[BUILD] CAPI VP28~500 Series~2-Stage Preamp~Official Support Thread

[Carl Cress]

If it is not already abundantly clear from my post, I should provide the following disclaimer: I have zero experience working with electronic components. The CAPI builds are my first venture into the field. I have wanted to do this for years and finally found the opportunity to. Please be patient with me.

I have a few questions for anyone that is kind enough to help out:

1) Is some sort of pad or double sided sticky tape required for the underside of the EA2622 input transformer? What happens if it is not used?

2) The circular gold ground plane of the +V socket of my preamp opamp slot has partially lifted from the PCB surface - Is a solder connection necessary between the pin and that gold ground plane or is it sufficient to have a soldered connection through the hole?

3) Any general or specific advice with regard to building the opamps? I thought I had them perfectly soldered yet one caused the PR2 and PR4 resistors to fry. The Q7 and Q8 transisters (black blocks with the holes) overheat on another.

I was so excited about building a pair of these vp28s and a pair of the vp312s.

1) no (it only makes it easier to solder in)
2) Solder from the back and re-float the top.
3) You have to be careful with heat around those transistors when soldering and they can get damaged.

Despite my experience here in this topic I have successfully built many pieces. I got lazy and used solder that was to big and a tip that was to big. I have decent soldering skills and thought I could overcome the need for smaller solder etc. , I was wrong. I will only buy pre-made and tested DOAs as they took me and 1hr and a half a piece to build.

 

[Allecstatic]

1) no (it only makes it easier to solder in)
2) Solder from the back and re-float the top.
3) You have to be careful with heat around those transistors when soldering and they can get damaged.

Despite my experience here in this topic I have successfully built many pieces. I got lazy and used solder that was to big and a tip that was to big. I have decent soldering skills and thought I could overcome the need for smaller solder etc. , I was wrong. I will only buy pre-made and tested DOAs as they took me and 1hr and a half a piece to build.

Thanks for your response, Carl!

With regard to your answers:

1) I have watched/read a few videos/instructions concerning the building of these pres. Some people say having some insulation between the input transformer and the PCB is "critical" because the ground is now on the front side? Here is a link to one such opinion:

http://studio939.blogspot.ca/2012/08/vp26-microphone-preamp-build.html

2) I did exactly that. I won't know if it worked until I can fix at least one of the 6 opamps I have built! :S

3) Do you mean Q7 and Q8? If so, do you mean heat from the soldering pen, or heat caused by a solder bridge?

I think my opamps are having problems because of either one or more of the following:

a) bridged connections that I can't see by eye;
b) components touching on the top side; or
c) the maxpins not soldered correctly. I simply dropped a bead of solder on them from the top side. There is no solder on them on the bottom side (where the connectors stick out to attach to the PCB.

I also did not twist the leads on the two diodes. I put all 4 in individually with the black (negative) side on the PCB board matched with the silk screen circle, as per the instructions. Hopefully not going the twisting route was not what caused my demise.


By the way: your opamp- that caused your PR1 and PR3 resistors to fry - was it irreparably damaged?


Thanks again for your time. I am hoping I can figure these things out. I was pretty excited about all of this but at this time I am just feeling discouraged by my inability to build at least one correctly following the instructions.

 

[Carl Cress]

As to the tape thing with the input trans I can still see air under mine and have installed dozens of these style on PCBs without issue but  I like to gently hold the piece against the PCB when soldering 2 of the leads and never clamp it hard as to do damage to the coatings on a PCB or have any stress on the leads. Chunger does some of the best tutorials ever and teaches good habits especially to people new to DIY so I should say that it's a good habit that would guarantee you don't dig in to the ground plain.

Yes I removed the resistors and cleaned the board and no damage. No other damage that I can note or see. The odd thing is I measured the resistors and they were still close even after the meltdown.

In respect to the opamps, the issue could be any of the issues you mentioned. Although you save money building them, if you destroy the board or other components you are about even or worse. I'm building 3 of these and will only buy pre-made for the remaining 2 and now for the one I have now. Enjoy the builds and remove any situation that will cause you grief ( such as as the opamp build) that's hard to test outside the pre.

And don't get discouraged. There can be hiccups along the way but solving them makes it even sweeter when it's working and helps build confidence to attack the next project. This forum has some great advise and help when you ask for it. Hang in there!

 

[Allecstatic]

As to the tape thing with the input trans I can still see air under mine and have installed dozens of these style on PCBs without issue but  I like to gently hold the piece against the PCB when soldering 2 of the leads and never clamp it hard as to do damage to the coatings on a PCB or have any stress on the leads. Chunger does some of the best tutorials ever and teaches good habits especially to people new to DIY so I should say that it's a good habit that would guarantee you don't dig in to the ground plain.

Yes I removed the resistors and cleaned the board and no damage. No other damage that I can note or see. The odd thing is I measured the resistors and they were still close even after the meltdown.

