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

[blackbird]

Dave,

the measuring part is just about cmrr,not more.The guide is printed even on the pcb itself,not joking.Pretty cool.But that's a part you can do later.First wait for the jig to get the module attached outside the rack.Way easier then.Once you have it trace through to the test points and see where the sognal stops.
Do you know how to do it?If not we will guide you through it,I'm sure you will make this baby work,it's really worth it (I know what I'm talking about because I have eight!).

Be patient and have a nice weekend,

best regards,

Udo.

Hi Guys.

Ive got my extention jig up and running, and ready to do the test points.
Do i need any signal going in for the tests? What do i use for ground when testing the points.
Where do the gain and fader knobs need to be?

Thanks!
Dave

 

[kante1603]

Thanks again Udo!

I did see some tips in section 12 of the Vpx build guide. I don't understand what a lot of that means, but I'm starting to figure it out. Is it safe to assume that it applies to the VP28 as well? Anything additional or different for the VP28?

You're welcome.

Had to read through this section 12 first.I "only" did the VP28s so far.
But yes,it seems to be right to do so.This is all about if the supply rails are o.k. so far.In case anything is shorted (zero ohms) or seems to be very lowish then something is definetely wrong.Fixing time then,don't try to attach the module(s) to the rack or a psu in this case.
The readings should be close to identical on both modules (you're building two,right?).

After powering up you can do a voltage check as described on the DOA sockets,so with none installed.

Again I highly recommend the extender cable,also for future use on other modules (you'll love I bet!).

Tell us what you get when done,

best regards,

Udo.

 

[blackbird]

A few more developments...

Ive managed to get a signal through now.. Its a bit dodgy though.. I have to have the phase switch in for it to work.

If i press on the pad when its out of the chassis, so putting downward pressure 'towards' the PCB, the signal jumps to life. It seems to be when im pressing on the Pad switch more so, but pressure anywhere around there seems to work.

Line input works well, as long as i have phase switch in. It drops 30 db with phase set to normal. When i put pressure on the pad when all is well, it jumps in volume a whole bunch, and distorts and thins.

Mic Input seems to do the same. Needs to have the phase switch in to work, and need to put pressure on the pad switch toward the pcb..

Ill check solder points on switch again right now.

Cheers
Dave

 

[kante1603]

A few more developments...

Ive managed to get a signal through now.. Its a bit dodgy though.. I have to have the phase switch in for it to work.

If i press on the pad when its out of the chassis, so putting downward pressure 'towards' the PCB, the signal jumps to life. It seems to be when im pressing on the Pad switch more so, but pressure anywhere around there seems to work.

Line input works well, as long as i have phase switch in. It drops 30 db with phase set to normal. When i put pressure on the pad when all is well, it jumps in volume a whole bunch, and distorts and thins.

Mic Input seems to do the same. Needs to have the phase switch in to work, and need to put pressure on the pad switch toward the pcb..

Ill check solder points on switch again right now.

Cheers
Dave

Yes,that looks like either cold joints or a broken switch.Reflow the joints with a little fresh solder tin/-flux and try again.

If that doesn't help measure if the switch contacts are working.

Best,

Udo.

Udo.

 

[blackbird]

A few more developments...

Ive managed to get a signal through now.. Its a bit dodgy though.. I have to have the phase switch in for it to work.

If i press on the pad when its out of the chassis, so putting downward pressure 'towards' the PCB, the signal jumps to life. It seems to be when im pressing on the Pad switch more so, but pressure anywhere around there seems to work.

Line input works well, as long as i have phase switch in. It drops 30 db with phase set to normal. When i put pressure on the pad when all is well, it jumps in volume a whole bunch, and distorts and thins.

Mic Input seems to do the same. Needs to have the phase switch in to work, and need to put pressure on the pad switch toward the pcb..

Ill check solder points on switch again right now.

