Hum Issues

[Category 5]

I will try too Brad...as soon as power gets restored here. Hurricane Wilma has knocked it out 7 days ago...and still nothing. Worst case scenario is Nov.22 - but at least while the power goes out I have no noise issues.

Shane

[bradb]

It seems pretty clear that the bad cables have the shield tied to pin 3 instead of pin 1. That's an easy mistake to make, and easy to fix.

Have SOME faith Tim! I did a quadruple check just now between the humming cable and the non-humming cable. there is NO continuity between any of the pins. and pin 1 is wired to pin 1, pin 2 is wired to pin 2, pin 3 is wired to pin 3.. all exclusively. Continuity-wise, both cables are the same...


The interesting question is why you don't see the same symptom with your other preamps. If the setup is exactly the same between the two units (physical position, AC outlet, power cord, mic cable, gain settings, etc.) I'd expect them both to hum with the bad cables. The shield is supposed to conduct noise signals to chassis ground. If the noise signals are applied to the amplifier inputs instead of ground, I'd expect them to be amplified, which is the symptom you describe. I'm not so sure the absence of hum from your other pres is necessarily a virtue. Are you absolutely sure they don't have a high-pass filter engaged somewhere?

There are no highpass filters on the Sytek nor on the Hamptone. I think we're running into a balanced cancelling problem, personally. Thats where my money is now.

Actually, the output impedance of a condenser mic, especially a transformerless one, is usually much less than that of a typical dynamic mic. A low output impedance means that the voltage on the cable is "stiffer", and it takes a stronger outside influence to disrupt it. So everything else being equal, the lower the driving impedance, the more noise immune the transmission line.

I see..

That's annoying. Is it present all the time, or do you have to crank up the gain to hear it? You may want to add a little extra shielding if it's a problem, let me know if you want to try it.

I'll get into this problem next... it occurs when the gain is cranked.

And one of my N72s has a slight buzz when i terminate with a dynamic... but not with the cable is unterminated

And this is with a "good" mic cable?

yes this is with the good cable, it doesn't hum as badly as the "bad" cable, however it is there and it stands out against all the other pre's which are dead quiet with the "good" cable(s).

I'm thinking about mailing you my good and bad cable, Tim.
-bb

[bradb]

We're in the home stretch here guys! I just tried a few more tests and I here are some more data points.

I powered up the hamptone tube pre and the 7th circle pre's and they're right next to each other, i plugged into the clean A12 on the 7th circles and cranked both pres. I retested my "good cables" and "bad cables" they were as before, everything same. Unterminated the good ones are DEAD quiet, the bad ones are very hummy.

I CRANKED the hamptone and plugged in the good and bad cables. The hum wasn't as loud as on the 7th circles but IT WAS THERE! I could see why I missed it before! i didn't have the hamptone absolutely cranked!! Another thing I noticed is that the bad cables are microphonic, while the good ones are not microphonic! if i stroked the cable (hah) i could hear the friction of my fingers in the headphones, if i stroked the good cable, i heard absolutely nothing. This is with both pres.

Soooo... again, all of these cables are wired right, 1 to 1, 2 to 2, 3 to 3, exclusively... But what I'm thinking is that when I built these cables, I never crimped the wires in the solder cups... I inserted the wire into the cup, heated the wire and cup up and then flowed a bunch of solder into them..... there was no physical connection in the beginning.. is this..WRONG?! How do you deal with solder cups anyways!! sheesh!

so i'm glad we're getting to the bottom of this....thanks for your patience Tim and everyone else.

[bradb]

Hi everyone,

I come here humbly to declare that I am an idiot. My bad cables have signal running on the shield wire. as in, i have pin 1 connected to one of the pair, pin 2 to the other pair and pin 3 connected to the shield. Continuity wise correct, but FAR FROM CORRECT. That also explains the differing measurements I was getting as well.. just looking at those measurements is pretty clear what is going on.

I'm just trying to figure out what the fuck is wrong with me.... I know the purpose of a shield, I could predict that this would happen... i guess it was just a marathon cable making session going late into the night.

my only hope now is that maybe i'm not the only one... Shane? have a look!

hehe


EDIT: and yes, i'm going to leave all my idiotic posts here leading to my epiphany and unmasking as a retard... kinda like in mad max beyond the thunderdome....thats wrong of me.

