need advice on H-pad optimum impedance with mics . . .

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strangeandbouncy

Well-known member
Joined
Aug 8, 2004
Messages
2,112
Location
West Sussex. UK
Hi Guys,

i am just about to build some in-line -20dB H-attenuators to pad down mic levels for some of my mic-pres, mounted in short XLRm-XLRf cables. I have all the relevent info to calculate the resistor values, but i am unsure whether to build -20dB H-pads at 200 Ohms or 600 Ohms, or at any other value for that matter, to give optimum performance. Mics in question are Shure SM57/8's,Sennheiser 421's,Yamaha MZ204/5's and Neumann U48's. ie no phantom required. Mic-pres include Helios, BBC/Glen, Audix, and my new home-made API312ish pre's with Neve (Marinaire/Belclere/Willesden) trafo's.

ANy help/suggestions would be greatly appreciated as ever,


All the best,

ANdyP
 
Don't use an H-Pad. Use a U-Pad. And H-pad is great for symmetrical impedances; but for a mic attenuator you want to reflect 1.5K-2K to the mic and 150-200 ohms to the preamp.

www.uneeda-audio.com/pads

There's a suitable 20dB switchable pad on the front end of my "One Bottle" preamp:

http://www.groupdiy.com/index.php?topic=6711
 
I built myself eight 20dB attenuators into those adaptor tubes with XLR male at one end and female at the other. U type attenuator with two 680 ohm and one 150 ohm resistors. They work perfect. :grin:

chrissugar
 
[quote author="chrissugar"]U type attenuator with two 680 ohm and one 150 ohm resistors.[/quote]

That's about 1.5K loading the mic. two 1.1K and a 220 is a little stiffer, and still looks like a low impedance source for the preamp's input.

Then again, chances are it'll make little difference in practice.

Peace,
Al.
 
hi,

I've just made up 6 XLR-XLR short leads with 1k/220/1k U-pads as suggested by NewYorkDave. Finally, I can patch any mic into any mic-pre, wherre before many were just too hot with gtr/bass cabs, toms, kicks and very hot output from U48's. :grin: I even tried some phantom powered mics. with Gefell UM70, voltage at mic is reduced from 44v - 38v, and the mic works fine! KM184 has 39v, and now can be used near a drum-kit(Why oh why didn't they include a pad, like the KM84?). U87 appears to have a reasonable voltage, but does not sound right to me(actually they NEVER sound right to me, but heigh-ho . . .). Being able to still use (some) of my phantom-powered mics is a great bonus! :grin:
just for the hell of it, I padded down the mix output today, feeding it through a variety of pres(Helios, my Neve-trafoed AAPi clones, and Pyes ) enabling a totally new "pallette" of colours . . . I'm in LURV . . . Looking for some extra "pizzazz" in da mix, build yourself some pads, and make use of those pre's that formally sat idle whilst in mix mode! Something for next-to-nothing(actually, 3 resistors and some solder!) Makes me want to build a "proper" passive mixer, rather than the lashed-up one on the patchbay! I think I'll use DOA's as buffer inputs, so I can patch any (Stereo) input in without messing up the balance(as happens at the 'mo).

God, I love this job! A year ago, I had scant few home-made toys. NOw, I have something new almost every month! They rank amongst my most treasured weapons in the studio, and with the knowledge,help, and confidance found within these hallowed walls, if I can imagine something I think might find useful, it is now probably within my grasp, AND within my pocket! I could never even have concieved my latest project, a dual mono/stereo, fully Class A, discrete SSL type mix compressor, switchable LR/MS . . . whatever next? perhaps a 2 or 3 channel discrete crossover network, dividing the signal, and 2 or 3 of the aforementioned ssl's . . . just think of the possibilities with each band switchable LR/MS! I am still very keen to play with passive crossover networks for this too, though loads of yous tried to discourage me from this(red rag to a bull!). All of this would have been mere pipe-dreams only a year ago . . . .


THANKS GUYS!


ANdyP :razz: :razz: :razz:
 
ok for a switchable attenuator for balanced signal, can I use the 220R shunt and just series up the resistors with the contacts?

I still have yet to find a decent model of a switchable attenuator(for MIC inputs), or even info on one!! I've been looking for forever!!
 
To maintain the input and output impedances at their design values, the answer is no. You must change the shunt resistor along with the series resistors.

The link I posted earlier gives the equations for a U-Pad.

www.uneeda-audio.com/pads

I'm afraid there are no shortcuts. :wink:
 
Also: you mentioned padding SM57/58s. You might want to make up some attenuators with 500-600 ohms in and 150 out. The 57/58s sound a helluva lot cleaner operating into lower impedances.

Peace,
Paul
 
WOB Had a nice switchable att that was on the web for a short time IIRC.
 
[quote author="pstamler"]Also: you mentioned padding SM57/58s. You might want to make up some attenuators with 500-600 ohms in and 150 out. The 57/58s sound a helluva lot cleaner operating into lower impedances.

Peace,
Paul[/quote]

That's really interesting, I had noticed an appreciable difference with 57s between my transformer balanced pres and the transformerless IC ones. Now it makes sense. I'm going to have to attempt to make some pads as you suggest.
 
FWIW, S. Dorsey* of Usenet fame maintains that the SM57 works best into a reactive load, not necessarily always low-Z. And many people (myself included) have noticed that the '57 seems to sound nicer with a transformer-coupled input, so he may be on to something. I don't doubt what Paul says, but I'm suggesting that it might go beyond the issue of simply "higher vs. lower" impedance.

You know, I could have sworn that we went into the "'57 Issue" in some detail in another thread, but I honestly cannot remember where.

*(I don't regard Dorsey as a living god, as some seem to do, but most of what he posts seems informed by experience and free of the talking-out-the-ass factor so prevalent on the Internet. :wink:)
 
Dave, you are right, as usual.

http://www.groupdiy.com/index.php?topic=11843

I completely missed this thread. I'll give it a thorough read-thru now. Sorry for the OT in this thread.
 
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