Argiriadis Analogue Electronics DU EQ 2022 User manual

DU EQ 2022
DU EQ 2022 USER GUIDE
STEREO

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Contents
Contents .......................................................................................................................................................... 1
PLACEMENT .................................................................................................................................................... 4
THE MAIN UNIT ........................................................................................................................................... 4
THE POWER SUPPLY UNIT ........................................................................................................................... 5
GROUND LIFT SWITCH ................................................................................................................................ 5
PRECAUTIONS ............................................................................................................................................. 6
OPERATING GUIDELINES CONTROLS & SWITCHES ......................................................................................... 7
(1) INPUT SECTION ...................................................................................................................................... 7
(2) PASSIVE EQ SECTION ............................................................................................................................. 8
(3) ACTIVE EQ SECTION ............................................................................................................................... 9
(6) MAIN DIFFERENCES BETWEEN SE & PP STAGES ..................................................................................10
BIAS CONTROL AND HOW IT AFFECTS DISTORTION AND CLIPPING IN THE SE DISTORTION SECTION.....12
(7) SINGLE ENDED DISTORTION SECTION .................................................................................................12
DIFFERENCES BETWEEN TRIODE & PENTODE ...........................................................................................13
(8) THE PUSH PULL (PP) OUTPUT STAGE...................................................................................................14
PREPARATION BEFORE SWITCHING ON/TESTING ........................................................................................16
switching on/testing .....................................................................................................................................18
(14) The Single Ended Distortion Stage (SE) .............................................................................................21
ABBREVIATIONS ........................................................................................................................................24

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PLACEMENT
THE MAIN UNIT
The location of the distortion unit (DU) is important.
Its location/mounting must allow adequate air circulation from below, the sides and above it to disperse
the heat that the tubes/semiconductors generate.
Avoid hot locations such as near radiators or other heating units.
IF THE UNIT IS MOUNTED ON A RACK:
(a) If mounted on a rack ensure that the unit below it does not get hot.
(b) Even if the unit below the DU runs cool, there must still be a distance of at least a 2U (3(1/2) inches,
90mm) of empty air space between the DU and the unit below. If the unit below gets warm this gap must
be increased by least 4U (7 inches,180mm).
(c) Leave a 4U distance of empty air space between the DU and the unit that is on top of
it.
(d) both sides of the whole rack must be open to allow air passage.
(e) Also, if the unit is mounted on a rack it may need extra support from below due to its weight. A
sturdy rack shelf is suitable the 1" feet will allow air to enter from below.
Keep the top clear of items such as papers or anything that could block air passage and
cause overheating.
PLACE THE UNIT AWAY FROM SOURCES OF INTERFERENCE
(a) AC power lines, fluorescent lights, fans etc.
(b) The power supply cords that are plugged to the power supply must always run from behind the unit
never along its sides to the power supply.
(c) If the room where the unit is operated is electrically and/or magnetically noisy, you may have
to slightly move it around and find the most suitable location in order to completely eliminate any noise.

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THE POWER SUPPLY UNIT
(a) Before you connect the power supply to the mains, connect the two power supply cords to the
power supply sockets. These are the two cables, one grey and one black, that come out from the back of
the main unit. Please treat both cords with care.
Both of the plug connectors at the end of these cables and their respective sockets on the power supply
front panel are mated in only one possible orientation. The smaller round high voltage plug at the end
of the black cord features keyway polarisation: it has a notch to make possible to fit in one way only.
The bigger plug on the grey cord and its socket (on the power supply) are also non-reversible, they
have a flat side in the middle to make them mate one way only.
Please examine both sockets and plugs before connecting to recognise their mating features and do not
use any force when inserting the plugs into their sockets.
(b) The power supply is also a source of electrostatic and magnetic interference and it must be placed as
far away from the main unit as possible, preferably on the floor.
(c) Please allow plenty of ventilation for the power supply unit too. Avoid hot locations such as
near radiators, heating units, hot central heating pipes etc.
GROUND LIFT SWITCH
If this switch is used incorrectly the unit will become susceptible to interference, noise and hum.
Ensure that this switch which is situated in the rear panel is in the on position which is with the (I) pressed
down. This is to ensure that the metal case is connected to the negative signal ground of the unit's
electronic circuits so that it (the metal case) works as a shield against external interference but also
prevents internal instability and oscillation.
In most studios and especially if the balanced (XLR) inputs and outputs are used it may never be
necessary to operate this ground switch, so there is a piece of tape to keep it in the ON position at all
times.
If you need to operate this switch (see notes on ground lifting & loops) just pull the tape off.
It is important to remember that unless the case is shielded (connected to the signal ground) by other
means if the GROUND LIFT SWITCH in is the off position the unit will be noisy.

