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Post by svart on Jul 6, 2016 7:14:42 GMT -6
So after a big tracking session this weekend, I was faced with a few days off work.. And a bunch of projects to catch up on. One of which was something I've wanted to do for a while, turn a couple old MXL603/991 mics into Royer mod SDC's. I had most everything I needed, so it was more of a "see how little I need to buy" type of project. I only bought a few things.. A pair of impedance matching transformers in the 12:1 range, a couple 5840 tubes, and a metal box. I already had everything else. I'm doing something different with these mics. I'm putting the transformer and cathode resistor inside the mic so I can run differential through the cable to lessen the amount of noise. I also didn't realize that I was out of 5 conductor mic cable. I used flexible MIDI cable instead. it's not very different from mic cable since it's shielded as well. I also ran one heater wire, the other side is grounded. Seems to work OK, no problem with hum that I can tell.  The power supply box. Pretty much the same as the royer schematic, but I had to adjust the resistor values to get the right voltages. I didn't feel like doing a layout, so I just used some breadboard. Now I remember why I hate breadboards. Attachment DeletedMy first prototype. I simply stripped one of the mic's original boards and used a nibbler tool to cut most of the board away. Since I put the transformer in the mic, I needed some more room.  The finished prototype. The mylar coupling cap is the largest thing in the mic. I need to see if there is a smaller one I can order. Attachment DeletedThe second mic. This one I cut out some copper clad to fit to make it more tidy inside. This pic also has the source for the transformer too. I got them off Ebay for 10$ each.  Cat tax. All in all, I spent less than 50$ to turn a couple mics and some other spare parts into a pair of royermod SDC mics. So far they seem quiet and sensitive. I need to do some testing and see how they sound on certain sources. I read that they can't be beat on toms and things like that. We'll see. |
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Post by svart on Jul 6, 2016 8:19:33 GMT -6
I should also mention that I tried both cathode-follower, and plate-follower styles of tube hookup. The transformer ratio is more suited to plate-follower topology, but I found that to be slightly noisy. That could have totally been my fault for not doing too much to find optimized resistor values.
I might still do a quickie etching layout to make things a bit nicer, but then again, I might not. If I do, I might do a MOSFET based B+ and heater regulator setup so that things are much better regulated, but I gotta say that these are pretty quiet as-is, so it'd be more for regulation when one or both mics are plugged/unplugged.
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Post by svart on Jul 6, 2016 8:57:26 GMT -6
One thing I did notice though, is these are self-quiet, but tend to hum when brought near anything that has an EMI field. LED lights, computer, monitor, etc, they tend to pick up hum really well. Not sure what to do about that. It doesn't seem to be a shielding thing overall, but it might be the fact that Royer used the grid-leak style of bias on the capsule, so that it's inherently sensitive to small current fields.
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Post by EmRR on Jul 6, 2016 15:35:16 GMT -6
I don't know if the classic Altec mics have the same hum problem or not. Might be a clue.
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Post by wiz on Jul 6, 2016 16:55:35 GMT -6
nice job Chris
cheers
Wiz
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Post by rocinante on Jul 7, 2016 11:29:34 GMT -6
Any chance you could do a comparison to other mics that are similar? No need to post any scientific results (we trust you) and its hard to distinguish audio online anyway. Just your thoughts would be plenty good enough.
I want to do this.
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Post by jcoutu1 on Sept 13, 2016 7:40:48 GMT -6
...how are these things? Any more info?
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Post by svart on Sept 13, 2016 8:28:25 GMT -6
I haven't really put them through their paces yet. I bought some smaller mylar caps to exchange into the mics, but haven't done so yet. I was thinking of using them as room mics, but I'd like to try them on acoustic, shooting them out against the KM184's.
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Post by markfouxman on Sept 15, 2016 0:01:43 GMT -6
One of the greatest challenges for SDC tube mic is a NOISE! That is apart from the obvious fact that one of the greatest tube challenges is reliability. No matter what tube topology you chose--anode follower, cathode follower, transformer coupled, direct coupled, cascode... whatever... you still get a NOISE a magnitude higher from SS!!! But even with a careful tube choice and selection you still never get over a RELIABILITY. At least, LDC's have less of the noise problem...
Sure, the tubes can make the circuit sound pleasant. On the other hand, the rightly implemented and optimized SS can also make it sound equally pleasant... without any drawbacks of tubes...
Not to discourage from a project--myself has been thinking about it for awhile... just wanted to point out some other design considerations...
