The AM Forum

What's the best way to go about interfacing a low impedance TX audio chain to a plate-modulated rig that was designed for a high-impedance dynamic microphone?
 
Thanks
 
- JT

Buy a $40 Behringer comp/EQ mic preamp. Many other brands too. Hit the musician websites and poke around. Then hit factory websites and download any specification sheets from models of interest.

They adjust for level, compensate as needed for bass-midrange-treble, and can take you from 600ohm balanced to 10Kohm unbalanced.

They chiefly cater to garage bands running Lo-Z mics into unbalanced mixers that in turn feed consumer grade recording devices. Some inexpensive mixers do it All-in-One.

Good Morning,

 If your just looking for a Quick lash up with out digging into the Transmitter or getting into alot of matching issues,  this little device aids in alot of areas.

http://w2ihy.com/ibox.asp

 very quick easy and you can build it yourself...hope this helps...

Paul, yeah.. I was looking at the Behringer Ultra-Compact series of gear because, to be honest, I really like the form factor and it'd fit in well in my shack.  However, looking at the output impedance specs of a lot of this gear, the impedance levels appear to be very low unless something is not registering with me.
 
Here is what I was looking at:

• MINIMIC MIC800 (http://www.behringer.com/MIC800/index.cfm?lang=eng) - Mic Preamp
• MINIFBQ FBQ800 (http://www.behringer.com/FBQ800/index.cfm?lang=eng) - Equalizer
• MINICOM COM800 (http://www.behringer.com/COM800/index.cfm?lang=eng) - Compressor

Each component is in the $35 to $40 range.

- JT

ZLR: I've heard good things about IHY's gear, thanks for the tip.
 
- JT

Well there's alot to consider, not just only impedance matching, but mic line level output, Hum, AC RF issues, all the little not nice things that show up when designing an Audio Rack.. it's a nice little attenuator that works very well...I've talked many times with Julius and he's good fella..

As Far as Processors I like the DBX series such as this:

http://www.zzounds.com/item--DBX286A along with their EQ, imported to the mic pre amp and it also has RF shielding....another thing to consider when building up a Rack...

Have fun...

I tried connecting a recorder playback to my transmitter through the microphone input, using a network of resistors and a volume control pot to reduce the 2-volt line level from the recorder down to microphone level.  No matter what I did, it had a fuzzy sound, so I used a UTC LS-50, single 15K plate to 500 ohm line broadcast quality output transformer, to match the unbalanced output of the recorder (rated by the manufacturer to work into a load anywhere between 10K and 50K) directly into the peak limiter that feeds the transmitter.  The quality of sound was much better, "no camparison".

I have long used balanced 500/600 audio lines to interconnect between units.  I have found it works better to get each unit in the audio chain to deliver a little too much audio, and insert 6 to 10 dB of attenuation using resistive pads between units to get the correct level.  The attenuator pads give a certain amount of isolation.  I have a few 500/500 ohm audio isolation transformers on hand, but I try to avoid using those whenever possible, since every transformer inserted into the audio chain degrades the frequency response and shifts the phase at certain frequencies.

JT, it probably helps that units like this typically use OpAmps, not transformers as output drivers, since the voltage source can ignore working into a higher impedance load by simply adjusting the gain and EQ to cover the impedance mismatch.

Quite different than trying to bolt up a low-Z microphone directly to a hi-Z mic input.  The microphone coil, in that case, loads down the mic input stage, causing a low level of audio and a very bassy sound.

My experience is with a Collins 32V2. I am using a Rolls/Bellari monaural EQ/comp (HR11C) that has selectable line/mic input and output, with 1/4" TRS and XLR for unbalanced/balanced.

http://www.rolls.com/data/hr11cman.pdf (http://www.rolls.com/data/hr11cman.pdf)

They quote the output impedance as 50 ohms, mic or line level (under Installation).

In practice, I use an EV 635A driving the balanced mic input of the Rolls/Bellari, and I use the 1/4" TRS unbalanced mic output to drive the mic connector on the front of the Collins.

Very easy lash-up and sounds good.

Ok Paul, very good.. many thanks, that helps to clear a few things up for me.
 
- JT