Microphone transformers converted to MC
step-up devices. Back row, left to right - RCA MI12399A, Altec 4722 and Altec
15095A. Front row, left to right - UTC P-1, Beyer Dynamic TR/BV 351015006 and
Dukane 3A55.
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| Tamura TKS 83 |
Many SET users are also analog aficionados and have discovered the
musical qualities of classic moving coil cartridges like the Denon DL103
series, Ortofon SPU, Fidelity-Research, Supex, Koetsu and etc. There are a few
brand new high quality MC transformers available from Tamura, Jensen, Sowter,
S&B and Lundahl. But chances are, you may be looking for something classic
or have a pair of obscure input transformers in your shelf that may do the job.
The Western Electric 618A, B and C input transformers are prized acquisitions
in Japan for use as MC step-up devices based on my perusal of MJ and Stereo
Sound Tube Kingdom magazines.
For the past couple of years, I have seen prices skyrocket for the Altec
4722 mic transformer [38 or 150:50K] since many have also discovered its merits
as a high quality MC step-up device. Others have even resorted to wiring its
companion line out transformer [15K:600 or 150] - the 15095 and 15095A -
backwards [150:15K] for MC step-up duty. However there are still many sleepers
out there like the RCA MI12399A and certain obscure models from Thordarson, UTC
and etc. Moving Coil cartridges behave very much like condenser microphones -
low impedance and low output - requiring a voltage boost. Step-up transformers
are passive devices and as long as they are wired properly, they are virtually
noiseless. To me a wide bandwidth input transformer is the most elegant way of
boosting MC output to MM phono level.
Back in the 70s and 80s when MC cartridges where at their peak in sales,
a lot of high end manufacturers sold black boxes wherein they inserted some
sort of step-up ratio transformer inside to boost the gain on an MC cartridge
into a typical 47K input phono preamp. Many of these transformers were probably
designed originally as microphone input devices. In general any transformer
with an impedance ratio from 150:15K [voltage ratio = 1:10], 150 ohms: 50,000
ohms [vr = 1:18] or even 250:50k ohms can work well as an MC step up. If the
primary has multiple taps [3-250 ohms] so much the better. Remember that the
square root of the impedance ratio = the voltage ratio. Another relevant
specification [if provided] is bandwidth, typically a good audio transformer
should have a flat response [less than +/- 1 dB] from 20 or 30 hz to 20 khz or
greater.
Most mic transformers have the wiring diagram printed on its case.
However there are some rare instances wherein they are unmarked. The best way
to sleuth a mystery transformer is to take impedance measurements between
terminals to determine the primary and secondary taps. After you have written
down the terminal arrangement, inject sine waves to the primary and view the
output from the secondary on an oscilloscope for proper phasing. Usually a
transformer that exhibits minimal phase shift or ringing when injected with a
1khz square wave [secondary loaded by a 47K resistor] will have good bandwidth
and generally good sonic performance.
I have learned through the years that loading the primary and/or
secondary of an MC step-up device with resistors to suit a cartridge
manufacturer's specification does more harm than good because it can induce
unwanted ringing in the transformer. The 47K input impedance at the phono
preamp is sufficient loading at the secondary.
Here is my typical wiring configuration for an MC step-up transformer:
*Grounding,
hum loops and etc.
The diagram above is my tried and tested arrangement to wire a
transformer as an MC step-up device. I discovered it by reverse engineering the
wiring of a Denon AU320. As long as the negative phases of the primary and
secondary are connected, I never encountered a ground loop. Some transformers
have a separate terminal for the shield in which case this should be connected
to a binding post wherein the tonearm cable ground lead should also be
attached. Another possible scenario is when the case itself is connected
internally to the shield. In this situation you cannot use an all plastic
chassis since the case has to be "grounded" onto the metal chassis
itself as well to ensure proper shielding.
As you can see from the pictures above I mount octal tube sockets and
RCA jacks in plastic project cases available from your local Radio Shack or
other electronic stores like Parts Express and wire them according to the terminal arrangement of the
transformer. If you use a metal case, make sure the barrel of the RCA jacks are
properly insulated otherwise excessive hum may result due to a ground loop.
Here is the wiring diagram for the Altec 4722.
Notice the switch on the positive phase of the primary, this was done so
that I can avail of the 38 ohm [5 & 6] and 150 ohm [4 & 6] inputs at
the primary.
Above is the diagram for exclusive 15095 or 15095A [150 ohm, parallel
connection]. To my ears the 15095 and 15095A do not sound exactly the same and
cannot recommend their use as a pair.
Sonic Impressions
I have been using the Tamura TKS83 and Altec 4722 as my main step-up
devices. My reference combination is the Denon DL103/4722 at 150 ohms and the
Ortofon SPU GME/TKS83 at 3 ohms. Sonically, I rate the 4722 at the same level
as the TKS83 with the Tamura sounding a bit leaner and detailed [perfect for
the softer sounding SPU] while the 4722 a bit warmer [tames the leaner
character of the Denon] with more air and decay. Others may find the 4722 a bit
euphonic depending on taste and system context, so try the 15095 and 15095A.
These I rank very close to the 4722 with just slightly less gain. In certain
ways the 15095 have more of that modern TKS83 character, especially the 15095A.
The RCA MI12399A also belongs to this same league with a bit more of the 4722
euphony.
The UTC P-1 and Beyer Dynamic are about as good as the Denon AU 320 with
the Beyer sounding leaner and detailed while the UTC is classically warmer and
with more air. The Dukane 3A55 is a decent performer but outclassed by the
others in this survey. My friend Ding in NYC found a pair of Thordarson
T-43606-A several years ago at a hamfest and this model actually sounded on the
same level as an Altec 4722 and the TKS83 when we did a shoot out at his place.
These are subjective observations gathered from extensive listening
through my two systems using the following MC cartridges, Denon DL103R, 103,
103C1 and Ortofon SPU GME and MC10.
Have fun and happy listening!