Tube Mic Royer Mod Build Thread

[dmp]

1) WHY just 330 ohms? A larger resistor is better filtering but more voltage drop. Doesn't the mike suck just a few mA? Can't that resistor be 1K, maybe 5K, without much drop?

I adjusted the value and used a 10k resistor for R1, since it would give better filtering, and with a 80 volt transformer this gave about 105v B+, which was in line with the value for the circuit Royer published in Tape Op. The 5840 tube has a max plate voltage of ~150v. And then with a B+ of 105v, the capsule polarization is about 52v.
The G7 psu uses the same filtering method, but with larger capacitors for the B+ in the PI filter.  I am posting that schematic for reference. I don't understand why the G7 has no buzz while this does? The larger caps, or could it be the heater voltage has somewhat poor filtering and is causing the buzz with the shared ground on the mic pcb? 

 

[Blue Jinn]

Quick clarification, the original schematic has a typo with 190v in. It is supposed to be more along the lines of 100-120v, and the revised schematic reflects this and BigUgly made the caveat that R1 needs to be adjusted to suit. With 80v in I found 270-330R and a nominal 200k-300k load to net about 118 volts using both the Duncan amps tool and an actual PSU build. (Which PSU is the bare circuit board, and hasn't been put into practice just yet.)

I'll double check don't seem to have any problem with hum or buzz, but I do need to let the mic "warm up" for about 10-15 minutes for the noise to go away. Note this is on my original PSU which is underrated somehow, and only gives me ~70vdc under load.

 

[dmp]

I tried changing the ground scheme to have the mic body connect to ground and the cable shield to float on the mic end (as described in other threads and a sketch by emperor tomato ketchup). This didn't change the buzz.
The level of buzz is low, but too much if the mics would be used for room mics or the like.
Anyone have suggestions for tests that would help determine if the buzz is due to the ground scheme or PSU filtering?

 

[Blue Jinn]

Tried out again today, no hum (and shorter  warm up time) some hiss, but actually the same amount of hiss as another unmodified 2001 that I have.* This is the cardioid only version, 5 pin XLR, common ground for B+ and Filament. The cable shield is grounded to the PSU chassis, and unconnected at the mic side. The mic however, is grounded (via the PCB.) My IEC ground is chassis grounded, but hte PSU grounds aren't (so they're floating (which there is an itch on my brain that says I shouldn't do that))  but I don't have any hum.

Interesting side note. The modded 2001 orig PCB had an unused place for a pad switch (like the ADK 51 and a few other whatever OEM model that is) the PCB's are 0553 and 0554. The unmodified 2001 I just got, doesn't have that, and hte PCB's are labed 0553A and 0554A.

 

[dmp]

A tried a series of tests to try to determine where the buzz was coming from (shut off the heater & shut off the B+, shut off one at a time, etc...).
I discovered that the buzz is caused by the power transformer being close to the psu circuit. The field from the transformer must be inducing buzz in components and/or the traces.
I am able to rotate it and mount it a little further away in the box and this should fix the problem. Maybe a shield in between as well.

Blue Jinn, thanks for the help!  I have some hiss as well - I have yet to measure / compare it to other mics.
I do have the psu ground connected to the chassis at this point.
Also, both ground schemes in the mics work fine. (I have two mics - one with each scheme now). I think I will go with the scheme where the mic body connects to the cable shield (isolated from ground in the mic).

Dan

 

[dmp]

So changing the position of the transformer reduced the buzz.

I have a Soundelux PSU that can power the mic as well and in comparison this PSU still has a little buzz.
Blue Jinn, possibly you do not have buzz because the psu ground is 'floating'?
With tests I've determined that the heater circuit and B+ circuit seem to have a ground interaction that shows up as buzz in the mic.
The test:
With the mic powered with B+ and Heater from the same PSU, I hear buzz.
When the B+ and the heater in the mic are powered from the two separate PSUs I built - NO buzz.  (ground scheme the same, w/ audio grounds connected between PSUs)
When the B+ and the heater in the mic are powered from the two separate PSUs BUT the heater on the B+ PSU (which was previously unloaded) is connected to a 50 ohm resistor (i.e. load like a tube)  - I hear buzz.

