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Old 13th January 2012, 12:43 PM   #1
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Red face Reduce output on a ECC88CC class A pre-amp !

Dear Members.
I have an old CAYIN SC-2 PRE-AMP . It has been modified with top class components , Siemens ECC88CC tubes, mkp caps, good resistors etc...
The pre-amp works great, sound quality is amazing.
The only problem i have is that it has too much output power. A slight move of the potentiometer can drive any power-amp at enormous levels (no distortion, sound is crystal).
Does anyone have a simple solution to lower the output a little bit in order to achieve better control when i move the pot clockwise ?? Slight turn at about 1/5 turn may force you to get out of the room. I used it with different power amps with the same result.
I searched for the circuit they used. It is very hard to find it.
Any Help would be appreciated.
Vassilis
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Old 13th January 2012, 12:53 PM   #2
DF96 is online now DF96  England
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You probably don't need a preamp at all. If you want to stick with it, then add an attenuator to get rid of all the gain. You will then be left with a little noise and distortion, which may or may not appear to improve the sound.
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Old 13th January 2012, 01:00 PM   #3
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There are lots of elegant solutions to reduce the gain. By far the simplest is to fit an attenuator.

I have assumed that 25K wont load the circuit. It is the ratio of the components that is important.

If 10:1 is too much (ie too quiet) increase the 2K2 accordingly.
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Old 13th January 2012, 01:04 PM   #4
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Well i don't want to get rid of the pre-amp as it gives me the exact quality of sound i want.
I know that there is trick at the circuit of the ecc88 tube (change a resistor value or removing a capacitor) but i don't know where to look or what to change to lower the output power a bit.

Quote:
Originally Posted by DF96 View Post
You probably don't need a preamp at all. If you want to stick with it, then add an attenuator to get rid of all the gain. You will then be left with a little noise and distortion, which may or may not appear to improve the sound.
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Old 13th January 2012, 01:05 PM   #5
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Thanks.
I will try it back home. Is it going to reduce the sound quality of the pre-amp?

Quote:
Originally Posted by KatieandDad View Post
There are lots of elegant solutions to reduce the gain. By far the simplest is to fit an attenuator.

I have assumed that 25K wont load the circuit. It is the ratio of the components that is important.

If 10:1 is too much (ie too quiet) increase the 2K2 accordingly.
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Old 13th January 2012, 01:08 PM   #6
DF96 is online now DF96  England
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Anything you do internally to reduce the gain is likely to change the sound of the preamp. An attenuator will cause little or no change apart from dumping all the gain you are paying for.
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Old 13th January 2012, 01:43 PM   #7
Doz is offline Doz  United Kingdom
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If there's a cap across the cathode resistor, you can remove this to lower the gain a little. It will alter the sound though.
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Old 13th January 2012, 03:23 PM   #8
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Quote:
Originally Posted by Doz View Post
If there's a cap across the cathode resistor, you can remove this to lower the gain a little. It will alter the sound though.
If the circuit is a common cathode, without cathode follower cascading it, this will move up the Z Out of the stage. Depending on the Z In of the power amp, it may degrade the sound quality.

A resistor attenuator at the input of the pre-amp may work best.
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Old 14th January 2012, 05:44 PM   #9
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Might be worth checking something...

Did you replace the pot when modding? You may have put in a linear pot instead of a log one.

Just a thought.
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Old 15th January 2012, 06:43 AM   #10
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Pot is 2x100K - ALPS BLUE VELVET, as it was from factory. It is connected at the output of the pre-amp.

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Originally Posted by mace1337 View Post
Might be worth checking something...

Did you replace the pot when modding? You may have put in a linear pot instead of a log one.

Just a thought.
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