Built a custom D8B external power supply

[RJH_MUSIC]

hen I tried the +/- 15 power supply i was able to adjust the positive side to +16 vdc, but since only output 1 is adjustable, i could not get the minus side past -15.33 vdc. when I hooked up the d8b, some of the faders on the video screen were jumping all over the place and the Vpots on the console would not work. the led's were also jumping all over the place. just not stable enough. I am posting a picture of the +/- 22 VDC power supply i just purchased and am waiting for.

[arjepsen]

Ah ok - I was only browsing over the dual output ones.

[RJH_MUSIC]

Yes, oddly enough, I could not find a dual output +/-22 VDC. I think that is why this one is ore expensive. I am waiting for TRC to ship it to me. They do not keep many in stock. TRC is located in Doylestown PA, (I have a lot of family there) and the company that makes this one is also in Northern PA. They said it was a 2-3 week shipping window, I am week 2

[arjepsen]

Yeah, I've been looking all over Mouser, Digikey, Farnell etc. All the suppliers I can think of here in Europe, but that one is actually the only one I've seen that has +/- above 15V. I can't find any dealer for that particular psu here in Europe.

[arjepsen]

Interesting update on my part:
I read a post on groupdiy, talking about important notes on how to use smps supplies - there's some interesting points:

1) Not all SMPS are good. SMPS noise is very dependent on layout and parts. But there are ones that are known to be good. In particular the MeanWell ones are popular. I once created a mic pre from an 48V SMPS for like $9 USD and it's noise floor completely flat and free of mains hum.

2) You must load an SMPS with at least 20 percent of the rated load or it will go into a "hiccup" mode where it modulates the output at low frequency. For smaller SMPS this frequently manifests itself as a whistle in the 1-3kHz range. For larger supplies, it can create massive noise at a wide range of frequencies. And note that at some point you can't just add a load resistor because it will need to be huge. So you need to pick an SMPS that supplies the right current to begin with. It should be selected for at 80 percent of load. SMPS work better when loaded. Even to 100 percent.

3) If you short an SMPS for even a few ms, the output protection mode will turn on where it modulates the output at a very low frequency. Meaning the device will get little bursts of power and generally not work at all. This is frequently caused by too much capacitance on the output such as when you connect the output directly to a board or multiple boards designed for a linear supply that have a lot of filter capacitors. The solution for this is to add a capacitance multiplier on the output of the SMPS. That will slowly turn on the pass transistor and let the filter caps charge up slowly and keep the SMPS output protection from kicking in. Note that for more than a few 100mA, a two transistor CM is required which incurs two diode drops in voltage but the larger SMPS frequently have a trim that can be adjusted upward to compensate (assuming you need the full voltage).

4) You should filter the SMPS input too. The switching noise of an SMPS can easily go backward out the input and then pollute things connected to it. This is especially true of DCDC converters with similar frequencies where all of the inputs are connected together and cause low frequency "beating". If you look at datasheets, you can frequently find filters on inputs that are required to achieve a certain EMI rating. Althogh personally I have never experienced this kind of noise so this part is somewhat theoretical. I have used SMPS without input filters and never found them to pollute other devices. So it's not required. But line filters are pretty cheap so if you're designing something from scratch, you might as well.

5) At one point I was advocating CLC fitlers on outputs. But now I'm leaning toward just using capacitance multipliers which will filter out the high frequency switching noise and keep the output protection from kicking in as described above. You don't need a regulator since an SMPS is a regulator. Or, if you need to bring the voltage down a little, then use a regulator.

If one follows these rules, SMPS are superior to linear supplies in just about every category. The only potential issue would be that they can emit EMI that can be picked up by nearby devices like guitar pickups, high impedance / high gain devices or unshielded magnetics. But depending on the amount of current being switched, the device would have to be fairly close. For a typical SMPS of a few 100mA or so, it would have to be a few cm. Even for a 5A supply you would probably have to be standing right next to it. But again, not all SMPS are good. If it's just some crap wall wart from Ebay, you're on your own.

The interesting point for me, was number 2 and 3. Particularly number 3.
I originally replaced the big caps on the power distribution board in the desk, with some nice new expensive Mundorf caps. My thinking is that these caps were quite old by now, and the desk would probably benefit from a change.
Anyways, the text I quoted states, that an smps supply can get quite annoyed with you, if you put large caps at their output - particularly when supplying a board normally made for a linear power supply (see bold text).
I had recieved some spare parts that I put in my desk - and originally got a bit of audio again. Unfortunately it seemed to die again.
Anyways, I just removed the two 4700µF caps from the power distribution box, and now the desk audio came alive. (woohoo). Unfortunately there still seems to be some hickups with the left output, but maybe i'm one step closer to have everything working

By using the +/- 22V supply, you would run it through the regulator circuit, so I think you should be ok there.

[arjepsen]

Someone pointed me to some interesting psu's:

https://capi-gear.com/catalog/product_i ... cts_id=567
http://collectivecases.com/?product=psu-cc-psu1

[arjepsen]

I have learned a few more things, over at groupdiy:

First of all: putting a large capacitor after a switch mode power supply (smps) like the ones we're trying to use, is not a good idea. The problem is, that the initial inrush current will make the supply think it is shorted, and it stops working the way we want.
Another problem is, that we want to filter the output, since they can be somewhat noisy.
The solution is to use a simple circuit called a Capacitance Multiplier.

Anyways, after learning this, I removed the big can caps on the power distribution board. I managed to make the desk work with just the +/- 15V from the tracopower supply, no need to adjust it up to 16V.
However, it has some noise, so I need to implement that Capacitance multiplier circuit, which will actually subtract about 1.2 volts - so I may end up with buying two single 15V supplies, adjust both up to about 17.2V, and "stack" them in a +/- configuration, and then filter it with the mentioned circuit.
Good news for this is, that those 15V supplies don't have to be very large - probably a Meanwell LRS-35-15 would have enough wattage. They're kinda cheap, and small. Now I only need to figure out the parts for the Capacitance Multiplier circuit.

[RJH_MUSIC]

Wow, and I thought this was going to be easy. Not being an engineer all of the above to me is kind of Over my head big time.

Got my power supply and now need molex connectors for it, so I have to wait another week or two to finish. I have one ace in the whole but hate to resort to it. If none of this works I could resign myself to just putting all the original D8B power supply parts into the new to wreck unit case I bought. The only game would be a lot more cooling and a smaller footprint and maybe quieter. But I would still be at the mercy of failing parts in the future. Here's a picture of the case I bought and how I'm building it up now.

[RJH_MUSIC]

Very well made chassie with sufficient cooling And not too expensive it was only about a $125 American.

[Carlo]

Hi RJH_Music

Fantastic job. Has this worked for you? Can you please write some instructions on how you built your PSU and what alternatives have you used. I'm noticing my Astec is failing.

Thank you

Carlo