Open source project: Little oscillator PCBs for LDCs shared on Osh Park

[midwayfair]

PS: RuudNL Yes, the RC time-constant with 1G/100nF would indeed be 100sec, but that's "only" the time the voltage reached 63%; i believe the widely-accepted(?) figure for it reaching 100% is 5 times the RC time-constant, so 500sec 

Is this the time constant for the capsule itself to come up to full voltage, or an argument for omitting the C6 and C10 filter caps after the 1G resistors (or using, say, 100M there instead)?

I think one of the K47 capsules I measured from the group buy was 170pF ... :/

 

[Khron]

It's an argument for not using 1G resistors to filter the bias voltages, at all

Ie. skipping R3 & C6 in here, altogether (and their correspondents on the negative "branch"):

https://jonpattonmusic.files.wordpress.com/2017/04/oscillator-for-schoeps-circuits1.png

See the "rise time" equations over here: https://en.wikipedia.org/wiki/RC_time_constant#Cutoff_frequency

Is this the time constant for the capsule itself to come up to full voltage, or an argument for omitting the C6 and C10 filter caps after the 1G resistors (or using, say, 100M there instead)?

I think one of the K47 capsules I measured from the group buy was 170pF ... :/

 

[midwayfair]

It's an argument for not using 1G resistors to filter the bias voltages, at all

Ie. skipping R3 & C6 in here, altogether (and their correspondents on the negative "branch"):

https://jonpattonmusic.files.wordpress.com/2017/04/oscillator-for-schoeps-circuits1.png

See the "rise time" equations over here: https://en.wikipedia.org/wiki/RC_time_constant#Cutoff_frequency

Fair enough. I might go in and jumper the 1G to see if there's a noticeable change in noise.

I should add here that the multiple filters aren't pointless. If there's some ripple in the phantom power, that will appear as a wiggle on the oscillator, which will then put out an inconsistent voltage. Multiple stages can ensure that this is never a problem. I guess a compromise would just be another 1M in place of the 1G or something. The 1G was also a carry-over from the Schoeps circuit itself. Phantom power was probably a lot dirtier in those days.

I don't have a good way to measure noise, but I would expect most of us have clean phantom power.

 

[Khron]

A few corrections:

1) The phantom power is usually supplied through a pair of 6.8k resistors in the preamp. Those, in conjunction with the pair of 1-2.2k "summing" resistors in the mic, and the first filter cap after those, form a preeeeeeeeetty "hefty" low-pass filter already.

Using 4.4k (3.4k from the two 6.8k in parallel, in series with a ballpark 2x 2k in parallel) and 10uF over here [ http://sim.okawa-denshi.jp/en/CRtool.php ], gives a low-pass cut-off frequency (-3dB corner) of 3.6Hz, and going down by 6dB/octave from there on. I'm gonna hazard a guess and say that not a lot of ripple's gonna make it to the input of this oscillator. And that's with VERY conservative values, i think you'll agree. Not that the oscillator would / will care all that much, anyway ;D

And that's even before the other voltage-dropping resistor (in this case, that 10-50k trimmer) plus the 220uF or however large it is, capacitor

<later edit>
Oh, but isn't the input to the oscillator taken from after the zener regulator on the mic pcb, anyway?  :
That's some extra ripple-"cleaning" right there...
<end of edit>

2) Not quite buying the "wiggle on the oscillator, which will then put out an inconsistent voltage", partially due to the above-mentioned reasons, and partially due to the filter capacitance on the output, coupled with the tiny tiny current it needs to supply.

3) "Carry-over from the Schoeps circuit"? I'm afraid you might've mis-read it - that's assuming we're talking about the original one. The only 1G resistor i see there, is the one from the JFET gate to the wiper of the bias trimpot. The "lone" series resistor that's on the output of the oscillator is a 1M one.

Fair enough. I might go in and jumper the 1G to see if there's a noticeable change in noise.

I should add here that the multiple filters aren't pointless. If there's some ripple in the phantom power, that will appear as a wiggle on the oscillator, which will then put out an inconsistent voltage. Multiple stages can ensure that this is never a problem. I guess a compromise would just be another 1M in place of the 1G or something. The 1G was also a carry-over from the Schoeps circuit itself. Phantom power was probably a lot dirtier in those days.

