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Why Ribbon Mics Sound Dull

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2nd January 2014, 06:08 PM

#1

sampleaccurate

diyAudio Member

Join Date: Mar 2010

Why Ribbon Mics Sound Dull

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.

ww.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.

ww.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.

ww.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

Last edited by sampleaccurate; 2nd January 2014 at 06:10 PM. Reason: typo

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5th January 2014, 06:17 PM

#2

bgruhn

diyAudio Member

Join Date: Feb 2008

Location: Medfield, MA, USA

Stephen,
I found your analysis of ribbon
mike performance to be most enlightening. I've been a ribbon mike user
since 1960, give or take a few years, starting with a Reslo with a
relatively hi-Z output impedance and a R to R recorder (vacuum tubes).
Most recently built some ribbons using neodymium bar magnets and Lundahl
transformers. Most of my test work was done with Gehringer (sp?)
preamp/mixer having a higher impedance than what you were talking about.
As far as I can recall I have not seen that rolled off, lack of sparkle,
performance. Sometime when I get back to playing with ribbons and
recording again, I will definitely explore the problem. So thanks for your
input. I didn't want your posting and work to go
unrecognized.

rgds,
BillG

Last edited by bgruhn; 5th January 2014 at 06:32
PM.

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5th January 2014, 09:41 PM

#3

gvasale

diyAudio Member

Join Date: Dec 2013

Location: webster, ma.

I've also used ribbon mikes. The first ones I
bought were Shure 330s, later got 1 SM33. I never got to play with them as
I hoped, and it's more than 35 years. I still have them. Most always used
with Shure's A95A transformer.

Last edited by gvasale; 5th January 2014 at 09:43
PM.

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5th January 2014, 09:58 PM

#4

Frank Berry

diyAudio Member

Join Date: Sep 2003

Location: Midland, Michigan

bgruhn,

Those Reslosound ribbon mics
were some of the best. Sadly, I lost a pair in my divorce in the mid 70's.
I simply forgot to take them with me. I'm certain that my ex put them into
the trash. Her mistake. They would be worth a fortune today.

__________________
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6th January 2014, 01:07 PM

#5

sampleaccurate

diyAudio Member

Join Date: Mar 2010

Quote:

Originally Posted by bgruhn

Stephen,
I found your analysis of
ribbon mike performance to be most enlightening. I've been a ribbon
mike user since 1960, give or take a few years, starting with a
Reslo with a relatively hi-Z output impedance and a R to R recorder
(vacuum tubes). Most recently built some ribbons using neodymium bar
magnets and Lundahl transformers. Most of my test work was done with
Gehringer (sp?) preamp/mixer having a higher impedance than what you
were talking about. As far as I can recall I have not seen that
rolled off, lack of sparkle, performance. Sometime when I get back
to playing with ribbons and recording again, I will definitely
explore the problem. So thanks for your input. I didn't want your
posting and work to go
unrecognized.

rgds,
BillG

Thanks.
I'd like to add (if I didn't mention it) that I put a potentiometer of 10K
across the secondary transformer output windings. Some transformers work
better with a resistive load to allow some current flow in the secondary
and dampen the response. With no load the transformer can have resonance
and even ring. By placing a 10K potentiometer in series with a 100 ohm
resistor on the input that shunts the input to ground it's possible to
present any input impedance from 10.3K down to 100 ohms.

If you
record something and turn the pot while doing so, as the transformer
sees different loads, the sound will change as the output coil "sees"
different impedances. There is an optimal spot where the frequency
response is the flattest and another optimal spot where your ears judge it
to sound best. Take your pick. Use a spectrum analizer, mount the mic in
front of a speaker making white noise in a non-reflective room (or
outside) and watch as the frequency response curve changes when you turn
the pot. It's difficult to get an absolute measurement on the response,
but relative measurements reveal what's going on. Your transformer may
need a higher or lower value resistor. There is an optimal load - you just
need to find it.

I just wish they had put a 4th pin on XLRs. I
could use split +/- phantom power and a special homemade phantom power box
with negative power, put the negative power on the unused pin, and have no
caps in the audio path of the circuit No modification would be needed for
the mic.

Ideally I'll use fixed resistors and switches to change
the input impedance so it can be exactly duplicated in a different
session. Using a pot at first will tell you what range you need so you can
spread the impedance over a range where it has an effect, for example,
100ohm, 200ohm, 500ohm, 1K, 1.2K, 1.5K, 1.7K, 2K, 2.5K etc. using
combinations of resistors to reduce the number of parts. You may find that
the mic sounds best over a very narrow range of loads.

Thanks
again.

sampleaccurate

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6th January 2014, 06:34 PM

#6

gvasale

diyAudio Member

Join Date: Dec 2013

Location: webster, ma.

There are 4 pin xlr connectors. It may be
your favorite dealer doesn't carry them.

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6th January 2014, 06:55 PM

#7

Pano

diyAudio Moderator

Join Date: Oct 2004

Location: Kona, Hawaii

Blog Entries: 4

Yes, I use 4 pin XLR for balanced stereo
lines. But I think Stephen meant on the mic itself, so no mods would be
required.

Thanks for your post, it makes a lot of sense. By using
low input impedance mic preamps, we are loading down the ribbon itself
causing an undesirable response.

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6th January 2014, 07:54 PM

#8

LineSource

diyAudio Member

Join Date: Sep 2003

Location: SiliconValley

sampleaccurate,

There are moving coil
phono preamps which parallel a few JFETs to obtain very low noise, and
enough gain to avoid the need of a step-up transformer. Each JFET has a
0.5NF, so four-parallel ~0.15NF. These circuits can be adapted to a ribbon
mic. Putting a JFET amp in the ribbon case would be possible/ideal.
Separate power supply or rechargable battery.

Aluminum ribbon, 2“
tall, 4mm wide, 2.5u thick ~ 0.14 ohms

1mv from the ribbon can be
amplified to ~400mv output into 50 ohms. ~52db gain.

LTSPICE
simulations show 0.00005% THD

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6th January 2014, 08:18 PM

#9

dirkwright

diyAudio Member

Join Date: May 2011

Location: Virginia

Quote:

Originally Posted by sampleaccurate

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.

The actual ribbon has a
much lower resistance than 2 ohms. It's on the order of 0.5 ohms or so,
based on what I have read.

__________________
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9th January 2014, 08:57 PM

#10

sampleaccurate

diyAudio Member

Join Date: Mar 2010

AC Impedance not DC Resistance

Quote:

Originally Posted by dirkwright

The actual ribbon has a much lower
resistance than 2 ohms. It's on the order of 0.5 ohms or so, based
on what I have read.

You're correct,
but I'm speaking about the AC impedance, not the DC resistance - they are
two completely different things. DC resistances don't reflect through the
transformer - DC is completely blocked.

The "AVERAGE" AC impedance
(across the audio band) of the ribbon motors I've read about is from 1 to
2 ohms.

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