Using Green Pre PSU as FETboy PSU? (and why you shouldn't do it)

[fucanay]

EDIT #2: After a lot of fussing around with this thing, I gave up on this PSU and went for an all JLM solution. Which includes the JLM TREX PSU boards along with the 48v 380mA SMPS. Using a star grounding scheme, this removed all of the oscillation and noise problem I was having with no other modifications needed. This is by far the easiest and cheapest solution. The Green Pre PSU just presented too many problems for this preamp.
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This is the Solution to the thread below. I think this is quite useful to many less experienced DIYers, so I'm amending the first post to show what has been changed to make the Green Pre PSU work for the FETboy. Or for that matter, anyone needing +24 and +48 from the same supply.

First, this is what the board looks like: Green Pre PSU configured for +24 and +48

This is an edited schematic that shows the circuit as I'm using it: FETBOY PSU Schem

1. The parts surrounded in yellow are omitted. These were for the -15V side of the PSU that the Green Pre needed.

2. The Green line within the yellow box is a jumper. This is needed so the ground plane on the PSU is connected with the PSU configured in series.

3. To get the +24 volts in this circuit, you'll need to adjust some resistor values around the LM317 regulator. In the original circuit R1 was marked 2k2 and R2 was marked 220R. R1 and R2 have been changed to 2k to coincide with the values suggested by the LM317 calculator.

Many, many thanks to everyone who helped me in this thread. You guys beat it into me and it helped me a lot.

EDIT: After switching to a 12-0-12 transformer, everything is good to go. Input voltage = 28.8VAC, rectified voltage = 38VDC, regulated voltages = +23.9VDC and +47.7VDC.

Matt

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The original post starts here
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matta gave me this drawing detailing how to set up Peter C's Green Pre PSU to work with the FETboy Preamps.

This details what parts to leave out for the negative side of the PSU (surrounded in yellow) and where to jumper it (in green, on the bottom right). Matt used a 48VCT transformer to do this.

The original schematic is here.

I've edited it to reflect Matt's changes here., except that I have not changed the transformer to 24-0-24. I have left the 15-0-15 in place. Because using the mnats PSU boards, you can use the same 15-0-15 transformer using a trimmer to get what ever voltages you need within the limitations of the regulators. But getting to +24V should be no problem.

Now, I'm not very good with these things and don't really understand how to adapt all of this stuff, so that is the reason for this thread. There is no place for a trimmer on the Green Pre PSU, so I would need to change some resistor in the PSU to get more DC volts to the LM317 regulator, I guess. But I have no idea which needs to be changed or to what it should be changed to.

Is anyone able to point me in the right direction?

Matt

 

[Gus]

Why only 1/2 the transformer? 15 VAC will not get you 24VDC

Calculating the resistor values for a 317 circuit is not that hard. Go to the national site and look up the 317 notes and spec sheet.

Just 24VDC?


Building a 3 term is a good way to learn. The math is not hard. Things to look out for.
To much voltage at the input of the 317 at startup.
Need a load on it when testing it.
Make sure to use a big enought heatsink

Build on perf or even terminal strips

Now take a 24AC transformer to a full wave bridge

24 x 1.414 = about 34

For high wall voltages maybe about 36VDC after the diodes at the first filter cap(depends on the transformer and wall voltage)

For the jfet amp I would use a simple zener, cap with a pass BJT or mosfet. Another easy nice circuit to learn on reg.

Good luck if the PS is causing problems selecting the fets should be fun.

 

[fucanay]

I'm sorry, I should have mentioned the requirements for the FETboy. That teaches me to post when it's late at night and I've had too many drinks. oops:

The FETboy takes +24V and +48V for phantom power. I know they LM317 can use that 15-0-15 transformer and and get a +24V output. I can do it on the mnats PSU board I have here just by adjusting the trimmer. Which led me to believe that I could do something similar here.

BUT, I will study the things you've pointed me too see if I can do it or not. I got the file you sent me in the PM, and I'm going to start looking at it now to see if I can learn it.

