Hum problem with measurement setup...

[ch83575]

I am using an Eric Walin mic preamp and an accelerometer for measuring resonance. I just noticed that all of my measurements have a large 60hz hum in them. Even when the accelerometer isn't plugged in the hum is there. As a matter of fact, the hum is there even when the soundcard input isn't plugged into the preamp at all. It only goes away when I ground the input. Shouldn't the hum go away when the input is plugged into the preamp? Could there be something wrong with the peramp? Or is it a problem with the soundcard? Please let me know if you know anything I can try to do to get rid of the hum.

Thanks,
-Chad

[ThomasW]

You're using a Wallin pre-amp or the Wallin jig?

The reason I'm asking is you reference a Xenya 802 in this thread...

http://www.htguide.com/forum/showpost.php4?p=403498&postcount=40

[ch83575]

You're using a Wallin pre-amp or the Wallin jig?

The reason I'm asking is you reference a Xenya 802 in this thread...

http://www.htguide.com/forum/showpos...8&postcount=40

No, my only preamp at the momment is the walin preamp. I am not sure if it is v1 or v2. I usually use it with a DIY panasonic mic, but I am currently using an accalerometer. The accalerometer and mic capsule work exactly the same way though, both use a small phantom power, I think the walin provides about 7v. If the preamp is the problem I would consider changing to a Beringer mic and peramp, but the accelerometer would still require a lower phantom power like the bare capsule mic. I guess some sort of adapter could probably me made to make the accelerometer work with the +48v, but I would much rather fix what I already have if possible.

-Chad

[ThomasW]

I don't think any adapter is necessary for 48V. I'd try it with the Xenya as a part of the process of elimination..

[ch83575]

The accelerometer I am using is the ACH-01. The spec sheet says the supply voltage can be from 3v to 40v, but you don't think it will have any problem with the 48v? Also how would it be wired? Right now I am using it as illustrated HERE in figure #2 into the Wallin preamp, but it looks like the xenyx only has xlr mic inputs. Would I hook up pin1=gnd, pin2=V+, pin3=Vout OR pin1=gnd, pin2=V+, pin3=gnd with Vout shorted to ground as in the RCA connection?

Also, that still requires me to purchase a new mixer, I was really hoping to find some way to get rid of the hum with the setup I already have. If there is anything anybody can think of to try with the Wallin preamp before I purchase the mixer I would appreciate it.

Thanks again,
-Chad

[ch83575]

One more small twist. If I have the soundcard plugged into the preamp, preamp on but not connected to the accelerometer the hum goes away. I don't think the accelerometer is reading a 60hz vibration... it is still there even if I hold the unit in my hand. I don't doubt that my hand vibrates a little, but at exactly 60hz? It also hums when just dangling from the leads. I plugged in my DIY mic that I usually use with the preamp, but my room was too noisy to still see the hum (its about -80dB with the rest of the noise floor @ -120dB).

Is the hum a result of the wiring configuration I am using?

-Chad

[pedroskova]

Have you connected it directly to your soundcard? If it still sets up a ground loop, your cable, or a mis-wired connection would be a likely culprit. And if you're really lucky, you might have an old soundcard, like some soundblasters, that provides a 5V bias on the center pin, so you wouldn't even need the preamp.

[ThomasW]

No your hand doesn't vibrate at 60Hz. Sounds like somewhere there's a open circuit or a ground-loop creating the hum.

Sorry I don't know anyone with the Wallin pre-amp or how to troubleshoot it..

The Xenyx has 1/4" TRS (= balanced) inputs as well as XLR

from the Libinst website don't know if it's applicable to your situation...

--often a strong component at 60Hz will be seen on accelerometer plots of speaker cabinet surfaces. I ran into this with the ACH-01, but quickly discovered the real cause -- the speaker I was measuring was sitting on a bench with other equipment. The power transformers in that equipment were vibrating at the line frequency and inducing a vibration into the speaker cabinet, and this vibration was faithfully picked up by the ACH-01. So when doing these measurements, it may be advantageous to place the speaker being tested on a solid concrete surface -- the ACH-01 can be very revealing...

