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Mic attenuator idea - someone should make this!
Old 29th December 2010
  #31
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Quote:
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....


Quote:
RCA OP-6 input impedance is 250 Ohms


That's a good question and I haven't yet found an answer I feel solid with. When attempting to measure the equivalent vintage pres I own I've gotten two very different numbers.

Using the -6db/variable resistor method on a similar RCA amp gives a number over 5K. Connecting an AC impedance meter to the primary gives essentially the rated Z # of the primary which was about 176ohms (speced @ 250ohm). I've always assumed that the 250ohm impedance is referring to the source Z the transformer expects to be terminated with and not the actual Z the mic sees. Of course a pad fixes the looking into Z and sets source Z.

Again, the question is of what source Z the transformer primary expects vs what the actual load (input Z) is presented to the mic. Bridging or matching?

Manual state as "source" or actual input Z ?


A good thread on the subject:

Are we loading our mics properly?



For JR's suggestion of a stepdown transformer you may want to talk to someone over here about the details. Cinemag as well.

Home




. . . check the OP 6 manual for suggested input level
Old 29th December 2010
  #32
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emrr's Avatar
A mic has an impedance that changes with frequency, and a transformer coupled preamp has an impedance that changes with frequency. So you have a variable recipe. If you need a pad, you are forced to insert a fixed impedance at all audio frequencies between a set of impedances which vary across the audio band.

The only real hairsplitting answer is to design the best custom designed pad for the mic in question and the pre in question. You'll need a group pads for a given pre; one for each mic. It may be the same pad for most mics, but I'd expect to need some different types.

You may still like none of them, in which case some other RCA tube preamps may prove to be better, meaning those with sane gain ranges, and meant to handle more than conversational radio broadcast vocal levels. Most of the time I use the standard 40 dB preamps, and a DI signal is usually on the verge of overload even there. A mic on the average amp will need a 20 dB pad. An OP-6 in this situation will be throwing away 75 dB of gain, more than most preamps even provide, so a pad with loud source is still part of a slippery slope into "less than ideal" land.

Only an idiot paid so much for a cosmetically hacked up OP-6 like that one offered by Mr. Sank. He certainly floated it out there for a long long time to get that sum. I think it was listed repeatedly for more than six months, while several others slipped by well under $1000.
Old 29th December 2010
  #33
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idylldon's Avatar
 

Quote:
Originally Posted by emrr View Post
Only an idiot paid so much for a cosmetically hacked up OP-6 like that one offered by Mr. Sank. He certainly floated it out there for a long long time to get that sum. I think it was listed repeatedly for more than six months, while several others slipped by well under $1000.
Then you're going to love this, Doug:

RCA OP-6 Mic Preamps (PAIR) OP6 Mic Pre - eBay (item 250748753815 end time Jan-27-11 17:55:56 PST)

hehheh

Cheers,
--
Don
Old 29th December 2010
  #34
Gear Maniac
 

pray some fool doesn't pay that.
Old 29th December 2010
  #35
Quote:
Originally Posted by emrr View Post
A mic has an impedance that changes with frequency, and a transformer coupled preamp has an impedance that changes with frequency. So you have a variable recipe. If you need a pad, you are forced to insert a fixed impedance at all audio frequencies between a set of impedances which vary across the audio band.
Thanks. That explains why I have always been disappointed when inserting ANY device between a mic and a preamp. 'Was hoping for a miracle.

Quote:
Originally Posted by emrr View Post
The only real hairsplitting answer is to design the best custom designed pad for the mic in question and the pre in question. You'll need a group pads for a given pre; one for each mic. It may be the same pad for most mics, but I'd expect to need some different types.
This was the reason for my original idea of a variable impedance pad, and why I started this thread.

