Altronics K 5171 Manuel - Page 3

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K 5171

ar as polypropylene or polystyrene dielectrics,

none of those capacitors are critical enough to

cause a measurable increase in distortion, as

demonstrated by our performance graphs.

But there are some capacitors with values be-

low 1nF where the dielectric is important and

this presents us with some difficulty, since

MKT capacitors with values below 1nF are not

particularly easy to get. However, we've found

them (see parts list) and that is what we have

used in our prototype, with good result.

If you can get MKP (polypropylene) capacitors

instead, those will certainly work well and we

would encourage that. But we have also men-

tioned the possibility of using NP0 ceramics.

We have tested these in the past and found

that they are just as good as the best plastic

dielectrics in situations where linearity is

critical. But be careful because many ceramic

capacitors are not NP0 (also known as C0G)

types, especially values above 100pF. Fig.5

shows a distortion plot for a simple low-pass

filter comparing two capacitors of the same

value, one polypropylene and one ceramic

(not NP0/C0G). As you can see, the ceramic

capacitor produces a lot more distortion. So

make sure you use one of the types specified.

Regarding resistance, you may find it a bit

strange that we have specified a 5kΩ volume

control potentiometer as values in the range

of 10kΩ-100kΩ are more commonly used.

But we have chosen 5kΩ because the ther-

mal noise contribution of the volume control

pot can be a major limiting factor in the

performance of a low-distortion preamplifier

and suitable motorised pots are available.

Op amps IC1a & IC2a buffer the signal from

the source so that it does not have to drive

the 5kΩ impedance; the op amps are more

than capable of driving such a load without

increased distortion.

If you can't get the 5kΩ motorised pot (availa-

ble from Altronics; see parts list), you can use

a 20kΩ pot instead; also a pretty standard

value.

In that case, we have made provision for two

4.7kΩ shunt resistors to lower the impedance

seen by the following stage, giving you most of

the performance benefits of a 5kΩ pot. These

have minimal effect on the pot curve, so it still

works well as a volume control.

Fig.6 shows the difference in distortion with

and without these shunts (the signal level is

lower here than in the other figures, hence the

higher base level). The performance with the

proper 5kΩ pot is slightly better again.

Remote control

Pressing the Volume Up or Volume Down

buttons on the infrared remote causes the mo-

torised pot to rotate clockwise or anticlockwise.

It takes about nine seconds for the pot to travel

from one end to the other using these controls.

For finer adjustment, the Channel Up and

ULTRA LOW DISTORTION PREAMPLIFIER WITH TONE CONTROLS

Channel Down buttons on the remote can be

used instead. These cause the pot shaft to ro-

tate about one degree each time one of these

buttons is briefly pressed. Holding one of

these buttons down rotates the pot from one

end to the other in about 28 seconds.If any

of these buttons is held down when the pot

reaches an end stop, a clutch in the motor's

gearbox begins to slip so that no damage is

done to the motor.

The code also provides a convenient auto-

matic muting feature. Press the Mute button

on the remote and the volume control pot

automatically rotates to its minimum position

and the motor stops. Hit the button again and

it returns to its original position. If you don't

want the pot to return all the way to its original

setting, you can simply increase the volume to

your desired new level instead.

So how does the unit remember its original

setting during muting? The answer is that the

microcontroller monitors the time it takes for

the pot to reach its minimum setting and the

minimum pot setting is detected when the

load on the motor increases at the potentiom-

eter end stop, as the clutch begins to slip.

When the Mute button is pressed again, pow-

er is applied to the motor drive for the same

amount of time, rotating it back to the original

position.

The orange "Ack" LED flashes whenever an

infrared signal is being received from the

remote, while the yellow Mute LED flashes

while the muting operation is in progress and

then remains on when the pot reaches its

minimum setting.

Circuit description

Fig.7 shows the main preamplifier circuit but

only the left channel components are shown,

for clarity. The right channel is identical and

the matching part designators are provided,

in brackets. The following description refers to

the left-channel part names.

The audio signal from the Input Switching

board is AC-coupled to the input of the first

op amp (IC1a) via a 22µF non-polarised (NP)

electrolytic capacitor and 100Ω resistor. A

22kΩ resistor to ground provides input DC

biasing and sets the input impedance to

around 22kΩ. The 100Ω resistor, ferrite bead

and 470pF capacitor form a low-pass filter to

attenuate radio frequency (RF) signals ahead

of the op amp input.

IC1a operates as a voltage amplifier with a

gain of two, due to the two 2.2kΩ feedback

resistors. The 470pF capacitor combines with

the feedback resistors to roll off the top-end

frequency response, with a -3dB point at

about 150kHz. This gives a flat response

over the audio spectrum while eliminating the

possibility of high-frequency instability or RF

demodulation. IC1a's pin 1 output is fed to

the top of volume control potentiometer VR1a

(5kΩ log) via a 22µF non-polarised capacitor.

The signal on its wiper is then AC-coupled

to the pin 5 non-inverting input of IC1b via a

4.7µF non-polarised capacitor. This coupling

arrangement prevents direct current from

flowing through any part of the volume control

potentiometer, VR1. Even a small direct

current can cause noise when the volume is

adjusted.

As mentioned earlier, the circuit was designed

for a 5kΩ motorised volume control pot as

this results in good noise performance but in

case you can't get one, you can use a more

common 20kΩ potentiometer and fit resistors

R1 and R2, so that the circuitry has a similar

impedance, resulting in the same overall

frequency response.

lC1b operates as a unity-gain buffer and

provides a low-impedance output regardless

of the volume control setting. Its pin 7 output

is fed to the tone control section and also to

switch S4a. When S4a is set to the 'tone out'

position, the output from IC1b is coupled via

the 22µF capacitor to output socket CON3,

via a 100Ω resistor. Therefore, the tone con-

trols are effectively out of circuit. The 100Ω

resistor isolates the op amp output from any

capacitive loads that might be connected, to

ensure stability. This resistor and ferrite bead

in series with the output also attenuate any

RF noise which may have been picked up by

the board.

Tone controls

When S4a is in the 'tone in' position, output

CON3 is instead driven from the tone control

circuitry, so potentiometers VR2a and VR3a

adjust the amount of bass and treble in the

signal. Op amp IC3a forms the active tone

control in conjunction with VR2a and VR3a

and associated resistors and capacitors. The

bass and treble tone circuitry is a traditional

Baxandall-style design. This is an inverting

circuit, so it must be inverted again by unity

gain buffer IC3b to restore the original signal

phase.

When the wipers of potentiometers VR2a and

VR3a are centred, the impedance between

output pin 1 of IC3a and each wiper is equal

to the impedance between the wiper and out-

put pin 7 of IC1b. So in this condition, IC3a

operates as a unity gain inverting amplifier

for all audio frequencies. Therefore, in this

case, the tone controls have little effect on the

signal – they just add a little noise.

Bass adjustment

The bass control (VR2a) provides cut (an-

ti-clockwise) or boost (clockwise) to low fre-

quencies. The impedance of each of the two

100nF capacitors for high-frequency signals