Hum madness, driving me wild... how do you fair?

Hi Folks....

Well the title says it all.

I know the best way to avoid hum/buzz in rack, esp pres is to have the transformer outside of the rack. It also helps when you only put 2 pres in a rack and as FAR away from the power transformers as possible.

The problem is that in 2 racks I've built, to save money I've put 4 pres in a rack with the PSU and torrid. The problem in both cases is that the pres closest to the torrid have hum/buzz, which can only be the torrid, deduced by the facts:

1>I've tried 2 different PSU's, a JLM AC/DC and Peter C, both the same.
2>Not the grounding, since everything is grounded the same way and the ones furthers from the torrid are hum free.
3>Moving the torrid outside of the box clears the problem.

I have tried the following in an effort to lower the hum

1>Rotated the torrid to find the null and least amount of hum
2>Twisted the AC lines (makes no difference)
3> Tried a metal can, with no luck.

Mu-Metal shielding seems to be my best option as moving the torrid out the box is something I'd prefer not to do, if at all possible.

Now maybe I'm just being pedantic, because I've seen many other projects that do the same thing I've done and no one seems to have complained about hum issues from the torrid.

Here is a pic of my Green Pre.



The Torrid is now where that mounting hole in the rack tray is.

What I did to eliminate a mic/external source as the issue I put a 150R resistor between Pins 2 and 3 and then cranked the gain to full gain (66dB), and recorded the outputs at +4dB directly into Pro Tools with the Fader set to 0dB and did a bounce down.

Here are the clips, Channel 1 is furthest from the Torrid, Channel 4 is the closest.

Green Pre Channel 1 (290kB)
Green Pre Channel 4 (290kB)

I then took the waves into Wavelab and looped them and did an FFT test and here are the graphs.





I tried the same test with my Spectratone (Spectra Sonics 101 Cards) rack and here are the samples and FFT results, Channel 1 being furthest away, 2 being closest.

SS101Channel 1 (530kB)
SS101Channel 2 (530kB)





The graphs look pretty similar, but the buzz if more evident in the channel closest to the torrid.

How do these samples fair to you pres? Is the hum acceptable for 'real world' recordings? Are yours better/worse? Would love to hear more.

And should they be too loud, do you think a strip of mu-metal around the torrid would lower/eliminate it? And if so any idea where to get some metal tape/strip?

Sorry for all the questions, I'm just going mad here and need to see how I fair to others.

Cheers

Matt

Hi Matt,


Nice built! :thumb:

I might be not the best person to reply here, so in case my remarks are garbich, feel free to ignore them completely ...
Have you tried connecting the pres per one only?
maybe you are taxing the PSU to heavy with 4 channels?
What metal can did you use?
A piece of thick iron plate, bend in L-shape, and tied to ground to screen the trafo could be a thing to try.
I listened to your examples, on my pc speakers I didn't hear any real problems... Have your tried of putting some real sound tru them? It might well be that the little noise is cranked up too far?

Only my 2 eurocent...
Just trying to help you troubleshoot any further as where you are now

Matt,

I always have hum issues with 1 rack cases... thats why most of my units now go into 2 rack units, where the torroid is mounted on the Side of the Chassis. Moreover, I put a metal divider further separating the PSU section from everything else -- I only use Avel Lindberg transformers.. too many issues with Avemco.

After giving it some thought (I stopped for lunch, then came back to this thread,) I think the two simplest solutions are:

1.) Make it a 3-channel preamp.
(not a good solution since your holes are already drilled.)

2.) Use an external AC-AC adapter (wall-wart.) The JLM PSU is made to work with one - and Joe Malone suggests this route in another thread.

here's the thread in which this quote is issued:

"It is also worth considering the wall mount or wall wart type external transformers as no mains wiring needs to be done and the problem of hum due to the power transformer being in the same case as the audio transformers is solved."

Kato

Hey, Matt,

I looked and I listened. The strange thing is the peak frequencies that pop out on your plots look like they are odd harmonics of 50Hz (150 and 250 being the strongest). Can you use your SW to pinpoint the frequencies? Do you have 50Hz power in SA? The reason this seems odd (haha) is that if it was EM hum wouldn't you expect the fundamental to be the strongest? And if it's a PSU issue like bad filtering or overloading you'd get mostly the even harmonics.

