Recycling low/mid power Receivers and Integrated Amps

onplane

What! No Wake???
As many of you know, I've been an advocate of passive bi-amping for many years. To me the huge advantage is gaining full control over the speaker's "voice" ... far, far better control than those built-in controls we find on some speakers. In order to impact the speaker's voice, however, the user must be able to control the power sent to each speaker half. This means that each amp must have gain or volume controls.

In addition, I've argued this does NOT have to be an expensive proposition. Indeed, many of us have old low/mid power receivers and integrated amps (even some tube units) collecting dust, which could be used to power the mids/tweeters. (I have at least 4 in my basement. Not even sure where they came from! I know my Dad and sisters often gave me their old units upon upgrading. Now, my Dad always considered it a huge "win" whenever he could move "stuff" from his basement to mine!)

In any event, the argument against doing this is that these low/mid power units become the new limit on total power available to the speakers. That is, even though we have a solid state "brute" driving the power hungry woofers, the "little amp" becomes the limiting factor. The argument goes along the lines that these low/mid power units have lower rail voltages and the "system" is thus limited since a full range signal will hit those lower rails sooner.

This argument is true providing you send that "little amp" a full range signal. What if you turn the bass control on the mid/tweeter amp full OFF? Since mids and tweeters don't reproduce those frequencies, why bother sending them a full range signal? (Indeed, I can make an argument that eliminating those very low frequencies is going in the right direction from an IM distortion perspective.)

Anyhow, today I wanted to share some scope traces of a mid/tweeter amp with the bass control full OFF. There are two pics attached. In both pics, you'll see two traces. The trace centered two divisions up from the bottom is the trace for the woofer amp (you cannot see the entire trace as it "bottoms out"). The trace on top (and not centered on a division, because I wanted to see the entire signal) is the mid/tweeter amp with the bass full OFF. Just prior to taking the pics, I had both amps with the tone controls centered and gain controls so that the traces were identical (scope is set at 0.5 volts/division). At that point the only adjustment made was to turn the bass control full counter clockwise (bass full off). What you see is the huge difference in voltage between the high frequency amp and the woofer amp.

In the first pic, the only instruments are piano, drums and double bass. That huge "spike" is a double bass note and not a particularly loud note at that. In the 2nd pic all that is playing is the piano. Even here you can see a difference! While the piano is not producing a bass line (in this case that's left to the double bass), it is producing fundamental frequencies clearly in the frequency range of bass tone control. (As a side note ... after messing with tone controls over so many years, it's really strange moving the bass control from end to end, yet no difference whatsoever in sound! The mid/tweeter xover just blocks those frequencies from ever getting to the drivers. Nevertheless, it's a strange sensation.)

My point in all of this is you can gain significant headroom in the mid/tweeter amp by turning the bass control full OFF in some music. Indeed, this is the case in most music. It would not be true if you listened to music consisting of violins and flutes, however, or any music whether there exists no "bass" frequencies.

Nevertheless, as you can see from my traces the, differences can be quite significant, so that "new lower limit" might not be so low after all.

Regards,
Jerry

PS: Those, who have single trace scopes, can view the same results by simply adjusting the scope gain or the amp gain on a full range signal to hit the scope's display limit. Then replay the track with the bass full off and watch how the resulting signal never even approaches the scope's display limits.
 

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