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Old 23rd January 2013, 05:29 PM   #11
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Quote:
Originally Posted by djk View Post
I was able to make a very high quality version of this amplifier, I will see if I can find the notes and post some suggestions when I have some more time after work.
I'm very interested in hearing about your project! Notes and suggestions would be very welcome if you have some free time.

Quote:
you don't just simulate but you build, you measure and compare. That gets +1 from me.

Interesting output stage design. The waveforms in your last simulation look really awful. My preference is still for the double emitter follower where Q1 and Q2 collectors are taken to their respective supply rails and the junction of R10 - R11 is not grounded but floating.
Thanks for your suggestion. I agree that the waveform is very awful. I made some experiments by arranging the output transistors into a Darlington configuration. Is this what you meant?
http://sound.westhost.com/articles/cmpd-vs-darl.htm#s6
darl1.png
darl2.png

I also tried modifying the original circuit slightly, by changing the values of R5 and R8 to increase the current going through the diodes. The graph shows the output voltage into the 8R resistor and the base currents of driver transistors.
comp1.png
comp2.png

Thanks for your comments
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Old 25th January 2013, 08:09 PM   #12
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I made the small tweak of changing the values of R7 & R8 by paralleling them with 22k resistors. So the total resistance ends up being (15⁻ + 22⁻)⁻ = 8.9k.

It was surprising to see such a big difference in how the waveforms appear on the oscilloscope. Especially the magnitude of sharp harmonic peaks was reduced in FFT analysis.

Here are some more pictures.
Driver transistors' base voltage levels (the channel connected to PNP is reversed in display):
imgp3404.jpg
Voltage across NPN driver transistor's 47R emitter resistor:
imgp3406.jpg
Voltage across PNP driver transistor's 47R emitter resistor:
imgp3408.jpg
FFT analysis of the output into 10R/5W resistor:
imgp3411.jpg
2x zoomed view of previous:
imgp3412.jpg
FFT of amp driven heavily into clipping. I only used +-200mA limited power supply as the output transistors only have tiny heatsinks, I don't want to fry them:
imgp3413.jpg

I still need to experiment with the Darlington configuration and a better constant-current device than a resistor. Maybe also try to implement some kind of short-circuit protection circuit with a 2N3904&2N3906 pair.

Last edited by d1030180; 25th January 2013 at 08:18 PM.
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Old 26th January 2013, 06:01 PM   #13
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Click the image to open in full size.

Here is the general idea, it will take a while to dig through my cold storage and find the actual project.
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Old 26th January 2013, 06:35 PM   #14
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Q5, Q6 make up a CCS for the bias of Q1, Q2. We want to run about 2mA through the the diodes biasing up Q1 and Q2. I used MPSA06/56 for all the small signal types, others may be suitable. The MPSA06/56 are rated at 500mA max collector current, so they have plenty of muscle to act as drivers for the outputs, they are also available in 1W types (the MPSW06/56 if you want to bias them real heavy. I think I ran about 4mA through Q1 and Q2, with 51Ω as their emitter resistors to ground, I used 150Ω feedback resistors to the output, and the caps were about 10nF.

The outputs were TO220 versions of the 3055/2955 and they were mounted with no insulators on a common heatsink, one for each channel. A TL072 was tried, but the NE5532 sounded vastly better so we bit the bullet as far as the increased cost, the TL074 were used elsewhere. The Zobel was the usual 10Ω and 100nF. The amplifier was deliberately overcompensated and had a bandwidth of about 250Khz. We used a 160VA 20-0-20 transformer to power the two-channel version, it sounded much beefier than the original 80VA we had planned on.
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Old 26th January 2013, 07:02 PM   #15
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The hand-wired prototype was evaluated with pure class B bias in the outputs, ie: none. This was done to eliminate any bias adjustment in the production line. I deliberately did not tell anyone this as "everybody knows class B doesn't sound good". The amplifier was evaluated on some very expensive speakers done by a top designer and everyone was stunned by how good it sounded, going so far as to bring in their own high $$$ hi-fi amplifiers only to find them suffering by comparison. Keep in mind this was designed for a computer sound system and fit into a half-height drive slot (external transformer). The product was shown at the Comdex Show in Las Vegas in 1997, and marketed as an OEM product to many different PC manufacturers.
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Last edited by djk; 26th January 2013 at 07:12 PM.
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Old 26th January 2013, 07:27 PM   #16
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I finnally finished a concept with discrete current feedback opamp on this configuration, slightly modified into compound type exit. You cannot imagine, how high is level of quality of reproduction on complex RLC load via my old HP oscilloscope and sweep of Wavetek 182. But there are many years (10) of elaboration and tests. There are also very good measure to made this concept with IC's directly, but with used mirror type active feedback for more quality phase response.
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