4 channel API 312 build, help me choose the right parts please

[joe_masi]

Since I am new to this forum: Hi to all of you!
After recapping and rechipping my Tascam M3500 with great results I now want to take it a step further and try to build my own 4 channel Api 312 style preamp. I only have some basic knowledge in electronics (one semester electrical engineering  ;D ) so I thought I go for kits. I am really on a budget so I don´t want to buy a lunchbox or a 51x rack.
Ok, my plan is:
- Buy four VP312 (or maybe 2 VP312 and 2 VP26) from classic api without the faceplate
- 4x gar2520 DOA Full Kit Bundle
- Buy a decent 1ru 19 inch housing like this:
http://tinyurl.com/3sw7yeu

So for the PSU whats the easiest, cheapest and best solution?
Get the JLM world transformer
http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=13
plus a JLM Powerstation?
http://www.jlmaudio.com/shop/index.php?_a=viewProd&productId=12

Or could I also use something like this:
http://tinyurl.com/3bs7p45
Sorry it´s in German. This seems like a convenient unit but I would need an extra solution for 48V pp.

If I would decide for the JLM stuff should I build the PSU in the same housing as the preamps? Or is it smarter to have an extra housing for the PSU and connect the preamp to the psu with 5 pin xlr or something like this? This way I could connect some more preamps sometimes in the future since the powerstation has 5 rails.

Every hint is helpful and it would also be great to know if someone in Germany sells kits or good power supplies here.
Thanks for your help,
joe masi

 

[radiance]

I would rather get the JML stuff.
A couple of things to keep in mind:
The JML transformer is 30VA. I don't know if that's enough for your needs. Find out what current the VP312 and  VP26 need.
Also, the JML transformer is 38mm high. You will not be able to fit that in a 1 unit case since the spacers between the pcb and bottom plate are going to be another 3mm or so....and because all cases I came across have an inner hight of less than 38mm.

So, if you decide to make an external PSU which is able to power more units (like 8 or so), you will want a bigger transformer.
Get a 80VA one or so, and connect that to the JLM Powerstation.

 

[Walrus]

80VA sounds well overkill to me! A fag packet calculation says 20VA will supply all 4 channels doing 28dBU into 600Ohms with all 4 channels phantom shorted to ground through their 6k81 resistors. Or am I missing something?

 

[Kevin]

hey 4 of VP312 will not fit in 1 RU 19 inch  the pcbs are a bit to big for 4 PCBs in one rack.. you could think about the access 312 with 4 units

 

[nielsk]

Or this:
http://www.groupdiy.com/index.php?topic=45313.0
a really good deal and you will help a brother out...
you can fit up to 8 of these in a 1 space rack.
the 9k p/s board from Gyraf will power 4 312s no problem, they draw very little current. Even a 10 VA P/T is sufficient.

 

[Hank Dussen]

Since I believe you're German I think it would be a lot cheaper and easier to use the SSL9K PSU from Gustav. (I see they're no longer in his shop, but maybe he's got some left. Ask him nicely here: http://www.groupdiy.com/index.php?topic=10776.0)
You might want to go for +/-15V so replace the 7818 and 7918 with 7815 and 7915.
Buy a 2 x 15v transformer for €20 from Farnell or so.
Check the SSL9K documentation for the rest.

 

[Meathands]

From the horse's mouth:

"The last unique feature of the VP312 to mention is the easy adaptability to have four PCB’s side by side on a 1RU enclosure. I have put two horizontal lines in the silk screen as guidelines for cutting. Once the 3/8” strips are removed from both edges, four PCB’s will fit perfectly in a standard 19” rack mount enclosure. That is also the purpose for the four extra mounting holes! "

 

[joe_masi]

Wow, thanks for all your replies!
The good thing is that 4 VP312 pcb´s would fit into a 1RU rack. Too bad that the transformer does not...
The access 312 and the other one seem really cool as well but I really like about the VP312 that you can buy it as a kit.
Regarding the current draw I found on gearslutz: "Current Draw per rail = 23.3mA up to 28.1mA, but with 600 Ohm it reaches 50.5mA"
Ok, noob question 1: It says per rail so is it 50.5mA for the "+"rail and 50.5mA for the "-"rail? Or am I on the wrong track here and it´s 50.5mA for the whole pre? If it´s for the whole pre 4 pre´s would draw about 200mA. So that´s 3VA.
Even if it was 50.5mA per rail it would still be only 6VA so with a 30VA transformer I would still have enough headroom to power another 4 pre´s. Right? Wrong? Half wrong? half right?

What I liked about the JLM world transformer is that it comes with fuse and everything. I have had some *very* bad experiences with 230V...
So I just don´t know how to wire a transformer from farnell in a safe way. Where would I put the fuse?

Ok, noob question 2: How do I wire the power rails to the mic pres? Is that in series?
Five Fish makes this psu:
http://www.fivefishstudios.com/diy-kits/psu-2448mk2-kit
Would that be more appropriate for my plan?
Thanks again,
jm

 

[Harpo]

Regarding the current draw I found on gearslutz: "Current Draw per rail = 23.3mA up to 28.1mA, but with 600 Ohm it reaches 50.5mA"
Ok, noob question 1: It says per rail so is it 50.5mA for the "+"rail and 50.5mA for the "-"rail? Or am I on the wrong track here and it´s 50.5mA for the whole pre? If it´s for the whole pre 4 pre´s would draw about 200mA. So that´s 3VA.
Even if it was 50.5mA per rail it would still be only 6VA so with a 30VA transformer I would still have enough headroom to power another 4 pre´s. Right? Wrong? Half wrong? half right?