I don't have a good way to measure noise, but I would expect most of us have clean phantom power.

 

[emrr]

Yes, if you want to generate a negative voltage rather than a positive one, you would reverse the diodes. The diodes only see AC voltage and the direction each pair faces changes whether they rectify to a positive or negative voltages.
May I ask what situation that would pop up in?

Just saw this, thanks.  Hacking in bias for figure 8 in a vintage 87 clone but with a single backplate capsule.  Several changes to make that happen, of course. 

 

[midwayfair]

Just saw this, thanks.  Hacking in bias for figure 8 in a vintage 87 clone but with a single backplate capsule.  Several changes to make that happen, of course.

I see. So you have a mic that already has a bias voltage generator, and you just need a way to create a SECOND voltage for the figure 8? If you have room, it might make more sense to remove the current voltage generator you have an use the version of this board that makes two voltages for you. They'd be identical voltages so you should get a better pattern.

 

[emrr]

I see. So you have a mic that already has a bias voltage generator, and you just need a way to create a SECOND voltage for the figure 8? If you have room, it might make more sense to remove the current voltage generator you have an use the version of this board that makes two voltages for you. They'd be identical voltages so you should get a better pattern.

True.  I'm cramming a vintage 87 circuit into scavenged Neumann metalwork for use with a modern Neumann single backplate K67, so space is at a premium.  I may have to breadboard the solution.  It'll end up at least partially like a U87AI in terms of bias approach.  There's a number of other things to change as well. 

 

[Blue Jinn]

Thread revival here. I just picked up some of the boards from OshPark, and a little confused. the Mouser BOM has four .1uF (100nF) caps, but only C8 is 100nF, C4-6 are 10nF, is the schematic off or the BOM off, or does it matter? (Use 100nF for C4,5,6,8?)

 

[midwayfair]

Thread revival here. I just picked up some of the boards from OshPark, and a little confused. the Mouser BOM has four .1uF (100nF) caps, but only C8 is 100nF, C4-6 are 10nF, is the schematic off or the BOM off, or does it matter? (Use 100nF for C4,5,6,8?)

Ah, I probably misread my own schematic, since I'm so used to using 100nF as filter caps in guitar pedals. I'm pretty sure it doesn't matter given the size of the resistors involved but I should fix the cart to alleviate confusion.

 

[Blue Jinn]

Thanks! The corresponding caps on the multi pattern are all 100nF so I'm going to go ahead and give it a shot.

 

[jsender]

Hi, long-time lurker on this project and the transformerless LDC mainboard sister project. I built a couple single sided LDC a couple years ago and am still using them almost every day
I'm pretty much just a solder jockey right now but trying to learn more about how everything works this year.
I was hoping someone could explain a bit what the trimpot is doing here? There is fluctuation in voltage as I turn the trim, but then the voltage always settles back at it's resting voltage once I stop turning the pot, whether it's left on 0ohm or 10Kohm.
So what is this resistor is doing, and way is it a trim? Aside from voltage fluctuating when I'm turning the pot, I can't tell any difference between the trim being left at 0k, 5k or 10k.
Cheers and thanks for the awesome project midwayfair!

 

[jsender]

Hi, long-time lurker on this project and the transformerless LDC mainboard sister project. I built a couple single sided LDC a couple years ago and am still using them almost every day
I'm pretty much just a solder jockey right now but trying to learn more about how everything works this year.
I was hoping someone could explain a bit what the trimpot is doing here? There is fluctuation in voltage as I turn the trim, but then the voltage always settles back at it's resting voltage once I stop turning the pot, whether it's left on 0ohm or 10Kohm.
So what is this resistor is doing, and way is it a trim? Aside from voltage fluctuating when I'm turning the pot, I can't tell any difference between the trim being left at 0k, 5k or 10k.
Cheers and thanks for the awesome project midwayfair!

okay whoops I was using the wrong points to test voltage when testing at the correct R2/R3 point, clearly the 10k trim is adjusting the polarization voltage making it easy to hit 60V.
but now that I've figured that out, I guess my new question is, why is 60v the target if the mics seems to function the same with different voltages -- simply a way of matching voltage when building a pair? or does the 60v target have other implication I should understand...

