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AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
A phantom-powered DI box using a JFET/BJT cascode
F
ollowing on from last month’s
column with the theme of
unusual experimental audio
circuits, I’ve dug another one out from
my notebooks.
This direct injection (DI) box circuit
is unusual in using a JFET and BJT in
cascode to handle the relatively high
phantom power voltage.
Electric guitars need to be loaded
with a high impedance, over 1MΩ.
Otherwise, they sound dull and the
high-resistance volume and tone
controls don’t work properly. This is
at least partly because they evolved
during the valve era, and guitarists are
conservative musicians.
I’ve tried to persuade some musicians
that a low-impedance balanced output
is the way to go for the best sound
quality, but they are wedded to their
legacy hardware. Also, the idea of
drilling a 22mm hole in their beloved
instrument for an XLR socket is
anathema to them.
The solution to this dilemma is a
direct inject (DI) box, which provides
a high-impedance input for the guitar
and a balanced, low-impedance output
to feed a low-to-medium-impedance
input, like a mixing desk input. Mixing
desk inputs typically have an input
impedance of 5–10kΩ.
Recording guitar
An important factor in getting a good
electric guitar sound in the studio
is to mic up a valve guitar amp and
mix it with the DI input to the desk,
as illustrated in Fig.1. Guitars do not
sound good recorded directly. They
need the overdrive distortion of a valve
amplifier and the multiple breakup
modes of a light-weight paper speaker
cone.
Multiple effects are also used on both
channels to give a massive sound with
stereo spread. This combination of two
mains Earthed inputs would normally
cause a hum loop between the amp and
Room acoustics
Fig.1: a typical
studio setup for
recording electric
guitar.
Guitar amplifier
(preferably valve design)
Guitar plus
FX pedals
Speaker
Microphone
Mixer channels
Reverb and other effects added
the mixing desk, so all useful DI boxes
have a transformer-isolated balanced
output. A direct feed from the guitar to
the amp has to be provided, as shown
in Fig.2.
Going active
Attempts were made in the 1980s to
make active, transformerless DI boxes
but they never quite eliminated the hum
because the output has to be referenced
to the ground of the guitar amp. Most DI
boxes are passive devices, using a 10:1
step-down transformer (Fig.3), which
needs expensive Mu-metal shielding
and gives a poor frequency response.
By using an amplifier to drive a
lower-ratio transformer, as done here,
these problems are eliminated. The
downside is that a power supply is
needed. This is normally a 9V block
(PP3) battery, which is expensive and
has a short life. The solution used here
is to take advantage of the 48V phantom
power available on most mixing desks
and computer studio interfaces.
Amplifier design
Since phantom power is a relatively
high-voltage, low-current supply, a
discrete amplifier design is preferable
to an op amp, as well as being more fun
to design. A JFET input is a good idea
for a high input impedance, since it
gives lower noise with the high source
impedance of the guitar. Most low-cost
JFETs are limited to 30V, which gives
a good excuse to employ a cascode.
A cascode is where two transistors
Guitar jack
output to
amplifier
Fig.2: a basic direct
injection (DI) box circuit.
Amplifier
DI Box
Isolation
transformer
Stereo output
to recorder
Input Output
XLR
balanced
3 – output
Guitar jack
input
2 +
Balanced
output
0V
1
Lift
Amplifier
50
DI (direct inject)
Ground lift
switch sometimes
fitted here
Practical Electronics | June | 2025
Special 1:1 to 25:1 step-up audio transformer
(Difficult to achieve more than 100Ω input impedance
at high frequency so guitar sound is compromised.)
600Ω output
impedance
Guitar input
3
2
Mumetal
shield
Balanced output
Male XLR socket
1
1kΩ
ground
lift
1nF
RF
bypass
Fig.4: four
examples of the
basic cascode
configuration.
Both active
devices can be
of any type
(bipolar, JFET,
MOSFET or
valve).
