I thought I'd give it a shot with my DIY tube amp. I'm wondering if anyone has tried anything similar, and if there are any tips / tricks / gotchas to doing this.
I decided to use REW, because it's free, and the current version has a 'spectrum' tab that can measure THD+N. Set up the generator for a 1kHz sine wave, disconnected the amp from the speaker and hooked it up to a 7.5ohm resistor, ran alligator clips from the resistor leads to the input RCA cable.
The first thing I discovered is that the signal generator in REW can be a little squirrely. Since the speaker wasn't connected, I didn't have an easy way of telling if the signal was on, or what frequency / level it was playing it. I have a cheap DMM and I was using that, set to VAC, to check for signal. As it turns out, if I type in a frequency into the generator window, it apparently doesn't register until I tab out of that field. And sometimes the start/stop button would stop doing anything. A few times I had to restart REW to get the signal to stop or start. Also, at these low signal levels, the DMM leads pick up noise from the air, I think, so sometimes the signal reading would jump higher if the DMM lead slipped off the resistor. It probably didn't help that I had a bunch of unshielded alligator clip leads all over the place.
Then, my first measurement had an OK THD (0.4% or 0.04%, don't remember, almost all H2), but the THD+N was up at around 5%. The spectrum showed 60Hz and its harmonics. It took me a little while to figure this one out. The amp has directly heated triodes on the output, which have AC on the heaters. This adds 60Hz hum to the output, the DMM says the hum level is 5mV. The last time I measured it with a scope (years ago), I think I got similar numbers. Since I've always used this amp to drive a midrange and tweeter, I never bothered trying to reduce that hum. Anyway, with the laptop volume controls set to about half-way, and the REW generator at the default -20dB, I was sending about 10mV to the amp. If I trust the DMM, the amp's gain is around 3 (it's a 4W SET), and the output was around 30-40mV. So the level was clearly too low.
Then I decided to crank up the REW signal generator level. It goes up to -3dB, which sent about 300mV to the amp, and I had about 900mV output. The REW distortion chart showed about 3% THD before it said 'clip' and stopped updating. So, was it my amp distorting, or the soundcard input? I don't know.
So then I decided to try a voltage divider to reduce the input level to the soundcard. I added a 0.27 ohm resistor in series with the 7.5, and took the signal off that to send to the soundcard. This caused the output signal level to drop by 90% as soon as I connected the ground lead to the soundcard. Sometimes in my test gear also started 'singing', because I could hear the 1kHz tone near the laptop. This really puzzled me for a while, but I think I finally figured it out. I'd put the 0.27 ohm resistor on the 'hot' side of the output transformer, and connecting that to the soundcard was grounding it... and either shorting something out, or I don't know. Once I switched the 0.27 ohm resistor to the 'bottom', and made sure the soundcard ground lead connected to the output transformer ground lead, the singing stopped, and the voltage drop went away. I hope I didn't sink a lot of current into the soundcard and kill it.
Anyway, by this point I think I was finally set up to take some measurements, and my laptop rebooted. It was late, so I decided to listen to some music before going to bed. I don't think I hurt the amp, because it played music fine. I'll pick this up again tonight and see if I can get some numbers.
Can REW do the distortion/frequency charts that I see people posting from SoundEasy? Speaker Workshop says it can do that, but I've never been able to make it work. Assuming it can't, what signal should I be using for this measurement? 1kHz seems like a marketing standard for reporting THD. I'll probably also measure it with a 500Hz signal, since that's the XO frequency to the woofer so that the nominal 'lowest' frequency the amp is playing. I measured FR for the amp a couple of days back using SW, and it's flatter than I expected (I built the amp years ago and haven't messed with it in a long time). Channel balance is OK (one side is 0.4dB louder than the other). I should probably also try to measure cross-talk.
I decided to use REW, because it's free, and the current version has a 'spectrum' tab that can measure THD+N. Set up the generator for a 1kHz sine wave, disconnected the amp from the speaker and hooked it up to a 7.5ohm resistor, ran alligator clips from the resistor leads to the input RCA cable.
The first thing I discovered is that the signal generator in REW can be a little squirrely. Since the speaker wasn't connected, I didn't have an easy way of telling if the signal was on, or what frequency / level it was playing it. I have a cheap DMM and I was using that, set to VAC, to check for signal. As it turns out, if I type in a frequency into the generator window, it apparently doesn't register until I tab out of that field. And sometimes the start/stop button would stop doing anything. A few times I had to restart REW to get the signal to stop or start. Also, at these low signal levels, the DMM leads pick up noise from the air, I think, so sometimes the signal reading would jump higher if the DMM lead slipped off the resistor. It probably didn't help that I had a bunch of unshielded alligator clip leads all over the place.
Then, my first measurement had an OK THD (0.4% or 0.04%, don't remember, almost all H2), but the THD+N was up at around 5%. The spectrum showed 60Hz and its harmonics. It took me a little while to figure this one out. The amp has directly heated triodes on the output, which have AC on the heaters. This adds 60Hz hum to the output, the DMM says the hum level is 5mV. The last time I measured it with a scope (years ago), I think I got similar numbers. Since I've always used this amp to drive a midrange and tweeter, I never bothered trying to reduce that hum. Anyway, with the laptop volume controls set to about half-way, and the REW generator at the default -20dB, I was sending about 10mV to the amp. If I trust the DMM, the amp's gain is around 3 (it's a 4W SET), and the output was around 30-40mV. So the level was clearly too low.
Then I decided to crank up the REW signal generator level. It goes up to -3dB, which sent about 300mV to the amp, and I had about 900mV output. The REW distortion chart showed about 3% THD before it said 'clip' and stopped updating. So, was it my amp distorting, or the soundcard input? I don't know.
So then I decided to try a voltage divider to reduce the input level to the soundcard. I added a 0.27 ohm resistor in series with the 7.5, and took the signal off that to send to the soundcard. This caused the output signal level to drop by 90% as soon as I connected the ground lead to the soundcard. Sometimes in my test gear also started 'singing', because I could hear the 1kHz tone near the laptop. This really puzzled me for a while, but I think I finally figured it out. I'd put the 0.27 ohm resistor on the 'hot' side of the output transformer, and connecting that to the soundcard was grounding it... and either shorting something out, or I don't know. Once I switched the 0.27 ohm resistor to the 'bottom', and made sure the soundcard ground lead connected to the output transformer ground lead, the singing stopped, and the voltage drop went away. I hope I didn't sink a lot of current into the soundcard and kill it.
Anyway, by this point I think I was finally set up to take some measurements, and my laptop rebooted. It was late, so I decided to listen to some music before going to bed. I don't think I hurt the amp, because it played music fine. I'll pick this up again tonight and see if I can get some numbers.
Can REW do the distortion/frequency charts that I see people posting from SoundEasy? Speaker Workshop says it can do that, but I've never been able to make it work. Assuming it can't, what signal should I be using for this measurement? 1kHz seems like a marketing standard for reporting THD. I'll probably also measure it with a 500Hz signal, since that's the XO frequency to the woofer so that the nominal 'lowest' frequency the amp is playing. I measured FR for the amp a couple of days back using SW, and it's flatter than I expected (I built the amp years ago and haven't messed with it in a long time). Channel balance is OK (one side is 0.4dB louder than the other). I should probably also try to measure cross-talk.
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