Help adding polar patterns w/ 47-style capsule (single backplate) on phantom

[midwayfair]

EDIT: Got it worked out when the front membrane is polarized. See my picture a few posts below this.

Sorry this is so long, but I wanted to get a handle on this BEFORE I mess something up beyond repair.

Trying to work out two things:
1. Make sure I understand how polar pattern switching operates.
2. The simplest way to implement figure 8 and omni patterns using a capsule without isolated backplates in a mic running on straight phantom power.

In most cheap tube mics, it's simple:
Like the 460: http://www.deeringamps.com/workbench/images/apex460_mic_lg.jpg
Backplate is held at B+V -- this is the primary output.
Front membrane is at 60V.
Rear membrane is at 120V in figure 8, 0V in omni.

It looks like I don't actually need any way for sound to go from the rear membrane to the input of the mic, just change the voltage relationships. How does the signal get from the rear membrane to the front?

RM - BP - FM
Omni: 0 - Med - 0
Card: 0 - Med - Med
8: 0 - med - hi

When the front membrane is as much higher than the backplate as the backplate is above the rear membrane, the phase of the membranes is reversed. So it looks like I can't use this switching with phantom unless I polarize the backplate to 24v. I don't like that idea much.

In the U47-N, it's even simpler -- the rear membrane is simply shorted to the front! Nothing even to drain off voltage when switched back to cardiod.

RM - BP - FM
Omni: 0 - Med - 0
Card: ? - Med - 0

No figure 8 available here, but the switching is easiest.

In the u87i, it's different and looks more complicated:
http://cdn.recordinghacks.com/images//mic_extras/neumann/U87-schematic.png

One main thing here compared to the other schematics is the addition of a capacitor from the rear membrane to front membrane. This makes it look more like two capsules smooshed together instead of one capsule with two membranes.

Front membrane is held at 0V through R6.

Front backplate is held at ... uhh, it's complicated and I'm not exactly sure. Something less than the 48V from phantom. There's a 33V zener in there, but is that doubled to 60V? I know it's doubled in the Ai based on a recent question I asked. Anyway, I'll call it the "polarization voltage" and just use percentages of it.

Rear membrane is connected to the front backplate's same polarization voltage except that it's got 1.68G between them. I assume that it's not drawing any current, so in cardiod the polarization voltage is actually the same between the front backplate and the rear membrane, and the exceptionally high resistance is to keep it separated from the voltage created on the front membrane.

Rear backplate in cardiod is connected to the front backplate through 68M, so it's at the same polarization voltage. This makes the two backplates look like one.

In figure 8, the rear backplate is switched to about half of the polarization voltage (a pair of 68M between the polarization voltage and the 300K to ground).

In omni, the rear membrane is polarized through a 1G to the 0.4% polarization voltage, and BOTH rear elements are at the same polarization voltage, so everything except the front backplate is now near ground.

Okay, that does actually mesh with the other switching schemes where in omni everything except that instead of switching the polarization between ground and half, we've switched to something slightly above ground and something slightly below half. And the Figure 8 is switching the back to half voltage instead of double. Same principal.

RM - RBP - FBP - FM
Omni: ~0 - ~0 - Hi - 0
Card: Hi - Hi - Hi - 0
8: hi - med - hi - 0

I'm having a hard time seeing how the back is exactly out of phase with the front. From this, it looks like the back half of the capsule is only 90 degrees out of phase with it. What gives?

Now let's say I have a capsule without isolated backplates in a circuit that doesn't already have a polar pattern. What's the simplest way to do the switching?

I need to polarize one thing, have another thing at ground, and switch the third thing between 1/2 polarization and another voltage.

I'm going to use this, because it actually has space for more switches and just needs a bit of metal nibbled out of the body to poke through:
http://aurycle.com/manual/a460-diy-schematic.pdf

Ignoring some different resistor and capacitor values, this has the front end of the u87 minus the polar pattern switching and necessary components, and the polarization voltage is the 48V from phantom instead of whatever it is in the 87.

The kit comes with a capsule without isolated backplates. (I'm not totally sure what the capsule is in the current kits as it's different from when they first started them, but i think it's the same one in the MXL920 -- that is, 67-style but the 32mm membrane.) My alternative was a 47-style capsule, in part because I have one on hand.

Here's a diagram I drew (it's large so I didn't embed it) of some possibilities:
https://dl.dropboxusercontent.com/u/9878279/Jon%20Patton%27s%20layouts/Circuit%20ideas/Polarization%20possibilities.png

Proposed Method 1

Sort of like the cheap tube mics, but some things are mixed up. Adds two resistors and that's it. I don't think this one works -- the mic input is the front membrane here instead of the backplate like it is in the cheap tube mics.

