Dave Royer SDC Mod trouble

[Travis1000]

Hello all, after tinkering with this on and off for a few weeks now, I am calling in for reinforcements! This is my first build, and after some initial cold solder joints, I got the mic up and running. After using it on a session or two, I noticed the mic pre needs to be all the way open to get signal that approaches 0VU on my DAW. This doesn't seem normal after reading other peoples responses to this mic. After reading this thread (http://www.groupdiy.com/index.php?topic=46396.0) I double checked my voltages.

At pin 2: 90V
    pin 5: 5.8V
    pin 1: 36V

According to the schematic in Dave's article, the voltages on pin 2 and pin 1 seem a bit low. Would this be the cause of low output level? If so, what would cause the voltages to be a few volts low?

I am doing two things differently than the article suggests. 1) I am using a Shure KSM109 as my donor mic, which I think is mostly irrelevant. 2) I am using a Cinemag CM-11021 (http://cinemag.biz/mic_output/PDF/CM-11021.pdf) which is what the folks at Cinemag suggested when I e-mailed them.

here is a link to download the original article.https://www.dropbox.com/s/r046jjs1v01jzwl/Another%20Tube%20Microphone%20From%20Barstow.pdf

Any guidance and suggestions would be greatly appreciated!

 

[bruce0]

Voltages need to be measured with mic attached to psu.  But I assume you are doing this since your volts are low. It is sort of low across the board, Is it possible that the transformer you are using is not 24V? Are you n 50HZ/220 land or 60Hz/110 land?  What AC voltage do you measure on the secondary of the transformer?

That said, low voltages (10% low or so) would not explain the problem you are having.  What tube are you using?  5480?

The cinemag transformer wired as 2:1 stepdown should be fine.  D Royer used the Jensen MB-D as a 2:1 stepdown.



bb

 

[zebra50]

Hi Travis,

Can you post some pictures of your build? It might help the guys here to troubleshoot.

Did you use the capsule from the Shure? Isn't that one an electret mic? If so is the electret polarised in the right direction? You may need to flip the capsule wires.

 

[Travis1000]

Hi guys, thanks for the replies. I am in the land of 110. On this last go round of measuring voltages, I did not check the secondary of the power transformer, but I seem to remember it measuring right at 24 Volts. And yes, I am using a 5840.

I am using the KSM109 capsule. I hadn't thought about the electret capsule. I will check that, and if that doesn't help, I'll post pictures of my messy build! : )

I went ahead and used the mic on a session the other day and I will say, even with the gain of the preamp cranked (in this case, and A-Designs Pacifica) it was dead quiet in terms of noise. And the mic sounded pretty good on the acoustic guitar that I was mic'ing.

 

[bruce0]

The electret mic capsule is a whole different thought, and an excellent one.

http://en.wikipedia.org/wiki/File:Electret_condenser_microphone_schematic.png 

Some fet's can take pretty high voltages, (and would drag the grid to ground in your schematic... thus more current thus your lower B+....  ) and even if you fried (or saturated) the little FET or one of the wires there, there might still be a charged electret backplate to provide some signal (the electret is permanently charged plastic) so you might get signal.

The KSM-109 specs on the shure site say that it is a "Permanently Biased Condenser", while they don't use the word Electret, I believe that is what they are revealing.  Nothing wrong with electret mic's, but I don't know how to use this circuit to amplify them.  It is usually done with a FET buried in a sealed capsule as shown (though there are other types, and sometimes folks cut and separate the FET to Ground link, and use, and they have a bias current provided by the external amp (various method for this, I have a schematic and a board layout posted elsewhere in this forum in case you want to reconstruct the mic (but I would be cautious about the capsule condition (if it is a typical one they are cheap to replace) after potentially putting high voltage across the fet, they typically run on 3V-14V)).

Attached is a good diagram of different ways to bias an electret capsule (like the little aluminum panasonic capsules) to get different level handling and noise performance out of them. These little capsules make excellent mics and are used in many if not most measurement mics.

A fella named Linkwitz has done some excellent work on modifying these capsules and has a lot on line. Using the attached schematics you might be able to figure out how to use the Royermod 2 circuit ( the one you are using) to amplify that capsule, but I would not depend on the FET, and I would be concerned about spacing of the wires and contacts with 90 volts... depending on how your capsule is configured. 

I think you might be the first to mod an electret mic into a royermod tube mic.  So welcome to the bleeding edge (the first pioneers get all the best arrows)... and let us know how it goes.

Be especially cautious about how you ground that mic body, (as you should be in any mic with High Voltage inside) because you are in the land of the untested and a dead short to the mic body could happen (The operative word being dead.). 

But there may be a way to do it using this capsule.

 

[zebra50]

I think you might be the first to mod an electret mic into a royermod tube mic.

Not at all! Check this from the man himself, in 1996, before the tape op articles.

http://www.jensen-transformers.com/as/as076.pdf

I made this circuit some years ago with an SM94 (?) electret capsule, and it worked fine!

 

[bruce0]

I don't see the electret capsule, is that what the thick capsule plate implies, I thought that was just the backplate.

So the idea is to ground the electret?

 

[zebra50]

Hi!

The country boy was originally done with an electret but is pretty much the same as the "Tape op 2" circuit.

You need to ground the -ve side of the electret, and connect the positive to the grid. It is a minimalist design, but it seems to work! You need just an electret capsule, not the kind with built in FET etc.

Stewart

 

[bruce0]

That is cool. 

I don't have any experience with the electret capsules except the ones that have the fet inside.

When I did the Royermod 2 (country boy) the only change I made was to separate the interface transformer into a separate little box, so I could use the same power supplies for both Royermod 1 and royermod 2 boxes.  (I needed to change the pinouts a bit for safety in case I plugged them in wrong), but it worked well, and isolates the output transformer from the psu in a separate little case.

 

[Travis1000]

UPDATE: I swapped the leads going to the capsule this morning and that definitely did the trick! With preamp gain at around 40dB I get just below 0VU on my meters in my daw. That is much more like what I am used to seeing! Thanks for your help guys!

 

[zebra50]

Good stuff! With pre-polarised capsules you need to get them the right way round!

How does it sound?