What component could be causing this audio distortion? - Page 1
Forum section: electronics / beginners

author: NiceProduction
author_rank: Newbie
timestamp: December 27, 2023, 01:35:40 pm
content: I'm trying to find the source of a weird type of distortion I've not encountered before in an audio signal. It's occuring in the signal from electret headset mics.The distortion appears to happen once the waveform exceeds a very specific level, before which a sine wave remains intact, and above which, the quarter portion of the waveform around the peak, appears scooped. The further the waveform exceeds the threshold, the more scooped that quarter of the waveform appears. By scooped, I mean it sags almost instantaneously, then reforms the shape of the sine wave once the level has lowered. It adds a kind of sawtooth look to the peaks of the sine wave.There is no classic clipping of the waveform, so I don't think any components are being overdriven. The waveform can and does exceed the threshold level by a large amount. Is this a capacitor physically "bottoming out" and discharging across the plates? Going by the passages in which the distortion occurs, I feel like it might be frequency-dependent, occuring when there's strong levels around 800 - 900 Hz.This is happening in 3mm electret condenser microphones used in headsets for musical theatre. I cannot access the capsule to see what components are inside, but it would be fair to assume there is an electret capacitor, a FET, and a resistor, as they are all standard components in an electret condenser microphone circuit.The capsules are rated for 5V. My wireless mic transmitters deliver 7 or 8 volts DC of bias voltage. The distributor ships the mics with adapters that include a 2V 0.5W zener diode wired in reverse between the signal and ground wires, to drop the bias voltage to the capsule. I originally thought the zeners might be responsible, but the same distortion happens when I run the mics terminated directly to a connector with no zener diode in the circuit.Signal path is: electret condenser microphone in a headset/boom configuration (claimed to be able to handle up to 153 dBA but no distortion specs available) -> 3.5mm TRS connector with signal pin linked to ground through a reverse polarity 2V zener diode -> Sennheiser EW DX SK digital radio mic transmitter with no internal gain -> Sennheiser EW DX EM2 digital radio mic receiver -> Midas DL431 preamp (24-bit, 96KHz).The audio source is actors speaking, singing and shouting. I've varied the Midas preamp gain from 0dB to +30dB with no effect on the distortion. I believe the distortion is occuring before the preamp stage in the Sennheiser EW DX SK transmitter, as I've heard the same sound on these mics using analog SK500 and SK300 FM transmitters. At the time I blamed the analog transmitters and their companders, but the fact that the distortion occurs with the digital transmitters makes me think it's inherent to the mics themselves.Here are some screencaps showing the waveform as it starts to distort, and the waveform in full distortion. The yellow lines are about -13dB in this audio editing software, but that's unrelated to the original signal level. The fundamental frequency is around 840 Hz:I don't have any standard EE test equipment here, unfortunately.I need to pre-emptively point out that as I mentioned in the title of this thread, I am interested to know what electronic component would fail in a way that produces the kind of distorted waveform in my attached image. I find that in threads like this, people post all kinds of responses that ignore the actual question being asked. I can't change the bias resistor in the capsule. I can't access the capsule to do anything about the components or circuitry inside. I just want to know what might be responsible for the distortion, so I have something to take to the distributor when I open up a dialog with them.The ring and sleeve are connected as per the diagram on page 33 of this documentation from Sennheiser, the manufacturer of the wireless mic systems I'm using:https://assets.sennheiser.com/downloads/download/file/4062/SK300G3_Instructionsforuse.pdf- this signals to the transmitter that the signal is mic level.Thanks for any help or suggestions you can make.

author: CaptDon
author_rank: Super Contributor
timestamp: December 27, 2023, 02:28:40 pm
content: You absolutely can't wire a zener that way to control the phantom voltage to the mike!!! Of course it will do weird things. They probably meant for the zener to go in series with the center lead where when placed in the proper direction it would in fact drop your 7 to 8 volt supply down by the zener voltage, if put in facing the wrong way it would drop the voltage by .6 volts. The anode end (arrow head) would go towards the mike and the cathode end (black line) toward the belt pack. You may have to adjust the gain in the belt pack also (if it has an adjustment). The way your zener is connected now the mike will only see the zener voltage and not even 5 or 7 to 8.

author: dmills
author_rank: Super Contributor
timestamp: December 27, 2023, 04:03:47 pm
content: Seconded, that Zener makes NO sense as drawn, and I am far from convinced it is needed at all.The transmitter pack runs from 2.4 - 3V or so (pair of AA batteries) so I would expect the mic bias to be something like a 1k resistor or so from that rail (The spec doesn't seem to say) which is a perfectly reasonable way to bias most such microphones.

author: CaptDon
author_rank: Super Contributor
timestamp: December 27, 2023, 05:21:29 pm
content: I am guessing your belt packs are powered from 9vdc batteries? That is where the 7 to 8 volts is coming from. 90% of my pro wireless stuff (mostly audio technica) are 9 volt powered. And by the way, all of my 9vdc battery powered stuff HATES those 'rechargeable' 9 volt batteries made with 6 nicad or nimh cells who's real voltage is a measly 7.2vdc!!

