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Old 7th December 2003, 11:56 PM   #1
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Default Lowering gain in 12AX7 SRPP preamp

I have a Jolida JD1501 hybrid integrated amp. Great amp, especially at it's price (US$650 retail), but it's input is way to sensitive. The most I can ever turn it up is 9-10 o'clock, the lowest is 7 o'clock. And, even turned all the way down, there is some bleed through. Also there is some channel to channel mismatch at the very lowest levels. I would like to get up more in the middle of the control. I believe it is an Alps pot.

So, how can I lower the gain? Here is the schematic:
Click the image to open in full size.

And, while I'm in there, are there any other obvious improvements I should make? I'm currently using NOS GE 12AX7 tubes, I also have some CV4004 laying around. I've read somewhere that a 12AT7 should work and would lower gain, but may increase distortion?

Thanks,
David
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Old 8th December 2003, 02:05 AM   #2
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Hi,

12AT7A should work but you could also cut C101-C102 to lower the gain at the expense of higher Zo.

Quote:
Also there is some channel to channel mismatch at the very lowest levels.
Probably the pot mistracking.

If this were to mine, I'd sell the unit and build something better from scratch or even a kit. No Jolida fan here.

Cheers,
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Old 8th December 2003, 05:21 AM   #3
Colt45 is offline Colt45  Serbia
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snip C101 and C102 is probably the easiest route.
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Old 8th December 2003, 07:11 AM   #4
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Default Reducing gain.

This one has voltage shunt feedback . If you raise R101 or reduce R103 the gain would go down. It might not be a good idea to drop the value of R103 because it will load the output of the tube even more. It would be better to raise the value of R101.
My simulation shows a gain of 8 and it drops to about 2 with R101 changed to 22k ohms. From what you explained , it might look like a drop in gain by two time may not be enough. So you can try a drop of about 4 times by using 22k. It will also improve the distortion figures. Input resistors in series with the input will contribute to the S/N ratio. But as this is a line level amp you may not be affected much by the increase in noise level--especiallly as the vol pot is after the amp.
Coming to c101 and c102. I would like them to go. In some circuits I tried the sound was much better with the cathode caps removed. As Colt 45 said the gain will also drop. I simulated it and a the gain dropped to 7 with C101 and c102 removed. That is not enough. You could also make R101 15K or so. Change R101 to something between 4.7K and 22K to get the levels you want. In any case just snip off the c101 and 102 first and see if it improves the souind. After that you can raise R101 to get what you want.
Cheers.
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Old 9th December 2003, 01:54 AM   #5
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Default Thanks!

Thanks for the suggestions. I'll dig around in it soon and see what I can do.

--David
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Old 19th December 2003, 07:49 PM   #6
ALBQ is offline ALBQ  United States
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Lightbulb Lower Gain of SRPP

Have you thought about trying a fixed bias srpp - decrease R1 to adjust gain. R1=2K is about unity gain.
ALBQ
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Old 19th December 2003, 11:18 PM   #7
PRR is offline PRR  United States
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> Here is the schematic:

With the pot settings you are getting, I would scrap everything shown. Well actually, throw away the 12AX7, disconnect the top of W1 from the 12AX7 circuit, and wire the wiper of S1 to the top of the volume pot.

You lose the tube sound, but gain a much higher input impedance. (It is under 10K now, which may be fine for 99% of solid state sources, but seems mighty low to my old eyes.)
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Old 19th December 2003, 11:19 PM   #8
Asen is offline Asen  Bulgaria
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To lower the gain of my SRPP pre I use a resistor in series with the pot. I put a 220k resistor at the input and a 10k pot right after it insted of using just a 250k pot. This way the whole circuit has no gain. It works as a follower. And additional feedback is avoided.
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Old 19th December 2003, 11:43 PM   #9
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Hi,

Quote:
This way the whole circuit has no gain. It works as a follower. And additional feedback is avoided.
Brilliant engineering...

What's the point??? Can't one use a buffer?

Mama mia....

Cheers,
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Old 20th December 2003, 12:31 AM   #10
AKSA is offline AKSA  Australia
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Why not re-engineer the power supply to give around 10mA, replace the 12AX7 with a 12AU7, replace R101 with 10K (leave R103 at 47K), lower the output pot to 22K, and remove the capacitors from the bottom cathode resistor?

This should give you a gain around 3.5, close enough for government work, while lessening the feedback factor around the SRPP maintaining a nice tubey sound, and into the bargain increasing drive considerably without the distortive effect of imperfect caps at the lower cathode...

You will need to fiddle with the cathode resistors, which will probably be around halved.

And you can keep the wiring the same as well, same pinouts!

Cheers,

Hugh
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