Backplate is polarized, input connects to front membrane, which is at 0V.

1. SPDT on/off/on switch
1 -- Omni. Connects to ground like in the cheap tube mics. Or the polarization voltage instead? Does it matter?
2 -- Connects to the rear membrane (does it need to be through a large value resistor?) Off in the center: cardiod.
3 -- Figure 8. Connects to center point of 68M/68M voltage divider between the polarization voltage and ground.

Here I'm going:
Omni: 0 - hi - 0
Card: x - hi - 0
8: med - hi - 0

So this doesn't have the "medium" voltage in the middle for figure 8. However, it does look a little like the 87 minus one of the elements.

Proposed Method 2

Same as above, but instead of connecting the rear membrane to a 0V point directly for omni, it shorts it to the front membrane like in the 47-N schematic.

Still not sure this works:
Omni: 0 - hi - 0
Card: x - hi - 0
8: med - hi - 0

The end result is the same as above.

I am more confident that the omni mode works, though.

Proposed Method 3

This is more like the 87 but ignoring the second backplate.

1. Add 470pF cap from the rear membrane to the front membrane.

2. SPDT on/off/on switch
1 -- Omni. Connects to ground
2 -- Connects to the rear membrane through a 1G. Center off (cardiod)
3 -- Figure 8. Connects to center point of 68M/68M voltage divider between the polarization voltage and ground.

Omni: 0 - hi - 0
Card: x - hi - 0
8: med - hi - 0

Not really sure what happens here. There's a clear path to the microphone output from both capsules, so I am not worried about that. The omni should work the same as in the 47 schematic, because both capsules are at ground potential, so they're in phase. I just can't figure out if the figure 8 pattern's produced.

This leaves out some components that appear in the 87, like a 10pF cap and the 300K from the switch to ground (not sure exactly what that's doing).

Method 4 -- not drawn

Polarize the backplate to 24V instead and switch the backplate between 0, 24, and 48V. Avoiding this because it loses a lot of sensitivity in all patterns.

Again, I'm sorry this is so long, but I've tried to be as clear as possible.

 

[Khron]

This is just my pair of pennies, but the switching itself would be easiest, IMHO, in a mic with a DC/DC converter that puts out both a negative and a positive voltage (like the one in the U87Ai).

I've went this way in my "custom" FET mics based on some stripped-down iSK BM600's with CK12 capsules. In addition, i've chosen to skip the input capacitor by taking signal from the (tied together) backplates. Front diaphragm is hooked up permanently to the positivie bias voltage, and the rear diaphragm is connected to the center pin of an on-off-on switch, to which the positive & negative bias voltages go.

 

[midwayfair]

This is just my pair of pennies, but the switching itself would be easiest, IMHO, in a mic with a DC/DC converter that puts out both a negative and a positive voltage (like the one in the U87Ai).

I've went this way in my "custom" FET mics based on some stripped-down iSK BM600's with CK12 capsules. In addition, i've chosen to skip the input capacitor by taking signal from the (tied together) backplates. Front diaphragm is hooked up permanently to the positivie bias voltage, and the rear diaphragm is connected to the center pin of an on-off-on switch, to which the positive & negative bias voltages go.

A negative voltage would definitely simplify things as long as it fits in the body. There's some extra room between the boards.

I know Studio 939 has a conversion board, but my understanding was that was 60v, not a negative supply. Are you still running it off phantom, or a separate power supply?

Got a link to a project or schematic you can share for the negative voltage maker?

 

[Khron]

For the switching, i've posted some before / after schematics, in the thread i started about those mics:

http://groupdiy.com/index.php?topic=61084

As far as the DC/DC converter is concerned, it's the same as the one in the U87A(i) - the lower half of this schematic:

http://recordinghacks.com/images/mic_extras/neumann/u87ai-schematic.gif

In the BM600's i used as donors, the inductors were two 220uH axial ones, no big deal. Also, you can tweak the output voltage by adjusting the values of the input resistors (R20 and R21 in the Neumann schematic).

 

[Khron]

In tube mics, they bias one of the "slices" of the capsule to 1/2 of the B+ voltage going to the tube, basically because they CAN And as you've noted in your original post, they can then switch the other "slices" of the capsule between B+ and ground, to obtain the various polar patterns.

 

[midwayfair]

For the switching, i've posted some before / after schematics, in the thread i started about those mics:

http://groupdiy.com/index.php?topic=61084

As far as the DC/DC converter is concerned, it's the same as the one in the U87A(i) - the lower half of this schematic:

http://recordinghacks.com/images/mic_extras/neumann/u87ai-schematic.gif

In the BM600's i used as donors, the inductors were two 220uH axial ones, no big deal. Also, you can tweak the output voltage by adjusting the values of the input resistors (R20 and R21 in the Neumann schematic).

