Blocking distortion in 18 watt amps
Posted: Sun 04/16/23 8:48 pm
I have looked at a few circuit diagrams where zener diodes are used to clamp the negative voltage across the 470K grid leak resistors on the output stage and I think the zener diode break over voltage is too great to kill off the crossover distortion in some drawings that I have seen on this site.
My understanding of this circuit (which might be wrong) is the zener conducts when negative voltage on the grid (with respect to earth) exceeds the Zeners break over voltage.
To put this into perspective, the signal from the P.I. stage increases till the EL-84 goes into forward conduction (the signal exceeds bias voltage), this causes the top of the signal to be clipped and for the coupling cap to charge to a negative voltage with respect to the P.I. side of the cap. Once the signal is below the cathode voltage the EL-84 ceases to conduct, though the cap remains charged as the 470K resistor is too large to dissipate the negative voltage in the required time. The grid now has a larger negative going voltage, as the signal voltage is negative going and there is the negative voltage on the cap. The combination of the negative going signal and negative DC voltage cause the valve to go into shut down prematurely when a sufficiently negative voltage is on the grid. As the other valve in the push/pull circuit is also in shut off there is a flat spot in the signal on the secondary of the output transformer.
The idea of the zener is to conduct when a certain negative voltage is reached, ideally this voltage is much less than the voltage required to send the valve into shutdown. Thus minimising the flat spot in the signal at the secondary of the output transformer.
In the circuits I have seen the zener is a 1N4744 which has a 15 volt break over voltage. This would mean that the signal would go to -15.7v with reference to earth or 28.7v with reference to the cathode of the EL-84. This seems excessive to me.
Have I misunderstood the operation of this circuit and if so could you explain the misunderstanding?
I have had the “feeling” that people have opted for the large zener diode valve as they think it will conduct before the EL-84 goes into forward conduction. However, if this hypothesis is correct, the EL-84 only conducts when the grid is more positive than the cathode (roughly 13V) and Zeners conduct when the grid is more negative than earth. Thus the Zeners can’t operate when the signal is positive going.
The other option to alleviate crossover distortion is a cathode follower. RG Keen came up with a circuit some time ago.
http://www.geofex.com/Article_Folders/m ... tfolly.htm
Thanks for your patience and I look forward to your responses.
Regards
Mark
My understanding of this circuit (which might be wrong) is the zener conducts when negative voltage on the grid (with respect to earth) exceeds the Zeners break over voltage.
To put this into perspective, the signal from the P.I. stage increases till the EL-84 goes into forward conduction (the signal exceeds bias voltage), this causes the top of the signal to be clipped and for the coupling cap to charge to a negative voltage with respect to the P.I. side of the cap. Once the signal is below the cathode voltage the EL-84 ceases to conduct, though the cap remains charged as the 470K resistor is too large to dissipate the negative voltage in the required time. The grid now has a larger negative going voltage, as the signal voltage is negative going and there is the negative voltage on the cap. The combination of the negative going signal and negative DC voltage cause the valve to go into shut down prematurely when a sufficiently negative voltage is on the grid. As the other valve in the push/pull circuit is also in shut off there is a flat spot in the signal on the secondary of the output transformer.
The idea of the zener is to conduct when a certain negative voltage is reached, ideally this voltage is much less than the voltage required to send the valve into shutdown. Thus minimising the flat spot in the signal at the secondary of the output transformer.
In the circuits I have seen the zener is a 1N4744 which has a 15 volt break over voltage. This would mean that the signal would go to -15.7v with reference to earth or 28.7v with reference to the cathode of the EL-84. This seems excessive to me.
Have I misunderstood the operation of this circuit and if so could you explain the misunderstanding?
I have had the “feeling” that people have opted for the large zener diode valve as they think it will conduct before the EL-84 goes into forward conduction. However, if this hypothesis is correct, the EL-84 only conducts when the grid is more positive than the cathode (roughly 13V) and Zeners conduct when the grid is more negative than earth. Thus the Zeners can’t operate when the signal is positive going.
The other option to alleviate crossover distortion is a cathode follower. RG Keen came up with a circuit some time ago.
http://www.geofex.com/Article_Folders/m ... tfolly.htm
Thanks for your patience and I look forward to your responses.
Regards
Mark