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My vintage Ribbon mic and input impedance of pres -what to do?
Old 21st March 2013
  #1
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Lorenzop's Avatar
 

My vintage Ribbon mic and input impedance of pres -what to do? Fethead vs Cloudlifter

I hope you guys in the high end can give me a better answer on this!
I am quite confused about getting my head around this:

As far as I knew low impedance output of ribbon mics (or any mics for that matter) is best matched, theoretically, on the preamp by an equally low imput impedance. Ideally the same, right? And generally, thats the reason why mic input impedance is usually lower than say line level and instrument level, right (well, also to minimise resistor noise levels, as gain needs to be high, as far as I understood)?

So why on earth in trying to find a preamp for my vintage 50ohm output ribbon mic, am I reading that good preamps (like AEA TRP for example) all have extremely high input impedance? Like 30kOhm (AEA) ?!?! Shouldnt they have low input impedance?

Which leads me to my next question:
Would I be better off then plugging my ribbon mic, for the time being, not in the mic input on my pre but in the Line level or even Instrument level input? If following the same logic, they all have higher input impedance as well.

Thanks for clarifications!
Best,
L.
Old 21st March 2013
  #2
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Santiago's Avatar
 

Hi, best input in your pre would be a mic input, (line will be worse in terms of low volume!). A preamp ideally needs to have a much higher impedance than the mic to work correctly, 4-5 times as much.

There are several impedance transformers that can be used to get the 50 ohms up to the 200 or so a standard preamp expects. Xaudia in the UK make a (I've got two for my old ribbon and other low-z mics, they increase the gain quite a lot with no noise when using my rme fireface, although don't put them right on top of power cables, they will pick up some hum).

Transformers

By the way, could you tell us more about your ribbon mic? (out of curiosity)
Old 22nd March 2013
  #3
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Hi and thanks.
I knew about the Xaudia matching transformer.

Still I dont understand the theory, why are "high end" preamps for Ribbons being sold with very high input impedances, as well as being stated that this high input inpedance will "open up the sound" sonically.
Shouldnt the input impedance just be matched? Therefore lowered (at least in the case of a vintage mic with 50ohm)?

My vintage ribbon is a Melodium 42b

By the way: instead of a matching transformer would I get good results from a Triton audio FetHead? Or is that just adding gain -but not "curing" the impedance mismatch, root of the problem??

Thanks!
Old 22nd March 2013
  #4
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This matter has some history to it. It is even older than I am!

First thing to clarify is the fact that we are less concerned today with "matching" impedances when talk about audio signals these days. We actually don't match impedances. What!! Sounds like heresy.

Matching impedances is a concept that goes back to the days of early long distance telephone circuits before the invention of amplifiers. They had to transfer the maximum amount of power into the electromagnetic telephone receiver at the far end so people could hear well enough.

They used a principal of electricity that goes like this:

If the output impedance of a circuit generating a voltage is X, what is the "load" impedance (the thing the electricity is going into) that will allow the maximum transfer of power.

Answer: the load impedance should be equal to X (matched impedances)

That's right with matched input and output impedances the maximum POWER is transferred.

But the side effect of that transfer is the voltage will drop by half. That is 6dB. But when you do the math with matched impedances (the voltage times the current) the power is Maximized.

But with audio we don't need power. We need the signal voltage. So we need a new question.

If the output impedance of a circuit generating a voltage is X, what load impedance is the lowest I can use that will not affect my signal voltage very much?

Answer: X times 10

So although your microphone has an output impedance of 50 ohms, it will perform best if the load (the input impedance) is greater than 500 ohms. With 500 ohms or greater the signal voltage will not be reduced very much.

And with microphones loading them heavily will cause the frequency response in the bass and high treble to suffer as well. And with electronic mics distortion comes earlier too.

So all that to say a hi impedance input for a mic is just fine ...

As long as it is quiet enough and has enough gain to boost the feeble mic signals up to the level your equipment needs.

