Edwin 20 watt (Elektor 1970 - May)

[steph_tsf]

hello,

the Elektor May 1970 issue is downloadable here : Verstärkermetamorphosen - ELEKTOR.de | Elektronik: Analog Digital Embedded Mikrocontroller Audio Messtechnik

Have a look to the Elektor Edwin 20 watt schematic (attached .jpg). There is a small class A amplifier nested inside a pure (zero bias) class B output stage.

Anybody still owning such amplifier ? What are the subjective results ?

I took some time deriving two modern versions of the Edwin 20 watt amplifier, using LTspiceIV. Version A should be easy to build, no adjustments, and uses an opamp as front-end. Version B is more conventional, with a differential NPN pair at the input, but needs care for getting a low DC offset at the output.
Performance looks decent on simulation.

During the simulation sessions, I have noticed unstabilities when using BD237/DB238 transistors in the VAS. I then realized it would be needed to have specific transistors in the VAS, instead of using the same as in the small class A. I have tried BF469/BF470 transistors, but still getting HF oscillations. The best stability came with 2N5550/2N5401 transistors in the VAS. I have the impression that Edwin is hard to stabilize because there is a steep open loop gain jump each time the class B gets activated. When testing the amplifier on simulation, it is essential to verify the voltage and current waveforms in the VAS, at different power levels like 1mV input, 10 mV input, 100 mV input and 1000 mV input. The compensation caps are mandatory. If you halve their values, you'll experince HF oscillations bursts at high power.

Cheers,
Steph

[Elvee]

In the seventies, semiconductor designers were pretty bold and confident; one might even say "gung ho".

They thought they could get away with unbiased output transistors.
The thing was in the air, and there are numerous examples dating from that era.

At the time, Hermann Schreiber was a contributor to the French magazine "Le Haut-Parleur", and he described a number of such circuits (BTW, H Schreiber was also an excellent author and electronician).

IC manufacturers also proposed this kind of circuit to boost the output of small amplifiers. Here is an example from National.

The belief behind those circuit was that no crossover distortion could appear, because the output was always connected to the signal, and the transition between the two modes was supposed to be smooth.
The cold truth is that the scheme is worthless, and they sound awful, even though figures can look decent.

[tiefbassuebertr]

Some guys claim, that this circuit was a simplefield circuit of the current dumping amp Quad 405
http://www.fralu.de/images/qaud405.jpg
But the only identical thing that I know is the no needed idle current adjust.

BTW this amp was my first amplifier project (arround 1976). It was a amp kit from Oppermann electronic include PCB.

[ilimzn]

Actually Quad 'current dumping' is a misnomer. Peter Walker patented it with it's proper name, which is a form of feedforward. As a result, many amplifiers that connect B-E of the output transistors across a resistor in series between the previous stage and load, were called 'current dumping'. Ironically, it is a fitting name for those, while they have nothing in common with the Quad topology.

[steph_tsf]

Originally Posted by tiefbassuebertr

Some guys claim, that this circuit was a simplifield circuit of the current dumping amp Quad 405.

That's not quite possible because Edwin got presented in Elektor in May 1970, and the Quad 405 got presented by P.J. Walker and M.P. Albinson at the AES in 1975. A possibility is that Walker or Albinson creative imagination got triggered upon discovering the Edwin amplifier in Elektor. Have you noticed, in the Quad 405, the integrated circuit used as preamp ? That's another diffuse similitude with Edwin ...

[Bonsai]

Hey Elvee, I recognize that book - its the Radio and Audio Handbook from Natsemi. Brings back memories!

[forr]

In the Edwin circuit, there is no idle current in the output trannies, they are in class C. They draw current only when there is sufficient voltage across the driver resistors to bias them.
The same idea was used in Phase Linear and ESS big power amps, and probably some others, and more recently, by Graham Nalty in his Apex amplifier described in ETI.