[Reggie]

How embarrASSing!

I hope Tim didn't lose too much sleep over this one.
I hope you had many sleepless nights.

But you got me curious about the solder cups now.....are we supposed to pinch the cups to the wire?

[Category 5]

Well, I messed around with the R84 tonight. It has a non-detachable 10ft cable so I know it is wired correctly.

I get the hum at full gain on the J99s (slot 7 and and A12s (slots 1-4) - I don't get any hum on the N72s.

Again, i can minimize the hum by turning the mic + side east. it is worst when facing west. The N72 is the only one usable for this ribbon though. It's a shame because the J99's gentle high frequency lift is just what the doctor ordered for this mic.

Tim, I'd like to try some additional shielding on my box. What do I need to do?

Thanks
Shane

[Category 5]

I thought maybe my snake was wired wrong, but it's shield to pin 1.

Every other cable I checked is shield to pin 1.

Shane

[tpryan]

Again, i can minimize the hum by turning the mic + side east. it is worst when facing west.

Do you get the hum on the same channels at the same gains with nothing connected to the inputs? IOW, what happens when mics and cables are removed from the equation?

I'd like to try some additional shielding on my box. What do I need to do?

Additional internal shielding will only affect the preamps in "slots" 1 and 2, if it has any effect at all. But if you want to add some more mu metal, send me an e-mail.

[Category 5]

I still get the hum with no mic cable connected. The hum on channel 1 (A12) is the absolute worst. Channel 2 is less noticable than 3 or 4, but has a much higher noise floor (hiss), probably because that one is using the real 2520 and 3 and 4 are using your SC25s (which are quieter).

The J99s in the last two slots still have the hum when nothing is connected so I am thinking it must be PS related. Perhaps it has never been a problem with condensors because the gain is so much lower. the above tests are at full gain (which I need for the ribbons).

I noticed that the newer kits get 2 iron noise isolators. One for the power cable and one for the internal power leads? I didn't get those with my kits over a year ago, but do you think they would help?

Thanks!
Shane

[tpryan]

As an experiment, can you disconnect everything but the J99s from the power harness and check again for hum? Does it make a difference?

[bradb]

i have a ferrite toroid on my power cable and one on the internal wires running to the modules. in fact the one on the internal power wires are looped around the toroid. i'm not sure how significant doing these things are, i didn't notice any changes.

shane, there's another post here where a guy had a bad input transformer. http://www.seventhcirclestudios.com/SCA ... =2531#2531

could this be it with your J99s.. take a look. I'm going to investigate this on my N72.

I moved my modules away from the PS02 in my rack so slots 1 and 2 are empty, but i guess thats not an option for you.

[Category 5]

I doubt I got bad input trannys since the pres sounds fantastic with all but my Ribbons (and I gues the 57s too).

I have not had a chance to pull it out of the rack and disconnect the modules, but I know my grounds are all continuous since I checked thoroughly. I have continuity between the PS, modules and chassis grounds.

I have no ferrite toroids, and winder if that may be what I need. The hum is pretty minimal even at full gain. I could certainly get by on electric guitar tracks and such by gating it. It would never be heard over signal (probably vox too).

The hum on the A12 in slot one is quite severe in comarison to the other modules though (so I am sure it has to do with radiated EM from the PS. Oddly, while hte self noise of the N72s is high (in the form of hiss) I am able to plug the ribbons to the N72 without hum.

So far I think my best recourse will be additional shielding between the modules and the PS. I still have to try disconnecting the other modules to see if the noise can be isolated to a particular modules, but again...this sounds like very typical radiated EM...and the only PS I can seem to relate it to is the one in the box with the modules.

[Category 5]

Still no luck.

I am wondering, is it possible that twisting my output pairs could help? Most projects I have seen have this done. I did not think to do it on any of the A12s or J99s?

Is there really anything to be gained by twisting the pairs?

Tim, how about hat extra shielding? How can I get some from you? Also, do you think the ferrite collar on the power harness inside would help? Aside from the channel in slot 1 the noise seems minimal...just bad enough to cause me grief.

[tele_player]

what output pair are you talking about?

[Category 5]

what output pair are you talking about?

The wires on the output transformers.