Manley TAPE HEAD PREAMPLIFIERS - Manual (Page 6)

6
TECHNICAL DETAILS
The tape inputs are unbalanced however the POLARITY switch governs which XLR pin is used
for “positive” signal from the “floating” head coil. Normally Pin 2 is signal, Pin 1 and Pin 3 are
grounded but if the switch is set “180” then Pin 3 is signal and Pin 1 and Pin 2 are grounded. The
input impedance is variable from 4K to 250K depending on the setting of the VHF control.
Notice that “loading” the head affects its hi frequency response.
The EXT inputs are unbalanced. The input impedance is 50K. Pin 2 is signal, Pin 1 and Pin 3 are
grounded. The line driver gain is variable and typically 15db . With an Ampex ATR series
machine the easiest location to get the signal is from the edge connectors in the meter bridge
(INPUT/OUTPUT PWA). We drill holes in the back panel to get the wire thru the chassis. Use
shielded cable - Solder the signal wire to pin 10 and the shield to pin 11.
The line outputs are also unbalanced. The output impedance is less than 200 ohms. The output is
capable of driving +36 dBv or about 32 dB of headroom. This is enough to “test” the next piece
of gear in the chain. Some OP-AMP inputs may be damaged if the designer did not expect better
than +/- 15 volt peak swings. This is unlikely, but it is likely that the input has nasty clipping
characteristics. We mention this because the first suspect if peak clipping is a problem is to check
the unit following the tube preamp.
The noise floor typically measures -75 dbv, A Weighted with an average responding meter. This
equates to a few dB improvement over most stock electronics and pretty close to theoretical
limits. There may be some worse very low frequency noise components - mostly due to
fluctuating AC mains.
The VU meter is factory trimmed for +4dBm. It can be adjusted internally on the METER PC
BOARD.
The tube complement is selected for low noise and microphonics. We expect that each tube
should last for 5 to 10 years, however sometimes they don’t. If replacing a tube is necessary then
either buy the tube from us or buy several very good tubes and select the quietest one.
Unfortunately the raw gain of this circuit and demanding performance requires better than
average tubes.
The first stage is particularly critical. This is a 12AX7WA located directly behind the HF control.
It is followed by another 12AX7WA located behind the GAIN control. The line driver uses a low
noise 6072A followed by a 7044. The line driver is the board closest to the back panel.
Best performance with most equipment means providing good ground paths.
With a device such as this with mucho LF gain, grounding may be critical. We have included
chassis and circuit ground terminals on the back panel. You may try disconnecting chassis from
circuit and/or running a separate ground wire to the monitor console / system. You may also try
disconnecting the shield at the XLR’s to eliminate any ground loops. In our experience most hum
is due to not enough shielding around the head and head wires.