[mjf]

QUOTE=tiefbassuebertr BTW this amp was my first amplifier project (arround 1976). It was a amp kit from Oppermann electronic include PCB

hello.
found something.........

[apexaudio]

I found this,
Regards

[steph_tsf]

here you will discover how an Edwin can become a Quad 405, same PCB, same output devices, only a few localized differences.

High-power distorsion :
*****************
Fourier components of V(edwin)
DC component:-0.00070647
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.252e+01 1.000e+00 -0.22° 0.00°
2 2.000e+03 3.725e-05 1.654e-06 2.06° 2.28°
3 3.000e+03 2.567e-05 1.140e-06 -82.88° -82.67°
4 4.000e+03 2.173e-06 9.649e-08 -61.81° -61.59°
5 5.000e+03 5.246e-05 2.330e-06 -86.07° -85.86°
6 6.000e+03 1.753e-06 7.785e-08 -40.68° -40.47°
7 7.000e+03 4.834e-05 2.147e-06 -84.56° -84.35°
8 8.000e+03 8.157e-07 3.622e-08 -29.85° -29.63°
9 9.000e+03 4.766e-05 2.117e-06 -84.18° -83.96°
Total Harmonic Distortion: 0.000431%

Fourier components of V(quad405)
DC component:-0.000729602
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.323e+01 1.000e+00 -0.27° 0.00°
2 2.000e+03 4.585e-05 1.974e-06 4.51° 4.79°
3 3.000e+03 8.381e-06 3.608e-07 14.11° 14.39°
4 4.000e+03 1.684e-06 7.251e-08 -56.56° -56.29°
5 5.000e+03 3.481e-05 1.499e-06 -79.20° -78.93°
6 6.000e+03 1.862e-06 8.017e-08 -31.06° -30.79°
7 7.000e+03 3.070e-05 1.322e-06 -77.28° -77.01°
8 8.000e+03 9.427e-07 4.059e-08 -25.73° -25.46°
9 9.000e+03 3.110e-05 1.339e-06 -77.89° -77.62°
Total Harmonic Distortion: 0.000313%

Low-power distorsion :
*****************
Fourier components of V(edwin)
DC component:-0.000698215
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.252e+00 1.000e+00 -0.22° 0.00°
2 2.000e+03 1.350e-06 5.996e-07 -9.77° -9.55°
3 3.000e+03 4.080e-05 1.812e-05 -90.40° -90.18°
4 4.000e+03 3.852e-07 1.711e-07 -23.20° -22.98°
5 5.000e+03 3.719e-05 1.652e-05 -89.95° -89.73°
6 6.000e+03 1.327e-07 5.891e-08 -56.46° -56.25°
7 7.000e+03 3.455e-05 1.534e-05 -89.54° -89.32°
8 8.000e+03 1.458e-07 6.475e-08 -141.97° -141.76°
9 9.000e+03 3.239e-05 1.438e-05 -89.17° -88.95°
Total Harmonic Distortion: 0.003231%

Fourier components of V(quad405)
DC component:-0.0007204
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 2.323e+00 1.000e+00 -0.27° 0.00°
2 2.000e+03 1.249e-06 5.378e-07 -5.24° -4.97°
3 3.000e+03 2.823e-05 1.215e-05 -82.00° -81.72°
4 4.000e+03 3.808e-07 1.639e-07 -18.17° -17.90°
5 5.000e+03 2.689e-05 1.158e-05 -84.54° -84.26°
6 6.000e+03 2.349e-07 1.011e-07 -29.63° -29.36°
7 7.000e+03 2.594e-05 1.117e-05 -85.41° -85.14°
8 8.000e+03 1.630e-07 7.017e-08 -47.12° -46.84°
9 9.000e+03 2.514e-05 1.083e-05 -85.76° -85.48°
Total Harmonic Distortion: 0.002289%

See the attached .zip containing everything for LTspiceIV.

Cheers,
Steph