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

[SoundOfRolf]

May I guess: the sine wave / pink noise signal generated in your DAW is Mono, and cancels out.
But the music is a stereo signal (at least I see stereo tracks in the screenshot)?
Low end is often near mono, this is why it cancels out much better.

Nope. Pink or white won't cancel either.. The audio file is stereo. Track is mono. Feeding mono into the pre.
And I'm talking about the cancellation of recorded phase on and phase off tracks. Not canceling to the original track.
So no user error there. I first noticed this using just a mic and the sound difference was big.

Well no can do.I'll take them apart and solder the joints the third time


Thanks for the input!

-Tony

 

[bulley]

I think I may have put some resistors in the wrong place. Now...it's going really well, built a 2nd one, everything works, no funny noises.....except the gain pot doesn't do anything, only a slight issue. Any suggestions for that?

Most likely a bad solder joint somewhere between the 200R, the 330uF cap and the pot itself. Not seating the opamps fully can also be the cause.

Hi, I've resoldered all those bits and checked the op-amp, also tried different op-amps. any suggestions?

Thanks,

Phil

 

[jsteiger]

Hi, I've resoldered all those bits and checked the op-amp, also tried different op-amps. any suggestions?

Post a pic or 2 of the top of the PCB so I can see the colors on the resistors.

 

[bulley]

Hi, I've resoldered all those bits and checked the op-amp, also tried different op-amps. any suggestions?

Post a pic or 2 of the top of the PCB so I can see the colors on the resistors.

Here's one....

 

[bulley]

and the other...

 

[jsteiger]

The R's look correct as far as I can tell in the pics. Either the gain is really low or really high. Do you know which?

What is your test rig?

 

[bulley]

The R's look correct as far as I can tell in the pics. Either the gain is really low or really high. Do you know which?

What is your test rig?

It's a low gain setting, although the output is 6dB quieter than the other one I've built.  I think there is a dodgy connection in there somewhere, the signal occasionally dies and bumping the level of the test tone up and down brings it back to life. Not much of a test rig, just a multimeter and DAW. I'll sort a way of doing a signal inject.

Thanks.

 

[jsteiger]

Sound like a bad solder joint. That is still my feeling for why the gain pot is not working. There is really not much to that part of the circuit. Are you sure the middle 3 leads for the gain pot are soldered? You would not be the first to have forgotten this.

 

[SoundOfRolf]

OK. Phase button problem described in post #716 FIXED !!!

The error was so stupid I’m ashamed to tell you, but if this helps anyone in a similar situation this might save them some time, sweat and tears.

I first noticed the HUGE difference in sound depending on the phase testing with a mic and HEADPHONES. That’s the clue. The phase change cancels audio inside your head!!! I’d never thought of that. I’ve worked in studios for over 20 years and never ever tried changing phase with my own voice and headphones on. There’s never been need for that. We’ll after some great help from Gunner666 and studying the matter that was it. Live and learn. BTW different phase can help singers performance as they hear themselves better.

Well how does this explain the phase canceling effect I clearly was able to record?
Because I was sure I had made a mistake I never thought about an other more plausible reason. Now I’ve tested several mic pres high and low end stuff and they all have this same feature. Some more than others. Then it hit me. It’s the bloody self noise that’s never the same even if your audio is……

So sorry for the trouble.
Again great pre, great documentation, super support.

Next time I’ll think before I’ll post.

Yours,
Tony

 

[bulley]

Sound like a bad solder joint. That is still my feeling for why the gain pot is not working. There is really not much to that part of the circuit. Are you sure the middle 3 leads for the gain pot are soldered? You would not be the first to have forgotten this.

Ha, my stupidity is probably at the root of all this but I did remember that bit. I think you're probably right though, I did a resolder and the gain pot works occasionally now so I just need to get it right

 

[Andersson]

  Hi! I just finished my first ever build (VP 26) and I'm having a bit of trouble.

  The fuse on my power supply blew twice when I tried to test my new preamp. I haven't used anything in the lunchbox before so I don't quite know if it's an issue with the supply or the preamp.  The power supply/lunchbox is a Fredenstein Bento 6.

  I couldn't find any visual evidence of shorting (no smell either) and my solders look ok and without bridges.Is there anything you would suggest that could be the cause of it in case the problem lies with my build ?

  Other than that I had a great time putting it together and would like to thank Mr. Steiger for making me take the leap into DIY'ing! 

  Cheers!

  Andersson

 

 

[jsteiger]

What is the opamp and who built it? Is it installed correctly? http://classicapi.com/catalog/DOA_Install.php

 

[Andersson]

  Thanks for your reply!

  It's the pre-built 2520 Red Dot. It's seated like the install guide says it should be. I have another unbuilt VP 26 kit with another Red Dot DOA that I will try as soon as I can get my hands on new fuses for the lunchbox.

 

 

 

[jsteiger]

If its a red dot then the opamp is most likely not the issue. I would think that only a direct short on the build would be tripping the PSU fuse. With the module loose, remove the opamp and check DCR from +V and -V opamp sockets to ground.

 

[Andersson]

  I went through the testing list yesterday and went through the whole list again right now and all measurements are above 200K Ohms. I don't have any power supply to complete the test list.

  I'll get new fuses today and I'll see if I can borrow a 500 series pre from my local music gear shop. That way I'll at least be able to find out wether it's my lunchbox or my build that's at fault.

  Thanks again and I appreciate all the help! 

 

 

[Andersson]

  FIXED! 

  I got some new fuses and voila! Preamp works perfect with both DOAs and sounds great! Tested on different channels in my lunchbox to be sure and no issues what so ever!

  I guess I'll just have to do a shootout with the Lindell Pre-82 I borrowed from the local gear store.

 

I first noticed the HUGE difference in sound depending on the phase testing with a mic and HEADPHONES. That’s the clue. The phase change cancels audio inside your head!!! I’d never thought of that. I’ve worked in studios for over 20 years and never ever tried changing phase with my own voice and headphones on. There’s never been need for that. We’ll after some great help from Gunner666 and studying the matter that was it. Live and learn. BTW different phase can help singers performance as they hear themselves better.

  You just made my life so much easier with that post (I'm a singer who records himself). Thanks SoundOfRolf!

  On top of it all, I got a message that my VP-26 Litz's have arrived aswell! This is the best weekend ever! 

  I forgot to thank Chunger for his wonderful guide that I used extensively for this build. Thank you Chunger!

 

[jgmille]

Hey everyone, brand new to this board and the world of DIY mic pre's.  Just finished a VP26 with the Gar op amp.  Feel pretty good about my build and plugged it in with smoke and a burned out resistor in the R14 placement.  I may have had a short created with the solder from the op amp millmax pin that is right next door to it. 

Really not sure how to proceed with testing, etc - I have zero experience troubleshooting this type of thing.  Any step in the right direction would be appreciated.

Jason

 

[jsteiger]

R14 on the VP26 PCB smoked? The 1/2W 10R? That's kinda strange.

There are a few tips in the first post of this thread. I would start there.

 

[jgmille]

thanks Jeff,  I double checked and its the R14 10r.  I'll check the whole thread more thoroughly tonight.

Jason

 

[jgmille]

actually to clarify, its the R14 spot on the GAR1731 op amp..