In respect to the opamps, the issue could be any of the issues you mentioned. Although you save money building them, if you destroy the board or other components you are about even or worse. I'm building 3 of these and will only buy pre-made for the remaining 2 and now for the one I have now. Enjoy the builds and remove any situation that will cause you grief ( such as as the opamp build) that's hard to test outside the pre.

And don't get discouraged. There can be hiccups along the way but solving them makes it even sweeter when it's working and helps build confidence to attack the next project. This forum has some great advise and help when you ask for it. Hang in there!

Ya my input transformer does not have any gap - it is directly on the PCB. Still not clear on what the implication is. Hopefully someone can chime in.

 

[Carl Cress]

The implication is that when installing it you scratch through to the metal underneath (which is the grounds Plate). Most likely not your issue. If you followed the instructions closely you should be ok. When my new opamps solve my issue you can feel a little better and maybe try the same. Mose issues you will experience are bad solder joints or bad grounds. I'll let you know tomorrow.

 

[Allecstatic]

The implication is that when installing it you scratch through to the metal underneath (which is the grounds Plate). Most likely not your issue. If you followed the instructions closely you should be ok. When my new opamps solve my issue you can feel a little better and maybe try the same. Mose issues you will experience are bad solder joints or bad grounds. I'll let you know tomorrow.

Thanks, Carl. Hoping the opamps work out for you.

 

[Carl Cress]

Allecstatic, I replaced the (4) 10 ohm resistors first . Note that after that amount of heat it is easy to lift one of the solder pads on the pcbs so dismantle to the point where you can access from the back and carefully remove the resistor with whatever desoldering tool you have and not forcing anything (be gentle). Once you are replacing the resistors solder and make sure that front and back pads are soldered and that goes for your opamp receiver as well. I would also note that the already assembled 1731 and 2520 had been soldered from underneath with a generous amount of solder with the solder bridges that are in the instructions but I hesitated to do when building. Long and short of it has fixed the issue and the pre is passing sound and well. Hope this helps.
After seeing your pics I would recommend that if you have some extra wire from your transformer you strip it back and resolder and cover with heat shrink. Good luck!

 

[Allecstatic]

Allecstatic, I replaced the (4) 10 ohm resistors first . Note that after that amount of heat it is easy to lift one of the solder pads on the pcbs so dismantle to the point where you can access from the back and carefully remove the resistor with whatever desoldering tool you have and not forcing anything (be gentle). Once you are replacing the resistors solder and make sure that front and back pads are soldered and that goes for your opamp receiver as well. I would also note that the already assembled 1731 and 2520 had been soldered from underneath with a generous amount of solder with the solder bridges that are in the instructions but I hesitated to do when building. Long and short of it has fixed the issue and the pre is passing sound and well. Hope this helps.
After seeing your pics I would recommend that if you have some extra wire from your transformer you strip it back and resolder and cover with heat shrink. Good luck!

Carl - happy to hear it worked out! Ya I was reluctant to make the solder bridges they suggest can be helpful to secure the Milmax pins as well. Do the opamps fit snugly into the slot or are they loose?

I cleaned up and resoldered the 6 opamps yesterday. 4 of 6 work. I am debating lightly soldering one pin of the opamp to the preamp so that they don't move around so easily. They sound great! When I replaced the resistors I lifted one pad on the top of the pcb board. I just made sure it was soldered through and on the bottom, as well soldering the top as best I could. If there was a gap between the resistor and the circuit line I would have bridged it but it was fine.

I'm not sure entirely what you mean by resoldering the transformer wires. Are you referring to how some of the wiring is exposed? The first preamp I made turned out that way but the other 3 I did what most would consider properly.

 

[Allecstatic]

How are you guys securing the opamps in their positions? Some of mine will stay on their own while others just slide out.

 

[Carl Cress]

Bump

 

[Potato Cakes]

How are you guys securing the opamps in their positions? Some of mine will stay on their own while others just slide out.

If you use the same size pin as sold on the CAPI website,  they shouldn't slide out. I've build dozens of DOAs from pretty much anyone that sells the PCBs and I've never had any trouble with them falling out. Maybe hot glue if you its going to be semi permanent. That would work better than tape I think. You could just remove the pins that are too small and replace them the correct size.

Thanks!

Paul

 

[Allecstatic]

I don't mean to be rude but this has been cover a million times and I have a page at the site about it http://capi-gear.com/catalog/DOA_Install.php

As someone that is new to these builds I did my best to print out and organize the documents provided on the webpage. Even though they may appear to be straight forward and organized in a logical way, it was still overwhelming to an amateur like me. Accordingly, I will concede that there were certain aspects that I did not understand at first but I eventually figured them out on my own; however, there were some issues that I was unable to resolve on my own within a reasonable time. I deferred to this support thread as it was listed on the VP28 build page as a exactly that, a support page.

The issue with the pins, according to the shop I brought it into, happened to be due to the gold coating on the interior of one of the slots restricting the pin from going in all the way.

