CLASS A amplifier output stage

[muddasirwaheedmalik]

Hi, i have two vintage sony TAN-330 Amplifiers it has simple two stage emitter follower output, with a bias current of 70mA, it is driven by STK 3102-III , the rail voltages are +/-56 Volts, The STK is driven in class A for VAS only and providiing base current to darlington driver transistors. Its rated around 100W into 8 ohm and is bridgeable.

The attatchment of both the IC and the amplifiers are here.

Now i want to enhance the output current capability using

1. separate series regulated power supply 1000VA for output stage.
2. separate output transistors, i have 2sc5200 complimentary pairs
3. CLass A operation , 10 transistors , 0.1 ohm emitter resistors with 200mA ICQ each=2A continous PD=100W per driver side, 200W per channel
4. Output 50W class A per channel and 100W bridged.=====so 2 , 100W monoblocks Class A.

How to do it, i intend to drive the 2SC5200 transistors from the emitter , fed to base via 2.2 Ohm resistor and getting to ground via 0.1 resistor in the sony Amp, but since Vref=7mv and higher current would trip the circuit i will replace the 0.1 with 0.01 resistor, so a current of 2 Amp will make 20mv across it. PD in the Sony 2SC1215 now acting as driver for 5200 would be 2Amp/20(Beta)=100 mAx56V =5.6 Watts only so the existin amp remains the same and i just take

Since the pre-driver is already in class AView attachment sony_ta_n55es_ta_n330es.pdfView attachment STK3122.pdfView attachment sony_ta_n55es_ta_n330es.pdfView attachment STK3122.pdfView attachment sony_ta_n55es_ta_n330es.pdfView attachment STK3122.pdf

 

[godfreyl]

Hi

Firstly, here's a picture of the Sony's output stage (so others don't have to download the 3MB PDF).

If I understand correctly, you want to change it from a double EF to a triple EF output stage, by adding 10 pairs of output devices (2sc5200 and compliment) per channel. The new output devices will each have a 2.2 Ohm base resistor and a 0.1 Ohm emitter resistor, and they will be driven from the emitters of the existing output devices (Q304 and Q305). Is that right?

If so, there's a few other things to take care of:

• The VBE multiplier will have to be modified to increase the bias voltage for the output stage. i.e. You need to change the resistors around Q301.
• R309 and R310 in the existing circuit can be left out. At the moment the protection circuitry (R311, Q306 etc) is connected to R310. With the new output stage, it can be connected to one of the new 0.1 Ohm emitter resistors. I don't see a need for an extra 0.01 Ohm resistor carrying all the output current.
• Add a resistor (e.g. 22 Ohms) between the emitters of the existing output transistors (Q304 and Q305). This is to help the new devices switch off quickly when the amp shifts from class A to class B at high output power. That will increase the power dissipation of Q304 and Q305, but they should be OK - It's still less than they dissipate now with high output.
• You're going to need huge heatsinks, duh!
• I'm not sure that regulating the supply for the output stage will be worth it. I expect that regulating the supply for the front end would give a bigger improvement to performance.
• Lastly, but probably the most important:
  You need to be very careful about high frequency stability. Triple EF output stages can be a bitch in this regard. It's not just the phase margin of the global feedback loop you have to worry about - there can be problems with local instability as well.

btw, It looks like some text is missing near the bottom of your post.

P.S. I strongly suggest you post this in the solid state section of diyAudio forum as well. There are much more knowledgeable and helpful people there than here, when it comes to this sort of thing.

