[BUILD] CAPI VP2X~500 Series~Preamp Kit~Official Support Thread

[Bassphil]

Hi, I have a quick polarity question.

I'm assembling the new rev of the VP26 and there is a 470uM Vishay capacitor that goes into C10. Which side is + and -? There are arrows running from one side to another, right to left if you're reading the writing. I'm assuming that they're pointing in the direction of + right? This cap doesn't have a strip on the negative side like it should...

Thanks!

-Phil

UPDATE: Never mind- arrows point to negative....  :

 

[poiesznbeats]

How "matched" should any given batch of VP312s be when at the same gain settings?  I have one pre out of six that gives about 1/3 less output at minimum gain with the same input as the others.  Its still very usable, but when I'm doing a stereo thing I've needed to avoid that channel because I'm giving it more gain to get the same signal.

I'm double checking the build as best I can, but thought this might be a worthwhile question in general. 

 

[jsteiger]

A stepped gain pre will match to withing about a .25dB, typically better. A variable pre can be a little farther off but should not be drastic like you are mentioned.

 

[Bassphil]

Okay, I've read every post on this great forum and haven't found a definitive answer that will let me sleep better tonight.

I just finished my first build of the VP26. Like a few people here in the thread, I also noticed the weird issue of the moving numbers on my Fluke multimeter when I got to the final step of checking the ohms of the op-amp sockets.

I have a nice Fluke 179, but I might very well be using it incorrectly! When I set it to ohms and touch the different sockets of the op-amp, the numbers are moving up. Is that normal? My Fluke is set to auto range.

I also noticed that the readings change depending on which lead of the Fluke touches which op-amp socket. For instance, if I touch the red lead to the o and the black lead to C, I get a reading of 1.6M that continues climbing from there. If I switch that around, I get a reading of -1.35M.

I want to make sure that, as of this point, everything seems to be normal. Here are my readings:

-V & +V = starts at 200K and climbs
-V & C = starts at 1.06M and climbs
+V & C = starts at 168K and climbs
C & O = starts at 1.6M and climbs if the red lead from the Fluke is on the O (like I mentioned)
+V & O = 480K and climbs
-V & O = 1.2M and climbs

I'm definitely not an electrical engineer, but I'm good at following orders. I was pretty thorough in checking the solder joints, and I organized all of the resistors with my multimeter, so those are definitely working and should be sorted correctly. I can't imagine I'd have any problems, though if there's one thing I've learned from reading this thread is that people with more knowledge than me can misplace a resistor... 

Anyway, before I take it to the next step, I just wanted to get a confirmation that the moving numbers thing when I was performing the first check was normal, and that the numbers I posted look to be okay.

Thanks!

-Phil

 

[jsteiger]

Phil, this is totally normal due to capacitor charging during the measurements. Looks like you are all good.

 

[JuniusRecordingCo]

Finished up two new VP26 with variable gain - and the first of my six to have the phantom LED. Question about those LED's behavior: both unit's LED lights immediately upon flipping the 48V on, but one of my units ramps down, slowly dimming after flipping it off, and the other unit turns off immediately. I inspected the boards and couldn't find an obvious visual difference. What is the correct behavior? Where might I look more closely to find what might be the cause of the difference?

Thanks - and enjoy your stay in Nashville, Jeff.

 

[qmp audio]

Finished up two new VP26 with variable gain - and the first of my six to have the phantom LED. Question about those LED's behavior: both unit's LED lights immediately upon flipping the 48V on, but one of my units ramps down, slowly dimming after flipping it off, and the other unit turns off immediately. I inspected the boards and couldn't find an obvious visual difference. What is the correct behavior? Where might I look more closely to find what might be the cause of the difference?

Thanks - and enjoy your stay in Nashville, Jeff.

I can't speak exactly for the new VP26 as i haven't built one ( I think I read that Jeff changed where the LED is power is coming from ),  but the vp312di's i have the LEDs do fade out when disengaging 48v.  This is due to the LED draining current from the cap.

 

[abechap024]

To me thats almost more useful, so you can see when phantom is actually off.

 

[JuniusRecordingCo]

Wanted to check in again on my "fading vs. fast out LED" question. I've since used both preamps in sessions, with 48v, and nothing seems awry. But still, the different behavior unsettles me.

Also, I have two RevA boards. I'm thinking of drilling the faceplates for a 48v indicator LEd, because it's a nice feature, and I like consistency. Looking at the RevA and RevB schematics, it looks like the LED can be introduced to a RevA board pretty easily. Just a 10k resistor prior to the LED, terminated in parallel to the chassis and to ground via a 10R .5W resistor.

Looking at it again, perhaps the short to ground from the switch's off position is making the difference between the two LED's behavior. I'll have to check the actual board to see.

 

[Terryb]

Jeff, I was following these instructions on my own build and encountered loss of signal at the + of c10.

*******

Next, check the + side of C10. This is the output of the opamp. The gain pot (or switch) should effect the level of the signal probed here.
*****

Signal was fine on pin5 of 2622.

I tried to reflow the c10 joint a couple of times, but still no signal

 

[jsteiger]

Jeff, I was following these instructions on my own build and encountered loss of signal at the + of c10.

*******

Next, check the + side of C10. This is the output of the opamp. The gain pot (or switch) should effect the level of the signal probed here.
*****

Signal was fine on pin5 of 2622.

I tried to reflow the c10 joint a couple of times, but still no signal

That to me would make look at the opamp.

 

[Terryb]

I did not have an opamp inserted when testing with the audioprobe.

 

[jsteiger]

Aha, that is why the audio stopped at the opamp. You will need to install an opamp to continue testing.

 

[Terryb]

Does it need to be powered in order to check the stuff post-opamp?

And if so, where can I find such a unit?

Thanks for all your help.  I feel like I'm really close to figuring this out.

 

[jsteiger]

Yes indeed. You will need a 500 series rack of some sort or a bench type PSU.

 

[Terryb]

Semi-success!

I plugged the vp26 into my 500 rack and then ran that into a pa.  I'm definitely passing audio and the gain pot, mute, and -15db pad are all working fine.

However, when I turn the attenuating pot I don't notice any difference.
Should I?

 

[jsteiger]

Yes, fully CW will be no attenuation. Turning fully CCW will basically pass little to no signal. Guys have forgotten to solder all the pins before.

 

[Terryb]

Ok, I also noticed my phase switch is not working.  When I press it, all audio cuts out.

So, i reflowed solder to switch-2 pads and rv1 pads.

Still don't work.  Is there a component that is upstrean of both that would not affect switch 4?
I will also continue combing this entire thread to see if anyone has had the exact same issue

 

[Terryb]

Jeff, disregard that last post, I figured out the problem.

Basically, like most other issues, it was an oversight on my end.  I hadn't soldered the 3 posts on SW4 furthest from the other switches.

Thanks so much for your time and patience.  I will absolutley continue buying your kits and hopefully get good enough at building them so I won't have to bug you with my ******* mistakes.

Thanks again!

 

[drew571]

I've put together several of the VP26's now and on one of them I'm having a problem.  It passes audio fine, all the knobs/switches work fine, but there is a very low level "clicking" or "popping" that is happening at a constant rate.  Like a 16th note.  Here's a link to the file.  It's subtle, but there.

https://soundcloud.com/drew571/vp26-clicking

I switched out the opamps to eliminate that issue.  The one I built was a gar2520, and I also tried a Red Dot.  Issue happens with both of them. 

Any other suggestions for what could be wrong? 

Thanks,
Drew