Topic: CA-UGLY bit the dust, need another preamp solution for DPA 4061s

[joeldotc]

I have just bought these boost converters and will be thinking if I can implement them somehow.

https://www.amazon.com/Comidox-Module-Voltage-Converter-0-9-5V/dp/B07L76KLRY/

Can these pass the AC signal back through to the recorder? I’m not sure exactly how it works, but that’s where I get confused in the circuit

Could a capacitor be added in parallel with the converter to pass through the signal while blocking any DC?

I could see how you could use these in a battery box to get a 1.2 V AA battery up to 5 V of bias, but I think using the PiP from a recorder leads to some issues? I’d love to hear if someone more knowledgeable could educate me!

[adrianb]

I have just bought these boost converters and will be thinking if I can implement them somehow.

https://www.amazon.com/Comidox-Module-Voltage-Converter-0-9-5V/dp/B07L76KLRY/

Can these pass the AC signal back through to the recorder? I’m not sure exactly how it works, but that’s where I get confused in the circuit

Could a capacitor be added in parallel with the converter to pass through the signal while blocking any DC?

I could see how you could use these in a battery box to get a 1.2 V AA battery up to 5 V of bias, but I think using the PiP from a recorder leads to some issues? I’d love to hear if someone more knowledgeable could educate me!

I received the boost converters today and can confirm they work perfectly. I have tested them with input voltages between 1.5v and 5.0v and the output voltage remains stable at 5.0v. They are also tiny, 10mm x 10mm.

How to make them work is another question entirely, and for the moment beyond my knowledge, but I’m determined to make it work. At some point I will open up my Zoom H1 XLR and have a look to see how much space is in there.

The Zoom has a USB-C input to power the device, but doesn’t supply external power. Assuming the USB PCB board is a normal type I might be able to add 5v external supply which I could then utilise in a battery box.

Also thinking about drilling an additional 3.5mm input with 5v PIP.

High possibility that I will end up bricking my new recorder 

Any suggestions?

[Gutbucket]

I received the boost converters today and can confirm they work perfectly. I have tested them with input voltages between 1.5v and 5.0v and the output voltage remains stable at 5.0v. [..snip..]

Any suggestions?

Try powering them using PIP from the recorder, and from a few different recorders if you have them available, confirming good output voltage.  I'd then modify a standard battery box circuit, replacing the battery with one of the converters.  Should be able to use a single converter in place of the battery.

1) +V in to the converter taps into Signal+ on the recorder side of the series DC blocking capacitor in the battery box circuit.  Do so through a series resistor, same value as the others in the circuit. 
2) +V out from converter gets connected the same as the battery it is replacing (through the resistor on the microphone side of the blocking capacitor).
^ Duplicated for both channels, you'll end up with 4 resistors total, in pairs arranged symmetrically on either side of the capacitors.
3) Try it.
4) Maybe add capacitance in parallel to the V+ out from the converter to achieve a cleaner supply to the mics if needed. Not sure if such supply filtering will be needed or not or what the appropriate value might be, but won't hurt to include it. Intent is to filter out any high frequency switching noise from the converter.  The converter's stated operating frequency is 150kHz. 

I'm not a circuit designer! Pretty sure the above should work, but fully open to correction from anyone who knows better!

If this works, it will eliminate the battery box battery and should work with any recorder that provides PIP.  PIP current draw from the recorder will increase, so check battery run time while recording.  Current draw for powering a pair of electret mics is pretty minimal, so hopefully the additional current draw due to the inefficiency mentioned below is a non issue.  The converter along with the two additional resistors and possible filter capacitors should be smaller than a battery.. guessing maybe half the size of a 9V.  Small enough that I'd look into building it into a small node placed at the Y junction of the mics>recorder cable.

No modification of the recorder is necessary as long as the recorder provides PIP on the desired input.  But of course you can also could build it into the recorder if you like.

I took a look at the link posted for the converters. Here is the most helpful review-

"They are super-simple to use (just 3-wires), and they worked well out of the box. The 5V is stable and clean (add a few caps, downstream, of course).

They are not at all efficient. Best case, when the input voltage is 4.5V+ (fully charged battery), they yielded about 84% efficiency. However (and this dramatic a drop was surprising to me), the efficiency drops as the input voltage drops. With 2.8V going in (like when a battery is nearing exhaustion), the efficiency is 44%. Around 2.8V they get a little wonky and won't up-convert voltage much below that.

So this means that as your battery drains, the efficiency drops, requiring more and more power from the battery to satisfy a constant load, for example. It's an accelerating cycle that drains the battery really quickly.

That's too bad, since these are tiny, easy to deploy units with a relatively clean output. They're kind-of battery killers, though. Modern DC/DC converters should be able to maintain 90%+ efficiency over their full range of input voltages.

For non-battery powered applications where power does not matter, or your load is very, very light, and all you need to do is convert a voltage between about 3V-5V to a stable 5.0V output, they'd probably be OK."

[adrianb]

Thanks for all that info Gutbucket, very useful but also a lot to digest.

I have already tested the booster with the PIP 2.5v from my Zoom and can confirm that it produces a clean 5.0v output.

I think what I will do first is utilise the Soundman A3 battery box, which utilises a 6v battery, and replace the battery with one of these boosters to make a battery free battery box. It should have all the blocking capacitors required already.

