I am using 4 channel 4xLM4780 GCs in bridged mode to drive 5-6ohm speaker drivers directly after active crossover (freedsp). DC offset is around 100mV for all amps and it sounds pretty good. PS rails are at +-34v. However, they do run a bit too warm even at low volume levels, around 45deg Celsius. The exact same amps are running less warm with classic xo.
Is this normal?
Sent from my MI 5s using Tapatalk
Is this normal?
Sent from my MI 5s using Tapatalk
Last edited:
It's parallel configuration actually. Instead of 680r and 270r resistors in audiosector schematics, I have 1k. That's probably why I have a bit higher offset. I found a post from Peter Daniel saying this is normal value for this kit. Not sure this is related to high dissipation, as the same configuration runs fine with passive xo. I will test with no load at the output.
Sent from my MI 5s using Tapatalk
Sent from my MI 5s using Tapatalk
In bridged mode the amplifier's PdMax is 4x conventional mode. What's the thermal impedance of the heat sink?
100mV is way too much offset as Andrew mentions -- current flows out of the input pins of the LM4780 (up to 1uA) so the IR product * gain shows up on the output. This is why TI (formerly National) showed a servo on their bridged designsheet.
Wait hold on, this in parallel configuration?
http://www.audiosector.com/lm4780 amp.pdf
0.2 ohms between amps... a 20mV difference between the 2 amplifiers will already result in 0.1A of current. Put things in perspective, the quiescent current of LM4780 is also 0.1A.
http://www.audiosector.com/lm4780 amp.pdf
0.2 ohms between amps... a 20mV difference between the 2 amplifiers will already result in 0.1A of current. Put things in perspective, the quiescent current of LM4780 is also 0.1A.
That's the one, yes. I think I might have an oscillation outside audible spectrum.
Sent from my MI 5s using Tapatalk
I should have written that, in bridge mode, the chip's dissipation is 4x conventional mode.
Here's a screen shot of NatSemi's Excel spreadsheet. If you want a copy PM me:
Attachments
I had a cap of 1uf at the input which I have shorted now. Freedsp's 50k output resistance is now parallel to 22k input of the gc, so now I have 80mV offset. I also tested my old gc kit from Peter Daniel which had too around 100mV offset with lower input series resistors, so I guess this is normal.
Sent from my MI 5s using Tapatalk
Sent from my MI 5s using Tapatalk
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.