DC-DC Converter for Multi-Pattern Phantom powered mic.

[Khron]

The first mic I want to bring this into is my km84-47 LDC mic. Basically a km84 circuit with a k47...

So...

https://groupdiy.com/threads/fet847...-jonathan-burtner-pcb-layout-by-poctop.61057/

So far making progress...

Still not sure if I need to add Q6 and passives but I wanted to start layout and can expand board if I need to do so.
View attachment 108704

Why such an utterly humongous transistor? Most chinese circuits use a 2N5551, so if you wanna go SMD, MMBT5551 (or whatever the SOT23-packaged one is).

Keep in mind phantom power can provide ~14mA into a dead short-circuit...

 

[RuudNL]

And how do you know (without any form of adjustment) that you will get + and - 60 Volts?

 

[SparkleBear]

And how do you know (without any form of adjustment) that you will get + and - 60 Volts?

True, I cannot be sure. I'm learning as I go here.. I have to do more research about this but this is where i've gotten so far. How would you achieve 60v at the capsule vs the 48.. and I know that early on you explained that it is not that much of an improvement over the 48v. But if i am making +/- rails for multi patterns, I might as well boost the voltage too

Thanks for tuning in Ruud.

So...

https://groupdiy.com/threads/fet847...-jonathan-burtner-pcb-layout-by-poctop.61057/

Huh, thats cool someone else did this too... I made this in the format of mxl990 body size. Gotta use the smaller transformer size but it sounds good. This design also only supports Omni as a second pattern, and I would like to be able to use bi-directional pattern too. This is the main reason I want to make the DC-DC board.

Why such an utterly humongous transistor? Most chinese circuits use a 2N5551, so if you wanna go SMD, MMBT5551 (or whatever the SOT23-packaged one is).

Ahh, sometimes I lose the scale of things when working in Kicad. The sot23 is indeed a way more reasonable size! ;P`

Keep in mind phantom power can provide ~14mA into a dead short-circuit...

Worth putting protection diodes into the design?

 

[Khron]

What "protection diodes"?

 

[ccaudle]

sometimes I lose the scale of things when working in Kicad

You make it sound like you are picking the footprint first. The reasonable way is to determine what parameters are needed for the circuit to operate properly, find a device which meets the needs you have determined, then use the footprint appropriate for that device.

Worth putting protection diodes into the design?

No, the point that Khron was making was that phantom power cannot supply more than a few mA, the corners being 48V at 0mA and 0V at 14mA. Your schematic shows a DZT5551 transistor which has a 160V collector break down voltage, and a max continuous collector current of 600mA. That does not appear to be a device chosen based on a realistic understanding of requirements.

 

[SparkleBear]

You make it sound like you are picking the footprint first. The reasonable way is to determine what parameters are needed for the circuit to operate properly, find a device which meets the needs you have determined, then use the footprint appropriate for that device.

That is generally how I like to do it but i've only just started getting into these circuits so understanding the operating conditions is not under my belt quite yet. Thanks for the guidance.

No, the point that Khron was making was that phantom power cannot supply more than a few mA, the corners being 48V at 0mA and 0V at 14mA. Your schematic shows a DZT5551 transistor which has a 160V collector break down voltage, and a max continuous collector current of 600mA. That does not appear to be a device chosen based on a realistic understanding of requirements.

Yes, I have chosen a smaller more appropriate component as seen in the new layout.


Anyway, I worked on the board a bit more today. Added the 9.1v zener and filtering, used a smaller transistor... Wanted to use a bigger value cap but also wanted to keep the footprint small. So went with a 22uf tantalum.





I would certainly like to learn more about proper understanding of current draw and knowing how to choose components that are designed for the right range for the application. I have a loose understanding, learning more every day.

 

[kingkorg]

Careful with tantalums, they are not as reliable as electrolytics. They have caused lots of crackling and noise for me, i gave up on them.

Go through this related thread.

https://groupdiy.com/threads/open-s...lator-pcbs-for-ldcs-shared-on-osh-park.65174/
Before ordering the pcb you should check the layout and see if selected inductors work as they should. It's not uncommon for Hartley Oscillator to act up.

 

[Khron]

Your schematic shows a DZT5551 transistor which has a 160V collector break down voltage, and a max continuous collector current of 600mA. That does not appear to be a device chosen based on a realistic understanding of requirements.

Yes, I have chosen a smaller more appropriate component as seen in the new layout.

My point wasn't necessarily about the parameters (the SOT23-packaged 5551's have the same voltage & current ratings). The chunkier package only accounts for the allowable power dissipation differences (2W vs 300mW), and i merely tried to point out there's no significant power to be worried about (~680mW into a short-circuit, all of which would be dissipated by the pair of 6k8 resistors inside the preamp).

https://www.diodes.com/assets/Datasheets/ds31219.pdfhttps://www.diodes.com/assets/Datasheets/MMBT5551.pdf

 

[SparkleBear]

Careful with tantalums, they are not as reliable as electrolytics. They have caused lots of crackling and noise for me, i gave up on them.

Go through this related thread.

I’ll check this out… thanks KK.

Regarding tantalum: the original circuit was calling for 330 uF polarized capacitor. The only way to keep the boards small was to use tantalum. I suppose I could just make it a bit bigger and use electrolytic. I’ve never used tants in an analog audio circuit, only as bulk caps on voltage rails for digital devices. Thanks for the heads up on those.

https://groupdiy.com/threads/open-s...lator-pcbs-for-ldcs-shared-on-osh-park.65174/
Before ordering the pcb you should check the layout and see if selected inductors work as they should. It's not uncommon for Hartley Oscillator to act up.