Quote:
Originally Posted by emrr View Post
You may still like none of them, in which case some other RCA tube preamps may prove to be better, meaning those with sane gain ranges, and meant to handle more than conversational radio broadcast vocal levels. Most of the time I use the standard 40 dB preamps, and a DI signal is usually on the verge of overload even there. A mic on the average amp will need a 20 dB pad. An OP-6 in this situation will be throwing away 75 dB of gain, more than most preamps even provide, so a pad with loud source is still part of a slippery slope into "less than ideal" land.
I hear you. Honestly, I've never heard a pad I liked. Sometimes it still sounds fine when the meter is pegged, but I worry about damaging the VU meter by letting it peg hard for hours on end. Do you think that would damage the meter?

Quote:
Originally Posted by emrr View Post
Only an idiot paid so much for a cosmetically hacked up OP-6 like that one offered by Mr. Sank. He certainly floated it out there for a long long time to get that sum. I think it was listed repeatedly for more than six months, while several others slipped by well under $1000.
In that case, there are a lot of wealthy idiots out there. There were 47 bids on that OP-6. I saw another one sell for $3250.00 not long ago... But I agree that it is too much to pay for any one preamp. And I don't care for the cosmetic changes they made to that one. They are great-sounding pres though...

.
Old 29th December 2010
  #36
Quote:
Originally Posted by idylldon View Post
$14,000?!

It's getting ridiculous! Those two don't even have the correctly shaped meters to fit the outlines on the face plates!
Old 29th December 2010
  #37
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emrr's Avatar
Fishing will catch a fool, sooner or later. These asking prices are exceeding some Western Electric, which tells you right away that it's nonsense. They made so many of these, for so many years. There are tons of them out there. Really, more than a BA-6A?
Old 30th December 2010
  #38
Gear Maniac
 

Well, if you are running a mic into too low an input impedance you could be overloading the mic's output stage rather than the pre itself.

In any case, padding a mic isn't great as it almost inevitably gives you a higher noise floor than you would get otherwise. Modern capacitor/condensor mics like to run into the maximum input impedance possible (the standard of a few k has more to do with avoiding random input thumps than loading the mic as such) and a pad in front of the kind of amp you're talking about will have a lowish input impedance so could introduce as many problems as it solves. Have a designer look into the amps and see if they have a multi-stage architecture that could allow you to put a pad in the middle somewhere, thus keeping the input and the first-stage amplification as low-noise as possible.
Old 30th December 2010
  #39
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emrr's Avatar
in these cases the 20+ dB gain from the input transformer will almost always immediately overload the 1st stage, which was never designed for so much level.


It should be more likely to design a pad for a transformerless mic and preamp combo lacking perceived effects than for a transformer filled combo.
Old 30th December 2010
  #40
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I know exactly what youre talking about

Justin:
Commercially available inline pads almost always sound wrong - reducing some air and transients. This is true of most pads in recording consoles.

The variable is the mic input transformer of the pre. These units have a varying impedence, usually rising towards the high end. The standard pad design assumes a constant impedence on the part of the mic input transformer. When the pad is engaged the mic sees, not the rising hi freq impedence of the mic pre transformer, but the flat impedence of the pad (the mic does still see some of the transformers unique impedence, but not much)

Anyway, the solution is to use an "O" pad, and the formulas in the Audio Cyclopedia, but use the mic pre's impedence at 6K to 10K in the formulas. With this approach you will not get the attenuation spec'd by the formula, it will be less by 3-4db. It is also helpful to parallel small film caps on the series resistors in the pad. This has to be done with help of test gear.

I could design a small box with various pad settings - the cost would be about $800. If interested, contact me at World Pro Audio [email protected]
Old 30th December 2010
  #41
Thanks Superfly! I'll look into that, and may want one of those.

.
Old 31st December 2010
  #42
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Tranny-Cap-Tube's Avatar
 

Does the API 312 pad have an 'O' design or is it considered poor?
Old 31st December 2010
  #43
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emrr's Avatar
I will need a good bit of convincing to buy that an O pad has any advantage over a U.

