Hi!
This week I have put together a couple of G7-ish tube mics, with the aim of doing Blumlein pair recording. I used the power supply and pattern control scheme from the G7, and a slightly altered amp circuit that is closer to the 'Royer' (i.e. 5840 tube)
The mics work nicely and sound good, but when checking them out in the studio I noticed an obvious difference between the sensitivity of the front and back capsules. For this application I want two well matched mics with accurate patterns (within the limitations of dual capsules!)
Looking back over the schematic, I noticed that there is a subtle difference in the way the polarisation voltages are handled.
Here's the G7 schematic: http://www.gyraf.dk/gy_pd/g7/gic_s.gif
For the backplates, the polarisation is set by a voltage divider (2 x 470k) which comes after a 10K dropping resistor. For the back diaphragm, the divider (2 x 100K) comes before a 10K resistor.
My measurements are as follows:
Omni mode gives 0-63-0, so no problem there.
I'm measuring 5V difference on cardioid - 63V vs 68V. 5V difference. Theoretically gives a slightly altered shape to the pattern, but inaudible to my ears.
However in Fig8 it's Front=0V - Backplate=63V - Rear=136V. So the difference between backplate and rear diaphragm is 73V, 10V more than the difference between the front and rear, which is certainly enough to hear an obvious difference in sensitivity between the two sides.
The fix is simple - either change one of the 470Ks, or put a dropping resistor before the 2 x 100K. I want well matched mics so I may try a pot in the PSU so I can set it accurately. I will report back when I've got good tested resistor values.
Cheers!
z50
PS I really don't want to seem to be criticising the G7 design - it has been absolutely inspirational to me and many others here. :thumb:
This week I have put together a couple of G7-ish tube mics, with the aim of doing Blumlein pair recording. I used the power supply and pattern control scheme from the G7, and a slightly altered amp circuit that is closer to the 'Royer' (i.e. 5840 tube)
The mics work nicely and sound good, but when checking them out in the studio I noticed an obvious difference between the sensitivity of the front and back capsules. For this application I want two well matched mics with accurate patterns (within the limitations of dual capsules!)
Looking back over the schematic, I noticed that there is a subtle difference in the way the polarisation voltages are handled.
Here's the G7 schematic: http://www.gyraf.dk/gy_pd/g7/gic_s.gif
For the backplates, the polarisation is set by a voltage divider (2 x 470k) which comes after a 10K dropping resistor. For the back diaphragm, the divider (2 x 100K) comes before a 10K resistor.
My measurements are as follows:
Omni mode gives 0-63-0, so no problem there.
I'm measuring 5V difference on cardioid - 63V vs 68V. 5V difference. Theoretically gives a slightly altered shape to the pattern, but inaudible to my ears.
However in Fig8 it's Front=0V - Backplate=63V - Rear=136V. So the difference between backplate and rear diaphragm is 73V, 10V more than the difference between the front and rear, which is certainly enough to hear an obvious difference in sensitivity between the two sides.
The fix is simple - either change one of the 470Ks, or put a dropping resistor before the 2 x 100K. I want well matched mics so I may try a pot in the PSU so I can set it accurately. I will report back when I've got good tested resistor values.
Cheers!
z50
PS I really don't want to seem to be criticising the G7 design - it has been absolutely inspirational to me and many others here. :thumb: