AUDIO
OUT
AUDIO OUT
L
R
By Jake Rothman
Op amp adaptor boards: surface mount and
through-hole versions
Fig.1. The original inspiration for these
adaptor boards: an SMD-to-DIL op amp
adaptor board from HobbyTronics.
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such as HobbyTronics Ltd – Fig.1, https://
bit.ly/pe-may23-dil – and I will produce
one of these too.
Replacements and upgrading
Audio engineers don’t like to waste
money, and like grumpy farmers, they
expect all equipment to last at least 50
years. Unfortunately, some op amp designs become obsolete and can be hard
to find, but on the up side, new versions
may well have much better noise and distortion figures. There’s no point scouring
eBay for replacement NOS (new-oldstock) quad-pack 741-style devices – it’s
much better to replace expired op amps
with some NE5532s.
Pin 1 for 14-pin
socket under board
Pin 1
IC1
IC1
we’ve described some generalpurpose audio op amp boards
designed for single and dual op amps.
While I was designing these, it also
seemed like a good idea to make some
general-purpose adaptor boards to allow
different package op amps to be adapted
to any socket. For example, for the
dual op amp board covered last month
I wanted to compare a pair of single
op amps with a dual device without
changing anything else. To achieve this,
I designed a special adaptor board to
allow two 8-pin single op amps to be
plugged into a socket with the pinout
for one 8-pin dual op amp.
I liked the idea and decided to take this
concept further, producing a board that
enables two 8-pin dual op amps to be
plugged into a 14-pin quad op amp socket.
Once I had sorted that out for DIL (dual-in-line) ICs I realised that it would be
just as useful to have SMD (surface-mount
device) versions of these adaptors. This
would not only make the assemblies more
compact, but also allow the use of some
new op amps that are only available in
SMD packaging. One important point to
note here: there are quite a few varieties
of SMD packaging, I have opted to use
the most-common 8-pin SOIC.
The result is four adaptor boards. (Note
that 8-pin-op-amp-SMD-to-DIL adaptor
boards are also available from companies
Pin 1
IC2
IC2
O
ver the last few issues,
Pin 9 for 14-pin
socket under board
Fig.2. Overlay for the two-duals-to-quad
op amp adaptor board.
J-FET replacements
A quick word or two of advice here. If
you’re working on old equipment that
uses J-FET input op amps, such as the
1980s Texas Instruments TL0xx Bi-Fet
series, then it’s essential to use J-FET
replacements because bipolar input
op amps have much higher input bias
currents. If a bipolar NE5532 is used
to replace a TL072 in a mixer channel
or preamplifier then then the result is
likely to suffer from switch clicks and
pot scratching. While the bias currents
could be blocked by adding extra coupling capacitors and DC path resistors,
it is a lot of work, probably involving
track cutting. Good audio J-FET op amps
tend to be very expensive, such as the
Burr Brown designs. There is only one
relatively low-cost replacement for the
TL072, the SMT-only Nisshinbo NJM8087,
distributed by Profusion. Mouser also
stock them, but they are pricey.
Texas Instruments have taken over a lot
of the National Semiconductor, Analogue
Devices and Burr Brown op amps. The TI
OPA134 SoundPlus FET series are one of
the best, but they’re expensive.
Discontinued early Fairchild J-FET
op amps (eg, the µA771 and 772) occasionally pop up in late-1970s equipment
such as mixing desks from Electrosonic
Fig.3. The two-duals-to-quad adaptor
board assembled.
Fig.4. The two-duals-to-quad adaptor
installed in an old Audio Developments
mixer channel strip.
and Soundcraft. Likewise, the LF351,
LF353, LF356, LF441 and TL082 are
similar second generation Bi-Fets. All
Practical Electronics | May | 2023
Pin 1 for 14-pin
socket under board
Pin 1
IC1
Pin 1
IC2
Pin 9 for 14-pin
socket under board
Fig.5. SMD dual-to-quad DIL adaptor
close-up. These are dual NE5532s – note
that the pin 1 end is identified with a bar.
Fig.6. Component overlay of the SMD
version of the two-duals-to-quad DIL
socket adaptor.
Fig.7. Surface-mount version of the quad
adaptor shown in a mixer. SMD versions
are smaller, so they are often easier to fit.
these resemble the TL072 and can be
satisfactorily replaced with the NJM8087
(maximum supply voltage ±16V).
When all the op amps were replaced with
5532s the hiss dropped 12dB and the
distortion dropped to a tenth at 0.002%.
However, there were some op amp upgrade side effects, mainly high-frequency
instability. This was fixed by putting
47pF capacitors on the feedback resistors
and several 100nF capacitors across the
power rails from positive to negative.
Also, the power consumption for each
channel tripled from 12 to 36mA using
the current-hungry 5532s.
Fig.10, installed in the dual RIAA board
described last month. Note the use of
solid wire (25 SWG) to raise the adaptor
board above neighbouring capacitors.
The SMD version is shown in Fig.11
and Fig.12.
Practical adaptor – quad devices
If you want to replace an old quad ‘741style’ device, such as the LM324, then a
two-times-dual-to-quad adaptor board,
which simply plugs into the original 14
pin DIL socket, is the solution. Its overlay
is shown in Fig.2. An assembled unit is
shown in Fig.3, and it is shown installed
in an Audio Developments broadcast
mixer channel in Fig.4.
