Headphone Amp Measurements ... Almost There!
InnerFidelity's second anniversary is coming up April 1st, and I'm bound and determined to get the amp measurement program up and running by then. Two years is just too long; the time is now.
I was spending a lot of time listening for the differences in headphone cables (not easy to hear) and wanted to try to do something while I'm listening. Turns out, programming is one of those things that doesn't like to share time with anything else in your head, so I had to abandon the listening and just carry on programming the tester. The good news is that I've made great progress, and the basic unbalanced solid-state amp test is working.
My basic philosophy of the test routine was to run an amp through its paces, but not to try to really stress the amp during testing. For example, I had initially wanted to test amps with a 16 Ohm load, but found that too many amps weren't very happy with this low impedance load. I want the test routine to be as universal as possible so that it would be easy to compare one amp to another, so I opted to make the test a bit less stressful on the amps so it could be used as broadly as possible. In the end, after looking at a half dozen amps or so, I think the test as it stands does a good job of identifying differences between amps, and will identify problematic amps fairly readily.
The Test Procedure
The basic test runs like this:
- The amp is connected to the ins and outs of the tester with the 150 Ohm load on the amp output. The tester outputs 0 dBu and the headphone amp volume is set to unity gain. Gain setting on the amp will be low for two gain setting amps, and medium for three gain setting amps.
- Shorting plugs are inserted into amps inputs and the noise spectra is measured.
- Tester outputs are reinserted into the amp inputs, and frequency response, phase response, SIMPTE IMD spectra, CCIF IMD spectra, and cross talk are measured.
- Single point measurements are made with 150 Ohm load with 0 dBu output for output impedance and channel balance.
- Volume is turned all the way up, and measurements are made for gain and noise at all amp gain settings. Gain setting is returned to low for two gain setting amps, and medium for three gain setting amps. Volume is reset to unity gain.
- THD+noise Vs. Frequency is plotted.
- Volume is turned all the way up. (This is the tricky bit.) The tester is put into THD test mode and is commanded to regulate the input voltage until the output THD is 1%. Then that input voltage is put into the maximum sweep voltage setting. The tester then plots amp output voltage vs. THD+noise up to 1%. (This will change for tube amps as the output distortion is much higher and it will be informative to see the distortion vs output voltage to a higher level. I'll determine the actual THD% setting when I start measuring tube amps.)
- With volume still at full, input is regulated until 1%THD is reached, then output voltage is measured and output wattage is calculated for single point datapoints.
- Input is set to 0 dBu and volume is reset for unity gain. Single point measures for THD+noise @ 1kHz, SMPTE IMD, and CCIF IMD are recorded.
- The load is changed to 32 Ohm and steps 6 through 9 are repeated. Load is again changed to 600 Ohms and steps 6 through 9 are repeated.
Here's the current look of the datasheet.
The current task is to run a bunch of unbalanced solid-state amps through the ringer to see if everything runs smoothly and to tweak any hick-ups. Currently I've measured the: Meyer Audio Corda Rock; Lake People G109; Apex HiFi Butte; Burson HA160D; HeadRoom Desktop, and HeadRoom Max. A .pdf of all the measurements can be downloaded here. Once it looks like everything is working, I'll start a Headphone Amp Datasheet download page and will post links to amp datasheets there, and will start commenting on amps measured in monthly updates similar to the way I do with headphones.
Once it seems everything is working well with the unbalanced solid-state test, I'll start the process of converting the current test to tests appropriate for balanced and tube headphone amps.
Once that's done, I'll start working on using the digital outputs of the Audio Precision tester so that I can measure headphone amps with DACS that have optical or coax digital inputs. USB amps will be the last to get tests as the AP does't have USB output capability. I can however create digital test tones that can be stored on the computer and called by the tester, but it's a more complex process with a bit of a learning curve so that will take a while yet.
Phew! I'm I glad to have made it to this point, it's taken far too long.