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Mic attenuator idea - someone should make this!
Old 24th December 2010
  #1
Mic attenuator idea - someone should make this!

I posted this in So Much Gear So Little Time but didn't find any answers there, so how about you guys?

I use vintage mic pres (like RCA OP-6 and Ampex 350) which have so much gain that pads are often required between the mic and the pre. I've tried several inline pads and they all degrade the sound somewhat, each in it's own way.

I'm told that ideally a pad should be built to match the specific impedances of the mic and pre being used.

So what I want is a high quality passive mic attenuator box with XLR I/O, selectable input and output impedances, selectable amounts of attenuation, and phase reverse switch.

Is there such a thing already in existence? If no, can someone tell me who can custom-build me one?
Thanks!

.
Old 24th December 2010
  #2
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emrr's Avatar
Do a calibrated test and match the levels exactly for both conditions, using tones. If you are off even 1/10th of a dB you will hear a difference, with the louder seeming better. Only when perfectly matched in level can you make this call. Until you've done this, it's nothing but a rumor.
Old 24th December 2010
  #3
Quote:
Originally Posted by emrr View Post
Do a calibrated test and match the levels exactly for both conditions, using tones. If you are off even 1/10th of a dB you will hear a difference, with the louder seeming better. Only when perfectly matched in level can you make this call. Until you've done this, it's nothing but a rumor.
Make what call - that the inline pads diminish the tone? I have satisfied myself that this is the case, and I can hear different colorations from different pads. I am aware of the importance of level matching in listening tests. I might need a different pad for each mic/pre combo...

.
Old 24th December 2010
  #4
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JohnRoberts's Avatar
 

It's not a product because you are talking about a pretty obscure application but what you want is a pad with constant input and source impedance so it loads the mic the same (nominally 1.5-2k) , and presents the preamp with the same source impedance (nominally 150-200 ohm) no matter the attenuation.

Not rocket science with a multi-position switch and a bunch of resistors.

JR
Old 24th December 2010
  #5
Quote:
Originally Posted by JohnRoberts View Post
It's not a product because you are talking about a pretty obscure application but what you want is a pad with constant input and source impedance so it loads the mic the same (nominally 1.5-2k) , and presents the preamp with the same source impedance (nominally 150-200 ohm) no matter the attenuation.

Not rocket science with a multi-position switch and a bunch of resistors.

JR
You mean like this?

Rapco Patchadap In-Line Attenuators Attenuation Devices at Markertek.com

It's the only one I can find that says it retains impedance...

.
Old 24th December 2010
  #6
Gear Maniac
 
Helsing's Avatar
 

The use of resistive pads has been long debated. The Rapco is probably the same as the Shure thats the same as most of the pads used in commercial mic pres. EG for a 20db pad they run the balanced signal though a pair of 1% resistors about 619-680 ohms with a 150 ohm resistor across the output side of these resistors. Yes there is a very slight effect to the insertion, but some people actually like it better. Perhaps the only way to not change the mic pre's sound is to never use a pad and just move the mic away from the source or use a different mic with lower output. Try a Shure SM7.
Old 24th December 2010
  #7
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surflounge's Avatar
Would back-to-back zenor diodes and a resistor work?
Old 24th December 2010
  #8
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JohnRoberts's Avatar
 

Quote:
Originally Posted by Trakworx View Post
You mean like this?

Rapco Patchadap In-Line Attenuators Attenuation Devices at Markertek.com

It's the only one I can find that says it retains impedance...

.
Perhaps, it says it maintains impedance so better chance than one that doesn't claim to. There are both input and output impedances to manage. If the mic sees the same termination impedance, and the preamp sees the same source impedance, then the sound should be identical except for level.

Changing the termination on the mic can affect frequency response. Changing the source impedance to the preamp can increase noise, generally not an issue when the signal is so hot it needs to be padded down, but we never want to increase noise.

This is probably the perfect first project to get you started on DIY. A handful of resistors from radio shack and repurposed mic cable, and presto you have a decent pad.

Using the simplest 3 resistor topology will converge on a range of attenuation. A pair of resistors in series with each mic line (XLR pins 2 and 3). A 3rd resistor connected between pins 2 and 3 at the preamp end provide attenuation and drops the pad source impedance back down to nominal range

A 4th resistor can be added between pins 2 and 3 at the mic end to drop down the termination impedance if needed for larger amounts of attenuation.

