old 2N3055 TUNGSRAM (Hungary) - your opinions

[joozwa]

tungsram hungary

Hello everyone,
I've created this topic just to learn about your opinions and experiences with those transistors. Some say they were lousy, having a border frequency of about 800kHz (which, I believe, doesn't matter in case of AF amplifiers, PSU, etc.).
I do invite you to share your opinions about them in comparison with 2N3055 assembled by other manufacturers.
Best regards,
joozwa

 

[Audioguru]

tungsram

Audio amplifiers must produce frequencies up to 20kHz with low distortion. If the distortion is high then the harmonics beat together and make many new frequencies that are audible and that mess up the sound.

Transistors make distortion. Negative feedback reduces distortion. But using negative feedback in an amplifier with low frequency transistors causes oscillation, so the the gain of the amplifier at high audio frequencies must be reduced which causes less negative feedback and causes the distortion to be high.

An OPA134 opamp has a bandwidth to 8MHz. With negative feedback it has very low distortion up to about 5khz. The distortion above 5kHz rises.

I have a stereo receiver that uses 2N3055 output transistors and it was made in about 1963. It sounds alright. Its distortion at 1kHz is about 0.1% and its distortion at 10khz is about 1% at almost full power.

 

[joozwa]

2n3055 tungsram

Thanks for your answer. It helps a lot.
Could somebody, who came across these transistors, write how they used to cope with higher wattages (with a proper heatsink of course)?

 

[LvW]

2n3055 max frequency

...............
But using negative feedback in an amplifier with low frequency transistors causes oscillation...........

Please, can you explain this statement ?

 

[IanP]

tungsram transistors

In the 80s and perhaps 90s it was the most commonly used power transistors in power supplies and audio amplifiers ..
Nowadays, I would look for something more recent .. unless you have ‘tons’ of 20355 on hand .. and, they are still very good for those applications ..

Rgds,
IanP

 

[FvM]

tungsram 2008

These more recent devices, e. g. MJ15003/15004 are generally 80th products. I'm not aware of better audio power BJT that could replace them. The main features compared to 2N3055 are
- factor 10 higher minimal Current Gain — Bandwidth Product
- voltage strength as required for high power audio (e. g. stage P.A.) applications
- doubled power handling per TO-3 case
- availability of true complementary parts

at least since the 90th, power amplifiers are mainly a MOSFET domain, I think.

 

[Audioguru]

Thanks for your answer. It helps a lot.
Could somebody, who came across these transistors, write how they used to cope with higher wattages (with a proper heatsink of course)?

A 2N3055 transistor can dissipate 115W if its case is cooled to 25 degrees C somehow (liquid nitrogen?) but in practise it is bolted tightly to a big finned aluminum heatsink with thermal grease in between where of course its case gets warm or hot so its power dissipation must be less. Maybe 80W is its max.

If an insulator is used to isolate it electrically from the heatsink then it prevents some of the transistor's heat from going to the heatsink so maybe its max dissipation becomes only 60W when its chip is at its max allowed temperature.
A fan will help cooling then more dissipation can be used. Frequently transistors are in parallel (with emitter resistors to equalize their spec's) to share the heat.

Added after 9 minutes:

...............
But using negative feedback in an amplifier with low frequency transistors causes oscillation...........

Please, can you explain this statement ?

A transistor at its max frequency has phase-shift. Other parts in an amplifier also have phase-shift. When the phase shifts add to near 180 degrees then the amplifier oscillates at a high frequency when negative feedback is used to reduce distortion. So amplifiers and opamps reduce gain at high frequencies so that at a frequency where the phase shifts add to 180 degrees there is no gain to cause oscillation.

If the output transistors are low frequency then the gain at high frequencies is reduced but then high audio frequencies are also reduced. With the gain reduced then the negative feedback is also reduced which increases the distortion.

 

[LvW]

Hi audioguru,
I am pretty aware of the oscillation condition for a feedback amplifier.
However, my question was how do you explain your former statement:
But using negative feedback in an amplifier with low frequency transistors causes oscillation.....

Thus, my point is not "distortion" but "oscillation" (or was it only a typing error ?).
Thank you.

 

[Audioguru]

Hi audioguru,
I am pretty aware of the oscillation condition for a feedback amplifier.
However, my question was how do you explain your former statement:
But using negative feedback in an amplifier with low frequency transistors causes oscillation.....

Thus, my point is not "distortion" but "oscillation" (or was it only a typing error ?).
Thank you.

In order to stop oscillation when negative feedback is added, the high frequency gain must be reduced. But with transistors that have a poor high frequency response the high audio frequencies are also reduced. Then the negative feedback is also reduced which increases distortion.

 

[LvW]

OK, let´s stop the discussion here, because I cannot follow your arguments. I am afraid we don´t come together in this point. Perhaps it is not too important.
Regards
LvW

 

[FvM]

@LvW: I really appreciate your insisting on precision in analysis of feedback circuits. I completely aggree, that the questioned statement isn't exact.

But using negative feedback in an amplifier with low frequency transistors causes oscillation...........

I however think to know, what is meant here, and I guess, you basically know, too.

The addressed problem, as far as I understand, is phase margin in feedback circuits and available compensation techniques. The above statement and the additional explanations are apparently assuming, that the 2N3055 limited current gain-bandwidth product is directly affecting the amplifier open loop gain, creating a respective pole. If this would be true and there are additional poles affecting the phase margin, reducing the loop gain may be the most obvious means to handle this situation.

There are however, several points contradicting these assumptions.
- the limited transistor current gain can be fight by local feedback
- if unavoidable, the output stage pole can be made dominant
- additional poles in amplifier gain can be shifted by pole-splitting

So a general correct statement would be to say, low frequency transistors are requiring a higher effort in amplifier compensation.

There's also a certain risk of dynamic instability, cause the transistor bandwidth is strongly current dependant. You have to assure, that you don't get sustaining oscillations after overloading the amplifier. Most catastrophic failure cases of audio power amplifiers are likely caused by effects of this kind.

 

[Audioguru]

Many old amplifiers used 2N3055 output transistors. I have one.
The distortion at 100hz is 0.03%, at 1kHz it is 0.1% and at 10kHz it is 1%. At 20kHz it is worse.
That is because the designer rolled off the high frequencies to avoid oscillation. Then there is hardly any gain at high audio frequencies for negative feedback to reduce the distortion.

 

[FvM]

Yes, you already reported about this amplifier. I also assume it's a profound, traditional design. The question is, if it would be possible to achieve a higher loop gain at 20 kHz. This can't be answered easily, I think.