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PostPosted: 02 Jan 2014, 16:19 
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Joined: 05 Jan 2010, 21:16
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I thought I posted this - I either messed up or it got moved. I would assume a microphone is considered a signal source.

Many ribbon mics have either gotten a bad wrap or are at a minimum not performing up to their ability, and it seems nobody (that I can find) understands what's going on when a ribbon mic has poor high (and low) frequency response.

I can’t find the information I’ve shared below – I had to figure it out myself but it wasn’t difficult and ALL of my experiments prove my theory correct.

Most professional mic preamps have a low input impedance, usually 100, 600, 1K or 1.5K ohms. I come up with about 2.7K output impedance for the transformer secondary on a ribbon mic (2 ohms reflected through a 37:1 turns ratio transformer is about 2.7K) that is being required to drive a low impedance load. This is not good for the reasons below.

If the preamps have 600 ohm inputs (switchable to 100 ohm for ribbon mics for more gain) how is a 2.7K source (transformer secondary) going to drive that? It can’t. Look at the transformer freq response specs that are posted for some of these cheap transformers and you’ll see that a source of A FRACTION OF AN OHM was used to drive them to get those specs. A fraction of an ohm source will reflect back to the secondary as a much lower impedance than 2.7K, so the freq response plots that are published are all bogus. NO ribbon mic motor has an output impedance of 0.1 ohm. Most are ten to twenty times that (about 1 or 2 ohms) so you can expect a good amount of high frequency roll-off if you use a mic preamp with a low input impedance. Remember, vacuum tube preamps were around when the ribbon mic was invented, with input impedances on the grid of the input tube of 1Megohm or more in many cases.

Fast-forward 70 years and we find that we now live in a world of condenser mics and preamps with very low input impedances - MUCH lower than vintage tube preamps used with vintage ribbon mics. But for some strange reason we fail to take into account these changes, and as a result we expect many ribbon mics to drive loads that they simply can't drive. A mic pre with a high impedance input would prevent the highs from “sagging” on the primary of the transformer due to the fact the current draw is much lower, OR. the output of the transformer secondary could be buffered and boosted INSIDE THE MIC using active electronics just as condenser mics do. I use a homemade TI chip with JFET inputs as a “pre”-preamp just like my phantom powered condensers use a "pre-preamp" inside of them.

Using a homemade low noise JFET input high impedance preamp/buffer on the secondary of the transformer causes the high frequencies to become much more pronounced and open compared to loading it down with a professional mic-pre having very low input impedances. The transformer gives a “noise free” voltage boost, but the output driving current is reduced by a factor of 37 – there is no “free power”. Conversely, condenser mics have an almost constant output impedance at the XLR mic connector with respect to frequency (active circuitry as opposed to a transformer coil). Transformers tend to have rising output impedances with frequency, so loading them down with a low impedance resistive load will simply create a low pass filter and make the mic sound dull and muddy.

If you have a high impedance preamp such a a guitar pedal with a 1 Meg input impedance try plugging (any) ribbon mic into it before going to the mic preamp. You might get a lot of noise due to the noise the pedal creates, but you’ll also notice that the treble is no longer being “sucked away” by your $5,000 mic pre that sounds worse than a $4 Texas Instruments Op-Amp circuit that costs about $15 to build. If you build one use a non-inverting circuit configuration to avoid resistor noise on the input and a TI LME49860 Op-Amp with an input noise density of 2.7nV/sqrt(Hz). This is one of the quietest op-amps you can buy. The input bias current is high, but with the *relatively* low impedance of the source (assuming 2.5K), current noise is not an issue.

www.lundahl.se/pdfs/datash/1927A.pdf (add a "w")

The Lundahl above specs a flat frequency response – flat with a 0.3 ohm source and a 10K load!!! These impedances are FANTASIES. In real life they are VERY different. Try about 2 ohms for the source (about 7 times higher than the 0.3 ohms used for the specs) which reflects to the transformer secondary at about 2.5K. So a 2.5K source is supposed to drive a load as low as 600 ohms on the input of a good mic pre PLUS the cable capacitance. NOT GOING TO HAPPEN without problems. This is basic stuff. READ THE DATA SHEETS.

