Cqwet Dbdfte
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I was not responding to you.
A Question About HUM
- Thread starter GLM
- Start date
Help Support GroupDIY Audio Forum:
Then why post information which is completely incorrect in the context of the thread?
Cqwet Dbdfte
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Think aboutr
Think about that...
DaveDC
Active member
Something to try that is safe and appropriate for a musician in rural Ireland who doesn’t own a voltmeter:
1. un-rack the WA273 and place it on a convenient surface. Make sure the chassis is not touching anything metal, since we are trying to isolate it from any conducted interference (per my earlier post.)
2. locate a pair of headphones, preferably high impedance ones like AKG K240s so as to not load the WA’s output too much. Looking at the online picture on the WA site, the WA273 appears to have transformer-coupled outputs, so it should be safe for the phones to have the WA drive them directly.
3. Make a cable that connects the sleeve of the headphone jack to Pin 3 of one of the WA’s XLR outputs, and connects both the headphone tip and ring together to pin 2 of the same XLR. So, two wires: sleeve to pin3, tip and ring together to pin 2. This puts the left and right headphone elements in parallel and driven by the WA in a way that that is isolated from the (potentially chassis-connected) XLR pin 1.
4. install the XLR shorting plug that Ian described into the input XLR input of the same channel as you have the headphones wired to.
5. With the WA's mains power switch OFF, plug the AC mains cord into the same wall socket you usually use. Configure the ground lift switch as you usually do. Confirm that the WA is isolated and not connected in any way to anything else besides the mains cord and the phones (per my earlier post.)
6. With the WA’s power OFF, listen to the phones to confirm that there is no sound, hum or otherwise. It is always a good idea to “proof” your test equipment like this to avoid making dumb mistakes.
7. With the WA’s gain set to minimum, output level minimum, phantom OFF, pad OFF, Mic input selected, and hi-pass filter OFF, turn on its power.
8. Listen in the phones for any hum as you slowly advance the gain and output level. If the WA is working properly, you be able to get the gain and output level to maximum and hear only the hiss of the WA’s natural noise floor, free of any hum.
9. Check both positions of the WA’s ground lift switch. If it hums with the ground not lifted, there could be an issue with the mains power. Unfortunately, I am not expert enough on AC wiring standards in Ireland to help much with this.
10. If you do hear hum, I think it is safe to say that the WA is not performing as intended when installed in your studio. Since you already had the unit checked okay by a qualified tech, pretty much the only conclusion is that there is something amiss with your mains power.
11. Have a *qualified* electrician check out your facility’s AC mains installation. Look for improper voltage (230 VAC +/- 10% typically), dodgy safety earth connections, etc.
Please report back on what you find so we can help you with next steps.
1. un-rack the WA273 and place it on a convenient surface. Make sure the chassis is not touching anything metal, since we are trying to isolate it from any conducted interference (per my earlier post.)
2. locate a pair of headphones, preferably high impedance ones like AKG K240s so as to not load the WA’s output too much. Looking at the online picture on the WA site, the WA273 appears to have transformer-coupled outputs, so it should be safe for the phones to have the WA drive them directly.
3. Make a cable that connects the sleeve of the headphone jack to Pin 3 of one of the WA’s XLR outputs, and connects both the headphone tip and ring together to pin 2 of the same XLR. So, two wires: sleeve to pin3, tip and ring together to pin 2. This puts the left and right headphone elements in parallel and driven by the WA in a way that that is isolated from the (potentially chassis-connected) XLR pin 1.
4. install the XLR shorting plug that Ian described into the input XLR input of the same channel as you have the headphones wired to.
5. With the WA's mains power switch OFF, plug the AC mains cord into the same wall socket you usually use. Configure the ground lift switch as you usually do. Confirm that the WA is isolated and not connected in any way to anything else besides the mains cord and the phones (per my earlier post.)
6. With the WA’s power OFF, listen to the phones to confirm that there is no sound, hum or otherwise. It is always a good idea to “proof” your test equipment like this to avoid making dumb mistakes.
