Fostex 250 - Impedance and Connectivity

NOTE: Many power amplifiers are rated at +4 dBm to +8
dBm (1.23 to 2 volts) input sensitivity. While it might not be
apparent, such amplifiers can be driven to full power by the
Model 250’$ -10 dBV outputs. Power amplifiers are an ex¬
ception to the rule; the input sensitivity specification of a
power amplifier is the level required to drive the amplifier to
its maximum rated output power. To avoid clipping distor¬
tion on peaks, nominal input levels to the amp should be 10
dB to 20 dB below the rated amplifier sensitivity. Given that
the Model 250 outputs are — 10 dBV nominal, and allowing
a 2 dB approximate correction for dBV to dBm conversion,
the Model 250 output level is about 12 dB below +4
dBm—just right. At times, it may be desirable to lower the
Model 250 output level (bring down the MASTER 1-2
FADER)—a surprise if one had casually compared the -10
dBVnominal output to the + 4 dBm amplifier input sensitivity.
IMPEDANCE IN GENERAL
Impedance {abbreviated "Z”) can be defined as the total
opposition to the flow of alternating current in an electri¬
cal circuit. In this context, alternating current (ac) refers to
an audio signal, not to the power mains. Impedance and
resistance are similar, both being expressed in ohms.*
However, resistance ("R”) is the opposition to dc current
flow, not ac; "Z" and “R” seldom are equal in a given circuit;
in fact, impedance changes with frequency. If the frequency
at which a rated impedance has been measured is not
specified, it is probably a typical value which is generally
measured at 1,000 Hz.**
Input impedance is also known as load impedance (the
greater the load, the lower the impedance.) If an input has a
higher impedance, it offers more “opposition" to the flow of
audio than would a lower impedance input. Consequently,
high impedance inputs do not draw as much power from the
signal source which feeds {they are less of a load on the
signal source). For example, if a given output has adequate
power to drive just one low impedance input, it may be
capable of driving five or more high impedance inputs.
Outputs impedance is also called source impedance. In most
cases, an output can be connected only to an input whose
load impedance is equal to or higher than the output’s source
impedance. For example, an output having a 600 ohm source
impedance could be connected to an input having a 600 ohm
load impedance or a 20,000 ohm or higher load impedance.
When the input impedance equals the output source im¬
pedance, the input is said to be matching the output. When
the load is approximately ten times the source impedance (or
higher), the input is said to be bridging the output. This distinc¬
tion is not critical, except in certain cases where the output
must be bridged to avoid overloading or distortion. (In some
passive equalizers, matching, not bridging, is required to
preserve proper frequency response.) A typical example
of a bridged output is a case where the source imped¬
ance is 2,000 ohms, yet the minimum load impedance is
20,000 ohms.
* One kilohm (abbreviated kohm) equals 1,000 ohms.
“Loudspeakers are an exception—there is no standard frequency at
which impedance is measured; a speaker is usually rated close to its
minimum impedance.
Do not connect a hlgh-Z output to a low-Z input.
Sometimes an output can be operated into a load of equal im¬
pedance (matching). It is seldom that an output can be
operated into a load of lower impedance that the output’s
source impedance. For example, an output with a 2 kohm
source impedance should not be connected to an input hav¬
ing a 600 ohm actual load impedance; distortion, and possi¬
ble overheating and failure of the output circuit could occur,
FIGURING IMPEDANCE WHEN ONE OUTPUT DRIVES
TWO OR MORE INPUTS
When a “Y” adapter is used to feed two inputs from a given
output, the combined input impedance seen by the output
is actually less than the rated load impedance of either
input alone. This places a greater load on the output. Rather
than explain the formula to calculate actual impedance in
these cases, we offer a simplified method. Assume that a
Model 250 output is being connected to more than one
high impedance input, and that each of those inputs has the
same impedance, for example 20 kohms. Divide the
input impedance (20 kohms) by the number of inputs to figure
the actual load on the Model 250 output.
Thus, if the output is driving two 20 kohm inputs, the load is
10 kohms (20/2). If it is driving three 20 kohm inputs, the load
is 6.7 kohms (20/3). if it is driving four 20 kohm inputs, the
load is 5 kohms (20/4).
Since the maximum load (minimum impedance) recommend¬
ed for the Model 250 is 5 kohms, the practical limit would
be a 4-way split—if driving 20 kohm inputs. (Higher input
impedances would allow more inputs to be driven, lower
impedances would lessen the number of inputs that could
be driven by a single Model 250 output.)
CAUTION: Do not connect two Model 250 outputs together.
Also, except as specifically noted for track reassignment,
do not connect Model 250 outputs directly to Model 250
inputs, as this could cause feedback.
THE RELATIONSHIP BETWEEN IMPEDANCE
AND LEVEL
We have discussed signal level and impedance separately,
yet are they related? Yes, but not directly. Outputs which
are rated at nominal +4 dBm line levels usually are capable
of driving low or high impedance inputs (e.g., they can
be matched or bridged). Outputs which are rated at nominal
- 10 dBV line levels usually are capable of driving only high
impedance inputs (they must be bridged). This impedance/
level relationship is not always the case; it is merely a com¬
mon design practice.
Insofar as actual engineering design, there is no set relation¬
ship between high and low impedances and high and low
levels. However, the maximum level available from a given
output may be reduced when that output is connected to a
low impedance input, compared to the level it will deliver to a
high impedance input. For example, consider an output rated
at “ + 15 dBV maximum into 10 kohms or more.’’ Such an
output might be useable with a 3 kohm load, although with
such a load it might be reduced to a maximum level of +8
dBV. Be careful though—the same 3 kohm load could cause
another similarly rated output circuit to fail altogether.
How many ohms constitute a high impedance? A low im¬
pedance? These terms are relative, and depend on the
nature of the equipment and whether an input or output is
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