royer modded mxl603 pair. Hum trouble.

Hello all,
I've been searching day and night and returned nothing. Still i apologise if the topic was already brought up...

I've done the second Royer mod to a pair of MXL603 mics. I've done the PSU on a eurocard format breadboard. As I intend to use them as a pair, I went for the "one psu box for the pair" option. Of course I'm in trouble now: they work great separately, but hum as soon as both PSU cicuits are powered.
Still, I love the box I used, it's a car battery charger I found and painted. I would hate to go the "one psu per mic" road, but I will have to do it if it's the only solution...



What I've tried so far:
-First I was using a 2x24V/30VA toroid transformer, with two separate secondaries, each powering its own mic. I experienced electrostatic buzz (it went up or down while turning the transformer), plus hum (the hum I'm having now). I tried two secondaries, or the secondaries in parallel, or only one, I still had the problem.

-bought a 2x12V/30VA "R" type transformer with secondaries in series (26V total with load) for both mics. Hum, but much less electrostatic buzz.

-one mic powered: no problem
-two mics: hum with both circuits working. If I unplug mic number 2(tube+capsule but leave the circuit powered), the hum stays the same in mic circuit number 1 (so I guess it has nothing to do with the heater supplies connecting to the same ground) and the power transformer voltage doesn't move (30VA has to be ok).

-the ground is a star gound to one only point.( Well of course in the picture you can see the screen of the power xformer is not star grounded, but it was when i was doing experiments.)



Do you think it could have something to do with the 1N4007 diodes that should maybe be paired? I'm at loss here, these mics have been on my shelves (like most of my "projects" i must admit...) for more than a year, I've tried 3 power transformers, and 2 pairs of OPtransformers, and they still don't work. I'm about to give up and make two ugly boxes with 2 24V transformers and dump my beloved enclosure.

Any advice greatly appreciated, thanks.

Hi

It might be not to helpful, but I did the Royermod with a pair of 603s, which uses one powersupply for both and do not experience hum problems like you. Well, the mics were humming at first, but the reason was the OP-transformers being to close to the power transformer. As soon as I seperated them, the hum was gone. The powertransformer lives in a seperate box now.
But your problem is probably different.
But one thing I don't understand: You write one mic at a time works ok, but two don't and when you unplug one mic the other one still hums???
What is the difference?
Well it seems to be impossible to introduce a groundloop with the second mic plugged in.

Sorry I can't help,  but one powersupply was possible for me as long as there was enough physical space from powertransformer to OPtransformers.
Tobias

Hi Tobias,
I've edited my initial post for clarification thanks to your remarks.
It already helps me to know it's possible to power two circuits with one power transformer. It's not hopeless!

could you post a schematic??? sounds like you might have a ground loop problem...


Bill

When I build a Royer type power supply I use a >=2 amp transformer more like a 50VA.  1 amp or so can just barely power some circuits I have used with that type supply without heater hum for one microphone.

  Is the hum heater supply related? 

I have hum problems with this mod as well. I've tried several different power transformers, changed tubes, different OTs, even rebuilt the PSU to different grounding schemes and layouts and the hum is still very present. My voltages come out low, wondering if the 6205 I'm using needs more juice than the 5840. When I move the mic cable it crackles like a bad guitar cord.

Gus, how can I tell if it is heater related?

Gus said:

When I build a Royer type power supply I use a >=2 amp transformer more like a 50VA.  1 amp or so can just barely power some circuits I have used with that type supply without heater hum for one microphone.

  Is the hum heater supply related? 

Click to expand...

My power transformer is a 24Vsec CT/ 30 VA. I'm afraid it is not heater related, because (let's say here I'm listenning to mic n°1):
-I have tried it with mic n°2 unplugged (so it's not drawing any heater current) and it makes no difference in mic number 1, the hum is still there.
-If i unplug the ground connection from mic n°2, the hum disappears in mic n°1.

I've tried other star ground points too.

The red and black flying wires on the top left of the first pic distribute the AC to the two circuits (except they're not soldered in this particular pic). Interestingly enough, the hum level seem to vary a bit depending if I connect the PSU to the bottom circuit or the top one. Still it only disappears when I disconnect one mic from the ground.

If there is a ground loop, I really can't find it. I've spent three more hours scratching my head on this last wednesday, and I think they're gonna end up in two separate boxes.

I've tried redoing the ground connections with solid copper wire to no avail, too.
I have replaced the 1N4007 diodes too.

BTW, I just realized I was talking about "my beloved enclosure, beautiful like the red roses on the sunday morning" while showing you its ugly innards. As you all know it's what's outside that counts and it doesn't look that bad when closed.