- Written by John E. Johnson, Jr.
- Published on 04 October 2010
- Manley Stingray iTube Stereo Integrated Amplifier
- Page 2: Design of the Manley Stingray iTube Integrated Amplifier
- Page 3: The Manley Stingray iTube Integrated Amplifier In Use
- Page 4: The Manley Singray iTube Integrated Amplifier On the Bench
- Page 5: Conclusions About the Manley Stingray iTube Integrated Amplifier
- All Pages
On the Bench
All distortion measurements were made within an 80 kHz bandwidth and into an 8 ohm load. The first few tests compare triode mode with ultralinear mode. The yellow graph lines represent the left channel, and the red lines are the right channel.
At 1 kHz input, and 5 volts output into an 8 ohm load, the measurement was 0.20% THD+N in triode mode, and 0.10% in ultralinear mode. Overall, there really wasn't much difference.
I used a 1 kHz sine wave stored on my iPod to obtain the graph shown below (the other test signals were generated by my Audio Precision). The iPod dock takes the analog output from the iPod. It does not take the digital bitstream. So, the input to the Stingray is from the output of the iPod's own DACs. The analog output from the iPod is at full volume, and you cannot lower it using the iPod's control panel. You will need to lower the input level using the input level adjustment mode described in The Design section above. The resulting spectrum is irregular, possibly due to the iPod's inexpensive analog output amplifiers being at full output. Here is a video of how to set up your iPod with the Stingray.
Inputting 19 kHz and 20 kHz sine waves, the B-A (20 kHz minus 19 kHz) peak at 1 kHz was 47 dB below the fundamentals in triode mode, and 54 dB below in ultralinear mode. There are also some slight differences in the height of the side peaks on either side of the 19 kHz and 20 kHz peaks as well as around the 40 kHz and 38 kHz harmonic peaks. Really, not much difference between the two modes, so I only measured in ultralinear mode from this point on.
IMD was 0.38%
My test environment has some electrical noise in the 20 kHz range, but the Manley rejected it as seen in the graph below. I turned the volume all the way up for this test, but no input signal was applied. Some amplifiers don't reject it, and when it shows up, it is usually about 10-15 dBv in height.
THD+N vs. Frequency indicated 0.3% or lower distortion from 20 Hz to 20 kHz. The differences in the channels was probably due to differences in the bias voltage of the eight output tubes. When I measured them, there was a variation of about 20 mv. I did not adjust them to exactly 250 mv. Although the instruction manual says that anywhere between 230 mv and 270 mv is OK, but I would suggest trying to set them all right at 250 mv or as close as you can get them.
Output of the Manley Stingray showed a hard knee at 30 watts into 8 ohms, and clipping (1% THD+N) at 32 watts.
The measured frequency response was flat between 20 Hz and 20 kHz.