Hope that helps

BF
Old 22nd March 2013
  #5
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Thanks Bf I heard about that idea of impedance matching being dropped off in favour of Voltage bridging, you explained it again very well.

So in other words, if I understand you correctly: putting my low impedance mic into a Intrument level (and not mic level?) is just as good ?

Ive been reading up on 2 interesting boxes also: the Cloudlifter and the Fethead.
Now here my question: the cloudlifter has variable imput impedance from 150 to 18k, the fethead is fixed at 20k

Now, as far as frequency response for my Melodium goes, what might I expect, if used with one of those boxes? Will it significantly change with those imput impedances?

And secondly: would you recomend one of those gain boxes, OR would I be better off putting a impedance matching transformer (from Xaudia, turning 50ohm up to 150ohm)) in front of my preamp?
Old 22nd March 2013
  #6
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radiovoiceone's Avatar
 

Original ribbon transformers could have output impedances of 30, 50, 150 and 250 ohms.
Some had transformers to raise output to high impedance.
RCA tube consoles and preamps, designed to be used with the companies ribbon mics had transformer primaries of those impedances which could be selected through different taps on the transformers.
I believe a lot of British ribbons had the 30 ohm impedance selected.
Most of these original ribbons today are set for 250 ohms.
Again, I'm talking about the classic ribbon mics, made in the first third of the last century.
Old 22nd March 2013
  #7
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Bump:

Should I use a Fethead / Cloudlifter OR a matching impedance transformer on my vintage ribbon??

Thanks
Old 22nd March 2013
  #8
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I never heard of Matching impedance on Mics...
It WAS a typical method for LINE level, 600 to 600...And this was for lower noise...For the most part..

So as mentioned, you want a MUCH higher than 50 Ohms..above 1K is a good example..
There is also a GOOD reason for using 50 Ohms than say 200 Ohms years ago as well..In some applications it's Still a GOOD idea..

Look at the specs some Ribbon mics..Old Or new makes no difference...
Old 22nd March 2013
  #9
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Yes but then why would Xaudia make little boxes with transistors to bring the impedance up from 50ohm to 150ohm (they say 150 is what modern preamps expect) ? Only to boost signal by circa +12db??
In that case -gain boosting- I guess a Fethead is better.
Old 22nd March 2013
  #10
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I can guarantee that the Xaudia box does improve gain in a clean (ie, improving the signal/noise ratio) in my case. I've tried that out on a number of ribbon and dynamic microphones (Lustraphone Stereolus 1950s ribbon microphone, Trix ribbon microphone and an STC 4021 dynamic microphone). The preamps I am using are those of my RME FF400.

I think that the FEThead, unless it addresses the impedance issue (ie, unless it has a lower impedance than that of your usual preamp, one that is suitable for 30-50 ohm mics), will only amplify the signal but not change the signal noise ratio (well ok, if the FEThead has a better preamp than your usual preamp it will improve it, but it could still be improved by having a preamp of the correct impedance). So recordings of quiet sources will still be noisier than they could be.

I can put up AB comparisons if people are interested.
Old 26th March 2013
  #11
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zebra50's Avatar
 

I just wanted to say that the xaudia boxes contain transformers, not transistors, and their are no active components in there. They swap current for voltage gain. The standard model will transform a 30 ohm mic to about 600 ohms, and give around 12 dB gain into a modern preamps, which tend to be designed with generous input specs to cope with modern mics. The transformers can be wound to any input and output as required.

As Mike says, this is the old solution to the problem and many manufacturers in the 50s and 60s offered matching trannies, or different taps on the ouput transformer in the microphone.

There is a money back guarantee on these so if they don't work for you then you can send them back. No risk! But they never come back,

Cheers
Stewart
Old 26th March 2013
  #12
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nikodemos's Avatar
 

to the OP....

