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Category: Audio Accessories for Transmitter & Receiver

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Review Summary For : MM1 IMPEDANCE MATCHER
Reviews: 8MSRP: 17.95
Remember that clean, smooth, broadcast quality audio that you use to get from your tube final transceivers? Well now with the MM1 you can get that same quality audio from your solid state radios with your D-104 once again. You'll also find that with a correct impedance match you'll have improved audio drive, and not have to use an audio pre-amp that picks up all the un-wanted background noise. The boards are small enough (1"X7/8") to fit into the base of any D-104 (even those with the audio pre-amps). The circuit works well at 8-13.8 V.D.C., but comes with a 9 volt battery snap already installed. Simple, easy to understand installation instructions come with every board.
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W9AC Rating: 2009-01-11
Good, but can be improved. Time Owned: 0 to 3 months.
I just installed a MM-1 into a D-104 mic. The MM-1 is a good value, but it can be easily improved with a few circuit changes.

The reason for a device such as the MM-1 in the first place is that through the evolution of time, the D-104 began as a Hi-Z grid-coupled mic to the input of a vacuum tube speech amp. As solid-state devices became poplar beginning in the '60s, the D-104 was no longer capable of producing high-quality audio into the lower input Z of solid-state transmitters. Much of this effect was masked over the years as solid-state circuitry and narrow-band SSB evolved together.

As delivered, the MM-1 is not configured as a true "source follower." Rather, the Drain is switched with the Source lead. For most audio applications, this does not present a problem. Still, the device should be configured properly from the start. I simply unsoldered the MPF-102 FET and reversed the Source and Drain leads. Now, the MM-1 is configured as a true source-follower.

Perhaps the "Achilles Heel" of the MM-1 is its incorrect termination of the crystal cartridge. The analog of a crystal cartridge is a voltage source, in series with a capacitor. Depending on the age, QC and type of crystal cartridge, the series capacitance may range from 800pF to 1800 pF. The low-frequency roll-off of a crystal cartridge primarily becomes a function of the series capacitance and its terminating Z value.

For example, let's assume that our D-104 cartridge has 800 pF of series capacitance. Even with 1.0 megohm cartridge loading, the low frequency turnover point (i.e., Fc = -3dB) is 200 Hz. The circuit actually needs to be lowered by one full octave for today's transceivers. Most rigs manufactured during the last five years are capable of excellent low-end response down to at least 100 Hz. For the male voice, the first fundamental generally appears between 70-85 Hz.

As delivered, the MM-1 presents far too much low-frequency roll-off for a crystal cartridge.
Like my other D-104 mics, I modified the MM-1 by removing the series capacitor, and removing the shunt resistor at the FET's gate. These are superfluous components in the MM-1 circuit, and only serve to degrade audio performance. Instead, I am using a 5.0 megohm resistor coupled to the gate with the only other component being a ferrite bead. In other words, the crystal cartridge is coupled to the FET gate with a 5.0 meg resistor. One end of the 5-meg resistor is inserted into the ferrite bead at the FET's gate. After the change, the cartridge loading Z goes up well past 10 megohms and the -3dB turnover point is lowered to approximately 50 Hz, depending on the characteristics of the individual cartridge.

On a positive note, the MM-1's FET is biased correctly. The bias resistor of choice is approximatly the reciprocal of the MPF-102's "Gm," or transconductance value.

Also, the MM-1 could use some additional RFI abatement. On the voltage supply line, I recommend a combination 1 mH RFC with a 0.1 uF film disc or capacitor to ground. Finally, I am using a 100 uH choke on the audio output lead with a 0.001 disc cap shunted to ground. In the alternative, the audio output line and shield could be bifilar-wound on a toroid as a common-mode choke with even better RFI abatement while maintaining audio integrity. In either case, these components should be included on every MM-1 and they would not add appreciably to the cost of the unit.

I could have circumvented all these issues in the first place by "rolling my own" interface although I decided to use the MM-1 as a building block since I did not want to ceate my own PC board -- although I could have used Vector board or "perf" board.

Last throught -- There are many similar D-104 buffer circuits shown by various authors on the Internet. To date, every single one I've seen is either deficient or incorrectly designed.

The MM-1 is capable of making a D-104 (or other crystal mic) into a studio-quality microphone -- it just needs a bit of help along the way.
VK3ZRT Rating: 2008-03-31
Great product, value for money, excellent service Time Owned: 6 to 12 months.
Eham reviews suggest many folks have been disappointed when their D-104 'lolly-pop' mike sounds 'tinny' when used with modern rigs, or is less than satisfactory even on some 'boat-anchors.' This is true even if their mike is amplified (factory buffered), but more so if it isn't. Larry's MM1 is easy to fit to any Astatic D-104, and enables the full low-frequency response range of this time-honoured and well-respected microphone, making it ideal for 'rag-chews' as well as retaining its well-earned Dx reputation. The MM1 is beneficial even with high input impedance boat-anchors, but is essential for modern low impedance rigs. While many tube rigs present around 1 Megohm load, the MM1 presents around 10 Megohms - most suited to this outstanding crystal mike. After receiving very positive on-air feedback under varying conditions over several months, I ordered four more for the remainder of my small D-104 collection. These high impedance mikes deserve to be heard at their best. The MM1 is value for money and Larry's customer service leaves nothing to be desired. Bob
CHARLIEC321 Rating: 2006-07-13
OLD D-104 SOUNDS NEW AGAIN!! Time Owned: 0 to 3 months.
i installed one in a tired old d-104 i have
and i gotta tell ya i sounds like new again
audio is much better!!!
easy to install my soldering skills are not that great but i had it going in about 10 minutes
if you love your d-104 you need this its great!!!
IZ4CTM Rating: 2005-11-28
Excellent in old TURNER mikes Time Owned: 0 to 3 months.
just ONE word: perfect!
i use it with a very old Turner454X and a 254c... it is a bomb! Nobody in my country believe that i use non amplified mikes!!! Strong and loud voice.
thanks to Larry.

Mirko (Italy)
K4CWO Rating: 2005-09-12
Excellent Time Owned: 0 to 3 months.
Wired mine to an older,amplified D104, being used on a Kenwood TS-570 SG. Using the existing battery in the D104 to power both the MM-1 and the pre-amp, thru a switch on the mic's grey base that I added. So far, I have received good reports. May add another MM-1 to an existing high impedanced Turner Desk mic. 73s, de Bobby
G0CTU Rating: 2005-09-03
two words "excellent audio" Time Owned: 0 to 3 months.
I fitted the MM1 in my d-104 and wanted some one to give me a real audio report, I was listening round and heard a station complaining about the quality of audio from people he was trying to copy, so I thought, "here is my chance" I gave him a shout and the first two words he said was "excellent audio" ENUF SED, well done Larry
SM0PCA Rating: 2005-08-27
New life on ASTATIC Time Owned: 0 to 3 months.
The combination of ASTATIC D-104 and MM1 gives great sound for DX. Not so bassy as "modern mikes". I have recived very good coments on my DX audio. It is a great combination with my T.T. Omni6+
Tommy /SM0PCA
WA2E Rating: 2005-05-06
Great upgrade for D-104 Time Owned: 0 to 3 months.
Tiny board with a huge improvment for a D-104. My unamplified D-104 has been sitting for years. As we all know it's not compatible with new rigs but this little board has allowed me to use my old mic connected to my TS-940. Now I get nice audio reports using the old favorite. Fits easily inside the base and can be powered via 9V battery or 8V from the radio. Scroll down Larry's page to view.