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Old 7th September 2008, 06:13 AM   #11
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No preamp...... the amp ( after the 50K input atteniator), inputs the signal into the grid a single triode of a 6SN7 input stage into the grid of a single triode of a 6H30 driver stage into a 6550.

The input voltage that I arrive at through the circuit that clips the output is .67V

I want to feed the source (cd, phono, tuner etc) directly into the amp.

That's why I need the "pre" attenuator to reduce the voltage to a level the amp can tolerate and use the 50K attenuator as the main volume control.

JD


Quote:
Originally posted by audiowize
I still vote that you add a zero to all those resistances and keep things at 100k. What are you using for a preamp btw?
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Old 7th September 2008, 07:27 PM   #12
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Can I suggest using a input transformer. I have had the same issues in two amps.
In the first i used a microphone transformer to step down in the voltage ratio of 9:1.
In the second I used a main toroidal at about 10VA rating with a similar step down of 9:1. Cost about 10.00 each
Both worked fanstastically and offered galvanic isolation with the reduced potential for group loop hum. I also used them to achieve phase splitting, but you don't need that.
You need the grid leak as well to eliminate any potential for ringing - something around 100K should do.
I guarantee a better result than a resistor voltage divider.

Shoog
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Old 10th September 2008, 07:05 AM   #13
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Shoog.... great idea...

Looking at som Lundahls for a 2:1 or 4:1 step down.


JD
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Old 13th September 2008, 02:25 AM   #14
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"Can I suggest using a input transformer. I guarantee a better result than a resistor voltage divider."

yes, or an volume control using an autoformer
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Old 13th September 2008, 03:21 AM   #15
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Basic design has already been built. set in stone, and cannot/will not be changed. THe 100K stepped attenuator stays in place.

Tha'ts why the input transformer may be a good idea as it has some benefits such as isolation and no further series resistance to make the system less transparent.

I have also been advised about the use of a grid choke, the Lundahl LL1670 in a 2:1 arrangemetn using it's center tap in place of the grid leak resistor into the first input tube (6SN7).


JD

Quote:
Originally posted by speakerfritz
"Can I suggest using a input transformer. I guarantee a better result than a resistor voltage divider."

yes, or an volume control using an autoformer
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