Well well well...lm3875 how the mighty have fallen!!!
Here we have a tubeman needing the warmth from the music (but mostly the filaments) and he mentions a (dare i say it) intergrated circuit........ and worse, he has one in his posession...boo.. hiss.. (I can hear the crowd getting fidgety).
Well if you must know, i developed a circuit inspired by a sony motional feedback amplifier, and a phillips motional feedback speaker. This cpould take a while to explain, so grab a coffee and settle back.
The lm3875 as you know is an operational amp. If we use a second op amp (393) as a presampler, we can get a sample of what the speaker did with its signal, against what it should have done with its signal. Any diference between the input sig, and the measured speaker cone accelerations is by definition distortion.
Knowing this we can sample the speakers realtime performance by one of two methods. We can put an accellerometer to measure what the cone is doing (phillips does this with a fet amplifier and microphone built into the center of the woofer cone) or we can measure the speakers back emf, which will reflect any difference between what we think it should have done to what it actually did.
We can then feed a compensating voltage difference signal back into the op-amp and force it to comply with what we have sent as signal.
Gosh why would we do this? As you can see if you have control over the cones performance, then you can compensate for the speaker box inadequacies, and room position of the speaker ie on floor, against wall etc. Each of these things influence how the speaker transduces the signal into the air.
In my case I designed a three channel treb,mid,bass amplifier, with motional feedback compensation, to make each speaker channel. So the tweeter has it's own amp with feedback as does the squawker and bass speaker. It allows accurate tracking of the signal to the speaker. Needless to say, you have to break up the input signal into three pieces before the three amps. This gives you an added level of control of how you want the signal to sound. You also get to change the amount of compensation if you wish to see the difference between straight thru and motional feedbak.
Makes a neat system at around 80 watts per channel.. If you start to go into non-linear zone,(distortion) the amp automatically starts to compensate by compressing the signal by default. Because of this, if you do choose to blast someone out of the room, you can pull the average full power up considerably without distortion, until it beams at you.. very impressive, but hard on the eardrums as it behaves like a very very much larger amplifier (coz the compression).
So yes, am very familiar with lm1875 I find it gets very hot with 32v +- on the power inputs. Thats my only complaint. If you are going to use it as a party machine, it needs pentium fins and fan, but what a machine.
And best of all, you can drive it with the 12ax7 front end....and yes you can get the grids rattling for a bit of ambiance and body in the music.
Well I'm worn out after that
............oztules