author: dmills
author_rank: Super Contributor
timestamp: December 27, 2023, 09:41:50 pm
content: Quote from: CaptDon on December 27, 2023, 05:21:29 pmThat was common on some of the older gear, but most modern Sennheiser use a couple of AA cells as you get way more energy that way.The common trap with the rechargeable batteries, apart from the slightly low voltage was always that due to the very flat discharge curve the time between a low battery light and the thing shutting down was inconveniently short.Back when I was in the game we used to have literal buckets full of half used primary cells after a run.

author: NiceProduction
author_rank: Newbie
timestamp: December 28, 2023, 06:33:52 am
content: Quote from: CaptDon on December 27, 2023, 02:28:40 pmYou absolutely can wire a zener in reverse to regulate voltage, as perhttps://www.electronics-tutorials.ws/diode/diode_7.html- anyhow the zener isn't the issue and I pointed out that the distortion happens when I use mics terminated without a zener diode.And it's not phantom power, it's DC bias voltage for the electret condenser capsule.I commented in my original post that there is no internal gain in the transmitter pack.I also made the point that people inevitably make a bunch of off-topic and irrelevant comments but never address the question in the title.

author: NiceProduction
author_rank: Newbie
timestamp: December 28, 2023, 06:36:04 am
content: Quote from: dmills on December 27, 2023, 04:03:47 pmAs per my other reply, the zener makes perfect sense, and is irrelevant anyway, as the distortion happens when using connectors that don't have zeners.The transmitter packs run on 2 x AA batteries but supply (as measured) 9.4V DC bias voltage to the mics.

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2023, 02:34:53 pm
content: Yes, we all agree, a zener CAN regulate voltage, that is basically their purpose and we don't need a link to prove it. The way you have that zener wired across the electret mic makes no sense!!! The max voltage across your mic could only be the zener voltage, go ahead and measure it. So it will only be the zener voltage and not 5 volts if that matters. As for answering your question of "What part could be causing this" maybe the aftermarket mic has to much gain and is overdriving the ADC converter? Maybe something in the beltpack is off tolerance and without a complete schematic we will require a crystal ball. Couldn't tell you "What part" may be bullocks for a million dollars without a full schematic. Yes, phantom power would generally be considered 48vdc through resistors done in a balanced manner in a typical mixer/mic setup, HOWEVER, isn't it true that ANY power being supplied to a microphone that isn't self-contained within the microphone would be considered as phantom? Yes, we know electrets require power or 'bias' if you prefer. They actually consume power if they are FET preamplified so that would be a power source more so than simply 'bias'. You really beat up on those trying to help you and I could assume most of us in the pro-audio (Clair Brothers / Showco Worldwide family) have already forgotten more about audio production than you currently know. For the amount of money that changes hands in a typical road tour it would be expedient to throw out an improperly functioning P.O.S. and replace it rather than some band-aid repair. The performers will thank you!

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2023, 03:03:42 pm
content: It looks like the receiver has both a 'GAIN' adjustment and an 'AF OUT' adjustment. If the aftermarket mic has a lot of signal output, more so than other Sennheiser mics you may have to reduce the 'GAIN' setting in the receiver to compensate. The distortion could be taking place in the receiver? Sadly the D series transmitters don't seem to be equipped with a mic gain feature in the belt pack but supposedly the DX series belt pack is adjustable? Via menu? I don't know as I don't use Sennheiser wireless gear. Only their studio mics.

author: NiceProduction
author_rank: Newbie
timestamp: December 28, 2023, 03:17:31 pm
content: Quote from: CaptDon on December 28, 2023, 03:03:42 pmThe EW DX transmitter has 134dB of dynamic range, and, being digital, doesn't use companding. Thus the design choice was to apply no gain at the ADC in the transmitter, since it's capable of converting mic input signals ranging from extremely low-level to high-level with very low noise. The gain in the receiver is to boost the output to levels similar to what the analog receivers from the previous G4 range would be outputting.I wrote in my initial comment that there is no internal gain in the transmitter.