All the above means two things: 1)The response curves and specs on transformers is MEANINGLESS, 2)You can fix the problem of a dull ribbon mic using a good transformer AND a buffer amp between the mic's transformer and the mic preamp. A buffer alone may solve all your problems better than a new transformer but it takes more time to build. An alternative is to use the Hi-Z input on a good mic preamp.

www.ti.com/lit/ds/symlink/opa627.pdf

The above is another good op-amp that won’t load down your transformer, the OPA627. This Op-amp has the advantage of a very low input bias current although in this application that spec isn’t as important than the voltage noise.

www.lundahl.se/wp-content/uploads/2013/05/7903.pdf

This transformer specs the freq response using a 30K ohm load!!! I believe that, but as soon as you plug it into a mic pre with a low impedance input the first thing that will happen is that your treble will be attenuated. A device designed to drive a 30K load will NOT PROPERLY drive the 600 ohm load of a mic pre. If your plugging a ribbon mic into a low impedance input mic pre you WILL experience loss of high frequencies. If you buffer the secondary so that it feeds a load in excess of 30K ohms then you’ll get a flat response. The main problem here is that the output impedance of the transformer is not flat with respect to frequency, and if the load is too large (low impedance) the high frequencies will drop off. Also, transformers are poor drivers of long cables. I also use a very short cable (about 5′) from the mic to the homemade buffer/preamp to avoid picking up noise or loosing treble in the cable capacitance.

Summary: Build a high impedance preamp/buffer with an XLR input and output. It need not be balanced. Set the circuit to get a reasonably high gain (40dB is a good compromise), and plug the output of the preamp/buffer into the XLR low impedance input of the mic preamp. Presto – the high frequencies are there is spades and the mic sparkles.


If you want to test this theory and have a mic pre with a good Hi-Z in here's how:

I own a Studio Projects VTB1 Mic pre. Not the best in the world but not too shabby - actually sonically it's a damn good preamp. It also has a high impedance (Hi-Z) unbalanced 1/4″ input that's 1.5 MegOhm. The XLR input impedance is 300 ohms by contrast.

I made a simple cord with an XLR on one end to connect to the mic and a 1/4 inch on the other so I could plug the MXL R40 into the Hi-Z input. Pin 3 (-) and pin 1 (ground) of the XLR connector are connected to the 1/4 sleeve and pin 2 (+) is connected to the tip.

Plugging the mic into the Hi-Z 1.5 Meg input yields a wider, flatter sounding response, and noticeably so using only the stock transformer. The bass and treble are enhanced, and there is less “boominess” in the lower mids. On a 12 string acoustic the difference is immediately apparent. The pick attack cuts through and the upper end sounds bright and airy, not choked or attenuated.

If you’re mic pre has a hi-Z input (usually for instruments like guitars and usually 1/4″) you can try this yourself. I’m not certain why the bass is enhanced, but the treble is simple physics and using a Hi-Z input with a ribbon mic is almost exactly analagous to a guitar player installing a buffer preamp with a JFET input in a guitar to prevent the cable capacitance from “sucking all the highs” out of the tone. The guitar pickups present a rising output impedance with frequency, but the preamp converts the output voltage to a low, constant impedance output. This allows a guitar to drive a low impedance input as well as eliminates the treble attenuation in the cable capacitance. When solid state guitar amps came out it was recognized by many manufacturers that the formerly 1 MegOhm + input impedance of tube amps sounded brighter than the new SS amps. It was quickly recognized that guitar pickups alone couldn't drive the low impedance inputs of either SS guitar amps or the direct in to mixing consoles so manufacturers started including hi-Z inputs to go along with their awful sounding guitar amps. Going into a console requires a direct box with a Hi-Z input or the guitar will sound dull due to the low input impedance of the console.

