Understanding condenser microphone bias adjustment and its audible effects

[joulupukki]

I'm in the middle of a couple of DIY KM-84 builds and am trying to sort out what makes a proper bias adjustment. I've watched this video a few different times trying to understand it:


It's got me curious what the effects are when the mics are in use. In the video he biased the microphone for least amount of THD without compromising that much headroom. ...and that was done by getting the 2nd harmonic as low as possible? Is that what most people do? When you actually use a mic, under what circumstances are you going to hear the differences between one that's biased for maximum headroom vs least THD?

 

[kingkorg]

I'm in the middle of a couple of DIY KM-84 builds and am trying to sort out what makes a proper bias adjustment. I've watched this video a few different times trying to understand it () though it's also got me curious what the practical effects are in practice. In the video he biased the microphone for least amount of THD without compromising that much headroom. ...and that was done by getting the 2nd harmonic as low as possible? Is that what most people do? When you actually use a mic, under what circumstances are you going to hear the differences between one that's biased for maximum headroom vs least THD?

Recording high SPL. For me it is usually drums where poorly biased mics show ugly side. Biasing will also depend on topology, some mics can be biased tor maximum headroom, and maximum spl, but that won't give you least THD at lower levels. THD can also be affected by other things, such as parasitic capacitance, capacitive pads etc. I have tested my own hearing, and i don't care about THD of 1%, which is plenty level for mics, and doesn't happen often. I start caring when i see asymmetrical clipping in my waveforms, clipped transients... I personally bias carefully the mics i know are going to end up on high SPL sources, but lately i've been using OpAmp based mics for that.

With u87 the tone of whole mic can depend on biasing, because de-emphasis network depends on properly biased fet, so you might end up with varying levels of high end depending on bias. Km84 circuit is similar. No de-emphasis network though. By choosing diferent fets and how you bias them you can choose if the mic will be optimized for highest possible output, or lowest possible THD. This will depend on fet Idss. Lots written about it already, you need to search for u87 and km84 bias/fet selection. Just using random 3819 fet is not enough.

 

[RuudNL]

That is what I usually say: you will find a point where distortion is at the lowest level, but this is not necessarily the point where you can handle the highest SPL. As with everything in life: it is always a compromise!...

By the way: the second harmonic is less harmful because it has a musical relation with the fundamental.
A thrird harmonic will be more disturbing!

 

[kingkorg]

That is what I usually say: you will find a point where distortion is at the lowest level, but this is not necessarily the point where you can handle the highest SPL. As with everything in life: it is always a compromise!...

Yup, this used to drive me crazy, a bit counter intuitive. I was certain i was doing something wrong until i read you were getting the same results.

 

[joulupukki]

Thanks for more info. I've definitely tried to search and read as much as I can find, but still a bit confused. Right now I've biased both of these DIY KM-84 mics by adjusting the trim pot resistor so that I can increase a 1kHz signal as high as possible without clipping and then once it does start to clip, I've got the sine wave clipping the same amount on top and bottom. I tried measuring Idss and Vgs(off) on 10 different 2N3819 FETs and chose a pair that was within 0.5% of each other on both readings. The THD I'm calculating is identical between the two mics with this bias setting.

The frequency responses I've tested with a sweep in the REW app look nearly identical. However, when using the mics with the preamps at the same level (same outcome if I swap preamps between the two mics) one mic puts out slightly more volume than the other. If I wanted to make their output volume exactly the same, would I just adjust the bias of both so that the output measurement on my scope is showing the same output peak-to-peak measurements?

 

[rogs]

.... I personally bias carefully the mics i know are going to end up on high SPL sources, but lately i've been using OpAmp based mics for that.

Using op-amps as impedance converters removes all the bias problems, and rail to rail output signals with very low distortion are difficult to match with FETs.
I'm guessing the main opposition comes form the idea that the self noise levels can be a bit hgher than the quietest FETs?
In reality, I found that using the OPA1641 opamp as a unity gain buffer introduces less than 10dB(A) of self noise -- still considered 'extremely low noise' in Neumann's paper HERE

With reliable sources of many 'famous' mic FETs becoming more and more difficult to locate, I'm a bit surprised op-amp converters - similar to the simple one the one I used in my OPIC project - aren't more common?

With a noise floor that's below ambient noise in most applications, very low distortion, decent headroom (rail to rail outputs), and a reasonable input capacitance (less than 6pF across the range), I find the OPA1641 works very well as an impedance converter.

Not really suitable if you're trying to 'copy' something else specifically of course - and it's not easy to inroduce any non-linearity into the simplest format.
But for basic DIY experiements - and especially for those just starting out on DIY mic building - using op-amps like the OPA1641 are a bit of a game changer, in my view?