Load
resistor
Practical Electronics | June | 2025
RL
22kΩ
RL
100kΩ
+114V
Vb
Fixed bias Vb
typically 2-5V
Output
+5.2V
2N5459
JFET
D
0V
Input
Input
Standard
bipolar
High-voltage
device
MP5A42
Fixed low-voltage
6.8kΩ
stops VCE modulation
0.6V
G
S
1MΩ
0V
0V
Gain = 160x
V+ 240V
V+ 12V
RL
82kΩ
V+
RL
(Inductor
eg, IF coil)
+120V
A
68kΩ
Output
470kΩ
Output
7.4V
15kΩ
G
Valve
½ ECC82
D
K
0V
4.9V G2
10nF
3N200
BF528
Dual-gate
MOSFET
0V
BFW10
G1
Input
S
Input
Fig.5:
Douglas
Self’s
cascode
amplifier
circuit
from
Wireless
World,
February
1979.
1MΩ
1MΩ
0V
0V
R5
6.8kΩ
47kΩ 220Ω
47nF
V+ +38V
330µA
14.6V
R10
22kΩ
C5
22µF
35V
11mA
+
The cascode was popular in topnotch Hi-Fi amplifiers from the 1970s.
The PE Gemini designed by Ferranti
engineers was the first in a magazine,
and it still sounds and measures very
well today. A cascode was necessary to
avoid an odd distortion arising in the
main voltage amplification stage (VAS)
transistor when the emitter-collector
voltage (VCE) swing was almost railto-rail.
This large swing caused the transistor
to modulate its own gain and junction
capacitance by a mechanism called
the Early effect (named after James
M. Early), where the thickness of the
active region of the semiconductor
varies with applied voltage. By using
a cascode, the voltage across the lower
transistor is effectively fixed, at a few
volts, preventing this modulation.
The effective capacitance is also
reduced, improving high-frequency
performance.
This configuration is still popular in
RF amplifiers and deflection amplifiers
in cathode ray tube (CRT) oscilloscopes.
The Early effect is less of a problem
with modern VAS transistors, many of
which were originally developed for
deflection transistors in CRT monitors
in the 1980s. The data sheets of such
transistors will often display a very flat
hFE vs VCE curve.
Those made by Toshiba, Sanyo and
V+ 238V
R9
22kΩ
C3 +
47µF
10V
R8
2.7kΩ
0V
R8
2.2kΩ
TR3
BC182
23.4V
R12
1kΩ
C4
15pF
1.4V
R7
680Ω
3mA
TR2
BC182
0V
Input
R1
10kΩ
Zin = 10kΩ
Max gain = 7x
inverting
Output
R11
2.2kΩ
0.5W
C6 +
22µF
35V
11mA
sink
C1
22µF
15V
TR1
BC182
0.6V
R2
1.5kΩ
0V
0V
AC
feedback
DC
feedback
0V
R3
22kΩ
C7
22µF
35V
24V
0.8V
+
The Early effect
220V
Output
Fig.3: a passive DI box circuit.
are placed one on top of the other, in
series, as shown in Fig.4. The supply
voltage drop is shared by both devices,
allowing a low-voltage device, in this
case the JFET, to be used for the lower
input device.
There’s nothing stopping you from
cascoding bipolar junction transistors
(BJTs), or even a combination of BJTs,
JFETs, Mosfets or even valves. For
guitar practice amps, I like to use a
JFET at the bottom with a valve on top.
A dual-gate Mosfet can be considered
a ‘cascode in a can’!
There are other advantages to the
cascode configuration that are not of
great importance here, but are worth
knowing. This simple circuit gives an
opportunity for experimentation.
V+
+
Guitar output
R4
22kΩ
C2 +
22µF
15V
0V
Volume
VR1/Rf
50kΩ
Log
CW
Hitachi are considered the best. In
the quest for maximum power, the
cascode is used less in discrete power
amplifiers because it loses a couple of
volts of headroom for a given supply
voltage. It is still used frequently in
high-quality JFET audio op amps,
where there is virtually no limit on the
number of transistors that can be used.
The first time I saw a cascode was
in Douglas Self’s 1979 Wireless World
pre-amp, where it was employed in
the main active gain stage (Fig.5).
This three-transistor circuit gave the
same performance as the NE5534 op
amp at a tenth the price (at the time).