7. With the WA’s gain set to minimum, output level minimum, phantom OFF, pad OFF, Mic input selected, and hi-pass filter OFF, turn on its power.
8. Listen in the phones for any hum as you slowly advance the gain and output level. If the WA is working properly, you be able to get the gain and output level to maximum and hear only the hiss of the WA’s natural noise floor, free of any hum.
9. Check both positions of the WA’s ground lift switch. If it hums with the ground not lifted, there could be an issue with the mains power. Unfortunately, I am not expert enough on AC wiring standards in Ireland to help much with this.
10. If you do hear hum, I think it is safe to say that the WA is not performing as intended when installed in your studio. Since you already had the unit checked okay by a qualified tech, pretty much the only conclusion is that there is something amiss with your mains power.
11. Have a *qualified* electrician check out your facility’s AC mains installation. Look for improper voltage (230 VAC +/- 10% typically), dodgy safety earth connections, etc.
Please report back on what you find so we can help you with next steps.
DaveDC
Active member
I occasionally take my own advice. So I decided to check to see if hum in the WA's noise floor would indeed be audible in headphones.
I set a transformer-output signal generator to 100Hz at a level of -50 dBu (2.45 mVRMS), and connected it to a pair of AKG K240s as mentioned earlier. Assuming the WA's (unpublished) EIN is similar to other quality preamp's at about -130 dBu, and the WA's maximum gain being 80 dB, the output noise should be around - 50 dBu.
My lab is not all that quiet, so I was not able to hear the 100Hz -50 dBu signal very well in the phones. However I was able to hear it clearly at -40 dBu. So if GLM's hum is more than 10 dB above the WA's noise floor, then this test should be valid. It seems likely that this is the case, since the hum evidently is loud enough for GLM to seek help online to fix it.
An improved test would be to use a battery-powered amplifying device to provide a bit of gain between the WA and the phones. A quality field recorder (Sound Devices MixPre, etc.) would be a good choice because the interference could be recorded and brought into a DAW for analysis. But it would absolutely need to be battery powered and isolated from any other studio gear so as to not be susceptible to conducted interference.
And of course a voltmeter with a 200mV AC scale would also see it. It could also check cable continuity, battery condition, etc.
I set a transformer-output signal generator to 100Hz at a level of -50 dBu (2.45 mVRMS), and connected it to a pair of AKG K240s as mentioned earlier. Assuming the WA's (unpublished) EIN is similar to other quality preamp's at about -130 dBu, and the WA's maximum gain being 80 dB, the output noise should be around - 50 dBu.
My lab is not all that quiet, so I was not able to hear the 100Hz -50 dBu signal very well in the phones. However I was able to hear it clearly at -40 dBu. So if GLM's hum is more than 10 dB above the WA's noise floor, then this test should be valid. It seems likely that this is the case, since the hum evidently is loud enough for GLM to seek help online to fix it.
An improved test would be to use a battery-powered amplifying device to provide a bit of gain between the WA and the phones. A quality field recorder (Sound Devices MixPre, etc.) would be a good choice because the interference could be recorded and brought into a DAW for analysis. But it would absolutely need to be battery powered and isolated from any other studio gear so as to not be susceptible to conducted interference.
And of course a voltmeter with a 200mV AC scale would also see it. It could also check cable continuity, battery condition, etc.
NoisyIndividual
Well-known member
A while ago I had a pair of Warm Audio EQPWA units for repair.
Turned out both were oscillating ultrasonically, but only when connected to a patchbay via the usual longish cables.
So it wouldn't just do it on the bench.
Apparently this is a known issue and there is a factory issued mod to try and fix it.
At the time I discussed this on a Forum, and very respected posters suggested the main problem was actually a poor choice/implementation of the transformers.
Now, I have repaired quite a few other Warm Audio pieces, and I am reluctant to say that on more than one occasion I have been more than surprised at the build/design choices employed.
I think the OP has pretty much demonstrated that he only has a problem with this particular piece of equipment in his studio. Everything else is able to cope with whatever condition his mains system is in. So it is likely the equipment is either faulty, or is not designed to cope with whatever he has.
