TRS Phantom Power box to power onboard buffer?

[bluebunny]

By the way, those graphics are great. Did you make all those images yourself and move them into place?

https://bancika.github.io/diy-layout-creator/

DIYLC is so cool it has its own lengthy thread right here at DIYSB.

[Big Fat Walrus]

Hello again all-

So I have decided to make the power box 12v (or even 18v) instead of 9v. I would like to make sure I can deliver the buffer at least 9+ volts even after the drop from filtering (but less than 24v). What would be the optimal component values you guys recommend? I tried to calculate this myself, but would like some validation (or correction) on my math - which is probably wrong.

And if I were to add the 47n C2 (ceramic) to the schematic, I would just lay it across the + and - lines just like antonis' diagram (but after the existing 270uf), correct?

And lastly, I really would like to have a pcb made for this that hangs off of the switch. While I do have access to Eagle, I have no idea where to begin designing something like this for production. Is there a service anyone can recommend (or anyone here who does this sort of thing for freelance?) where I provide a layout or schematic and the service will turn that into a Gerber file, etc., for me?

Thanks again for all of your input!

[antonis]

if I were to add the 47n C2 (ceramic) to the schematic, I would just lay it across the + and - lines just like antonis' diagram (but after the existing 270uf), correct?

There is no "before" or "after" for 2 elements placed in parallel..
(but place 47nF "after" 270μF if it makes you feel comfortable..)

P.S.
In case of difficulty getting 270μF cap, proceed to 220μF one..

[R.G.]

You might want to look at
http://www.geofex.com/FX_images/Onboard_Preamp.pdf
which was designed specifically for some of the issues brought up here. It works on two coin cells, and was designed for low current use, smaller than nearly all opamps. There are coin cell sockets that will let you push cells through an exposed slot in a pickguard.
I did the prototype with through hole components, but an SMD version could probably be mounted to one side of the coin cell holder under the pickguard. No routing of the body.

[Big Fat Walrus]

There is no "before" or "after" for 2 elements placed in parallel..
(but place 47nF "after" 270μF if it makes you feel comfortable..)

Thanks for the reminder, antonis. I guess I'll live dangerously then; if I do include it I shall place it before!

So am I wrong to think that for a 12v (or even 18v) box, some of these values should be changed? Say, for R= 100, C1= 400u, C2= 65n (again, my math is probably wrong)? The buffer current is approximately 3mA.

You might want to look at
http://www.geofex.com/FX_images/Onboard_Preamp.pdf ...

Thanks, RG. Actually that was my original plan. But the more measurements I took, the worse things looked. This bass has very little room left inside a very shallow rear cavity route (and no routing under the pickguard). Plus I simply cannot bring a schematic to PCB- way above my skill set. So I went with a jack-mounted option. But I am glad you posted that article here. Should anyone who has more space in their cavity and more skills come across this thread with similar questions, they should certainly go down that route.

Q: Is there such a PCB design service where you can send out a layout or schematic and receive a printable file in return? Something ready to send to, say, OSH Park? As much as I would like to learn PCB design, it is thousands of hobby hours down the road from now

Thanks!

[FiveseveN]

Is there such a PCB design service where you can send out a layout or schematic and receive a printable file in return?

Peeps on this forum offer it sometimes, maybe for free if it's simple enough. And sure there are professional establishments that could also help you out with fabrication and assembly but it will cost a lot.
Anyway, for 3 components you really don't need a PCB, you can solder them directly to the jacks. Or if you're thinking about the buffer try asking the guys in this thread: https://www.diystompboxes.com/smfforum/index.php?topic=110259.0

am I wrong to think that for a 12v (or even 18v) box, some of these values should be changed?

Yes, that's wrong. Same values, just make sure the caps have the proper voltage rating (e.g. 25 V for 18 V supply).

[Big Fat Walrus]

Yes, that's wrong. Same values, just make sure the caps have the proper voltage rating (e.g. 25 V for 18 V supply).

Hmm. Well, I thought my math was wrong so no surprise there. I guess that makes things easier.

Anyway, for 3 components you really don't need a PCB, you can solder them directly to the jacks...

Yeah, I know. I just wanted the sturdiness of something hanging off the foot switch. I have looked, but can't seem to find any proto board with hole spacing that will fit a DP or 3PDT. Maybe I'll just keeping digging for something on that end.

Thanks!

[antonis]

So am I wrong to think that for a 12v (or even 18v) box, some of these values should be changed? Say, for R= 100, C1= 400u, C2= 65n (again, my math is probably wrong)? The buffer current is approximately 3mA.

As 57 already answered, LPF items values only depend on cut-off frequency of interest, which is 0.159/(R x C)..
but..
Cap(s) voltage rating depends on working voltage (the one across its legs) and Resistor value concerns affortable voltage drop across it (caused by supply current.. - 3mA in your case..)
Also, R power rating depends on its actuall value and supply current (I² x R) but this shouldn't bother us here 'cause it's slightly lower than 1mW

You can realize that R value should be a compromise between voltage drop from power supply to buffer and respective cap value in the mean of undersizing R value (for less voltage drop) results into oversizing C value (to maintain constant R x C product..)
But caps value (not voltage rating) is both cost and space related where resistor value (not power rating) is irrelevant both to cost and space..

