Hum Issues

[bradb]

Hello everyone,

I've been using my 7th circle pre's for several months now. I'm getting different hum/noise behavior out of the pre's depending on what type of microphone I have plugged in.

1. No input cable plugged into the module. >>No hum
2. Mic cable plugged in >>Lots of hum (this is irrelevant, but just a data point)
3. Dynamic microphone plugged in >>Same hum, but quieter
4. Ribbon microphone plugged in >>same hum, but quieter
5. Condenser microphone plugged in >> No hum, no noise, clean!
6. Condenser microphone plugged in WITH +48V >> No hum, no noise, clean!

When I move the mic cable around with the ribbons/dynamics, I can get the noise to increase or decrease. However, its never as quiet as with the condensers.

Do you guys have the same experiences? any advice on beating this problem? I love using ribbon mics, but the hum can build up and then they "beat" with each other when its on more than one track... nasty!

thanks

[tele_player]

Were all those observations made without changing the gain settings?

[bradb]

thanks for the reply... i've been trying to log on all day, resetting passwords, that not working...

ok... back to the hum...

the gain settings are the same, totally unchanged. Its amazing when I plug a condenser in to the pres.. they are totally quiet.

[tpryan]

Have you tried a similar test with any other preamps you may own?

[bradb]

Good idea, I'll do that with the Sytek and Hamptone Tube pre and report back.

[bradb]

Ok, after long last, i finally did some testing.

the setup is this(using the J99 furthest from the PSU):

SM57>J99>Digidesign002Rack_chan1
SM57>Sytek>Digidesign002Rack_chan2
SM57>Hamptone TUBE Pre>>Digidesign002Rack_chan3

With the gains roughly equal on the sytek and the J99 I get a typical noise signature from the Sytek, just a nice low hiss. Even less noise from the hamptone (good design by scott hampton, to pull this off with tubes!). From the J99 I get the hum that's been a constant with all of the modules.

What drives me so crazy is that when I have the output connected and nothing plugged into the input, all the 7th circle pre's are quiet, no hum/buzz, just a nice typical hiss.
if I CRANK all the knobs to MAX and go from module to module (with no input cables plugged in anywhere) I just get a typical hiss, except on the first A12 next to the PSU, i get a quieter hum there (not such a big deal).

Could it be a mic cable grounding issue? these observations happen with all of my cables (neutrik/canare)

Some other observations. If i crank a pre and take a mic cord with no mic plugged in and coil the cord on top of the 7th circle rack, the hum COMPLETELY goes away (i know the "no mic" tests aren't so relevant, but i found this interesting)

The sytek with no mic plugged into its cable makes no hum or buzz. Neither does the hamptone. In fact, both pres get quieter with no mic plugged into their input cable.

Any more tests that anyone out there can recommend. I'd appreciate some help or advice, please!!

thank you!

[Category 5]

This is the same issue I have had with dynamics and ribbons. Ultra quiet with the condensors no matter what.

it hasn't been too much of an issue since I have just been using the Presonus MP20 with my ribbons until now.

I have just ordered an AEA R84 though, and I am looking forward to using the SCA pres for that so I guess it will be back to the drawing board.

I have considered getting another chassis and a PS03 and seeing if that has less radiated noise from the PS (assuming that's where the noise is coming from). I was going to build some C84s anyway so I can get the PS for free again.

What's weird is that in the control room I can turn the ribbon toward the east wall and get no hum...toward the west I get maximum hum. Toward the north and south I get a medium hum. In the booth though, even to the east i get a bit of hum that makes the ribbon useless except with loud sources like guitar cabs.

Why would condensors work perfectly, but dynamics and ribbons have this issue? I know using ner maximum gain doesn't help, but the fact that I can cancel the noise by pointing the mic east makes me wonder if the PS is the culprit, or in fact something else.

Shane

[tele_player]

Category - in your case, I'd check whether noise level is affected by proximity to the PS02. Try a long mic cable.

[bradb]

Shane,

The AEA r84 is my favorite mic on most amp'ed sources (guitar, organ). Watch the proximity effect, placement is EQ, but its very drastic. Its a bummer that its such a hassle using it with the 7th circle stuff. The worst is when the hum is on one track and it beats with another hum on another track.

Like you experience, when I use my 7th cir pres with dynamics and I orient the mic cable a certain way, the hum level changes. I usually spend an awkward 30-40 seconds coiling/uncoiling/moving my cable around while the performer waits.

I don't remember how I have my ground jumper hooked up, but perhaps the shield should be disconnected at the input?

The thing that is making me question the effectiveness of an external power supply is how totally quiet the pre's are when there is no mic cable plugged in!!! No other pre's that I have get so hummy when I plug in an unterminated mic cable. This says to me that there is some kind of flaw in how the input or grounding is handled.

[bradb]

It would be good to hear from people with the PS03 what their dynamic mic hum is on their 7th circle pres. I've always heard that switching power supplies are noisy, but it seems like its high frequency and could be out of the audio band. There _is_ a small tranny on the PS03...

from the main site:
Low Noise High frequency switching design eliminates any possibility of audible hum. State of the art soft recovery rectifiers, RC snubbers, and dual-stage LC filters on each output minimize switching noise.

an PS03 users out there?

