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Old 3rd October 2005, 10:07 PM   #1
Devius is offline Devius  Portugal
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Default Balanced pre-amp attempt

Hello

I'm just starting into valves and would like to make a pre-amp. The thing is I'd like it to have ballanced output and maybe input too. I've seen some such preamps on this forum but I'd like something different. I'm thinking of a Isodyne phase splitter since it seems to have very nice features such as low output impedance. I don't know if it's meant as a pre-amp but I'm guessing it will work just as well right?

The attached schematic is what I've done so far. Just a little bit of modification on the original design to lower the gain. According to the simulator it should work as intended.

Any thoughts? Is it very bad? Am I doing something stupid? I really don't know much about valves although I've been reading info on the subject.
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Old 3rd October 2005, 10:11 PM   #2
SY is offline SY  United States
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The balance will be awful. You can improve things by tying the two cathodes of the input tube together (for heaven's sakes, use a more linear tube than a 12AU7!), then tie them to a current sink running to a negative rail. The negative rail doesn't have to be very large, -5 to -12V will be fine.

Or, even better, use an input transformer to get near-perfect balance and galvanic isolation.
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Old 3rd October 2005, 10:21 PM   #3
Devius is offline Devius  Portugal
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Thanks for the advice

I don't want to use a transformer since I can't get one...

Any suggestions on valves for replacing the 12AU7? I don't have access to many... maybe 12AX7? or some of the ECC8x type?

I'll fiddle around with your suggestions and place the updated schematic as soon as I can.
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Old 3rd October 2005, 10:30 PM   #4
SY is offline SY  United States
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Depends on the gain you want. You'll get similar gain and better linearity from a 6CG7/6FQ7 at a similar operating point.
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Old 3rd October 2005, 11:53 PM   #5
Devius is offline Devius  Portugal
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No, I can't get those you mentioned easily. Here's a list of what I can get: 12AX7, 12BA6, 12AV6, 12BE6, ECC81, ECC85. not a great list to choose from...

I would like around 18dB of gain or less.

And here's the schematic with the current source. There is just one thing I'm not quite sure (I'm very bad at electronics), is the current at each anode equal to half the current supplied by the current source?
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Old 4th October 2005, 12:01 AM   #6
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Unless you're in Lower Volta, there are a great variety of tubes available via mail order. If you can mention your location, perhaps some of the members can pipe up with some sources that you can use.

The current source will force the two halves to have equal and opposite AC current. You've got two DC balancing controls which is probably redundant. And the 12AX7 is going to be huffing and puffing trying to drive a load. You want something more like the easily-available ECC88 in that position. In fact, you could use it at the input, but a 6CG7 (or a 6SN7) will have lower gain, closer to what you're looking for.
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Old 4th October 2005, 12:07 AM   #7
Devius is offline Devius  Portugal
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My location is visible to all and the side bar. The valves I mentioned are from an electronics repair store in my town. The guy will sell them to me all at the same price.

I'll look into that mail order thing when I have the time

Thanks for all the help
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