I currently use two microphones mixed together in proper phase, with a homebrew tube type 2-channel audio mixer. One mic is a vintage hi-Z Astatic D-104 and the other is an Electrovoice model 670 dynamic that otherwise might have been tossed out because it somehow developed very poor high frequency response. I mix in some audio from the dynamic to give the D-104 a little more bass response and mellow out the characteristic shrillness.
Each channel of the mixer pre-amp has two hi-mu triode stages of amplification using a 12AX7 tube. Since the audiophools have driven up the price of n.o.s. 12AX7's to ridiculous extreme, a good alternative is a pair of older tubes, the 6F5 or 6SF5, which are electrically identical and have a mu of 100, just like the 12AX7. The D-104 has a built-in acoustical pre-emphasis curve, achieved by a response peak, that gives it the unique D-104 sound. But I added some additional pre-emphasis to the D-104 by careful choice of cathode resistor by-pass in the 12AX7 stages, so that the pre-amp has a rising characteristic starting about 800~ and rising up to about 9 dB at 2000~ and then levelling off. The combination of electrical pre-emphasis and peaked response curve of the D-104 gives it good punch by emphasising the sibilance frequencies of the voice. The bass response of the dynamic balances out the shrillness of the D-104 to give the overall audio a pleasant sound, at least with my voice. I use a low-pass filter in the audio line between the pre-amp/mixer and the compressor/peak limiter. It gives me the switchable choice of two passive L-C filters. One, which I use most of the time, has a gradual roll-off, beginning at about 5000~ with complete attenuation at about 7500~. For congested band conditions I switch to a sharp cut-off brick-wall filter that is flat out to about 3300~, but near complete attenuation at 3400~. I can switch out the filter entirely, but rarely run that way because I know some of the "broadcast quality" audio transformers in the chain have measurable phase shift distortion beginning around 10K, and this could cause some of the push-pull stages to generate splatter and distortion around 10 kHz from my carrier frequency. Besides, very few receivers used by amateurs would respond to 10 kHz audio because of the necessary selectivity for listening on the ham bands. Even with the 3400~ filter, the rising characteristic and peak in the 2-3.5 kHz region compensates for the lack of highs above the cut-off frequency while balancing out the low frequency response of the dynamic, and I routinely get reports of "broadcast quality" even when everything is cut off above 3400~. Most signal reports say there is only a subtle difference between the 3400~ and 7500~ cut-off, although with wide-open selectivity at the receiver the difference is said to be readily noticeable. The filter is followed up with a UREI BL-40 Modulimiter, which is a processing device built for AM broadcast use. It keeps the modulation level near 100% without overmodulating, and I don't have to ride the gain and closely watch the scope at all times to maintain a good modulation percentage without pinching off the carrier. Reports say the limiter is very transparent and has little effect on the sound of the audio. I do not try to compress or limit excessively in hopes of increasing the average power level in the sidebands; that produces distortion with little benefit. I just follow the manufacturer's recommendation for broadcast operation. The Modulimiter has two limiter stages, the first one called "RMS limiting", which is basically the same thing as compression, followed by "peak limiting" which is pretty much the normal meaning of the term. All units in my audio chain: pre-amp/mixer, low-pass filter unit, Modulimiter, line amplifier and transmitter are linked together with balanced 500/600 audio lines. Click on the link below to see where I first got the idea for my audio response curve when I built the mic pre-amp some 30+ years ago. I didn't follow his instructions exactly, and I don't guarantee that George's theory is 100% correct, but using it as a starting point for trial-and-error experimentation I got the best sounding audio possible with the microphones I had on hand. Since then I have tried some expensive broadcast type microphones running the pre-amp at flat response, and the audio was always inferior to what I get with my two-mic combination. Note: results may vary with different voices. I have monitored my audio while visitors were at the mic, and while some sounded great, others sounded like crap. I firmly believe you need to tailor your audio to sound best with YOUR voice over the air. http://www.amwindow.org/tech/pdf/eam.pdf _______________________________________________________________ This message was typed using the DVORAK keyboard layout. http://www.mwbrooks.com/dvorak/ http://gigliwood.com/abcd/ ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
