Ultrasimple mm/mc RIAA preamp 2 - Page 18

ra7 · 12th November 2009, 01:55 AM

Salas,
I'm afraid I'll take a day or two to put together what you have shown, with the corrections.
I already soldered the current circuit.

What I need to look for is 10 Vds on the second stage JFET? Is this correct?

ra7 · 12th November 2009, 01:56 AM

Shunt reg? Yes, I've ordered a couple of the B1 boards, which are also thanks to you!

Will give it a shot!! Be in touch!

Salas · 12th November 2009, 02:00 AM

Quote:

Originally Posted by ra7

Salas,
I'm afraid I'll take a day or two to put together what you have shown, with the corrections.
I already soldered the current circuit.

What I need to look for is 10 Vds on the second stage JFET? Is this correct?

Yes, you look for 10VDC from that point I show and ground. If you have one channel still somewhat quiter, you can play with its R8 trimmer until it matches output to the other channel. In that case, the quiter channel may show less than 10VDC after you trim it.

ra7 · 12th November 2009, 02:28 AM

Got it!

I'm trying to understand what's happening here...
by putting a resistor on the source pin, you've reduced the current
from drain to source (or ground) for a given voltage at the base.
Is this correct?

Salas · 12th November 2009, 02:46 AM

Its called degeneration. Now the resistor controls the current along with gm by creating some little local NFB. The matching will be easier for gain between channels, the THD lower because of that source degeneration, and of lower level than before, was a waste anyway.

ra7 · 13th November 2009, 12:20 AM

Salas,
Can you help once more?
I got new Nimh batteries.... now I got close to 30V from the battery. How do I drop the 6V?

I want to put a resistor in series with the battery, but I don't know what value unless I know how much current is flowing.

For some reason, I can't measure the current in my existing circuit.

Salas · 13th November 2009, 12:50 AM

Easy, and you filter better too.

ra7 · 13th November 2009, 01:20 AM

How did you do the calculation... please tell me... I want to understand it.

Thanks again. You're great!

Salas · 13th November 2009, 02:32 AM

Ohm's law. 1k2 drops 6V for 5mA that each stage will roughly use. 1k-1k2 will get you there depending on your batteries top voltage and how much around 5mA each of your fets really draws in practice. The 220uF caps make a long enough RC time constant with the given resistor ballpark value so to bypass even under 10Hz. Your first stage especially needs that because batteries have enough impedance to pass modulation from the stronger swinging 2nd stage. So you do two good things with one needed Vdrop move.

ra7 · 13th November 2009, 02:34 AM

excellent... many thanks!!!

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12th November 2009, 01:55 AM	#171
ra7 diyAudio Member Join Date: Feb 2009 Blog Entries: 1	Salas, I'm afraid I'll take a day or two to put together what you have shown, with the corrections. I already soldered the current circuit. What I need to look for is 10 Vds on the second stage JFET? Is this correct?

12th November 2009, 01:56 AM	#172
ra7 diyAudio Member Join Date: Feb 2009 Blog Entries: 1	Shunt reg? Yes, I've ordered a couple of the B1 boards, which are also thanks to you! Will give it a shot!! Be in touch!

12th November 2009, 02:28 AM	#174
ra7 diyAudio Member Join Date: Feb 2009 Blog Entries: 1	Got it! I'm trying to understand what's happening here... by putting a resistor on the source pin, you've reduced the current from drain to source (or ground) for a given voltage at the base. Is this correct?

12th November 2009, 02:46 AM	#175
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Its called degeneration. Now the resistor controls the current along with gm by creating some little local NFB. The matching will be easier for gain between channels, the THD lower because of that source degeneration, and of lower level than before, was a waste anyway.

13th November 2009, 12:20 AM	#176
ra7 diyAudio Member Join Date: Feb 2009 Blog Entries: 1	Salas, Can you help once more? I got new Nimh batteries.... now I got close to 30V from the battery. How do I drop the 6V? I want to put a resistor in series with the battery, but I don't know what value unless I know how much current is flowing. For some reason, I can't measure the current in my existing circuit.

13th November 2009, 01:20 AM	#178
ra7 diyAudio Member Join Date: Feb 2009 Blog Entries: 1	How did you do the calculation... please tell me... I want to understand it. Thanks again. You're great!

13th November 2009, 02:32 AM	#179
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Ohm's law. 1k2 drops 6V for 5mA that each stage will roughly use. 1k-1k2 will get you there depending on your batteries top voltage and how much around 5mA each of your fets really draws in practice. The 220uF caps make a long enough RC time constant with the given resistor ballpark value so to bypass even under 10Hz. Your first stage especially needs that because batteries have enough impedance to pass modulation from the stronger swinging 2nd stage. So you do two good things with one needed Vdrop move.

13th November 2009, 02:34 AM	#180
ra7 diyAudio Member Join Date: Feb 2009 Blog Entries: 1	excellent... many thanks!!!

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