DIY cheap 9v preamp for dual source MIC

[rschultz]

I bought a Cioks Sol (5 isolated outputs) thinking that it would eliminate the ground loop buzz that I experience with my 9v mic preamp when plugged into a power supply. And it did... but the noise floor is significantly higher than the battery version. Like 2-3 times higher, white noise. The noise floor on the battery version is essentially the same as anything else I use: helix, FRFR speaker, pedals, etc. But when I plug the preamp in, it's a noticeable hiss. I've tried this with the helix, and direct into the FRFR. I don't even have to plug the mic in... oh, that's interesting, it's not the mic then. So it's just the way the 9v circuit is wired. I think I need a more sophisticated powered circuit.

Where can I learn about this stuff?

Input on the left, output and 9v jack on the right:

[Doug Young]

You might benefit from a shielded box.

[HogsNRoses]

Great stuff! Some suggestions:

1) For the hiss, add a low ESR capacitor across the positive and negative power input. Maybe 47uF or higher, and use a good capacitor like Panasonic FC.
2) you can also put a film capacitor in parallel with the one in suggestion 1. Or a ceramic, preferably a C0G. Cheap or SMT ceramic capacitors can ring, even with an isolated power supply.
3) your mic sounds like it doesn’t need a lot of juice, but the phantom power spec would use a smaller resistor and allow more current: 6.81 kΩ for 48 V, 1.2 kΩ for 24 V, and 680 Ω for 12 V (Wikipedia). Furthermore, with your higher resistor value, the mic might see quite a bit lower voltage because the resistor makes a voltage divider.
4) to save space in the case, you could use panel jacks like these: https://a.co/d/5fR4yER. I use Neutrik ones soldered in a PCB to keep the pins from slipping, but you can prototype and risk damaging a few for now.
5) the electrolytic capacitor you are using now to block DC should be high quality - either FC like above or it can be a film. Even the green ones are OK, and anything 0.1uF or bigger.

[rschultz]

Originally Posted by HogsNRoses

Great stuff! Some suggestions:

1) For the hiss, add a low ESR capacitor across the positive and negative power input. Maybe 47uF or higher, and use a good capacitor like Panasonic FC.
2) you can also put a film capacitor in parallel with the one in suggestion 1. Or a ceramic, preferably a C0G. Cheap or SMT ceramic capacitors can ring, even with an isolated power supply.
3) your mic sounds like it doesn’t need a lot of juice, but the phantom power spec would use a smaller resistor and allow more current: 6.81 kΩ for 48 V, 1.2 kΩ for 24 V, and 680 Ω for 12 V (Wikipedia). Furthermore, with your higher resistor value, the mic might see quite a bit lower voltage because the resistor makes a voltage divider.
4) to save space in the case, you could use panel jacks like these: https://a.co/d/5fR4yER. I use Neutrik ones soldered in a PCB to keep the pins from slipping, but you can prototype and risk damaging a few for now.
5) the electrolytic capacitor you are using now to block DC should be high quality - either FC like above or it can be a film. Even the green ones are OK, and anything 0.1uF or bigger.

Wow, a treasure trove of suggestions. A one comment I'll make and then I'll start working on them.

The wikipedia page link. Plotting those values in Excel, it's an exponential function R = 0.2854 e^0.0652v. So for 9v, that would be a 513 ohm resistor. Although my Cioks has higher voltages so I could go that route too.

[rschultz]

Originally Posted by Doug Young

You might benefit from a shielded box.

I put the circuit in the metal box from the original post. No change. But thanks for the suggestion.

[rschultz]

Originally Posted by HogsNRoses

1) For the hiss, add a low ESR capacitor across the positive and negative power input. Maybe 47uF or higher, and use a good capacitor like Panasonic FC.

I had a 100 mfd (16v) in my box, no change with negative lead of cap on ground. I tried hooking it up backwards with the positive lead on ground... and about 15 seconds later, I let the magic smoke out of the cap

So besides wiring it up the correct way, what do you suggest?

[rschultz]

On the battery version, I swapped the 2k2 for a 470 ohm thinking it would increase current and therefore signal. But it really didn’t change any.

[rschultz]

Originally Posted by HogsNRoses

Great stuff! Some suggestions:

1) For the hiss, add a low ESR capacitor across the positive and negative power input. Maybe 47uF or higher, and use a good capacitor like Panasonic FC.

OK, did a little research. Here's a circuit from Shure. Similar to HogsNRoses suggestion of using a 47 mfd... except it has 3 of them in series/parallel with resistors. It then uses a 750 ohm resistor based on a 12vdc supply, which is close to the 680 ohm standard for 12v.

So I'll buy some caps and try to put this together. Or maybe



Shure ALSO had a schematic for a battery powered one… very similar to what I have been doing, but didn't have the 10 mfd cap on the output.

[rschultz]

The Shure circuit worked! See circuit diagram in previous post, or that link. I had a scrap piece of pcb and some 47 mfd caps, so I soldered it up... quick, dirty, and rather ugly.

The noise floor is now the same as with the battery. I plugged the guitar in (with internal mic) and it sounded like a mic'd guitar!

It will take some time for me to get a better breadboard and enclosure design, etc. But it works!

[rschultz]

I've been talking to some of the guys over on allaboutcircuits.com. They helped me improve the Shure circuit, like this:

Noticeably quieter. Imperceptibly quiet. The first two caps eliminate AC ripple current, while the .33mfd is a HPF I think to further eliminate any 50-60hz buzz. I tested it using an old OneSpot, but my Cioks should be even better.

I'm going to work on sticking this inside a box.

[rschultz]

I cut a small piece of prototyping pcb and soldered up this circuit, it's about 1" x 3/4". Try as I might, I couldn't get it in a 1590L size box (2"x2"). But I printed a 2x2.5" and that worked. I changed the (yellow) cap to a 10 nf, which gives a cutoff frequency of 16 hz and a high pass filter of 80 hz... assuming 1M input impedance on my Helix (which I set it as).

Pedalboard below that; with the Becos is in the FX loop. The Cioks is held in place by a 3D printed bracket; white filament, with top spray painted black.

[rschultz]

I’ve been busy. I designed a circuit to T-power, preamp and HPF an internal electret condenser mic. Then I created a PCB and designed an enclosure layout. It fits in a Hammond 1590A which is 1.5” wide x 3.5” long. Gain on left (1 - 5.5x) and HPF on the right with a range from 70 - 680 Hz. Although both of those values can be tweaked.









Last picture is mic and circuit going straight into a PA, no other processing. I set the gain to about 30%, and I’m guessing the HPF is at around 450 Hz. Sounded great.

The HPF is a 2nd order Sallen-Key filter I learned about over on allaboutcircuits.com.

[Cuki79]

Nice job, please share the schematic!

[rschultz]

Originally Posted by Cuki79

Nice job, please share the schematic!

First one is the Eagle simulation schematic:



This one is Eagle schematic for layout with the dual op amp:



Last one is Eagle layout:

[rschultz]

If anyone wants a PCB, I’ll send you one. If you want all the part and components to build your own, I’ll sell them to you at cost. Probably in the $25 range.