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Old 8th January 2013, 04:46 PM   #1
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Default 50W BJT amplifier

First of all: I don't have much experience with solid-state amplifier circuits or SPICE simulation. I'm interested in the 50W amplifier circuit shown on this website:
2N3055 50W Audio Amplifier

After downloading the SPICE models for the transistors and op-amp IC I wanted to simulate the circuit with LTspice IV.

After running the simulation on the original circuit, one can notice a difference in the output transistors' Collector currents. Especially at high frequencies (>3 KHz) there is a big difference.

I also simulated the circuit with transistor types that are (much) more easily available for me; 2N2222 & 2N2907 as driver devices and BD243 & BD244 as output devices. This time there is no high frequency increase in Collector currents, but the 0.8 A difference remains.

I wonder if this would be a feasible design for a simple solid-state amplifier. Perhaps BD243 & BD244 pair could be used at lower supply voltages and output power?

LTspice circuit files are attached.
Attached Images
File Type: png sim.png (85.6 KB, 591 views)
Attached Files
File Type: zip components.zip (10.9 KB, 27 views)
File Type: asc 50w 2N3055 741.asc (5.4 KB, 29 views)
File Type: asc 50w BD243 BD244 741.asc (5.4 KB, 32 views)
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Old 8th January 2013, 05:02 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

This thread details a similar but better design :
Rega Brio (one channel dead)

More designs of the type here :
A Paul Kemble web page - PW Texan amplifier and the PE Rondo.

rgds, sreten.
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Old 21st January 2013, 08:58 PM   #3
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I built a breadboard prototype and the circuit seems to work. I used two lab power supplies to to power the circuit and performed some analysis using an oscilloscope. I changed the value of resistor R9 from 10R to 470R and noticed a 10dB reduction in levels of the harmonic "spikes".

Circuit: imgp3351.jpg

Output voltage into a 10R/5W resistor
R9: 10R, imgp3355.jpg
R9: 470R, imgp3356.jpg

FFT analysis of output:
R9: 10R, imgp3357.jpg
R9: 470R, imgp3358.jpg
Attached Images
File Type: jpg imgp3353.jpg (684.3 KB, 457 views)
File Type: png sim.png (46.6 KB, 406 views)

Last edited by d1030180; 21st January 2013 at 08:59 PM. Reason: schematic
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Old 22nd January 2013, 12:04 AM   #4
Bigun is offline Bigun  Canada
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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.
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Old 22nd January 2013, 12:26 AM   #5
CBS240 is offline CBS240  United States
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there is so much wrong with that circuit I wouldn't know where to begin.....
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Old 22nd January 2013, 01:24 AM   #6
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Dunno about the author of the first schematic linked in #1 but their parts list was out of date in about 1970. 2N2222/2N2907 are are also dinosaurs used as generic/default parts in simulators and Radio Shack. I guess any silicon will function in a crude audio amplifier but these parts are not well suited to audio. You would have to dig through really prehistoric stock to find 2N3053/2N4037!

Try to locate more credible sites and designs before wasting your time and money on the mediocre, ancient parts and ideas that would be better as static museum exhibits. We have plenty of threads and designs to search that will sweep these away in any performance categories you care to name. So does ESP and other reputable audio design/forum sites, for that matter.
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Old 22nd January 2013, 04:32 AM   #7
Jay is offline Jay  Indonesia
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I have designed many amps with this topology. Why many? Because this is not a plug and play circuit. Different opamp, different circuit. Different driver, different circuit. Different output stage, different circuit. The topology is not safely stable. This is a classic topology and you will find many failed amplifiers using this. If you want to use it, a protection scheme must be used.

Slow parts (741) must go with slow parts (3055), fast with fast.

The issue with this topology is that it requires strict operating condition. This is not a simple issue if our simulation models are not precise and we don't have proper measurement tools. Because our amp, even tho working, may have instability issue such that we must implement the protection circuitry to be safe.

Well, the fact is, the rate of failure of this topology is very high, whichever the circuit.

I like this topology because it is where slow parts can sound good. Instrumentation opamps are slow but often good sounding and can be used here.
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Old 22nd January 2013, 04:38 AM   #8
Jay is offline Jay  Indonesia
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BTW, I used darlingtons in my last endeavor with this topology. Look at OnAudio work, the HAL OPB amp.

And of course you don't want to miss the Audio Analogue Puccini.

Last edited by Jay; 22nd January 2013 at 04:42 AM.
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Old 22nd January 2013, 11:39 AM   #9
effebi is offline effebi  Italy
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It's a (CFP) buffer with gain . You need gain because you need to bring your maximun +-15 V signal coming out from the OP-Amp to output rails level.
This configuration, as many said (also there is an article on the ESP site) may work if well executed, but is troubleful to stabilize. Maybe you should consider to add a normal VAS stage and bring back the gain of the buffer to 1.
Interesting experiment, though.

Last edited by effebi; 22nd January 2013 at 11:41 AM. Reason: clarification+typos
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Old 22nd January 2013, 07:28 PM   #10
djk is offline djk
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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.
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