The basic assumption with a differential output stage is that it can only work in pure class "A". This is logical and what it is designed to do. If you need class AB1 or AB2 don't try to implement a differential.
Shoog
Johan Potgieter said:
Johan & Shoog,
Thanks for your responses.
At the moment I have been drawing up some ideas. I have long thought that since I like diff pairs anyway, then the output stage should conform to this ideal. Last night's effort involves three stages with each on a cascode CCS. The output stage (triode strapped YL1071s) will pass 180mA.
To save money, the idea is an isolation transformer to provide about 320V HT and a smaller toroid to give -80V, thus providing (including bias) about 340V for the output stage and a nice -ve rail for the other sinks.
The amplifier is not otherwise very interesting. I might try though to re-draw it to be all DC-coupled, but that will take a bit of time as I am a little "rusty"!
7N7
I tried three versions with 8 V RMS output @ 8 ohms load:
1.) 560 ohms resistors in each cathode. k = 1.45%
2.) 560 ohms with 470 uF in each cathode. k = 0.26%
3.) 50 mA current generator (LM317) with 470 uF in each cathode. k = 1.22%
So the best option is what Shoog advised. I don't understand why the CCS gave higher distortion than a simple resistor.
1.) 560 ohms resistors in each cathode. k = 1.45%
2.) 560 ohms with 470 uF in each cathode. k = 0.26%
3.) 50 mA current generator (LM317) with 470 uF in each cathode. k = 1.22%
So the best option is what Shoog advised. I don't understand why the CCS gave higher distortion than a simple resistor.
Seems like the capacitor is the dominate distortion generator.
Have you tried it with film bypasses. Its also a good idea to place a film cap directly between the two PP cathodes.
Since you seem to have a better measurement setup than me, I would be interested if you could lash up the differential setup and give us a distortion measurement.
Shoog
EDIT: I just thought - the simple resistor is introducing degenerative feedback, and so linearising the valves output. The CCS has no degenerative feedback so it is bound to have slightly higher distortion. The cap probably has very little to do with it.
Shoog said:
The distortion was lower with bypassing capacitor, perhaps because of the higher voltage gain that needed lower driving voltage. In unbypassed configuration the driving voltage was 53 V RMS, with bypassed it was 26 V (for the same 8 V output).
My measurements were done with 20 uF polystyrene capacitors (WIMA MKS-4) bypassing ROE 470 uF/63V electrolytic caps.
I will try the differential setup, but I need to modify the circuit a bit. Now I have a center tapped secondary winding, and the lower ends of the bias resistors (or CCS) are connected to each end of the secondary (to the speaker terminals) cross-coupled. This is approx a 1.5 dB negative feedback. Since it does not do much, I will try without it.
I did not try the CCS without bypassing. Will I get any gain?
yagoolar said:
No, I was agreeing with you - for example as far as I am aware, a resistor does not have "gain" as I understand it , but I know that it can introduce distortion!
7N7
oshifis said:
A complementary symmetry emitter follower output stage has no [voltage] gain. There are lots of people that would be happy to know that these produce no distortion. 🙂
As a valve ages, a ccs would adjust the bias to ensure the cathode current remains constant as the cathode material gradually wears out. This would have to be a good thing.
yagoolar, oshifis is using a separate CCS under each tube. If those CCS's were unbypassed then there could be no AC signal current through any of the tubes. The current would be constant DC no matter what signal was applied to the grids.
The voltage at the cathodes would respond to signal at the grids (cathode followers!) but there would be no AC signal current at the plates for the output transformer to respond to; there would be zero output at the speaker.
-- Dave
The voltage at the cathodes would respond to signal at the grids (cathode followers!) but there would be no AC signal current at the plates for the output transformer to respond to; there would be zero output at the speaker.
-- Dave
- Status
- Not open for further replies.