So I have a Keithley 2015 THD analyzer, which I am trying to use to set the bias on a Yamaha B2.
The Keithley, with internal sine generator connected straight from output to input, gives about 0.01% THD. Even the manual suggests this number will be about 0.02%. Is it me or does this suck? For some frequencies and magnitudes, the Keithley actually does approach 0.02% THD when simply reading itself.
Now, the Yamaha AMP is supposed to achieve 0.005% at most sane volume levels. So I hook up the Keithley output through my Adcom GFP-750 (set in passive mode, of course, so I can adjust volume and hit 30W continuous output according to the Yamaha Service Manual), to the Yamaha, and back to the Keithley, and get about 0.013% THD. I can send it 1Khz, 10Khz, 20Khz, anywhere from 0.5V to 2.0V. Problem is, the Yamaha never seems to output THD number that is any different from what the Keithley reads when looped back on itself. In other words, the Yamaha is adding no significant measurable distortion, according to the Keithley. I can sweep the bias from 40mV to 100mV, and nothing really changes. It makes it hard to set the bias for lowest distortion when the Keithley doesn't seem to be able to measure the change.
Am I doing something wrong here, is the Yamaha amp really that good, or is the Keithley simply that crappy?
For comparison purposes, if I flip the GFP-750 into active mode, THD jumps to about 0.6% to 0.9% - huge distortion from the Adcom, but perhaps that's a problem for another day.
The Keithley, with internal sine generator connected straight from output to input, gives about 0.01% THD. Even the manual suggests this number will be about 0.02%. Is it me or does this suck? For some frequencies and magnitudes, the Keithley actually does approach 0.02% THD when simply reading itself.
Now, the Yamaha AMP is supposed to achieve 0.005% at most sane volume levels. So I hook up the Keithley output through my Adcom GFP-750 (set in passive mode, of course, so I can adjust volume and hit 30W continuous output according to the Yamaha Service Manual), to the Yamaha, and back to the Keithley, and get about 0.013% THD. I can send it 1Khz, 10Khz, 20Khz, anywhere from 0.5V to 2.0V. Problem is, the Yamaha never seems to output THD number that is any different from what the Keithley reads when looped back on itself. In other words, the Yamaha is adding no significant measurable distortion, according to the Keithley. I can sweep the bias from 40mV to 100mV, and nothing really changes. It makes it hard to set the bias for lowest distortion when the Keithley doesn't seem to be able to measure the change.
Am I doing something wrong here, is the Yamaha amp really that good, or is the Keithley simply that crappy?
For comparison purposes, if I flip the GFP-750 into active mode, THD jumps to about 0.6% to 0.9% - huge distortion from the Adcom, but perhaps that's a problem for another day.
The 2015 source is compromised as a 12 bit DAC. It was adequate for cell phones 10 years ago. The ADC and analyzer are good to close to .001% in my experience.
You can use a good DAC as a source which should be better.
You will not get lower distortion with a DUT in the system than from a loopback.
You can use a good DAC as a source which should be better.
You will not get lower distortion with a DUT in the system than from a loopback.
Thanks for the responses.
Yes, my family is Belgian.
I ended up using my Schiit Modi DAC, which ouputs sine waves that measure 0.001% THD by the Keithley. Funny that a $99 device can absolutely smoke the generator in the big dollar Keithley.
So, this Yamaha actually does measure 0.005% at 20Khz, 30W continuous output, just as it is supposed to.
Yes, my family is Belgian.
I ended up using my Schiit Modi DAC, which ouputs sine waves that measure 0.001% THD by the Keithley. Funny that a $99 device can absolutely smoke the generator in the big dollar Keithley.
So, this Yamaha actually does measure 0.005% at 20Khz, 30W continuous output, just as it is supposed to.
I looked into a DAC upgrade but its not a real option. I started on some software to automate a suite of measurements with it however the DAC is a limitation. Someone could link a DAC as a source and the 2015 as an analyzer. The 2015 is supposed to be able to show individual harmonics but I could not figure out how to extract them. Here is a link to source code etc: PC Software control for Analog Audio Analyzers Its all in Microsoft visual Studio.
The cell phone market has gone way upscale. The converters in some of the higher end phones push the limits of what an AP 2722 can measure. The Keithley 2015 was relatively inexpensive, fast and easy to integrate into test rigs. Interestingly enough, the former GM of Keithley is the new CEO of AP.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.