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PRECAUTIONS
 Please do not leave the unit switched on when not in use
Each time:
(a) you increase any gain settings
(b) change from triode to pentode,
(c) operate in OFF LOAD mode
(d) operate in positive feedback (PFB) mode
 Ensure that you decrease the OUTPUT P14 control and/or use the attenuator (OUTPUT
ATT) switch (SW21, in the right position) so that the output level of the unit remains as constant
as possible.
 Until you get to know the unit, always turn the OUTPUT control (P14) down to zero whenever
you change anything and then gradually turn it up again.
 Ensure that output overload LED OD3 is always off.
 Always set the S-BY/ON switch on S-BY (up) before you use the TRI/PE1/PE2 (P15) rotary
selector switch and then press S-BY/ON down again (ON). This procedure is explained in the
'THE OPERATING GUIDELINES' section.

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OPERATING GUIDELINES
CONTROLS & SWITCHES
DUAL MONO
(a) The Tube amplifiers in this unit do not employ any form of negative feedback (NFB) in order to retain
the harmonic sonic character of each tube and to be able to overdrive it to its full extent softly or harshly
depending on how the controls/switches are set.
NFB not only reduces distortion but it also offers accurate amplification by controlling parameter
tolerances of each tube. The gain (and other parameters) can vary from tube to tube (of the same type).
Components like the potentiometer controls, signal capacitors, etc have small tolerances too.
For these reasons individual minor fine adjustments of each channel may be necessary in order to make
them produce exactly the same output in some operating modes. For instance, sometimes, a
drive/level/output control in one channel may need to be set at 5(1/3) (= a third of the distance between
5 and 6 on the knob dial) while the same control is set at 5 in the other channel.
(b) Due to the complexity of the unit I personally found it easier (when operating for the first time) to
fully test one channel first, then the other and finally align them both in stereo mode.
(c) When I switch on each unit for the first time, I also prefer testing each section separately (in each
channel) and then mix them altogether. For instance, test the whole channel without the SE (single
ended) and active EQ sections then bring in the SE section, then the active EQs and so on. It is possible to
test each section separately.
(1) INPUT SECTION
P1: INPUT LEVEL CONTROL, it adjusts the amount of signal level that enters the unit from the unbalanced
(jack) input
SW1: INPUT STAGE SELECT SWITCH
This switch selects the operating mode of this stage. In the up position is triode (TRI) in
the down position cascode (CASC) mode
In triode mode, at low P1 settings, the input stage will produce a clean sound, but with 'musical' low order
even (mainly second) harmonic distortion, the amount being directly proportional to the signal level set by
P1. The more you turn up P1, the more harmonic and intermodulation distortion is generated, resulting in
noticeable sound colouration.
Cascode mode, is to be used for harsher and dirtier sound. Cascode is a low noise pentode made out of
two triodes (= CasCaded triode), that produces more distortion even at lower P1 settings. The distorted

