Internal->External PSU mod

Discuss Yocto construction and related issues

Internal->External PSU mod

Postby antto » Nov 29th, '16, 17:41

some background: the yocto appears to be designed to be powered by a 15VAC adapter, and has built-in half-wave rectifier.
afterwards, there's been a thread suggesting a modification to adapt an internal PSU with full-wave rectifier link
my yocto kit came with this modification on a nice green PCB made by xlarge:
Image

it turns out however, that there's some PSU-related noise going on
the problems, as far as i can tell, are two:
1) when the yocto GND is connected to EARTH (via the loop breaker) i get a constant buzz, even when the yocto is switched off (my switch turns off the 220VAC going into the transformer)
- now, if i just remove that connection - the buzz goes away, however:
2) when the box is closed back together - i still get hum/buzz, this turns out to be picked up electromagnetically, the transformer is basically sitting a few centimeters from the whole mainboard
- moving the green board with the transformer away from the yocto PCBs fixes that issue. about 15 centimeters of distance is sufficient

with that said, i should also say that i'm not too familiar with 220VAC/mains/ground loops/earthing
i've read some thing about ground loops and hum/buzz, and it seems it's possible that in some houses, the "earth" pin may not be exactly what it should be.. so in this regard - i haven't checked if mine is "proper"
i also don't think i have another similar device (an audio device which takes 220VAC power (all three pins) and has a metal case) to use it as a reference and to see if it also hums/buzzes in a similar scenario
but in any case, even if i solve problem #1 somehow, i don't think solving #2 would be easy.. i already tried making some "shield" around the transformer but it made absolutely zero audible difference

and so i decided it might be easier to just move the internal PSU outside of the box
prerequisits:
- my yocto has the metal case which has holes for both the barrel jack and the 220VAC power socket, as well as an ON/OFF switch
- i'm currently using the power switch, and the power socket with built-in fuse holder
- "green board" contains the blue "box" transformer, rectifier, two fuses, and the loop-breaker
the idea:
- move the green board out of the box
- the green board shall be put in its own box, probably plastic
- the "PSU" will have a 220VAC power cord on one end
- the "PSU" will output filtered +15VDC, GND, and -15VDC (and maybe optionally mains earth) on a cable
- this cable should have a connector, and it should be such a connector that is hard to mistakenly plug into something else
- the power socket on the yocto shall be replaced with the same sort of connector, it should be such a socket that is hard to mistakenly plug something dangerous
- the ON/OFF switch on the yocto case may stay, it can be used to switch OFF the +15 and -15 supply
- the two large capacitors on the IO board (2200uF) will be ommited or replaced with smaller ones (but not thrown away, see below)
- the "PSU" will include large capacitors on its end, to filter the +15 and -15 voltages (the two large caps from the IO board may go here)

now, the two important things are the schematic, and the choice of connector
so far, after some discussions, we picked XLR as a good candidate
depending on whether 3 or 4 pins are going to be used, there's XLR3 and XLR4
the XLR male connector (with exposed pins) will go onto the yocto, and the female connector will go onto the cable, it will be a good idea to label them since they will be used for exotic purposes!

Image
Image
so, if anyone has any comments or thoughts - throw them on the table
i'm almost sure this can get rid of the hum and buzz, but i'm not fully sure if it is a good idea in terms of safety

* i'll update this post with some more pics
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Re: Internal->External PSU mod

Postby pfaffian » Dec 8th, '16, 00:04

Something doesn't sound right to me in this scenario. I have installed a large 17V 1A non-toroidal transformer inside of my unit. It sits about a cm from the board and there is no hum.
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Re: Internal->External PSU mod

Postby antto » Dec 8th, '16, 19:09

mine isn't toroidal, as you can see in the pic
i'm not familiar with transformers too much, but i think i read somewhere that toroidal transformers are supposedly better insulated or something
also, are you with a metal case and are you using the "loop breaker"?
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Re: Internal->External PSU mod

Postby jeroenbvo » Dec 11th, '16, 13:27

The only purpose of a groundloop breaker is to avoid ground loop hum when you interconnect different audio devices together with all of them connected to mains.

Image

So, if you say you have no hum with the groundloop breaker disconnected, that means that the loop breaker circuit has a fault.
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Re: Internal->External PSU mod

Postby antto » Dec 11th, '16, 21:27

jeroenbvo wrote:The only purpose of a groundloop breaker is to avoid ground loop hum when you interconnect different audio devices together with all of them connected to mains.

i'm interconnecting the yocto together with two x0xb0xes (on AC-AC adapters), a korg ES-1 (AC-DC adapter without earth connection), the computer (pretty normal chunky old PC), and the mini-HIFI (no earth connection)

So, if you say you have no hum with the groundloop breaker disconnected, that means that the loop breaker circuit has a fault.

well, no idea, i also didn't expect that
btw, i'm talking about buzz and hum which is quite quiet, but it pretty quickly becomes a huge issue when i want to push the gain up and distort the drums

for the actual hum/buzz tests i did, i did not plug the yocto into the normal setup, but only into the HIFI's microphone input (which has a gain knob) ... the HIFI itself was still connected to the computer (which itself was connected to the rest of the setup)

so that is some extreme amplification gain, surely, but there definately is buzz (with the loop-breaker) and hum (picked up electromagnetically)
when i exclude the loop-breaker and move the "green board" 15-or-so centimeters away from the rest of the stuff - all that is left is the noise floor of the yocto (which is pretty clean)
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Re: Internal->External PSU mod

