How do I connect a 5 wire transformer for my MXL 2001 Royer mod PSU?

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bsconz

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Jul 17, 2012
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I bought a transformer for this project which has 5 wires (obviously the 5th is a center tap on the secondary). The PSU schematic from Royer only shows 4 wires. Can anyone guide me (a mic modding noob)?!?!

I am following the original schematics which were in Royer's Tape Op article for both the mic and the PSU.

Attached a picture of the transformer next to the PSU which I started putting together on some perf board.
 

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The side w/ two wires is most likely the primary. Assuming you are going with the Royer PSU, and this trafo is a 24v centertapped, meaning 12v-0-12v on the secondary. You should be able to just isolate the center tap (which should be a different color from the other two) . Isolate with perhaps whatever those twisty things are called you splice household circuits with (can't think of the name at the moment) and electrical tape. Then wire according to the schematic.
 
I believe Blue Jinn was referencing wire nuts. If you are certain of the primary side, you could apply power to the primaries, and test the secondary sides to find which two gives you 24V. Make sure you don't let any wires short together or the transformer will blow. Do you have the datasheet from the transformer manufacturer? All of the info you are looking for should be there.

-James-
 
@HellfireStudios: So I think you answered my question. I am looking for the 24V AC voltage not the centertapped 12V. That makes a ton of sense now that I think about it. Maybe I could use that 12V center tap for something else on the power supply like another pilot light to monitor the transformer instead of just wire nutting it.

Anyway, thanks for all the help guys! Just waiting on the PCB's I ordered from Blue Jinn now to complete the mod...well that and the tube and Jensen.

 
You can't use the "12v center tap" for a pilot light. It isn't another voltage in that sense, but a tap in the middle of the secondary winding. Looking at the picture again, I'm assuming the white lead is the center tap. So you'd measure ~12vac between white and red #1and 12v between the white and red #2, and 24vac between red and red.  (Again, I'm assuming the transformer is "24VCT"  or some such nomenclature or "12v-0-12v"

You would use the center tap with two diodes to rectify 12v.

See this link:

http://www.engineeringslash.com/power-supply/full-wave-power-supply.html

High voltage tube power supplies e.g. use a center tap, that becomes the 0V reference, and the two outside wires  go to the plates on the tube, just like the diodes in the example linked to above.

The Royer circuit isn't really like those above, more like a pair of half wave rectifiers put together, so you can use one transformer and get two voltages.

Download this tool, it will give you some visual guides:

http://www.duncanamps.com/psud2/index.html

I avoid point to point as a matter of personal preference, so I opted for the high voltage transformer and used the PCB's. You could also do the G7 trick of back to back transformers.  I emailed you the G7 schematics. Take a look at that PSU and you'll see what I mean. (Note, that is based on European mains of 220-240, so you need to adjust accordingly.)

I also have a thread at homerecording.com on my royer mod. You need to have an account there to see the pictures though.

Shameless plug: I have a few transformers with 80v and 9v secondaries and 135v and 9v secondaries, specially wound for these circuits. They put out a bit higher then spec'd and the 80v will give net about 118v under a 100k load on the BigUgly psu board (with about a 300ohm resistor.) Let me know if you want one.
 
Since the center tap is unnecessary, just cap it off. Both of my transformers, used for the SDC and LDC versions of the Royer mod, are from the air conditioning industry. They have multiple primaries (115, 240, and 203VAC) and a single secondary at 24VAC. So, here in the US, the 240 and 203VAC leads are wire-nutted off. Same idea with you transformer. If it isn't needed, don't use it. BTW, I know I can use the multiple primaries for European power (240VAC). No use, yet, but that is why the wires are still available, if needed. Hope that helps.

-James-
 
Thanks for all this advice guys. I am going to do the most basic PSU and mic mod that I can for my first one though. I am following the original Royer schematics to the T just for simplicity's sake. Maybe I will try some more complicated stuff on future projects (I have 2 MXL 991s, a Behringer B1, an MXL 990, and some cheap dynamic mics too).

I thought of one more question about the PSU. How come a 2-prong AC cable is used? Why not use a 3 pronger and connect the ground to the circuit ground points? My only guess is that it would possibly introduce noise into the circuit. If this is the case then I will stick with the original schematics. Originally Royer put his PSU in a metal ammo case and probably grounded the circuit to the metal of that but I am thinking of putting this into a small plastic pelican case and using a 3-prong power cable.

Any thoughts on that?
 
Wait... I hooked the AC ground to the circuit ground and chassis in my recent Royer Modded LDC. Should I have disconnected the AC ground? I figured 0V was 0V, and the voltages sent to ground needed somewhere to go. Am I wrong on this? I have yet to hook the mic to circuit, so if I screwed up, the mic will survive. If someone who knows more about grounding PSU's could chime in here, I'm sure we would both appreciate it.

