isolated DC-DC converter

[pokus]

Hey there,
short question. Would a guitar pedal benefit from an isolated DC-DC converter brick in regards to noise, hum or anything other than the boosted/inverted voltage?
I just wonder, wouldn't any standard pedal in the chain make it an unisolated one by connecting the two grounds together (like the dashed line shows) trough its own power supply and signal GND? Let's say the two or multiple pedals power supplies are daisy chained and not separately isolated, which would make the isolated converter unnecessary anyway.

[iainpunk]

that would be a great option for pedals that draw big currents, to relieve some of the ground path noise, or to eradicate any ground loops.
the biggest thing here is: does the benefit outweigh the cost/hassle/time/effort or is a tiny bit of ground loop noise acceptable?
ground loop noise can be really bad, but generally, other noise sources are often louder.

cheers

[Rob Strand]

I just wonder, wouldn't any standard pedal in the chain make it an unisolated one by connecting the two grounds together (like the dashed line shows) trough its own power supply and signal GND? Let's say the two or multiple pedals power supplies are daisy chained and not separately isolated, which would make the isolated converter unnecessary anyway.

There's different forms of isolation and different motives to isolate.       The pedals have to connect to each other so there's no need for isolation *between* the pedals. 

For pedal power supplies isolation is about isolating the ground currents.  The idea is to prevent power supply currents in the power grounds from flowing down the signal ground wires.   If power and signal share the same any noise and pulses on the power rails will get super-imposed on the signal and you end-up hearing it as hum, whine, ticks, clicks even hiss when digital circuit and switch-mode power supplies.

In the bad old day of unregulated supplies the ripple from filter caps causes current pulses at twice mains frequency.  When you connected a number of pedals together back to the same supply those pulsing power supply currents ended up flowing down the coax wires between the pedals and that created hum.   (The hum from current pulses in grounds is different to, and added on to, the hum from old pedals which were sensitive to the ripple *voltage* on the supply rails - that's a different thing and that's why you use power supply filters and regulators.)

With regulated supplies you get far less problems with hum even when connect to the same supply.  The hum mainly gets in through ground loops.  It can still get in but it's far less aggressive than the old unregulated power supply current pulses.
[FYI, one pedal can create power supply noise and that can can go through the grounds to other pedals.]

The motivation for isolated supply is to isolated the *power* so ground loops cannot form and the power of one pedal doesn't affect the signal on another.     The outputs are usually regulated and clean.    The isolation can also help with noise from the mains but it depends how the noise gets in.  With those schemes in place there should be not noise sources which can cause issue.

One exception is switch-mode power supply noise.    The high frequencies and magnetic fields in the circuits can find their way into sensitive circuits.   Things which look unconnected to the casual eye become coupled by magnetic fields or connected together by stray capacitances.    The capacitance between the primary and secondary of transformer in a place where noise can couple across a transformer.   If you have  a 100kHz switchmode power supply the noise voltages produced by magnetic fields are 2000 times greater than the voltages from 50Hz hum loops.

[imJonWain]

Here's an old thread chatting about them a bit.

https://www.diystompboxes.com/smfforum/index.php?topic=124998.msg1188212#msg1188212

[anotherjim]

Way back in time I had my first-ever dual pedal setup and wanted to run both from the same PSU. I hacked another 2.1 plug onto the DC cable and got a hum problem. The cure was to cut the 0v wire to one of the DC plugs. So one pedals power was having to return via the signal cable to the other pedal.
Rather than cause a problem, power return via signal ground solved it!

[pokus]

Let's see if I get this right. The isolated DC-DC converter basically cuts the ground loop, that would have been there if the second pedals ground would make contact with the first pedals power supply ground through signal ground, and then a path back to pedal 1 power supply ground again trough the daisy chain?
I was just confused because the 12V will appear between the +- of the power supply of course, but also between COM and + now, if there is another pedal connected. So I haven't consider it "isolated".


In the linked thread and on a few datasheets there was mentioned to put a small cap between the two GNDs. For EMI suppression reasons. Is there any downside of doing this?


I used a dual output DC-DC converter (100kHz) with 1u MLCCs from each rail to COM on the output and from + to - on the input. It works just fine and I don't get any noise other than the typical breadboard noise, I was just curious about the functions of it.

[iainpunk]

Way back in time I had my first-ever dual pedal setup and wanted to run both from the same PSU. I hacked another 2.1 plug onto the DC cable and got a hum problem. The cure was to cut the 0v wire to one of the DC plugs. So one pedals power was having to return via the signal cable to the other pedal.
Rather than cause a problem, power return via signal ground solved it!

for that reason only, i have 2 patch cables where the ground and shield is only connecting to one side's plug only, to alleviate ground loops in sensitive pedals.

cheers

[Rob Strand]

Let's see if I get this right. The isolated DC-DC converter basically cuts the ground loop, that would have been there if the second pedals ground would make contact with the first pedals power supply ground through signal ground, and then a path back to pedal 1 power supply ground again trough the daisy chain?

Yep, that's the basic idea, and visa versa for pedal 2 and pedal 1.   When there's two ground paths there's a potential for undesirable side-effects.   (I'll admit in some rare cases it can improve things because two equally bad effects can cancel out.)

I was just confused because the 12V will appear between the +- of the power supply of course, but also between COM and + now, if there is another pedal connected. So I haven't consider it "isolated".

It's easy to get misleading results.   The input side has to sit at some voltage relative to the output side.  Since there is some weak coupling between the input and output so it's not surprising that the output side sits at a voltage close to the input side.  The high-impedance input of the multimeter means the meter doesn't load the voltages down.

There's no need but there a few checks you can do to convince yourself the output is isolated.     There's no need to do them you can just treat these as thought experiments.

As a sanity check you could measure the resistance between the input and output but it's almost certainly open circuit. Only do this with the circuit unpowered.  (On real some circuits you might not get an open circuit since there may be a resistance path from input to output in the whole system.)

*If* you had to convince yourself of isolation you can do an experiment:    Take a 9V battery and put say a 100k resistor in series with the + terminal then wire the battery - to the input ground and the hanging end of the 100k to the output ground you should be able to move the potential between the grounds.   Now reverse the battery see the potential between the input circuit and output circuit change polarity.   What that's saying is the output circuit isn't wired to the isolated side.   You can can also check there's no voltage across the 100k.  A voltage would mean there is a current path between the input and output (FWIW this test is a little more convincing than the ohm meter but noise affecting the multimeter can also cause false readings.)

In the linked thread and on a few datasheets there was mentioned to put a small cap between the two GNDs. For EMI suppression reasons. Is there any downside of doing this?

Only the fact you have to add caps and they take-up space.  It's wise to put those parts in.