Cheers
Dave

Yes,that looks like either cold joints or a broken switch.Reflow the joints with a little fresh solder tin/-flux and try again.

If that doesn't help measure if the switch contacts are working.

Best,

Udo.

Udo.

Thanks udo. Done.
The pre is working almost as it should now.
Few things..
When I use condenser mics, so 48v on, the level drops off. The signal is clean and nice, just a volume drop compared to where it should be.(checked against other pres).
Also, when I engage phantom, there's one hell of a thump that goes through the system.

Any ideas?

Cheers
Dave

 

[kante1603]

Hi Dave,

always turn down your monitors when switching phantom on or off-I mean that!

What do you mean with the mentioned level drop "compared" to other preamps?How can you compare it with no technical readings/measurements or even no printed scale on the gain switch?
Physical pot positions are mostly very different on different preamps.
It also might be a question of the input impedance etc........

Can you confirm the unit has unity gain?And have you done the cmrr adjustment so far?
For unity gain check just apply a known signal,say a sine 1kHz at 0dBu,set the mic switch to "off",output fader to zero or middle position and the gain switch to "unity"(blue marking).
You should get the very same output signal.This will confirm the unit is working correct.

Happy you solved the switch issue btw.,best,

Udo.

 

[blackbird]

Hi Dave,

always turn down your monitors when switching phantom on or off-I mean that!

What do you mean with the mentioned level drop "compared" to other preamps?How can you compare it with no technical readings/measurements or even no printed scale on the gain switch?
Physical pot positions are mostly very different on different preamps.
It also might be a question of the input impedance etc........

Can you confirm the unit has unity gain?And have you done the cmrr adjustment so far?
For unity gain check just apply a known signal,say a sine 1kHz at 0dBu,set the mic switch to "off",output fader to zero or middle position and the gain switch to "unity"(blue marking).
You should get the very same output signal.This will confirm the unit is working correct.

Happy you solved the switch issue btw.,best,

Udo.

Thanks udo.

The level drop wasn't explained well. Sorry bout that. I was swapping between a dynamic and condenser, making sure all was well with both phantom and non phantom mics.
The re20 was louder than the C414 on the vp28 with equal gain, whereas on the API 4312 it was the other way round. I tried on the TG2 and Mercury m72 as well, as I thought the mic not be getting enough power.

I'll do the test again tomorrow, but test against all preamps I got.. the c414 isn't a mechanical switch for the pads and polar patterns. It has a led that shuffles across the options. It was kinda like the pad was stuck on.. I'll do more tests.

The white noise/ sub blast when switching on phantom isn't normal. I'm sure we all accidentally do it occasionally when monitoring at a decent level. With all my other pres, the spike is there, but not too scary.. With the VP28, it's a whole bunch louder, and takes a good few seconds to settle down. Not sure how loud it spikes, as my converters have a soft clip that hide the true peak of it.

It does run unity fine. I had a signal generator running outta tools, through VP, and back again. The levels were correct.

What is the cmrr adjustment? I can see it mentioned here n there, but struggle to work out what it is, what it calibrates exactly, and curiously, what it stands for. I'd love to know.

I'll post tomorrow after I've played more..

Thanks guys!

Dave

 

[kante1603]

Dave,forget about the cmrr ( which is common mode rejection ratio btw.).This Preamp doesn't seem to have a trimpot for it,my bad,excuse me.I think I swapped it with the LC53 or so.....

About phantom again:
Be aware of what you're doing.Don't switch it on if not needed,only with powered mics and d.i. boxes etc.
Of course it could happen but......never ever listen to it.
Basic audio rules,guess what might happen if you have bazillion kilowatts P.A. system attached instead of your monitors......

If and how loud it pops depends on the input load meaning what mic. or D.I is plugged in.,how the preamp input is designed,if it is a ramping phantom power etc.,and last but not least cmrr on input:If it is too bad a dc offset will accompain your audio.
Lots a variables,I know....but easy to handle with monitors down or off.
If it sounds right it is right,and you confirmed the unity gain test,pretty good.