Research the differences between Impedance Matching and Impedance Bridging
Old 26th March 2013
  #13
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emrr's Avatar
Quote:
Originally Posted by nosebleedaudio View Post
I never heard of Matching impedance on Mics...
The vast majority of vintage remote broadcast mixers with multiple inputs and one output use matching unbalanced ladder attenuators in a passive mix bus in front of a single channel preamp. Most could be ordered either 30/50 ohm or 250 ohm input.
Old 26th November 2014
  #14
Gear Nut
 

old HMV ribbon mic with triton fethead

heyho!!

I just got my trition fethead today, to use it with my 40s / 50s HMV ribbon mic...

I made some measurements, to check the gain won by the fethead...

and I heard/measured, that depending on the impedance of the preamp, the sound changed extremely...not a bit, but a LOT!

I put pink noise on my studio speaker with the mic in front...

first I used my Neumann V476 preamp, which provides up to 76dB of pretty clean amplification...with and without fethead...

I got about 25dB of gaim from the fethead, but it chopped of the high and low end :(

Then I tried with my Universal Audio 610 preamp, where you can choose between 500 Ohm or 2kOhm...

The lower the impedance, the less the sonic difference...


With fabfilter, I matched the EQ curves, to see what exactly was going on...

With the neumann , the highend had a difference of around 10 dB! sounded totally dull with fethead...

with 500Ohm on the UA, there was only around 1-2 dB of difference...

Very interesting...

Does anyone of you guys know of a good mic preamp with very low impedance? lower than 500 Ohm?

BTW: I expected to get less hiss with the fethead, because I dont need to crank the gain on the preamp...but it hardly made a difference...(when volume matched/compared)

That sucks, because it is said that the ribbon mic doesnt generate noise itself, it is only the amplification circuit...but even with the fethead, it is a bit noisy for let´s say quiet solo acoustic guitar...



cheers,

Patrick
Old 26th November 2014
  #15
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Hi
Amplifiers designed for lowest noise are generally very low input impedance as it is a function of resistive 'Johnson' noise. Unless it features a form of bootstrapping to make the input impedance 'appear' high you will need to use a low resistance design.
As transformers are passive components they are essentially constant POWER devices so some of the arguments for using high impedance amplifiers fails as the situation is different and you need to extract the greatest signal POWER from the mic.
Due to impedance of microphones that will be a combination of resistance capacitance and inductance this will interact with cable capacitance (primarily as it is greater than it's inductance or resistance) and affect the overall response. Having a high impedance load (amplifier) may tend to accentuate part of the response whereas a low resistive load will tend to 'flatten' the response.
You should appreciate from these comments that there are a number of opposing situations going on and much of it is conditional on the gear and cables used.
A Ribbon mic does have very low 'self noise' (Johnson) BUT it takes a really good amplifier to get the best signals from it. A transformer is a reasonable start but this can introduce unevenness in frequency response as the signal is still low level and capacitance and inductance of following cables etc will make what is probably a very 'flat' response from the ribbon element into one that is more prone to 'influences'.
Matt S
Old 26th November 2014
  #16
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Santiago's Avatar
 

Quote:
Originally Posted by tidalwave View Post
heyho!!

I just got my trition fethead today, to use it with my 40s / 50s HMV ribbon mic...

I made some measurements, to check the gain won by the fethead...

and I heard/measured, that depending on the impedance of the preamp, the sound changed extremely...not a bit, but a LOT!

I put pink noise on my studio speaker with the mic in front...

first I used my Neumann V476 preamp, which provides up to 76dB of pretty clean amplification...with and without fethead...

I got about 25dB of gaim from the fethead, but it chopped of the high and low end :(

Then I tried with my Universal Audio 610 preamp, where you can choose between 500 Ohm or 2kOhm...

The lower the impedance, the less the sonic difference...


With fabfilter, I matched the EQ curves, to see what exactly was going on...

With the neumann , the highend had a difference of around 10 dB! sounded totally dull with fethead...

with 500Ohm on the UA, there was only around 1-2 dB of difference...