The other option is wiring the accelerometer to a RCA then directly to the mic input on the soundcard. This is explained on the Libinst website

[ch83575]

Have you connected it directly to your soundcard? If it still sets up a ground loop, your cable, or a mis-wired connection would be a likely culprit. And if you're really lucky, you might have an old soundcard, like some soundblasters, that provides a 5V bias on the center pin, so you wouldn't even need the preamp.

Thanks! Connected it to the soundcard directly... no hum. So, just the preamp = no hum; just the accelerometer = no hum. The only other thing in the loop was the janky alligator/RCA monstrosity I had cobbled together to hook the accelerometer to the preamp. Swapped out for a real cable and no more hum!!

Thanks again!
-Chad

[Dennis H]

The only other thing in the loop was the janky alligator/RCA monstrosity I had cobbled together to hook the accelerometer to the preamp. Swapped out for a real cable and no more hum!!

Ah, that explains it. Glad you got it to work. There's a reason they build mic cables with shields.

So which wiring did you end up with? Hooking the sensor output to ground like on the Liberty page seems strange.

[pedroskova]

The only other thing in the loop was the janky alligator/RCA monstrosity I had cobbled together to hook the accelerometer to the preamp. Swapped out for a real cable and no more hum!!

Thanks again!
-Chad

The added resistance of your shiny stainless steel alligator clip (clinging on to whatever it was clinging on to) probably threw a spanner in the works.

Glad it all worked out...now...go out and create the ultimate non-resonant something or other. :B

[ch83575]

So which wiring did you end up with? Hooking the sensor output to ground like on the Liberty page seems strange.

I am still using it as illustrated on the liberty inst. website. The unit seems to be very similar to a electret mic capsule. It uses an internal FET to swing the bias voltage (sorta like the transformer in a ribben tweeter I think... impedance mismatch). I am no EE by any stretch (I still have to look Ohm's law ocassionally), but I think by grounding the drain you are using the FET as a common source amp. I think it is basically using the vibrating element to control how much of the V+ gets shunted to ground and that is what the preamp amplifies. I have been thinking of trying different resistors between the drain and ground, but I don't really know where to start. If anybody with more EE skills has any ideas for making the accelerometer run more linear please let me know.

As for what I am watching vibrate, I am actually working on a new and revolutionary sex toy. Just kidding. I am measuring Linn LP12 plinths using a tactile transducer to see what makes some plinths sound better than others. Probably not exactly up everybody here's alley, but if anybody is interested I can post the results when I am done.

Thanks for all the help,
-Chad

[pedroskova]

I am measuring Linn LP12 plinths using a tactile transducer to see what makes some plinths sound better than others. Probably not exactly up everybody here's alley, but if anybody is interested I can post the results when I am done.

Cool! The main resonance of the spring / plinth system may fall below the resolution of your accelerometer, but any secondary resonances may show up. How are you going to "excite" the system. Pink noise? Sweep?

[ch83575]

Cool! The main resonance of the spring / plinth system may fall below the resolution of your accelerometer, but any secondary resonances may show up. How are you going to "excite" the system. Pink noise? Sweep?

I think you are right about the system resonance. I would guess the system resonance is somewhere in the 5-10hz range, at least I think that is where it is supposed to be. I am actually trying to simplify the matter by only testing the plinths themselves.

The whole thing started when the tech at the local hifi shop told me he is working with a dude who makes his own plinths that sound better than the Linn plinths. Tom (the tech) is one of those guys who has been doing this for a long time, knows his stuff, and is perfectly willing to tell you if something is crap... if it really is. His standpoint on most tweeks is that 99% of them make the table worse, but he loves this new plinth. The dude that makes them is making them in lots of different woods and the all sound different. Some slightly better than the Linn, some dramatically better.

"What accounts for the difference?" I asked Tom. "Is it weight, density, hardness?" None of the above. Tom just picks the new plinth up and taps it with his finger, then picks up an old Linn plinth and taps that one. The new one is warmer, more resonant and much more like tapping a musical instrument. The Linn is kind of muted, lower in frequency and made more of a thud than a musical sound.

