Newbie needs help plz - Very low output on D-87 (Diy U87) Needs 60-65 DB gain

[jazzandvo]

Hi Gang
I finally got around to getting this D-87 DIY mic put together with the help of my brother. Used Chungers SYT body(Love it) and
Cinemag CM 13113 Transformer, Mouser parts found on ww.vintagemicrophonepcbkit.com and a BeesNeez capsule. Mic looks great and when gain is pushed hard on my mic pre, the mic sounds good but the issue is I have to really crank my gain up to get a level, decent sound. My other mics (even cheap ones) get a good level at even 35-40 DB but I need to crank this D-87 up to 60-65 to get a decent level. At 50 DB, you can't even hear mic. Any idea what might be the issue gang?
Thanks so much!!

 

[poctop]

Hi Gang
I finally got around to getting this D-87 DIY mic put together with the help of my brother. Used Chungers SYT body(Love it) and
Cinemag CM 13113 Transformer, Mouser parts found on ww.vintagemicrophonepcbkit.com and a BeesNeez capsule. Mic looks great and when gain is pushed hard on my mic pre, the mic sounds good but the issue is I have to really crank my gain up to get a level, decent sound. My other mics (even cheap ones) get a good level at even 35-40 DB but I need to crank this D-87 up to 60-65 to get a decent level. At 50 DB, you can't even hear mic. Any idea what might be the issue gang?
Thanks so much!!

the D87 is 10db lower output than modern mic but not more than that , this mic has been built here hundreds of time
i suspect a bad bias or transformer wiring,  or capsule wiring , or bad capsule ,
hope this helps,
Dan,

 

[microphonefolks]

Hello, if you wired the transformer in backwards you would load down the output of the FET.

Also, if you the capsule is not getting polarized correctly then you will also significantly drop the output level.

If the circuit is identical to a U87 there is a point where you can inject a signal into the front end of circuit and then measure the output level of the microphone.  My guess is its the same circuit as a U87 you should measure about 35db of gain between the input to the fet and the output of the fet.  There will be a 22db drop from the fet/transformer side to the output of the transformer.

If you post the schematic I can tell what the polarization voltage should be and where you can measure it.  You can't measure it directly on the capsule and must measure on the other side of the blocking resistor.  the original U87i had about 40v getting to the capsule and the U87ai has 60v on the capsules . 

Cheers, Dave