It's not so much the load itself, but the way it is connected. An unbalanced load will dump more HF into the "ground" than a well-floating load. Shorting one leg of the secondary to ground, without any load, may start or kill an oscillation; even which leg is shorted to ground makes a difference.ricardo said:
Output stage oscillating
- Thread starter Ilya
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Help Support GroupDIY Audio Forum:
> output stage is just an emitter follower with a constant current load.
It is a White Cathode Follower, without the cathodes or tubes. As Abbey says, push-pull. Peak output current can be nearly twice the idle current.
It is a White Cathode Follower, without the cathodes or tubes. As Abbey says, push-pull. Peak output current can be nearly twice the idle current.
Ok. Turned out that the cap did stop the oscillation, but the output stage still doesn't want to drive anything lower than ~2.5K. It behaves like this: the heavier the load, the more the output level increases, and THD increases as well. The only way to stop this is to ground the transformer primary. But in this case the overall output level is 3 dB lower which is undesirable.
Is there anything I can look at to make this work with 600R loads?
Is there anything I can look at to make this work with 600R loads?
That is definitely a sign that the negative impedance circuitry is not properly balanced. The ideal situation would be that the negative impedance compensates exactly the primary resistance. In practice, in order to avoid issues, the compensation is set a little lower, to make sure that there is no risk of unstability.Ilya said:
Wet finger says R33 should be about 1/10th of the DCR of the primary. Is that the case?
This is not normal. Gain should change by a fraction of dB only. Again that's because too much positive feedback. I think it's because the xfmr you use has a much lower DCR than the original.
What do you mean " the output stage still doesn't want to drive anything lower than ~2.5K"? What happens? Does it clip or cease completely to function?
Simulation indicates it should be capable of delivering about 3.5Vrms into 2 k (+13dBu) and about 1Vrms (+2 dBu) into 600 ohms.
It seems the only solution is to decrease the value of the resistor in the emitter of teh bottom output transistor R37.
27 ohms there suggests 3Vrms into 600 ohms (+12 dBu), and 15 ohms suggests 5Vrms.
The gain of this circuit is normally governed by the ratio of the NFB resistor R28 to the input resistor R7. The calculation gives a ratio of 8.5, or 18.5 dB. The additional 3 dB you get at the moment are due to excess of positive feedback, which should not be considered normal operation.
abbey road d enfer said:
DCR of the primary is 18R, so 8R2 looks like too high for it.
You are correct, it seems that I got those additional 3dB from positive feedback.
It looks like clipping. If I connect 600R resistor across the output, the level increases and THD goes up 10-fold.
Indeed, changing R37 improved driving capability of the circuit and dropped THD significantly.
I was able to solve this (sort of, see below). One way was inserting 68R resistor in series with transformer primaries, another way was reducing 8R2 to 3R (the lowest I had at hand). Both solutions worked the same, and I removed the 22p cap and the output didn't oscillate.
However, there came another problem. After all these manipulations I get a wide droop around 8kHz-14kHz, down 2dB at ~11kHz. It appears in both cases - with 68R in series with primaries and when changing 8R2 to 3R.
If I return everything as per original circuit (and insert a 22p cap to stop the osc), the frequency responce is flat.
I woukd think that inserting a resistor in series with the primary solves the unstability problem at the detriment of drive capability. I believe you should go a little further in reducing R33. My sim says the optimum value is about 2R.Ilya said:
This circuit is really stubborn! You may try putting a capacitor across R31, a few nF.
abbey road d enfer said:
Will put 2R resistor when I get one from the store!
As for frequency responce, turned out it was carried on from the input stages. The output stage is linear (as far as I can measure). I think that changing 8R2 resistor to 2R will be the most convenient solution. Thanks!
I'd like to revisit this output stage circuit.
I need to limit it's bandwidth. I've come up with a couple of caps across the transistor as shown in the attachement. I couldn't find any negative effects of these caps. However if there's something that I haven't noticed please chime in.
I need to limit it's bandwidth. I've come up with a couple of caps across the transistor as shown in the attachement. I couldn't find any negative effects of these caps. However if there's something that I haven't noticed please chime in.
Attachments
Attachment?
Seems ok to me.