Pretty much nothing uses an O pad for mic input. Shure's barrel pads used to be O pads, of a matching design, but have been changed to a U pad.
Old 31st December 2010
  #44
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Quote:
Originally Posted by RHSuperfly View Post
I could design a small box with various pad settings - the cost would be about $800. If interested, contact me at World Pro Audio [email protected]
Or you could go to radio shack and roll your own, for more like $8.

JR
Old 31st December 2010
  #45
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Quote:
Originally Posted by JohnRoberts View Post
Or you could go to radio shack and roll your own, for more like $8.

JR

thumbsup +10 thumbsup

. . . though someone's liable to feel shortchanged. blessing or burden.
Old 31st December 2010
  #46
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mister sunshine's Avatar
 

I've been reading this thread and sort of wondering...

What does the fact that the preamp has 90dB of gain have to do with anything?

It sure seems to me like the OP is describing a problem with headroom at the first(?) stage.

It seems like keeping that perspective may narrow down the options for modification.

Just reading and learning... I didn't even know anyone had an interest in these old pres.

best regards,
mike
Old 31st December 2010
  #47
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surflounge's Avatar
Quote:
Originally Posted by mike_mccue View Post
What does the fact that the preamp has 90dB of gain have to do with anything?

It sure seems to me like the OP is describing a problem with headroom at the first(?) stage.
Give me the Stagetec True Match without converters (doubt possible). Just analog with a dynamic range of over 150dB
Old 31st December 2010
  #48
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Quote:
Originally Posted by JohnRoberts View Post
Or you could go to radio shack and roll your own, for more like $8.

JR
i'm not sure if you are being sarcastic, but if you could design/build a transparent pad that accounts for the reactive components of transformers for $8 i and many people would buy them by the truckload heh
Old 31st December 2010
  #49
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Quote:
Originally Posted by johnnyc View Post
i'm not sure if you are being sarcastic, but if you could design/build a transparent pad that accounts for the reactive components of transformers for $8 i and many people would buy them by the truckload heh
Would LOVE to see how someone can design a pad with the SAME input impedance from 20 to 20KHZ as the input trans....
The fact is Most input transformers input impedance goes down by more than 1/2 at both low(20Hz) and HF(20KHz)...
This loads the mic at both LF and HF differently than a resistor pad...
Different levels also change the sound of the input transformer, esp at LF...
Old 31st December 2010
  #50
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emrr's Avatar
Quote:
Originally Posted by mike_mccue View Post
I
What does the fact that the preamp has 90dB of gain have to do with anything?
OP-6 is quoted as -24 VU max input, +19 VU max output, for 1% RMS distortion. We have 90 dB gain, and a 43 dB minimum gain window. Throw away 47 dB one way or another, probably several ways. Many modern sources are over -24 VU. With many older preamps, that max input is reflective of input transformer level handling capabilities (at lowest frequency of concern), so there's nothing you can do but pad before the preamp. A condenser mic into an OP-6 is going to blow it up. This applies to virtually all vintage high gain preamps.

Yeah, a variable box that allows one to tweak the impedance curve sounds like a great way to audition settings infinitely, and never get anything done.

"does it sound that same as without the pad? who knows?"
Old 31st December 2010
  #51
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I knew there was a reason I stopped designing mic preamps using transformers back in the '70s... If they vary that much in the audio band it's remarkable you get any usable sound at all?

Getting back to the Op's question.. If he has that much gain to throw away even a simple resistive pad can have relatively high input and relatively low output impedance to minimize interaction at both ends.

I have wasted more time than I should on this. IMO this is a subjective pursuit as a perfectly transparent pad, will never sound like the original preamp driven to the edge of saturation. Even the different (reduced) amount of room noise will affect listener perception.

Quote:
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.
As I think I answered already generally.

Yes to XLR I/O
Yes to selectable input and output impedance
Yes to selectable attenuation
yes to phase reverse
but no to all of the above with both high quality "and" passive.

We can play all kinds of games to increase or decrease both input and output impedance, but without an active gain stage in the middle we can't reliably deliver a useful range of attenuation and low noise independently of the others.