Have you scoped your DC rails with the unit on and the gain cranked to see if tony is right about perhaps loading the PSU too heavily (or under doing the heatsinks on those regulators)? Do the heatsinks get too hot to touch after a couple of minutes of operation?

Good luck!
A P

I quickly run a high-resultion FFT from the WAV-files you provided so that we can see what's really going on: [removed]

The strange thing is the peak frequencies that pop out on your plots look like they are odd harmonics of 50Hz (150 and 250 being the strongest).

Click to expand...

That's normal if it's induced by a magnetic stray field.

Samuel

"It is also worth considering the wall mount or wall wart type external transformers as no mains wiring needs to be done and the problem of hum due to the power transformer being in the same case as the audio transformers is solved."

Click to expand...

Wall Warts may stop hum in the box, but can also cause problems. Most Wall Warts have no shielding round the transformer & consequently radiate a significant magnetic field. Unless carefully sited they can cause hum in other parts of the system, as I have found in my studio. Personally I try to avoid them where possible. Where they are used it is a good idea to move them around to check that they are not inducing hum.

What often happens in cap input filter PS's: the peak currents cause momentary saturation of the core and it "goes away"---and hence the assumption of self-shielding for which toroids are justly prized becomes unwarranted. So even if you have a very nicely uniform winding, and small emission from the lead-out region, you still get nasties.

The solutions are many and mostly painful. Distance always helps as the radiated power falls off quickly (but as pointed out radiates then into other things in proximity).

Magnetic shielding. Painful, expensive, and pointless unless it almost completely encloses the assembly, except for special cases where the geometry/symmetries of the situation allows something simpler.

Reduce the peak currents: I've resorted to this, but it's painful in terms of requiring loss---some R in the circuit in series with the AC, to brute-force limit the charging currents, hence requiring a higher voltage and higher VA transformer. You can use a choke but it will also require a transformer change, usually, as you will not be getting the same charge-to-peak voltage out that you had. Also, the choke is another potential radiator, and is bulky and expensive.

Use a bigger toroid than apparently required. Expensive, maybe impossible to fit into the existing mechanical design, and it takes a mighty big one to not saturate on the charging peaks.

Source a toroid with a gap or effective gap (from a distributed material). Difficult to get, usually less efficient, etc.

Make your circuit immune to pickup by minimizing loop areas. Painful, nontrivial; with parts you can see and work with readily there is always some unavoidable loop area. Obviously the loops in the highest gain portions need to be given priority.

Make the unavoidable loops cancel by symmetrical arrangements and proper orientation. Painful, assumes you know where the sources of the disturbing fields are.

It's interesting that Audio Precision has migrated to a very-carefully-designed custom switching power supply from a well-trained manufacturer for their latest line of multichannel instruments. There is still the need for the downstream circuitry to have some local bypassing and decent power supply rejection---Bruce Hofer told me there were a few millivolts of noise riding on the d.c.---and they also paid very close attention to the conducted noise common-mode chokes, but it represents a sea change for Bruce, who had heretofore shied away from switchers, with good reason. I should add that it is not a cheap supply and is in a separate enclosure within the instrument. But he mentioned that there is less hum per se in the new product compared to previous ones with mains-frequency toroid-based supplies. It is just much easier to shield against high frequency magnetic fields.

Thanks, Brad--very good summary. Do you have an idea how much resistance for a given DC current we should get to reduce peak input currents to a reasonable value?

Samuel

[quote author="matta"]Sorry for all the questions, I'm just going mad here and need to see how I fair to others.[/quote]

Hmm, although I see the hum in your FFTs I probably wouldn't care too much. I mean, this is at 66dB of gain, so the peak is at -130dB EIN (assuming 18dBu = 0dBFS). That's not that bad.

