Dynaco A 25 Owners Manual

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APE RIODIC
LOUDSPEAKER
SYSTEM

DESCRIPTION

The Dynaco A-25 aperiodic loudspeaker
is a compact, high performance two way
speaker system. A 10 inch extended excur-
sion woofer is matched to a specially de-
signed soft dome tweeter with a dual section
high pass filter at 1500 Hz, in a cabinet de-
,sign which achieves a uniquely non-resonant
bass response.

The aperiodic, or non-resonant character-
istic of the A-25 provides an unusually uniform impedance characteristic
throughout the bass range of less than 2 to 1. This design utilizes an acoustic
impedance system which provides variable volume action rather than the
totally enclosed cabinet of the acoustic suspension principle or the tuned port
of the bass reflex. The Q of the system is lowered through a high friction
venting action, and the amplifier is now working into a more resistive-and
more linear-load impedance. This enables higher power transfer with low
distortion at low frequencies, and provides superior articulation or transient
response in the bass range. This speaker does not exhibit the boomy or heavy
bass of conventional compact loudspeakers which depend on resonance for
extended lows, yet critical listening will confirm its truly impressive bass
reproduction accuracy.

When measured in an anechoic chamber, the response of the A-25 drops
only about 5 db at 50 Hz even without the normal augmentation of response
which a reflecting surface will add at low frequencies. With normal room
placement, response will be substantially uniform to below that frequency,
with useful output at 30 Hz without doubling. At high frequencies, the dome
tweeter gives smooth response beyond 15 kHz, and its excellent dispersion
and high power handling capability contribute to its big sound and
excellent stereo imaging.

Dynaco has designed the A-25 to take full advantage of the drive capabil-
ities of both solid state and vacuum tube amplifiers. It may be used with
amplifiers having continuous power ratings in excess of 60 watts RMS per
channel, and yet it is efficient enough to be used effectively with amplifiers
of more modest power, such as the Dynaco SCA-35 and Stereo 35. The
A-25s nominal power rating for short duration sine wave signals is 35 watts.

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INSPECT FOR CONCEALED DAMAGE

This loudspeaker system, including the wooden cabinet, was care-
fully tested and inspected before it was packed at the factory. If
damage is evident, follow the recommendations at the end of these

instructions. Any damage to the contents of an unopened carton as
a result of rough handling in transit is the responsibility of the
carrier. This may occur even though the outside of the carton
appears intact. Therefore, any claims for damage must be placed
immediately by the consignee against the carrier.

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INSTALLATION

The most commonly used speaker connecting cable is #18 stranded two
conductor lamp cord (zip cord"). Do not use wire smaller than #18. If the
speaker leads exceed 50 feet, it is recommended that heavier leads be used,
such as #16 lamp cord, or #14 BWG cable.

The connecting terminals on the A-25 will accept standard banana plugs
(including double ones with standard 5% spacing), but it is easiest to wrap
the lead around the terminal or to push the bared end through the hole in the
side of the post, tightening it with the cap. Stranded wire should be twisted
tightly before connecting it to the terminal so that loose ends do not touch
the adjacent terminal. (It is a good idea to tin the twisted leads to avoid
fraying, if a soldering iron is available.)

The nominal impedance of the A-25 is 8 ohms. Thus one wire is connected
to the 8 ohm amplifier output terminal, and the other to the common or
ground terminal.

Two A-25 speakers may be connected in parallel to the output of each
channel of a transistorized amplifier without concern, because the speakers
impedance characteristic is very uniform, and remains substantially above its
nominal value. If a tube amplifier provides a 4 ohm tap, this should be used
for two speakers connected in parallel.

In stereo use it is necessary to have both speaker systems in phase. This
means that the sense of the connections between corresponding terminals
on the amplifier and speakers must be correctly maintained. Thus the red
speaker terminal is normally connected to the 8 ohm or hot terminal on the
amplifier, and the black speaker terminal is connected to the common or
ground terminal on the amplifier. With lamp cord, phasing is easily main-
tained because one lead is coded, with a tracer thread wound around one of
the wires under the insulation, or with a molded ridge on the outer insulation
of one conductor, or with different color conductors.

To check the phasing, play a mono sound source, or set the amplifier con-
trols to provide an A + B signal. If phasing is correct, the sound will be full
bodied (more bass) and will appear to emanate from a point between the
speakers. When moving back and forth between the speakers, you will find
that the sound moves smoothly with you. Incorrect phasing will cause the
sound to jump abruptly between the speakers as you move, and there will
not be a well centered image when you are equidistant from the speakers. To
change the phasing, only the connections between the amplifier and one of
the two speakers should be interchanged.

In most rooms, the best placement for the speakers will usually be at ear
level with respect to the listening position, against the wall, with the speakers
separated by not more than 10 feet. Some listeners may prefer less bass out-
put. This may be controlled by moving the speakers away from the wall.
Those who favor heavier bass will prefer a location nearer the floor, or in
a corner.

The high frequency level is raised or lowered by a screwdriver Operated
switch on the back panel. Each clockwise step is an increase of 3 db at 10 kHz.
Choose the position that suits you. The center position is marked Normal

If you wish to hang the speaker flush on a wall, 2 brackets are provided
which accept #6 round head screws (not supplied) projecting about 1/4 from
the wall, to engage keyhole slots. These brackets should be placed in the
corners, over the depressions which accommodate the mounting screw heads.
Self-adhesive felt feet are provided to protect adjacent surfaces. The Dynaco
nameplate can be rotated if the speaker is placed horizontally.

Where the placement of two speakers in a normal stereo system will not
yield accurate stereo imaging (as when the speakers are very widely sepa-
rated), a third A-25 may be used for a center channel.