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Should one preamp affecting behaviour of others in my lunchbox?

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Should one preamp affecting behaviour of others in my lunchbox?

Postby peddlebreaker » Fri Feb 28, 2020 1:03 pm

Hey folks, we have a lunchbox at the studio which houses 6 x preamps. We have 1. API 512, 2. CAPI VP26, 3 & 4. A designs P-1's & 5 & 6. Neve 1073's.

Love em all, they are smashing, however, I noticed something rather odd yesterday. The signal meter lights on the Neve's were on all the time as soon as I turned on the lunchbox unit. I faffed around and found that when I turned on the phantom power of the CAPI preamp (in slot 2) and messed with the gain it would directly affect the Neve preamps and the signal light would come on and off. What's happening here? Some srt of cross-talk between the clots within the lunchbox? Is the lunchbox (API original 6b) not powerful enough for all these preamps?

I have not noticed any degradation in signals going through any of the preamps by the way.

Many thanks in advance
Benny
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Re: Should one preamp affecting behaviour of others in my lunchbox?

Postby CS70 » Fri Feb 28, 2020 1:08 pm

No they shouldn’t, unless you’re drawing more power than the lunchbox supply can provide?

I’ve heard of this kind of interactions and they often are down to a faulty module.

You can debug by removing all but the neve and then reading modules one by one.
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Re: Should one preamp affecting behaviour of others in my lunchbox?

Postby Hugh Robjohns » Fri Feb 28, 2020 1:15 pm

Ooh... intriguing!

Lots of possibilities for that kind of behaviour, but I think the most likely is noise (or HF oscillations) on the power supply rails, and that could indicate a fault either in the rack itself, or in the CAPI or Neve modules.

If the CAPI and Neve modules were adjacent it might have been direct capacitive crosstalk but you appear to have other modules in between, so it's probably not that.

Have you checked the current requirements in the module specs and made sure that both the total for all six modules, and each individual module, is within the limits for the Rack? Those early API racks were/are quite restricted compared to more modern designs.

But I think Cristian is right, I'd take out and replace the modules in an experiment to see whether it is just the CAPIs that interfere... and whether it's in all slot positions or just some.

I think my gut reaction is that one or both CAPIs are faulty -- HF instability in DOAs is not uncommon. Were these DIY builds or ready-made and tested?

H
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Re: Should one preamp affecting behaviour of others in my lunchbox?

Postby Sam Inglis » Sun Mar 01, 2020 11:28 am

Is it an actual API lunchbox, or another manufacturer's? I seem to remember we had a problem at the studio when we added a second Neve 1073LB to a third-party lunchbox. Not sure if it was the power draw that was the cause but it was fully solved by upgrading to a bigger and better lunchbox from a different manufacturer.
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Re: Should one preamp affecting behaviour of others in my lunchbox?

Postby peddlebreaker » Sun Mar 01, 2020 9:24 pm

Hey folks, yes I think it is strange that it is affecting both of the Neve's specifically. The only other preamp in the rack with a level signal meter is the API 512 however, the CAPI is not affecting that preamp.

It is an original API Lunchbox 500-6B, I have had it for around 5 years now. I am not entirely sure how many mA's that unit dish out. It would be handy if there was a list detailing the power consumption of all the 500 series modules.

Admittedly I do not know the much about electronics egineering, this is probably the steepest and longest running learning curve I personally face that is associated with the 'craft' fo sound recording, I feel like the problem could well lie with the hand built DIY CAPI VP26 pre that I bought from a fella on Ebay. I may contact him to ask him if he can tell me more about the design and build. Could there be a way to regulate/decrease the current draw from that module?

Luckily we are fortunate to have a man that can in this area locally, so I should probably ask him to take a proper look at it altogether.

If the lunchbox is a bit pathetic at powering the format of device it was built to power, what would be a better solution that has more power?

Thanks so much for your replies, it's always super helpful to me!
Kindest
Benny
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