It produced a marked improvement
in the signal-to-noise ratio of the mixer.
Quad SMD
I’ve also created an SMD version of this
adaptor, which is smaller and easier to
install, as shown in Fig.5. The overlay is
shown in Fig.6 and its mixer installation
in Fig.7. Using these boards, I was able to
compare the OnSemi SMD versions with
their Texas DIL counterparts. Strangely
the distortion of the SMD parts was 30%
higher. I suspect the die may have been
shrunk, a common cost-cutting procedure.
If you are interested in comparing op
amp distortion figures, Swiss audio engineer Samuel Groner plotted the results
for dozens of devices in his epic analysis
at: https://bit.ly/pe-may23-ao
Two-singles-to-dual adaptor
Fig.8. Two-singles-to-dual op amp
adaptor board.
Pin 1
IC1
IC1
Pin 1 for 8-pin
socket under board
IC2
Pin 1
IC2
Pin 5 for 8-pin
socket under board
Fig.9. Overlay for the two-singles-to-dual
adaptor board.
Practical Electronics | May | 2023
In Fig.8 you can see the little board for
adapting a dual op amp to two single
op amp ICs. This is a handy option for
many applications – for example, if
you wanted to replace a dual NE5532
with two NE5534As, which would provide lower noise, lower distortion and
double the dissipation it can handle. Do
remember that when using the NE5534A
with gains below five then it’s necessary
to add compensation capacitors across
pins 5 and 8.
This board is also useful for exploiting
the full ±24V rail voltage rating of the
single-pack OPA604 because the dual
version, the OPA2604, has recently been
downrated by Texas to ±20V.
The overlay for this design is shown
in Fig.9, and you can see it in action in
Soldering the adaptors
Standard square-pin 0.1-inch headers
(eg, BKL from Rapid, part 50-8086) act
as the ‘plugs’ into normal leaf spring IC
sockets. If turned-pin sockets are used
on the ‘motherboard’ then you have to
use round-pin headers on the adaptor
Fig.10. Two-singles-to-dual adaptor
board installed in an RIAA preamplifier.
Note the use of tinned copper wire to plug
into turned-pin sockets.
Pin 1 for 8-pin
socket under board
Pin 1
IC1
Pin 1
IC2
Pin 5 for 8-pin
socket under board
Fig.11. Component overlay of the SMD
version of the two-singles-to-dual
adaptor board.
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Fig.12. Assembled SMD version of the
two-singles-to-dual adaptor board.
Compensation capacitors for NE5534 op
amps that have been added. Sometimes
a power supply decoupling capacitor
should be soldered on as well.
board (eg, TruConnect from Rapid, part
22-1704). These are also available from
Tayda (part number A-4662). I had run
out of round-pin stock, which is partly
why I had to use the 25 SWG tinned
copper wire mentioned above. It fits
well, but can bend easily.
There is a logical order for soldering the
adaptors, such as soldering the headers
before soldering the IC sockets on top
(see Fig.13). In Fig.2, you can see pins
1 to 7 could end up under the op amps
and be very difficult to solder with bits
of plastic blocking the soldering iron tip.
Sourcing the boards
All four adaptor boards are available in
packs of five from the PE PCB Service.
Fig.13. The best way to solder pins to an adaptor board is to first put them in a socket on the ‘motherboard’. This holds the pins in alignment
and provides a stable platform for soldering the adaptor and op amp(s). Note the 8087, which replaced a TL072 in a mixer channel.
Desoldering component puller
M
any electronic engineers
make their own little jigs and
tools to deal with the odd jobs
that come their way. As somebody who
deals with studio equipment, I have a
vast junk box full of control knobs of
all different sizes and shaft diameters.
A lot of these are collet knobs, which
have a clamping system similar to a
drill chuck. These are very useful for
holding things such as 3mm drills for
stripboard cutting and the de-soldering
aid described here.
When the curtain twitcher next door
died, her hideous net curtains were
thrown into a skip. I found a use for
the plastic-coated coiled-spring steel
wire used to hold them up (see Fig.1).
It’s 4mm diameter, so fits perfectly
into a standard 4mm Sifam S15 collet
knob, as shown in Fig.2. The wire is
coated with a plastic sheath which
should be peeled off to get a good fit
in the knob.
Cut about 45mm of wire, then fashion
a hook on the end for inserting under a
component lead-out (Fig.3). Don’t use
your precision Lindstrom side cutters
for this job; a tough old electrician’s
pair is needed. The tool is made short
so it can be pulled against the board
using one hand with the soldered joint
being heated underneath at the same
time, as illustrated in Fig.4. The beauty
of this technique is that only two hands
are needed, and it stops you getting
burnt fingers.
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Fig.3. Close-up of the hook.
Fig.1. Always check skips! This net
curtain wire is useful for all sorts of jobs –
it’s tough and springy.
Fig.2. A 4mm collet knob is attached to
the stripped net curtain wire.
Fig.4. It’s easy to pull the resistor out
of the board with one hand, leaving the
other hand free for the soldering iron. (I’m
happy to supply knobs, curtain wire and
even completed tools.)
Practical Electronics | May | 2023