I upped the power rail on mine to the
standard telecom/phantom 48V and
used a Bourns 91 conductive plastic
volume control.
An interesting tweak is the extra
resistor to boost the current to the lower
transistor (R8), since this stage usually
saturates first. Sadly, this lovely preamp was stolen one jazz night at the
Hebden Bridge Trades Club in 1993.
I’ve been meaning to build another
ever since.
51
“Root canal” fuzz
One overlooked problem with the
cascode is its horrid overload waveform;
modern power amps are expected to
clip cleanly. I use it here to give an
unusual fuzz effect, where the top of
the square wave starts collapsing in on
itself (Fig.6), generating a rash of extra
‘harsh’ high-frequency harmonics.
A practical circuit
Fig.8: the
prototype circuit
on stripboard.
Note that the Vigortronix output
transformer has a 2:1 step-down ratio,
reducing the minimum gain to unity,
while doubling the output current
available.
The fuzz switch, SW2, just removes
all NFB, giving full gain. A treble boost
function is obtained by bypassing some
of the NFB to ground via capacitor C3.
This function is switched in with SW1.
Interestingly, jazz guitarists like the
effect, while rock guitarists hate it.
The whole circuit is non-inverting
and the output transformer should
be wired to ensure the phasing is
correct. Most mixers have a phase
invert button.
If yours doesn’t, it may be worth
incorporating one on the output
transformer, since some guitar amps
are inverting.
+36V
3.2mA
R3
22kΩ
R5
15kΩ
52
C6
100µF
35V
+14.7V
6.8kΩ
13V
1.2mA
TR1
2N5457
*T1
VR2
1kΩ
R6
6.8kΩ
*Output transformer
Vigortronix
600Ω 2+2:1+1
Repanco/OEP
TT3 3.6:1+1
LT44 Economy
Gain
CW
+
Max
Treble
boost
NFB
C4 +
22µF
15V
+1.7V
VR2
1kΩ
Trim
sym
clip
ON
SW1
6.8kΩ
R9
100Ω
R7
750Ω
2mA
+10.7V
0V
0V
Feed phantom
power via mixer +48V
Integral 6.8kΩ resistors)
13V
+11.4V
R4
10kΩ
Output to
guitar
amplifier
TR3
BC559
TR2
BC549
Cascode
stage
VR1
1kΩ
Fig.6: the strange
clipping effect of this
cascode circuit.
There is not much to say here since
the circuit is experimental, but it lends
itself to stripboard construction, as
For the power supply, a centre tap on
C3
330nF
C2
470pF
Construction
Power supply
C1
10nF
R2
2.2MΩ
the output transformer is needed. The
phantom power is fed through 6.8kΩ
resistors mounted in the mixer (shown
faded on the diagram). This supply is
decoupled by capacitor C7. Pin 1 (0V)
on the XLR connector is fed through
ground-lift resistor R11 and bypassed
for RF by capacitor C8, then connected
to signal ground.
The internal connection between
the phantom power supply 0V and op
amp power supply 0V in the mixer
could cause an Earth loop. It might
be worth fitting a ground-lift resistor
here as well.
Originally, phantom power was
designed for microphones, which have
no mains Earth connection. This is an
area that needs further investigation.
+
Finally, we get to a buildable
circuit, shown in Fig.7. The input is
AC-coupled by capacitor C1, while
the combination of resistor R1 and
capacitor C2 act as a low-pass filter to
provide a degree of RF rejection. The
input impedance is defined by resistor
R2; I find that 2.2MΩ gives the best
results with most guitars.
The input signal is applied to the gate
of N-channel JFET TR1. This can be any
low-current, low-IDSS type, such as the
2N5457 shown here. The J202, J305 or
BF245 are also suitable. The popular
2N3819 and similar types need too
much current, which would overload
the phantom supply.
Since the gate source voltage (VGS)
for a given channel current (ID) varies
so much between individual JFETs,
preset VR2 is needed to trim for
symmetrical clipping. Trimmers are
bad for mass production, but interesting
for tweaking. Some guitarists like
asymmetrical clipping for the even
harmonics it produces, similar to how a
12-string guitar has extra strings tuned
an octave higher.
