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Friday, March 31, 2017
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  How to measure the Phono Pre-Amplifier



Generator
Frequency Hz
Phono Amp
Uout
Phono Amp
dBout
209,23 volts+19.30 dB
308,51 volts+18.60 dB
407,76 volts+17.80 dB
507,08 volts+17.00 dB
606,38 volts+16.10 dB
805,31 volts+14.50 dB
1004,52 volts+13.10 dB
1503,27 volts+10.29 dB
2002,57 volts+8.20 dB
3001,88 volts+5.48 dB
4001,55 volts+3.80 dB
5001,35 volts+2.60 dB
8001,08 volts+0.67 dB
1,0k1,00 volts0.00 dB
1,5k0,85 volts-1.41 dB
2,0k0,74 volts-2.61 dB
3,0k0,58 volts-4.73 dB
4,0k0,47 volts-6.56 dB
5,0k0,39 volts-8.18 dB
6,0k0,33 volts-9.63 dB
8,0k0,25 volts-12.04 dB
10,0k0,21 volts-13.55 dB
15,0k0,14 volts-17.08 dB
20,0k0,10 volts-20.00 dB
The test circuit is built according to plan ( the scoop not really necessary ).

The sine generator is set so (out of balance can be no question), that appears at the output of the phono amplifier at 1kHz an AC voltage of 1V. In following the sine wave generator, only the frequency is changed, the amplitude remains the same.

Now, driven by the following frequencies and compare the measured voltage with the figures shown here. The closer the measurement result at this data, the better, with deviations of ± 0.12 volts quite acceptable.

When performing any tests on an audio system, some form of measuring device is essential. Digital multimeters are not useful, since they will not give the true picture of what is happening, and most have a fairly limited frequency range. An oscilloscope is the ideal tool, but not all hobbyists can afford the outlay for a scope.

  1. Audio Generator - with a range of 10Hz up to 100kHz and high amplitude stabilisation
  2. AC millivoltmeter - calibrated in dB - with a range of 30V down to 3mV full scale (80dB range)
  3. Oscilloscope is ideal, but not all hobbyists can afford the outlay for a scope




RIAA equalization
 




reversed RIAA equalization

  Reversed RIAA EQ Network

The previously described method for the measurement may, perhaps for one, two amplifiers are still acceptable, but if one more phono amplifier built, then the effort for this significant.

It would be better if they possessed a signal source, which already provides the distorted signal at the correct amplitude.

The frequency response of the current through the network formed by R1, R2, C1 - C4 corresponds to the inverse of the standard RIAA curve.
Resistor R3 determines the output voltage or the overall voltage gain.
R3 influences the frequency response at the upper end. If R3 becomes too high, high frequencies are reduced. With R3 = 1.5k the reduction is less than 0.1dB at 20kHz.




Circuit anti-riaa network
Component values

IdealActualAlternative
R1883k909k910k
R275k78.7k75k
C13.6n3.3n3.3n
C2-180p270p
C31n680p1n
C4-270p-
R301.48k1 - 1.5k


The use of close tolerance components in the filter network is absolutely necessary.





  This calculator finds the closest standard resistor or capacitor value.


Desired Value:

 
IEA Series Closest Value Error Comments
E6 most capacitors, some inductors
E12 some capacitors, 10% resistors
E24 5% resistors
E48 2% resistors
E96 1% resistors
E192 0.5%, 0.25%, 0.1% resistors