SYMPRE2 preamplifier

this is a “conventional” design using operational and instrumentation amplifiers (OPA134, OPA627 and INA217) with single ended inputs, balanced input and balanced output. It has become my main preamplifier, as I need a symmetrical output to drive my new bridged power amplifier. This time I am showing a complete schematics with component values.

Circuit description

The following image shows a circuit diagram of the preamplifier. As the image may not be well readable, it is possible to
download schematics as pdf file.



The preamplifier has 3 single ended inputs switched by K1 – K3 relays. Each unused input is grounded via 680 ohm resistor, this improves crosstalk rejection considerably. K4 relay connects the balanced input with IC1, IC2 instrumentation amplifiers. A signal from the selected input goes to IC5, IC101 input buffers. Their outputs go to P1 blue Alps volume control potentiometer. The potentiometer is followed by IC4/IC3, IC102/IC103 opamps. They create a symmetrical output of the preamplifier.

There are two regulated power supplies. One of them gives 2 x 15V for active devices, the second one makes 12V to supply input selection relays. Both power supplies are independent and their 0V terminals are connected at one point. Two power supplies are used to minimize coil/contact crosstalk in the relays.

Specifications

input impedance SE 100 kohm
input impedance balanced 66 kohm
max. input voltage 8 Vrms
voltage gain 6 dB
max. output voltage 16 Vrms
THD/10 kohm/1Vrms/(50Hz - 10kHz) < 0.001%
Signal to noise ratio re 1 Vrms 106 dB(A)

Inside

Measurements

I made some basic measurements of distortion, however my measuring system has higher distortion than this preamplifier. Thus, the distortion plots are shown for information only. Oscilloscope measurements of maximum output voltage and 10kHz square response are shown as well.

Fig. 1. THD vs. frequency at 1Vrms output voltage

Fig. 2. Spectrum of 1KHz sine tone at 512 mVrms output voltage

Fig. 3. Maximum balanced output voltage

Fig. 4. Response to large signal 10kHz square wave

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