Hrm, that might be a lot of circuitry for me to fit with through-hole when there are already two PCBs in there. I'll have to see how big it ends up. I'm tempted to say I'd have to redo the entire PCBs so I could fit it, which is not really something I'm inclined to do.

Can someone clarify for me how the u87 gets an out-of-phase signal when the rear membrane is not the reverse polarity of the front membrane but just at half its polarization voltage and/or what role the second backplate plays there? If I can understand that then I will first try the simplified u87's switching, which is just a few extra parts.

One thing I thought of overnight was figuring out how to squeeze a DPDT in there for the pattern switching. If I could do that then I think I can work out how I could have the 48V polarization for omni and cardiod, but switch to 24V polarization for the backplate ONLY for figure 8: Just swap the mic input to the backplate instead of the front [now the backplate is at 0V, and the front is at 48V], then one pole switches the rear between 48V (card) and 0V (omni and fig 8), while the other pole raises the backplate to 24V in figure 8. No extra caps are involved, I don't need a second 1G for the rear membrane, and it's just a couple high-value resistors I already have in my parts bin. A 6dB sensitivity drop is not outrageous -- I'd still end up with a figure 8 mic that's way more sensitive than a ribbon. I'm lacking any other phantom-powered fig 8 mics at the moment, and while this isn't the ideal setup, it does give me something more portable/less fragile than a tube mic or ribbon if I need a figure 8 for live recording -- a situation where a less-than-ideal polarization voltage is a small concern.

 

[Khron]

What exactly is the mic you're trying to implement this into?

Just these days, i've been working on designing the circuit boards for a "full retrofit" for three of my sE 2200A's, and i think i'm pretty much done. Three circuit boards - one for the audio circuitry, one for the DC-DC converter (which is a bit of a Schoeps-Neumann hybrid), and one to hold the three switches (high-pass, pad and pattern select). I'm still trying to figure out the optimal(?) way to solve the padding, though...

 

[midwayfair]

What exactly is the mic you're trying to implement this into?

Aurycle FET mic.

Here's a picture showing the switch holder (from their site):
http://aurycle.com/manual/A460DIY_3.JPG

The plastic holder actually has 4 ports; the switches go inside the plastic holder and the wires poke through the bottom. There are two ports cut on the microphone face, but filing out a third is pretty trivial. A normal mini DPDT I think is slightly too big to fit, but if I can find a thinner one then it would make things a little easier.

A third board can fit between the sandwich as long as it's all flat components.

I'm aware that there are PCBs already available for the u87 layout etc., but I wanted the option of keeping the source follower.

 

[Khron]

Ah... In that case, yeah, you're in a bit of a tough spot, due to their... laziness, perhaps?

With a bit of persistence, i'm positive all those components COULD have fit onto a single PCB easily A few of the resistors placed vertically, and bam! - you've saved yourself a whole lot of real-estate.

The sE 2200A's only have one pcb with everything on it (although granted, mostly SMD and no DC/DC converter), and on the back side is just a blank pcb (no copper), just with the logo and model silkscreened onto it - i'm guessing for mechanical reasons.

 

[midwayfair]

With a bit of persistence, i'm positive all those components COULD have fit onto a single PCB easily

Yeah, as a person who builds pedals in 1590As, it is annoying to see wasted space requiring extra wire runs like that. But even if I design my own PCBs for the mic, assuming I get the dimensions exactly right, an OSH Park order for them would set me back an extra $40 or more. That's cheaper than the other option, which is to ditch only the second board, lose the source follower, replace the transformer, and use a capsule that doesn't require a deemphasis circuit. I could maybe squeeze the output section AND the voltage multiplier onto one PCB, but that's a LOT of components to squeeze.

I'll have a dig for a smaller DPDT switch before I do anything like that ... I think it's the most elegant (and least noise-prone) solution.

 

[Khron]

Depending on how big of a hurry you're in, other companies like Itead could be an option Either them, or another one i've got bookmarked will be my choice for these 2200A retrofit pcb's. I managed to make all three of them smaller than their smallest (5x5cm) and cheapest (10$ for 10pcs) choice, so i think i'm good with that

 

[midwayfair]

Actually, I think I got it with the SPDT on/off/on ...

The +/-60V is still pretty likely to be better if it can fit. I'll give some thought to making a layout that uses it. If I get a layout working properly I'll share it on Osh Park.

When 1 and 2 are connected, 1G's connection becomes 24V through the two 330K resistors, and the rear membrane is held at 0V through the 68M; the rear and front membranes are completely out of phase and the backplate is in the center: Figure 8.