Cheers - Godfrey

 

[muddasirwaheedmalik]

Thanks. I have a confusion when i add resistor 22Ohm between the existing output transistors (Q304 and Q 305), how do i know whats their collector current , how much is going to throught the 22 Ohm resistor and how much into the base of the the output stage ??? its a bit confusing for me that how VBE adjusts bias current i used to think it only sets bias voltage ???? current setting ????
Regards

- - - Updated - - -

Instead what if i simply enhance the number of output devices on a separate heat sink maintaining the same topology double EF

1. The existing R307 is 4.7 Ohm , what should be value when i feed it to ten output devices.
2. the existing VBE multiplier Q 301 would then be isolated from the heat sink of output devices ????
3. Is it a better option the triple EF, but can Q302 drive 10 transistors or i need to replace them or put them on heat sinks , with VBE or with Output device ????

 

[godfreyl]

its a bit confusing for me that how VBE adjusts bias current i used to think it only sets bias voltage ???? current setting ????

The Vbe multiplier sets the bias voltage for the output stage. In the existing circuit, the bias voltage must be enough to switch on Q302, Q303, Q304 and Q305. Assuming Vbe for each of those is 0.6V, then the bias voltage must be about 4 * 0.6 = 2.4V.

If the bias voltage is increased slightly then those transistors will conduct more current and there will be a slightly higher voltage across R309 and R310. So changing the bias voltage changes the quiescent current of the output devices. The quiescent current can be checked by measuring the voltage across R309 or R310.

This is a very sensitive adjustment. You said that the quiescent current of the output transistors is 70mA. That means there is only 7mV across each of the emitter resistors, so a very small change in the bias voltage can easily double the quiescent current.

Even if the quiescent current is set correctly when the amplifier is cool, it is likely to change when the amplifier warms up playing music. The problem is that Vbe decreases by about 2mV / degree centigrade of temperature increase.

To keep the quiescent current constant, Q301, Q302, Q303, Q304 and Q305 must all be at the same temperature. It helps to put them all on the same heatsink, but even then the temperature compensation is far from perfect.

If Q301 is mounted on the same heatsink as the power transistors, it will compensate for changes in heatsink temperature. However the internal temperature of the power transistors is higher than the heatsink temperature, so there is always some error.

When you design an output stage, you have to be very careful to avoid thermal runaway, which can cause the quiescent current to keep on increasing until the transistors burn out.

It's caused by this vicious circle:

1. Higher power dissipation causes higher temperature.
2. Higher temperature causes lower Vbe.
3. Lower Vbe causes higher quiescent current.
4. Higher quiescent current causes higher dissipation.
5. Back to point one.

The heatsink helps reduce the effects of point one to some extent.
The emitter resistors of the output devices control point 3. So to avoid thermal runaway, you have to make sure that the emitter resistors are big enough.

With multiple output devices like you're planning, the emitter resistors help to avoid another problem - current hogging.

The cause of current hogging is similar to the above. Imagine if one output transistor is conducting slightly more current than the others (for whatever reason). That will cause it to get hotter than the others, which in turn will cause it to conduct even more current. If you're not careful, this can result in one transistor conducting almost all of the current, while the others conduct almost nothing. The emitter resistors help to get equal current sharing between the output transistors.

I have a confusion when i add resistor 22Ohm between the existing output transistors (Q304 and Q 305), how do i know whats their collector current , how much is going to throught the 22 Ohm resistor and how much into the base of the the output stage ???

I think you already calculated the base currents. The voltage across the 22 Ohm resistor is the same as the voltage between the bases of the output transistors i.e. about 1.2V. So the current through it is about 1.2V / 22R = 55mA.

Instead what if i simply enhance the number of output devices on a separate heat sink maintaining the same topology double EF

You could do that but Q302 and Q303 will not be enough. With 10 pairs of output devices, I think you should use at least 2 pairs of drivers and they will need to be heatsinked. Q301 should probably be on the same heatsink as the output transistors, depending on your choice of emitter resistors.

I wonder if it wouldn't be better to leave the Sonys alone and build the new amps from scratch. Most of the cost of an amplifier goes to the power supply, heatsinks and output devices. The rest of the circuitry doesn't cost much. Since you're planning to replace all of the expensive stuff, you won't be using much of value from the original amps, which seems a waste.