-James-
 
Here is a good thread and diagram:

http://www.groupdiy.com/index.php?topic=36857.0

Note, I inadvertantly "floated" my V-

the 3rd wire from the wall and the shield wire on the XLR (I used 6 conducter w/ shield and 5 pin XLR for the microphone connection)  goes to chassis, but the PS and signal grounds don't touch the chassis.
 
My PSU is complete! Decided to just go ahead with the original schematic and get it done. Still have couple of questions:

1) I know the voltages measured will not be correct without the rest of the circuit load (the mic) but does anyone know what the PSU will read at the post for the heater supply unloaded (pin 5 on the original Royer schematic)? I get about 38VDC (as an aside I get about 115VDC at pin 4 for the grid voltage but I'm not worried about that).

2) I am curious if I should also connect the ground lug of the 5-pin XLR connector the to the PSU circuit ground (this is done on the 3-pin XLR and shown on the schematic).
 

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Ok. After checking the schematic for the Royer LDC mod again. It shows direct continuity beteween the Common Negative (Pin 3 from the mic), The ground lug on the 3 pin XLR, and the Chassis. Now, his BOM only calls for a two lead AC cord. My concern is that I used a 3 leaded AC cord, and attached its ground to the chassis for star grounding. Can this hurt anything? Should I disconnect it? What about the shield lug on the mic input? Should this go to chassis ground, too? It is technically hooked to the Common Negative at the mic end. Thanks.

-James-
 
If you look at the original schematic (and the Neumann U47) you'll see a chassis ground on the microphone and the PSU. If you look at the digram in the thread I posted above, you'll also see this chassis ground. I don't know why the AC plug is drawn omitting the ground, but I don't think it was intended to be an ungrounded two prong adapter.
 
If the Bill of Materials from the TapeOp article says a two lead AC cord, it probably was intended that way by David Royer. I just don't see how this would be problematic as a 3 prong setup with star ground. I guess, when I get chance to test out the mic, I'll cut the ground from AC cord when/if hum occurs. Does this sound like a safe proposition? Thanks.

-James-
 
@Hellfire: I agree with your thinking about the original BOM. And the circuit would actually be grounded by chassis grounding it. I think that a 3-prong earth ground would introduce noise. It really is a safer method of grounding but not necessary.

Could you measure the voltages on your PSU without the mic plugged in? I am curious to see if you get the same higher heater voltage (~38VDC) that I get without the mic plugged in. Also, my supply voltage is a little high (~115VDC) but that is ok given the max allowed for the 5840 tube.
 
Unfortunately, my original lighted power switch is buzzing like crazy. It just happens to be scavanged off some old radioshack inverter, and is larger than the current ones available at radioshack (the hole for the switch is, unfortunately, already cut). So until I find a replacement (I hope mouser has something that will work), it is a no go.
Also, when I first tested it, there was no mic attached, and even days later there was still enough voltage stored on the heater supply to cause a spark when I inadvertently touched it to the chassis. I've checked my wiring multiple times, and everything looks right. My assumption is, with no mic attached, the voltage has nowhere to dissipate (like the tube's heater). Is this correct, or is there something wrong with PSU? Thanks.

-James-
 
That sucks. I hope you can find a replacement. If you would be so kind to measure those voltages I would really appreciate it!!!

I believe you are right though. The potential will be higher on the heater circuit without a load (the tube heater) which would draw some amount of current and dissipate heat in terms of watts. If our PSU circuits are the same we should get close numbers for this measurement with the mic unplugged which would put my mind at ease about connecting up my mic and turn the power on.
 
I'll see what I can do. If someone else has a fully working PSU, they might be able to get the answer you are looking for faster than I can. Also, check out google. These are not the only forums to cover the Royer mod. The other forums could contain the info you are looking for.

-James-
 
you got me thinking outside the box. I looked at the tube data sheet which says the 5840 tube nominal heater voltage is 6.3VDC and current draw is .15A. That means the heater resistance is about 42Ohms. I only had a 47Ohm 1Watt resistor so I connected one from pin 5 (heater supply) to pin 3 (ground) to simulate the heater load. Then I measured the voltage drop across the resistor and it came out to be about 6.8VDC (just outside the tube tolerance but now in the right ballpark).

I am guessing that I need to adjust the resistance by adding another one in parallel. Anyway, I feel better about hooking up the mic now.

On a side note, the 2 wire wound resistors get way hotter under load than I thought they would. I may try to design a small sub-circuit to utilize the center-tap off the power transformer to drive a small fan to keep air moving through the PSU case.
 
Check the resistors' operating temperatures on their respective datasheets. Then, use an infared thermometer to check the operating temperature of the resistors. If under spec, you all good...

-James-
 

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