Very interesting...

Does anyone of you guys know of a good mic preamp with very low impedance? lower than 500 Ohm?

BTW: I expected to get less hiss with the fethead, because I dont need to crank the gain on the preamp...but it hardly made a difference...(when volume matched/compared)

That sucks, because it is said that the ribbon mic doesnt generate noise itself, it is only the amplification circuit...but even with the fethead, it is a bit noisy for let´s say quiet solo acoustic guitar...



cheers,

Patrick
I'm interested as well in supper low impedance preamps, it's a good idea - but don't you get already enough gain with your Universal Audio preamp set on low impedance?

Failing that, have you thought of changing the transformer in the mic? Perhaps a matched modern transformer can give you more gain without too many artifacts. It may change a bit the character of the mic (wider frequency range, basically less colour), but that would allow you to use other preamps.
Old 26th November 2014
  #17
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Regarding (very) vintage Ribbon mics and impedances - these were often designed with a low impedance input in mind and the frequency response of the internal transformer and the acoustic baffles, grilles, etc. expects a certain loading on the ribbon. Using a high impedance pre-amp input can leave the ribbon less damped than what it was designed for and lead to a less flat frequency response.

Of course, cables, parasitics, etc. will not be exactly the same between the original expected mic pre inpt and a modern mic pre input with a "matching" transformer in the middle. So, some experimentation will be needed to find the optimum result.

Even with vintage condensers, you can sometimes hear a big difference in their responses with different loadings. Especially if they are a simple tube / trafo affair. I had a mic that sounded most happy at 1200 to 1500 ohms loading. Less than that and it sounded muffled (the lower the mic pre input impedance the worse it got). With a higher input impedance, I didn't expect so much trouble, but there was still an effect. It sounded like you were moving the mic closer to the instruments - but in a bad way, albeit quite a subtle effect. Again, an issue with not enough damping. Maybe some ringing on the trafo that could have been fixed with a Zobel. So, I'd be a little wary of these super-high input impedance mic pres with older mics.
Old 26th November 2014
  #18
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More gain in the transformer equates to a change in impedance relationship.

Very interesting observations about the fethead. I'll say (again) that quoted input z and measured input z are usually very different. In many cases I can measure a 200 ohm transformer input with an audio precision unit and get readings closer to 20k. I'm leaving LOTS of details out for sake of brevity, go dig through my other posts for more details. The 200 ohm input is specified for a 200 ohm source, which is critical to the response of the transformer itself. That's all the spec relates to in almost all old gear. The UA and everything modern quotes impedance in an entirely different manner.

I have not been a believer in devices like the fethead. Generally if the preamp isn't working with the mic, you need a different preamp altogether. If no known preamp works then the mic is also wrong for the source. I think we all know this; there is no magic fix.

The fethead will present a uniform impedance to the mic, whereas the transformer input will not. That in itself is an EQ to a dynamic mic, so if you generally like the mic/preamp combo, the fethead will rob you of that basic relationship.
Old 26th November 2014
  #19
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Ok. Dazed and CONFUSED ... again!

So, theoretically vintage ribbons were designed with low impedance (50ohms often) and their matching pre was also low impedance, called "impedance matching".

Now, today most preamps are designed to EXPECT a mic with at least 150 to 200 ohm load.
However, they are also NOT designed to "impedance match" but "impedance bridge" hence the usual minimum impedance of 10x more so around 1200 to 2000 or more kohm.

So far so good, if I'm correct.


Now for the confusing part:
Some modern preamp designers sell us the pre with "a very very high impedance" (8000 to 10000 or more ohms, like AEA TRP and such) because they say this allows ribbons "to breathe".

Problem is: what ribbons are they talking about? Presumably NOT vintage (50ohm) ribbons but modern 250ohm ones.

So the question arises:
1. Will a low z ribbon still benefit from such a huge impedance bridging? Presumably this will alter its freq. response considerably.
Or is it best "left as intended" ?