The only way I check my circuit these days Is by getting a little prototype PCBs from JLC… Would you put it on a bread board or something? Otherwise, How would you check the layout? Spice or some other emulation?

I’m pleased how he layout looks so far, but looks are not what we are going for in the long run ;p

Also, curious what is the difference between rf inductors, chokes and power inductors of the same Henry value?

My point wasn't necessarily about the parameters (the SOT23-packaged 5551's have the same voltage & current ratings). The chunkier package only accounts for the allowable power dissipation differences (2W vs 300mW), and i merely tried to point out there's no significant power to be worried about (~680mW into a short-circuit, all of which would be dissipated by the pair of 6k8 resistors inside the preamp).

Thanks for clarifying khron, that is very great info for me to keep filed away for future designs.

Appreciate the feedback y’all.

 

[Khron]

Regarding tantalum: the original circuit was calling for 330 uF polarized capacitor.

Which "original circuit"?

Keep in mind, we're dealing with, at best, single-digit mA figures, so there's not much reason to go quite so overboard with the filtering there. Especially if you're planning on using zener regulation only for this separate circuit.

https://www.opic.jp137.com/index-ldc.html
And regarding the layout, the workings of this oscillator rely on the magnetic fields of the two inductors coupling with each other. That's why you'll often see through-hole inductors used in otherwise all-SMD circuits, stood off the board and gooped together with hot-glue - remembering my t.bone SC140's here.

 

[RuudNL]

The circuit uses 'mutually coupled inductors'.
If one of the inductors is wound in the 'wrong' direction, there will be no oscillation. Even with inductors from the same brand this can happen.
You may have to find out if a combination works for you by trial and error.

 

[ccaudle]

The circuit uses 'mutually coupled inductors'.

Can someone explain why side by side inductors work better in that case than end-to-end inductors? I would have thought that mounting inductors so that their ends are very close together, or even touching (which would not work well in automated assembly but should be no problem by hand) would be more similar to the original designs which have a tapped inductor on a single core.

 

[Khron]

Might also depend on the style of inductor. These monolithic SMD types are not actually wire wound as such.

https://www.core.com.tw/data/2019_PRODUCT/SMD_Chip_Bead_Inductor/CL-3.JPG
You'd need to carefully look through the manufacturer's documentation to see how a particular inductor is constructed.

Even with through-hole axial inductors, you can probably get the magnetic axes closer together if the bodies are parallel, than end-to-end.

 

[SparkleBear]

I am going to ditch the tantalum as per @kingkorg 's recommendation. At that point in the circuit (c2 below) what voltage rating should I spec my electrolytic capacitor? Also, if i need to use the higher voltage choice, how much can i lower capacitance before sacrificing filtering usefulness?

Right now I am using 50v capacitors. Does the zener pull it all down to 9.1v at that point so I could maybe use a lower voltage capacitor at that position? That would save a lot of space.

Which "original circuit"?

The zener section was pulled from the 'Pimped Alice Mic'. It seems to be the same circuit that i was drawing from in the first place. (HL Pimped Alice V.40 and DC V.56 PCBs)


So interesting about the inductors for this oscillator. I will certainly have to experiment with the vertical THT ones even though the rest is SMD. Such wonderful information here guys, thanks!

 

[Khron]

Does the zener pull it all down to 9.1v at that point so I could maybe use a lower voltage capacitor at that position? That would save a lot of space.

Yes.

 

[SparkleBear]

Seems that in the Pimped Alice Mic the inductors are laid out end to end like so:


where as @Drosselmeier pointed out these chinese layouts with them side by side:


A blurry image of the u87ai DC-DC board:


I could not find any examples of smd DC to DC circuits with the THT inductors.

Regardless, i made another revision of the board with the 330uf 10v Electrolytic capacitor AND the standup THT inductors. Hopefully this one will do the trick.

 

[Khron]

I could not find any examples of smd DC to DC circuits with the THT inductors.

The t.bone SC140 i mentioned earlier.

Are those ceramic capacitors, like... 0201-sized, or 01005? And D5 is utterly microscopic as well. Of course, that doesn't matter if you're planning on having these boards also assembled by the PCB fab, but...

I mean, if they're that tiny compared to that sot23-packaged transistor...

 

[SparkleBear]

The t.bone SC140 i mentioned earlier.

View attachment 108790


Are those ceramic capacitors, like... 0201-sized, or 01005? And D5 is utterly microscopic as well. Of course, that doesn't matter if you're planning on having these boards also assembled by the PCB fab, but...

I mean, if they're that tiny compared to that sot23-packaged transistor...

I think the smallest footprint I picked was 0603. I’m pretty good at SMD assembly by hand with hot air station. Definitely not planing for fab house to assemble. I’ll take a look tomorrow but I’m pretty sure it’s all 0603 or larger.

 

[Khron]

If R1 and R2 are 0603, C5 and C6 are 0805, as is R3, and all the other ceramics are 0201 at best...

Or "worst case" 0805 / 1206 respectively, and 0201 or 01005. D5's maybe SOD723 or whatever package is so minute.

 

[RuudNL]

A blurry image of the u87ai DC-DC board:
View attachment 108785

This is not a picture of the U87ai DC-DC board, but a picture of the (often hated) input circuit of version 04 or 05 of the U87ai. (The red wire is the input, connected to the capsule.) Later Neumann has gone back to a single 2N3819 FET. (From version 06)