If it makes you sleep better, here is a plot of my latest tube pre with an EI core tranny:

(at 60dB gain, input terminated with 150R, +18dBu = 0dBFS)

Okay, I will change the tranny to toroid, but even now the hum is not that loud and all the other noises when used with a mic clearly mask it.

If course, tranny outside the box is always optimum solution, if you can live with the idea of having to handle to separate boxes.


BTW, for those speaking German, here is a nice article on hum-filtering power supplies: http://www.elektronik-kompendium.de/public/schaerer/lnpowsup.htm

Olaf

Hi Guys,

Thank you ALL so much for your input, a lot has been said and covered, I'll try answer the questions posed to me.

Have you tried connecting the pres per one only?
maybe you are taxing the PSU to heavy with 4 channels?
What metal can did you use?
A piece of thick iron plate, bend in L-shape, and tied to ground to screen the trafo could be a thing to try.
I listened to your examples, on my pc speakers I didn't hear any real problems... Have your tried of putting some real sound tru them? It might well be that the little noise is cranked up too far?

Click to expand...

I hadn't done it channel my channel! Good call, I had swoped out the NE5532's on 2 channels for OPA2604's, which draw more, and so swoped them back to NE5532's, and no difference. So i tried it channel by channel, while having ONLY the channel closet to the torrid connect did make a small difference, it wasn't substantially quieter than having all 4 pres connected.

As far as I recall the Greens (these are V1) draw around 80mA, and I'm using a 30VA torrid, which I think should be sufficient?

I will try and see if I can get some 'L' shapped steel and see if it makes a difference.

The noise isn't even noticeable till around +40dB's of Gain and even then... barely, so I guess for more recording applications it is suitable, but not for say Ribbon Mics or Mics that need High Gains.

I put a metal divider further separating the PSU section from everything else -- I only use Avel Lindberg transformers.. too many issues with Avemco.

Click to expand...

I have mine wound for me by Gardner's, now called Torrodial Technologies here in South Africa, they have been a big improvement over the Chinese stock transformers available at our version of Radio Shack/Maplin.

2.) Use an external AC-AC adapter (wall-wart.) The JLM PSU is made to work with one - and Joe Malone suggests this route in another thread.

Click to expand...

I plan to do something similar on advise from Joe on a 4 channel Neve rack I'm working on, external transformer, internal PSU.

Have you scoped your DC rails with the unit on and the gain cranked to see if tony is right about perhaps loading the PSU too heavily (or under doing the heatsinks on those regulators)? Do the heatsinks get too hot to touch after a couple of minutes of operation?

Click to expand...

Nope, not scoped the DC rails, the heatsinks are fine and the regs, even with all 4 pres is only warm to the touch, could keep my fingers there for ages, unlike my single rail +24V Neve's!

I quickly run a high-resultion FFT from the WAV-files you provided so that we can see what's really going on: greennoise.pdf

Click to expand...

Thanks Sam! Your results look FAR better than mine and the 'spike' in the low end shows up better on high end unit than my simple Wavelab

Brad, way too much good stuff there. Thanks!

I hear you on the stray induced/radiated by surrounding fields... the other day the hum was worsened and I wondered why and it turned out to be because the unit was on my workbench right next to my solder iron, the base station is plastic and contains an EI core open Frame Trano that worsens the hum as the pres were picking it up.

Okay, I will change the tranny to toroid, but even now the hum is not that loud and all the other noises when used with a mic clearly mask it.

If course, tranny outside the box is always optimum solution, if you can live with the idea of having to handle to separate boxes.

Click to expand...

Thanks Olaf, it will make me sleep a little better. I've not had all that much 'real world' use out of the pres hence it is all under test load and am wondering what is or isn't acceptable for buzz/hum in pres.

Thanks again guys for all the input and suggestions, you all rock, and I love this place :thumb:

Cheers

Matt

I mean, this is at 66 dB of gain, so the peak is at -130 dBu EIN (assuming 18 dBu = 0 dBFS). That's not that bad.

Click to expand...

But please don't conclude that the EIN of this preamp is -130 dBu--in fact it will be several dBs worse. You can't easily derive a full-bandwidth figure from a FFT analysis.

