# Magnetic dose stereo preamplifier - IPRS Baneasa - Prospect SME8902

#### What does the project present?

The assembly can be used for signal amplificationr obtained from a pickup (vinyl player) with magnetic dose. The vintage DIY electronic kit presented is intended to familiarize the electronics technician with the performance and construction of preamplifiers with the corrected frequency characteristic.

#### How does the assembly work?

Specific to this electronic kit, is the introduction on the reaction path of a variable impedance with frequency.

The recording on AF (vinyl) of the AF signals is performed keeping constant, at medium frequencies, engraving needle speed, and at the extremities of the audio range, its displacement (fig.1).

The determination of the frequencies f1, f2, f3 or of the corresponding time constants t1, t2, t3, (f = 1 / 6.28 xt) is done by standards. The playback amplifier will need to have symmetrical response curve relative to the recording curve relative to the "0" dB axis.

The RIAA norm defines the time constants ti = 318us, t2 = 75us; t3 = 318us corresponding to the frequencies: f1 = 500 Hz; f2 = 220 Hz; f3 = 50 Hz. To obtain this characteristic, the components R7, R8, C5 and C6 were introduced in the reaction network.

Time constant t1 (in microseconds) is determined by the product R8, C5 where the resistance is in KOhmi and the capacity in nanophases. It causes an increase in amplification starting from the f1 frequency with a slope of 20 dB / decade.

Time constants t2 and t3 are obtained with the help of a second degree equation knowing that:

t2 x t3 = t1 x R8 x C5
t2 + t3 = t1 + R7 (C5 + C6)

They cause a decrease in amplification with a slope of 20 dB / decade starting from the frequencies f2 and f3 respectively. Summarizing the influence of the 3 time constants the idealized response curve is obtained, presented in fig. 2.

It is observed that between f3 and f1 as after f2 the amplification decreases with increasing frequency and between f1 and f2 it remains constant due to the compensation of the effects of time constants t3 and t1. In order to obtain a frequency characteristic according to the RIAA norm between the components there must be the relations: R8 = 0.145 x R7; C5 = 318 / R8; C6 = 0.34 x C5

#### Technical characteristics of the assembly

• Rated output voltage: min. 0.7V
• Frequency characteristic: RIAA
• Distortions: max. 0.5%
• Input impedance: 470 KOhm
• Signal to noise ratio: min 52 dB
• Nominal sensitivity: 5 mV
• Crosstalk attenuation between channels: min. 75 bB
• Current absorbed from the source: max. 20 mA

#### List of required components (with recent equivalents):

• R1, R11 - resistor 1 KOhm
• R2 - resistor 56 KOhm
• R3, R7 - resistor 270 KOhm
• R4 - resistor 82 KOhm
• R5 - resistor 27 KOhm
• R6 - 120 Ohm resistor
• R8 - resistor 33 KOhm
• R9 - resistor 100 KOhm
• R10 - resistor 3.3 KOhm
• C1 - electrolytic capacitor 2.2uF / 63V
• C2, C7 - electrolytic capacitors 4.7uF / 40V
• C3 - ceramic capacitor 10pF / 20%
• C4 - electrolytic capacitor 100uF / 25V
• C5 - ceramic capacitor 10nF / 10%
• C6 - ceramic capacitor 3.3nF / 10%
• T1 - transistor BC 413 or BC 550
• T2 - transistor BC 415 or BC 560
• T3 - transistor BC 172B or BC 548
• Printed wiring or breadboard test board

Note: All resistors will have a nominal dissipated power of min. 0.25W and tolerance of the nominal value of +/- 20% except for the positions R6, R7, R8 which will be +/- 10%. Capacitors C5 and C6 will have a tolerance of +/- 10%.

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

In order to carry out this project in our own laboratory, we will also need printed wiring PCB layout From lower:

To have a clearer view of this vintage electronic DIY kit, I have attached below a picture with assembled assembly:

Also Mister. George-Alexandru Marinescu sent us a picture with the assembly assembled using other component capsules:

##### sources:

http://www.emil.matei.ro/

4,287 total hits, 2 hits today

1. marius b say:

Bravo Dane, you turned them upside down. I don't have it in my collection st I know it and I had it through my high school years… but I don't know where to get it, so the information comes from you, so that I can have it at least as a prospect.

2. Cristian N. say:

This is how we learned electronics in the past, with pieces like this and as you can see, R2 has its foot glued ... because they were so bad and how you warmed them up more, they gave up… 🙁

3. Dan Alx say:

Yes, I remember too. I was at the children's club and due to the lack of materials we reconditioned the resistors by adding terminals at their ends 🙂

4. Raphael u. say:

Who makes electronics today, apart from Chinese robots? Now a great electrician is called a doctor: What does he need to know the amplification factor of a transistor or what does a sinusoid look like on an oscilloscope !? Yes, it gives you lessons 🙂

5. RobertP. say:

Yep .. resistors with 1 / 2W carbon film with noise factor as Mont-Blanc, electrolytes with equally grandiose losses, transistors with hFE approximately… clean "haudiophilia"!

6. Dan Alx say:

Yes, the components had a pretty high tolerance! Probably so could the manufacturing processes of those times!

7. Valentine A. say:

What a headache the "green resistances" gave me! I found the "cold solder" on the board, I repaired the solder, the "foot" came loose from the body of the resistor.

8. Dan Alx say:

This is because the terminals were extremely thick and a higher extraction temperature was required.

9. Joseph P. say:

It is known that the installation is "Made in Romania" after the capacitors, resistors and transistors used!

10. Catalin B. say:

In general, they all differ… Romanians, Russians, former Czechoslovaks, Japanese, etc. Identify them with just a glance.

11. George a. say:

Isn't it more comfortable with an LM 387, LM 381, etc.? It would be a smaller and simpler installation!

12. Valentin P. say:

Yes, if you want "professional"! But, if you want to live the nostalgic scent of those times, you do it as it is presented here! (No offense, if you haven't lived through those times, you can't understand… I repeat, no upset)!

13. Caesar O. say:

These were children's kits. The parts with high tolerances that did not enter production were capitalized. The idea was very good and aimed at training and learning through practice.