Ribbon Mics and Preamp Impedance

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Somnium7

Somnium7

Noise Criminal
Since I have never owned a ribbon mic before i have been digging around the internet doing some research. I see some articles discussing which impedance level to set the preamp at for the best sound. Some say use a very high impedance and others say use very low.

So this leaves me wondering. What impedance is best?
Is there a definitive rule for this?
Is it based on the performance of the individual mics?

What is the story?

Does it really matter?
 
I had read here at HR that you should use a preamp that is (or can be adjusted to,) 3 to 4 times that of the mic.

Example:

If you have a mic that is rated at 250ohms, your pre input should be at 750 to 1000 ohms. (3x250=750) or (4x250=1000)

I am no expert. In fact I am still learning about this myself. I am starting to get into ribbon mics so, I have questions too.

Can anyone else shed more light on this? Is the impedance something you can tweak by ear? (on adjustable micpres) How do you know when it is set to optimum? Does the mic get louder or just sound fuller, richer, sweeter?
 
RawDepth said:
Is the impedance something you can tweak by ear? (on adjustable micpres)
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Yes.

How do you know when it is set to optimum?
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Technically speaking, you would simply set it as high as possible, since that loads the mic the least. However, that might not be the best sound.

Does the mic get louder or just sound fuller, richer, sweeter?
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It gets louder, yes, but the tonal effects change with the change in impedance. Experiment.
 
Alright, but does the sound get dirtier with a lower impedance?
I know that the CMRR of a preamp degrades at lower impedances and that results in more common mode noise. So isn't playing around with the impedance not such a good idea in retrospect?

The main reason I am so curious about this is because I will soon own one of these mics and am planning to build a custom preamp for it. Building variable impedance into the design is tricky, expensive and can lead to imbalance which wrecks CMRR. I need to know if it's really that important a feature.
 
Somnium7 said:
Alright, but does the sound get dirtier with a lower impedance?
I know that the CMRR of a preamp degrades at lower impedances and that results in more common mode noise. So isn't playing around with the impedance not such a good idea in retrospect?

The main reason I am so curious about this is because I will soon own one of these mics and am planning to build a custom preamp for it. Building variable impedance into the design is tricky, expensive and can lead to imbalance which wrecks CMRR. I need to know if it's really that important a feature.
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Are you doing a transformer-balanced input? That would solve both those problems . . .
 
mshilarious said:
Are you doing a transformer-balanced input? That would solve both those problems . . .
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Nah, I can't afford them right now, so I'm sticking to parts-on-hand and going direct (I just ordered new monitors :eek: ). The good part here is that there is no phantom power so I won't need all those expensive capacitors which wind up costing almost as much as a transformer. During the prototyping phase of this project I am going to experiment with the bootstrapping concept...

http://www.jensen-transformers.com/an/ingenaes.pdf

If this stuff holds it's water I won't need to worry about degrading CMRR with varying impedances.
 
Somnium7 said:
Nah, I can't afford them right now, so I'm sticking to parts-on-hand and going direct (I just ordered new monitors :eek: ). The good part here is that there is no phantom power so I won't need all those expensive capacitors which wind up costing almost as much as a transformer.
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What cap would you buy that is as much as a Jensen :confused: I mean really, you can pay as much as you want for anything, but a good electro bypassed with a poly film of moderate value ought to do the trick. You are presumably going to use caps somewhere in the design, to decouple the (transformerless) output if nothing else :confused:

I would seriously reconsider your decision not to incorporate phantom power. That seriously limits the utility of a preamp which you will spend considerable amounts of time and effort.

PS monitors are much easier to build than preamps, with much larger cost savings.
 
mshilarious said:
PS monitors are much easier to build than preamps, with much larger cost savings.
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Now that I'm interested in!
 
mshilarious said:
What cap would you buy that is as much as a Jensen :confused: I mean really, you can pay as much as you want for anything, but a good electro bypassed with a poly film of moderate value ought to do the trick. You are presumably going to use caps somewhere in the design, to decouple the (transformerless) output if nothing else :confused:

I would seriously reconsider your decision not to incorporate phantom power. That seriously limits the utility of a preamp which you will spend considerable amounts of time and effort.

PS monitors are much easier to build than preamps, with much larger cost savings.
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I have never built a mic preamp before, most of my audio design experience is in modular synths. I assumed that the caps used to decouple phantom power would be very high value (20uf) poly type which cost around $32 each. I had no idea they use bypassed electros for this. Thanks for that!

I think I might just put in P48 after all.

