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emrr
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by emrr » Mon Apr 12, 2010 12:56 pm

Trivia: the Mackie uses the same amp for both preamp and line, and I am remembering the only practical difference (ignoring the differences for phantom power) is the addition of (2) 5-7Kish series resistors inline for the line input. I don't believe it is actually a U-pad.
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JR.
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by JR. » Mon Apr 12, 2010 8:13 pm

That is pretty common practice in value gear. Just pad the line down and send it to the input. This way pad and polarity switches (that Mackie didn't have) work for mic or line inputs.

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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by mediatechnology » Tue Apr 13, 2010 6:59 am

Trivia: the Mackie uses the same amp for both preamp and line, and I am remembering the only practical difference (ignoring the differences for phantom power) is the addition of (2) 5-7Kish series resistors inline for the line input. I don't believe it is actually a U-pad.
I think those series resistors work against the preamp bias resistors to form the voltage dividers. The pad that's formed is likely a "double L" or whatever a U-pad with a center-tap in the shunt's called. Works better than a U on a line input since the source can be unbalanced and the ring of a TRS possibly floating ungrounding the other end of the U. (The other objective is to raise the input impedance for the line path since the bias resistors should be kept low.)
I remember Pro Audio Review looking at this question maybe a decade back, and I think their hottest discovery at the time was a TLM103, at something like +13 dBm.
Thanks for that data point. I wonder what the SPL was. If I calculate their measurement of +13 dBu relative to 0.775 Vrms then the peak voltage was something like 4.9V!

Question: Let's say a typical preamp's minimum gain is 14 dB and it overloads at +21 dBu.

If you had to chose a mic pad value on your ideal preamp would it be 10, 15, 20 or some other dB value?
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by Bruno2000 » Tue Apr 13, 2010 9:48 am

I like 20 dB pads. Gets you out of trouble in a hurry. My $0.02.
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mediatechnology
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by mediatechnology » Tue Apr 13, 2010 9:57 am

Bruno2000 wrote:I like 20 dB pads. Gets you out of trouble in a hurry. My $0.02.
Bruno2000
I agree. If you have a 10 or 15 dB dB pad and it's not enough then what do you do? You're kinda boxed in to unpatching and inserting an additional in-line.
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by JR. » Tue Apr 13, 2010 11:45 am

This is a tough call... and depends a little on the application.

For live use I'd go with 15 or 20 dB and be done with it.

In recording where I want to get the best S/N performance, I lean toward the 10-15 dB pad range.

One possibility if you have a patch bay in your system is to put an additional 2k:200 10dB pad in one position of the patch bay and that way you can stack 2x 10db for the rare problem input, and get by with 10 db for most. Perhaps add a hi Z pad for radioactive line inputs too.
========
To answer for a specific preamp design I would measure with all three pad ratios and see what the cost in S/N actually is. Since at lower gains the noise floor can be dominated by other than the very input stage. It could be instructive to compare a 10 dB pad with minimum gain to a 20 dB pad with 10 db more than minimum gain. If the overall S/N is not 10dB worse the data will inform your decision better than general theory. Noise floor at low gains is a somewhat different animal.

JR

edit- another old trick I used at peavey was to put different features on different console inputs. While a little awkward, you could make most pads 10dB and 20 db on only one or two channels. I designed some value mixers that only had pads on a few channels to save cost. /edit
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by emrr » Tue Apr 13, 2010 12:41 pm

I've always thought you were stuck using a minimum pad value close to 20dB, to preserve the loading characteristics for the mic with the average preamp.

In practice and ignorance early on, I put a four position switch that added dual build out (for a U pad) to a 200 ohm shunt, for 0, -6, -12, -18 settings. Ears seem to confirm the theory that the lower loads increase the lpf factor on most dynamic mics, and lower the headroom and decrease distortion point on most condensers. I use -12 occasionally, and almost never the -6.

JLM has a diagram for a continuously variable mic pad, that kind of creeps me out. He says it shouldn't be an issue, and maybe my math is wrong, but it certainly appears to vary the load factor quite a bit in the areas most in between 0 and -20. 600 ohm load in the middle, with 2Kish load to mic at -20 and 2k15 in parallel with preamp load at least setting. Maybe an issue with some mics, maybe not with others.

http://www.jlmaudio.com/Variable%2020dB%20PAD.gif
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by JR. » Fri Apr 16, 2010 4:44 pm

Sorry, I've been distracted by taxes..

I don't know how to make a variable pad that doesn't alter input termination, which IMO must be held constant so the mic character doesn't change.

Mic preamps are designed to expect 200 ohm source, but there is little benefit to adding a buildout R after the pad to raise this impedance back up to 200 ohms. About the only thing affected might be an input RF filter. If concerned about this a cap could be added across the pad shunt leg.

Larger amounts of attenuation will result in a lower source impedances which will slightly lower input noise.

I still think it would be instructive to compare the S/N of the preamp with and without pads but normalized for same overall gain.

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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by mediatechnology » Fri Apr 16, 2010 5:39 pm

The pads in the THAT1570 DN-140, which are switchable (0 or -x dB), are constant load. They're 20, 15, 13.6 and 10 dB. I don't recall if we settled on 2K or 3K termination. IIRC it was 2K. (Been awhile since I read it and other people changed it considerably.) To make it all of the above would require lots of poles

Doing it with pots seems tough though.

EDIT: 20 dB (and oddly 13.6 dB) seem to just "fit" as values though. (13.6 dB is a "round" number due to the gain structure of the 5171.)
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Re: Electronic Design IFD: Modified Schoeps Capsule Design

Post by JR. » Sat Apr 17, 2010 10:29 am

Yes, there is a single sweet spot component selection that maintains nominal input and output terminations. Since I am arguing that a lower output termination is generally harmless and could even deliver lower input noise (at the preamp), the pad amount criteria depends on external signal level expected, and preamp headroom.

I would still look at normalized S/N performance if going for deeper than typical pads. If there is no performance penalty, go for it across the board, if there is a penalty, use it only as needed. It shouldn't be needed in too many inputs at any one time.

JR
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