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2-channel instrumentation input amplifier.



IA21 is a 2 channel electronic balanced input amplifier for audio.
The circuit is a 3 OP-AMP instrumentation amplifier with bootstrapped DC bias for high common-mode input impedance.
To avoid the use of very expensive precision resistors, a trimpot us used to trim the CMRR.
Some of the design parameters are:


IA21A schematic.
Fig.1: IA21 schematic ( channel 1 shown, add 30 to designators for channel 2 ).

J21 is a female XLR connector. Neutrik A series and other types with similar footprints can be used.
The 2 nets CH1L and CH2L are brought out on solder pads near the panel and can be connected to the panel with solder tabs. This is useful for connector types where pin 1 and chassis are not internally connected to chassis.
The solder-terminals LI+, LI- and LIG can be installed in stead of the XLR.
R11, R12, C11 and C12 is input HF-filtering. The capacitors should be matched for best CMRR at high frequencies, however the prototype maintained 70 dB CMRR at 20 kHz with 5% types from the same batch.
R13 and R14 are DC bypass for the input capacitors C13 and C14.
The value of C13, C14 may seem very large for the application, but this is required to maintain good CMRR at low frequencies with 20% tolerance capacitors.
They are 50 V bi-polar types to allow for up to ± 50 V DC on the input as required by the N10 broadcast standard. If you have little or no DC on the input you can use ordinary electrolytics.
R15 sets the differential input impedance at 10 kΩ.
R17..R20 provides DC bias for U11. R23, R24 and C15, C16 is the bootstrap circuit.
The common-mode input impedance for this circuit is around 400 kΩ up to 1 kHz, falling to around 40 kΩ, at 20 kHz due to C11, C12.
D21..D28 are input over-voltage protection. The circuit will protect against overload transients, but not against abuse ( like connecting the input to a power amplifier ).
D21, D22 are connected between the OP-AMP's input and output to prevent them from conduction and creating distortion under normal operating conditions.
In case of an over-voltage transient on the input, D23 or D24 will conduct and clamp the input to a little over 13 V.
R21 and R22 are HF-stoppers for the OP-AMPs.
U11 is a OPA1678. This is only available in a SMD package, but the board is made with a very large footprint, so they are easy to solder.
You may be able to use a NE5532A ( I have not tested it ), but the OPA1678 is generally cheaper.
The circuit around U12 is a standard balanced amplifier. It has a gain slightly above 1 to compensate for the attenuation in R11..R15.
R33 is the CMRR adjustment. Its range is just sufficient to trim out worst-case tolerances in the rest of the circuit.
The trimpot is a 10-turn type as I could not find space for a single turn, top-adjustment type on the PCB.
CMRR is adjusted by applying a common mode signal and adjusting R33 for minimum output. CMRR measured this way is around 85 dB at 16 kHz.
Unless you use a meter with a high-pass or band-pass filter, you will need a non-metallic screwdriver to avoid too much hum on the output.
Without the CMRR adjustment, you should expect a CMRR of a little over 40 dB. You can leave it out by shorting R33 and replacing R27 with 698 Ω.
U12 is a NE5534A with a second-order compensation network to maintain low distortion a higher frequencies ( the THD-N measurements on the circuit are mainly noise, not distortion ).
C19 is a DC blocking capacitor for the output. If you can live with some mV DC out, you can short it.
R31 and R32 provides an impedance balanced output.

IA21A schematic.
Fig.2: IA21 schematic for power supply.

D1, D2 and R3, R4 are 12 V regulators for the input over-voltage protection circuit.
H1..H3 are PCB mounting holes. H1 can not be used when XLRs are installed. C1..C3 are HF decoupling between the PCB and the chassis. They are essential for good HF CMRR as the XLR pin 1 is connected to the chassis only ( not to circuit ground ).

Component selection.


C13, C14 are bipolar electrolytics with a 50 V rating and 2.5 or 5 mm pin-spacing.
Other electrolytic capacitors are general-purpose types with 25 V rating and 2.5 mm pin-spacing.
Decoupling capacitors C1..C3, C22.. C25, C52..C55 must be ceramic types.
Low value capacitors C11, C12, C17, C18, C20, C21 must be ceramic NP0 types, preferably with a voltage rating of 100 V or more.


The diodes are standard signal diodes like 1N4148 or 1N914.
The zener diodes are 0.5 W, 5% types.


All resistors are 0.25 W..0.6 W 1% metal-film types.


All ICs are in SOIC-8 cases.
Use the types suggested. They are quite inexpensive.


