Silicon Chip – July 2019

siliconchip.com.au Australia’s electronics magazine July 2019 99

This guitar preamplifier uses six un-
buffered inverters in a single package
(74HCU04) to provide amplification,
and runs off a USB 5V power supply.
This IC can be used either as low-
delay digital inverters or high-gain
inverting amplifiers (as in the TOS-
LINK to S/PDIF Converter, October
2010, siliconchip.com.au/Article/319
& High-Sensitivity Magnetometer,
December 2018, siliconchip.com.au/
Article/11331, although the latter used
a similar 4069UB IC).
“Unbuffered” refers to the fact that
each inverter consists of just a single
stage, ie, two small Mosfets in a totem-
pole configuration with their gates and
drains tied together, acting as a non-
linear high-gain inverting amplifier.
A guitar practice amplifier does not
need to be highly linear nor particu-
larly loud. CMOS logic chips have a
high input impedance well-suited for
connecting to an electric guitar pickup.
So that IC is suitable for this applica-
tion, if a bit unusual.
Most guitars have volume control
pots, as do some headphones used by
musicians, so this circuit has a fixed

Guitar practice preamplifier based on inverters

gain. The guitar is connected via jack

socket CON3. A 10MW resistor pro-

vides ground biasing while keeping

the input impedance high, and the

100pF capacitor filters out any RF

picked up by the guitar or lead.

The signal is then AC-coupled to the

first gain stage via a 100nF series ca-

pacitor. This gain stage is built around

inverter stage IC1f and has a fixed gain

of around two times, set by the ratio

of the 1MW and 470kW resistors. This

works similarly to an op amp based

inverting amplifier but is less linear,

as the open-loop gain is a lot lower.

This stage is self-biasing because the

two internal Mosfets conduct a simi-

lar amount of current when the input

and output pins are near mid-supply.

So the DC level at the input tends to

settle at around 2.5V.

The output of this stage is then fed

to a second gain stage comprising the

remaining five inverters (IC1a-IC1e)

connected in parallel. This stage has

a gain of around five times, set by the

ratio of the 100kW and 20kW resistors.

A 10W resistor in series with the

output of each inverter ensures that

they share the load current more-

or-less evenly. The five inverters are

paralleled so that they can drive 32W

headphones to a reasonable volume.

The headphones at CON5 are fed

via a 100μF capacitor, to remove the

2.5V DC bias from the amplified signal,

with a 10W series resistor to slightly in-

crease the impedance seen by the am-

plifier and to improve stability.

The signal is also sent to output

CON4 via another 100μF capacitor

and a 100W series resistor. This can be

used to connect an amplifier to drive

a small speaker.

The circuit is powered with 5V ap-

plied to either DC barrel socket CON1

or USB socket CON2. The supply is

decoupled by two small capacitors

(the USB specification limits the ca-

pacitance directly across a socket) with

LED1 indicating power is applied and

diode D1 protecting the circuit against

reverse supply polarity.

100μH inductor L1 forms a low-pass

filter in combination with the following

220μF capacitor, reducing supply noise

which may otherwise be fed through

to the sensitive amplification stages.

Petre Petrov,

Sofia, Bulgaria ($70).

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