 

[Khron]

However, when using the mics with the preamps at the same level (same outcome if I swap preamps between the two mics) one mic puts out slightly more volume than the other.

That will most likely be the tolerances of the capsule showing up.

 

[joulupukki]

That will most likely be the tolerances of the capsule showing up.

Very interesting. You're totally right. If I switch just the capsules, the louder output follows the capsule. It's not much different, but it's there.

 

[Khron]

If you really wanted to, you could perhaps tweak down the capsule bias of the louder one, until it gets "close enough" to the quieter one.

 

[McIrish]

Like the OP, I've always set the bias for the most output with symetrical clipping. They might be mutually exclusive in some ways, where I might get more output but the clipping becomes asymetrical.

 

[joulupukki]

It's interesting that the MP SDC-84 I've been comparing with definitely doesn't clip symmetrically. Here's a pic of the scope with the input signal a bit past the point where it starts to clip and I'm assuming that's all part of the design of that particular mic.

 

[Matador]

But for basic DIY experiements - and especially for those just starting out on DIY mic building - using op-amps like the OPA1641 are a bit of a game changer, in my view?

Totally agreed! It may be that such circuits are 'too clean', and behave too cleanly. I think discrete JFET's may be more forgiving of large signals like plosives, and distort 'nicer' than an opamp in the same situation.

Also, people seem to hate SMD parts.

 

[k brown]

Is it even possible to clip an op amp with the teensy signal from a mic capsule?

 

[Matador]

Is it even possible to clip an op amp with the teensy signal from a mic capsule?

LDC capsules can spit out a volt or two when close miking loud, percussive sources.

 

[k brown]

LDC capsules can spit out a volt or two when close miking loud, percussive sources.

I've heard that LDC mics can put out that much; but the capsule itself?

That is, in a typical mic circuit using an op amp as an impedance converter, the input of the op amp is seeing only a few millivolts under any conditions.

 

[rogs]

Totally agreed! It may be that such circuits are 'too clean', and behave too cleanly. I think discrete JFET's may be more forgiving of large signals like plosives, and distort 'nicer' than an opamp in the same situation.

Also, people seem to hate SMD parts.

The deliberate introduction of non linearities into the signal path - or how you experiement to find 'nice' distortion is a whole different ball game....
It's a bit like the guitar world and tube amps. Lots of deeply held views on what sounds 'right'.
Difficult to be objective though - and that can open the gates to the 'snake oil' brigade!

SMD is a pain for simple experiments - especially where stripboard is used for prototyping.
I've found simple 8 way SMD/DIL converter boards quite useful. And not difficult to use....
A tiny dab of Blu Tack (I think it's called 'poster putty' in the US?) applied to the underside of the 8 pin SMD package holds it in place on the PCB before any soldering ... Even my clumsy hands can manage to make those quite easily.
(Attached photo shows an adaptor with pin 3 removed, to allow the 1G resistor connected to pin 3 of the op-amp to be kept away from the stripboard!)

 

[k brown]

No, it's called Blu-Tack in the US, too.

 

[joulupukki]

The deliberate introduction of non linearities into the signal path - or how you experiement to find 'nice' distortion is a whole different ball game....
It's a bit like the guitar world and tube amps. Lots of deeply held views on what sounds 'right'.
Difficult to be objective though - and that can open the gates to the 'snake oil' brigade!

Yeah, makes sense. I've re-biased my mics for the least THD once it's into the clipping range and swapped the capsules to balance out the output volume just a hair. The clipping is just barely on the asymmetrical side now and speaking of tube amps ... I just recorded some loud distorted guitar through one of my hand-wired amps with the MP and DIY mics simultaneously. When I had them biased for symmetrical clipping I could more easily tell that the MP sounded a little different (added a bit more distortion/dirt ... in a good way). After biasing these DIY mics so they are clipping just barely asymmetrical, the loud guitar through the amp is extremely difficult to tell the difference between the two mics. I think right here's where I'll leave them for a while. I'm mostly recording acoustic bluegrass instruments anyway so the SPL isn't that high. Pretty pleased with these DIY mics.

Thanks for the extra info on biasing.

Edit:
Correction ... with this type of biasing (Least amount of THD) I could hear a difference when recording acoustic guitar more drastically. I went back to a more symmetrical clipping bias and there's less of a difference. In any case, the mics sound great.

 

[soliloqueen]

I've heard that LDC mics can put out that much; but the capsule itself?

That is, in a typical mic circuit using an op amp as an impedance converter, the input of the op amp is seeing only a few millivolts under any conditions.

yes, i have used some capsules with headamps at unity gain and it does definitely get this loud

 

[Matador]

I've heard that LDC mics can put out that much; but the capsule itself?

Yes, the capsule itself can put out a volt or two easily (again, next to a loud source with sharp transients, like close miked drums).