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

[jsteiger]

Remove A2 (4558 chip) from that pre and measure again.

 

[jakmusic]

Jeff I'm going to ask another question about measuring with my dmm before I reply about removing the 4558 chip. 

Here's something I just did, and I don't understand why it happens.
My DMM reads
O/V- 16.5
O/V+ 14.8
If I hold the probes on the O/V- it will read 16.5V then drop as long as I hold the probes there. I held the probes there until my DMM  read around 14v Then I switched the probes to O/V+ and the voltage was around 17V  (but it was around 14V when I first measured it minutes ago).  By holding the probes on O/V-, am I changing the amount of electricity stored up in a capacitor and that's causing the O/V+ voltage reading to change? 

 

[Terryb]

Jeff, can you recommend a bench psu for testing preamp, opamps, etc?

 

[jsteiger]

Terry, I have a JLM 5 rail Powerstation that I use.

I also have a new 51x extender card kit that I need to post to the store. You can use it with any 500 or 51x series rack. I have everything here for it, just need to get pics and get it set up. I already sent one to 'Chung so he is gonna get a few build pics even though it is a simple as can be. I posted a pic on FB a few weeks ago.

 

[kante1603]

Hi Jeff,

yes,a powerstation I had left over was used on my little testbox too.
This way one has the "main" 5 voltages handy all the time.
Ideal for testing and adjustment,perfect when doing lots of modules,hahaha.......
For my racks I have yours and Volker's "real" psu stuff (Floorbox and 2 dual psus).
About the new extender card you're talking about,is there a link you can give us non-FB- members?

Thanks in advance,

Udo.

 

[jsteiger]

Here it is Udo.

 

[kante1603]

Great stuff Jeff!
The first "real" extender card,very nice indeed!
When I did my first extender cables I used the protoboards,but they were hard to plug in when say just 1 module slot was open.

The switch and resistors,are they for cmrr (hard to read this printed instructions on the board)?

Cheers,

Udo.

 

[jsteiger]

When I did my first extender cables I used the protoboards,but they were hard to plug in when say just 1 module slot was open.

Yes, this one is easy with only 1 slot open. Plus it's simple for 15 or 18 pin card edge connectors.

The switch and resistors,are they for cmrr (hard to read this printed instructions on the board)?

Yes, exactly. Simple instructions for CMRR calibration are on the board. All parts will be included with the kit. There is a status LED for the CMRR switch that can't been seen in this pic.

 

[kante1603]

Wow,cool!

Udo.

 

[Craig32768]

Hi Guys - just built my 2nd VP28 and I have a strange issue. I wonder if anyone else has come across anything similar so I know where to troubleshoot.

Only the Phase LED lights up when buttons are pressed. And - with the phase button pressed and the LED lit, it you press any of the other buttons - 48v / Pad etc - the Phase LED goes out.

Anyone had a similar experience / thoughts on the likely culprit?

Thanks in advance. Craig

 

[jsteiger]

All of those status LED's are in a string so if some are backwards, this will happen. Only the working one is the right way around.

 

[Craig32768]

All of those status LED's are in a string so if some are backwards, this will happen. Only the working one is the right way around.

Hi Jeff - thanks for that. I've just plugged in a Dynamic mic and something is not happy. I've attached a link to an MP3 - First part is the VP28 that's fine. 2nd part is the one with the non-lighting LEDs.

https://soundcloud.com/orange-orb/vp28-issue

I'll take a look at the LEDs now but I'm pretty sure they're in the correct way.

Thanks for your support.

 

[jsteiger]

Well, the direction of the LED's will have nothing to do with the audio path so there must be something else going on.

 

[Craig32768]

Well, the direction of the LED's will have nothing to do with the audio path so there must be something else going on.

Yeah - checked the LEDs and they're all in the correct way. Any likely areas I should be checking on the board that could cause this type of issue Jeff? - I'll take the metal work off and take a quick look for solder bridges etc, but this was a cleaner build that the first one that works ironically! 

 

[jsteiger]

Well, the LED thing does not make sense to me. The string starts at the lowest LED which is phantom and ends with the polarity LED. When a switch is out or disengaged, it is merely shorting out that particular LED. When the respective switch is pressed, the LED is introduced into the string. So, with the last LED (polarity) illuminated, if pressing any of the lower switches causes the polarity LED to go out, they are in backwards. I can't think of any other reason that this could happen.

