Class A amp...is this normal?

[redrabbit]

Headphone amp (from AudioXpress, years ago)

I just built it.
...... But this is happening:

* With a load at output (34 ohm resistor(s)), it gives perfect waveforms.

* With no load (open jack), there is +10v at the jack.....that sinks very slowly. The waveform is there on the scope, but is riding on that +voltage.

* When powered down (no load) , a negative voltage (-5v ?) is measured at outputs that will dissapate slowly (cap charge, if I short the output to ground it's gone).

Here is the circuit, and the voltage measurements at No Load:


Could the T2,T3 be too powerfull, and the drain resistors (100 ohm, 100k) are overwhelmed?



H...E...L...P

=FB=

[MikeB]

Redrabbit, this behaviour is "perfectly normal" for an amp using an output cap. This output cap needs to be charged with the 15v to give a zero volt reading at the output. This takes quite long through a 100k to charge 2200uf. With the 34 ohm load this charge is much faster. (This charge needs to be done through the load)

To be honest, i don't think that this is a good circuit... The whole operation point of the output stage is set by the hfe of T2/3 and the 220k. Using MJL21194 for T2 is overkill, it operates on ~1.5ma.

Mike

[traderbam]

What you see is what you should expect. It's the output capacitors.

Before the amp is first switched on the voltage across the 2200uF/10uF caps is zero. Capacitors maintain a constant voltage across their terminals unless a current flows through them. A current causes a capacitors voltage to gradually change.

When you switch the amp on the emitter of T3 rises quickly to about 15V and the output side of the caps will also rise to the same voltage. Current will flow through the 100k resistor and the caps and this will gradually change the voltage across the caps until the output is zero. This will take several minutes with the values you've got. Once the output is zero the caps have 15V across them. When the amp is powered down the emitter of T3 goes quickly to zero but the caps keep 15V across them so the output drops towards -15V and then gradually rises due to the, now opposite, current in the 100k resistor.

The upshot is that the speaker will sound a "crack" at switch on and at switch off. The capacitors will charge and discharge mainly through the speaker because it has a much lower resistance than the 100k resistor.

[Nexus]

Hello Redrabbit,

you can lower the 100K resistor so the output cap will charge/degharge quicker, but it is better to short the output (the speaker) with a relay during switch on/off for a few seconds.

than the capacitor charges/discharges, and your speaker will be protected.


best Regards:
Nexus

[bobo1on1]

I would rather switch a 10 ohms resistor with the relay, because shorting the output might cause excessive currents.

[Tube_Dude]

You need to put another 220 Kohm resistor from T2 base to ground , for the emitter of T3 to be biased at around half the supply voltage...

[ilimzn]

Originally posted by Tube_Dude
You need to put another 220 Kohm resistor from T2 base to ground , for the emitter of T3 to be biased at around half the supply voltage...

T3 will remain biassed more or less as is regardless of any (sane valued) resistors from it's base to ground.

This schematic has that 'designed by ear without covering the basics' look to it.
For starters, the coupling capacitor betwen T1 and T2 is not necessary - assuming one designed the T1 stage properly. The second stage darlington is completely over the top. 2N3055 has a fairly low gain, and even more so for low collector currents, such as would be seen by T2. 2N3055 could be used for T3 if nothing else was available, vut these days, there is plenty. Either way, T2 E to T3 B junction really needs a resistor to ground.
Considering that there is a resistor to pull down the output, better results may be obtained by increasing the DC drop across it, this will somewhat decrease distortion at the expense of available output swing. The resistir in parallel with the output should be decreased. It is likely 1k will not make a signifficant difference (especially if the above changes are implemented) to output driving capability, but it will to the length of the power-up or power-down 'thump'. Things can also be improved somewhat by chosing the input stage power rail filter cap properly.

[Tube_Dude]

Originally posted by ilimzn


T3 will remain biassed more or less as is regardless of any (sane valued) resistors from it's base to ground.

How without the 220 K resistor from T2 to ground (to put T2 base at apr. half the supply voltage ) the voltage at the emitter of T3 will stay at half the supply voltage , as I have told in my previous post?

Later on you say

Either way, T2 E to T3 B junction really needs a resistor to ground.

[redrabbit]

Thanks for the replies and suggestions.

This circuit was originally designed by Rudi Stor http://www.rudistor.com/ (using 2N3055's) and augmented by the author of the AudioXpress article using his available transistors. I am using my available transistors.
Rudi Stor no longer posts his schemos (I couldn't find them), and he now sells his units commercially.


Do you have a more sensible transistor choice?
I have accsess to 2N types, as well as TIP types.

(I will listen to it as-is, for the first time, with real phones tonite.)
The author also thought that the circuit would never sound good, until he simulated it, and built it.....and claims it's quiet, and sounds very good.
I am as skeptical as you guys are.
...but, we'll see.

=FB=

[Wavebourn]

Originally posted by Tube_Dude
You need to put another 220 Kohm resistor from T2 base to ground , for the emitter of T3 to be biased at around half the supply voltage...

...and get more distortions. I'd rather lowered emitter resistor instead of base current and voltage. The more current you can afford through emitter follower, the less distortions you will get.