Transducer amplifier circuit

The power requirements for driving light aircraft flightsim transducers is not so high; about 20W (real Watts, RMS power). Attention has to be paid to the amplifier circuit to make sure that the amplifier has sufficient low frequency bandwidth. Single ended amplifiers with capacitive coupling between amplifier and speaker would require very large capacitor values to reach 5Hz. Amplifiers with direct coupling between amplifier and speaker are preferred. You can either go for amps that have + and – supply that drive a speaker to ground reference, or you can choose Bridge Tied Load (BTL) structure, where the speaker is connected between two amps, where the amps have opposite polarity. BTL is most easy, supply- and component-wise.

Building small power amplifiers is very easy with currently available IC amplifiers. They require very few external components, have various protections build-in and are cheap. You can choose from SGS-Thompson , Philips Semiconductors , National Semiconductors to name a few.

All you have to care about is total power, gain, max supply voltage, speaker resistance, heatsinking.

For my transducers I used Philips TDA1554Q which in BTL configuration has two channels. As I wanted to adjust the "volume" of each transducer function separately, I used 2 IC’s, leaving me with one unused channel. (You could use this channel for driving a separate subwoofer).

(Similar device is TDA7374B from SGS Thompson, do a search on the part number)

Subwoofers and transducers do not really need stereo information, so I mixed Left and Right together, added a low pass filter stage and then dedicated volume adjustment for each transducer.

The two rudder pedal transducers need very little power, so I put them in series, driven by one amp.

Transducer amplifier circuit diagram


resistors 56k = 56000 Ohm, Capacitors: 100uF = 100 micro Farad

The LM358 is a standard small signal operational amplifier, with two amps in one 8 pin package.

Manufacturers: National Semiconductors, SGS Thompson, …

The TDA1554Q is a class B output amplifier, with 4 power amps in one 17 pin plastic power package. Manufacturer: Philips Semiconductors. Go to and type "TDA1554Q" in the search box. You will also be able to find all similar devices.

Pins 9 and 15 are not connected. Connect the other pins as shown. Capacitors on the supply (470uF and 0.1uF) need to be close to the IC. Keep in mind that the electrolitic capacitors have polarity (see top one). Although the amp outputs and speakers have polarity as well, it does not make a difference how you connect them, as transducers are on separate areas anyway, so do not cancel.

For the volume potmeters, I used the PCB mounted type, not wired to the front. If you wire them to a front panel, you need to use shielded cable. (Ground = shield)

I needed the gain stage as my sound card (Sound blaster 16) did not have sufficient output signal to fully drive the output amplifier, which has a gain of 26 dB (20x).

The 1uF input coupling capacitors make a high-pass filter with the output amplifier’s 30k input resistance at about 5Hz.

The gain stages make a low pass filter of 100Hz (determined by 0.047uF and 33k)

The max power that each transducer can receive from the amplifier depends on the amplifier supply voltage: (2x higher voltage is about 4x higher power) and speaker impedance: (2x higher impedance is about 2x lower power). With supply voltage +12V and speaker impedance shown, the 4 Ohm chair speaker receives about 15W max, the 8Ohm throttle speaker about 8W max, and each 4 Ohm pedal speaker about 4W max (they are in series).

The 12V supply needs to be able to deliver about 4 Amps max. I used a 70W switch mode power supply "brick" as used for notebooks. They are getting cheap, and are (mostly) short circuit proof.

The output amplifiers need to be mounted on a heatsink of about 2.5oC/W (I used 100x100mm black anodized, with 12 x 20mm fins)


If you use a higher gain output amplifier, (45dB or so), the gain stage can be omitted, and replaced by simple passive low pass filter as shown below. This assumes a sound card output with reasonably low output impedance (<100 Ohm, most of them are able to drive headphones, so should be OK)

Below circuit with higher gain output amplifier like TDA1557Q. This IC is even simpler to use, has only13 pins. Pin 12 is not connected.

The passive low pass filter rolls of at 60Hz. I did not build this circuit as I could not get the TDA1557Q IC, but it should work. For IC spec, go to the Philips Semiconductors web site mentioned earlier.

Note on the supply capacitor (470uF) for each IC: If you don’t use a switch mode power supply "brick" but ordinary transformer with diode bridge, the capacitor value needs to be increased dramatically, about 2200uF/25V type per IC.

Transducer amplifier with TDA1554Q

All of my circuits are build on experiment boards, placing the components in a logic fashion and connecting them with solder (for adjacent pads) or 0.3mm f transformer (enamelled) wire. (you can remove the isolation at the wire ends by heating them with your soldering iron for a few seconds). This method works quick, and the transformer wire isolation is good enough to avoid shorts over crossing wires.

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