Thanks Gus

Matt

 

[Ptownkid]

15 x 1.414 = 21.21VDC

That being the case, if your mnats is putting out 24VDC, your xfmr is putting out more than 15VAC

 

[rodabod]

[quote author="Ptownkid"]15 x 1.414 = 21.21VDC

That being the case, if your mnats is putting out 24VDC, your xfmr is putting out more than 15VAC[/quote]

Yeah, the maths doesn't add up.

If it is a 15+15, then I'd just wire them in series for 30V and if the regs run too hot then add a series resistor to soak some of the power up.

 

[fucanay]

Ok, I'm seriously confused. I keep trying to understand it and keep reading and just don't get it.

I started this thread months ago, and I thought that I understood that AC volts don't have anything to do with DC volts. So looking at what the mnats PSU was doing with the LM317 regulator, I thought I could do the same thing with the Green Pre, and therefor be able to use the 15 - 0 - 15 transformer I already have.

So what I should be doing is just buy a +24V transformer and just let it be what it is? Maybe I'm missing some notes on what Matt A did with his. I thought that maybe it would be easier to just change the Green PSU to use the adjustabilty of the LM317 regulators.

It's ok to call me a moron, I sure feel like one right now. I just can't get my head around it so far. Everytime I think I do, I don't. Bah!

Matt

 

[matta]

Hey Matt,

I never really had great luck using it the way it shows your transformer. I ended up using a 24-0-24 VAC trano and using the center tap as show, but leaving of the - rail parts, so using fullwave rectification, or at least wired for it, but not stuffing the - rail.

I would suggest the easiest thing to do would be to get a JLM AC/DC board and wire combine the windings for 30VAC as Roddy said and wire Joe's board for full wall rec from half wave.

Personally I would also keep the power trano outside of the box because at full gain those FETboys will pick up any EMF from the transformer and you may end up with hum/noise.

Cheers

Matt

 

[Ptownkid]

This'll straighten you out

http://www.kpsec.freeuk.com/powersup.htm

 

[fucanay]

I guess this is the key.

Note that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.

So I really do need something pretty damned close to the right voltage. If I were to wire the transformer in series, it should give me a little more than +30V. THEN I should be able to adjust the resistance to the LM 317 to get my +24V, correct?

Alternately, is there a PSU schematic I could try to build that'll give me what I'm looking for simply? I could order the JLM, but it's on the other side of the planet and would like to get moving on this. Time to hit the Metas again.

Thanks everyone.

Matt

 

[Ptownkid]

yeah, you could use the 30VAC and regulate it to 24VDC by choosing the appropriate resistance for both regulators.

However, if you say that you can get 24VDC with your mnats psu and a 15-0-15 wired in parallel...what's the problem?

 

[originalmusician]

Here's a LM317 resistor calculator if you are interested:

http://www.muzique.com/schem/lm317.htm

 

[fucanay]

[quote author="Ptownkid"]yeah, you could use the 30VAC and regulate it to 24VDC by choosing the appropriate resistance for both regulators.

However, if you say that you can get 24VDC with your mnats psu and a 15-0-15 wired in parallel...what's the problem?[/quote]

The problem is lack of confidence and a desire to not blow things up. :grin: :grin: And the fact that the mnats PSU has no phantom power. So, I'm trying to adapt something I already have,even though I'm in no way qualified to do so. Half fun, half frustrating.

All of the PSU parts are coming in tomorrow and I'll be digging into this and a few other projects. I'm fortunate that I have enough project going at once that this one can't slow me down, yet.

Matt

 

[fucanay]

[quote author="originalmusician"]Here's a LM317 resistor calculator if you are interested:

http://www.muzique.com/schem/lm317.htm[/quote]

This helps a ton. I was looking for something like this earlier today and didn't find it.

Thanks for the link.

And thanks to everyone for talking this out with me. I think I'm starting to understand this much better.

Matt

 

[originalmusician]

I'm not extremely skilled at this, but I'll tell you what I think.

Measure the output of your transformer so you know whether or not you can use one side or if you need both. If it truly is a 15-0-15, you will need both sides. Either way, look at the documentation for the transformer, and it should show you how to wire it for the proper output.

