Taken from the AEA TRP manual. (this has little to do with phantom powered ribbons, although it is talked about, but mostly its the loading of ribbons, note the TRP two channel ribbon pre has a 18,000 ohm input impedence)
Appendix A
Ribbon Mics and Mic Pre-amps
© 2004, 2006
by Wes Dooley
A RibbonMics.com Publication
Studio ribbon microphones offer smooth and versatile performance that sounds good on a wide variety
of music. They’ve proven themselves in daily use for more than 70 years. However, to achieve this
great sound quality, using them with the right mic preamplifier is essential. This presupposes an understanding
of how mic pre-amps interact with mics, in this case, with an emphasis on ribbon and other
dynamic mics. Which mic pre-amps are best? Although this appendix will present some criteria for you
to evaluate them, you should trust your ears and start listening carefully to mic pre-amps you own, borrow,
or are considering purchasing.
As you gain experience, you’ll discover that a quiet, distant, solo acoustic guitar, for example, will
need 70 dB or more of clean, quiet gain. Older classics such as the ‘class A’ Neve units were designed
to provide this gain. Likewise, many contemporary, stand-alone mic pre-amps sound good and can be
ordered with extra gain.
High performance, specialty pre-amps start at around $700, and choices multiply amazingly as you get
to $1,000 per channel. Ribbon mics deliver extraordinary dynamic range, especially with percussion.
Your experiences will tell you whether a specialty pre-amp might help you best capture the sounds you
value.
Mic pre-amps with transformers tend to have a sound of their own and lower input impedance. Transformerless
designs tend to be more neutral and open sounding, generally are quieter, and have higher
input impedance. The input impedance should be at least 1,000 ohms for ribbon mics, although 1,500
ohms is better. The lower range of impedances influences wider ribbons less, such as those found in
AEA and Coles mics. Narrower ribbons, such as in the RCA 77 and the Speiden/Royer stereo mics, are
more susceptible.
Ribbon microphones with integrated, DC-powered active electronics, such as the old Cambridge or the
new Royer P48 powered ribbon mics, reduce the influence of a mic pre-amp’s input impedance. Transformers
are still used between the ribbon and the internal electronics, but these internal electronics
keep the recording electronics from being affected by a ribbon’s low frequency resonances.
As always with P48 powering, the 6,800 ohm current limiting resistors place a limit on the maximum
current or voltage available. Ribbon mics have the potential, especially on percussion, for handling
extreme transients and dynamic ranges. Thus in many cases it is better to use a ribbon mic with usercontrolled
external electronics. Completely transformer-less ribbon mic signal chains are possible, but
their current requirements would be high enough that P48 powering would be impractical.
Mic output loading is another term for the interaction between a mic’s output impedance and a mic
pre-amp’s input impedance. Richard Werner of RCA published an AES paper in 1955 showing that the
overall frequency response is altered by this interaction. He measured an RCA 77’s impedance variation
with frequency, and an RCA mic pre-amp’s input impedance at the same frequencies. His paper
demonstrates how this changes the overall frequency response.
The nominal source impedance of a balanced output professional mic can be as low as 20 ohms, is
typically from 150 to 300 ohms, and occasionally is as high as 600 ohms. A balanced low impedance
design minimizes hum pickup, and reduces high frequency loss and slew rate limiting caused by the
higher capacitance of longer mic cables.
Mics are not designed to drive a matching load. If your mic’s output impedance is 300 ohms and the
pre-amp’s input load is 300 ohms, you‘re attenuating the mic’s signal by six dB. Raising the input
impedance to 2700 ohms, reduces that attenuation to one dB, and increases the system’s signal to noise
factor by five dB.
Mic output and pre-amp input impedances can vary with frequency, sometimes significantly. Richard
Werner’s AES paper documented a worse case scenario: He paired a 250 ohm output RCA 77D,
narrow ribbon mic, with the 1,500 ohm input of a transformer-coupled tube mic pre-amp. The mic’s
nominal 250 ohm output impedance soared to 1300 ohms at its 50 Hz resonance. At this frequency the
pre-amp’s nominal 1500 ohm input impedance sagged to around 600 ohms. The result was a deep hole
in the bass response of this combination below 100 Hz.