If it is way too loud then something is wrong.My first spot to look at would be the two 6,8 k resistors and measure them if they are closely matched (or a resistor has the wrong value).They should be in a range of 0,1% or better.To do this lift one side of them each (R20 & R21 on the main board) to not have them in circuit and measure them.

I don't have my modules handy here nor schematics.At this point Jeff might chime in?

Best,

Udo.

 

[tonycamp]

i've forgotten to turn down my monitors and gotten a pretty gnarly bump when engaging phantom, but i normally turn down no matter what preamp.

 

[blackbird]

i've forgotten to turn down my monitors and gotten a pretty gnarly bump when engaging phantom, but i normally turn down no matter what preamp.

Hey guys.

I agree it happens with all preamps. I do turn down my monitors when engaging phantom (most the time).. All my other pres (API, vintech, RCA, mercury, heritage, chandler), just make a click sound. It's loud, but it's just a click, so it's ok. 
VP28 is a full scale burst that hangs around for about 3-4 seconds. It just makes me feel like something isn't right. The noise is a sub blast, with a white noise burst. Even with the monitors quiet, it puts the fear in me.

Do all you guys have the same sound when engaging phantom on VP28?

Cheers
Dave

 

[kante1603]

Dave,

can you tell us if this happens with nothing attached to the input or maybe the same microphone plugged in?
Also did you check the phantom resistor values?
It comes to my mind that it might be the load (your mic,the 414?) because there should only be a short(!) pop sound,not lasting for seconds and having a noisy tail.Looks like something is not charging correct.
Since it is a thing about time a capacitor could be faulty.
Try another condenser mic if you can and see if it is the same behaviour.This way we can sort out wether it is the mic or the preamp.

Tell us what you get,

Udo.

 

[Commander Fluffypants]

I ordered the extension card and in the meantime I built one 2520. I think I did well.

 

[kante1603]

I ordered the extension card and in the meantime I built one 2520. I think I did well.

Cool,but don't you need two DOAs? ;D

Best,

Udo.

 

[blackbird]

Dave,

can you tell us if this happens with nothing attached to the input or maybe the same microphone plugged in?
Also did you check the phantom resistor values?
It comes to my mind that it might be the load (your mic,the 414?) because there should only be a short(!) pop sound,not lasting for seconds and having a noisy tail.Looks like something is not charging correct.
Since it is a thing about time a capacitor could be faulty.
Try another condenser mic if you can and see if it is the same behaviour.This way we can sort out wether it is the mic or the preamp.

Tell us what you get,

Udo.

Hey Udo..

So, it happens with both my 414's. Its alot more subtle with my other condensers.
With no mic plugged in, its completely quiet when I switch phantom on and off.

I also just put a multimeter on the mic lead going in with phantom engaged, and read 55.7volts in the hot and cold pins. And 0 in the earth..

55.7 seems a bit too far from 48v, or is that within normal tolerance.

Cheers
Dave

 

[kante1603]

Hey Udo..

So, it happens with both my 414's. Its alot more subtle with my other condensers.
With no mic plugged in, its completely quiet when I switch phantom on and off.

I also just put a multimeter on the mic lead going in with phantom engaged, and read 55.7volts in the hot and cold pins. And 0 in the earth..

55.7 seems a bit too far from 48v, or is that within normal tolerance.

Cheers
Dave

Hi Dave,

Holy cow!

55,7vdc is too high,definition for phantom power is 48 vdc +/-4.
It seems the mic tries to bring the overvoltage down,the noise tail might come from caps charging/discharging.

What kind of rack is it or what psu is the supply voltage?
Do you have specs for it or built it yourself?
Please measure the dc without any modules in the rack directly on a multipin connector.
Pins are 5 and/or 13 for 0vdc,pin 15 for +48vdc for phantom.
Here's the pinout data as a proof:

http://51xaudio.com/alliance/pinout.html

Be careful not to short pins doing it.
Easiest with an extension cable of course,but reachable in an empty rack.