 

[midwayfair]

okay whoops I was using the wrong points to test voltage when testing at the correct R2/R3 point, clearly the 10k trim is adjusting the polarization voltage making it easy to hit 60V.
but now that I've figured that out, I guess my new question is, why is 60v the target if the mics seems to function the same with different voltages -- simply a way of matching voltage when building a pair? or does the 60v target have other implication I should understand...

Different capsules like different voltages, and/or will tolerate a higher voltage. If the voltage is too high, then some extra pressure or wind will pin the diaphragm it to the backplate and you'll hear a sucking noise until the voltage disacharges (turning the mic off usually works). Too low a voltage and your signal to noise will suffer ... at some point. I have one mic that sounds quite good splitting only 48V (no voltage board like this one to get +-60), but the ones running the 60V in either direction are a heck of a lot higher output.

The 60V target is somewhat typical for the types of capsules I expect people to put in these. If you put something nice in there, ask the manufacturer what a good polarization voltage is. Like Tim Campbell recommended 50V to me for his C12-style capsule.

 

[jsender]

Different capsules like different voltages, and/or will tolerate a higher voltage. If the voltage is too high, then some extra pressure or wind will pin the diaphragm it to the backplate and you'll hear a sucking noise until the voltage disacharges

Ah I see thanks for the info! This maybe explains what was going on with 1 of the 3 that I built in 2020. I got parts for three, including three 12-style LDC capsules from alibaba. The three capsules were very different pF, about 70pF, 80pF, and 100pF. The build that was using the 100pF capsule would only work for a few minutes before a weird sound happened and the signal cut out until the mic was left off for a while, then repeat. I swapped in the capsule from an old Aurycle build and it worked fine, so I just assumed I got a faulty capsule.
But maybe instead the higher pF capsules would have worked just fine with lower polarization voltage? Any ideas if that would be always be the case here? Higher capsule pF means lower your voltage? I would love to try but can't seem to find the my old discarded 100pF capsule...I know it's around here somewhere

 

[kingkorg]

Ah I see thanks for the info! This maybe explains what was going on with 1 of the 3 that I built in 2020. I got parts for three, including three 12-style LDC capsules from alibaba. The three capsules were very different pF, about 70pF, 80pF, and 100pF. The build that was using the 100pF capsule would only work for a few minutes before a weird sound happened and the signal cut out until the mic was left off for a while, then repeat. I swapped in the capsule from an old Aurycle build and it worked fine, so I just assumed I got a faulty capsule.
But maybe instead the higher pF capsules would have worked just fine with lower polarization voltage? Any ideas if that would be always be the case here? Higher capsule pF means lower your voltage? I would love to try but can't seem to find the my old discarded 100pF capsule...I know it's around here somewhere

Nope, it doesn't directly relate to capacitance necessarily. However if you know how capacitors work, it makes sense that if the spacer between backplate and diaphragm is thinner, and diaphragm closer to the backplate you do get higher capacitance and it is easier to get the diaphragm sucked to the backplate. However, sometimes to counter this effect the diaphragm is tensioned tighter, so it can take higher polarization without getting stuck. If you do that though you lose some of the low end. Neverending loop of capsule construction compromises.

I wouldn't be surprised if that capsule you are talking about is the Rayking capsule i have. That one wont take a volt above 60 without getting stuck, and has caused me tons of trouble simply because i wasn't aware of it. However it does have huge low end.

 

[mihi_fuchs]

Here is another set of values you can check. Worked out very fine with me. Just make sure the inductors are close to each other (as stated above already). The voltages you supply via high value resistors to the capsule. Feel free to use!

Best regards
Michael

 

[metalockpick]

For the record: this type of oscillator depends kinda crucially on the coupling between the two inductors. All the ones i've seen in factory mics, even the ones using SMD components, have through-hole (axial) inductors, parallel and right up next to each other (and hot-snotted together).

I've found that out "the hard way" - i've been... having a few headaches with my own design (partially down to my selection of SMD inductor and/or part layout)...

The best variants of this use a transformer, both inductors on the same core. In those cases, the schematic is correct, but misleading.