To hear a mic as its designer intended, the pre-amp’s input impedance should be at least 1,500 ohms
and consistent across the audio band. Even in the ranges above and below the 20-20 kHz audio band,
the input impedance should remain high, as out-of-band load changes can affect in-band sonics. A rule
of thumb is that a mic’s load impedance should be five or more times the mic’s source impedance. For a
300 ohm mic, such as the Coles 4038, this would translate to a 1,500 ohm load. For a 150 ohm source
mic, such as most Neumann mics, this would mean a minimum load of 750 ohms, however, higher is
even better.
Thus, microphone impedances and pre-amp loadings seem to be a perpetual source of confusion. Ideally
the mic’s source impedance should be as low as practical, and the load impedance it feeds should be
as high as possible. Microphone pre-amplifier input impedances have been at 1,000 ohms or above now
for decades. (Although 19th century voice-powered carbon telephone transmitters had to be loaded
into matching 600 receivers for maximum power transfer, even the BBC abandoned that approach well
before the 20th century ended.)
As mentioned above, with such low impedance loads, P48 phantom powered electronics run out of
current at high sound pressure levels. With such mics, the sound can improve significantly if loaded
into a higher input impedance. The lower pre-amp input impedance a mic must drive, the harder it has
to work. For condenser mics or others with built in electronics, driving too low of a load impedance
is heard as decreased headroom, increased distortion, and especially when driving long mic cables, as
high frequency limiting at high SPL.
With the recent resurgence in popularity of “vintage” equipment, some contemporary mic pre-amp
manufacturers provide selectable mic input impedances, as if this is a useful feature or a wonderful
effect. Unless you are working with antique 30 or 50 ohm microphones, however, it is neither. Impedance
matching is bad for signal to noise, increases distortion, and reduces headroom. The 1955 STC
4038 30-ohm version, built for the BBC was the last major studio microphone designed to be operated
into a matching load. More recent versions have a 300-ohm source impedance. Both the 30 and 300-
ohm versions, however, operate more efficiently with a bridging rather than a matching load, and the
BBC has now recognized this. Studio mics are not designed to feed a matched impedance load. You
don’t get the sound you paid for when you force your mic to operate into loads that are less than five to
ten times their own source impedance.
In 1985 AEA built a limited run of transformer-coupled, stereo mic pre-amps with 84 dB of gain.
These pre-amps had a user adjustable input impedance switch so that different mics could achieve their
best square wave response. A number of mic pre-amps now offer user adjustable input impedances.
Such mic pre-amps can alter the load a mic drives in a variety of ways. These changes range from
simply varying the resistive loading a mic must drive, to the complexity of changing the turns ratio
(impedance transformation) of an input transformer, and / or its secondary loading.
Changing the turns ratio and secondary loading of a mic input transformer, can change a mic’s sound
on transients from dull, to open, to ringing. Mic pre-amp designers typically aim for transients that are
neither over-damped, i.e. dull and lifeless, nor under-damped with excessive ringing. However these
designers have no control over which mics their pre-amps will be used with. Thus the same mic, used
with different pre-amps, can yield a quite different sound.
Transformers, like ribbon mics, look simple, but are deceptively complicated. Here are five common
and uncommon events that will change the sound of any transformer.
1. When the source impedance (the mic output in this case) changes
2. When the turns ratio is changed
3. When the load on the transformer’s output side (secondary) changes
4. When the audio is softer or louder than the transformer’s linear operating range
5. If DC through a winding magnetizes the core material and it becomes less linear
These variables can alter a pre-amp’s overall sound in quite complex ways, both in the 20-20kHz audio-
band frequency response, and the frequency response above and below these points. Interestingly
enough, changes in the out-of-band response will impact a pre-amp’s sound, especially with regards to
transient response. As with any signal processing, the judgment is yours whether this is a change for
the better or worse.
To summarize everything above, the most relevant aspects of a mic pre-amp’s input impedance are:
1. How does it change the useable output level and linearity of a mic?
2. How does it alter the frequency and transient response of its mic?
For more than forty years, I’ve listened to, set up, used, repaired and manufactured ribbon mics. The
RCA-44 and RCA-77, among others, were the high performance standard when I was young and began
being supplanted by moving coil and condenser mics as I learned my craft. More recently ribbon mics
have enjoyed a renaissance. Throughout all these years, however, a number of competent audio engineers
have continued to use them daily. Why? Because they deliver a warm, flattering sound in a wide
range of situations that cause them to be the mic of choice for engineers, producers and musicians.
with all of that extra gain in reserve. 