2. What about a modern ribbon, will it benefit from such huge bridging or not?

3. Could it be that such bridging is useful only for modern 150+ ohm ribbons?

It all warrants experimentation but tbh it's expensive to buy AEA preamps and others just to mess around!

My solution (proposed by Xaudia who services my mics
Put not a preamp with high z (Fethead) but a passive matching transformer which bumps ohm from 50 to 150, and use your normal (modern) preamps with it. I'm very happy, and just the matching transformer gives circa +12db of totally "free" noiseless boost anyway.
So I can easily operate the normal preamp within its decent noise free limits (c. 55db boost) so I'm happily at +65. And if it ain't enough who cares? Turn more DIGITAL gain ITB, it's noiseless and accomplishes exactly the same thing!
Old 26th November 2014
  #20
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Hi
The ribbon 'element' will have an impedance (mostly resistance) of less than 1 Ohm (it is only about half an inch of aluminium foil!), so most units will have a transformer included within the housing to bring it up to 30, 50 or whatever impedance. A second transformer in a preamp would typically raise the impedance and level to as high as possible (without massive HF loss) for a valve input or typically around 1 - 5 K Ohms for a transistor/IC input.
Newer 'all transistor ' inputs can be made that give gain without the 'iron' and it's associated problems.
Originally POWER matching was important but with low noise devices and more research/ interesting designs alternative VOLTAGE maximising techniques can present a 'flatter' response.
The only true way to sort it out is to try combinations of gear to 'hear' what suits your application best.
Very high (more than several K Ohms) input impedance will not alter the response once you are above a certain 'threshold' but WILL start to have issues with excess noise. A form of 80:20 rule. It was long established that if the preamp impedance was around 10 times that of the mic then you are getting about the best performance possible, bearing in mind that other factors such as cable capacitance will also start to intrude.
Matt S
Old 26th November 2014
  #21
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Most old mics and preamps were not matching situations, but bridging. The remote mixers referenced in this thread are more often the exception. Nothing on average has changed in the mic/preamp Z equation since the introduction of the ribbon in the early 1930's. Yes, you can make along list of matching type preamps that existed, but they are the minority.

My 200 ohm input preamps from the 1930's-1960's all read Z 5k and higher. They read LOWER, not higher, when you insert a typical 20db U pad with series resistance around 1k5.
Old 28th November 2014
  #22
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Both AEA and Royer recommend high impedance mic pre inputs to avoid loading down the ribbon and preserving sonic integrity. This is consistent with professional industry knowledge.
Old 28th November 2014
  #23
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Quote:
Originally Posted by emrr View Post

I have not been a believer in devices like the fethead. Generally if the preamp isn't working with the mic, you need a different preamp altogether. If no known preamp works then the mic is also wrong for the source. I think we all know this; there is no magic fix.

The fethead will present a uniform impedance to the mic, whereas the transformer input will not. That in itself is an EQ to a dynamic mic, so if you generally like the mic/preamp combo, the fethead will rob you of that basic relationship.
Glad to see you posting this. I've been down that road as well and found those devices to not be beneficial.

Can't help wondering Doug, on a side note, if there's any particular pre's you tend to reach for with the MF65?
Of course, so much is source dependent but thinking lower signal voice / acoustic stuff. I have one coming soon.
Old 28th November 2014
  #24
I always figured where the Fethead may provide benefit would be amplification as close to the ribbon as possible, say one foot or less. The cables on the AEA R84s, for example, are really short for a reason, and lengthening this distance effects performance IME.
Old 28th November 2014
  #25
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Yeah, if I was on a remote job with hundreds of feet of mic cable and there was obvious interference, I would reach for a fethead type product to improve S/N across that long line, assuming I couldn't put the preamp close to the mic.

MF65: always some ancient tube thing. RCA BA-2 or Gates SA-whatever most frequently for vocals and acoustic guitars.
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