I am now trying to finish this build as I have been excited to use the preamps.  I was actually encouraged to buy these preamps and seek help in the forum if needed, as Jeff was supposed to be quite supportive. I will continue to look through these documents I have already printed to figure the remaining issue. This was a lot of fun and a great learning experience for me. I really enjoy the units I have finished. Thank you to all for the help!

 

[Potato Cakes]

The shop you took it to is mistaken regarding the pins. Brand new sockets require the initial break in, then after that, swapping out and seating new DOAs is easier, but it still requires a little bit of force. Once seated they should never come out on their own if the proper size pin and socket is used.

Again: http://capi-gear.com/catalog/DOA_Install.php

Also, Jeff, along with all the wonderful people here, who offer these amazing design so others can build world class studio gear at a fraction of the cost and get some electronics education in the process, have been and are extremely supportive with the products they offer, not supposedly.

Thanks!

Paul

 

[Allecstatic]

The shop you took it to is mistaken regarding the pins. Brand new sockets require the initial break in, then after that, swapping out and seating new DOAs is easier, but it still requires a little bit of force. Once seated they should never come out on their own if the proper size pin and socket is used.

Again: http://capi-gear.com/catalog/DOA_Install.php

Also, Jeff, along with all the wonderful people here, who offer these amazing design so others can build world class studio gear at a fraction of the cost and get some electronics education in the process, have been and are extremely supportive with the products they offer, not supposedly.

Thanks!

Paul

Hey Paul,

Thanks for the input. I appreciate these builds, which is why I purchased them in the first place. I was not bashing anyone on that front. I also appreciate the assistance just as much as you or anyone else does. I was only addressing what appeared to be a bit of frustration with my question.

I clarified I was new and I had already viewed and printed all of the necessary documents. A friend that has built these units was adamant that as someone new to electronics, the capi pres were a good start. He also insisted that I defer to the forum and to Jeff if I had any problems. So I did. The first response I received was the one quoted above that has since been deleted.

I only responded to express my first impression and further, that my intention was clearly not to frustrate someone but rather to get help and finish these awesome pres. If someone wants to say I was wrong for asking questions here and further that I was wrong for being a bit discouraged with the first response I received, then go ahead.  I have no other experience with someone from capi.

As to the pins - I had put much force pushing the opamps into the sockets, like the others (which went in securely). These other ones would not go in, despite how much I pushed. In any event, the pins will be replaced. I have managed to fix the opamps that were not working (solder bridges) and ordered a vp26. I will only buy premade opamps now to be sure. Where I am in the US seems to take a bit for the order to get here so not sure when I will be able to work on the vp26. I look forward to one day trying the vp312.

 

[jsteiger]

Wow that is strange. I have not deleted anything on this thread.

 

[geardude]

Hey guys,

I'm having a problem with one of my 28's on this build go around.Having a slew of these under my belt, I've never encountered this error before  but here it is.

Apparently this unit is quieter when activated in line mode when sending a 1khz sine wave I'm getting a return output exactly 3 db quieter. Besides this the preamp is functioning just swell. I've attached some pics.

The greyhill knobs are in the correct orientation & using red dot op amps; possible cold joint somewhere in the line switch signal chain? I tried touching up the pins on that particular switch but no differences.

All capacitors are in their correct positions and orientations comparing them to other builds. And I can't seem to spot any solder brides anywhere.

http://s1067.photobucket.com/user/_Geardude_/embed/slideshow/

 

[jsteiger]

I would double check the location of all R's specifically the 160R's and 4k99's just behind the mic/line switch. Next I would check all R's behind the preamp gain Grayhill. After that look for a solder bridge on the preamp gain Grayhill. Exactly 3dB is far too suspicious since all but the lowest 2 gain settings are 3dB.

Are you 100% sure the preamp gain knob is orientated properly?

 

[geardude]

I would double check the location of all R's specifically the 160R's and 4k99's just behind the mic/line switch. Next I would check all R's behind the preamp gain Grayhill. After that look for a solder bridge on the preamp gain Grayhill. Exactly 3dB is far too suspicious since all but the lowest 2 gain settings are 3dB.

Are you 100% sure the preamp gain knob is orientated properly?

Yes indeed, well stupid me, after thoroughly checking those and confirming everything is gravy I realize that my totalmix fx settings changed...and you guessed it...had the lineoutput 3db quieter. UGH go figure. Now line level is the same going out and as coming in.... however now after building trying out this test on a bt50 with the proper RME fx settings, & my line level return is -16.5 dbfs instead of -18 as it should be. again all knobs are in their correct orientation. I'll post this over in that forum

 

[Allecstatic]

I just realized the light indicators on the 4 vertically aligned push buttons are not working even though the buttons themselves are. I resoldered to be sure and visually inspected the area - still not working. Anyone run into this problem or know what the issue may be?

 

[Potato Cakes]

Polarity of LEDs soldered backwards?