I just cant leave things like that alone, so I asked him if I could do some measurements on the plinths to see if I could actually isolate the behavior that makes the new ones sound better. My plan is to use this little guy to excite the plinth while suspended by a string with no sub-chassis or top plate installed. So far I am testing using pink noise. I tried to calibrate for the exciters FR, but depending on where on the exciters surface you placed the accelerometer and how firmly you held the assembly the FR would change dramatically. So I have no reference probe, and no cal file... just raw doggin it. Luckily the pressure that you use to hold the exciter to the plinth seems to change the FR of the measurement very slightly, but the CSD looks identical regardless of the pressure on the transducer (sigh of relief). The results I am getting look like this:


They are consistent and repeatable on my test sample (the old plinth), so I think I am ready to get the rest of the plinths and do the full set of measurements.

If you have any ideas about other things I could be testing for or better ways to get accurate results I would love to hear them.

-Chad

[pedroskova]

Cool!...squared...I'm repeating myself.

OK, turn your spectrum upside down, and it looks like a device that follows the 3/2 power law ...i.e., like a triode, with a textbook diminishing of harmonics...I'm hand waving here...they probably don't match up perfectly. Now, do the same thing on an installed plinth. The suspension should muck things up a good bit. My WAG about the other plinths would be that they have an even stronger resonant character that withstands the "mucking up factor" of the suspension just a little bit better, and imparts a SET-like quality (that a lot of people find pleasing) to the playback.

OTOH, I could be totally out to lunch.

[ch83575]

Cool!...squared...I'm repeating myself.

OK, turn your spectrum upside down, and it looks like a device that follows the 3/2 power law ...i.e., like a triode, with a textbook diminishing of harmonics...I'm hand waving here...they probably don't match up perfectly. Now, do the same thing on an installed plinth. The suspension should muck things up a good bit. My WAG about the other plinths would be that they have an even stronger resonant character that withstands the "mucking up factor" of the suspension just a little bit better, and imparts a SET-like quality (that a lot of people find pleasing) to the playback.

OTOH, I could be totally out to lunch.

To be honest I am worried that it will come down to what you are guessing; a system behavior, not an individual component behavior. Unfortunately if that turns out to be the case it will be really hard to measure. First the way I am attaching the transducer and accelerometer (with very little, very light clamps) would not work with a setup table, but also I don't know if Tom would be interested in dropping his table in that many plinths. What I am hoping is that I can find a resonant behavior that correlates to the perceived quality of the sound and then possibly use that information to accentuate that quality in further plinth designs. As long as I find a strong correlation that might be the limits of my engineering prowess at the moment, to find out WHY it sounds better will require a lot more research.

Your SET-like theory is interesting. I never consider the idea of euphonic distortion. In this case, if I follow you correctly, perhaps it isn't adding more distortion in a euphonic pattern, but making sure that the system resonances that are always going to be there follow the 3/2 power law. Very interesting indeed. This might turn into a larger project than I had originally thought.

-Chad

[JonP]

Very cool experiment...

Hmmm... thinking on the "system resonance" concept... I wonder about trying to mount the accelerometer on the arm, at (above) the cartridge? Thereby you are measuring the resulting vibrations at the point where it (probably) matters the most. Also, with the plinth mounted on it's supports, with table, etc.

Of course the accelerometer weighs a few grams (?) so it would shift the resonance of the arm a bit, but it should be pretty close. Might be a good evaluation method. Get a dab of Quake Hold or similar and tack it to the top of the tone arm... see what you see...

Of course, maybe I dont' know what part of the table is a plinth... I'm thinking the platform where the turntable and arm are attached. But the same idea applies.

Overall, I'd think the best one would be one that damps vibration and has very little at any frequency, but if you are going to have some, the even harmonics of course would be better. If I was going to design one, doing laminates of different density woods, or better, laminates of damping and stiff materiels (constrained layer style) would be the way I'd go. Rather tricky to get the mass loading to resiliant layer ratio right, I've read.