[Big Fat Walrus]

Hey antonis!

Sorry, I just saw your reply now. Thank you, that is very informative. And also thanks to 5 7 for your explanation. Maybe this is also a good time to learn Eagle with a seemingly simple first project...

I recently stumbled across this link (http://www.muzique.com/news/dc-power-filter-box/) to an interesting AMZ widget. Am I crazy for thinking that this would be a nice addition to an external power box like we are discussing? Or am I just overthinking this?

It seems like there is no down side to making sure A) the power ripple is minimized and B) the power has been 'cleaned up' the best it can be - especially if I start with a voltage that allows sufficient drop (12v would allow 3v loss, etc). Is this correct?

Basically, this house is pretty electrically noisy and I would like to keep as much 'junk' as possible from making its way into the audio path. And I suppose overkill is acceptable in a situation like this?

[antonis]

I've just realize that you didn't say the way for 3V drop..!!
(which drop, by the way,  is fine for a voltage regulator ...)

[Big Fat Walrus]

OK, maybe poor explanation on my part

What I mean to say is: As long as I can get above 9v out of this box into the TRS jack, everything is good. So even if my over the top desire to filter and clean the power supply (wall wart) cost a few V along the way, that is OK...

[antonis]

If so, then overkill is more than welcome..

For 3mA current draw, a resistor of 470R and a capacitor of 100μF (3.4Hz cut-off frequecy) shoud be fine..

Of course, if you consider yourself serious overkiller, implement Jack's LC T-bridge filter with a resistor (100R - 470R) in series with input inductor..
Cap will serve both for ripple reduction (with R) and for RF & clock noise (with Ls)..

P.S.
Under your own risk, of course..!!

[Big Fat Walrus]

Nice.

I assume the 'risk' comment is sarcastic though. Like I said earlier, my knowledge of these things is 'just enough to be dangerous.' So I would like to avoid any risks 

But thank you for all of your advice - I believe I have almost bothered you enough here. I will now attempt to draw up a schematic (first time, so will need someone to double check it) so that I can attempt to make this into a PCB in Eagle. If it all works I'll post the Gerber here for all to use as this forum has been so helpful.

Thanks!

[Big Fat Walrus]

So I am beginning to use a schematic layout program. Baby steps, but I guess you've got to start somewhere.

Before I get too far along here, does this look like what you are suggesting? (obviously the switch and the LED/CLR, jacks, etc are omitted)

[antonis]

For 3mA load current, you'll have 1.4V drop across 470Ω resistor so about 7.6V availiable for your load..
If it isn't OK, reduce R1 value down to 100Ω  - RC  LPF new corner frequency at 3.4Hz should also be fine..)

P.S.1
I should ommit inductor or place it after 470μF cap or increace significantly its value..!!
(as it is it raises cut-off frequency at about 404Hz..)

http://sim.okawa-denshi.jp/en/RLCtool.php

P.S.2
Value of 330 μH taken from Jack's filter: http://www.muzique.com/news/dc-power-filter-box/ refers on LC T-bridge LPF..
(it has different cut-off calculation formula..)

[antonis]

Could you plz post a schematic of circuit of interest and power supply intended use..??

It should be more easy for us to suggest you the power supply filter..

[Big Fat Walrus]

Hey Antonis,

I don't have a diagram of the buffer circuit I'll be using inside the guitar, as I purchased it from a company. But it can run on 9 - 24v and draws 3mA.

I was trying to draw a schematic for the external box to power it over TRS cable based on your suggestions earlier in the thread, and Jack's filter box. Obviously, I have never designed a circuit before (or even drawn a schematic), but I thought this would be a good place to start. And then hopefully I could create a PCB for it as well. An easier project to start on vs., say, a delay pedal.

I didn't know this (https://www.emgpickups.com/parts/es-918.html) even existed a few weeks ago - I got the idea from an older Alembic someone I know has. But I would rather build one myself, and that EMG seems like a lot of money for a few parts that I already have mostly in my cabinet. I just want to take the same principle, but filter the power ripple and hum, and add an LED and an On / Off switch. As long as the box outputs above 9v through the ring terminal...

-Oh- I mislabeled that power rail in the picture. I meant 12v. I thought I could just use a standard wall wart 2.1mm adapter to power the box.


Thanks for your patience!

[antonis]

-Oh- I mislabeled that power rail in the picture. I meant 12v. I thought I could just use a standard wall wart 2.1mm adapter to power the box.

If so, retaing R1 value, get rid of inductor and place another cap (1nf or so) in parallel with existing ones...
(you'll be proud for your super-fancy-quiet filter..)

[Big Fat Walrus]

OK, so this will take care of all the filtering? I'll just need to use the switch and led to the schematic?

[antonis]

OK, so this will take care of all the filtering?

Have you the contract to be signed prepared..??

I'll just need to use the switch and led to the schematic?

Yeappp..