BTW, an addition to Brad's list: reducing peak currents can be accomplished by reducing the smoothing capacitor value. I usually specify them for about 1 V ripple--if you go for much less, transformer efficiency decreases and the danger of saturation is much more pronounced.

Samuel

[quote author="Samuel Groner"]But please don't conclude that the EIN of this preamp is -130 dBu--in fact it will be several dBs worse.[/quote]

Maybe I shouldn't have used the abreviation EIN, so my sentence should read: At 66dB of gain the 50Hz peak equals a noise in front of the mic that would be -130dBu.

Olaf

[quote author="Samuel Groner"]...BTW, an addition to Brad's list: reducing peak currents can be accomplished by reducing the smoothing capacitor value...[/quote]
Great info, along with Brad's list. Like yourself, I'm also curious about the resistance for a given DC current for reducing peak input currents.

It would be interesting to lay out the basic math (if possible) for all of this stuff concerning rectifier-filter-regulator type power supply circuits. I could put it in the Power Supply Meta.

Im very interested too on how you can reduce the peak currents to minimize the harm done from having the toroid in the same box as your micpres. im willing to try anything to reduce the hum/buzz im having problems with.

How big of a resistance do need, and how much bigger speced does my toroid need to be for me to try this..?

Do i connect the resistor to the secondary side or the primaries?

Sorry if this is a stupid question.

Thanks!
/J

In the last few month I was working on a (actually four of them) 8 channel mic preamp(s), and I did some extensive hum & noise testing. As I thought the results are of some interest (perhaps as reference to measurements of other designs) let me post them here.

First a few details on the design: it's similar to the "Super Green", i.e. transformerless and fully balanced. The linear power supply with 7 different voltages is internal as is a 8 channel AD board. The power transformer is a custom design with full mu-metal and static shield. The smoothing capacitor values where chosen for about 0.4 V ripple (I didn't had sufficient specs of the AD board, that's why it's lower than my usual 1 V), and the power transformer specified for about 2.5x the actual power need.

Maximum gain is 66 dB and all measurements were done with this setting, running into a MOTU 896 with known reference level.

First I disconnected the AD board to avoid any interference and run a measurement on channel 1 and 8 (the later being closest to the PSU with about 10 cm distance to the transformer and 5 cm to the rectifier diodes) both with 0 ohm and 150 ohm source impedance. Spectral analysis gave the following results:

0ohm.pdf
150ohm.pdf

For a 20 kHz BW, EIN was as follows:
channel 1, 0 ohm: -137.2 dBu
channel 8, 0 ohm: -132.8 dBu
channel 1, 150 ohm: -130.7 dBu
channel 8, 150 ohm: -129.4 dBu

Repeating the measurements with the AD board connected again:

0ohm_AD.pdf
150ohm_AD.pdf

channel 1, 0 ohm: -137.3 dBu
channel 8, 0 ohm: -132.4 dBu
channel 1, 150 ohm: -130.7 dBu
channel 8, 150 ohm: -129.3 dBu

The hum in channel 8 is audible if amplified, but nothing I'd be concerned of. In channel 1, the hum is almost completely masked by the residual noise. The increase in high frequency noise in channel 8 is perhaps a result of diode switching noise. Just as an example channel 1 and 8 with 150 ohm source impedance, AD on, amplified by 60 dB:

c1.wav
c8.wav

And a practical hint when doing such measurements: take care where you place your gear--if I put this preamp on top of the MOTU 896 (which has the most crappy mains transformer I've ever seen), the hum is about 15 dB (!) worse.

For those interested in technical details: the FFT analysis used 64k bins and a Kaiser window with beta = 50, averaged over 30 seconds.

BTW, any idea what the 18 Hz peak in the FFTs could be?

Samuel

A quick update on this in case anybody is interested: I was able to improve the noise performance of channel 8 by bypassing the inductor on the input RFI protection filter--presumably it must have picked up a good deal of diode switching noise. This improved the EIN for 0 ohm by 1.5 dB and the HF rise startes about one octave later: 0ohm_AD_mod.pdf

Samuel