U87 low output

[Greenie]

I have a U87 build (vintagemicrophonepcbkit.com) that has a fairley low output compared to anothe build I have (Vamisound). Is there any part that contrubutes to lower output or is it just the way they are constructed?

 

[Khron]

Do they have identical capsules? Or can you swap capsules between them?

 

[OneRoomStudio]

What transformer did you use? Is it wired for 50ohm or 200ohm impedance? What capsule did you use? What values did you use for the C3 cap? Lots of variables here...

 

[pasarski]

What is Varmisound? Maybe it just has more output than U87?

 

[OneRoomStudio]

What is Varmisound? Maybe it just has more output than U87?

It's another U87 board set: https://vamisound.com/pcb/u87-style-pcb-s-se/

(Jan and Milan at Vamisound do good work)

 

[ElSmurf]

How did you bias the fets? Also, did you compare the schematics and component values?

IIRC the Vami 87 isn't a 100% recreation like the one from Dany.

 

[pasarski]

It's another U87 board set: https://vamisound.com/pcb/u87-style-pcb-s-se/

(Jan and Milan at Vamisound do good work)

I see. Don't know why I saw an "r" in the name. Should be about the same gain then.

 

[Voyager10]

The U87 circuit has a 'calibration' facility, where you can test the electronics by connecting a signal generator across the 560R resistor R10 and measuring the output signal.

The headphone output of a phone works well as a signal source (there are several 'tone generator' apps), and you can measure signal levels in your DAW or with audio test software (https://www.roomeqwizard.com/ is one of the best). It looks like both mic PCBs have an R10 to which you could connect wires.

None of this needs precise setup if you're just comparing the two mics' relative levels. Just keep the headphone volume and mic preamp gain the same.

If the levels are very different the causes might be:

• Different capsule capacitance (try swapping the capsules?)
• Different feedback capacitor C3 (smaller value gives higher gain)
• Different FET
• Different bias current
• Different output transformer ratio (you can check by connecting signal generator directly to transformer primary)

If the levels are similar when fed from the signal generator, then either the capsules themselves have different sensitivity (related to physical design), or the polarization voltage is different. You can measure this at either end of 330K resistor R13, it should be somewhere over 40V, depending on the current drawn by your DMM. If it's noticeably lower on the less sensitive mic, that's a circuit fault and can probably be fixed.

Good luck, people here are always happy to answer questions if you have them.

 

[Greenie]

Hi and thanks for answers. I have a T13 Tab Funkewerk in the low output one and a Moby Bv13 in the louder one. I have tried the bias pot, but with no big difference and I have trierd different fets with same result.

 

[RuudNL]

I suppose you are not comparing a U87i with a U87ai? (n that case the difference in output is about 11 dB.)

 

[Voyager10]

There's a post here which compares the original U87 with the -A version. There are several changes which increase sensitivity but the big one is taking the rear capsule out of circuit in cardioid mode. On the original it was always connected and basically acts like a 6dB pad.