G7 transformer feedback...

... I want to understand how the g7 mic uses a two channel transformer. Is there some kind of feedback going on? Or what other principal is at play... Thanks

To my knowledge, it is a feedback like U67.
But chefducuisine wrote  it in this thread:
http://groupdiy.com/index.php?topic=56304.0

Directly from Gyraf's G7 page on his website

"On the PCB board there's an option for adding a trimmer and a capacitor in order to introduce cathode feedback to the tube amplifier stage. This lowers both distortion figures and output impedance, but cancels out a lot of the "tubey" sound - the reason we bother to make a tube microphone in the first place. The reason I have'nt removed this option, is for purely educational reasons - if you want to do some experimenting yourself. This is also where you can put in various equalizations if you like that. But for the best sound - at least to me - leave the tube undisturbed by feedback and other dirty tricks."

Naive1073,

The G7 is a mixture of the M49/U67/U47 circuits.
The Lundahl transformer that has been specified by Jakob is a 1+1:5 that is used 'in reverse' to transform the output impendance of the EF86 to a usable region for the following mic-pre.
This means the '5' side is the primary winding and one of the '1' sides is the secondary and leaves us with a further '1' winding.
This further winding can be used for an equalization feedback.

The useage of this winding depends on the capsule you put into your microphone.
If you are going to use a C12 / K49 or an M7 capsule type no equalization is neccessary and you can omit this winding.
If you are using a K67 type of capsule there is a built-in high frequency lift that needs to be tamed. Otherwise you'll get the famous chinese harshness. If you use this winding for feedback be shure that the phase of it must be 'reversed' to work correctly.
This is what Charlie's and MetalD posts both refer to.

Please check the U67 schematic to get the idea.

I've built two G7 variants. Both have an original K67 in it and I therefore had to connect the feedback network.
If you use a trimmer as Jakob suggested you can even fine tune the response to the talent.

Believe it or not: These G7s are in heavy usage here and beat even expensive available mics freightently easy.

Be warned: A G7 is only as good as the components you put in - go for quality here.
The G7 has been one of my favourite mics for years now - Jakob has done a truly outstanding job!!!

You can see my built here:
http://twin-x.com/groupdiy/displayimage.php?album=455&pos=1

Regards,

Chris

Edit:
Can you please change the thread title to something more usable (i.e. G7 transformer feedback). This makes it easier for others to find if they have a similar question. Many thanks.

Hello chefducuisine !
Thank s for sharing Your nice Build !
Did You  design the PCB by your own?
When You use the Feedbackwinding to tame down the high Frequency Rise from a K 67 Capsule how did you calculate the R in the R/c Filter ? I know 1/  2 pi * R*C  is the Cutofffrequency in a Highpassfilter and you just want to send the high Frequency into the Feedbackloop,to get rid of the Highboost, no? Is R the secondary Trannywinding, the 2,2k poti or the 1,6 k/220 kathoderesistor or a Combination of all ? So how to calculate C ( in the Schematic the  100uF Couplecondenser in the Feedbackloop) Or did you experiment with different Values like 1 uF, 100 nF, 10 nF until you were satisfied with the Result? Of course it depense on the Frequency Chart of a given Capsule.
I also have build  me 4 G 7 Mics (  2 with a Ck12 style Capsule and 2 with Dale M 7 ) and can confirm that they sound fantastic and can stand up against every Tubemic ,no matter of Price. For me they are the Missing Link between a C12 and a 47 style Sound.
Thank s.
Greetings
Lothar

Hi Lothar,


I designed the PCB myself because the ones that where available are simply too long for most commercial available housings that could serve as a donor mic.
It follows the original design except for the transformer - I decided to mount it 90° to the circuit as you can see on the pictures to save some space inside the mic body.

As for the calculation of the corner frequency on the feedback filter:
I had the same problems as you!
While the formula you suggested is the same that I used I struggled on how the feedback winding of the transformer influences the freedback network or transforms it back to the circuit (the winding ratio is squared for impendance).
To be honest I still don't understand the theory and if this influence is caused by the trafo ratio, the inductance of the winding or something else.

Maybe someone more intelligent can chime in and explain this? :-[

Instead I went the trial and error route on the feedback and installed a capacitor together with a trimmer.
I ended up with a restistance around 1kOhms and a 100uF cap if my memory serves me right.

Please note that I did not use the specified 1+1:5 Lundahl tranny. Instead I used an LL1576 (1+1:7). I assume that this also influences the feedback.

Regards,

Christian

Thanks to all for the very helpful info and for making my dumbass thread more generally useful.