A little progress, finally. Ordering a couple new toroids from Antek, then I need to pick a chassis...
Building an ultimate low power class A amplifier – my way
- Thread starter tombo56
- Start date
Build is basically done!
I first used the amp at 250mv across r17 as written here:
0.8 A 160 - 170 mV
1 A 250 - 270 mV
1.2 A 340 – 360 mV
Later I tried cranking up the bias current to see how it would sound, but one of the channels wont flow more than 254mv across r17, while the other will go above. Any ideas what could be causing this, or where I should look around to diagnose?
It's a wild sounding amp. I've never gotten so much detail out of music.
Very nice build. 
Max. Bias current depends on R10 and J113 Idss, which varies. Seems that you got one J113 that needs lower resistor. You could change R10 to 47R or add 100R in parallel.
Exactly that. And with adequate loudspeakers, sound stage is equally incredible.
Max. Bias current depends on R10 and J113 Idss, which varies. Seems that you got one J113 that needs lower resistor. You could change R10 to 47R or add 100R in parallel.
Thank you!
I ordered a couple 2215s.
Super excited to see more builds popping up! I've been waiting on this one for a while. I had to tear myself away from the bench this morning to actually go outside. The horrors!!!!
^ Changed R7 and R8 per the master's previous post for protecting that little critter.
Nice and simple amp.
I was curious how Microcap matches other simulation programs...
Also to see if it results in unrealistic low THD values, what it obviously does not.
...and how different amps compare to my own stuff in simulation.
I had to increase the 2.7k to get the desired 1A bias. Maybe some bad model.
What is the diode for?
THD: 1Wp -121dB, 10Wp -110dB
Different opamps did not change anything.
I was curious how Microcap matches other simulation programs...
Also to see if it results in unrealistic low THD values, what it obviously does not.
...and how different amps compare to my own stuff in simulation.
I had to increase the 2.7k to get the desired 1A bias. Maybe some bad model.
What is the diode for?
THD: 1Wp -121dB, 10Wp -110dB
Different opamps did not change anything.
Thanks. And it sounds very nice too.
Regarding simulation, LTSpice was calculating only 6 dB lower distortion than measured. Your numbers are almost the same as I got with LTSpice.
It is likely J113 model issue. R11 shouldn’t affect bias. Try with R11 = 2K7 and reduce R10/RV1.
Diode is used to account for 0.6 V higher Vgs of P MOSFET, compared to N part.
True, not much at 1 kHz distortion, but they would for slew rate and overall stability, especially with capacitive loads.
Ok, understand, however with diode I get 385mV offset on the opamp output, without diode it is only 23mV.
Would it be possible to use only the OUTPUT STAGE, Without Global NFB, driven by any choice frontend? I understand that the wide bandwidth can be a potential issue. Thanks.
Output stage alone was tested and listened to in the first development phase. It works very well and higher supply voltages can be used. Distortion is low and harmonics profile is monotonic. However, damping factor is in the low range (didn’t actually measure for the OS alone).
So, yes, output stage can be used with other frontends.
So, yes, output stage can be used with other frontends.
tombo56, I couldn't have expected a better reply. It is just what I need. I use FostexFE168Sigma speakers in ML-TQWT enclosures. They need amps with higher output impedances. And I love to listen to amps and preamps with zero NFB because of their natural sound and the way the sound envelope grows when the music hits crescendos. Thank you very much.
I forgot that these boards were not UMS compatible. DOH!
Waiting on a drill / tap set.
Waiting on a drill / tap set.
I scoured my local area for a brick and mortar store that had an M3 tap set on hand... so... Bezos' bookstore will deliver tomorrow.
PSU in the chassis... layout pretty much final and "inspired" by the origin build.
PSU retested in the chassis with the dim bulb / variable transformer and all other safety nets removed. All good.
Pretty lights...
Drill and tap 4 holes tomorrow or Sunday... and if all goes to plan... music!
For those that like pics...
Right now, the R21s have ~2V5 drop unloaded (22V0 prior set to 19V5 after). I'll adjust as necessary when I add an amp-sized load. Hopefully they'll drop into that sweet spot of ~1V5 drop.
PSU in the chassis... layout pretty much final and "inspired" by the origin build.
PSU retested in the chassis with the dim bulb / variable transformer and all other safety nets removed. All good.
Pretty lights...
Drill and tap 4 holes tomorrow or Sunday... and if all goes to plan... music!
For those that like pics...
Right now, the R21s have ~2V5 drop unloaded (22V0 prior set to 19V5 after). I'll adjust as necessary when I add an amp-sized load. Hopefully they'll drop into that sweet spot of ~1V5 drop.