Regarding monitors, I had Mackie HR824s for awhile but they always sounded anal-retentive to me so I went back to using my home-made monitors which had a great sound. Unfortunately, those monitors died. I don't have the time or the woodworking tools I used to so I decided to just buy new after falling in love with the HS80Ms. :rolleyes:
 
Somnium7 said:
I have never built a mic preamp before, most of my audio design experience is in modular synths. I assumed that the caps used to decouple phantom power would be very high value (20uf) poly type which cost around $32 each. I had no idea they use bypassed electros for this. Thanks for that!
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Often they are 47uF/63V electros, usually not bypassed, that would be a custom detail. Use of large-value polys is unusual in preamps. They are so extremely large that the designer would look towards a transformer at that point. You see that sort of cap mainly in crossovers where only a few caps are needed, and space is not an issue.
 
mshilarious said:
Often they are 47uF/63V electros, usually not bypassed, that would be a custom detail. Use of large-value polys is unusual in preamps. They are so extremely large that the designer would look towards a transformer at that point. You see that sort of cap mainly in crossovers where only a few caps are needed, and space is not an issue.
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Or in McIntosh tube amplifiers. Found one in the trash once, all banged-up like a truck ran over it. The caps I scavenged from it made some really nice sample & hold and envelope follower modules for my synth.

My design specs for this project are CHEAP, CHEAP and CHEAP but sounding pretty good and not taking up all of what little time I have for such things.

I do have some decent output transformers now from the parts of my console I recently caniballized in order to keep it's more critical modules working. When i can afford a good mic transformer, it will go with these OPTs into a tube preamp I have on the drawing board.

I am really looking to test out the bootstrapping concept I mentioned earlier too. It sounds promising but they left out just enough information to make it very difficult to actually build. i'm feeling inspired though. Maybe i can get bootstrapping to work as advertised and reap the benefits of a transformerless preamp with the performance of one with transformers. :p
 
that mcintosh stuff is great. we use their amps exclusively at my studio here.
 
mshilarious said:
monitors are much easier to build than preamps, with much larger cost savings.
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mshilarious, any pointers on this? I'm well past due for a monitor upgrade (using M-Audio BX5's currently). Did you build your own speakers? If so can you elaborate or point to a previous thread on this? I'd love to build my own monitors and save some dough.
 
nuemes said:
mshilarious, any pointers on this? I'm well past due for a monitor upgrade (using M-Audio BX5's currently). Did you build your own speakers? If so can you elaborate or point to a previous thread on this? I'd love to build my own monitors and save some dough.
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Wow, yeah there are some threads on it, but it was a while ago, and I'm not sure how to search for them :confused: Also, I'm talking about passive monitors here, you can DIY a power amp, but I think that is not as efficient in terms of cost savings. I scored an incredible deal on a boutique 6 channel power amp years ago, so I never bothered with that.

Anyway, it depends on what you are looking for, but there are some projects out there. Check out the Madisound site, there are a ton of kits there you can buy without cabinets, and bang together some MDF cabinets for cheap, you could do a killer pair for $400 or even less, depending on what you want. From other monitors I've heard, those would compete in the $800-$1,000 range, easily.

My first pair of monitors, I pretty much told them what I wanted and the target cabinet size, they did the crossover design and sent me the drivers. Those were nice boxes (6.5" woofers), but I want bigger now, so I'm doing mains with 10" woofers in a BIG box, much bigger than the typical monitor, mounted in wall. Also I am building a sub the size of a small refrigerator! I like big cabinets :o
 
petimar said:
http://www.wesdooley.com/pdf/TRPOwnersManual_1.1.pdf

Download this, the owners manual of the AEA TRP (The Ribbon Preamp). Starting on page 4 there is a good discussion of impedance and ribbons.

By the way, the TRP is wonderful. It is my favorite pre for ribbons.
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Thanks! this is looking pretty useful so far.

I have the core of my preamp working now. I have been testing it at full gain (1000) with encouraging results. Only 20 microvolts of noise with carbon resistors on the breadboard. Once this circuit is properly built with a PCB, chassis, and good quality components the noise level will drop considerably lower.

All the testing is being done with a dynamic mic right now. I can't wait to get my ribbon so I can see how the circuit performs with it.
 
Somnium7 said:
Thanks! this is looking pretty useful so far.

I have the core of my preamp working now. I have been testing it at full gain (1000) with encouraging results. Only 20 microvolts of noise with carbon resistors on the breadboard.
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You're at -94dBV on a breadboard? Man. The electrical noise in my house alone is usually good for -76dBV in an unshielded design! Is this a fully balanced topology?
 
mshilarious said:
You're at -94dBV on a breadboard? Man. The electrical noise in my house alone is usually good for -76dBV in an unshielded design! Is this a fully balanced topology?
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I'm using a bog standard differential amp made with monolithic transistor arrays and a single opamp for nulling and feedback. The signals are balanced up to the opamp stage.
 
Somnium7 said:
I'm using a bog standard differential amp made with monolithic transistor arrays and a single opamp for nulling and feedback. The signals are balanced up to the opamp stage.
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I didn't see mention of a flux capacitor there. You really might want to rethink that design. ;)
 
if impedance adjustments have the same effect on ribbons that they do on condensers, then yes, it can make quite a difference in the tone

i don't use any formula or trick when setting the impedance for something - i just listen and turn the knob, and find where it sounds the best
 
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