TOC

Preface
In previous article I have desribed restoration process of vintage vacuum tube integrated amplifier Pioneer SM-83. The next obvious step it is to put newly "refurbished" device under throughput tests. Since I'm was keen to upgrade my PC which I use as media center with high-quality sound card. Integrated Intel ICH9 828011 audio and Creative Sound Blaster Live fall too short in terms of truly h-fi audio.Finally I have opted for 2 very similar semi-professional PCI audio cards - M-Audio Audiophile and ESI Juli, both seem to be compatible with Windows and Linux. Couple of weeks later 2nd have arrived from Germany.
Card is built exceptionally well, with swappable inout/output sockets (unbalanced RCA -10dBv or balanced TRS +4dBu),
Asahi Kasei Microsystems Semiconductor 8-channel multibit 24-bit 192 kHz DAC AKM AK4358, dual bit delta-sigma stereo 24-bit 192 kHz AKM AK5385A, and DSP VIA Envy24HT-S (24-bit, 192 kHz; interfaces: three output I2S/AC-links, two input I2S/AC-links). Its back panel is quite thick, and that have given a trouble with my stylish all-aluminium PC enclosure - it was impossible to snap this card before retooling PC case with flat screwdriver and hammer. Since I use dual-boot Linux/Windows system, I had to install and configure drivers twice. On Linux everything was a snap - drivers for these chips are already compiled as loadable kernel modules. Unfortunately, ESI card utilities are not available on Linux, so configuration options are very limited to basic audio in and out thus limiting card functionality. Installing drivers on Windows drove me mad - it took over 3 hours (need to say, my PC is assmebled using top-notch components like Intel Quad-Core 2.4GHz, Gigabyte P45DS5 ultra-durable mainboard, etc.)! First of all, there are no ESI driver installer, they are installed with "Add Hardware Device" utility in 2 pass routine. Second, it didn't work - Windows was simply freezing at startup and vice versa. Even complete re-format, re-install and downgrade of Win from SP3 to SP2 did not helped. I started to suspect taht card had some hidden defects. After all, I managed to get it working by means of swapping PCI slots between 2nd Ethernet card and ESI Juli. Once it started to work, I had no more trouble with it whatsoever.

Software

There are many audio measurmenet software available, commercial, shareware and freeware. I used 2 packages - freeware RMAA (RightMark Audio Analyzer) and shareware ARTA. The latter is more advanced and allows precise voltage calibration of both input and output. Of course, all this software is Windows-only.
PS. You can download Pioneer SM-83 schematic diagram here.

Testing Methodology & Caveats
I have decided to test
pre-amplifier (1x12AX7 in each channel) and power amplifier (1x12AX7 + 1x6AN8 + 2x7189a) separately for Frequency Response, THD (Total Harmonic Distortions) and IMD.(Total Intermodulation Distortions). One of the aspects that drove me to this decision was the fact that trebble/bass control is not capable of linear frequency response +1/-1 dB no matter what. Additionally, you should take into account few other moments:
1) Input impedance of ESI Juli is only 10K on unbalanced RCA, yet coupling cap in the output stage of SM-83 is only 0.1 uF, thus, it will become high-pass filter with corner frequency of 159 Hz. Solution - connect SM-83 pre-amplifier and ESI Juli RCA input via 100k+ resistor (or replace coupling capacitor).
2) In order to test power amplifier, one will need 8 OHm dummy load with voltage divider and protection with 2x4V Zenner diodes. I used 2 pcs of 10 OHm Russian-made 25W rheostats, turned input signal on, and lowered output voltage to the safe level before each test.
3) Input impedance of SM-83 pre-amplifier is 470k, so cable length and capacitance does matter!

Tube Rolling
One of the things I'm was eager to do since I have acquired my first tube gear is to conduct tube rolling with both sonic and engineering tests. Since I had 3 sets of tubes for pre-amp - vintage Matsushita 12AX7, Sovtek 12AX7 LPS (Long Plate Spiral Fillament) and 6N2P-EV. Unfortunately, in Pioneer SM-83 each triode from 12AX7 tube connected to different channel, so it was impossible just to put one 12AX7 variant in left channel, and another in right, and perform instant sonic test just with L/R switch. For power amplifier I have 3 different 6AN8 tubes - Hitachi, RCA and Sylvania.

Power Amplifier Test
Frequency Response (20Hz - 20KHz)


Frequency Response (10Hz - 50KHz)


Frequency response curve is good but not perfect indeed. I do not think the amp will be capable of 10Hz-100KHz +/- 1dB as advertised.

Total Harmonic Distortions (10W Output Power)


Total Intermodulation Distortions (10W Output Power)


Pre-amplifier Amplifier Test

Frequency Response (10Hz - 50KHz)



Frequency Response (20Hz - 20KHz)


Total Harmonic Distortions (150mV Input Signal)



Total Intermodulation Distortions (150mV Input Signal)


Conclusion (Measurements)
Overall, SM-83 is a good performer, but not perfect. The dropout of low frequencies caused by low input resistance of ESI July card, and on high frequencies probably ba parasitic capacitance of long audio cables. Since I used PC with sound card, I had to use quite long interconnects.
THD of power amplifier is very typical for tube push-pull, its really very good that THD is less than 1%. Intermodulation distortions is where SM-83 falls short compared to other high league tube gears (McIntosh, Marantz, Sansui, Luxman), which typically have 0.5% IMD.
As for pre-amplifier, more expensive and advanced units like Luxman CL or Marantz 7C have a considerable edge over SM-83, with only 0.02% - 0.15% THD.

Tube Rolling Tests
Tests were done with 0.5V of input voltage, pre-amplifier handles this without any problems.

Total Harmonic Distortions (20Hz - 20KHz)
Matsushita 12AX7 - green, Sovtek 12AX7LPS - orange


Total Harmonic Distortions (5KHz - 10KHz)
Matsushita 12AX7 - green, Sovtek 12AX7LPS - orange




Total Intermudalation Distortions (20Hz - 20KHz)
Matsushita 12AX7 - orange, Sovtek 12AX7LPS - green


Total Intermudalation Distortions (20Hz - 20KHz)
Matsushita 12AX7 - orange, Sovtek 12AX7LPS - green


Conclusion (Tube Rolling)
Sonic difference between Matsushita 12AX7 and Sovtek 12AX7LPS can be barely heard, it is really subtle. I think this only because of extra slight IMD level distortions of Matsushita at 14 - 17 KHz range. BTW, I cannot tell which tube is sonically better, since they are very close to each other.

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