Another simple BJT quasi-complimentary Amplifier

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Inspired by a number of excellent threads on quasi-complimentary amps in this forum, I just finished one.

The target is to build 1 with output power rated at about 50W/8Ohm, in the meantime use as much as possible components that I have in my inventory.

The circuit is essentially based on P12 of Rod Elliott's site, with the biggest difference being the output stage is npn-based using MJ21194 (as I only have the npn, not the pnp parts). Also I used a very old Toshiba 2SD756 in the front stage.

The sound is rather different from what I am using currently (Nelson Pass version of JLH). Focus is quite sharp and very clean sound. A small surprise with the minimal dollar spent.
 

Attachments

  • Main_Amp_big.jpg
    Main_Amp_big.jpg
    112.8 KB · Views: 684
This circuit has an inherent difficult to maintain DC null at the output: drift in the first stage and the reference zener will conduct to a speaker blown. I would prefer the classic differential pair.

You are correct and the DC offset is not as stable as I wish. After 2 hours of listening it drift to the positive side by about 50mV. Not big enough to blow the loudspeaker but it would be nice to tame it a bit. Currently a loudspeaker protector is installed but that is of course not a proactive track.
 
Feeling a bit concerned about the DC offset, I looked into improvement. In the solid state forum I found a number of solutions. It seems that a Vbe multiplier might solve the problem. I tried this out and was very glad about the result. After warm-up, the DC offset only drift within a range of a couple of mVs. Much more robust compared to the zener solution.
 

Attachments

  • Main_Amp_v2.jpeg
    Main_Amp_v2.jpeg
    111.4 KB · Views: 524
It helps bias stability.
An increase of Ibias, increases the voltage drop across the 2 x 0.33 ohm. This voltage increase is "seen" by the base spreader ( here a Vbe multiplier ) that will counter act the Ibias increase.

The bias stability is very good indeed. I thermally coupled the Vbe multiplier transistor with the BD139 driver transistor on the +ve side of the output stage. The drift is within 1-2% only.
 
The bias stability is very good indeed. I thermally coupled the Vbe multiplier transistor with the BD139 driver transistor on the +ve side of the output stage. The drift is within 1-2% only.
Quasi complementary has inherent good thermal stability thanks to 3Vbe base spread.
( Versus 2 with CFP, 4 with 2EF, 6 with 3EF ).
With the 2 x 0.33 ohm that give immediat biais correction, and the thermal coupling, this design is very safe from thermal runaway.
 
Just start to play around with the ARTA software. Not sure if I made the measurement in a correct way but a THD and THD+N check is interesting. The signal is 1kHz with output just above 1W into 8 Ohm load. The distortion figure is rather decent and noise might be a bit high. Second harmonics is definitely low and so that explain the different sound characteristics I experienced from this amp. In the past most of the amps I built was based on Nelson Pass's designs with a richer 2nd harmonics content.
 

Attachments

  • 20190516_02.png
    20190516_02.png
    25.4 KB · Views: 252
Osvaldo,
You are correct. However, there are provisos.
Straying offset is irritating but has little sonic effect, and no damage to the speaker.
You can improve it with an opamp servo.
But a conventional LTP does have disadvantages; poor transfer factor at high amplitude, viz bass, pretty much same levels of H2 and H3, and not wonderful resolution.
I've built both, and I find the singleton has more bass, more H2 than H3, and more resolution.
But these are subjectives, you need to compare for yourself.
Greetings from southern Australia.......

HD
 
I like the circuit it actually has a very long history RCA did this in the early 70,s
I also like the fact that you have taken the thermal drift challenge and met with it
Diff pairs seem to be the standard way it works leave it alone attitude However we are a diy forum full of innovation so lets innovate
Trev
 
Osvaldo,
You are correct. However, there are provisos.
Straying offset is irritating but has little sonic effect, and no damage to the speaker.
You can improve it with an opamp servo.
But a conventional LTP does have disadvantages; poor transfer factor at high amplitude, viz bass, pretty much same levels of H2 and H3, and not wonderful resolution.
I've built both, and I find the singleton has more bass, more H2 than H3, and more resolution.
But these are subjectives, you need to compare for yourself.
Greetings from southern Australia.......

HD
Apparently I have "more" H3 than H2 in the THD measurement. Impression from listening test also points to this. Seems that quasi-complimentary configuration cancels H2 more effectively?
 
Member
Joined 2010
Paid Member
I think AKSA was talking about the singleton (single transistor) input stage as having the better spread of harmonics than a differential pair. Quasi-complementary output stages also tend to produce more even harmonics than odd.

I think there is something wrong with your soundcard measurement procedure or settings though. I don't see the harmonics aligned correctly with the horizontal scale.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.