ReVox A 78 MK II: Adjusting Input Balance
I did recently mount my main loudspeakers to the walls. However, after some listening tests I got the feeling that something was amiss; for some reason, i felt that the stereo center wasn’t really centered. The balance turning knob was set correctly though, so that couldn’t be the issue.
Thankfully, the A 78 has gain controls for each input channel. But how can one measure the error, and adjust those gains accordingly?
You’ll need two tools. The first one, obviously, is for measuring: a VU-Meter. For me, this role was filled by the lovely A 77, which has two of them integrated, one for each channel. If you don’t have one of those (rather slim chance, since if you got here and most likely have the amp, and thus, the complete “A” series devices), there are a multitude of available on both the internet and electronics retailers. You don’t need a expensive one; but if you get a digital one with LED’s, choose one with more than just five indication levels. After all, you’ll want accuracy that’sÂ at least of some use.
Now, the second tool, or rather, thing, is a test signal, to be more exactly a sine wave. There are dedicated professional devices for this, but a recording Â suffices perfectly (there are hundreds of places to download these from). In my case, I installed some random test sound generator App on my iPhone (there are many of these available for free). This test signal will cause the VU-meter to display a constant level.
Calibrate the VU-Meter (somewhat)
Having aquired these tools, the first step is to adjust the input gain for each channel of the VU-Meter, in my case the AÂ 77. (If your VU-Meter has only one channel or the channels have a fixed gain, you can skip this step).
I connected the iPhone to the machine and presented a sine wave of 1kHz to it. After that, it’s just setting the gain for each channel accordingly. Since the response isn’t quite linear accross the frequency spectrum, I chose some more frequencies, above and below 1kHz, readjusting if the disparity was too big between the two channels.
Of course, one might argue that both channels have their very unique response and that adjusting at one particular frequency results in a wrong setting at some other frequency. While this is technically true, it’s negligible, as this difference is kept in a particular tolerance range by the manufacturer (barring defects).
So, the previous procedure will have the effect that disparities between the response curves of the two channels, which are not detecable visually at some frequencies (which energize the VU-Meter less) can be seen with different frequencies and corrected accordingly.
The actual adjustments
After that, I connected my test signal source the the Amp, and the A 77 to the A78’s “Tape Monitor” Output. And guess what: I was right, the input gain of the right channel was WAY off, several dB’s.
Checking the output at several frequencies as above, I adjusted both channels to the same level. The only problem were the aged potentiometers; at some settings, they lost contact completely due to oxidization.Â Since I was too lazy and moreover not prepared to disassemble the amp and replace them, I resorted to just turning them up and down several times, hopefully scraping away the oxid. It worked – but I’ll still have to replace them at some point in the future Which is a tale for another time.