His mains system may well not be optimal, but this is not uncommon.
And its possible the isolated transformer he referenced will sort this one out.
Testing by other engineers on their own bench and there not being a hum does not prove the unit does not have issues IMHO.
Many of us who have repaired valve amps will have come across a fault of ultrasonic oscillations. Although you can't actually hear the oscillations, you can hear the power supply ripple change when the oscillations start, and this will indeed be 100Hz plus harmonics.
The OP states that the level of hum changes, it is not constant. Maybe he could mess about with the EQ and see if there are some settings that cause more hum or reduce it. That would be a clue and a half.
Oh, and I hope he will have taken the hints here that having even a cheap meter to measure voltage and resistance is not really an option
Turned out both were oscillating ultrasonically, but only when connected to a patchbay via the usual longish cables.
So it wouldn't just do it on the bench.
Apparently this is a known issue and there is a factory issued mod to try and fix it.
At the time I discussed this on a Forum, and very respected posters suggested the main problem was actually a poor choice/implementation of the transformers.
Now, I have repaired quite a few other Warm Audio pieces, and I am reluctant to say that on more than one occasion I have been more than surprised at the build/design choices employed.
I think the OP has pretty much demonstrated that he only has a problem with this particular piece of equipment in his studio. Everything else is able to cope with whatever condition his mains system is in. So it is likely the equipment is either faulty, or is not designed to cope with whatever he has.
His mains system may well not be optimal, but this is not uncommon.
And its possible the isolated transformer he referenced will sort this one out.
Testing by other engineers on their own bench and there not being a hum does not prove the unit does not have issues IMHO.
Many of us who have repaired valve amps will have come across a fault of ultrasonic oscillations. Although you can't actually hear the oscillations, you can hear the power supply ripple change when the oscillations start, and this will indeed be 100Hz plus harmonics.
The OP states that the level of hum changes, it is not constant. Maybe he could mess about with the EQ and see if there are some settings that cause more hum or reduce it. That would be a clue and a half.
Oh, and I hope he will have taken the hints here that having even a cheap meter to measure voltage and resistance is not really an option
+1
A meter is truly a must in any studio. Useful for tackling all sorts of issues (from cable connectivity to circuit troubleshooting and everything in between). And a simple DMM does not have to be expensive at all...
thor.zmt
Well-known member
I doubt that.
There is a HUGE hint. Earth loops and anything leaking from mains is 50Hz (except over there, where it's 60Hz).
100Hz is RECTIFIED MAINS.
I'd say somewhere, somehow a piece of power circuitry turned Totally Incapable To Support Usual Performance (TITSUP).
Oscillation as has been suggested can also be responsible, but it more often picks up 50Hz than 100Hz, because normally with working power supplies, 100Hz is at very low levels. Of course, some people like to build Power supplies with very large loop gain and un-necessary performance for audio while being deficient where it matters and an oscillating Power Supply would dump a lot of 100Hz into the circuit, that still falls under "powersupply went TITSUP".
Thor
Last edited:
warpie
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Here's a thing you can try.
Take the mains lead out of the unit and measure resistance between Neutral and Earth on the mains inlet of the equipment.
There shouldn't be anything, it should be open circuit, infinity ohms.
If there is, this can induce hum through your entire system.
The Neutral is usually connected to earth where it enters the building, though in rural Ireland this might actually be anywhere on the planet.
But if you connect Earth to Neutral locally by accident, you could create a massive Earth Loop that will confuse you til you die.
The Isolation transformer you mentioned should actually make a difference if this is the case, either fitted to the offending unit or the whole system.
Edit: In fact given your circumstances I think I would get one for the studio anyway.
It could also be that your "Earth" connection is a pole hammered into the ground outside your house.
Sometimes watering it can help so I've heard, if it isn't boggy enough already.
I don't get it. Filtered power inlets have a resistor across Live and Earth. Some megaohms if I'm not mistaken.
