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Source follower IRF720 or DN2540
Source follower IRF720 or DN2540
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Old 7th April 2018, 12:42 PM   #61
triode_al is offline triode_al  Netherlands
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Location: Westland
I finally settled for the DN2540/53K and not for the IRF830/CCS.
[Just finnicky: the IRF830/CCS of 5mA had a slight ring that of course cannot be heard as above the transfer of the output transformer]
This combo with R load is very easy. Smooth edges.

I have a CCS for the driver: I soldered a 2SK30 (selected pairs for 3,9 mA) on the source and gate leads of the DN2540. That connects with 820 ohm to the gate. Yes, 2x12V zeners to source on the pins... (not before the gate stopper.) So slight delta to Ale's CCS schema.

IMG_0615.JPG

There is no slew rate limiting I can see when driving the tube to the limit. I have an Rk of 200K on the 300B; could go lower.

Bandwith to output depends on the current of the DN2540 CCS: with 1,3 mA it is 47K [With IRF830 also 46 KHz.]; with 4 mA it is 53K. Funny, I would have expected the output follower / 300B would dominate the picture. But the DN2540 input capacitance is evidently a factor.

Depending on the driver tube I get different amplification.
Amplification ( input/output 300B, Lundahl 1k8) is 21,3 dB for 5751 and 3,5 dB more for ECC83 - I like it lower and the 5751 has more spatiality.

Advantage of CCS on anode: decoupling the kathode of the driver is not needed, such a capacitor is a major source of slew rate limiting (undershoot up to 40% sag).
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SPU --> tube MC/RIAA;
Pass buffer -->300B SE + DN2540 CCS/Buffer; --> 165GMF in TQWT; working on a HYPEXed sub.
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Old 16th April 2018, 02:16 PM   #62
triode_al is offline triode_al  Netherlands
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Sounds great, depth of 300B single ended is indeed delivered. And the driver 5751 gives a slow decay of environmental sounds (the ECC83 is much more abrupt and gives a bit more very highs).
One strange thing: the anode current is 3,06 mA measured over the cathode resistor.
De CCS on the anode originally gives 3,95 mA (the Idss of the 2SK30). Where did that 1 mA go?
there is no leak to the gate of the DNS2540 of coursenor from cathode to heater of the driver. I tested those.
Can the deterioration of the CCS be due to the lack of a gate stopper on the DN2540 and 2SK30? (very HF, >1GHz)? Ale's diagram has those. But I have such a compact soldering . .
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SPU --> tube MC/RIAA;
Pass buffer -->300B SE + DN2540 CCS/Buffer; --> 165GMF in TQWT; working on a HYPEXed sub.
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Old 29th April 2018, 10:24 AM   #63
triode_al is offline triode_al  Netherlands
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I took two pictures of the driver
- with IRF830 (first) & an active load (DN2540);
- and (second) with DN2540 & resistor.
The first one has an incredible risetime, but that exposes the output transformer resonance that is about 176 kHz. (Lundahl 1k8); there is even a bit of crosstalk of that resonance through the earth as you can see.
The second one has a slower response but the resonance is less present.
For that reason I retained the second one.
Driver = 5751 tube in parallel.

IRF830 & active load:
IMG_0621.jpg

DN2540 & 43K:
IMG_0622.jpg

The square wave is at about 7 KHz

(I must say in apology there are the slightest differences in ground connection between the two mono chassis)
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SPU --> tube MC/RIAA;
Pass buffer -->300B SE + DN2540 CCS/Buffer; --> 165GMF in TQWT; working on a HYPEXed sub.
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