 

[tchgtr]

Have any of you built this circuit with a dual capsule yet? If so, are you happy with it, and where did you get the 250K, 10 step pot?
Thanks

 

[dmp]

Have any of you built this circuit with a dual capsule yet?

Yes, I used the CK12 style capsule from a group buy here and the sound quality is great.

where did you get the 250K, 10 step pot? Thanks

It is not a pot - rather a rotary switch. I used 11 positions in a 12 position rotary switch from mouser.  You need an odd number in order to have a 'middle' cardioid setting. R ~= 250k / (#positions-1)

 

[tchgtr]

Thanks dmp,
    I'm thinking of building this circuit with the capsule I took out of a Nady 1050, and replaced with one of those very same group-buy C12 copies. I still have one more, and wanna do a C12 circuit, but that's for later, if I ever actually have disposable cash again. Fun, ain't it! Thanks for your answer.

Have any of you built this circuit with a dual capsule yet?

Yes, I used the CK12 style capsule from a group buy here and the sound quality is great.

where did you get the 250K, 10 step pot? Thanks

It is not a pot - rather a rotary switch. I used 11 positions in a 12 position rotary switch from mouser.  You need an odd number in order to have a 'middle' cardioid setting. R ~= 250k / (#positions-1)

I see.
I found this: http://www.calradstore.com/25-424.html which seemed to match the BigUgly schemo, but that is obviously not what you used...

Trying to cobble this together with parts on hand, including a nice Thordarson TX that puts out 125vac and 6.3vac., and some nice PSU filter caps of 180uf (?!) that I have lying around. PRRs input above inspired me to get creative, but I need to read it a few more times.
Already have one Royer cardioid build from a 2001, and figured this combo of parts might make a nice match for that to do MS recordings, as they are all center-terminated capsules, and Royer kinda copied the U47 circuit. I already have everything except the Cinemag, and they're across town.
It's nice to see more discussion of mics around here, even if we do get ridiculed by the actual geniuses every once in a while.

 

[dmp]

I found this: http://www.calradstore.com/25-424.html which seemed to match the BigUgly schemo, but that is obviously not what you used...

I think that would work. Just remember that it will have ~100v so take care. The resistance doesn't need to be exactly 250k. The lower the resistance the more it loads the B+, the higher, the less.
The Soundelux PSU that I bought from another member here has a pot, not a switch. There are +/- to each approach.
You can turn the pot while monitoring the sound without getting pops (+ for pot)
You can have exact recall and know exactly what your positions are with a stepped switch (+ for rotary switch).

Incidentally, I examined the Soundelux circuit and it uses an approach more like what was described by PRR above. Four stages of B+ filtering, then a zener diode to set the B+ value, then two more stages of filtering. Two stages for the heater before the regulator. Seems very robust and well designed.

With 135v AC, your B+ will likely be quite high. You may want to think about how you are going to get the B+ you need.

 

[tchgtr]

All good points. I've been examining the G7 PSU schemo option also, with 110/9v and 110/15v AC line trafos. Very cool idea. Might be the smartest way to go, but my local surplus store doesn't have those values.
Should save the Thordarson for a preamp, though...
This build is still a month or two away for me, so I'm just window-shopping now.

 

[Blue Jinn]

So changing the position of the transformer reduced the buzz.

I have a Soundelux PSU that can power the mic as well and in comparison this PSU still has a little buzz.
Blue Jinn, possibly you do not have buzz because the psu ground is 'floating'?
With tests I've determined that the heater circuit and B+ circuit seem to have a ground interaction that shows up as buzz in the mic.
The test:
With the mic powered with B+ and Heater from the same PSU, I hear buzz.
When the B+ and the heater in the mic are powered from the two separate PSUs I built - NO buzz.  (ground scheme the same, w/ audio grounds connected between PSUs)
When the B+ and the heater in the mic are powered from the two separate PSUs BUT the heater on the B+ PSU (which was previously unloaded) is connected to a 50 ohm resistor (i.e. load like a tube)  - I hear buzz.

