FEELER - Solid-state microphone upgrade boards (OpAmp, JFET, U87A, FET847)

[jp8]

- output-disconnect circuit to prevent spurious power-up/down noises

Reading your post again, I noticed this interesting requirement. I've been playing with this idea too, but what I saw as the ideal solution just did not work as intended. And if it would have worked as intended, then only upon connecting the XLR. The idea was to have two P-MOSFETs in series with the emitters of the Schoeps output BJTs, which would slowly turn on after a few hundred ms when phantom power was applied. The resistors in this circuit would be in the MOhm range, so the DC voltage on the signal lines would initially drop just a few mV by the load of the Mohm resistors, instead of jumping from 48V to, say, 35V or less. Once the MOSFETs start to slowly turn on, the mic will continue to power up. Guess what: it did not help. The plops and clicks were just as loud as when I hot-swap other mics. Maybe charging the 1nF RFI suppression cap on the mic output was already sufficient to cause the plops and clicks, I don't know. Anyway, this idea was short-lived. But it does make me curious as to how you are trying to solve this issue. If you don't want to tell and keep this as your little engineering secret, no problem.

Schematic of my flawed circuit depicted below, so you get the idea. The 4.7 MOhm resistors connect to Vdd, so when the MOSFETs turn on and Vdd starts to rise, it will slow down turning on the MOSFETs completely. Basically, this worked, but the clicks and plops remained... :-(



Jan

 

[Khron]

I was spurred on to look into that by reports of power-up/down noises from @kingkorg that he discovered during his opamp circuit experiments. At least in LTspice, i managed to pull it off with a pair of N-channel MOSFETs (on the driven output of the opamp-Alice circuit). See the datasheet for DW01 battery protection chips for at least part of the idea

 

[micolas]

Yeah, reasonably sure the Nova would need its own board design(s). I'm still at work for another 6h or so, but when i get home i'll double-check / confirm whether those MXL 440 dimensions check out with the ones i suspect or not

In M-audio Nova the fixing rails can be made straight from the Head to close to the screws of the transformer shielding cylinder and a greater transverse distance is obtained. Or the cylinder can be completely dispensed with. That's what I did. Then yours PCBs will probably be compatible.

 

[Voyager10]

Yeah, reasonably sure the Nova would need its own board design(s). I'm still at work for another 6h or so, but when i get home i'll double-check / confirm whether those MXL 440 dimensions check out with the ones i suspect or not

No worries about the Nova, it's a bit of a long shot anyway. If you're supporting the MXL 2001, that's also pretty good mechanically IMO.

 

[MicMaven]

In a couple of these cases, going full-SMD was necessary in order to manage to fit all the needed components that allow all the tone-shaping possibilities i wanted. The SMD ones i'm seriously considering offering fully-populated (for the most part, anyway - unless otherwise requested).

Good Day Mr. @Khron --

Gratuitous Serving Suggestion: Perhaps you could make/provide partially completed boards with surface mount components, but not through-hole components already installed. This would make it easier for others to build, and allow greater some latitude for experimentation by substitution of through-hole components? It might even lower the cost by producing partially populated boards. Um ... er ... I think. (A number of kit suppliers do this in other fields, so there is substantial precedent for this approach.) James /K8JHR

 

[Khron]

Good Day Mr. @Khron --

Gratuitous Serving Suggestion: Perhaps you could make/provide partially completed boards with surface mount components, but not through-hole components already installed. This would make it easier for others to build, and allow greater some latitude for experimentation by substitution of through-hole components? It might even lower the cost by producing partially populated boards. Um ... er ... I think. (A number of kit suppliers do this in other fields, so there is substantial precedent for this approach.) James /K8JHR

In a couple of these cases, going full-SMD was necessary in order to manage to fit all the needed components that allow all the tone-shaping possibilities i wanted. The SMD ones i'm seriously considering offering fully-populated (for the most part, anyway - unless otherwise requested).

My apologies for the lack of clarity - i did indeed mean (or imply, at least) precisely what you advised, the SMD ones (possibly) coming pre-populated, but the through-hole ones coming only as bare boards.

I don't think i have any reason to bother kitting up sets of through-hole parts, since they're expressly NOT anything special or "esoteric" Less work for me, and more flexibility for the users Especially in the cases where there might be some tone-shaping part values to select "by taste"...

