Simplified U67 circuit

[LevinGuitar]

Can this schematics work as a simplified U67 circuit?
Without S1, S2, S4 and all filtering caps? Is the new deemphasis switch placed correctly (before or after C17)?

I find a bit easier to use C12 negative bias way instead of the U67 with chinese power supplies, guess it will work the same, right?

 

[gary o]

I cant help but would also like to learn about the U 67 circuit.

 

[dmp]

As long as the tube is biased, it will work. Sound the same? Build one both ways and compare. I'd be curious what you find. The bias scheme of the tube is a significant effect on the character of the mic, in my experience.
S1 and S2 are for low frequency control. With no low freq control, removing the CRCRC is fine, I think.
S4... I don't see?
S3 is position (omni, fig8, and cardiod)
S5 is a pad, can be omitted if you want.
defeating C17, probably better to not have the switch be on the high voltage side. It will "work" either way. But S1 was designed to have 75v on the switch, so not that big a deal.
There have been many other threads here on 'simplified' U67s you could search for.

 

[Moby]

Can this schematics work as a simplified U67 circuit?
Without S1, S2, S4 and all filtering caps? Is the new deemphasis switch placed correctly (before or after C17)?

I find a bit easier to use C12 negative bias way instead of the U67 with chinese power supplies, guess it will work the same, right?

I can't see a problem with this. C17 with switch is disconnected before or after, does not matters IMO, but why do you put that switch? I think you can trim it to the taste and leave it.
Also, so called "C12 negative bias" can also work, but maybe you will need some filtering if you pick up the noise or humm though the cable.

 

[gary o]

I have been messing with a point to point version of U67 circuit with a suspected broken Max Bv12( I posted in another u67 thread) saying the tertary measured 12 ohms & that I couldnt Ihear any de emph going on.... well having worked out the pin out of the transformer from photos I have got that wrong, my real tertary winding measures open circuit, for the hell of it I connected them anyway, to my surprise there are now less highs, I instaled a switch too, I also have a switch to switch in different C17 values for and my old ears c17 makes little if any difference, i e no capacitor of 150 pf

Can anyone make sense of this....to recap (pardon the slight pun) when I connect my seemingly dead tertary winding with or without C17 connected quite a lot of the highs go missing , kind of working as should I guess , but how

Thanks in advance Im enjoying learning about this great circuit.

 

[Gus]

A little off topic
If you don't have a good U67 transformer(one that works like a real 67 transformer. Think about how the higher frequency Primary to Feedback C17 bypass cap interacts with the how the transformer is wound and how the lams were made)

I don't understand building a u67 clone microphone without the HPF. Without the HPF and good transformer it is another microphone

If you don't want that part and don't care about the feedback and want to keep the + heater supply
You could build a cathode biased VM1, Valvet etc like circuit with >= 470uf cathode bypass cap and then add a LPF. There was a 67 like design posted here without the HPF
OR
for fixed bias
Use the m49b/3-s heater circuit adjust the voltage divider in the heater section for the bias you want. I do not understand why Neumann did not add a cap from the grid and voltage divider node to ground in that bias method to help reduce noise. One could use a bias voltage divider with a trim control to find tune the bias and then used fixed resistors.

 

[LevinGuitar]

Is the S2 the HPF you are talking about?

I have no experience with this circuit so don't know if I would like to have the S2 connected, but I guess is easier to get rid (with EQ) of some low end excess in case it would be too much. And would like to keep the circuit at minimum for easier diying.
For the transformer I'll get a good one, look if it fits the donor mic (B2) and build the circuit around it.

Modding chinese psu to achieve negative bias I found to be easier with the C12 way: less filtering and changes needed rather than turning the heater negative plus the divider.

If the C17 switch will do little difference, will get rid of it too, I thought it was a big part of the negative feedback. But since I will use a chinese K67, that is supposed to be brighter than the original capsule used in this mic, guess that I would need to increase the value to get closer to the original response.
I'm not geting into the headbasket shape for now, even it will make more difference than the C17 cap value.

 

[Gus]

The C12 biasing is OK.