The input connections can be made with Neutrik A or B series connectors or solder terminals can be used.
If you use a connector similar to the Neutrik connector from another manufacturer, make sure that no metal on the connector shorts to the pads for the input solder-terminals.
The power and output connections can be solder terminals, three 3-pin pin-rows or a 11-pin pin-row. The pin-spacing for the pin-rows is 2.54 mm.


This is the specification for the IA21A prototype.
Supply is ±15 V from a low-noise lab power supply ( <-92 dBu ( 18 µV ) noise in the 22 Hz to 22 kHz bandwidth ).
The unit dBu is dB referred to 0.775 V.
More detailed measurements are in the file IA21A_Measurements.ods in the design file download.

Table 1: Specification for IA21A. Unless stated, source impedance is 40 Ω and load impedance 100 kΩ.
Supply current, no signal, no load18.2 mA
Input impedance, 20 Hz..20 kHz10.0 kΩ..10.1 kΩ
Voltage gain, 1 kHz, channel 1-0.03 dB
Voltage gain, 1 kHz, channel 2-0.02 dB
Linearity, 20  Hz..20 kHz, ref. 1 kHz±0 dB ( Note 1 )
Output clip level, 1 kHz, <1% THD+N21.8 dBu ( 9.6 V )
THD+N, 1 kHz, 22 Hz..22 kHz BW, 6 dBu ( 1.5 V ) input.0.0008%
THD+N, 1 kHz, 22 Hz..22 kHz BW, 20 dBu ( 7.7 V ) input.0.0011%
THD+N, 20  Hz..20 kHz, 10 Hz..80 kHz BW, 6 dBu ( 1.5 V ) input.<0.002%
THD+N, 20  Hz..20 kHz, 10 Hz..80 kHz BW, 20 dBu ( 7.7 V ) input.<0.0025%
IMD, SMPTE, 60 Hz / 7 kHz, 4:1, 6 dBu ( 1.5 V ) input.<0.002%
IMD, SMPTE, 60 Hz / 7 kHz, 4:1, 20 dBu ( 7.7 V ) input.<0.002%
Cross-talk, 1 kHz-133 dB
Cross-talk, 20  Hz..20 kHz<-115 dB
Output noise, 22 Hz..22 kHz, Rs=0 Ω<-111.4 dBu ( 2.1 µV )
Output noise, 22 Hz..22 kHz, Rs=1 kΩ<-111.2 dBu ( 2.1 µV )
Output noise, CCIR-468 Q-peak, Rs=0 Ω<-100.4 dBu ( 7.4 µV )
Output noise, CCIR-468 Q-peak, Rs=1 kΩ<-100.0 dBu ( 7.8 µV )
CMRR ref. IEC60268-3, 50 Hz.81 dB
CMRR ref. IEC60268-3, 1 kHz.81 dB
CMRR ref. IEC60268-3, 20  Hz..20 kHz.>70 dB
Board size including XLR connectors ( length / width / height )81 mm / 48 mm / 31 mm ( Note 2 )

Note 1:Below the resolution of my audio analyzer.
Note 2:The board is <46 mm wide allowing a channel spacing of 23 mm.


IA21AAA photo.
Fig.3: Photos of the mounted board.

I have boards available for this project. See the PCBs page.


IA21A design files.

Known Issues / updates.

No known issues.

Appendix A: CMRR adjustment.

CMRR adjustment setup schematic.
Fig.A1: CMRR adjustment setup.

G is a sine-wave generator with a frequency in the range 10 kHz to 20 kHz and 5 V to 10 V output voltage.
M is ideally a meter with a band-pass filter tuned to the generator frequency, but an ordinary AC voltmeter can be used.
The meter must be able to measure at the generator frequency. Its accuracy is not important, but it should be able to display down to below 1 mV.
RHP and CHP is a high-pass filter at 5 kHz that allows you to hold your fingers near the circuit without upsetting the measurement too much.

Simply adjust R33 for the lowest meter reading.

Appendix B: Some test circuits.

These are some simple circuits to help adjusting and measuring the amplifier.
They can easily be built on strip-board or solder-less breadboard.

Sine-wave generator for CMRR adjustment.
Fig.B1: Simple sine-wave generator for CMRR adjustment (only simulated - not tested).

Simple voltmeter for CMRR adjustment.
Fig.B2: Simple voltmeter for CMRR adjustment (only simulated - not tested).
Power supply connections as in Fig B1.

Switch for CMRR measurement.
Fig.B3: Switch for CMRR measurement. See [1], page 24 for test setup.
For testing bare IA21 boards, the ground of the generator must be connected to IA21 circuit ground as pin 1 of the XLR is separate from circuit ground.


[1] Audio Transformers by Bill Whitlock.

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E-mail: diy@poulpetersen.dk

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