My 50W class A design!

[gustav]

Hello again,
the problem is not BD137. It is 2sc2240 which only can handle something like 0.3W.
All you need to do (in the first scm) is put a resistor (maybee 2kOhms) in the collector of the 2240 !

[gustav]

Sorry , BD137 should have been BD139

[Lelak53]

Dare I disturb?
Whatever is the purpose of the circuit, lowering the value of R30 would make more reliable design. Bypassing it with a capacitor would speed-up the reaction.

[PanzerLord]

Thanks Gustav, have inserted a 1.6k resistor in series with the collector.

[Steven]

PanzerLord,

I think it is time to post my solution for the Q25/26 problem.

The circuit below shows it. I only added two resistors R5 and R8 and two diodes D6 and D1 (strange default labels BTW). The diodes are Infra Red LEDs of the GaAs type with a forward voltage of 1-1.2V, like CQY36. Most GaAlAs types have a higher Vf of about 1.3-1.5V and cannot be used, just as other LEDs with visible colors.
For small output currents the voltage across R3 and R4 is determined mainly by the Class A biasing and Q2 and Q4 keeps that voltage around 0.6V. The Vf of the LED is too high to have influence. If the output current is increased by a bigger input voltage or a heavier load the voltage across R3 increases for positive signals and the voltage across R4 decreases, for negative signals the other way around. Without the LEDs the maximum output current would be twice the bias current, since at that moment the voltage across R3 (or R4) becomes zero and the voltage across R4 (or R3) cannot increase beyond VbeQ2+VbeQ4. But with the LEDs the contribution of the voltage across R3 to the Q2/4 is gradually clipped, leaving some voltage left for R4. For negative signals the other way around.
This can be seen on the two graphs. The top one shows the output current through R3 and R4 with 10Vp input sine. The output stage is clearly in class A with a bias current of 70mA. The second graph shows the same but for an input signal of 30Vp. Now the stage is in class AB. But note that the current through R3 and R4 never reaches zero, just as the current through the output transistors will not go to zero. This is a "non-switching" class AB, i.e. the current is never switched off completely, nor reverse biased as you can get with normal fixed voltage biasing (Vbe multiplier).

Instead of using IR LEDs for this trick, you can also use a normal small signal Si diode and a Schottky diode in series, giving a Vf of about 1V. A third method is to use a Vbe multiplier with a multiplication factor of approx 1.6. The bigger the Vf, the smaller the minimum current will be in the output stage. If Vf becomes more than 2Vbe it will have no effect.
Of course, distortion will rise as soon the output current halves are "shaped", actually pre-distorted by the action of the LEDs. But this rise in distortion will not be so dramatic as the currents through R3 and R4 suggest. They will be subtracted from each other. The output voltage will still have a distortion less than 0.1% (open loop!). In this simulation most of the distortion comes from R1 and R9 due to varying base currents of Q1 and Q3. This will improve by using darlingtons for the ouput transistors, as in the diagram at the beginning of this thread.
The emitter resistor values are quite high in this example and only small signal transistors are used, but that is only because of my lack of models for power transistors in my free downloaded Circuitmaker program. The circuit can easily be used in power amplifiers. The value of R5 and R8 can be from 20 Ohm - 1kOhm. A small value will keep the output longer in class A without starting to shape the currents, a bigger value will increase the influence of the LEDs. This is simply because R5 and R8 are voltage dividers with the Rd of the LED as second resistor.

Steven

PS Just an idea: if you use optocouplers for the LEDs you can use the output current as an indicator whether the amplifier is in class A (no current) or class B (current). Of course these should be IR optocouplers, but I have not checked yet if these exist (or are these always IR?).

[Steven]

As the above biasing scheme is a little off-topic for this thread and its use not limited to the amplifier that is discussed in this thread, I started a new thread for this biasing proposal here: http://www.diyaudio.com/forums/showt...339#post282339

Steven

[PanzerLord]

Oki, some new modifications, added a cascode in the VA stage and VA CS (ok, I cheated, I stole a bit from peufeu`s design), the cascode at the input stage has also been transformed in to a hawksford (thanks Andy C.). I`ve replaced the BD139/140 with
MJE340/350, it may become a 100W class A......, -higher rail-voltage.
CC`s have been scrapped due to the excellent stability!! MAN, look at the bandwith and phase (without zobel-network)...
The distortion curves are almost flat throughout the audiospectrum!

I`m designing the PCB as I speak, I`m making an option for CC`s and VA without cascode, just in case the simulator`s fooling around with me).

[PanzerLord]

AC analysis, red line is without zobel, blue with zobel. There will be an additional low-pass filter at the input.

Magnitude is dB. It`s flat all the way past 10MHz!

[PanzerLord]

The 2nd and 3rd harmonics (all simulations with 8 ohm load at full voltage swing +/-28volts peak-peak).

[Konrad]

Hello
Here is one eksample of aktive loading to the innput stage.
Q1- Q2 is input transistors, Q3 Q4 is the aktive load. and Q5 Q6 the Vamp stage. Resistor values has non realistic values.