Tube Mic Royer Mod Build Thread

Starting a build thread for the Royer mod boards being sold in the black Market.
http://www.groupdiy.com/index.php?topic=46004.0
I just finished building this board in a MXL2001 and it has low noise and normal level BUT very little bass.
I used a Beyer 5:1 output transformer wired backwards. I'm wondering if this is causing a HPF because it isn't a high enough ratio (a 12:1 is recommended in the TapeOp article)? The output cap is a 2.2uF WIMA. Tried a couple tubes with the same problem and a different capsule.
This is the mic schematic.

And this is a picture of my build.

You have the Beyer wired as  a 5:1? The Apex mod (Dave Thomas' the upcoming project) uses a 5:1 with a 2.2uF cap, but off hte cathode follower. So I'm assuming a higher ratio is preferred here.

I used an inexpensive BV11 knockoff 11.5:1 in mine with a 1uF. They're $15 with shipping from Ebay, and slightly larger than the stock 2:1 that came with the mic. I have some extras too. I don't have access to Ebay at the moment or I could send you a link.  Do a search on ebay for KCM transformer. I had to drill the base of hte  mic a bit to m ake it fit in the bell. Just a few notches for hte corners. It was a tight fit.

Did you look in the meta?  There are Royer threads
http://www.groupdiy.com/index.php?topic=256

I am not surprised you have a bass issue.  You have some reading to do.
Or use the parts listed in the build

EDIT  Blue Jinn

Blue Jinn said:

The Apex mod (Dave Thomas' the upcoming project) uses a 5:1 with a 2.2uF cap, but off hte cathode follower. So I'm assuming a higher ratio is preferred here.

Click to expand...

5:1 with a CF?

I had a wiring mistake on the Beyer transformer. With that fixed, the mic sounds great.
Noticeably better than the unmodified. Now to modify the unmodified to complete the pair.

I used the 5:1 because I had them on hand and the G7 design uses a 5:1 in the same configuration with the EF86. It doesn't create as low an output impedance (and I've read in a thread that a 10:1 is a good mod for the G7) but I have a G7 now that works great with the 5:1. 

I would love for chunger to do his detailed build of this mic and PSU

Gus said:

EDIT  Blue Jinn

Blue Jinn said:

The Apex mod (Dave Thomas' the upcoming project) uses a 5:1 with a 2.2uF cap, but off hte cathode follower. So I'm assuming a higher ratio is preferred here.

Click to expand...

5:1 with a CF?

Click to expand...

Yes,  although per Foxaudio it is actually a constant current draw amplifer, with about a 600 ohm output impedance. Dave Thomas also indicated that the stock Apex circuit is borrowed from a IMSMC a McIntosh preamp. (Sowter has a 3:1 for the same purpose, there is another thread on that.)

http://www.foxaudioresearch.ca/CCDA.htm

how are the nylon "plugs" (for lack of a better term) being sourced? (the 3 holes at the top of the board)

Gonna try and put my materials list together this weekend so I can hack at this thing next week.

I got these from Mouser:
Teflon standoffs 534-11160

I believe I got the part number from Blue Jinn.
Note that they are too large for the board and I had to drill out the holes slightly.

I have an order in to Mouser, but I'm pretty sure this is the part number:

534-11216

I'll report back any problems.

dmp: where did you get the tube sockets?

dmp: where did you get the tube sockets?

Click to expand...

I posted in the black market and another member had a couple.

i can see the sockets and the tubes being an issue... hmm.

i can see the sockets and the tubes being an issue... hmm.

Click to expand...

Why???
Having the tube in a socket makes it easy to try different tubes without having to solder.

There's plenty of 5840 tubes around and the sockets come up on ebay now and then.
I just bid on 10 sub min sockets today and if I win the auction I'll have more than I need. I can pass a few on.

dmp said:

i can see the sockets and the tubes being an issue... hmm.

Click to expand...

Why???
Having the tube in a socket makes it easy to try different tubes without having to solder.

Click to expand...

Insulation resistance of the socket might be an issue.

Insulation resistance of the socket might be an issue.

Click to expand...

For noise? I don't think I am seeing that with the two mics that I've just built.

The problem I am having is a 60Hz type buzz that is noticeable in the noise floor.
This is the same in both mics that I built, so I believe it is a design problem.
When I shut off the PSU while listening to the mic, the buzz immediately stops (while the mic continues to work for a few seconds... ) so I think it is verified to be a PSU problem.
I have a G7 that does NOT have this problem, and the psu circuit is slightly different. The B+ filtering caps are larger (470uf vs 220uf) and the heater and B+ do not share the return ground. I'm not sure what the problem is yet...

This is the PSU schematic

There's plenty of 5840 tubes around and the sockets come up on ebay now and then.
I just bid on 10 sub min sockets today and if I win the auction I'll have more than I need. I can pass a few on.

Click to expand...

I would be most interested in one if all goes well on the bay. 

> This is the PSU schematic

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?

2) Even so, a single filter stage can only do so much. For LARGE ripple reduction cost/bulk increases directly as "large" increases, and with exponential hearing this really means 3X or 10X cost/bulk for every little bit of buzz reduction. (And there are limits with ESR and stray wire resistance.)

Here's rule-o-thumb. Use as many R-C filter stages as there are amplifier stages from here to the loudspeaker in a simple system (like guitar-amp). This mike might drive a Fender directly. There will be 2 or 3 small stages and a power stage, at least 4 amplifier stages. Even though your mike lacks all the later stages you need about this many R-C filters to get the buzz down.

Here's rule-o-thumb. If not gasping for supply voltage, allow 10%-20% drop in the filter. Any benefit from a mere 20% rise of B+ is likely to be hidden behind buzz.

(If you like math and can compute raw ripple and mike-amp's PSRR, assume 20db-30db is a good target for each R-C stage. Less for high current low loss (EL34 screens), higher for low current (preamps).)

Below I've drawn your supply with several more R-C filters. I used your "330r" value and cut it in fourths to spread it around.

However 330r seems teeny. What is the mike's actual draw? There's 100K so the whole thing can't act like less than 100K. The 100K "fights" a tube so the sum is probably more like 200K. The supply is an astonishing 260V. Total demand is near 1.3mA.

By the 20%-drop rule we should use around 50K total drop resistance. Say 10K per R-C. Or since caps are cheap, maybe less. 4K7, 6K8, 8K2, whatever is handy.

Another rule-o-thumb. Use 1uFd per mA of current, to 10uFd/mA for small loads and low-cost caps. 220uFd is absurd. Five 220uFd 350V is large, and the mike will stay-on a very long time after AC is cut. R-o-T suggests 1.3uFd-13uFd. 22uFd is convenient and cheap. Since you already have two over-size 220uFd on board, I only used 22u in the middle, but you could re-do with only 22u and 5K.

Another rule-o-thumb. 3,000-pixel images don't fit most PC monitors.

me> the whole thing can't act like less than 100K. ...probably more like 200K

More R-o-T suggests the head-amp may be near 150K.

Also: I overlooked the Pattern pot, 250K.

Total load may be like a 90K resistance.

Four 5K resistors is another 20K.

This gives an 18% drop. Or 215V at the connector.

This does not hurt electrical headroom. Capsule bias is gotten from the B+. Therefore capsule output falls with B+. The buffer swing as fraction of its supply does not change for large change of B+.

And of course the Pattern trim is also ratiometric so supply is not critical.