Audio Technica has made a major step forward in advancing ribbon microphone performance. Why it took so long is anyone's guess.

If anyone would like a schematic of what to stick inside a cheap ribbon mic using phantom power to dramatically improve the mic's performance let me know and I'll post it. I'm currently working to design a PC board that's small enough to fit in an R40 and plan to convert all four of my R40s using my preamp/buffers and new transformers. I'll probably try Shannon (on order), Cinemag, and Edcor (although the RMX-1 I just received had the leads pop off from the poor gluing job they did ruining a $40 transformer - what a waste.

I have two designs. The first uses two capacitors in the signal path to isolate the DC phantom power and can use the phantom power of any preamp. This would allow use with ANY preamp having phantom power that is quiet enough. However, large value non-electrolytic caps are physically big, so it may be a tight squeeze for the output capacitor. A non-electrolytic cap is HIGHLY RECOMMENDED. PIO would be ideal but will probably be too big to fit. Metal film, polystyrene or polyester caps would be better than electrolytic or ceramic - avoid ceramic at all costs) The voltage rating on the caps needs only to be 25 volts - enough to handle the highest peaks of the output of the mic transformer.

The second design uses no capacitors in the signal path, however, it requires a separate split power supply that must be constructed and a special cable with an extra wire to carry the negative voltage to the op-amp (or some other means to bring the power into the mic. It's easy to find multi-conductor cable. The hard part is routing a extra wire into the mic. One alternative here would be to drill a small hole at the base of the mic and make special cables with an XLR jack plus a wire on one end to connect the mic, and a special 4 conductor connector on the other end that hooks to a box with a split power supply (the cable can be reasonably long in this case since the signal will be buffered in the mic).

If you want the REALLY EASY way out, buy a Studio Projects VTB1, make a short (less than 2') cable to plug it into the Hi-Z input, and presto! You have now just improved the sound of your mic considerably, unless you prefer the dark, treble deficient sound of ribbon mics that is OFTEN created by impedance mismatches and NOT the mic itself, motor OR transformer. It's that $5000 preamp designed for condenser mics that's causing your problem.

I don't know why this isn't common knowledge by now, but I searched far and wide and have not once found the suggestion to buffer the poor mic transformer before asking it to defy the laws of physics.

Any comment or suggestion would be appreciated. I would be particularly interested in the experiences of those out there who own a ribbon and a preamp or quality interface with a good Hi-Z input who will take to trouble to jump two pins on an XLR connector and solder a 1/4" plug on the other end of a short cable to make an adapter cable and compare the recorded sound of the Hi-Z vs. the low Z inputs. I KNOW this works on a STOCK MXL R40 and a VTB1. With other mics and pres YMMV, but in general you should hear an improvement in the high end and obtain a flatter response. The deep bass is also enhanced slightly using the equipment above. I'm not sure why that is, but it's the open, clear, clean high frequencies that I'm after and I've found them - buried under decades of misuse.

Consider also two other things. Active electronics that were quiet enough to be used in this type of audio application have only been available for 15 or 20 years (if that) due to the tiny voltages involved. So ribbon mic users did NOT have the option of active buffering inside a ribbon mic until recently. I think that's one reason we still see the misuse of ribbons with preamps designed for condenser mics. 37:1 seems to be the upper limit of transformer ratios that still perform well. Higher ratios won't work mainly because the output impedance gets so high and non-constant with respect to frequency that the mic sounds dull with almost any preamp or long cable. Secondly when transistor amps and consoles were introduced the ribbon mic was already fading from use, and the low input impedance of the new SS consoles combined with the fact the condenser mics sounded great due to their ability to drive SS preamp inputs but many ribbons did not relegated the ribbon to seldom used status.