A simple 3 resistor pad off the top of my head 2x680 ohm in series, with 220 ohm shunt at the preamp end will give approx 18 dB of pad and meet nominal impedance criteria for both ends.

For less attenuation (around 12 dB) I am inclined to just drop the 680 ohm series resistors to 620 and increase the 220 phm shunt to 360 or so. This will slightly miss the source impedance target at the preamp to the high side but this is a lesser evil, IMO than loading down the mic.

Note: for good common mode rejection match the series resistors to 1% or better.

Merry Christmas

JR
Old 24th December 2010
  #9
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Quote:
Originally Posted by Helsing View Post
... Perhaps the only way to not change the mic pre's sound is to never use a pad and just move the mic away from the source or use a different mic with lower output. Try a Shure SM7.
This is my thinking too, and I'm curious as to what is causing the overload. What mics and what source?
Old 24th December 2010
  #10
Quote:
Originally Posted by [email protected] View Post
This is my thinking too, and I'm curious as to what is causing the overload. What mics and what source?
Take my RCA OP-6 pre for example -

Rebuilt RCA OP-6 Ribbon Mic Preamp All Tube 1930s RARE - eBay (item 110626277009 end time Dec-28-10 12:16:20 PST) (this one's not mine BTW)

- it was built in the 1940s for broadcast use. It's likely optimized for RCA ribbon mics and does sound amazing with my AEA R44CE. It has 90dB of gain and even on it's lowest setting it is too hot for recording a vocal with my U67. Even a passive ribbon mic on a loud amp is too hot for it. But the preamp sounds so good that I want to be able to use it. All the inline pads so far have diminished the sound. So I need a quality, flexible mic level attenuator with variable impedance so I can use various mics with it and my other vintage pres. I have similar problems with my Ampex 350 and Ampex 400 pres.

Thanks,
Old 24th December 2010
  #11
Quote:
Originally Posted by JohnRoberts View Post
Perhaps, it says it maintains impedance so better chance than one that doesn't claim to. There are both input and output impedances to manage. If the mic sees the same termination impedance, and the preamp sees the same source impedance, then the sound should be identical except for level.

Changing the termination on the mic can affect frequency response. Changing the source impedance to the preamp can increase noise, generally not an issue when the signal is so hot it needs to be padded down, but we never want to increase noise.

This is probably the perfect first project to get you started on DIY. A handful of resistors from radio shack and repurposed mic cable, and presto you have a decent pad.

Using the simplest 3 resistor topology will converge on a range of attenuation. A pair of resistors in series with each mic line (XLR pins 2 and 3). A 3rd resistor connected between pins 2 and 3 at the preamp end provide attenuation and drops the pad source impedance back down to nominal range

A 4th resistor can be added between pins 2 and 3 at the mic end to drop down the termination impedance if needed for larger amounts of attenuation.

A simple 3 resistor pad off the top of my head 2x680 ohm in series, with 220 ohm shunt at the preamp end will give approx 18 dB of pad and meet nominal impedance criteria for both ends.

For less attenuation (around 12 dB) I am inclined to just drop the 680 ohm series resistors to 620 and increase the 220 phm shunt to 360 or so. This will slightly miss the source impedance target at the preamp to the high side but this is a lesser evil, IMO than loading down the mic.

Note: for good common mode rejection match the series resistors to 1% or better.

Merry Christmas

JR
Thanks for the info! Much appreciated. thumbsup

.
Old 24th December 2010
  #12
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JohnRoberts's Avatar
 

If using the pad with an old school ribbon mic preamp, you might go even lower value R on the back end of the pad.. say dropping the 220 ohm R down to 100 ohm or lower. This will give you more attenuation without dramatically affecting the mic termination.

I will resist the temptation to suggest the obvious, reduce the gain of the mic preamp. While that will reduce the preamp noise, it would definitely change the sound of that preamp.

JR
Old 25th December 2010
  #13
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brianroth's Avatar
 

My friend Rick Chinn has the numbers nailed down for a 20 dB pad with approx. 1500 Ohms in and 150 Ohms out:

Uneeda Audio - Build your own attenuator pads

Best,

Bri
Old 27th December 2010
  #14
Gear Maniac
 

Quote:
All the inline pads so far have diminished the sound
Which ones would those be? The Shure inline pad will be basically the same as any custom pad that you could have built.

