Drastic sound difference with transformers in PM1000?

[eanderso13]

Hey there. I'm new here...been reading Tape Op for years, though and love it! I recently decided to try the old PM1000 thing, I racked a few for a guy years ago and he loved them.

What I've noticed recently is that running unbalanced outputs to my MBox, with a Cascade Fathead ribbon mic, the sound is pretty nice, but there is an annoying ringing going on (prbably grounding...need to check) that doesn't matter with louder sources, but quieter sources, like acoustic guitar or vocals, it is an issue.

So I added some Altran C-4000 output trannys. Now if I run from the XLR output the ringing is greatly reduced, but the sound with the Fathead is very tinny and brittle sounding almost. Also makes the sound a little "distant"...

A few points:
1) I took the direct outs from C37 in the PM1000 circuit. I also ran this to the high of the primary on the Altran.
2) I know ribbon mics can be delicate...could mine be damaged to effect the sound that greatly? It was recently on a mic stand and was a bit unstable and slowly fell a few feet horizontally into a wall...

Any insight would be greatly appreciated!

Thanks.

[The Scum]

What's the nature of the ringing? A little added bell-tone after transients? A constant low level whine?

Tell us about your power supply.

Have the modules been recapped?

How do you have the transformer outputs wired? Do you have a Zobel/load resistor? And how are you unbalancing them for the MBox? If you left the cold output floating, that'll make the sound tizzy.

If you have other pres, diagnose the ribbon by running it through them. If they're ugly sounding too, it's probably damaged.

[eanderso13]

Thanks for the reply! To answer your questions:

1) the ringing is a constant low-level whine. it makes it with no input plugged in, and I hear it through my monitors or headphones. The computer monitor does not affect it. I've seen other posts about a 2250 Hz or so whine and most people attributed it to bad grounding.

2) The power supply is the Fivefish Studios PM1K kit with torroidal transformer.

3) I recapped the modules with Wima caps replacing the tantalums, and new nichicon electrolytics replacing the electrolytics.

4) What's with the Zobel/load resistor? The transformer pinouts are here:
I wired the negative of C37 to Pin 4 on the tranny, Pin 6 to ground. Pin 1 of the tranny I wired to pin 2 of the XLR, Pin 3 of the tranny to pin 3 of the XLR and pin 1 of the XLR to ground.
And I'm not sure what you mean by unbalancing the outputs for the MBox, my MBox has XLR inputs on the back....not kosher? Should I run the XLR outputs of my preamp to in-line impedange matching transformers and then into the DI input of the MBox?
And for the cold output floating comment, do you mean not connecting Pin 1 of the XLR jack to ground?

Thanks for the help!

[digitaldrummer]

when I used the altran c4000's I used 1&3, 4&6. nothing required grounding on the output side.

you can always wire a 1/4" unbalanced in parallel with pins 1&3 of the tranny - this gives you the option to go unbalanced either for testing or tonality reasons.

Mike

[The Scum]

Is the level of the whine independent of the input gain? What about the fader?

Are they near a CRT, which might add interference? Is the whine there with all of the lights out?

Do you have an oscilloscope? Look for hash on the 44V rail...heck, walk through the circuit, seeing if you can identify where the hash is coming from.

And for the cold output floating comment, do you mean not connecting Pin 1 of the XLR jack to ground?

No - you say the sound gets tinny when you use the output transformers. If you're running it into a balanced input, it would be OK. If you're running it unbalanced, then you need to make sure that the unused line (XLR pin 3) from the transformer gets grounded. If it's left hanging, not connected to anything, the result is usually a very drastic highpass response...hence tinny.

[eanderso13]

when I used the altran c4000's I used 1&3, 4&6. nothing required grounding on the output side.

Wouldn't pin 3 of the XLR be grounded? I know this wouldn't touch the secondary of the c4000.

For the direct outs I wired it in parallel with pins 4 and 6 on the C4000...since the output on the secondary is a high, low and ground, and unbalanced is just high and ground wouldn't I need to wire it to the primary?

Thanks for the input.

[eanderso13]

Is the level of the whine independent of the input gain? What about the fader?
Are they near a CRT, which might add interference? Is the whine there with all of the lights out?

The whine is totally independent of gain or fader level. It seems to change slightly when I adjust the treble in the tone. It is there when the unit is not even turned on, but when I unplug the unit the whine drastically decreases. There is a CRT nearby but if I turn it off it has no effect. I thought maybe the computer might be doing it, but I moved mic cables around with no effect. I'll have to check with lights out.

No - you say the sound gets tinny when you use the output transformers. If you're running it into a balanced input, it would be OK. If you're running it unbalanced, then you need to make sure that the unused line (XLR pin 3) from the transformer gets grounded. If it's left hanging, not connected to anything, the result is usually a very drastic highpass response...hence tinny.

The XLR Pin 3 is always connected to ground how I have it wired. But it seems that I get a highpass response when I use the XLR, not the unbalanced.

Thanks!

[mixedupsteve]

I wouldn't connect the secondary center tap to ground. The shell of the transformer should be connected to chassis ground as should pin one of the output male XLR jack.

[eanderso13]

I wouldn't connect the secondary center tap to ground. The shell of the transformer should be connected to chassis ground as should pin one of the output male XLR jack.

Yup! That's how I have it.