Tube microphone with ef80 tube and oep transformer.
- Thread starter peterbach
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RuudNL
Well-known member
Although some capsules can withstand a polarisation voltage higher than 60 V, I usually don't go higher.
In some cases even a bit lower...
The sound of the capsule will change depending on the polarisation voltage.
With a higher voltage, the membrane will be more attracted by the backplate and this will influence the tensioning.
With a very high voltage, the membrane will be sucked to the backplate and in extreme case will even arc.
In some cases even a bit lower...
The sound of the capsule will change depending on the polarisation voltage.
With a higher voltage, the membrane will be more attracted by the backplate and this will influence the tensioning.
With a very high voltage, the membrane will be sucked to the backplate and in extreme case will even arc.
Also, the bottom side of R6 should go to ground instead of the cathode.
Understood, but you should know what voltage is required for your capsule.peterbach said:
As a Typical LDC, 60V seems to be the optimum value.
You don't need the 100k potentiometer. There's no point in adjusting precisely the bias voltage. Just make R3=120k and connect R3 to R4 directly. You may need to alter the value of R3 or R4 if your B+ is different than 200V.
You don't need the 100k potentiometer. There's no point in adjusting precisely the bias voltage. Just make R3=120k and connect R3 to R4 directly. You may need to alter the value of R3 or R4 if your B+ is different than 200V.
I seriously doubt the LF performance of the OEP transformer driven by such a high impedance.peterbach said:
Yes you can; now depending on what choice you want to have, it may be simple or a little complicated. The easiest is card/omni, which just involves connecting the two sides of the capsule together to get omni.
Getting figure-8 involves creating a half voltage bias connected to the backplates, the diaphragms being connected, one to ground, the other to the full bias voltage. A switch that connects the two diaphragms via a capacitor sets the fig-8 pattern.
You should look at the U47 and U48 schematics.
mhelin
Well-known member
There is another OEP transformer which is 1:6 mic input transformer with following inductance measurements from another thread:
OEP Mic input 1:6 model Z21808
Using handheld meter: Primaries 5.83H
Using HP 4262A meter:
Primaries at 120Hz - 10.5H, at 1K - 6.9H
Secondaries at 120Hz - 318H
Haven't used that tx but EF80 with 6:1 should be ok if you find proper operating point/bias.
OEP Mic input 1:6 model Z21808
Using handheld meter: Primaries 5.83H
Using HP 4262A meter:
Primaries at 120Hz - 10.5H, at 1K - 6.9H
Secondaries at 120Hz - 318H
Haven't used that tx but EF80 with 6:1 should be ok if you find proper operating point/bias.
mhelin
Well-known member
Datasheet: https://docs-emea.rs-online.com/webdocs/0d72/0900766b80d72828.pdf
Pin view = bottom view
When wired as 6:1 pins 6 and 9 are your input pins, and pins 2 and 4 your output pins (pin 3 is missing) . I guess at least pin 5 should be grounded also. The odd 1:1.8 output pin is propably there for current mode applications when used as line input transformer for feedback.
This transformer is available from RS at least, google for Z21808E part number (then one without mumetal shield) or Z21808C if you want one with the shield. In addition to RS this link might work:
https://www.enrgtech.co.uk/buy/search?search_by=keyword&terms=Z21808E
(the images are not correct, E-model doesn' have shield)
It's still only 5k4:150 ohms transformer by design and doesn't have too much headroom. Have to be seen how it performs in real life.
Pin view = bottom view
When wired as 6:1 pins 6 and 9 are your input pins, and pins 2 and 4 your output pins (pin 3 is missing) . I guess at least pin 5 should be grounded also. The odd 1:1.8 output pin is propably there for current mode applications when used as line input transformer for feedback.
This transformer is available from RS at least, google for Z21808E part number (then one without mumetal shield) or Z21808C if you want one with the shield. In addition to RS this link might work:
https://www.enrgtech.co.uk/buy/search?search_by=keyword&terms=Z21808E
(the images are not correct, E-model doesn' have shield)
It's still only 5k4:150 ohms transformer by design and doesn't have too much headroom. Have to be seen how it performs in real life.
