Altronics K 5171 Посібник - Сторінка 5

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K 5171

is low and so they can bypass VR2a entirely.

Any change in the position of VR2a's wiper

will thus have little effect on high frequencies.

For example, at 1kHz, the 100nF capacitors

have an impedance of 1.6kΩ each. That is

considerably lower than the 5kΩ value of the

half of the potentiometer track that they are

connected across when VR2a is centred and

therefore the capacitors shunt much of the

signal around VR2a. But at 20Hz, the 100nF

capacitors have an impedance of 80kΩ and

so minimal current passes through them;

almost all of it goes through VR2a. Therefore

VR2a has a significant effect on the amplitude

of a 20Hz signal and so it provides much

more boost or cut at lower frequencies.

When VR2a is rotated clockwise, the resistance

from output pin 1 of IC3a to its wiper increas-

es, while the resistance from the wiper to the

input signal decreases, providing increased

amplification. And when rotated anti-clockwise,

the opposite occurs, decreasing amplification.

Because the capacitors shunt a different

amount of signal around the pot at different

frequencies, this gain is also frequency-de-

pendent.

The 1.8kΩ resistors set the maximum boost

and cut range. They have been chosen to

allow up to ±15dB adjustments at around

20Hz, dropping to around ±1dB at 1kHz. The

measured frequency response with the con-

trols at minimum, centred and at maximum is

shown in Fig.3.

Treble adjustment

Treble control VR3a operates differently to

VR2a. It is configured to have more effect on

higher frequency signals. This is achieved by

connecting capacitors in series with the pot

channel, rather than across it.

At low frequencies, the 15nF capacitors have

a high impedance, eg, 106kΩ at 100Hz. This

is very high compared to the 10kΩ channel

resistance and so most of the feedback

signal at this frequency will flow through the

bass network, which has a DC resistance of

13.6kΩ and therefore a much lower imped-

ance. So VR3a will have little effect on the

gain at low frequencies. At high frequencies,

the 15nF capacitors have a lower impedance,

eg, around 1kΩ at 10kHz and so the treble

controls are effectively brought into circuit,

providing adjustable gain similarly to the cir-

cuitry surrounding VR2a. The 1kΩ resistors at

each end of VR3a set the maximum boost or

cut for high frequencies, up to around ±15dB,

similar to the bass control. You can see this

in Fig.3.

The 12kΩ and 1kΩ resistors between

the bass and treble potentiometer wipers

minimise the inevitable interaction between

the two controls. Note that while the treble

potentiometer is isolated from direct current

flow due to the 15nF capacitors in series, the

bass potentiometer requires two extra 100µF

ULTRA LOW DISTORTION PREAMPLIFIER WITH TONE CONTROLS

capacitors. These do not affect the action of

the bass control; they are just there to block

direct current flow through VR2a. This is for

the same reason that DC is blocked for VR1;

to prevent noise during adjustments.

The 1MΩ feedback resistor between pins 1

and 2 of IC3a provides DC bias for the pin

2 input, while the 47pF capacitor prevents

high-frequency oscillation of the op amp by

reducing the gain at ultrasonic frequencies.

When S4a is set to the 'tone in' setting, the

output from IC3b (reinverting IC3a's signal

inversion) is then fed to the CON3 output

as mentioned above. Another pole of the

switch (S4c) controls the indicator LED that is

contained within the switch. It is powered from

the ±15V supplies via a 10kΩ resistor and

therefore receives about 3mA.

Jumper link LK4 can be removed to prevent

this LED from lighting, or moved into one

position or the other to invert its function. In

other words, LK4 selects whether the LED

lights when the tone is in or out. Note that the

'tone out' position of S4 is when the switch is

pressed in. In other words, it acts like a defeat

switch.

Remote control circuitry

The Remote Control circuitry is also shown in

Fig.7. Signals from the handheld remote are

picked up by infrared receiver IRD1. This is

a complete infrared detector and processor.

