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(Hopefully) useful various sysadmin and other stuff.

ReVox A 78 MK II: Adjusting Input Balance

When the Balance fools you.

When the Bal­ance fools you.

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?

Tools needed

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’s 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  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  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. 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.

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