Impedance 101: Part Two
Page 1, 2, 3, 4

EXAMPLE 3:
EATING CONSUMERS FOR LUNCH
When I started in this business, interfacing hi-fi to pro was a deadly combination. Then, consumer equipment was “hi-Z,” slang for high impedance, while console input and output impedance was lo-Z, 600 ohms. Now, the interface impedance between consumer and pro gear is more compatible. Operating levels are the primary difference, consumer gear being standardized at -10 dBv, while pro operates 11.78 dB higher at +4 dBm (for vintage 600-ohm gear) or +4 dBu (for modern gear). Then, the impedance mismatch dropped the level further and created a highpass (bass roll-off) filter in the process.

Note: The “V” and the “M” designated two references, 1-volt RMS and 1 milliWatt (mW), respectively.

Figure 3: An exaggerated example of how excessive loading can reduce low frequencies (click for larger view).

Figure 3 depicts the insert points from a Trident Series 65 console. The “source” could be either the mic preamp or line input amp, pre- or post-equalizer, all determined by switches. In each case, the output op amp feeds a 100-micro-Farad (µF) capacitor and a 100-ohm resistor, much better choices for the application. (The Tascam circuit example was focused on R117 being too large to tolerate excessive cable capacitance. Note that the series capacitor in that circuit, C106, is 2.2µF.)

Micro-Cap’s simulation successfully shows what happens in a worst case scenario, the effect of excessive resistive loading of the 100µF output capacitor, creating a highpass (bass roll-off) filter. The very same filtering effect might occur if the output capacitor deteriorated, a very common ailment that plagues older equipment and discussed in last year’s column on “Upgrades and Maintenance Issues.”

The “load” ranged from an unlikely 100 ohms to the more typical 10.1 kil- ohms. Ignoring the 100-ohm load results for a moment, the Table details the frequencies that fall at the “-3dB” (half-power) point for each of the other load values.

Note: The internal 100-ohm resistor combined with the external 100-ohm load creates a 50% voltage divider, dropping the level 6 dB as indicated by the double arrow in Fig. 3.

FINALE
In Fig. 3, the lower graph depicts the simultaneous phase changes as the frequency is swept. Phase is one of the less tangible effects of filtering and equalization. Modern digital filters can be made sans phase shift. I have not had the opportunity to make a side-by-side comparison.
Though it has not been stated directly until now, you should walk away from this article knowing that a low-source impedance and a high-destination impedance are normal for transformerless gear. Even transformerless mic preamps have a 5-kilohm to 10-kil-ohm input impedance, some are variable. (Microphone impedance is typically 200 ohms.) Meanwhile, when interfacing transformer-based vintage (or retro) gear with modern technology, remember that the output transformer should be terminated, preferably at the destination. Speaking of which, at 2,500 words, I am outta here!

Eddie continues to thank Dave Hill (at Crane Song), Michael Shields, Shep Siegel and Dan Kennedy (at Great River Electronics) for their geek help and support. By the time you read this, EC will be a dad for the second time. Visit tangible-technology.com for baby pics and to have a virtual cigar.

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