As for the audio, start with the usuals. Rack slot, cabling, converter settings and then to swapping the opamps and general trouble shooting that is above in this thread.

 

[Craig32768]

Well, the LED thing does not make sense to me. The string starts at the lowest LED which is phantom and ends with the polarity LED. When a switch is out or disengaged, it is merely shorting out that particular LED. When the respective switch is pressed, the LED is introduced into the string. So, with the last LED (polarity) illuminated, if pressing any of the lower switches causes the polarity LED to go out, they are in backwards. I can't think of any other reason that this could happen.

As for the audio, start with the usuals. Rack slot, cabling, converter settings and then to swapping the opamps and general trouble shooting that is above in this thread.

Hi Jeff - found the issue I think.

Firstly there was some solder from the LEDs that had bridged a couple of the pushbuttons. Fixing that solved the LED issue.

And I swapped the op-amps from the working VP28 and it looks like a 1731 was causing the distortion. So just need to have a look at that 1731 and see what the issue could be.

Thanks again for your help. Nearly there!

 

[jsteiger]

Cool. Glad you are making some progress!

 

[Craig32768]

Cool. Glad you are making some progress!

Yes! progress but then another glitch. Built the replacement 1731 and now the 2nd VP28 works perfectly. However when swapping the opamps out of the first VP28 (Which was working!) to try them in the 2nd something must of happened. As the 1st VP28 lights up but there is no signal passing through it whatsoever. Don't even get the quick green flash of the signal LED with switching mic in and out while the phantom is engaged or when the lunchbox powers up.

I thought there may be a cold joint on the opamp pins on the PCB so re-did though but nothing.... bummer. I've got a Sound Skulpture Kit on its way and I ordered an extender cable as I guess this will be the only sensible way I can check for cold joints in the signal path.

Anyone know the signal path through the board that I could check to see if I have dodgy joint? - I can't see any damaged components.

Oh well - Enjoyment and frustration at the same time. I tracked some vocals today with the working unit and it was amazing. Can't wait to get this one up and running (again) now.

Cheers
Craig

 

[New Soul Rebel]

Well, when you have phantom on, there is usually a pop or thump when engaging the standard pad. After all, when switching DC there will be some popping. It has always been a standard practice to engage phantom and the like with a pre's output muted by some means or not connected, It really can also be said for the mic/line switch.

Except that there must be something wrong... as I don't get this thump at all on my other VP28's.

Its an almighty thump Jeff... I know what you mean about a pop or thump when engaging mic lines or pads, but this is an unusually big thump! Any other ideas what might be causing this. Like I say, the pre works perfectly in operation. But I want to resolve this.

Hi Guys

:-[

I STILL haven't managed to resolve this thump that happens when engaging the mic button.

the vp28 works perfectly once switched... but it is an almighty thump. not a bump that you get like when switching on phantom by mistake. this is a thump.

the peak meter on the motu HITS the roof when I switch the damn thing on.

I've got two good opamps in the unit. so its definitely not an opamp problem.

Matt

 

[iacntspell]

Hey guys, quick question.  I built two VP28s (well, I built one and a friend built the other).  Everything appears to be in working order, but when I was testing them out, I decided to really try to crank them and was getting some weird results.  I was using a pair of condensers, so phantom was on and mic level engaged.  I wasn't using the pad, and I had the input gain cranked to almost max and the output gain pulled way back.  At low signal levels, things were alright, but as soon as I got loud or there was a transient (clap or tapping the grill of the mic) I got a really nasty chirping sound.  Zoomed in on the waveform and I've got a very nasty looking square wave going on.  Is this just the preamp getting overdriven because I've got it cranked so hard, or is something wrong with the build?  This is happening on both preamps which were built by different builders, so I'm hoping its not a failure in the builds.  At more standard operating levels, particularly with the pad engaged, things seem to be fine (but more testing is in order).  I wasn't clipping at the converter stage, to my knowledge. I suppose there could've been some inter-sample peaking going on, but I was being pretty careful to not overdrive the converters.  Thanks, guys.  Fingers crossed!