"U67"-ish microphone

[Matt Nolan]

...until you have to replace a part that has a leg that forms a bus. Desoldering gets messy fast. I'm a fan of PCBs myself.

Another solution to that is to not use part legs as buses.

 

[OneRoomStudio]

Another solution to that is to not use part legs as buses.

Fair. I was responding to the pictures in post #98.

I'm building an M49c on turrets right now, so I'm not opposed to getting away from PCBs. For more complex circuits and/or circuits that use a lot of transistors, PCBs seem to be cleaner though.

 

[Kaskelot]

Getting ready to power before connecting M7 capsules. Will I need a load to protect the transformers? Dont anticipate any strong AC signal and the cap is rated 1kV Just want to check V readings.
And yes I anticipate hum before I get the enclosures.

 

[OneRoomStudio]

Getting ready to power before connecting M7 capsules. Will I need a load to protect the transformers? Dont anticipate any strong AC signal and the cap is rated 1kV Just want to check V readings.
And yes I anticipate hum before I get the enclosures.
View attachment 126449

Are you building a microphone or a radio control tower?

Just kidding! Very interesting technique, and not a bad way to test voltages. I'd anticipate that the hum will be as loud/louder than your signal though.

 

[kingkorg]

Getting ready to power before connecting M7 capsules. Will I need a load to protect the transformers? Dont anticipate any strong AC signal and the cap is rated 1kV Just want to check V readings.
And yes I anticipate hum before I get the enclosures.
View attachment 126449

That plate under the capsule will affect it's frequency and pattern response in a really bad way.

 

[Kaskelot]

That plate under the capsule will affect it's frequency and pattern response in a really bad way.

Will be 5 mm machined brass.

 

[Kaskelot]

Are you building a microphone or a radio control tower?

Just kidding! Very interesting technique, and not a bad way to test voltages. I'd anticipate that the hum will be as loud/louder than your signal though.

Bazooka ! Have ordered brass mesh and the housing is 2 mm brass.View attachment M7 Mic från sidan3_1.jpg

 

[kingkorg]

Will be 5 mm machined brass.

Doesn't help, it will cause reflections anyways, and cause cancelations in high midrange. You'll get quite substantial nulls and spikes in very audible range. If it absolutely needs to remain this way, you should elevate the capsule some more, and make a dome that would difract the soundwaves away from the capsule.

 

[Kaskelot]

Doesn't help, it will cause reflections anyways, and cause cancelations in high midrange. You'll get quite substantial nulls and spikes in very audible range. If it absolutely needs to remain this way, you should elevate the capsule some more, and make a dome that would difract the soundwaves away from the capsule.

Thanks! I am aware of the problem with standing waves and coloration. Every cavity and every piece of metal has a resonance giving a "wowel" as in the vocal tract or a trumpet, ogan pipe etc. Maybe you didn't see that the top is a ring, so the paralell surface is not so big. I could slant the top ring maybe 10 degrees to deflect.
Thankfull for all suggestions.
View attachment Plates 3_2.jpg

 

[OneRoomStudio]

Thanks! I am aware of the problem with standing waves and coloration. Every cavity and every piece of metal has a resonance giving a "wowel" as in the vocal tract or a trumpet, ogan pipe etc. Maybe you didn't see that the top is a ring, so the paralell surface is not so big. I could slant the top ring maybe 10 degrees to deflect.
Thankfull for all suggestions.
View attachment 126476

The problem isn’t (only) with the top ring and standing waves, it’s comb-filtering caused by primary reflections off of the surface of the plate below the capsule.

Sound will enter the microphone and simultaneously hit the capsule and the bottom plate, then some of those waves will bounce off the plate and strike the capsule immediately after, causing phase cancellation with the waves that directly hit the capsule. The only way to prevent this is to do as @kingkorg said - move the capsule higher and/or add diffusion/absorption to the surface of the plate below the capsule.

 

[Екатерина]

Hello RuudNL, I'm quite interested in this circuit, but I don't have an EF86. Would it be possible to use EF800 or EF184 as a substitute?

Just for fun, I recently put together a "U67"-ish microphone.
It is a kind of mix between a U67 and a U87.
The microphone works as expected, sounds good and I don't hear any difference between this microphone and a U87ai.
Measured self noise is 2 dB higher than a U87ai. Not too bad, but maybe a better EF86/EF806 could improve this.
I used an original Neumann capsule that I had as a spare.
The microphone is powered from an existing tube power supply, delivering +120 V for HT.
De-emphasis is through feedback from the output. (15 KHz is 5.5 dB down referred to 1 KHz.)

For those who are interested: the schematic is in the attachment.

 

[abbey road d enfer]

I'm quite interested in this circuit, but I don't have an EF86. Would it be possible to use EF800 or EF184 as a substitute?

It may work, somewhat, meaning it would probably make some sound. Note that the heater current is higher and the cathode resistor may need to be adjusted for proper operating current. Also the pin-out is different.
I don't think it's ideal. These tubes are designed to operate with about 10mA plate current, which puts them in the small power category. They are not particularly low noise.

 

[Екатерина]

It may work, somewhat, meaning it would probably make some sound. Note that the heater current is higher and the cathode resistor may need to be adjusted for proper operating current. Also the pin-out is different.
I don't think it's ideal. These tubes are designed to operate with about 10mA plate current, which puts them in the small power category. They are not particularly low noise.

Thank you for your reminder. I think I should order an EF86 produced by EH.

 

[RuudNL]

I suppose this is possible. The pinning of the EF800/EF184 is different than the EF86, but you can check this with the datasheet. It might be that you have to change the value of the cathode resistor for correct biasing.

 

[My3gger]

Thank you for your reminder. I think I should order an EF86 produced by EH.

Where you live NOS tubes like Tesla EF806, PF86 or EF86 should be cheaper, lower noise and last longer in a microphone. In my experience, EH EF86 and the other version with gold pins is good tube for preamps where grid resistor isn't higher than 1Meg or so.