That is weird.  Does the separate PSU, heater loaded with resistor buzz doing that with both supplies? (I.e. swap the one that has the resistor?) I'm reaching here.

Even with the grounds floating above earth, they are still common and connected at the PSU and on the mic cable. Have you actually tried floating it to see if that changes anything?


FWIW, I had a MXL V69 (traded it for some other stuff from "mista min" in Germany) It had a weird cabling issue, that I think I fixed, but it had a slight amount of hum. I've never seen a schematic for that PSU.

 

[Blue Jinn]

Followup for dmp:

You have two psu's I & II. Wired thus: I : B+ / Htr shunted  II: B+ open / Htr wired and you get buzz. Have you tried all the possible combinations here:

B+ connected/open/shunted (200k resistor or so?)
htr connected/open/shunted

 

[dmp]

Hi Blue Jinn,
I actually didn't try all the different permutations, but both PSUs behave identically, i.e.
I: B+/Htr = buzz
II: B+/Htr = buzz
Then
I: B+/htr_open w/ II:htr = buzz

I can't go back and do more because I have modified the boards significantly to try different experiments.
I have lifted the grounds on the PSU and connected them with wires (ref the grounding 101 thread) and I have one unit working without buzz. I want to get both working this weekend.
Basically this PSU board is designed with both the B+ and Htr connecting to a single ground plane, which apparently can be problematic for creating a quiet PSU. Both the Soundelux and G7 design have the B+/ Htr grounds connect in a single spot, without a single ground plane. 

Note also that the buzz is very low - it is perceptible in comparison to a mic without buzz, but is hard to tell if it is even there at all without comparison. It changes the tone of the noise floor.
For reference, I am recording in a quiet room with 55dB of gain - then boosting 30dB in my daw to listen to the noise floor.

 

[david-p]

PRR's version of the HT (B+) supply has plenty of smoothing.  Bit it may be that you need more on your heater supply.  Try putting 47 µF across Rv1.  If that improves matters, but is not enough, then put an extra R-C filter between C5 and the LM317.  I am not sure what your heater current is, but if you put a resistor that drops a volt or so in the line and then a large C to ground (2200µF or 4700F) on the right hand side of the resistor, you will improve the buzz on the heater as well as it can be done.

I am using an EF14 that takes 300 or so mA, and thus needed to do this.

David

 

[dmp]

Try putting 47 µF across Rv1.

Tried that - didn't change the buzz

put an extra R-C filter between C5 and the LM317

Tried that - didn't change the buzz

I also added an extra RCRCR stage to the B+ between the filter caps - didn't change the buzz.

The reason the experiment I've described above was so illuminating was because the heater on it's own didn't have buzz, and the B+ on it's own didn't have buzz. It is only when the tube is powered from  the B+ and heater on the SAME PCB that I get buzz. Once this happened, it made me think the heater and B+ smoothing circuits are sufficient independently, but interact somehow on the pcb in a bad way.

Thanks for the suggestions!

 

[Blue Jinn]

It is only when the tube is powered from  the B+ and heater on the SAME PCB that I get buzz. Once this happened, it made me think the heater and B+ smoothing circuits are sufficient independently, but interact somehow on the pcb in a bad way.

Thanks for the suggestions!

Did the buzz go away when you lifted the PCB ground from the earth ground? What happens if you lift the IEC ground?

 

[dissonantstring]

not a good idea (a very bad idea) to lift the mains (IEC) ground.

 

[david-p]

Is this a 120Hz or a 60Hz buzz?  It sounds like maybe something is NOT grounded...  Have you tried connecting various spots with a piece of insulated wire?

Does it change when you get your hands near the PCB?

Or, is it perhaps the layout of your PCB?

David

 

[Blue Jinn]

not a good idea (a very bad idea) to lift the mains (IEC) ground.

Agreed. But I'm curious if this isn't a ground loop somewhere else in the system. I don't seem to have the same buzz with the same PCB. (although admittedly, I'm not measuring it with 80some dB of gain.)