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signal contains both even and odd products. Bass, drum-loops and keyboards/synths sound interesting in
this mode. However, it also amplifies more.
The SW1 (TRI/CASC) converts the input stage (as already explained) from a Triode to a cascode circuit
with more distortion and gain.
During the change the operating conditions of the two input tubes change rapidly and the signal
may disappear for a very short period of time.
(a) in order to avoid premature failure of either or both of these tubes please disconnect the high
voltage (B, HT) through setting the S-BY switch in the up position before operating SW1 and re-connect
the high voltage afterwards by pressing the S-By down again.
(b) The changeover is a noisy procedure so the MUTE (SW18) switch can be pressed while operating SW1
(and the HT switch) to silence the output of the unit.
Procedure (a)&(b) above is the same as the one used for changing from Pentode to triode (and vice
versa) in PP output section. This procedure is discussed in detail further in the text. (see PRECAUTIONS,
OPERATING GUIDELINES, and SWITCHING ON/TESTING.
SW2: ATT, an attenuation switch that reduces the signal level coming out of the input stage by 12dB's.
This is to avoid overloading of the following stages, when the input stage (especially in cascode mode),
produces a high-level signal, when (and if) this type of overloading is not wanted.
(2) PASSIVE EQ SECTION
This is a passive EQ, with an inductor in the midrange section, that can emphasize certain harmonics,
eliminate others and add a certain character to the sound.
P2: BASS, a shelving type control for the low frequencies.
SW3: L /H, bass frequencies select switch, when P2 starts having a noticeable effect
L = 70Hz, H=100Hz
SW4: ON, bass on /off switch, it is not a 'kill' switch, but it cuts off the low frequencies.
P3: MID, midrange frequency control.
SW5: L/H, midrange frequencies select switch, see text below
SW6: ON, mid control (P3) on /off switch
P4: TREBLE, a shelving type control for the high frequencies.
SW7: L/CENTRE/H, treble frequencies select switch when P4 starts having a noticeable effect
L= 4.5kHz, centre =9kHz, H = 7kHz
SW8: ON, treble on/off switch it is not a 'kill' switch, but it cuts off the high frequencies.
This EQ interacts both with the stages before it and the stages after it. For instance, you can:

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a) overdrive the input stage, before the EQ and use its controls to either boost or filter out harmonics that
have been generated by the input stage, especially in cascode mode.
Another option may be to:
b) keep the input stage clean, (Triode Low distortion mode) and cut or boost certain frequencies of the
signal that enters the following stages (SE & PP DU's) and overdrive these stages.
c) Most commonly used option is a combination of a) and b).
The Bass and Treble is a Baxandall tone control, switch SW3 selects the low frequencies below which
maximum variation occurs, switch SW7 selects the high frequencies above which maximum variation
occurs. When SW7 is in the centre position the frequencies controlled by P5 are too high (often called “air”
frequencies) and the variation is not very audible.
When SW3 is set in H and SW7 in L, P2 and P4 become more responsive and interactive.
When SW5 is set in L control P3 affects a wide range of low mid- frequencies centred at 500Hz and the
sound becomes fuller on high P3 settings. When SW5 is set in H, P3 affects a narrower range of higher
midrange frequencies centred around 1kHz.
By turning P3 fully anticlockwise, or by switching off the mid switch SW6 most of the midrange
frequencies disappear and the Bass & Treble (P2, P4) controls become more effective.
(3) ACTIVE EQ SECTION
(a) BASS Variable Frequency Booster (VFB)
This is an active Variable Frequency Booster (VFB), a high Q ( + 27dB) band -pass filter at a frequency set
by SWEEP control P6. It uses both Negative and Positive Feedback mechanisms to function and swings
between the two. This circuit works in a way as if it is “trying” to oscillate at the chosen frequency but
never actually gets there. P6 selects this frequency from 30Hz to 350Hz, and the BOOST control P5 sets
the output level entering the EQ mixing stage.
This is a hybrid circuit, made out of a 12AX7 tube voltage amplifier and a high voltage transistor buffer.
The 12AX7 is producing a small amount of harmonic distortion and its unique grid current behaviour set
by the operating conditions of the circuit plays a role in the sound.
P5: BASS BOOST control
P6: FREQ, bass frequency SWEEP control
SW9: ON bass frequency booster on/off switch, when pressed down the boosted signal enters the EQ
mixing stage; it can also be used as a KILL switch.
(b) HIGH MID Variable Frequency Booster (VFB)
Same as the previous circuit but its frequency range (900Hz ... 4.2kHz) covers the high midrange and
presence frequencies.
P7: FREQ, high mid frequency SWEEP control.