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Old 17th May 2012, 06:23 AM   #1
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Default Pre amp output, would this work?

First of all, I am very new in circuit design, if it can actually be called that.

I am building a simple single stage single ended headphone amp, like this:

Click the image to open in full size.

I am wondering if I can tap a pre amp output from the plate with just a capacitor (the dotted box)? The tube is 6H30. There is a 2200 ohms resistor across the OPT secondary so that it is loaded at all times. I can have a switch at the proposed pre amp out, I imagine it has to be disconnected when the headphone is plugged in.

Would this work at all? Am I better off tapping the pre amp out at the OPT secondary with impedance matching parallel resistor?

Thanks.
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Old 17th May 2012, 07:54 AM   #2
tinitus is offline tinitus  Europe
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oh, I thought the preamp would go before the headamp
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Old 17th May 2012, 08:07 AM   #3
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Quote:
Originally Posted by Navyblue View Post

Would this work at all?
I see no reason why not. Don't forget to add a 100k resistor between the output side of the cap and ground.
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Old 17th May 2012, 08:56 AM   #4
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Quote:
Originally Posted by analog_sa View Post
I see no reason why not. Don't forget to add a 100k resistor between the output side of the cap and ground.
and small series resistor ?
not sure why, but aren't there ususally one ?
isolation maybe

hmm, 100ohm ? wouldn't that be a rather small grid resistor ?

and the 10K IT(OPT), wouldn't that very much depend on tube ?

Power(gain) loss from step down impedance ?
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Old 17th May 2012, 09:03 AM   #5
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why not cathode output, and another gain stage ?
might do with smaller(better) signal trafo ?
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Old 17th May 2012, 09:58 AM   #6
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Originally Posted by tinitus View Post
oh, I thought the preamp would go before the headamp
I prefer an active preamp at this point of time.
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Old 17th May 2012, 10:00 AM   #7
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Originally Posted by analog_sa View Post
I see no reason why not. Don't forget to add a 100k resistor between the output side of the cap and ground.
Glad that I can actually come out with something that works.

I sometime see a high value resistor to ground at the output, what is it for anyway? For stability?
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Old 17th May 2012, 10:06 AM   #8
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Originally Posted by tinitus View Post
why not cathode output, and another gain stage ?
might do with smaller(better) signal trafo ?
You mean a gain stage, followed by a cathode output, and followed by another gain stage?

I considered another gain stage, but unless I commit a serious power tube, I don't get much more voltage swing. And the additional capacitor and triode in the signal path does not seem to make it worth the trouble.

As for the grid resistor, I am actually following a schematic, I don't really know what is a good value.

The OPT is custom made for headphone btw, and I already bought it so it's not open for discussion.
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Old 17th May 2012, 10:14 AM   #9
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Quote:
Originally Posted by Navyblue View Post

I sometime see a high value resistor to ground at the output, what is it for anyway? For stability?
It's there to provide a discharge path for the cap. Bad things can happen if the pre is plugged into a solid state power amp with the cap still charged...
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Old 17th May 2012, 10:24 AM   #10
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Originally Posted by analog_sa View Post
It's there to provide a discharge path for the cap. Bad things can happen if the pre is plugged into a solid state power amp with the cap still charged...
Thanks. Glad that I asked, there are many basic things that I have yet to find out.

Btw, what is an appropriate value for the output capacitor. In the V cap website, there is a calculator. With a 50k ohms load, I came out with 2uF. Somebody here has an approximation that goes like capacitor size = 0.1/load impedance , which turns out to be 2uF as well.
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