tertiary feedback - output transformer ISA 110

Hi everybody!

This thread is about the Focusrite 110 preamp topology with its special output transformer.

My question will be, if there is a possibility to use another output transformer in this topology.

But first the background information:
This "pumpkin" toroidal output transformer used in the ISA 110 has a second output winding which is used to send a feedback signal back into the output stage; I guess to cancel out distortion. Here is the schematic:


You can find this schematic (in a bigger and more readable version) and more information at Kevs very nice webpages (called the DIY Factory; thumbsup for Kev!); at the following URL there is that special page about building a 110 based preamp:

http://www.diyfactory.com/projects/diy110/diy110.htm

As you can see there, forum member Okko has built some 110-type preamps using these pumpkin output transformers.

There also has been a discussion here at the prodigy forum regarding that special output topology:

http://www.groupdiy.com/index.php?topic=4485

Now to my "problem":

Two weeks ago I bought 4 of them pumpkin output transformers here at the black market. The seller claimed that they would be perfect for a Focusrite 110 project, so I thought the transformers would have the second output winding for the tertiary feedback topology.
Waiting for the transformers to arrive, I made the PCBs for four channels, and stuffed and finished two of them.

When the transformers arrived, I discovered that they did NOT have two secondary windings - so no winding for the feedback! Of course I have been disappointed... :sad:
(BTW: everything is cool with the seller; he has just not been aware that the pumpkins for a 110 preamp have to have that second output winding; and he will take back the transformers if I don´t want them anymore).
The transformers I got are on a NEVE EV11381 card (out of a serie 66 console, as the seller said).

Now to the question:
Can I just use them in this circuit, and leave the feedback connection open? Can anybody tell me what will happen soundwise (I´m sure it won´t explode :grin? Or do I have to modify the output stage?

Cheers and thanks for your help; Christoph

The feedback is the hole point for building it! It wont burn without it. But it have a impact on the distortion. But what rations does the original transformer use?

wind your own tertiary winding on the existing transformers!
do you have info for the original?
the winding for feedback is often low impedance - ballbark figure 50 to 100 turns should do.

ioaudio said:

wind your own tertiary winding on the existing transformers!
do you have info for the original?
the winding for feedback is often low impedance - ballbark figure 50 to 100 turns should do.

Click to expand...

My guess is that they use a standard transformer with two symetric output windings.

bace said:

ioaudio said:

wind your own tertiary winding on the existing transformers!
do you have info for the original?
the winding for feedback is often low impedance - ballbark figure 50 to 100 turns should do.

Click to expand...

My guess is that they use a standard transformer with two symetric output windings.

Click to expand...

Guys, thanks for your response! (This is funny - I´ve posted this in July, now I get a response in December!)

I have no information about the original transformer. I will post some pics of the transformer I got (and of my build) in the next days. Perhaps Max (or anybody else) can guide me through winding my own tertiary winding, that would be cool.
And @bace: I know that the feedback topology seems to be the whole point for building it, but as written I thought I would get the original transformer, and had built the thing before I got the transformers...  I would like to try to get the best out of it.

Cheers, Christoph aka Toff

Hi. This is a quote from my earlier post:

Hi. I found the datasheet of the vt30408 and it says 1:2+1 the +1 being the tertiary winding. Primary can take +20dBm and secondary is rated for 600ohms. That's all it says.

Click to expand...

I've lost the datasheet since but I'm sure this is correct.
I'd say the vt30408 has a distinctive sound but I don't know if it's in the construction or the way it's used. I like the way it sound but I'm sure you could build something useful too by winding a extra secondary to your existing tx. Sure it would be cooler than buying a ready made one . Can't tell how it would sound though.

-Okko