Now due to cheap Chinese motors they are making a comeback, but WITHOUT the inherent advantage the condenser mic has using an active buffer inside the mic to preserve the sound picked up by the diaphragm. You can wait for a phanton powered Ribbon, or you can DIY and make your own. An internal buffer will do more than a new transformer in most cases. Even the stock transformers pass the signal from the motor, or most of it. Unfortunately the signal (especially treble) from the motor is being lost by the mismatch of the transformer and preamp and is never reaching your recording device or your ears.


Stephen Cole, P.E.
B.S. Degree, Electrical Engineering
Audio Producer/Engineer/Musician
Specialty: Vacuum tube and SS audio circuit design

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PostPosted: 21 Feb 2014, 16:36 
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Joined: 21 Feb 2014, 16:26
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HI
Thanks for all the info. Do you have any schematics, layouts for making this op amp buffer using LME49860
for ribbon mic. I build few ribbon mic and want to improve them as much as I can.


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PostPosted: 21 Feb 2014, 19:09 
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Joined: 06 Jun 2008, 18:23
Posts: 5318
Location: Australia
Shamadan wrote:
HI
Thanks for all the info. Do you have any schematics, layouts for making this op amp buffer using LME49860
for ribbon mic. I build few ribbon mic and want to improve them as much as I can.

I would like to see images of your ribbon mics and maybe a quick overview of how you make them. I'm about to make mics our of 2" Aluminum/Magnesium cone wide range speakers. I'm placing them in a small speaker box and using a single transistor preamp to boost the initial signal.

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Projects: "Sortilege" - 180W Tripath Class D Mundorf upgrade amp | “Mantra” - tube MM phono preamp | "Carbon” - MM phono preamp | 180W RAW Class D amp |


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PostPosted: 22 Feb 2014, 05:59 
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Joined: 21 Feb 2014, 16:26
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Very simple design. I was thinking about a buffer to improve signal / noise ratio.
Do You think that Non-inverting amplifier would help. What single op amp do You recommend?


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PostPosted: 03 Apr 2014, 16:31 
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Joined: 03 Apr 2014, 16:16
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Fantastic, thanks for sharing that knowledge Stephen.
sampleaccurate wrote:
If anyone would like a schematic of what to stick inside a cheap ribbon mic using phantom power to dramatically improve the mic's performance let me know and I'll post it.

Would you be able to post up that schematic? :)

Tashi


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PostPosted: 07 Sep 2014, 04:18 
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Joined: 07 Sep 2014, 03:13
Posts: 2
Some things that Stephen mentioned in his post are incorrect.
Firstly he writes:
"NO ribbon mic motor has an output impedance of 0.1 ohm. Most are ten to twenty times that (about 1 or 2 ohms) "
This is incorrect a properly designed ribbon mic motor should have below 1 ohm impedance. But he probably hasn't
measured with sensitive enough equipment, you won't see below 1 ohm with a $20 multimeter.

Secondly: "The Lundahl above specs a flat frequency response – flat with a 0.3 ohm source and a 10K load!!! These impedances are FANTASIES"
Again these specs are not fantasies there are preamps like the AEA TRP that provide up to 18k load.

Thirdly: "You can fix the problem of a dull ribbon mic using a good transformer"
Most of the time a dull sounding ribbon mic will have one of a number of other problems that have nothing to do with the transformer.
The ribbon element is not in good shape ie. the ribbon is sagging, old, damaged in some way, not properly corrugated, too tight, the ribbon material is too thick like in the cheapest chinese ribbon mics some of which use ~6 micron foil.
If the mic is an older mic like pre 1980's. Then it most likely has weak magnets that should be replaced if possible
with new neodymiums.
So once you know that none of these things are causing the problem of dullness, then you can look at the transformer.
Personally I favour passive ribbon mics, if properly designed they are close to being no noise mics.


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