Single supply, three-pole compensated class-B retro amp

LKA

Member
2014-12-28 5:31 pm
I never saw 3P compensated amplifier so I tried to build one. The basic design is come from the local forum's guru, Danhard. I made fine-tuning to adapt it to the real conditions. Three boards were built, the biggest problem was the correct placement of components and routing so as not to cause distortion degradation. The measured 1kHz/4R thd is under 0.0001%, the 10kHz/4R is about 0.0005%.



As you can seen the loopgain slope falls 60db/decade View attachment 1044059
H-ENG-60V-sch.JPG
 

Attachments

  • H-ENGv3-loopgain1.png
    H-ENGv3-loopgain1.png
    21.4 KB · Views: 292
  • H-ENGv3-loopgain2.png
    H-ENGv3-loopgain2.png
    21.2 KB · Views: 293
Last edited:
  • Like
Reactions: 4 users

LKA

Member
2014-12-28 5:31 pm
9 transistors, no CM/CCS/CASCODE, only topology elements known from 60's
omitted emitter resistors at the output, idle 3mA, overall 33mA
world champion ;)
 

Attachments

  • henh-v60-sch.png
    henh-v60-sch.png
    35.9 KB · Views: 415
Last edited:
  • Like
  • Thank You
Reactions: 6 users

BSST

Member
2018-07-07 2:21 am
Thanks, LKA!

I recall building something with a similar single-transistor input stage, from Popular Electronics, circa 1970. I believe that design came from an RCA app note. I've no idea re its distortion performance. This amp is way more impressive.

I see the 3rd order compensation network. Do you have a copy of the original schematic, or a Link? I'm curious about other compensation subtleties as well.

Again, impressive work!

Many thanks,

Steve
 

LKA

Member
2014-12-28 5:31 pm
  • Like
Reactions: 1 user

dadod

Member
2006-04-18 3:11 pm
Zagreb
I never saw 3P compensated amplifier so I tried to build one. The basic design is come from the local forum's guru, Danhard. I made fine-tuning to adapt it to the real conditions. Three boards were built, the biggest problem was the correct placement of components and routing so as not to cause distortion degradation. The measured 1kHz/4R thd is under 0.0001%, the 10kHz/4R is about 0.0005%.



As you can seen the loopgain slope falls 60db/decade
I see you are using simple voltage probe to simulate Loop Gain, better to use Tian probe it will give you more accurate result.
If you prefer simple voltage probe try to plot -V(out)/v(b), it simpler to read the phase margin.
 

LKA

Member
2014-12-28 5:31 pm
The presented loopgain captures the EF output at a quiescent current of 80 mA (linear region). BTW, the circuit can operate in AB mode with emitter resistors 0.1R-0.15R. (the main goal was not to use emitter resistors). This is the second version of the PCB, assemled with those resistors. (for testing and proving the AB mode)
 

Attachments

  • H-ENGv2-ClassAB.jpg
    H-ENGv2-ClassAB.jpg
    493.2 KB · Views: 130
  • Like
Reactions: 1 user