opamp input very low distortion audio power amplifier
Fig.1 : Kunthrandum schematic diagram (TOO OLD and wrong, put here for the aesthetics only)
NOTICE: Though crawling due to lack of resources, this project now having some major developments.. including the PCB design.. I'll post it here as soon as it gets finished.. working stable, not bursting or smoking, safe for the public! :) DO NOT FOLLOW THE ABOVE GIVEN DIAGRAM... ITS OUTDATED. please come back later...
Beware that.. once you came to hear and know what is good low distortion audio to some dB depth, there is no going back. From thereafter, everything else will be sounding noticably bad to your new wiser ears. All those cheap mp3 players, music mobile phones, DVD players, televisions, 5.1's,... etc, most of them, most probably all of them..! will be distorting audio so much that you might want to avoid listening to. Decide whether to be a little mad about audio fidelity.
Kunthrandum is an extremely low distortion audio power amplifier with opamp input stage. According to the LTspice simulation results, this circuit produces 0.00001% Total Harmonic Distortion at full power output with the fantastic opamp LT1810 from Linear Technology. Though that is under computer simulated ideal conditions, it should operate well somewhere near that in the real world. It is a simple straight forward design with two opamps in the input stage to solve the non-linearities in the class-AB MOSFET power output stage by negetive voltage feedback. The quality of the output is highly dependent on the opamps used, the better the opamp the better the waves at the output will be. There are many good opamps available arround to taste with this circuit.. try Linear Technology LT1810, LT1469, LT1122, Analog Devices AD8599, AD8692, AD823, Burr-Brown OPA2134, National Semiconductors LME49720, etc.
When you are turning it on for the first time... do it like this:
Set the 'quiescent current adjust' preset to the middle (not zero).
Connect voltmeter across one of the MOSFET source resistors.
Connect input to ground and output to a dummy load of 8ohm(1/4 watt) resistor.
Turn on the power supply. (always use fueses..)
Adjust the 'quiescent current' voltage so that meter reading is between 30mV-100mV. (ie. 30mV / 0.5ohm = 60mA)
Wait 5-10min for the amp to heat up and check whether the current is stable. If it is increasing rapidly, lower the preset.
Set the current somewhere stable above 100mA (depends on your heatsink size).
If the dummy survives with no damage, the amp is ready to test with a speaker.
Keep the meter connected and play some music... and watch the readings as long as u can.
Quiescent currents between 100mA and 250mA is better, a big heatsink and a strong power supply is highly recommended. Go for higher quiscent currents if you can provide forced air cooling. When the output current is below quiscent current the amp works in class-A region, and produces extremely low distortions, reaching near the op-amp specifications. I've been able to set the quiescent current as high as 800mA with a cooling fan attached to the heat sink.
In an amp with +/-50v power supply, when the output is at 10V :
the voltage across the MOSFET is 40V.
current through the circuit = 10/8 = 1.25A
power to the load = 10 x 1.25 = 12.5W
power dissipated at MOSFET in the form of heat = 40 x 1.25 = 50W ...!
at 25V it reaches the maximum, 78Watts... heat.
This is from the DC point of view. ................
I'm utter much sure that I won't be the one to enjoy the real quality of this amp.. when there is someone else to try building this amp.. who have reach to much better components than I use here.. most importantly the one via which it all comes out... good speakers... :)
1. Use fuses wherever they are needed..!
2. Try high quality opamps like Linear Technology LT1810, LT1469, AD8599, AD823 or OPA2134.
3. Minimize lead and track lengths of decoupling caps to reduce noise.
4. Use polyester caps in parallel to electrolytic coupling caps.
5. Use thick wire for power and speaker lines.
6. Do not operate this amp near seismographic monitoring stations.
7. Keep coming back to this site to check for updates or corrections.
8. Feel free to e-mail me.
Thanks to Linear Technology LTspice... which with kunthrandum is developed.
Copyright 2011 ronybc's advanced audio amplifier design laboratory located near Indian ocean (RAAADeLLIO)