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Make sure that the grounding crown of the RCA plugs fit snugly around the
grounds of on the preamp inputs. A good connection is important for low noise
performance.
Watch for acoustic feedback that can arise if the speakers are too close to the
turntable. Increasing the space between the speakers and the turntable will help.
The signal to noise ratio (SNR) of a record isn’t what you’re used to with digital,
it’s less. Still, the music on the records mostly masks the noise, except in soft
portions, or if the record is worn or dirty.
Is one channel out? Swap phono outputs and preamp inputs to get to the bottom of
the problem. This will let you figure out if it’s a turntable/cartridge problem or a
preamp problem.
A frequent source of “one channel out” problems in the turntable/cartridge system
comes from bad connections between the wires in the head-shell and the phono
cartridge. The push-on connectors become flakey. If this is the issue, you can
often renew the connection by carefully rotating the push-on connector on the
cartridge’s input pin:
o
Be careful not to bump the stylus or catilever.
o
Hold the cartridge body in one hand.
o
Rotate the push-on connectors around the input pin using needle nose
pliers. This will clean the crud, and also let you tell if the connections are
so loose as to also cause a problem.
Need more information on your old turntable? Visit
www.vinylengine.com
. They
have an extensive collection of turntable service manuals.
Why Vinyl? Because it seems to have a higher SAR (Soul to Annoying Artifacts
Ratio) than most digital.
Gain Tweaks
I think you’ll find that the phono preamp has just the right amount of gain for you, which
will put the PAT-4 volume control at a nice play when you play LP’s. However, if you
find that the phono preamp has too much gain for your setup (cartridge and power-amp),
you can drop it by up to 6 dB by increasing the value of both R4 and R17 to 300 Ohms.
Be sure to use a high-quality metal film resistor.
More Tweaks
The design is unique in that the collector resistor (R6 and R19) in the output stage can be
made as low as 1K Ohm (even lower, actually) with no other adjustments to the phono
preamp. However, you will need to change the power supply to assure that you have
adequate current available. The advantage of lowering this resistor is the availability of
more current to drive the feedback network, further lowering distortion below the already
low numbers.