Balanced Preamp

[denny9167]

Wanted to try my hand at a balanced preamp using 4 6SN7's, the first two being the gain stage, the second two simply a buffer stage. Below is a pick let me know what you think. It was simulated single ended, just to do a basic AC sweep, I tried to spice the balanced topology, but I kept getting some quirky readings. The gain single ended seems to show around 11.5db.Thanks

 

[speakerfritz]

I see 1 signal path....where is the other? Balanced you need a non inverted as well as inverted signal path.

 

[denny9167]

I see 1 signal path....where is the other? Balanced you need a non inverted as well as inverted signal path.

I know the stage for the inverted signal isn't shown, it would be a mirror image with both the first tube and third tube receiving a negative bias through their cathode resistors.

 

[speakerfritz]

it wouldn't be truely balanced if neg and pos power rails are not used.

 

[SY]

it wouldn't be truely balanced if neg and pos power rails are not used.

Really?

 

[SY]

I know the stage for the inverted signal isn't shown, it would be a mirror image with both the first tube and third tube receiving a negative bias through their cathode resistors.

With that scheme, you basically throw away the main advantage of balanced-mode operation: common-mode rejection.

 

[denny9167]

With that scheme, you basically throw away the main advantage of balanced-mode operation: common-mode rejection.

It would be better for me to post a complete schematic, I'll try to have that up in the next day or so, I'm not sure if the benefits of a balanced design outweigh the cost of additional tubes. I may be just blowin in wind on this one.

 

[denny9167]

Here is the schematic, I decided to eliminate the buffer stage. Let me know if this will work. B+ will be about 200V, negative bias will be around -100V.

 

[enzoastro]

just put a dn2540 under the first pair with a 500r pot to ballance them and buy lots of anode load resistors so you can closely match them

 

[denny9167]

just put a dn2540 under the first pair with a 500r pot to ballance them and buy lots of anode load resistors so you can closely match them

Wouldn't having the the ccs do away the need to have a negative bias?

 

[denny9167]

I calculated the gain at 23db with a balanced input, and 13db unbalanced.

 

[SY]

Wouldn't having the the ccs do away the need to have a negative bias?

As long as you can always maintain the minimum voltage across it for it to operate under all signal conditions.

 

[enzoastro]

gain at each anode should be halved compared to se. i dont think the bias voltage on a 6sn7 would be enough to comfortably use a dn25 ccs at least not with a ballancing pot. there are some ccs that might though

 

[jazz]

Hi,

If that's the sort of circuit you're thinking about, have you had a look at Allen Wight's RTP3 or 5?

link:

grtz
Joris

 

[denny9167]

gain at each anode should be halved compared to se. i dont think the bias voltage on a 6sn7 would be enough to comfortably use a dn25 ccs at least not with a ballancing pot. there are some ccs that might though

I decided to go with the 6922. I can fit both channels on one board, thanks.

 

[denny9167]

Hi,

If that's the sort of circuit you're thinking about, have you had a look at Allen Wight's RTP3 or 5?

link:

grtz
Joris

I'll take a look at it thanks.