Circuit Explanation
The EQP-200B is two independent equalizers. The circuit has an electronically
balanced input stage that is followed by a passive equalizer. The function of the
equalizer can be thought of as three different sections. These are: a low frequency
attenuate and boost section, a high frequency boost section that has an adjustable
bandwidth, and a high frequency attenuate section. This type of equalizer is made of
resistors, capacitors and inductors. The equalizer has approximately a 20dB gain loss
which must be overcome. This gain loss is overcome by the vacuum tube amplifier. The
equalizer circuit feeds the first stage of the tube amplifier and the second stage of the
amplifier has a cathode follower for low output impedance. The cathode follower drives
the discrete 990 output stage. The tubes are operated class A with negative feedback.
The power supply is all solid state and regulated. The tube heaters are D.C. operated.
A Word About Tubes
The tubes that are used in the EQP-200B are selected to give the best possible
performance in the position that they are in on the printed circuit board. Switching them
to different positions will cause performance deterioration on the audio path. Replacing
them with “gold” or high end tube types may not be desirable. In cases that we have
measured, these tubes have shown higher distortion and noise as compared to the tubes
we have selected. In some cases, the so called “gold” or high end tubes have made the
unit unusable. The reason for this is some of the “gold type” tubes are selected for high
distortion in guitar amplifiers. Using gold tubes is no guarantee of better performance.
All of these so called “Brand X” types are selected with 6.3 volts AC on the heaters,
whereas Summit Audio uses 5 volts DC on the heaters for longer tube life and lower
noise. The reduced gain can raise the noise floor, increase distortion, and reduce
headroom.
For proper performance from a tube, the replacements must be selected using 5
volts DC on the filaments. The tubes we use are selected in the circuits that they are used
in to ensure proper operation, long life, and low distortion and noise.
In at least 50% of the cases we have tested, gear that has been used for several
years actually have lower noise and distortion levels than when new. This makes the
question of when to replace tubes difficult to answer. If a tube turns micro-phonic, the
distortion will be obvious and the tube must be replaced. However, tube life will most
likely be greater than 10,000 hours of operation. Tubes are generally very reliable; don’t
replace them just because they are old. In the EQP-200B there are gain adjustments that
will need to be checked when the tubes are replaced, or else the metering could become
inaccurate and the noise floor could change. Replacement should be done on the bench
with a distortion analyzer attached to ensure that the distortion levels are proper and it is
comprised of second order harmonics.
Before replacing the tubes in your EQP-200B, please talk to your dealer, call
Summit Audio, or find a technician who has experience working with tubes and high end
audio equipment.
(8 pages)
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