Wrong. These current draw numbers are DC current. A transformer is AC. For the usual psu circuits with fullwave bridge rectifier factor in times 1.8.
Looking at the VP312 schematic, from the +/-16VDC supply there is a relais drawing about 10mA/ch. on the positive rail as well.
A phantom powered mic might draw up to 10mA + 4.6mA for the status LED. For the power supply circuits in question, this +48V is comming from a lossy (factor 4) voltage doubler circuit.
Ignoring +/-10% mains line under-/overvoltage and power wasted in voltage regulators, your transformer would at least need a
4 preamps *( (0.05A+0.01A)*1.8 + (0.01A+0.0046A)*4 ) *(16V+16V) = 21.3VA AC rating for all 4 channels simultanously driving +26dBm into 600 ohm. For a more normal use into nowadays higher impedance loads, a 20VA rating seems sufficient, but as always YMMV.

What I liked about the JLM world transformer is that it comes with fuse and everything. I have had some *very* bad experiences with 230V...
So I just don´t know how to wire a transformer from farnell in a safe way. Where would I put the fuse?

Have a look at the datasheet of your specific transformer (wires colour coding is not standarized and this has nothing to do with farnell). For a dual primary winding transformer (115V+115V) you connect these windings in series for your 230VAC mains voltage. A slowblow fuse (150mA/t - 200mA/t for a 20VA toroidal transformer) will either be inside the IEC socket or you use a separate mains fuse holder.

Ok, noob question 2: How do I wire the power rails to the mic pres? Is that in series?

In parallel.

Five Fish makes this psu:
http://www.fivefishstudios.com/diy-kits/psu-2448mk2-kit
Would that be more appropriate for my plan?

Maybe, (your up to 60mA current demand from the +48V rail might get tight with C6 and C7 100uF), but have a look at Owels 'Download PDF file of Assembly Guide/Kit Instructions', especially for your 230VAC mains transformer hookup question to get the idea.


/*typo fixed

 

[joe_masi]

Wrong. These current draw numbers are DC current. A transformer is AC. For the usual psu circuits with fullwave bridge rectifier factor in times 1.8.
Looking at the VP312 schematic, from the +/-16VDC supply there is a relais drawing about 10mA/ch. on the positive rail as well.
A phantom powered mic might draw up to 10mA + 4.6mA for the status LED. For the power supply circuits in question, this +48V is comming from a lossy (factor 4) voltage doubler circuit.
Ignoring +/-10% mains line under-/overvoltage and power wasted in voltage regulators, your transformer would at least need a
4 preamps *( (0.05mA+0.01mA)*1.8 + (0.01mA+0.0046mA)*4 ) *(16V+16V) = 21.3VA AC rating for all 4 channels simultanously driving +26dBm into 600 ohm. For a more normal use into nowadays higher impedance loads, a 20VA rating seems sufficient, but as always YMMV.

Ahh, ok. After reading that a couple of times things have cleared up. Thanks!
I now have decided to have the psu in the same case as the pre´s. Seems somehow easier. Just to be on the safe and cheap side I will go for a 230V / 2 x 15V 50VA transformer from here:
http://www.musikding.de/product_info.php/info/p2883_Ringkerntrafo-230V----2x-15V-50W.html

Hopefully this one will fit into 1RU. 35mm high. I will call them on monday to check.

Have a look at the datasheet of your specific transformer (wires colour coding is not standarized and this has nothing to do with farnell). For a dual primary winding transformer (115V+115V) you connect these windings in series for your 230VAC mains voltage. A slowblow fuse (150mA/t - 200mA/t for a 20VA toroidal transformer) will either be inside the IEC socket or you use a separate mains fuse holder.

So what fuse rate should I use with 50VA? something like 400mA/t - 500mA/t?

Maybe, (your up to 60mA current demand from the +48V rail might get tight with C6 and C7 100uF), but have a look at Owels 'Download PDF file of Assembly Guide/Kit Instructions', especially for your 230VAC mains transformer hookup question to get the idea.

I checked it out but it didn´t answer all my qustions so i tried good old google and I found this:
http://sound.westhost.com/psu-wiring.htm
Under 2.2 they have the pic I was looking for so I will do it like this. Of course only as far as to the transformer.
So now the only thing left is the psu for the rails. Is the 9k psu from gustav and gyraf the same? I couldn´t find out.
sorry about all these little questions but I am trying to understand as much as possible.
jm

 

[radiance]

80VA sounds well overkill to me! A f*g packet calculation says 20VA will supply all 4 channels doing 28dBU into 600Ohms with all 4 channels phantom shorted to ground through their 6k81 resistors. Or am I missing something?

Well, it might be overkill but the price difference  (@RS)  between a 30VA and a 80VA is only a couple of euro's.
80VA will run less hot, last longer & is able to power  more modules...

 

[Walrus]

80VA sounds well overkill to me! A f*g packet calculation says 20VA will supply all 4 channels doing 28dBU into 600Ohms with all 4 channels phantom shorted to ground through their 6k81 resistors. Or am I missing something?

Well, it might be overkill but the price difference  (@RS)  between a 30VA and a 80VA is only a couple of euro's.
80VA will run less hot, last longer & is able to power  more modules...

Maybe. But a 20VA will fit in a 1U rack! ;D

 

[bobschwenkler]

You can't get anything between 20 and 80VA there? There's gotta be somewhere...

I don't know what shipping is to Europe from here, but Avel Lindberg has a great selection and will have plenty of the VA you need that will fit into a 1U.

 

[joe_masi]

Just to finish this: psu pcb from gustav is ordered as well as a rack and a 30VA transformer and a case. And if I would have been faster ordering the kits I would have saved some more money because the dollar/euro rate is really bad now. well.
Thanks again for all your help!!