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In Reply to: RE: This circuit appears capable of that voltage swing. posted by Ralph on December 21, 2021 at 13:22:23
Hi Ralph,
My question actually is on 5687 voltage amplify stage, which has 1K unbypassed in cathode What is the main purpose for the designer to insert 1K ohm unbypassed in this stage. Distortion reduction?
In not allowing loop NFB, I always find a simple RC voltage amplify stage necessarily swinging around 140V P-P, creates certain harsh sound in mid high, surely it is distortion, so do you think in this example, the 1K would have significant distortion reduction result?
apart from adding a resistor in cathode, what other approach would you use to reduce the harsh high, if no loop NFB allowed
Follow Ups:
"In not allowing loop NFB, I always find a simple RC voltage amplify stage necessarily swinging around 140V P-P, creates certain harsh sound in mid high"
The Chinese amp you're working on is not representative of quality design. You can't draw generalized conclusions like this from its performance.
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Buy Chinese. Bury freedom.
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> > What makes you think the harshness is coming from this circuit?
Except for the interstage (originally in the amp), all other parts are warm sounding, I rebuilt the entire circuit without loop feedback, I attach here, so I still think the harshness may come from tthe 1st or 2nd VAS stage, 300B and 845 seem typical
In 2nd stage Ecc99, I insert 200R for degeneration, attempting to reduce distortion, but current result still have certain harshness
IMO you might do yourself a favor and get a good simulation program and drop this circuit with these values into it. Then take a look at the distortion that results. Go for a distortion signature that favors the lower ordered harmonics (2nd, 3rd and 4th) and minimizes the higher orders.
You'll find that by varying some of the values (cathode values in particular) that you can alter the distortion quite a lot. I would not be changing any plate resistances without changing the cathode values as well. You'll also find that there are optimal operating points for each tube type. Sometimes the current required might be more than you really need, so you'll have to adjust the plate resistors higher and adjust the cathode values accordingly.
Simulation is awfully handy for this sort of thing! The way you seem to be going about it is unlikely to get results IMO.
Once that is done then you can play with the fancy parts!
Hi Ralph,
I am just wondering if increasing higher resistor like 360R instead of 200R would further help to remove harshness.
I only have left hand to work due to the stroke. So every modification I want think about expected outcome before I move this heary beast.
A quick estimate is that the negative feedback is only 3dB, so it's not what I would call "significant", though quite possibly audible.
The other way to reduce driver distortion is to use a high plate load resistance. The 50K resistor is about 10 times the plate resistance, assuming the plate current is around 3mA (an even cruder guess). That's fairly high already. A current source load would help, but not very much.
> > The 50K resistor is about 10 times the plate resistance
The cathode has 1K unbypassed, the effective plate resistance is nearly 20K
How do you calculate the 3dB feedback when 1K unbypassed in a 5687?
My remarks on plate load were intended as an alternative to an unbypassed cathode resistance - sorry I wasn't clear on that.
For the feedback, I estimated the voltages and currents based on the resistances and the 400v supply, then calculated the gain with and without the unbypassed resistance.
Incidentally, the current is very low, maybe 3mA for a 5687 with ca. 150v plate to cathode. So that triode is operating in a relatively high distortion area, perhaps to obtain some distortion cancellation? The 1K resistance could possibly be for fine-tuning that.
Or maybe the designer just tried a lot of things until it sounded right to them.... :^)
Hi Paul,
I saw someone said degeneration work really good, from Allen
That is why I want to try on the Ecc99
Allen Wright was genius and I agree with his approach... personally the 5687 is one of my favorite driver tubes... I have never ran that tube @ 3 ma.... usually run it @ 20 ma. The tube is very powerful, Eric.
The Mind has No Firewall~ U.S. Army War College.
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