• From: Houston, TX
  • Registered: 2007-05-11
  • Posts: 1,192

Topic: Possible to make appropriate -6dB input pads for the ADI-8 Pro?

This is likely an easy question for some of you, but I'd like to know what value resistors would be advisable for making a -6dB "H-Pad" (or whatever type pad would be appropriate here) that would be appropriate for padding the ADI-8 Pro's inputs.  I've tried using the H-Pad calculators available all over the web, but the resistor values get all crazy with smaller attenuation levels like -6dB, and I'm not exactly sure where to begin with the target impedances.

The Preamps to be padded are a slew of Seventh Circle Discrete modules that can output +25dBu and beyond IIRC (I think the N72 is +27dBu and 30v p-p), and are mostly transformer coupled outputs.  I want to get more transformer saturation and gain-stage color when desired w/o clipping the ADI-8's inputs.  IIRC, the Seventh Circle pres are happy with a load impedance of 600 Ohms or greater (the modules have internal 600 Ohm Jumpers for connecting to higher impedance inputs).  I'd prefer not to go too far beyond -6dB to -8dB  as then I will run into clipping well before I hit 0dBfs (I tend to track with conservative levels regardless).

I have a bunch of -14dB Mic pads (using 470 Ohm on each leg and a 220 Ohm across them as mentioned by a Neumann developer), but these are too drastic for a Line Pad (to much attenuation - can't get above -6dBfs w/o hard clipping), and I believe the impedance is not ideal for a line output involving a transformer.

I've seen that Little Labs offers the Attenuator 8-Pack, but it is very expensive, I'd need 2 of them, and I already have a bunch of empty Neutrik XLR barrels to make these pads from if the appropriate attenuation values and impedances can be obtained with such a pad...

Thanks for any help on this seemingly simple (but somewhat harder in practice) conundrum.  I have indeed done much research sprinkled among many years, but most online pad talk seems to be aimed at Mic applications, and don't seem offer the smaller attenuations that I'm after (if even possible).

Sorry to be so wordy (some might say "Thorough" fryingpan ).  You guys rock!!! cool

MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8

2 Reply by MC

  • MC
  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
  • Posts: 25,670

Re: Possible to make appropriate -6dB input pads for the ADI-8 Pro?

For transformer 'saturation' you should give it some load, so the -14 dB Mic Pads are basically ok. Just seem to damp a bit too much. I wouldn't investigate too much science here (the varying load will change the transformers output level, hard to add to the calculation) and simply replace the 220 Ohm with a 1,000 Ohm. That should work.

Regards
Matthias Carstens
RME

MC's Website

  • From: Houston, TX
  • Registered: 2007-05-11
  • Posts: 1,192

Re: Possible to make appropriate -6dB input pads for the ADI-8 Pro?

That was easy!  I will give that a shot.  What effect will replacing the 220 with the 1K have with regard to the amount of attenuation?

Thanks for your quick reply, MC! :-)

MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8

4 Reply by Randyman... (edited by Randyman... 2010-03-11 05:29:42)

  • From: Houston, TX
  • Registered: 2007-05-11
  • Posts: 1,192

Re: Possible to make appropriate -6dB input pads for the ADI-8 Pro?

Bump - Just curious on what the attenuation value will be (ballpark) if I replace the 220 with a 1K as you recommend?

I also got a reply from Tim at Seventh Circle (he makes the preamp kits I'm using), and he recommended something like a 150-300-150 pad.  There is obviously no set standard for audio pads - so which way would you go?  I see you mentioned you wouldn't want to load up the transformer with a very low impedance - so I'm assuming your 1K recommendation is a better option?

I can obviously make some of each type (470-1K-470 and 150-300-150) but I'd prefer to keep them consistent and make about 8-10 of these that are identical.

MC - Thanks for your time and expertise :-)

Edited for accuracy (MC poonted out the error in my resistor listings above)

MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8

5 Reply by MC

  • MC
  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
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Re: Possible to make appropriate -6dB input pads for the ADI-8 Pro?

Your values are wrongly quoted. The original -14 dB pad is 470-220-470 Ohm. Now take this and replace 220 with 1,000. Instead of 14.4 dB this gives 5.7 dB damping, which would fit the scenario described by you above.

A 150-300-150 is of course the same damping (ratio 1:2) as my proposal (470-1000-470), but gives more load to the transformer. Choose what you like :-)

Regards
Matthias Carstens
RME

MC's Website

  • From: Houston, TX
  • Registered: 2007-05-11
  • Posts: 1,192

Re: Possible to make appropriate -6dB input pads for the ADI-8 Pro?

Sweet!  I see that I mixed up my values above - thanks for the correction.  -5.7dB is purrr-fect.  I guess I'll go ahead and build some of each and see which ones I like best on the different modules.  Thanks so much for your time.  Hopefully this thread will help someone in the future with my same dilema.  I'll post back with my comparisons between the 2 options...

cool

MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8