I have an LM1875 based kit amp I built in 2014. The company hasn't existed since 2015 or 16.
It's got a Toroid-based power supply with a 12V output. I used one channel of this amp for years in a mono setup... The other was unused.
Now I'd like to modify it for a different project to still be a mono (small sub) amp, but double power. (into 8 ohm speaker) I have a diagram that supposedly shows what I'd need to do....
Since I can't find any verification of this...is this really how bridging 2 LM1875s should work? Just add the 2 resistors at the appropriate locations on the board and change the output setup? (See pic.)
Also, can I run an accessory off of 1 side of the power supply? I'm looking at buying a subwoofer preamp/low pass board that needs single sided DC supply...is there any risk of harm or problem if I run this off of one side of the DC supply for the LM1875 boards? Or do I need to find a separate DC source for another <100ma (guessing) load?
Thanks for any help...I don't fully follow how exactly this stuff works...so just want to verify with someone who knows more than I do!
-Nate
It's got a Toroid-based power supply with a 12V output. I used one channel of this amp for years in a mono setup... The other was unused.
Now I'd like to modify it for a different project to still be a mono (small sub) amp, but double power. (into 8 ohm speaker) I have a diagram that supposedly shows what I'd need to do....
Since I can't find any verification of this...is this really how bridging 2 LM1875s should work? Just add the 2 resistors at the appropriate locations on the board and change the output setup? (See pic.)
Also, can I run an accessory off of 1 side of the power supply? I'm looking at buying a subwoofer preamp/low pass board that needs single sided DC supply...is there any risk of harm or problem if I run this off of one side of the DC supply for the LM1875 boards? Or do I need to find a separate DC source for another <100ma (guessing) load?
Thanks for any help...I don't fully follow how exactly this stuff works...so just want to verify with someone who knows more than I do!
-Nate
It would be much easier seeing it as a circuit diagram rather than a layout but it looks and sounds reasonable. One amp has to invert the signal and the speaker is connected between the two 'hot' outputs. The 100 ohm effectively shorts the second amps non inverting input (the normal audio input) to ground, the 22k feeds the signal of one amp into the inverting input of the other with unity voltage gain.
Running a low current preamp should be fine as long as the voltages are OK. Remember a 12 volt AC winding will give 17 or more volts DC at light loading and the rail is not particularly well smoothed and is not regulated in any way.
Running a low current preamp should be fine as long as the voltages are OK. Remember a 12 volt AC winding will give 17 or more volts DC at light loading and the rail is not particularly well smoothed and is not regulated in any way.
Thank you.
Makes sense. Looking at a board that's "10-24 VDC" so should be fine, voltage wise. Would be nice to cram it in the same enclosure as the amp, so this helps!
I found this which might explain it better to you. You can see the 22k marked RG is feeding the output of the first amp into the inverting input of the second. This 22k and the existing 22k feedback resistor give a gain of -1 (minus denotes inverting). The 680 ohm feedback return resistor is left in place and this increases the 'noise gain' of the configuration and ensures stability because the LM1875 like most power chips is not unity gain stable. The 680 ohm fools the chip into thinking it is working at higher gain.
The normal audio input input on pin 1 of the second amp has been terminated to ground, in this case with a 22k.
So this looks pretty much like what you have.
The normal audio input input on pin 1 of the second amp has been terminated to ground, in this case with a 22k.
So this looks pretty much like what you have.
True, 12V AC output from the transformer, and ~16VDC from the rectified supply.