So far I'd stop using any phantom powered gear on this rack/psu until this problem is solved.

Very curious now,

Best,

Udo.

Btw.,I don't believe in any issues with your VP28 anymore,seems up and healthy,cool!

 

[tonycamp]

just to reinforce, no matter the mic, my vps make a quick pop when i engage phantom with the monitors turned up, no where near 3-4 seconds, somethings not right there Dave, sounds like udo's got you steering in the right direction.

 

[kante1603]

just to reinforce, no matter the mic, my vps make a quick pop when i engage phantom with the monitors turned up, no where near 3-4 seconds, somethings not right there Dave, sounds like udo's got you steering in the right direction.

Yes,should be a short spike only.I told that before,I have eight of them,all do behave this very same way.
Thanks for confirming!

Best,

Udo.

 

[blackbird]

Hey Udo..

So, it happens with both my 414's. Its alot more subtle with my other condensers.
With no mic plugged in, its completely quiet when I switch phantom on and off.

I also just put a multimeter on the mic lead going in with phantom engaged, and read 55.7volts in the hot and cold pins. And 0 in the earth..

55.7 seems a bit too far from 48v, or is that within normal tolerance.

Cheers
Dave

Hi Dave,

Holy cow!

55,7vdc is too high,definition for phantom power is 48 vdc +/-4.
It seems the mic tries to bring the overvoltage down,the noise tail might come from caps charging/discharging.

What kind of rack is it or what psu is the supply voltage?
Do you have specs for it or built it yourself?
Please measure the dc without any modules in the rack directly on a multipin connector.
Pins are 5 and/or 13 for 0vdc,pin 15 for +48vdc for phantom.
Here's the pinout data as a proof:

http://51xaudio.com/alliance/pinout.html

Be careful not to short pins doing it.
Easiest with an extension cable of course,but reachable in an empty rack.

So far I'd stop using any phantom powered gear on this rack/psu until this problem is solved.

Very curious now,

Best,

Udo.

Btw.,I don't believe in any issues with your VP28 anymore,seems up and healthy,cool!

Hey udo..

So you feel the problem is more likely to be the power supply? Does the VP28 just directly route the 48 v from it's own power supply? I'll check the phantom power voltage on the heritage audio pre and see what's going on there. Silly I didn't check it before I left for the night. Will check tomorrow.

The power supply and rack is a 8 slot Radial workhorse.
I have the extender card, so can do all these tests asap.

Thanks for your help guys!

Cheers
Dave

 

[kante1603]

Hey udo..

So you feel the problem is more likely to be the power supply? Does the VP28 just directly route the 48 v from it's own power supply? I'll check the phantom power voltage on the heritage audio pre and see what's going on there. Silly I didn't check it before I left for the night. Will check tomorrow.

The power supply and rack is a 8 slot Radial workhorse.
I have the extender card, so can do all these tests asap.

Thanks for your help guys!

Cheers
Dave

Hi Dave,

I took a quick look at the Workhorse.So it has an external psu,right?
Don't know it o far,maybe it has a trim pot for phantom?
Seems like it is an smps....I don't trust them anyway due to bad experiences,I always build linear types.

When checking phantom on other preamps remember that without anything plugged to the input the measurement might be a tad higher because of the higher dmm input impedance.It should go down with a load,e.g. a condenser microphone.

Could happen that you must contact Radial Engineering.

Best,

Udo.

 

[Commander Fluffypants]

I ordered the extension card and in the meantime I built one 2520. I think I did well.

Cool,but don't you need two DOAs? ;D

Best,

Udo.

Well, FOUR, but I'm taking my time. Wanna get it right. Funny, the 2520 didn't really seem that hard. I just went slowly and carefully.