[tpryan]

If i crank a pre and take a mic cord with no mic plugged in and coil the cord on top of the 7th circle rack, the hum COMPLETELY goes away (i know the "no mic" tests aren't so relevant, but i found this interesting)

What this suggests to me is that the chassis has a very small 60Hz voltage on it. When the cable is completely within the field, everthing is riding up and down on the same voltage, and you hear no hum.

Some things to check: Make sure the front panel bolts are tight. The resistance between the bolts and the safety ground at the rear power connector should be essentially zero. The same goes for pin 1 of all the XLR connectors. Make sure that the chassis ground lead is making a good connection. Basically, any exposed metal on the chassis should connect to the safety ground via a short circuit.

If everything is grounded well and you still get the hum, try removing C23 and C25 from the PS02. These are 2 of the 3 black "safety" caps at the rear of the supply. There may be enough current flowing through these caps to create a tiny voltage across the ground wire of the power cord. Let us know what you find.

[tpryan]

I've always heard that switching power supplies are noisy, but it seems like its high frequency and could be out of the audio band. There _is_ a small tranny on the PS03...

Much like the first generation of transistors got a bad rap in audio, so have switching supplies. But there's no reason a well-designed switching supply can't be used in high-performance audio applications, and the list of companies using them is growing all the time. Great River, FMR, RME, Rupert Neve Designs, Universal Audio, and I'm sure a bunch of others are all using switchers in some way.

Whatever slight electrical noise the PS03 makes is at 132KHz, well beyond the audio band, even if you're sampling at 192K. Since it doesn't have a big 50/60Hz toroidal transformer, it doesn't radiate an audio frequency field and doesn't require the same amount or type of shielding.

[bradb]

Some things to check: Make sure the front panel bolts are tight. The resistance between the bolts and the safety ground at the rear power connector should be essentially zero.

check, all resistances from these points to safety ground are 0.00 on my meter.

The same goes for pin 1 of all the XLR connectors. Make sure that the chassis ground lead is making a good connection. Basically, any exposed metal on the chassis should connect to the safety ground via a short circuit.

check, all pin 1s are 0.00 on my meter to the safety ground

If everything is grounded well and you still get the hum, try removing C23 and C25 from the PS02. These are 2 of the 3 black "safety" caps at the rear of the supply. There may be enough current flowing through these caps to create a tiny voltage across the ground wire of the power cord. Let us know what you find.

this is next, ill let you know.... thanks!

[bradb]

So on Saturday night I removed the two caps C23 and C25 from the PS02 and did some tests.

I did a quick hook up on one of the A12s, into protools with an SM57 and the hum was gone! WOW. but wait... whats changed? I realized in my haste to hook everything up, I had used a 4' patch cable made from mogami 2552 and neutriks. This was the one piece in the signal chain that was different. SO I switched the patch cable with one of the original cables I always get the hum with. The hum was back and as normal. The missing caps didn't make a difference, it was mic-cable related. I switched back to the mogami patch cable and the hum was gone. Even without the cable being terminated, there WAS NO HUM with the mogami!!

I then went thru ALL of my microphone cables. All but 2 of the canare/neutriks hummed like mad. My el-cheapo's didn't hum. But why?

Now at this point i became a little confused. NONE of my cables hum with my other preamps. But with the 7th circles, SOME of my cables hum.

So i did some measurements on two identical cables, one hums, the other is silent. Both cables are 20' Canare, L-2T2S. With standard Neutrik XLRs.

No-hum Cable
Capacitance
1-2 1.00 nF
1-3 1.01 nF
2-3 .566 nF

Resistance
1 0.4ohm
2 0.7ohm
3 0.7ohm

Humming Cable
Capacitance
1-2 0.566 nF
1-3 0.993 nF
2-3 0.982 nF

Resistance
1 0.7ohms
2 0.7ohms
3 0.4ohms

Notice anything?

It seems like there is an imbalance between the 2 and 3 lines on the humming cable and more of a balance on the non-humming cable. I would say that the fault is with my cables, but even the "unbalanced" cable doesn't hum with other preamps.

Could the inputs on the 7th circle stuff be such that a slight difference in input impedance could cause a hum... or incomplete cancellation?

Perhaps the condensers are quiet because their output impedances are large making the "imbalance" less consequencial?


After making these measurements, I reinstalled C23 and C25 and verified that they were not causing the problems. It is strictly a microphone cable issue.

Shane, see if you get the same results. You don't need to terminate the cable... the "balanced" ones will completely not hum.

On a side note, the A12 RIGHT next to the PS02 has a hum, even with no input cable, probably coupling from the power supply. And one of my N72s has a slight buzz when i terminate with a dynamic... but not with the cable is unterminated... weird..

Tim?

[tpryan]

I would say that the fault is with my cables, but even the "unbalanced" cable doesn't hum with other preamps.

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.

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?

Perhaps the condensers are quiet because their output impedances are large making the "imbalance" less consequencial?

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.

On a side note, the A12 RIGHT next to the PS02 has a hum, even with no input cable, probably coupling from the power supply.

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.

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?