A few random u47/u48 questions

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

salomonander

Well-known member
Joined
Aug 28, 2011
Messages
913
Hey there,
Im building a u48 and i could not find answers to some questions.

- psu: what inductor value should the choke have? The original uses 2x8H. Should i get a 32H or 16H replacement?

- can i get away with 30 Ohms for R4 instead of 29 Ohms

- having a hard time finding the 150M resistors. Could i replace them with the 200M i have lying around without a compromise?

Cheers
 
Pretty sure lots of people have use 27 and 30 ohms in place of the 29. It will affect your bias. I would get both values and measure. If you have the room, why not allow for two resistors to combine for the value that gets you closest to the 1.1V bias.

200M would be fine for your backplate bias. Have you thought about doing a variable pattern when in fig-8?

I read somewhere that EMI would modify their U48s for 63V capsule bias. I did that with my EF800 U47/48. Should be a fairly minor change with a 2W 510 ohms or so resistor and a 126V supply.
 
Thanks Neil,
any idea where i can find more info on the mod? I assume its to compensate for the loss in cardioid?

And do you know what current rating my mains transformer and choke have to be? Im also wondering about the choke‘s inductance value. As far as i know inductors in series only add up if they are magnetically separated - which i assume the original 2x8H part was not?

Thanks!
 
Found my 29R resistor. But still wondering about the chokes inductance/rating and if i may replace those 150M with either 178M or 200M. Anyone?
 
I tried reading on the cardioid thing and gave it a shot. I cant upload here for some reason and had to use and external hosting site.


I copied the voltage divider from the U47 schematic and drew an external 120v line for the back diaphragm (should it be 126v?). I really dont know about the values of R2 and R5. I copied the 100M from a m50 schematic, but im clueless. Please help with the value of these two. I can easily access 100M, 200M and 300M



https://ibb.co/vQHFy0C
 
Not quite.

The U47 has a capsule bias of 63V, U48 is 52.5V. Now, you may not prefer 63V over 52.5V, but if you did, you would need a 126V supply. Bringing a seperate feed from the power supply that is variable (like M49) allows you to achieve variable polar patterns when your switch (or relay if your body doesn't have a switch) is set to figure 8. This allows variable patterns and the cardioid level boost of switching out the back diaphragm.

I haven't thought about it too much with a VF14 but it should be simply adding a second B+ dropping resistor before the 1780R. Pull your capsule bias from before (the 126V side) the extra dropping resistor. R5 connects to the variable pattern voltage (0-126V) from the power supply. R8 and R6 connect after the dropping resistor to acheive the 105V for the VF14.

There may be other ways to do it. There may be drawbacks to this way. I'm not knowledgeable enough to say for sure. If you do this, make sure the extra dropping resistor is of high wattage. At least 2W, preferrable 5W if you can find a nice compact 5W resistor of the correct value.
 
Thanks a lot for your input. I think i would prefer to not install any additional high watt resistors inside the mic body for heat reasons. I should have mentioned it earlier, but i do have one of the hand wound drop resistors and i want to not mess with the heat dissipation.

Would it work when changing the voltage from 120v to 126v on my schematic?

Anyone on R2 and R5?

I assume i could also get away by simply changing the voltage divider to u47 specs, giving me 63v in cardioid and sacrifice some of the symmetry in figure eight?
 

Latest posts

Back
Top