Pads are often a compromise of some sort but it's impossible for anyone to tell exactly how your performance has "diminished".

You can tweak pad values a bit here or there to raise the Z the mic sees or lower the Z the preamp sees . . I don't know that that will overcome the problems of a hot condenser like a U67 into a 90db preamp - under most any scenario. It may be useful on other lower output mics.

I'm going to go out on a limb and guess your definition of "diminished". I've got a 4 stage tube pre similar to the OP-6 - 90db gain. Heavily padded and used with ribbon or low output SM-7 it has a tendency to sound 'choked' - the sound under this condition doesn't 'bloom' (harmonic content?) like it does without a pad. Solution? Really back to the advice others have posted so far.

Other possibilities of "diminished" would be a wacky freq response - a possible rise in the hi freq response, thinning out of the mids, loss of lows etc. All possibilities depending on what mic & pad combination the input transformer sees. I think most would consider the interactions there complex. I'm throwing in a vote for experimentation with a test jig - line up the pads on a breadboard, grab some alligator clips and see what happens. There may very well be a happy sound to be had.

You don't even have to get bogged down with the intricacies of calculations here if you don't want. Just vary the shunt resistor from say 30r to 1K. Vary the series legs from say 470 to 10K. Go for what sounds right vs what's supposed to be right.
Old 27th December 2010
  #15
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JohnRoberts's Avatar
 

It will be very difficult to accurately judge the sound quality associated with a pad, unless you very rigorously manage and normalize SPL during any listening tests.

All pads by definition "diminish" the amplitude, If they maintain constant termination to the mic, the signal coming from the mic will be unchanged. If they maintain a constant source impedance feeding the preamp, that will sonically be unchanged too. So an optimal pad, will change only the final output, and headroom inside the preamp.

Clipping or overdriving the preamp, can sonically alter the sound as much or more than mis-loading the mic termination.

JR
Old 28th December 2010
  #16
Quote:
Originally Posted by lassoharp View Post
Which ones would those be? The Shure inline pad will be basically the same as any custom pad that you could have built.

Pads are often a compromise of some sort but it's impossible for anyone to tell exactly how your performance has "diminished".

You can tweak pad values a bit here or there to raise the Z the mic sees or lower the Z the preamp sees . . I don't know that that will overcome the problems of a hot condenser like a U67 into a 90db preamp - under most any scenario. It may be useful on other lower output mics.

I'm going to go out on a limb and guess your definition of "diminished". I've got a 4 stage tube pre similar to the OP-6 - 90db gain. Heavily padded and used with ribbon or low output SM-7 it has a tendency to sound 'choked' - the sound under this condition doesn't 'bloom' (harmonic content?) like it does without a pad. Solution? Really back to the advice others have posted so far.

Other possibilities of "diminished" would be a wacky freq response - a possible rise in the hi freq response, thinning out of the mids, loss of lows etc. All possibilities depending on what mic & pad combination the input transformer sees. I think most would consider the interactions there complex. I'm throwing in a vote for experimentation with a test jig - line up the pads on a breadboard, grab some alligator clips and see what happens. There may very well be a happy sound to be had.

You don't even have to get bogged down with the intricacies of calculations here if you don't want. Just vary the shunt resistor from say 30r to 1K. Vary the series legs from say 470 to 10K. Go for what sounds right vs what's supposed to be right.
I have tried several inline pads - Shure, Audio-Technica, Pro Co, Schoeps, D.W.Fearn (custom made). Each changes the sound in a different way. All of your descriptions of "diminished" fit, depending on the mic/pre/pad combination. There are so many possible mic/pre/pad combinations - that's why I wish for a single box with variable impedances and attenuation so I could dial it in for each combo. I'm not sure that it would even work, which is part of why I posted this thread. Thanks!

.
Old 28th December 2010
  #17
Gear Guru
Telephone

Hi Justin. I think it would be difficult maybe impossible to include variable impedance in your design. I have a slightly similar difficult with a V76 and U67. The V gain steps are too big for comfortable level setting.
I have an answer on my workbench, I just need to replace the banana inputs with XLR's. It is an attenuator made by Standard Telephone and Cables, STC, in London. Three switches, 10-90dB, 0-9dB, and 0-0.9dB.
It states Impedance 600 Ohms, balanced.