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Post by svart on Sept 15, 2016 6:54:22 GMT -6
One of the greatest challenges for SDC tube mic is a NOISE! That is apart from the obvious fact that one of the greatest tube challenges is reliability. No matter what tube topology you chose--anode follower, cathode follower, transformer coupled, direct coupled, cascode... whatever... you still get a NOISE a magnitude higher from SS!!! But even with a careful tube choice and selection you still never get over a RELIABILITY. At least, LDC's have less of the noise problem... Sure, the tubes can make the circuit sound pleasant. On the other hand, the rightly implemented and optimized SS can also make it sound equally pleasant... without any drawbacks of tubes... Not to discourage from a project--myself has been thinking about it for awhile... just wanted to point out some other design considerations... Indeed! I normally wouldn't have looked at SDC tube mics, but I had the unused mics and enough parts leftover from other projects that buying another 50$ worth of parts to try the Royer mod mic build was worth the time and money. They seem relatively quiet, but I haven't really had a chance to put them through their paces yet. I'm in the midst of a couple recording projects and they've been requiring me to leave the mics and gear set up a certain way until they are done. |
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Post by svart on Mar 21, 2019 11:57:56 GMT -6
So I'm actually looking at getting back around to these. I built one mic and power supply and it worked great. I added the second mic and the power supply started to really stress out. While I adjusted the dropper resistor values, etc, this really needs to be a regulated supply as things started to get HOT.
So with that, I'm thinking of whipping up a regulated supply using a TL783 or something for HV and LT317 for heaters and make them adjustable.
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Post by svart on Jul 6, 2020 14:52:49 GMT -6
I actually finished these a few weeks ago. I used some EBAY switching power supplies. One of the supplies was a 80V-200V step-up for nixie tubes and the other was a 2-10V step-down. Both needed 12-15V input so I used a small wall-wart I had lying around.
First measurement showed fairly small ripple on unloaded outputs, but when plugged into the mics the ripple increased to unusable amounts. Some experiments showed that it just how they worked with the long cables as the regulation is probably too slow to account for cable length and resorts to a sort of oscillation. I added some unknown inductors from a couple junk computer power supplies and some big electrolytic caps to filter the outputs more and the ripple decreased to 10-ish mV with mics plugged in. Now the output of the mics is very quiet at normal preamp settings, something around -90dB for the gain I normally use for room mics.
Since these MXL mics are generally a bit zingy sounding with stock electronics, it's important to note that they no longer sound like that. They're quiet smooth now, which I don't know if the strange Russian tubes I'm using are the reason for this, or if it's just the lack of whatever was used.. I've never really bought the thought process of swapping "higher quality" parts in these mics as a fix for mismatched capsules and electronics, or at least I've never heard as much difference as others claim to have heard.
Anyway, these are quite nice now. I might still try to whip up a small carrier board for the EBAY switchers to make mounting them easier.
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Post by svart on Jul 7, 2020 9:30:59 GMT -6
Well, I had thought about doing a quick little carrier board for these switchers I used but it looks like the specific one I used for the HV is no longer available and there are only larger and more expensive options available. I had thought about just releasing the gerber files for anyone wanting to use this setup for the Royer SDC's since I don't want to sell them or make kits or whatever, but I also don't want to support another project either.
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Post by svart on Jul 17, 2020 12:32:54 GMT -6
Ok, so I waffled on the PCB thing for a few days and decided to whip something up. I found some chinese step-up switchers on ebay for about 10$ each that could serve the purpose. They seem to be similar to the one I used, but the size and connectivity are different. I ordered a couple and they'll be here within the month. I have a handful of the little switchers I use for the 6V supply so I was able to measure them fairly accurately for size and connectivity. I'll need to get the high voltage ones from china before I can measure their dimensions accurately but their ebay site had rough dimensions which I used to create a rudimentary outline that I'll adjust once I can measure them. The board will use a normal circular DC input and the switchers will solder right to the top of it. LC filters clean up the output a bit and there's a pair of connectors for the outputs. 4" square.  |
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Post by svart on Aug 3, 2020 9:02:45 GMT -6
Went ahead and finished up a board for this because I actually want a power module I can use for a tube preamp as well. LV can be 6-12v as long as the input voltage is a couple volts higher than the desired output voltage. HV can be 90-230V but the higher the voltage the less current is available, roughly 30ma at 200V according to the module seller. I added a switch location just in case as well. I ordered a few more HV modules and 15 boards because I want a few for my own projects, and I'll build a few to sell if anyone wants them.  |
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Post by wlinart on Aug 3, 2020 11:59:01 GMT -6
That is really interesting for almst every tube microphone. May i ask which modules you used?
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Post by svart on Aug 3, 2020 12:06:47 GMT -6
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Post by wlinart on Aug 3, 2020 12:15:28 GMT -6
Looks nice to make a quick psu. Thanks!
