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Suggestion for a tube buffer
Suggestion for a tube buffer
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Old 9th January 2018, 03:03 PM   #21
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by DF96 View Post
Your diagram shows a decoupling cap at the junction of the 220k and 68k. Assuming an appropriate value for this, the 51k only applies at DC and subsonic frequencies.
Headslap-to-self. Of course. OK, "no 51 kΩ add-in" That decoupling cap axes the value. Thanks! GoatGuy
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Old 9th January 2018, 03:49 PM   #22
indaco is offline indaco
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Quote:
Originally Posted by DF96 View Post
The diagram says medium to high mu triode. For good performance as a cathode follower you need high gm, not high mu. For use with lowish supply rails and lowish value load resistances (as seen in that circuit) you need lowish mu.


Where does the 51k come from?
About what you say, is that diagram not suitable for a buffer with ECC88 tube?
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Old 9th January 2018, 03:54 PM   #23
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by kodabmx View Post
I use this design myself. Active loaded CF Buffer. Adjust the 1u cap to suit your load and desired LF cutoff.
That is a nice design. The only real reason I chose "1 valve" is because the OP seemed interested in something simple and low in tube count.

It hardly remains simple when one takes the time to add components that allow "it" to work safely in stand-alone who-knows-how-it'll-be-connected-in-the-future mode. See diagram attached.

GoatGuy
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File Type: jpg simple_buffer.jpg (105.7 KB, 168 views)
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Old 9th January 2018, 04:01 PM   #24
stocktrader200 is offline stocktrader200  Canada
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C3 should be no bigger then 3.3uf which provides a low pass of 20Hz at a load of 2500 ohms.
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Old 9th January 2018, 04:02 PM   #25
DF96 is offline DF96  England
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Quote:
Originally Posted by indaco
About what you say, is that diagram not suitable for a buffer with ECC88 tube?
The ECC88 has very high gm and mid-range mu so should make a good cathode follower. It is designed to work well at lowish voltages.

Last edited by DF96; 9th January 2018 at 04:02 PM. Reason: confused!
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Old 9th January 2018, 04:54 PM   #26
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by stocktrader₂₀₀ View Post
C₃ should be no bigger then 3.3uf which provides a low pass of 20Hz at a load of 2500 Ω.
It was selected to give –1 dB at 600 Ω load; if 2,500 Ω load is the target, then of course you're right. It just depends on the load. GoatGuy
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Old 9th January 2018, 06:01 PM   #27
indaco is offline indaco
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My idea would be to set it in in the first stage of the amp (gainclone), other thing setting it before my headphone amp, with <10Kohm impedance
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Old 9th January 2018, 06:19 PM   #28
GoatGuy is offline GoatGuy  United States
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OK. Then use a polypropylene, film capacitors. Key is 250+ VDC and ≥ 1 μF for a GainClone type chip amp. Chip amps tend to have 10 kΩ input impedance.

Representative, for $1.90 ea: DPM2W1K-F Cornell Dubilier - CDE | Mouser as an example of an excellent capacitor for a darn-good price.

(But note: as soon as I say excellent, out from the closet will come a bunch of folk who will rebut this and offer no end of even better capacitors. Its up to you.)

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Old 10th January 2018, 12:05 AM   #29
campsquire is offline campsquire  United States
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I have a four year old hybrid amp design that uses a tube front end that provides all the signal voltage amplification. It's capacitively coupled to class AB solid state stage with a gain of 1. I call this SS stage the "tube buffer". It can deliver 70 watts of power with modest heatsink. It uses complimentary darlington pair transistors in the output driven by some high voltage albeit smaller driver transistors. The output stage has discrete current sense and protection - quite indistuctible. There are a total of 7 transistors in the tube buffer. There is no global feedback used. Oscillation in the output is prevented with a single cap in the output bias section.
This approach eliminates that large/heavy/expensive tube output transformer.
I started out using the 6SN7 for tubes but found it to be very microphonic. This amp is a guitar combo amp. (I like warm clean, not distorted). I changed from the 6SN7 to the 6SL7 which is similar but has higher gain and is way less microphonic. I did not detect a change in the sound quality. I suppose if your amp is seperate from your speaker, microphonics isn't much of a problem.
I found my B+ couldn't be much less than 200VDC in order to get enough excusion to create 70 watts RMS into 8 ohms. Even then I had to use a "White Cathode follower" as the last tube stage in order to get the most swing out of 200VDC B+.
I'd be happy to pass along the design to any individual.
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Old 10th January 2018, 09:08 AM   #30
indaco is offline indaco
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Hi campsquire, from what you say you coupled SS + tube that would be the opposite that I intend to. In fact my aim is to add some "tuby taste" before the amp stage, I don't care about high impedance source like guitar, etc. I'll never link in the chain. Anyway your contribuition wuold be appreciated, just as to compare with the others ideas. Thanks, Indaco
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