HF reduction on royer mod???

[ihscoutlvr74]

Gents,

I'm looking for pointers on a HF de-emphasis circuit that could be used with the royer mod?  Turns out the capsule I'm using is a 67ish style that needs some HF tweaking in the circuit.From what I've read, the 67 or 87 style is not possible as they both use feedback. 

Is there a way to tone down the highs a bit without using feedback or is there another way to implement this using a 5840 tube?

I did read somewhere in the mic meta about possibly just puttting a cap to ground at the transformer.  Would this work? advantages/disads?  Is there a value to start out with?

Any help is greatly appreciated, Nathan

 

[ioaudio]

I did read somewhere in the mic meta about possibly just puttting a cap to ground at the transformer.  Would this work? advantages/disads?  Is there a value to start out with?

yep - 500pF to 1500pF

-max

 

[Gus]

Did you build the microphone or is the post from what you read on the web? 

If you built it did you try it with the stock 32mm capsule like in a mxl2001?
Did you use the jensen DI transformer?
 
There are issues with using a cap to ground at the plate.

 

[ihscoutlvr74]

Gus,

Many thanks for you chiming in - seems you have been a huge help for many on this forum. 

I was given a Studio Concepts C1 from 797 audio that was performing less than stellar to play around with.  After perusing many meta threads, etc. I find that the capsule in it is a u67ish copy and that they do not perform great unless some tweaking to the high end as in the original schematics with feedback.

I followed the basic royermod pdfs from long ago, however this is not the same design capsule as in the 2001.  Transformer specs are still a little mysterious re: impedence v. ratio..., but I am using a Freed 15k:500/200/50 - tapped at 200, +-2dB 20-20k, peak working v. to ground 175v, circa 8/24/1960 in a mu-metal can the same as the small UTC cans of the era.

I found some information on using a grid resistor and I put a 1 gig to gnd - also adding a cathode bypass of 100uF/1uF to the 750ohm.  This settled down the high end, but it seems to have lost some bottom end too - from what I've seen, adding these bypass resistors changes the output impedence in a possibly bad way. 

I notice a massive difference in highend omitting the film 1uF across the 100uF(cathode bypass), droppping signal strength and freq response in a bad way.  When I swapped the 1uF film to a different cap(GE 1.0uF@50v plastic), the high end was far less dampened while still using an elna 100/6.3v to gnd as well.  It does seem that the low-end is reduced using these 2 caps.

What are the drawbacks of using the cap on the output?
BTW, I'm using an RCA 5840w tube, 1uF Vitamin Q plate-transformer cap, 5.9v dc heaters, 96v to plate at psu.

Thanks again, Nathan

 

[ioaudio]

There are issues with using a cap to ground at the plate.

What are the drawbacks of using the cap on the output?

the cap shunts the high freq to ground - so the tube has to work into low impedance in the HF region, distortion rises.