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Old 16th March 2004, 06:26 PM   #1
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Default buffer circuits

What does a buffer circuit do?? Sorry for the newbie question.. tia

Dave
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Old 16th March 2004, 07:40 PM   #2
SY is offline SY  United States
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It isolates one circuit from another. Normally, a buffer is something with unity voltage gain, a high input impedance, and a low output impedance. One very common example is a cathode follower. HTH
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Old 17th March 2004, 02:17 AM   #3
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Default buffer circuit

Ok, so does a buffer circuit increase the strength of the signal from the preamp to the amp, as compared to a completely passive preamp? (due to the low output impedance).. Also, what is unity voltage gain? Is that an increase in the output of the preamp voltage? So, ultimately what does the buffer circuit do for the sound? I have seen a few preamp projects on this forum which are very simple (like the thread a little below this one called 12B4A linestage)- they consist of a volume control, input, and output, plus a buffer circuit based on the 12B4A tube, which is supposed to be very neutral and detailed with little coloration. How would something like this compare to the sound of a full-blown preamp, which has various gain stages? Thanks a bunch

Dave
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Old 17th March 2004, 10:30 PM   #4
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Hi,

Quote:
Ok, so does a buffer circuit increase the strength of the signal from the preamp to the amp, as compared to a completely passive preamp? (due to the low output impedance)..
Compared to a passive preamp which is just a volumecontrol, a buffer behind it transforms the output impedance seen by the next stage to a much lower value making it easier to transmit the signal correctly as the input capacitance of the pot and interconnect cable are not seen by the input of the amplifier, it is essentially isolated from the following stage.

Quote:
Also, what is unity voltage gain?
Basically this is a 1:1 relationship, what goes in goes out. In reality the "gain" is slightly less as the tube buffer will present a small insertion loss, .95 to .98 at best.

In practice this is nothing to worry about as the previous stage, mostly a CDP, will have ample gain.

Quote:
How would something like this compare to the sound of a full-blown preamp, which has various gain stages?
Even the 12B4A preamp has more gain than you'll ever need from a line preamp. The 12B4A being a very linear tube you have every opportunity there to make one of the best preamps ever, bar none.
Naturally the endresult will depend on the execution of the design, PS and passive components used.

Hope this helps,
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Old 18th March 2004, 02:29 AM   #5
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Default power supply

oh you need a power supply to use a buffer circuit? That's not good.. i have no experience with making any of these, and didn't notice it in the schematic for the 12b4a line stage right away

Dave
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Old 3rd November 2004, 05:44 PM   #6
Tyimo is offline Tyimo  Hungary
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Default ECC85 or ECC88?

Hi!
Could somebody give me some info about the attached schem: could I use it for tube buffer? Is it possible to use the circuit with ECC88 tube too? What would be the lowest voltage for?
Thanks

Tyimo
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Old 3rd November 2004, 07:21 PM   #7
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Default Re: ECC85 or ECC88?

Quote:
Originally posted by Tyimo
...could I use it for tube buffer? ...

Tyimo
Hi,

The question should be: why and what for do you need a buffer ? The schematic of the stage you have attached is usually used as a voltage gain stage. It requires more supply voltage than usual CF (cathode follower) and is more difficult to implement than CF. It is like active load in plate of the lower tube (constant current source).

You can read a lot on the stage here:
SRPP Deconstructed

... and on buffers:
Power buffers @ tubecad
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Old 4th November 2004, 04:09 PM   #8
Tyimo is offline Tyimo  Hungary
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Hi Yagoolar!
Thanks for your answer!
I would like to use a buffer between the CD and a SS amps (Aleph5, JLH, Hiraga) to get "warmer" tube sound!
I am looking for a simple, but effective tube buffer circuit, possible with ECC88 and on low voltage. I found that ECC85 circuit somewhere in the forum, but I forgot where. (The schematic was colled:tube buffer ECC85.)

Tyimo
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Old 20th March 2005, 02:06 PM   #9
Tyimo is offline Tyimo  Hungary
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Default need more info

Hi!

I have this schematic somewhere from the forum, but I forget already :-)) from which thread. Could somebody give me more info abaout it? Is it a buffer or a preamp?

Thanks

Tyimo
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Old 20th March 2005, 03:32 PM   #10
sajti is offline sajti  Hungary
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Default Re: need more info

Quote:
Originally posted by Tyimo
Hi!

I have this schematic somewhere from the forum, but I forget already :-)) from which thread. Could somebody give me more info abaout it? Is it a buffer or a preamp?

Thanks

Tyimo

Szia,

Yes, it's buffer, with gain about 1. But I think, that the lower part is not really necessary. Keep the upper, and use 22-27kohms cathode resistor to the -85V rail. The other half of the valve can handle the other channel. This cathode follower is simple, and universal. You can try some different valves, and You have to modify the heaters only.

Sajti
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