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I have a VAC Auricle preamp which uses 6 tubes. VAC told me that 3 of the tubes are in active gain stages (2 phono, 1 line) and 3 of the tubes are in buffer stages (2 line and 1 phono).
My question, is it a waste of NOS tubes to use them in a buffer stage? Should I save my money for tubes in active gain stages and just put inexpensive Chinese tubes in the buffer stage, or are the tubes in buffer stages important (audible) too?
TIA,
John
Follow Ups:
Ther good news is that if they are a buffer then you can use a tube that is a lower grade. It needs to be quiet of course. But not lowest in microphony. This means you can spend less $$$ on new old stock tubes. Assuming your tube dealer grades them as such.
"Ther good news is that if they are a buffer then you can use a tube that is a lower grade. It needs to be quiet of course. But not lowest in microphony."Maybe. Maybe not. A lot depends on the amount of excitation the tube receives, which is dependent on construction technique, physical layout, socket type/material, etc. when considering microphonics.
A "buffer" as is being described here is an impedance transformation device, a.k.a. a current amplifier; high input impedance, low output impedance. The cathode follower configuration has a gain of less than unity (1), and it has virtually 100% feedback. Any non-linearities in the tube tend to be reduced by the feedback, which in turn tends to "homogenize" any tonal differences. As a result tubes used in buffer stages sound much more alike than the very same tubes used in a voltage amp where the feedback is much less than in a cathode follower.
So buffer stage tubes are quite important - but it's different criteria that is important.
Edits: 04/07/12
I find it difficult to believe that so many believe the cathode follower stages are unimportant. The principle reason is that the power supplies feeding those stages are relatively ignored, leading to a homogenization of the sound, at least in my experience. It is also important to note that many current designs advertise zero feedback circuits..... Personally I find that all output tubes make a bigger sonic difference than the inputs, but of course YMMV
Stu
When you say "So buffer stage tubes are quite important - but it's different criteria that is important.", what criteria is it that is more important for a buffer stage tube?
TIA,
John
Transconductance, no doubt.
And for safe operation plate/screen current capacity, heater/cathode voltage limit, etc...
Would you say then that your "Driver" grade 12AX7's would be best suited for this task? Your website says for driver stage of an amp or buffer stage of a DAC. It doesn't mention the buffer stage of a preamp. Po-ta-toe, Pot-a-toe?
even in the 12A_7 group. A 12AY7 will sound better, as would a 12AT7, and IIRC both will give a lower Z out in CF mode.
While a 12AU7 has more current, the design has intrinsically higher distortion than other low-mu/high-gm twin triode novals. There are a few U7's that sound good enough Viz the Brima 13D5 (it's not there!), some NoS RCA's, and Aussie AWA and AWVs, but many just are not good enough for audio, IMO.
Note that a post in response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Changes in the buffer tubes will be audible but to a lesser degree than the gain stages. Usually the first tube in a preamp has the biggest effect on sound. This is even true for most power amps
Alan
Typically, one of the functions is to flavorize the signal so it sounds "better". Since buffer stages sometimes do more bad things sonically than good things, I'm not sure why these tubes aren't sonically important. I would think, just the opposite.
Please help me, here?
"You don't need to be a Weatherman to know which way the wind blows"
If Im not mistaken, my Ah Tjoeb 99 CDP uses the two tubes in buffer stage. I can tell you that there are definitely sonic differences between different tubes in that player. I think my older Jolida CDP (603) uses the tubes as pre's. I actually think that tube rolling in the Tjoeb has a larger sonic impact although they use different tubes, 6922 in the Tjoeb and 12AX7 in the Jolida. I also have a Dynaco tubed CDP which used 6922's. Not sure as buffers or otherwise. But tube rolling in that CDP probably has more impact than in the other two players. So maybe ... it just depends on the tube and the rest of the circuit. Atleast it seems that way to me.
Edits: 04/04/12
That the main function of a tube buffer is to help impedance mismatches. If this is true-- the buffer tubes are not really there to aid tonality. I know this "tube flavor" is a popular assumption, e.g. that all one need do is throw a tube into the signal path to give the final sound a "tube" character.
That may be true in part, but again, if my friend (who designed and built a commercially successful preamp) is right, the function has more to do with providing optimal conditions for the signal to travel through. I can't tell you why this is the case, I am afraid my knowledge is qualitative, at best.
...I have to say that I tend to agree with you. I got this much from Wikipedia: A buffer amplifier (sometimes simply called a buffer) is one that provides electrical impedance transformation from one circuit to another. Two main types of buffer exist: the voltage buffer and the current buffer.
So it would appear as if the buffer tube is not actually in the signal path. I think their purpose is to help match the input impedences with the active tube circuitry.
For Laudauum, I also have a CDP with a tube buffer stage. I always liked the sound of this player and it came with Siemens 6922's in it when I bought it, but I never experimented with different tubes here.
I think Kevin Deal's answer on this thread has been quite helpful. For now I think I will use some NOS Sylvania 12AX7's in the buffer stages at about $25/tube they seem reasonable enough. I'll save the Telefunkens and Bugle Boys for the tubes that are actually IN the signal path.
"So it would appear as if the buffer tube is not actually in the signal path"
That is incorrect. The buffer stage is definitely in the signal path.
Alan
In the case of a preamplifier or a CD player with a tubed output stage the "buffer" separates the voltage gain stage from the following stage. In the case of a CD player the following stage could be a preamp or integrated amp; in the case of a preamp the following stage will be the input of the amplifier. While the voltage amp stages amplify the signal, the "buffer" stage increases the current. In this case it is to drive the cables to the following device. As such, the buffer is definitely in the signal path; it is simply lowering the output impedance of the device making it more capable of delivering the signal through the cables to the next active device.
A cathode follower is the usual configuration for a tubed buffer although a transformer can be used just as in a tube amplifier. The cathode follower has a gain of approximately .9, so the tubes used would generally be those with low gain and high transconductance. A 12AX7 has neither, with a gain of 100 and very low current, it is not generally used as a cathode follower for obvious reasons; although it is occasionally used by some.
This would imply that in the VAC amp the 12AU7 is the "buffer". You will have to experiment to see if changing brands changes the sound but 12AU7s are cheap and plentiful so there is no reason not to. I don't believe that "low noise" tubes will buy you much in this position, but you definitely don't want microphonics.
However, VAC told me that the single 12AU7 in this preamp is the line gain stage. They also told me that it used (2) 12AX7's in the phono gain stage. The other (3) 12AX7's are used in buffer stages, (2) in the line stage and (1) in the phono stage.
So to re-cap, there are (5) 12AX7's and (1) 12AU7 in this preamp. (2) 12AX7's (Phono) and the 12AU7 (Line) are in active gain stages while (3) 12AX7's are used in buffer stages. VAC also stated that changing the 12AU7 would have the most significant sonic changes. (Un)fortunately, I have a LOT more 12AU7's to experiment with, but few 12AX7's. I do have some more 12AX7's on the way though.
Thanks again.
Interesting. Im not one to argue about things I dont fully understand but regardless of what their main function is, and Im not doubting your EE friend, tube rolling does change the sonics. Atleast in the CDP/CDP's that I have that use them as a "buffer" (Tjoeb and, I think, the Dynaco).
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