proper values though. Or try a LC lowpass filter(better no voltage drop). C is normally 220uf-470uf , R is normally low 10 ohm.
Try searching for power decoupling tutorials, you will learn alot from it. Also used a good cap for c (BLackGAte,ELna, Muse etc). You used a 10uf Elna only that is actually bypass only. Doesn't totally isolate the stages from one another( sk170 at input and the sk170 at the output). Decoupling is much better. For bypass try blackgate high q nx work like magic but cost like hell. Just my two cents
P.s batteries supply has high impedance , use caps to bring it down. High speed caps is a must here
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Old 3rd August 2002, 04:19 PM   #4
HDTVman is offline HDTVman  United States
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Hi Onvinyl

NickC is on track with his advice. Are you adding another gain stage to what you have now to give you the extra gain you need for the MC pickup?

If so decoupling the power supply between stages and bypass caps should make a big difference. Bypassing needs to be done locally at the gain stage for it to do any good and every gain stage needs it in low level, high gain amps.

You may want to think about using pcbs with a good ground plane. This may also help. Others may think differently about this but it has always worked well for me.

Good luck

Bruce
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Old 3rd August 2002, 05:18 PM   #5
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Default Gate damping resistors!!!

Try them sometime. Then thank me.

Jocko
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Old 3rd August 2002, 08:41 PM   #6
dice45 is offline dice45  Germany
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Jocko,

have to agree, my one FET MC headamp also went better with a grid stopper oops gate stopper.

BTW, longtimenosee!

Onvinyl,
spotted your QQ on vinyl_lebt, thought you were in good hands asking hifidaddy/analogguedaddy

My one FET single-ended MC-headamp used 12V supplied by AA NiCd batteries, it used one 2SK43 per channel, 2SK240 also is usable. drain resistor was 1k, input load resitir also was 1k, both tantalum oxide. Output coupling cap was 0.5 µFSiemens MKV 25839.
PS caps were huge Siemens electrolytics, 100.000. µF bypassed with 2x1nF silver mica.
If you use the search engine, you may even find a pix i posted.

Gate stopper was added later so i remember, nonmagnetic SMD, 100Ohm. Or was it 50?
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Bernhard
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Old 4th August 2002, 07:02 PM   #7
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Hi,
in my case, (correct) decoupling was the way to go. Found two 47R Resistors and soldered them in.

@HDTVman:
It's a three-stage design now. The last two are as described in the circuit, the first one as described in my first post.

What do you all mean by 'gate stopper resistor'? 100 Ohms in series? This is already the case.

@NickC:
What are 'high speed caps'?

Bernhard, found your pic of your mc-prepre. These are some decent electrolytes....

The whole thing is a breadboard design by now. I will not use a PCB finally, rather p2p as Bernhard did it.

althoug my design works now, sonically I should get quite more out of it.

Thank you for your great suggestions,
Rudiger
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Old 4th August 2002, 10:30 PM   #8
dice45 is offline dice45  Germany
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Onvinyl,

email me if you need some micas, Might have some left.
Oh and please try out those MKVs for coupling caps. They are magic. witchcraft.
Unfortunately uncheap. Bürklin seels them.
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Bernhard
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Old 5th August 2002, 08:02 AM   #9
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Hi Bernhard,

got my Coupling-C's from steinmusic, shouldn't be too bad...
For the mica's: well, I'll check out and let you know.
Currently, I'm looking for good 'soldering stripes' and thinking about a good (wood) housing, probably with a coppper shielding. Any hints here?

Rudiger
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Old 5th August 2002, 11:43 AM   #10
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The phono amp looks easy to build for mm. Could i use sk369 instead of sk170 as i can order them from vacummstate? or where can i order sk170 matched?
Hows the sound of the phono amp?
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