diytube.com

Here is a concept unit I have that is fully balanced. It uses 2 6SN7 stages cascaded and a PPP output stage. The feedback is taken from the 8 ohm tap for one phase and the common for the other.

If this how feedback is implemented in a balanced design? Do you recommend I use a cathode follower for the second stage? I get more swing in the simulation if one is used. Any other comments?

If it is something new, why don't you have it patented (before sb else does it?

It's very interesting IMHO.

Its not new.

Hi,

This is an Allen Wright design, IIRC.

A cathode follower isn't really needed here since you're running AB1 and drawing no grid current (wich the coupling cap would block anyway).

With no input transformer though, you will have no isolation and mediocre CMRR, which is what balanced is famous for and its greatest strength.

Cheers!

http://www.sacthailand.com/AmpNewGlowMasterKT88.html

If you see this picture on this website, I am doing something along those lines.



How will using two gain stages instead of a phase splitter (the signal coming in is already split) not provide good CMRR without a transformer? I thought a design like this is fully balanced innately and transformers are used when the design is not truly balanced.

Hi,

kt88seamp wrote:How will using two gain stages instead of a phase splitter (the signal coming in is already split) not provide good CMRR without a transformer?

The nature of the beast!

At best, a design like this can provide when tweaked perfect, 60dB of CMRR and in reality, probably 40dB. Even a $10 Edcor WSM can give you 100dB.

I studied active vs. passive balanced inputs in detail before taking on a tonne of commissions demanding balanced inputs. I had been up to eight tubes (practcally an op-amp) before I was at 70dB CMRR.

The invention of active balanced was on wide bandwidth oscilloscopes. HP was hitting 100MHz before the rest were breaking 5MHz. CMRR was not that big of an issue, since the deflection plates were insensitive to common-mode signals. But we did get some bang-up balanced amplification techniques out of their research and the above circuit is a greatly simplified version of it.

I thought a design like this is fully balanced innately and transformers are used when the design is not truly balanced.

If that were true, Bogen and other PA equipment and studio broadcast mixing panels wouldn't have transformers in them

Cheers!

Broadcast consoles haven't had transformers on the input or about
for about 25 years. Opamp circuitry on the input and output
has been perfected.

Hi Tom,

TomMcNally wrote:Broadcast consoles haven't had transformers on the input or about
for about 25 years. Opamp circuitry on the input and output
has been perfected.

It should be noted that these are instrumentation op-amps, not regular ones.

I repaired a Tascam a couple years back for a party rental company that had a pile of little Sowter-like donuts in their line-inputs. So some models must still have them.

Cheers!

You said "broadcast" ... Tascam is cheap stuff with single ended
power supplies in most of their stuff, so no good balanced opamps.

Transformers come in handy now and then, I have some here and
there at the dozen or so radio stations I'm involved with, but actually
none are in the on-air audio chain. They are mostly used as a
quick way to convert an unbalanced input to take a balanced
feed without any issues. I hate unbalanced stuff in that environment.

TomMcNally wrote:...Tascam is cheap stuff with single ended power supplies in most of their stuff...

Actually, that's what was gone in the one I repaired

Cheers!

As far as the balanced idea goes, I am revisiting this idea, and will probably use it as a revision for my current KT88 PPP amplifier. The main points are to:

- Have the preamp on my computer desk and the power amps off somewhere else, aka long XLR wires.

- Fully balance everything from my turntable, to microphones, to my preamps, etc and have the signals not touch ground.

My main question is how do you apply global negative feedback to my new driver stage? The output stage is the same and the driver is just a phase splitter with a 12AX7 tube and cathode followers.

Driver:



Output Stage:

I am doodling actively with balanced technology. I am making a preamp and later a power amp. I am using Edcor transformers. I will unveil the schematics as son as I get them working.