In the center, there's no connection. The 1G is connected to ground through the 330K, and the rear membrane is held to ground by the 68M; only the front membrane and backplate are out of phase: Cardiod.

when 2 and 3 are connected, the 68M is connected to the 330K to +48V. This makes a voltage divider that puts it close enough to 48V.

I can't believe it took me over an hour to figure that switching out. 

Is there anything wrong with this that I'm not thinking of? Does the resistance between the backplate and +48V need to be higher for any reason? I just used 330K because it was the number in the 87 schematic and it's not like it's a problem to use a 1M like the front membrane.

 

[Khron]

Of course, one thing to keep in mind is that those 48v will never be 48v, if whatever's supplying the phantom power adheres to the standard (ie. supplying it via a pair of 6.8k resistors). With something in the region of 2-2.5mA quiescent through the PNP driving the transformer, something around maaaaaybe 40v (generously) is the best you can hope for, i'm afraid.

Either way, your switching arrangement reminds me of the Trion 8000 - http://recordinghacks.com/pdf/cad/trion-8000-schematic.pdf

 

[Khron]

PS: Regarding the retrofit of a DC/DC converter into a 2-pcb mic, perhaps this might give you an idea?

http://audioimprov.com/AudioImprov/Mics/Entries/2013/8/8_Mic-Parts_RK-47.html

And since we're on this topic / page, in the schematic there, you'll see that the output of the oscillator is coupled through a capacitor. Tack on another identical one there, attach a diode pointing the other way, and voila! A negative voltage is born

On my pcb, i've also added diodes to/from ground, like in the Neumann circuit.

 

[midwayfair]

Of course, one thing to keep in mind is that those 48v will never be 48v, if whatever's supplying the phantom power adheres to the standard (ie. supplying it via a pair of 6.8k resistors). With something in the region of 2-2.5mA quiescent through the PNP driving the transformer, something around maaaaaybe 40v (generously) is the best you can hope for, i'm afraid.

Indeed. It's one of the problems with retaining the emitter follower, but a replacement transformer is an expense that would have to wait its turn. I am kind of surprised that they didn't use the same transformer that's in the tube mics, actually, but I guess manufacturing a second, but cheaper transformer, is cheaper than making more of the more expensive transformers.

Interesting post ... looks like the Monoprice microphone body. That's hilarious that the other circuitboard was "superfluous" -- why screw in a second PCB if it doesn't have any components!?

 

[Khron]

Actually, i think i recall seeing some passive components having been mounted on that board. Most likely, as in the case of the "blank" boards in my 2200A's, purely for mechanical support? A single PCB screwed onto those rails might've been too flimsy, even for cheap chinese stuff...

 

[Matt Nolan]

Can someone clarify for me how the u87 gets an out-of-phase signal when the rear membrane is not the reverse polarity of the front membrane but just at half its polarization voltage and/or what role the second backplate plays there? If I can understand that then I will first try the simplified u87's switching, which is just a few extra parts.

Having two separate backplates means you can polarise the two capsules completely independently. The ELA-M251 was one of the earlier mics to use this scheme.

Zero on the backplate and positive on the diaphragm gives one polarity while positive on the backplate and zero on the diaphragm gives the other polarity.

If you share a common backplate between 2 diaphragms then you can't do this and have to use double or negative voltages on one diaphragm to get a phase invert.

 

[midwayfair]

Can someone clarify for me how the u87 gets an out-of-phase signal when the rear membrane is not the reverse polarity of the front membrane but just at half its polarization voltage and/or what role the second backplate plays there? If I can understand that then I will first try the simplified u87's switching, which is just a few extra parts.

Having two separate backplates means you can polarise the two capsules completely independently. The ELA-M251 was one of the earlier mics to use this scheme.

Zero on the backplate and positive on the diaphragm gives one polarity while positive on the backplate and zero on the diaphragm gives the other polarity.

If you share a common backplate between 2 diaphragms then you can't do this and have to use double or negative voltages on one diaphragm to get a phase invert.

Thanks for the explanation.

 

[Khron]

After a bit more browsing, i came across this (scroll down to the second schematic):

http://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic.html

The top ~half is exactly what you're aiming for, i believe, including pattern switching Read the description below that if some parts of the circuitry look odd to you.

 

[midwayfair]

After a bit more browsing, i came across this (scroll down to the second schematic):

http://audioimprov.com/AudioImprov/Mics/Entries/2015/12/20_Modding_a_BM-800_Mic.html

The top ~half is exactly what you're aiming for, i believe, including pattern switching Read the description below that if some parts of the circuitry look odd to you.

That's a good and simple circuit. But it lookes like it works off 9v. The caps are bigger, but I might be able to deal with that. I'll work up a little layout for it after I find some reading materiaal that describes how the circuit works. I think someone linked to it in another thread, where I asked how the 87ai got its voltage.