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October
INTRODUCTION
If you want design
your amplifier you need a measurement instrument to verify the result.
Also if you think to have calculated and simulated the circuit there are some
factors that you can forget.
Actually I am using the expensive Clio system by Audiomatica for all my measurements but my version
is limited to 22KHz.
In this last years the evolution of common sound cards allow to use these in the
audio measurements.
These sound card have good resolution 24bit and good frequency band until to 96KHz.
Using an external sound card with separated power supply you can keep the noise
very low.
I think the best freeware software for this kind of measurements is
Arta.
INPUT CIRCUIT
The sound cards accept only low
voltage input, about 1Vrms, so in order to protect the sound card and to measure
high voltage it is necessary a good input circuit.
This following circuit use:
- 1. an input capacitor to eliminate any DC voltage until 2KV
- 2. 1,1Mohm resistor to discharge the input capacitor if you disconnect the input terminal
- 3. 990Kohm resistor to reduce the input level and reduce the current on protection diodes
- 4. stepper attenuator with a max output impedance of 33Kohm to get a large band driving the op-amp input capacity
- 5. op-amp with 32dB gain to compensate the voltage lost on attenuator (990K / 33K about 30dB)
- 6. input impedance 550Kohm to create a very low load
- 7. support input voltage until 1KV rms
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Follows some plots of the
input circuit simulation using WinSpice
(click here to download the file). The first is the low frequency cut-off created by the 0.47uF and the 990Kohm, 0.7Hz at -1dB. The other two plot are the high frequency cut-off created by the attenuator in the worst positions, 33Kohm and 10Kohm. |
SOUND CARD
It is possible use many sound cards for this project but I suggest the EMU 0404 USB because it is a pure 24bit 192Khz with good converter chips and it is valid also to create a Hi-resolution player and to rip vinyl LP.
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Follows some distortion measurements in loop configuration using the
Arta software and my input
circuit. To get a low value of distortion, as in my following measurements, keep the input vu-meter of EMU 0404 on the green led increasing or reducing the level on input circuit with the two stepper attenuators. |
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Keep the two input regulators of the EMU 0404 on the 0dB/Unity Unbalanced position. Use RCA Female (Jack) to 1/4 Inch Mono Male (Plug) to use normal stereo audio cables to connect the input circuit to the EMU 0404. |
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There is only a little
noise coming from the network (in Italy 50Hz 220V) at (87-10)=-77dB and THD is
very low 0.0091%.
Follows the my setting in the configuration panel.
Follows the same spectrum measurement at different frequencies.
About the frequency
response there is a little problem because on output stage of EMU 0404 there is
a high frequency filter so if you don't make modification to the sound card are
possible only differential frequency measurements.
Using the dual/differential frequency measurements the software plot the
differences of the two input so any limitations of the sound card is
compensated.
Follow the single channel frequency response.
Follow the dual channel frequency response.
Follow the single channel frequency response with Arta Steps program.
Follow the dual channel frequency response with Arta Steps program.
Follow the single channel frequency response with Arta Steps program with output stage modified like my Hi-resolution system with Lundahl transformers.
As alternative to the Lundahl output transformers it is possible to use this
simple differential amplifier.
The two capacitors should be adjust to get the desiderated frequency band.
If you will use this active stage for the output you need a separated dual power
supply with
+12v and -12v.
Follows some measurements on my
GainClone amplifier at full power
(14Vrms on 8ohm).
For the frequency response has been used both methods: singe and dual
channels (differential).
Follows some measurements on D3a
driver amplifier at 50Vrms output voltage, power supply 170v, Rk=100ohm, Ck=1000uF
and 70H Fiat choke.
For the frequency response has been used both methods: dual
channels with pink noise and dual channels Steps.
Follows some measurements on my 813/GM70 amplifier.
813 anode THD at 1100Vdc and a signal of 350Vrms.
Output frequency response in dual channels Steps.