The upper transistor of the cascode is
a high hFE device because the partition
noise is less. Since the voltage is
shared, a low-voltage 30V transistor
can be used, such as a BC549C. The
cascode stage gives a gain of around
15×. The signal is then directly coupled
to a second PNP BJT transistor,
boosting the
open-loop gain
Fig.7: the
to around 400×.
complete circuit
The final gain
for the JFET
is controlled by
cascode DI box.
negative feedback
(NFB) through
resistor R6. The
minimum gain is
around 2× with
the gain control
High impedance
pot, VR2, fully
R1
guitar input
1kΩ
anticlockwise.
XLR output
to mixer
3 –
C5
22µF
15V
+
2
Fuzz
(open loop)
C7 +
100µF
50V
1
R10
100Ω
C8
1nF
ON
R8
100kΩ
SW2
Ground
lift
Practical Electronics | June | 2025
NEW!
5-year
collections
2019-2023
Fig.9: the wired assembly,
tested but not yet in a box.
shown in Fig.8. The output transformer
is quite heavy, so it is a good idea to
use a separate mounting PCB for it,
such as the ones described in Audio
Out, November 2022.
Fig.9 shows the PCB wired up to
the transformer, connectors and pots
before they were installed in a case. A
6.35mm TRS jack could be substituted
for the XLR connector.
I was unable to create a proper PCB
or detailed construction plans since
I just moved and haven’t completed
my horrendous house/workshop
repairs (the old wiring needs a lot of
attention!).
While on the subject, AOShop is now
located at Manchester House, Market
Street, Craven Arms, Shropshire SY7
9NN. The phone number and email are
still the same as before: 01597 829 102
PE
& jrothman1962<at>gmail.com.
Parts List – Phantom-powered DI box
1 2×2-inch (~50×50mm) piece of Veroboard
1 Vigortronix VTX-101-002 2.4kΩ to 600Ω transformer, 2+2:1+1 (T1)*
1 male XLR 3-pin chassis-mount connector
2 6.3mm mono jack sockets
Qty Value 4-band code 5-band code
2 miniature SPST toggle switches
1 2 .2 M W
1 10kΩ logarithmic potentiometer (VR1)
1 100k W
1 1kΩ trimpot (VR2)
1 22k W
Semiconductors
1 2N5457 N-channel JFET, TO-92 (TR1)
1 BC549C NPN transistor, TO-92 (TR2)
1 BC559 PNP transistor, TO-92 (TR3)
1
1
1
1
1
2
15k W
10k W
6 . 8 kW
1 . 0 kW
750 W
100W
1 15kΩ metal film (R5)
1 10kΩ metal film (R4)
1 6.8kΩ metal film (R6)
* Alternatives:
Repanco TT3/OEP E187B 3.6:1+1
Jaycar MM-2532 1:1 (cheap)
Practical Electronics | June | 2025
PDF files ready for
immediate download
2018-2022
All 60 issues from Jan 2018
to Dec 2022 for just £49.95
PDF files ready for
immediate download
2017-2021
All 60 issues from Jan 2017
to Dec 2021 for just £49.95
PDF files ready for
immediate download
2016-2020
Capacitors
1 100µF 50V electrolytic (C7)
1 100µF 35V electrolytic (C6)
2 22µF 16V electrolytic, aluminium or tantalum (C4, C5)
1 390nF (preferred) or 330nF polyester (C3)
1 10nF polyester (C1)
1 1nF ceramic (C8)
1 470pF NP0/C0G ceramic (C2)
Resistors (all ±1% ¼W)
1 2.2MΩ (R2)
1 100kΩ (R8)
1 22kΩ metal film (R3)
All 60 issues from Jan 2019
to Dec 2023 for just £49.95
All 60 issues from Jan 2016
to Dec 2020 for just £44.95
PDF files ready for
immediate download
1 1kΩ (R1)
1 750Ω (R7)
2 100Ω (R9–R10)
Eagle LT722 1.4:1 (cheap)
600Ω 1:1 modem transformer (cheapest)
See page 41 for further
details and other great
back-issue offers.
Purchase and download at:
www.electronpublishing.com
53
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