This seems ideal, I believe the 76 and the 67 are both designed for 600 ohms.

I could take a photo of say the decade section if you wish. Maybe someone could reverse engineer it. In any case a constant impedance design shouldn't be difficult.

I would sell mine for the price of say, Ireland.....

Might be worth contacting Markus or Andy at http://www.hestudiotechnik.de/
The use such pads in their Rack solutions for old broadcast gear.

DD
Old 28th December 2010
  #18
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JohnRoberts's Avatar
 

What you are asking for is entirely possible (within a range of adjustment) and while the design engineer inside me says Hell No, the customer is always right.

First my objection... The effects of mic loading are known to the mic design engineers and they pretty much voiced the microphone based on an expected load termination, similarly for the preamp designers, the preamp is optimized for source impedance and gain range.

That said, I am more than a little suspicious that some esoteric preamps intentionally mis terminate the mics to sound "different". If your marketing juju is strong you can argue this different sound is better.

For what you ask, you can easily terminate the mic with less than ideal, likewise send the preamp more or less than nominal impedance, but you can't provide too much range without an active buffer which is just not going to happen in a passive pad.

Why don't you buy a couple low impedance pots and play around, you shouldn't be able to hurt anything.

JR
Old 28th December 2010
  #19
Quote:
Originally Posted by JohnRoberts View Post

All pads by definition "diminish" the amplitude, If they maintain constant termination to the mic, the signal coming from the mic will be unchanged. If they maintain a constant source impedance feeding the preamp, that will sonically be unchanged too. So an optimal pad, will change only the final output, and headroom inside the preamp.

Are you saying that a simple universal 20dB pad that maintains constant termination to the mic and constant source impedance to the preamp can be built? One that would work in every mic/pre combination?

Forgive my ignorance. I am more of a user of gear than an expert on electronics. It's just that all of the available inline pads (except the Rapco) specify the input/output impedances, like the shure for example:

Input Impedance: 1 kOhm
Output Impedance : 150 ohm

So I wonder why they are not all made to be universal...

.
Old 28th December 2010
  #20
I see that we were writing at the same time. Thanks to all of you for your expertise! I'm going to show this thread to my local tech and come up with a solution if possible.

.
Old 28th December 2010
  #21
Gear Guru
T

T-Pad Attenuator. I am sure John will be right back with the design.
I am sure these guys would design and make exactly what you want.
http://www.goldpt.com/
DD

Last edited by DanDan; 28th December 2010 at 07:52 PM.. Reason: Found it.
Old 28th December 2010
  #22
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JohnRoberts's Avatar
 

Quote:
Originally Posted by DanDan View Post
T-Pad Attenuator. I am sure John will be right back with the design.
I am sure these guys would design and make exactly what you want.
Goldpoint Level Controls
DD
Don't be so sure I will design circuitry for free...... I have better uses of my time.

I will try to inform the community but I won't shovel your driveway or mow your lawn.

JR
Old 28th December 2010
  #23
Gear Maniac
 

Quote:
So I wonder why they are not all made to be universal..

They are made to interface two unequal impedances. The resistor values determine what impedance each side sees. Yes, they are constant - as are pretty much all pads using fixed value resistors.

So, a 2K to 150ohm pad is 'universal' for everything that's 'happy' with seeing a 2K load on one side and a 150ohm source on the other.

For the hundreds of other impedance combinations = hundreds of other pad combinations.

I'm just trying to simplify here to illustrate the potential impracticalities of such a 'universal' box.

The problems you are experiencing likely have more to do with the general situation - which is - a pad in front of a transformer input. Transformers do not have a constant impedance across the audio band. Resistive pads can't change that. Pads of 20db or more are considered the best bet for avoiding skewed response, dist, etc. If they still are unacceptable I don't know of anything else to remedy the case of pad in front of transformer input.

One other thing that will work for attenuation is a pot or fixed attenuator feeding the grid. This means it's connected across the input transformer secondary as opposed to being in front of the transformer. It may introduce some noise but in practice it works ok. Probably not a good thing to do in your case because it involves hacking into a very expensive vintage amp.
Old 28th December 2010
  #24
Gear Guru
Oops

Sorry John, didn't mean that literally, just a link is what I had in mind.
I think this does it-
http://www.fmsystems-inc.com/manuals/TPADart.pdf

DD
Old 28th December 2010
  #25
Gear Maniac
 

Quote:
T-Pad Attenuator
T-Pads reflect equal impedances in both directions and are generally used where you have equal source and load Z.

