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Old 23rd May 2006, 09:43 AM   #1
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Question What are the diferences between these ways of output?

Hey hey !

I'm looking for a tube buffer for my GainClone, so I started to search about it... I understood how tube works and everything, but I don't know yet the diferences between these two ways of output.

In the first pic I get the output signal from the Plate, and the Second from the Cathode!


Should I use the same way of output for Inverted or Non Inverted amplifier?



I need also some sugestions of tube buffers! I see tube buffers that work since 24v to 160v, what are the diferences in sound?



Thank you a lot !

edit: Please, read my other post above!

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Old 23rd May 2006, 09:44 AM   #2
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Second picture !
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Old 23rd May 2006, 09:48 AM   #3
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Take a look at SY's Heretical Linestage

The Musical Fidelity X10D schema + psu is also somewhere in the forum, if you like low voltage.
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Old 23rd May 2006, 09:57 AM   #4
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Heh ! No problem with High Voltage, it's a buffer, so I'll not need a big transformer to get a high voltage, it's low current, right? I just want a cheap buffer that can sound well !


I'll will drive a BPA200 amplifier (LM3886 "200W" power amp), WITHOUT it's buffer. It have already a buffer in the manufactures schematic, because due the four CI's to be driven, the imput impedance is relatively low.


Thanks !!
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Old 23rd May 2006, 10:03 AM   #5
arnoldc is offline arnoldc  Philippines
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Dang, I didn't answer your question

The first one is the Grounded Cathode, which depending on the tube used, will give you gain, and phase inverts. You may want to look for the 12B4 linestage which some of us built.

The second one is the Cathode Follower and will not provide gain regardless of tube used, and does not phase invert. Decware has one based on 6N1P which you may want to check out.

Let's wait til the experts chime in to this topic
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Old 23rd May 2006, 10:30 AM   #6
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Quote:
Let's wait til the experts chime in to this topic
While we're waiting, I thought I'd just add my 2c. The second circuit, the cathode follower, has a much lower output impedance than the first circuit - maybe around 1 or 2k, depending on the tube. A cathode follower is useful to feed an interconnect cable.
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Old 23rd May 2006, 11:19 AM   #7
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Hi Nando

What are you expecting from the tube buffer? If you want 'tube' sound SY herethical line stage (buffer) won't work - search the thread and look at the distortion figures... Than it may be better to use an ECC88 with low voltage - a la Musical Fidelity Cans. Or Joe Rasmussen's buffer with ECC88.

Erik
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Old 23rd May 2006, 04:42 PM   #8
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Arnoldc ! Thank you for answering my question !

My idea is replace the buffer U5 in that attached schematic with the tube, so I go for non inverted. The gain can be one, like the U5, but if I can get a little more will be nice, two is fine!

The tube have to be the less expensive possible ! I shouldn't spend money on this project, I have others priorities heh ! BUT I'll spend some for sure ! Case, transformers and PCB are all already done and mounted! Today I'll buy the capacitors and resistors and put this thing to work !


I do not expect "sound tube" from the amplifier, I don't believe that chips can have this sound just because of a little tube in the middle of circuit... But, I expect more spaced sound, with less saturation ! Should I expect it? Not THAT sound, just some improvement.





Thank for all of you !!
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Old 23rd May 2006, 05:40 PM   #9
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Hi there,

The second schematic, the cathode follower, will be a better choice. I think impedance is even lower than 1K. It is some fraction of the cathode resistance, if memory serves. Cathode resistances are usually some 300 to 600 ohms (1/transconductance).

Anyway, in this instance, you will probably get a warmer sound, simply because it is a tube.


Then, because feedback is considered to be a little over 100%, you would technically get no distortion.

So, you can concievably replace your U5 with the tube. But use the follower.

A buffer is defined as an amplifier with no voltage gain, but some current gain. So the cathode follower can take a 3-4 volts input at 0.05 milliamps and put out a 3-4 volt signal at 1-2 mA. In the case of the cathode follower, it has a slightly negative gain, which means if you input 2 volts, the output might be 1.8 volts.

Now, I would suggest using a 12AU7 or 12AT7, since you can set them up with a higher current output. They match your situation better.

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Old 24th May 2006, 12:42 AM   #10
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Hi there Nando,

I have a slightly different concern!

The circuit in your post #8: You have a very high amplifier input impedance as a result of the input voltage follower, so why "destroy" that with a low 10K input resistor. More serious for me is the input electrolytic capacitor (unless you use a relatively expensive polypropylene). I do not tend to get frenetic about the use of electrolytics as some, but a polyester is better here, and there is no reason (at least that I can see) for not using e.g. a 100K/330nF or even 220K/150nF. Would you still need a buffer then?

Secondly the cathode follower is better as already said, but I am a little worried about the low (35V) anode supply voltage quoted. I did not calculate, but it looks suspiciously low to put you in the high 2nd harmonic distortion operating area. (OK, you have full negative feedback; still.) If voltage is not a problem, for triodes I would personally prefer that to be over 100V.

But my first paragraph stands.

Regards.
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