 

[Khron]

No worries about the Nova, it's a bit of a long shot anyway. If you're supporting the MXL 2001, that's also pretty good mechanically IMO.

Well, as it turns out, i was quite pessimistic - at least the opamp-Alice-cardioid-only actually fits the mounting holes both for the Nova and the MXL 440 pretty comfortably! And only had to nudge a couple resistors on the opamp-alice-multipattern, to get them to fit there too

 

[Voyager10]

Great work! I'd certainly be interested in all of the boards as blank PCBs.

Personally my ideal SMD board would leave the op-amp site unpopulated (as a test fixture for comparing op-amp performance) but I'd also be happy to desolder the chip otherwise.

 

[Khron]

Great work! I'd certainly be interested in all of the boards as blank PCBs.

Personally my ideal SMD board would leave the op-amp site unpopulated (as a test fixture for comparing op-amp performance) but I'd also be happy to desolder the chip otherwise.

I'm looking forward myself to bringing these to a practical conclusion; just too bad I can only shoehorn in some design time here and there, between the 9-to-5, my amazing fiance, and some other occasional gear repairs, plus the mandatory R&R... Never enough hours in a day, am i right?

And if anything, i would myself order the SMD boards populated with the passives and install the opamps myself, just to avoid the risk of fake-marked opamps getting sourced from wherever So that may well be an (or THE) option, anyway...

 

[mrgrooves666]

I hope your very interesting project PCB's fit the HL-77 ali body. Its really my favorite body right now, its cheap, yes it has the ring issue but can be dealt with, and the basket is pretty transparent, quality is good for the buck, and material seems to be brass, not el cheapo nickel alloy stuff. Particularly those opa alice or 84, with multipattern capabilities and the filtering options sounds like a very interesting platform for me personally, for a couple of Campbell transmitters, if I can get take a couple of them from Tim's busy hands, or if Guosheng's put an end to the rumors with his

 

[Khron]

I hope your very interesting project PCB's fit the HL-77 ali body.

Those two dimensions highlighted in light blue i'd still need, as well as the internal diameter of the (top of the) body tube.

If you could provide the measurements indicated in my previous reply i quoted here (and the spacing to the second set of mounting screws, if applicable - expand to see the complete quote!), I'll see what i can do

 

[Khron]

Just about done with the multipattern version as well... Sorta-kinda half-ass "to scale", but oughta be enough to give SOME idea..?

 

[MicMaven]

if anything, i would myself order the SMD boards populated with the passives and install the opamps myself, ...

So... um ... ahem ... is it possible to install IC Sockets on the boards? Assuming the op amps are not, themselves, smt components, that would allow quick and safe substitution by operators in the field. Headphone amp enthusiasts do this so they can "roll their own" options and tailor results to taste.

This, of course, presumes op amps under consideration have predictable, standardized dimensions, number of pins, electrical properties, etc.?

Or, have I wandered way off the trail this time? / James /

 

[Khron]

is it possible to install IC Sockets on the boards? Assuming the op amps are not, themselves, smt components,

Well, unfortunately, the intended ones (JFET input, low-enough current consumption, low noise etc) are indeed SMD-only. That, plus a socket would just about nullify the efforts put into the high-impedance area (socket leakage etc), and/or complicate assembly even further. The footprint is indeed standard, though.

 

[Khron]

FET847 (cardioid-only) anyone?



(A couple resistor values pending testing though, but anyway...)

 

[Voyager10]

Will there be space to fit a SOT23 adapter board in the FET location, as a lot of FETs which are readily-available and good are SMD these days? Or even a dual footprint TO92 and SOT23 on the PCB?

(Also, TO92 FET pinouts aren't 100% consistent e.g. J305 vs 2SK170 - if the pads were in a triangular layout rather than in-line it would be easier to fit different devices).

I'm really looking forward to all of these, BTW!

 

[Khron]

I suppose, if you really wanted to..? Although even if you have a surface-mount JFET on an adapter, you'll still want to not have the gate soldered onto the adapter board, so... Might even end up easier to just solder a few resistor legs onto the board pads and solder the SMD JFET straight onto the ends of those? Just a thought...

Through-hole ones you can maneuver and twist around to get the legs in the right places, though. For what it's worth, the footprint is of a J113.

 

[Khron]

U87A version's more or less done too...