Audio mixer - IPRS Baneasa - Prospect 8203

Audio mixer - IPRS Baneasa - Prospect 8203

Reading time: 5 minute

What does the project present?

The vintage DIY electronic kit presented is an electronic assembly called "Audio Mixer", because with the help of voltage amplification stages it is made "mixing" of signals audio frequency provided by microphone, tape recorder, electromagnetic dose or crystal. Of course, the assembly can be adapted for other types of inputs.

This electronic kit complements the other two mounting sets "Amplifier 10W" leaflet 8109 and "Preamplifier with tone corrector" leaflet 8201. The audio mixer is made with 6 transistors according to the presented electrical diagram, distributed on several floors.

How does the project work?

First floor, made with transistors T1 and T2, forms a preamplifier for tape recorder head, ensuring the amplification of the signal and its correction depending on the frequency, in order to compensate the recording curve on the magnetic mandate.

The frequency correction is made for two speeds: 19.05 cm / s and 9.53 cm / s, the choice being made with the help of the K computer.

Second floor, made with transistors T3 and T4, forms a preamplifier for magnetic dose with which it amplifies and makes a frequency correction, RIAA type, of the signal provided by the dose.

Third floor, made with transistors T5 and T6, forms a microphone preamplifier, linearly amplifying the signal generated by it.

The three floors it is coupled at the output of the assembly through resistors R10, R20 and R29. Potentiometers P1, P2 and P3 allow independent adjustment of the signal levels provided by the three amplifiers.

Ways of interconnection

The audio mixer can be used to amplify and mix up to three different signal sources. The connection of the signal sources to the floors of the audio mixer is done accordingly figure 1.

The capacitor C19 forms together with the inductance of the playback head a resonant circuit of parallel type, damped by the resistance R1 in order to accentuate the high playback frequencies. Resistor R30 allows the connection between terminals 3 and 2 of the input socket and a crystal dose. The characteristics of this input will be: sensitivity - max. 200 mV, input speed - 1 MOhm.

However, the tape recorders have a "standard" output (approx. 500 - 800 mV), instead of a direct output from the playback head, the interconnection to the mixer in this situation will be done according to the diagram shown in figure 2.

It is observed that the tape recorder preamplification stage is no longer used, the connection being made by means of the resistor R31 between points D (from the input of the plug) and E (from the potentiometer P1).

The characteristics of this entry will be:

  • sensitivity: max. 800 mV
  • input impedance: approx. 50 KOhm

For other needs of using the mixer we are sure that you will find the interconnection ways yourself.

Technical characteristics of the assembly


  • tape recorder head: 6.5 mV
  • magnetic dose: 4 mV
  • microphone: 3.5 mV

Input impedance:

  • tape recorder head: 27 KOhm
  • magnetic dose: 56 KOhm
  • microphone: 22 KOhm

Output voltage: approx. 60mV
Response curve: 40 Hz – 18000 Hz
Efficiency of the mixing system: 100%
Supply voltage: 18 - 20 V
Current absorbed from power supply: approx. 5 mA

List of required components (with recent equivalents):

  • T1, T3, T5 - transistors BC 413 or BC 550
  • T2, T4, T6 - transistors BC 173 or BC 549
  • C3, C9, C16 - electrolytic capacitors 100uF / 6.3V
  • C1, C2, C4, C5, C7, C8, C10, C11, C14, C15, C17, C18 - electrolytic capacitors 4.7uF / 40V
  • C6, C12 - 22nF ceramic capacitors (min. 50V)
  • C13 - 10nF ceramic capacitor (min. 50V)
  • C19 - ceramic capacitor 3.3nF (min. 50V) or 4.7nF (min. 50V)
  • R3, R4, R7, R13, R14, R17, R23, R24, R27 - resistors - 220 KOhm +/- 10% 0.25W
  • R2, R12, R22 - resistors - 680 Ohm +/- 10% 0.25W
  • R5, R15, R25 - resistors - 18 KOhm +/- 10% 0.25W
  • R8 - resistor - 6.8 KOhm +/- 10% 0.25W
  • R18 - resistor - 150 KOhm +/- 10% 0.25W
  • R19 - resistor - 10 KOhm +/- 10% 0.25W
  • R28 - resistor - 4.7 KOhm +/- 10% 0.25W
  • R1 - resistor - 27 KOhm +/- 10% 0.25W
  • R11 - resistor - 56 KOhm +/- 10% 0.25W
  • R21 - resistor - 22 KOhm +/- 10% 0.25W
  • R10, R20, R29 - resistors - 100 KOhm +/- 10% 0.25W
  • R30 - resistor - 1 MOhm +/- 10% 0.25W
  • R31 - resistor - 47 KOhm +/- 10% 0.25W
  • P1, P2, P3 - potentiometer - 10 KOhm
  • Buttons (3 pcs.)
  • Mother plug with 5 contacts (3 pieces) - or other types of plugs
  • Printed wiring or breadboard test board
  • Tin or connecting threads

Download the original IPRS leaflet Baneasa 8203

For a better understanding of the operation of the circuit we will need electronic scheme presented below:

To carry out this project in our own laboratory, on printed wiring, we present below two variants of PCB layout:

1 variant

2 variant

Many of you are probably wondering how this DIY electronic kit was packaged or distributed. Below I have attached some pictures with packaged product (new) and the contents of the envelope. Thank you Mr. Marius Balauta for providing pictures!


We also thank you Mr. Adi Bud for the extracted (clear) image of the wiring of variant 1.


  1. I assembled the preamplifier but I put linear potentiometers, it seemed cooler. I also made the case but it stayed somewhere, I hope I can recover it someday.

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