It picks up the 38kHz pulsed infrared signal

from the remote and amplifies it to a constant

level. This is then fed to a 38kHz bandpass

filter, after which it is demodulated to produce

a serial data burst at its pin 1 output.

The resulting digital data then goes to the RB0

digital input (pin 6) of PIC16F88-I/P micro-

controller IC5 for decoding. Depending on

the button pressed on the remote, IC5 either

drives the volume control motor (via an exter-

nal transistor circuit) to change the volume, or

sends one of its RB6, RB7 or RB5 output low

to select a new input.

The input routing is controlled by the Input

Selector board which is connected via CON7.

IC5 is programmed for a remote control which

sends Philips RC5 codes. It supports three dif-

ferent sets of RC5 codes, normally referred to

as TV, SAT1 or SAT2. You must also program

the universal remote control with the correct

number for one of these sets of code. For the

Altronics A1012, use a code of 023 or 089 for

TV mode, 242 for SAT1 or 245 for SAT2.

Driving the pot motor

IC5's RB1-RB4 outputs (pins 7-10) drive the

bases of transistors Q1-Q4 via 1kΩ resistors.

These transistors are arranged in an H-Bridge

configuration and control the motor. The

motor is off when the RB1-RB4 outputs are

all high. In that state, RB3 and RB4 turn PNP

transistors Q1 and Q3 off, while RB1 & RB2

turn NPN transistors Q2 and Q4 on.

As a result, both terminals of the motor are

pulled low and so no current flows through

it and it won't rotate. The emitters of Q2 and

Q4 both connect to ground via a common

10Ω resistor, which is used for motor current

sensing. The transistors operate in pairs so

that the motor can be driven in either direc-

tion to rotate the potentiometer either way, to

increase or decrease the volume.

To drive the motor clockwise, RB2 goes low

and turns off transistor Q2, while RB3 goes

low and turns on Q1. When that happens, the

left-hand terminal of the motor is pulled to

+5V via Q1, while the right-hand terminal is

pulled low via Q4. As a result, current flows

through Q1, through the motor and then via

Q4 and the 10Ω resistor to ground.

Conversely, to turn the motor in the other

direction, Q1 and Q4 are switched off and Q2

and Q3 are switched on. As a result, the right-

hand motor terminal is now pulled to +5V via

Q3, while the left-hand terminal is pulled low

via Q2. Regardless of the direction of rotation,

current flows through the 10Ω shared emitter

resistor and so the voltage across it varies with

the current drawn. Typically, the motor draws

about 40mA when driving the potentiometer

but this rises to over 50mA when the clutch is

slipping. As a result, there is about 0.4-0.5V

drop across the 10Ω resistor.

This is ideal because the motor is rated at

4.5V and the result of subtracting the resistor

voltage from the 5V supply is that it provides

the correct motor voltage.

Current sensing & muting

Once the potentiometer has reached full travel

in either direction, a clutch in the motor's

gearbox begins to slip. This prevents the motor

from stalling and possibly overheating if the

button on the remote continues to be held

down. The clutch mechanism also allows the

user to rotate the pot shaft manually.

As mentioned earlier, when you press the

mute button on the remote control, the

volume control is rotated fully anti-clockwise.

Microcontroller IC5 detects when the wiper

reaches its end stop by detecting the increase

in the motor current when the limit is reached

and the clutch slips. That's done by taking a

sample portion of the voltage across the 10Ω

resistor using trimpot VR4. The voltage at

VR4's wiper is filtered using an 18kΩ resistor

and a 100nF capacitor to remove the motor

commutator hash and is applied to lC5's

analog AN3 input (pin 2). IC3 then measures

the voltage on AN3 to a resolution of 10 bits,

or about 5mV (5V ÷ 2

Provided this input is below 200mV, the PIC

microcontroller allows the motor to run. How-