Unaddressed pad Q

[Jeff Goodman]

I've searched but haven't found an answer to this.

Should a mic pad be matched for the source or destination impedance?

Right now I'm racking up some Neve 3415's. 1K2 input impedance. Should I figure the pad for 1K2 or for mic output impedance? Say 250 ohms?

Which would be the most transparent? Either way it can't be the same as plugging the mic into the preamp directly, can it? that would be 250 (or so) into 1K2, bridged. If I make the pad for 250, then the mic will always see 250 when the pad is engaged, equal power, as opposed to bridged.

So 1k2, right? Will that change the sound or gain of the amp? With the pad in, the amp will see 1K2 as the source instead of 250.

Which should I use?

Thanks,
Jeff

 

[NewYorkDave]

You're making the mistake of thinking that you must use a "symmetrical" pad with equal port impedances. But what you need is a pad that reflects >1K to the mic and ~ 200 ohms to the preamp. An L-pad (or its balanced equivalent, the U-pad) will do this. See http://www.uneeda-audio.com/pads for much more info, including ready-made pad designs.

Also, you can see a typical 20dB pad design on the front end of my "one-bottle preamp"
http://electronicdave.myhosting.net/miscimages/onebottlepreampschem.gif
This reflects about 2K to the mic and about 200 ohms to the preamp. It'll work well with most mic preamps.

 

[Jeff Goodman]

Thanks, NYD.

I do always think of balanced pads first, because I never really understood the application for the L or U pads. That link explains alot.

I looked at your schematic, I think I would wire it differently though. Tell me what you think: Take the input and Y it to both sides of a DPDT, one side directly to the poles, one side through the U-pad circuit into the other poles, and use the wiper out into the input transformer.

Does that make sense?

Thanks again,

Jeff

 

[pstamler]

[quote author="Jeff Goodman"]I looked at your schematic, I think I would wire it differently though. Tell me what you think: Take the input and Y it to both sides of a DPDT, one side directly to the poles, one side through the U-pad circuit into the other poles, and use the wiper out into the input transformer.

Does that make sense? [/quote]

If I interpret your descripion correctly, not really, because when you're in unattenuated position you have both the pad's impedance and the preamp's impedance hanging on the microphone, in parallel with one another.

Peace,
Paul

 

[Jeff Goodman]

Hmm, yeah I guess you're right.

But in NYD's One Bottle Preamp schematic, if I'm reading it right, both of the series resisters in the pad are always in circuit also, you're only switching the shunt in and out. Will this make any difference?

Thanks again, and sorry if these are stupid questions..

Jeff

 

[NewYorkDave]

Look again: in the "bypass" position, the shunt is disconnected and the series resistors are shorted out, therefore bypassed.

 

[Jeff Goodman]

Yes I saw that they were shorted, but I was wondering whether there would still be some resistance left in the circuit.

I guess by you're answer and the fact that you are using that pad in your design, that there isn't any.

Thanks again to both you and Mr Stamler for patiently putting up with my plebian questions.

Jeff

 

[NewYorkDave]

Current takes the path of least resistance. Just for fun, imagine we're using a really crappy switch with oxidized contacts, and the contact resistance is a whopping 1 ohm. Even in that scenario, with 1 ohm shunted across a 1000-ohm resistor, it's as if the resistor isn't even there. In real life, the contact resistance of the switch is more like milliohms.

 

[Jeff Goodman]

The path of least resistance didn't even occur to me. Most often I find that the practical application of even the simplest concepts is where the art lies.

I wired up the input circuit of the preamps today, including the pad. I will say that the elegance of that pad is most apparent when it's wired up. 1 dpdt and 3 resisters, clean and easy. Very nice.

Thanks again.

Jeff