Mics to Mic pres generally don't follow that scenario. That's why U or O pads are chosen.

If you have a mic that is truly optimized for driving a 600ohm load and you arrange the mic pre input to give a nominal(only at midband) 600r load - then H or T pad should be fine. Hardly universal though.


EDIT: standard T-pads can be designed for use between two impedances of unequal value. I was thinking of bridged T which are for use with equal source and load impedances only

Last edited by lassoharp; 30th December 2010 at 04:44 PM.. Reason: correcting mistake on info
Old 28th December 2010
  #26


Ok, I'll ask it.

What is the input impedance of these crusty old preamps? I can't imagine that preamps designed for mics with 50 ohm output impedances and 90dB of gain use 2k input impedances....

Besides, you can always back off the mic.




-tINY

Old 28th December 2010
  #27
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JohnRoberts's Avatar
 

Quote:
Originally Posted by tINY View Post


Ok, I'll ask it.

What is the input impedance of these crusty old preamps? I can't imagine that preamps designed for mics with 50 ohm output impedances and 90dB of gain use 2k input impedances....

Besides, you can always back off the mic.




-tINY

OK, I wasted some time on that too, and I was not able to find a standard source impedance or termination for "all" ribbon mics. (the defacto nominal standard for dynamic mics is not formal). My presumption is that they are lower source impedance "but" back in the (not so) good old days, these mics/preamps may not have been designed for bridging (10x) termination to maximize voltage transfer, but instead transformer coupled to maximize power transfer.

The simple answer is, there isn't a simple answer, further there isn't a correct answer IMO since the OP, it trying to interface a different mic into a preamp not designed for that type mic, because he likes the "sound" of the preamp. In my judgement this is already off the page regarding design for optimal fidelity (accuracy), and instead an exercise in what works and sounds good subjectively to the OP.

I have already given my best advice, get some low value pots and have at it... There is no correct answer without imposing personal judgement, which in my case is not enthusiastic about the original task.

JR

PS: Perhaps a step down transformer will get the dynamic mic down in the ribbon mic level and impedance range, but specifying and sourcing a transformer for that application is not trivial or cheap.
Old 28th December 2010
  #28


Well, my inclination would be to open up the pre-amp and look at adding an inter-stage pad...

But that's just me.



-tINY

Old 28th December 2010
  #29
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Quote:
Originally Posted by tINY View Post


Well, my inclination would be to open up the pre-amp and look at adding an inter-stage pad...

But that's just me.

or is it possible to pad the output of the pre? might sound better (or worse) than padding the mic.

as a side note it can certainly be possible that the inline pads alter or "degrade" the sound. simple resistive dividers neglect the reactive components of both the mic and mic pre. a true match can be much more complicated than simple resistor ratios.
Old 28th December 2010
  #30
RCA OP-6 input impedance is 250 Ohms.

Of course I back off the mic when appropriate, but that can alter the sound and/or introduce bleed problems in many situations.

The U67 into RCA OP-6 preamp is just an example I chose to illustrate the problem in it's most extreme form. Actually, pads are needed in much less mismatched scenarios with these pres. A passive bass through a passive DI box can peg the meter on an OP-6 with the OP-6 gain set to 1 (it goes up to 20). An R-121 on a medium-volume bass amp will peg the meter too.

Bass is my favorite use for these pres. As one producer standing here put it, "These have more fat than a pig in heat!". 'Problem is that the pads I've tried all make that bacon a bit leaner.

I wouldn't bother you all with this stuff, but these pres really are worthy of some extra effort.

I'm not the only one who thinks so:

RCA OP-6 powerhouse...

Look at this auction that just ended today:

Rebuilt RCA OP-6 Ribbon Mic Preamp All Tube 1930s RARE - eBay (item 110626277009 end time Dec-28-10 12:16:20 PST)

$4,861.00 paid for a single channel!

I got my pair for much less before the word got out about these "holy grail" pres. I just want to avoid "choking" them whenever possible, and I don't want to change their internal design.

Thanks again for all your helpful information!

.
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