author: NiceProduction
author_rank: Newbie
timestamp: December 28, 2023, 03:26:39 pm
content: Quote from: CaptDon on December 28, 2023, 02:34:53 pmThe zener is irrelevant, since the distortion happens when the mics are terminated without them. Don't get hung up on it. It works fine for what it does.QuoteYes this is the crux of the matter, and I rather stupidly assumed that an experienced electronics engineer might recognise the very distinctive type of distortion as being particular to a certain component. Much like I would be able to tell the difference between the waveform of a signal being distorted by a saturated transformer core compared to one clipped by exceeding digital 0dBFS. Alas I am not anywhere near experienced enough to recognise this characteristic distortion, and that is why I was asking on this forum. I relate it in my mind to my own skill at determining the failure mode of a radio mic channel in a live PA by the sound it makes. A fuzzy splat is guaranteed to be RF-related, most likely dropouts from interference over the frequency it's tuned to, whereas a loud, sharp crack is most likely a broken wire in the cable or connector, periodically breaking and making contact and passing sudden voltage spikes.If you are not experienced in matters relating to this type of distortion, I thank you for your input so far, and advise that you don't waste your time any further by continuing to respond in this thread.QuoteI already stated that the signal is not clipping and can go much louder than the level at which this distortion occurs, without incurring the same distortion.QuoteThis is occuring in multiple mic channels.QuoteGreat, then we're agreed that you don't need to post any further.QuoteAbsolutely not, and even suggesting this makes me question your expertise in this domain.QuoteYou ignored most of what I wrote, and acted most rude.QuoteSnide insults and shitty implications thrown into a conversation. Great. Thanks for your time.

author: themadhippy
author_rank: Super Contributor
timestamp: December 28, 2023, 04:18:21 pm
content: silly idea but why not give senni a shout,there,at least in the uk,very nice people.

author: mikerj
author_rank: Super Contributor
timestamp: December 28, 2023, 04:39:19 pm
content: Quote from: NiceProduction on December 28, 2023, 03:26:39 pmYour total lack of understanding of the implications of the zener wired in parallel with the microphone suggests you have zero expertise in this domain. Just think about what this will do to the signal.

author: NiceProduction
author_rank: Newbie
timestamp: December 28, 2023, 04:40:23 pm
content: Quote from: themadhippy on December 28, 2023, 04:18:21 pmThe problem isn't with Sennheiser gear. I'm certain it's something to do with the headset mic, which is a different brand.

author: NiceProduction
author_rank: Newbie
timestamp: December 28, 2023, 05:18:00 pm
content: Quote from: mikerj on December 28, 2023, 04:39:19 pmDID YOU READ THE PART WHERE I SAID THIS IS HAPPENING IN MICS WITHOUT A ZENER DIODE IN THE ADAPTER?I don't have to think about what it will do to the signal. I've used the mics both with and without the adapters that have zeners in them. The only difference was the signal level was lower with the zeners.Do you have some arcane knowledge of my audio that I wasn't privvy to while mixing these shows?And of course I don't have experience in the domain of which (non-zener) components are causing the distortion. That's why I'm asking.

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2023, 06:12:35 pm
content: I have seen exactly that waveshape of distortion when an amplifier was asked to output more than its capacity. When an amplifier runs out of headroom due to voltage limits it tends to hard clip. When it begins to run out of headroom due to lack of current capacity the sinewave does sort of a distorted soft 'knee-over' like your waveform. One place where I have seen this is when the input to a transistor amplifier is capacitor coupled and the base bias resistor provides insufficient base current. There comes a point where the input signal must provide a significant portion of base current and you see sort of an RC slope at the positive peak of the sine wave, then since the capacitor now has a different charge across it as the signal goes toward the negative peak the transistor starts to move toward cutoff and distorts the lower half of the cycle. Can you actually use a real Sennheiser mic cartridge and get a clean signal? Perhaps the aftermarket mic wants to see a radically different impedance than what the beltpack provides and is causing the aftermarket mic to go into soft knee-over clipping? The receiver should have both GAIN and AUDIO OUT adjustments via menus. What if the receiver is going into compression from to much DAC signal? It sure looks like some form of compression. Does the waveform look clean at lower SPL levels?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2023, 07:04:13 pm
content: I agree with CaptDon, something is being over driven. Most likely the FET in the Mic.I have modified some electrets for use on different transmitters and have seen distortion especially when the supplied volts are too high. It is possible to change the amplification of these mics by changing the supply voltage, but that is not your question. If supply volts are too high you get distortion.I know you don't want to talk about the Zener, but can you check it with a Diode checker someway?Otherwise the next problem would be Impedance Mismatch.. The Sennheiser calls for 40 K mic impedance, we do not know what the replacement mic has.

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2023, 11:28:06 pm
content: What if the Senn mic had 250mv pk to pk output and the new mic provides 500mv pk to pk or even 500mv rms, that would tend to lead to over driving some aspect of the transmtter / receiver chain although one would expect to see no distortion at low SPL levels. It could be the new mic is starving for 'bias' voltage? Case in point, if the Senn beltpack says 40K ohms input impedance that would be a far cry from the typical 1K to 5K input impedance typically found due to the 'power' being supplied to the electret mic and its FET preamp through a 1K to 5K resistor. What is the actual voltage across your aftermarket mic when hooked up and powered up. Could be it is only seeing .5 to perhaps 1 volt D.C. and that would surely be a problem!!

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 30, 2023, 12:26:46 pm
content: This might help:https://assets.sennheiser.com/downloads/download/file/3728/mke2wiring.pdf