Alternative input coupling cap suggestions for Supergreen preamp?

[kato]

Hi folks, I just stuffed a couple of old Supergreen preamp PCBs, but the input coupling caps are too tall to fit in my 1U enclosure.

(They're 700v 10µF Vishay polypropylene film caps.)


I want to rack and test them now without ordering new film caps. Would it be better to use:

• 10µF electrolytics, with the anodes oriented toward the input jacks (as in the Green mic pre.)
• or 2µ2 polyester film caps? I'm not sure how much low-end I'll be losing.

Thanks for any suggestions!

 

[Rob Flinn]

Do you need 700v versions of these caps ? Look for a lower voltage version & it will likely be smaller. Maybe something like this 1x 10uF 5% Metallised Polypropylene Film Capacitor 250V HiFi Audio Axial Speaker | eBay

 

[kato]

Do you need 700v versions of these caps ? Look for a lower voltage version & it will likely be smaller. Maybe something like this …

That's a great suggestion Rob, thanks. I suspect 3v would be sufficient. 700v = overkill.
I ordered based on the 37.5mm lead spacing; didn't consider the height at all.

It would be nice to box them up and test without ordering any new parts. But I'll pursue your suggestion in the long term. Thank you.

 

[kato]

For $12.50, I can get thousand layer cake. That's the correct specification for SuperGreen, right?

 

[kato]

When doing the capactative reactance calculation, do I assume 10k for the resistance? (The input impedance?) Or the 56k to ground after the capacitor?

 

[Rob Flinn]

There is no R in the capacitive reactance formula (XC = 1/(2πfC) ) therefore I'm not really sure what you are talking about. Also if these are the input caps to the mic amp. i.e where the mic plugs in then 3v caps would probably not be sufficient if they have to block the 48v phantom power, but since I don't have the diagram & you haven't posted it I don't know.

 

[Khron]

If it's a mic preamp, 1k would be a lot closer to the "usual" input impedance (needed for figuring out the expected high-pass frequency). 63v caps are pretty standard, and just fine for blocking the 48v phantom power voltage.

 

[kato]

Thank you for the advice, friends!

I've replaced them with 250v film caps that will allow me to close the lid.

They're only 2µ2, but I'll do a sweep to see how much low-end I'm losing.

(And thanks Rob for the correction. I'm still learning, slowly from youtube videos.)

 

[rankot]

What kind of opamps did you use? NE553x or something else? I was thinking to redesign the PCB so it can use only two dual op amps, but not sure why did they use two NE5534 for output? Perhaps to prevent cross talk?

 

[Matt Syson]

Is the 'supergreen' the mic preamp design that copied the mic input circuitry mr Neve put together for AMEK (first seen on the Mozart RN15 modules?) I think I had a copy of that supergreen schematic but lost it.

 

[Rob Flinn]

Is the 'supergreen' the mic preamp design that copied the mic input circuitry mr Neve put together for AMEK (first seen on the Mozart RN15 modules?) I think I had a copy of that supergreen schematic but lost it.

It's as below I believe but with the addition of a 4 led meter.

 

[Matt Syson]

Thanks Rob. Anyone spot the 'designed in' failure mode?

 

[Khron]

Thanks Rob. Anyone spot the 'designed in' failure mode?

Max (open-loop) gain if the wiper gets intermittent?

 

[Rob Flinn]

Thanks Rob. Anyone spot the 'designed in' failure mode?

Or potentially if one uses a BBM switch it's going into orbit between gain positions.

 

[Matt Syson]

Sorry, no, the gain would drop dramatically if the switch went open circuit and is actually fairly close to unity gain; The problem is that if you short the inout taking one leg or the other to ground, when phantom power is on you can damage IC1 which either gets noisy or burns completely. Mr Neve's solution was 'always switch phantom off before patching/plugging cables; My solution was a bit more practical and featured in the later incarnations.