All I am trying to do is simplify to process by removing another battery from the setup. It seems that it might have a detrimental effect on the life of the recorder batteries, but with the small recorders I use I normally get 15 hours of use so should be okay.

The A3 is easy to disassemble and to access the wiring and small PCB.   

[joeldotc]

1) +V in to the converter taps into Signal+ on the recorder side of the series DC blocking capacitor in the battery box circuit.  Do so through a series resistor, same value as the others in the circuit. 
2) +V out from converter gets connected the same as the battery it is replacing (through the resistor on the microphone side of the blocking capacitor).
^ Duplicated for both channels, you'll end up with 4 resistors total, in pairs arranged symmetrically on either side of the capacitors.
3) Try it.
4) Maybe add capacitance in parallel to the V+ out from the converter to achieve a cleaner supply to the mics if needed. Not sure if such supply filtering will be needed or not or what the appropriate value might be, but won't hurt to include it. Intent is to filter out any high frequency switching noise from the converter.  The converter's stated operating frequency is 150kHz. 

So I guess the circuit (for each channel) would look like this? With those two caps between Vi and Vo to ground being potentially optional depending on if the converter can be stable without them or not.

I think what I will do first is utilise the Soundman A3 battery box, which utilises a 6v battery, and replace the battery with one of these boosters to make a battery free battery box. It should have all the blocking capacitors required already.

I think you could use one converter for each channel to increase your stereo separation. One would probably work though but I’d wanna test it to see if it sounds fine. If you opt to just use one, i believe it would be the same schematic I drew but with a duplicate circuit of resistors/dc blocking cap into the second channel (edit: added a second pic)

[Gutbucket]

The non-circuit designer in me hesitates to reply and defers to someone more circuit savvy..

With those two caps between Vi and Vo to ground being potentially optional depending on if the converter can be stable without them or not.

Not sure what additional components may be needed to achieve converter stability. 

As far as power supply filtering to the microphone goes, I believe those additional capacitors in the circuit diagrams above will create a low-pass filter that not only affects the converter output as desired, but also the audio signal which is not desirable.  So if needed, and if correct, the fc corner frequency of that filter will need to be tuned high enough to filter supply ripple while not adversely affecting audible frequency response. 

I'm not sure if filtering of the converter's output might instead entail placing a capacitor in parallel with the resistor between Vo and Mic+ on the mic side of the converter (lots of not sure hand-waving here).  I don't think one is need on the Recorder+ side, unless the PIP supply voltage from the recorder powering the converter also needs filtering for some reason.

I think you could use one converter for each channel to increase your stereo separation. One would probably work though but I’d wanna test it to see if it sounds fine. If you opt to just use one, i believe it would be the same schematic I drew but with a duplicate circuit of resistors/dc blocking cap into the second channel (edit: added a second pic)

The purpose of the resistors on the recorder side is to maintain separation between channels across the shared power supply connection to a single converter or battery.  If using two converters, one per mic channel, it may be possible to eliminate the resistor(s) on the recorder side, since each converter will be supplied with PIP via its own separate channel.  If using a single converter you'd want to retain the resistors, same as in a (single) battery powered supply.  But maybe there is a good case to be made for keeping them in there either way.

Again, I very much defer to someone more circuit savvy.

[joeldotc]

I'm happy to publicly display my lack of knowledge I like being proven wrong, and I hope it's clear anything I ever say should be taken with a grain of salt since I'm not even close to an expert! Being wrong in public is a great way to get someone smarter to chime in

Not sure what additional components may be needed to achieve converter stability. 

As far as power supply filtering to the microphone goes, I believe those additional capacitors in the circuit diagrams above will create a low-pass filter that not only affects the converter output as desired, but also the audio signal which is not desirable.  So if needed, and if correct, the fc corner frequency of that filter will need to be tuned high enough to filter supply ripple while not adversely affecting audible frequency response. 

I looked more into the converter itself and it looks like it already has filtering caps at the output (and they're between the output and ground btw). Also found an additional teardown of the converter which confirms this and recommends an additional filtering cap at the output, but that may not be required in our case. Probably best to do some circuit calculations to ensure all the additional components have no effect on the response.

boost converter teardown (note: this is the 3.3 V converter, so the values might not match the 5 V version):
https://dzrmo.wordpress.com/2017/04/26/ce012-step-up-boost-whats-inside/

converter spec sheet:
https://archive.espec.ws/files/ME2108%20Series.pdf

I think you're right that the capacitor on the input to the converter is not required since I'm sure the recorder is already filtering the PiP voltage output.

If using two converters, one per mic channel, it may be possible to eliminate the resistor(s) on the recorder side, since each converter will be supplied with PIP via its own separate channel

I think you're right here as well - probably only required for stereo separation when using one converter. But, the resistors might be required if the PiP supply is too hot for the input to the converter (?) - based of Adrian's tests it seems like that's not the case though.

[Gutbucket]

I'm happy to publicly display my lack of knowledge I like being proven wrong, and I hope it's clear anything I ever say should be taken with a grain of salt since I'm not even close to an expert! Being wrong in public is a great way to get someone smarter to chime in

Totally with you!

[snip..] But, the resistors might be required if the PiP supply is too hot for the input to the converter (?) - based of Adrian's tests it seems like that's not the case though.

Those resistors are arranged in series with the load, so they'll limit available current, but won't decrease voltage.  If they were in parallel with the load they would decrease voltage and not limit current.