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Post by markfouxman on Aug 8, 2020 23:40:05 GMT -6
Interesting! What is the frequency of those? I have a few buck and step up converters here. Did not get to check them out, yet. I thought those operate somewhere in 100kHz range and the sellers claim they do not require any further filtering. But still, it is much easier to filter higher frequencies rather than 60Hz... Best, M |
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Post by svart on Aug 9, 2020 13:15:27 GMT -6
Interesting! What is the frequency of those? I have a few buck and step up converters here. Did not get to check them out, yet. I thought those operate somewhere in 100kHz range and the sellers claim they do not require any further filtering. But still, it is much easier to filter higher frequencies rather than 60Hz... Best, M The small switcher is running 30khz with filament load. Haven't tried to see what it does with a light load. The larger switcher for HV is running in discontinuous mode due to the light loading, which is normal for these kinds of controller chips. If the load is very light it'll burst small amounts of PWM trains to keep the overall average voltage up, but it's rather noisy in doing so, which is what the majority of the noise needing to be filtered out is. It allows a 100khz PWM burst every few khz, but it's not constant. |
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Post by svart on Aug 11, 2020 7:50:27 GMT -6
So I received my boards last night. They look good, however I found a few issues. One is that the shutdown control is actually opposite what the datasheet for the HV board says.. Two, the switch footprint is off a little. Three, the HV output came up with no load but under load really fell apart.
Turns out the new batch of HV boards can't regulate under load. Further investigation found that they're identical in overall design despite the different layout, but they stuffed inductors with "471" printed on top whereas the old boards had "470". Pulling up Ebay, some of the same boards shown in a search have 470 and some have 471..
That means they stuffed 470uH inductors instead of 47uH inductors.
Why is this a problem you ask?
Well, the circuit is switching current through a MOSFET and during each cycle, it's pulling a specific amount of current through the inductor based on the programmed switching frequency, and with this part (UC3845B) that frequency looks to be about 50KHz-80KHz but it can only do UP to 50% dutycycle.. If any part of this balance is off, then it can't optimize the amount of current through the inductor and you end up with poor regulation and/or inability to even reach the correct voltages since it would take a lot more current saturation than this IC is able to provide due to the limitation of 50% duty.
This results in the unit just sitting there getting hot, which is exactly what it was doing.
I'll swap in a 47uH inductor and give it a test soon, or I could look into changing the switching frequency to match the 470uH, but I think that would be too low to be of use.
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Post by svart on Aug 12, 2020 8:24:18 GMT -6
Got home a bit late last night and needed to do other things, but found a few minutes to unmount the inductors and measure them with my LCR meter. Indeed, they used 470uH instead of 47uH. I swapped them on the boards and tried it.. No go. Something else is also different. I measured all the caps and resistors on the boards and most of them are the same, or very similar.
There's two differences that don't seem like much, but could be an issue.
one is the current feedback network on the original board uses 1K and 1nF as an RC filter. The new board uses 1K and 500pf. This would allow some more voltage peaking to happen and it's possible that the higher peaks could cause it to shut down before fully regulating.
Two is that the resistor for the oscillator is 3.3K and not 2.4K on the new board. The capacitor for the oscillator is the same though. 3.3K shifts the frequency lower by about 15KHz, which shouldn't necessarily cause it to not work. It's certainly not right for 470uH either though.
It's also possible that these are just bum chips too, unable to regulate properly under any condition. I'll end up swapping the chips around if nothing else seems to work.
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Post by svart on Aug 13, 2020 6:59:45 GMT -6
Did some more testing last night. I put 2.4K and 1nf on the new board and it still didn't work. Swapped the ICs and it didn't work. Swapped most of the parts over from the working board and it still didn't work. At this point it seems to be something else unseen, perhaps the layout in some way. I see a lot of thermalling around the ground pads of the parts, which is never a good thing for switching devices. I'll try adding more ground to the parts and see what happens.
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Post by Guitar on Sept 7, 2020 20:13:31 GMT -6
This is really cool! Love to see a little bit of the technique that goes into it. The only problem I would have with those mics are the capsules themselves. I did some mods on some of those mics and in the end, solid state, I felt that the capsule coloration was holding me back ultimately. If you plot the frequency response there will be weird peaks and dips in the top end, it's just something inherent to those capsules. Microphone Parts claims to have fixed this and they sell capsules from their web store, with a supposedly flatter response, but I never tested them. I am wondering if there are other capsules out there that would fit those bodies, since there are so many of those kinds of mics floating around.
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Post by svart on Sept 8, 2020 11:53:09 GMT -6
This is really cool! Love to see a little bit of the technique that goes into it. The only problem I would have with those mics are the capsules themselves. I did some mods on some of those mics and in the end, solid state, I felt that the capsule coloration was holding me back ultimately. If you plot the frequency response there will be weird peaks and dips in the top end, it's just something inherent to those capsules. Microphone Parts claims to have fixed this and they sell capsules from their web store, with a supposedly flatter response, but I never tested them. I am wondering if there are other capsules out there that would fit those bodies, since there are so many of those kinds of mics floating around. Honestly I don't mind these mics with the tubes. They sound just fine. The problem now is that out of the power supplies I bought only a couple even powered up and those promptly stopped working with any kind of load on the output. Guess that's what I get for ordering chinese boards. I still need to make an order for some brand new ICs and see if those bring these back to life. I really should have just done my own boost supply design but I was interested in cutting development time off and I had such an easy time with my first chinese boost board I figured it would be a slam-dunk. Oh well. |
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