Any progress with this? Over at Parts-Express Tech Talk, a user has the Lepai 168 and the adjustable crossover doesn't seem to do anything (as others have noticed on this unit).
We're going to try reverse engineering it to see what's up. Any progress you've already made would be welcome.
I'm working from Photos of the Lepai 168 for a member on the TechTalk forum.
So, far I've been able to discern that the xover is a 1st order passive filter buffered by op-amp (U2-1) before going on to the power chip. The xover pot varies the resistance in the RC filter (in series with a fixed resistance to provide the xo "range". The problem is the cap is SMT w/o markings. So I have no idea its value. But the consensus is it "it does nothing". I take that as the installed smt cap is to small and the filter is rolling off much higher frequencies that the sub's usable frequency range.
The L-R signals go through the master volume and on to the sub channel summer and then to the Sub volume control pot. From there they go thru an op-amp (U2-2) circuit and then on to the passive xo, above. This op-amp is harder to reverse engineer as the traces are covered SMT component. Still working on that.
Note your markings are likely different. They changed the silkscreen between board versions. I looking at a Ver 5.0 board.
Here's a human interest story for ya' ... another poster here was looking for a 2.1 amp with full range on the ".1" channel. We settled on a particular TPA3116 amp I already knew and pulled the caps in the subs LP filter to get full range.
He could have just as easily bought a Lepai 168.
Here's a copy of my post on Tech Talk on the Lepai 168 ...
"Here's the deal with the Lepai; The L-R inputs go through the master volume pot then to L-R summer resistors to get the combined sub signal. From there it goes thru the sub volume control and then to the first op-amp circuit. This circuit is a fixed, 1st order LP filter. From there it goes to the XOVER pot. This pot , in series with a fixed resistor and a capacitor forms a passive 1st order LP filter. After that it goes through an op-amp buffer and then the signal goes to the power chip. Varying the XOVER pot varies the resistance in an RC network and, hence, its xo point.These cascaded 1st order filters form a 2nd order LP filter, only one of which is variable.
The Good, the Bad and the Ugly ... I've penned the circuit and should have a schematic shortly matching the silkscreen component IDs (and there might be a spot to add some gain to the sub channel, with a resistor swap). I can't read all of the values of the SMT resistors from the photo. There are no markings on the smt caps.
Without the component values, there is no way to calculate the LP filters' XO point. The consensus here, and over at diyaudio, is that the pot does nothing and the sub seems to get full range. This implies the smt caps are the wrong values (to small) and the XO point is way higher than the sub can handle. Testing some replacement cap values could confirm."
The gent I doing this for has limited electronic skills. I don't own the Lepai to experiment so we're kind of stuck at this point.
If you can find the time to try some capacitor swaps I can pinpoint some values to try.
"Here's the deal with the Lepai; The L-R inputs go through the master volume pot then to L-R summer resistors to get the combined sub signal. From there it goes thru the sub volume control and then to the first op-amp circuit. This circuit is a fixed, 1st order LP filter. From there it goes to the XOVER pot. This pot , in series with a fixed resistor and a capacitor forms a passive 1st order LP filter. After that it goes through an op-amp buffer and then the signal goes to the power chip. Varying the XOVER pot varies the resistance in an RC network and, hence, its xo point.These cascaded 1st order filters form a 2nd order LP filter, only one of which is variable.
The Good, the Bad and the Ugly ... I've penned the circuit and should have a schematic shortly matching the silkscreen component IDs (and there might be a spot to add some gain to the sub channel, with a resistor swap). I can't read all of the values of the SMT resistors from the photo. There are no markings on the smt caps.
Without the component values, there is no way to calculate the LP filters' XO point. The consensus here, and over at diyaudio, is that the pot does nothing and the sub seems to get full range. This implies the smt caps are the wrong values (to small) and the XO point is way higher than the sub can handle. Testing some replacement cap values could confirm."
The gent I doing this for has limited electronic skills. I don't own the Lepai to experiment so we're kind of stuck at this point.
If you can find the time to try some capacitor swaps I can pinpoint some values to try.
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