You can see by Matta's diagram that 24 * 1.414 = 34v went to the input of the LM317 and needed to be regulated down to 24v. The resistor Matta changed from 220 to 120 is R3 on the schematic you supplied (the first resistor in the calculator I linked). R1 and R2 add up to 2420 ohms to make up the second resistor in the calculator (you can see them in Matta's diagram. They are the 2k2 and 220 just above/right of the LM317). So Matta's voltage is regulated to 26.5v. By straight-wiring the 220 (R2), you can bring it to 24v. Or you can leave the 220 (R3) instead of swapping it with Matta's 120 and calculate values for R1 and R2 to get to 24v. Either way works.

If you use both sides of your 15v transformer to get 30v, you will probably need to cool the regulators. The tab of the regulators have the output voltage on them. So put heatsinks on them, but don't ground them. I think you can probably just cut a chunk of aluminum, drill a hole, put on some heat sink compound, and bolt one to each regulator. I'm not sure how effective that will be, but it seems to me that if you input 30 * 1.414 = 42.5v, you will be dissipating a lot of heat to get down to 24v, so you will need to do something there.

Someone with more experience is going to have to chime in and say what kind of voltage will be coming from those 1n4007 rectifiers before the phantom if you go into them with 42.5v? I guess since the cap it feeds is 100v it should be okay?

If you decide to buy a different transformer, you don't need to buy a 24v. I just built a 24v PS for a Neve and used a 22v. It was a lot cheaper than the 24v, and the LM317's didn't need to work as hard. I suppose I could have used an 18 * 1.414 = 25.5. I think that is the miminum size you can use with the LM317, because it requires and extra 1.5v above what you are trying to regulate to. I went with the 22v in order to play it safe. But I think you can make the 30v you already have work.

 

[fucanay]

I missed your reply and went ahead and built the thing. I left the 220R as it was and changed the R1 and R2 to 2k. I got the numbers using the calculator.

I paralleled the secondaries of the transformer and it only measured 18.3VAC. I went by the instructions for my transformer, listed here. The part number I'm using is 62063. I don't know what's wrong here. I tried two transformers and they both measure the same. Probably my stupidity, but I don't know where.

Speaking of stupidly, I did check the DC volts coming out before I checked the AC volts coming in and it's not that far off on the +24V. It came in at 22.9V. The +48V came in at 24.5 Volts, but I didn't put one of the 1N4007s in, thinking that I might blow the regulator if it was too high.

Any help is appreciated. Thanks

Matt

 

[fucanay]

I don't think paralleling the secondaries is doing anything. I rewired them to be in series and I get 18.6 VAC on the to positive leads. This doesn't make any sense to me.

According to the datasheet, It says I should wire green/brown and red/blue for parallel. I put the red/blue to positive and the green/brown to 0V. Am I doing something wrong here?

Matt

 

[Gus]

Google makes it easy lots of links

http://www.educypedia.be/electronics/powerelectronicssupplies.htm

look at C 8

http://www.onsemi.com/pub/Collateral/HB206-D.PDF

 

[fucanay]

I'm not quite sure what I was supposed to read in that pdf. It's 118 pages long. Don't have that much time right now. I'll hang onto it and read it when I have some more time. Where is the C 8 you were talking about?

Also, I think maybe I got some things confused. Should I be wiring it in series and then combine the green and blue leads? All of the PSUs I've build before required 15-0-15, so I've never built one like this before. I'm thinking that maybe they should, now.

I have so much to learn. Why aren't there more than 24 hours in a day?

Matt

 

[Gus]

chapter 8

also check the ST power supply basics pdf

lots of good links at the page

 

[fucanay]

That is great stuff on PSUs. I need some time to absorb it though. But those links should be added to the PSU meta with notes on the chapter to read.

In the meantime, while disregarding the PSU circuit itself, can anyone tell me how to wire this 2x15 toroidal to output +30? Is is just a matter of combining the two 15V wires? That kind of scares me as I don't know for sure that's how it works. I've googled, I've searched here, and I just can't find it.

Matt