Postby hofmann25 » Dec 14th, '16, 09:55

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Re: Internal->External PSU mod

Postby antto » Dec 14th, '16, 19:15

as i said in the first post, i already have that mod, my yocto came with it (sort of)
now whether it is implemented the same way - i can't tell
with this thread i am talking about taking that mod and moving it "outside" the yocto case

i read some stuff about mains voltage, earthing, safety, etc
things appear so messy
in my country it appears we use the so called "TN-C" standard, newer buildings (should) have 3 wires going to each power outlet, but all of the older ones typically only have two - Live and Neutral, and the earth pin on the "schuko" socket is typically connected to the Neutral (which as far as i understand is the least safe option)

now, i sort of understand that the metal case should be earthed if there is a chance for the 220V Live to touch it by accident, which may be very possible if there is 220V going into the device
sadly in practice in my situation, that "earth" will end up being equivalent to the Neutral, which as far as i understand could be bad for the buzz/hum situation
i also read that with audio electronics devices the buzz and hum are very hard to deal with pretty much all over the globe no matter which mains standards scheme is used

i'll have to stirr this information some more to really get closer to some clearer answers to some of my dillemas...
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Re: Internal->External PSU mod

Postby jeroenbvo » Dec 16th, '16, 12:49

I think the easiest way for you to go is to go back to the original design by Vincent. Use the external power brick.
You will be perfectly isolated from the mains. And probably have no hum/buzz.

Reason why some people go for build in power supplies is reliability. You don't want that flimsy 16V plug fall out of the instrument during a gig in front of 500 people.. :mrgreen:
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Re: Internal->External PSU mod

Postby antto » Oct 28th, '17, 11:32

update: my latest iteration:
Image

Image

so, the PCB is actually two boards that can be chopped off (or not)
- mains voltage goes on the bottom, into the transformer, rectifier, and some smoothing capacitors
- the result goes to connector K1, rectified 15VAC, which should actually be more like 21VDC
- from K1 it goes to K2 on the smaller board, where there's a voltage inverting regulator which would generate the negative voltage
- the final result comes at the connector at the top:
--- negative rail (-17V should be enough to drive the 7915 regulator on the yocto IO board)
--- positive rail (even more filtered version of the rectified 15VAC, to drive the 7815)
--- a current-limited version of the positive rail, which can be used to drive the 5V regulator
- the smaller board also includes the optional loop breaker

the two boards can be chopped off and connected via a cable with XLR3 connectors, the smaller one inside the yocto, while the bigger one outside (that's what i intend to do)
or they could be kept as a single board and stuffed inside the yocto.. but you'd get chances for electromagnetic interference from the transformer

important: the transformer i picked is rated 6VA, and might be too small for the job
my yocto certainly draws less current than a "normal" yocto because i used bigger current-limiting resistors for the LEDs (3.3k) than what was according to the BOM (220ohms iirc)

i measured the current consumption from the power rails on my yocto as it is currently, but my measurements might have been non-ideal (and thus wrong) because i measured between the rectifier and the first reservoir capacitors, so at that point in the circuit there would be violent currents to begin with
still, what i measured was roughly:
> 96mA on the positive rail
> 85mA on the negative rail
> 7.7 to 12.5mA on the GND rail depending on how many LEDs are lit and whether the sequencer is running or not
so i expect more consumption on the positive rail in a "normal" yocto due to the LEDs
i'll try to re-do the measurement, i'll add additional reservoir caps after the rectifier and measure the current after them

initially i was going for a transformer with a 14VA rating because that should certainly do the job, and i found a 18VA transformer from ANG
sadly it's huge, it's roughly the same size as the blue transformer, and that would make the bigger PCB yet even bigger
i also considered the option to use a chassis-mounted transformer, that could make the PCB very small
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Re: Internal->External PSU mod

Postby antto » Jan 19th, '19, 19:45

i finally got time to revisit this, and i modified the circuit and PCB slightly since last time
i fixed a few little issues, removed some components, made the PCB a tiny bit smaller

Image
Image
Image

splitting the two boards was easier than i thought.. i just ran a knife on the top to cut the copper tracks, then i pushed the weak spot onto an edge..

i made a small adapter plate for the metal case, to allow me to mount the XLR3 socket onto the panel hole for the mains socket
..in librecad, got it fab'ed locally

Image

the bigger board is now sitting sealed inside a plastic box, up-side-down (it's mounted onto the lid)
it's see-thru, so i can see the power LED

Image

on the inside, some of the cables are still kinda "temporary", i'll get better connectors later

Image

the schematic:

Image

i was worried that the transformer i chose might not be strong enough to supply the yocto
..it's actually good enough
the big board gave about 26VDC with no load, and falls down to 20VDC with the yocto running
my local electronics shop only had an SMD version of the switching regulator chip, i put it on a SOIC-to-DIP8 adapter, but it works fine
you might have noticed that there are some fusible resistors, i couldn't get them so i used just a jumper wire..
there should be 22ohm between the transformer and the rectifier, it would limit the charge currents i think (saw it in the tr-808 schematic)
the regulator generates -16.9VDC when the trimpot is centered, just like i expected

so..
even the way it is now, i finally don't have mains voltage in the yocto, and i can close it and i don't have any hum, or buzz in the audio, just an innocent noise floor
the whole case is connected to protective earth

interesting observation - the loop-breaker seems to add mains buzz, i left it disconnected
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