I repeat my suggestion that the OP, just get some resistors and experiment. I won't predict what "he" will hear, nor will I insult him with my specualtion.

JR

PS: regarding this veer into making a pad more like transformers or other non-ideal components, I don't see why bother, so amuse yourselves. I have real problems to solve.
Old 31st December 2010
  #52
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emrr's Avatar
It's a problem that won't be solved, other than in fantasy. In this sort of dual transformer coupled situation, each affects the response of the other, just like a speaker driven by a tube amp. The system is in essence an EQ network, which varies depending on the items put into the chain.

As soon as you strap a shunt load resistor across an OP-6 input transformer (250 ohm for stated source Z, for -6 dB) you get a measured change in frequency response. Some vintage tube preamps develop a high peak bump with almost any shunt load R up to 1K, where the shunt R has long since passed the point of meeting the stated goal of delivering desired source Z. The same preamps will measure differently if fed a 150-250 source Z without a shunt R. You won't build a reasonable pad that delivers desired reduction without the shunt element of the voltage divider.

In general, if you use a 40-50 dB line pad into a vintage transformer coupled preamp, you will see a loss of high frequencies due to the high series resistance, no matter what the shunt R may be.

The only place I have ever used an O pad was a 95 dB AGC intercom preamp that advised a U pad with series resistances up to 100K to set operating level. In that case I added the O element (shunt R to the mic) so the mic saw something substantially less than 200K when set for the highest input level. It works for that case, which is inherently noisy ITFP.
Old 1st January 2011
  #53
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Quote:
Originally Posted by nosebleedaudio View Post
Would LOVE to see how someone can design a pad with the SAME input impedance from 20 to 20KHZ as the input trans....
The fact is Most input transformers input impedance goes down by more than 1/2 at both low(20Hz) and HF(20KHz)...
This loads the mic at both LF and HF differently than a resistor pad...
Different levels also change the sound of the input transformer, esp at LF...
yes that was my point, if the OP is unsatisfied with the sound of commercially available resistive pads, it is doubtful any custom or diy all resistive network will be any better. apologies to JR if it came across as something else.

this theoretical pad can actually be designed using a combination of resistive and reactive components, but it is far from a trivial matter to do so.
Old 1st January 2011
  #54
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Just a note: The JT-16 varies very little from 20 to 20Khz, one of several reasons I use/Love it...
Old 1st January 2011
  #55
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It seems like you smart guys have drawn the same conclusion I did.

I'm guessing that if RCA had put the pad in the design in the first place... I'll bet it would have sounded a lot better.

:-)
Old 1st January 2011
  #56
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Quote:
Originally Posted by mike_mccue View Post
It seems like you smart guys have drawn the same conclusion I did.

I'm guessing that if RCA had put the pad in the design in the first place... I'll bet it would have sounded a lot better.

:-)
But some mics used back then had a VERY low output..ribbons for example..
They also placed mics further away than most do today...
Old 1st January 2011
  #57
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emrr's Avatar
and they were only used for speech, on radio. nothing loud was ever intended.
Old 1st January 2011
  #58
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mister sunshine's Avatar
 

Understood.

Thanks.
Old 1st January 2011
  #59
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I realize this would change the sound somewhat, but what about replacing the input transformer altogether? Something with a 1:1 ratio, or even 2:1, rather than padding the input to the existing one? If I'm following the discussion, the OP-6 has an input transformer with some gain,correct?

This would be changing the vintage preamp into something more custom (and probably worth less, even if more functional!)
Old 1st January 2011
  #60
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emrr's Avatar
why own an OP-6 at all then?

it's not a realistic idea, for impedance matching reasons. it's cart before the horse. We need fewer tubes for less gain....different preamp. Use the OP-6 where appropriate, end of story. I put various OP-6 obvious mod ideas in the other thread; I consider them all to be BAD IDEAS because it destroys the monetary value of the unit. Use the right unit; no square pegs in round holes.
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