Buffered Passive and 12AX7s

[Shadowfax]

Hey all, I don't totally understand just how my VAC pre works. It uses 2 12ax7s and 2 12au7s in normal and extra gain modes. But when in the Buffered Passive mode, it just uses the 12ax7s.

My understanding of Passive, is volume control only so I am unsure what is going on in the Buffered Passive mode.

I am asking because I was thinking of rolling some 12ax7s because I do find that I use the buffered mode at night because it gives me more control over lower volume levels. I enjoy the sound but was wondering if rolling the tubes will change anything or are the tubes used in a different way in this mode?

I am currently tubed with what Keven Hayes puts in his new units. The AUs are JAN Phillips NOS, and the AXs are supposed to be a Chinese NOS but are not really marked with much.

Anyway, if anyone feels I can boost the sound quality by rolling some 12ax7s, what are your favs and why.

If anyone can clearly tell me what the pre is doing in the buffered mode, that would be great.

Pre is the VAC CLA1 MKII with the outboard PS.

Thanks!

 

[Face]

Buffered mode=no gain, but makes it more capable of driving a power amp. Someone feel free to correct me if I'm wrong, but sometimes pure passive(unbuffered) preamps have a difficult time driving amps with a low input impedance.

 

[roncagg]

My understanding is that without the buffer (so it is just a volume control) then input impedance = output impedance, so it won't be ideal on both ends (ideally high on one, low on the other). I don't know how tubes achieve this (my Pass buffer uses jfets, resistors and caps). I also have a niteshade passive that didn't match my amp on paper but still sounded good (I like the Pass better, but is that because I built it? Perhaps...)

 

[Shadowfax]

Thanks for the input, gentlemen. I am unsure what the tube is doing in that mode also. Rolling tubes in that spot may or may not have as much effect as rolling the AU7s in normal gain mode. I guess I will have to drop a few of my old Sovteks in and see if it sounds worse or just different. I was hoping for something technical to back it up tho.

 

[Face]

Brian,

The signal is still passing through the tubes, so it will make a difference.

Ron,

The B1 does sound nice. If you want to go all out, throw a Goldpoint attenuator in there and check out Claritycap's MR line.

 

[Shadowfax]

Brian,

The signal is still passing through the tubes, so it will make a difference.

Ron,

The B1 does sound nice. If you want to go all out, throw a Goldpoint attenuator in there and check out Claritycap's MR line.

Not sure what the B1 is and correct me if wrong, you are suggesting a re-cap of the VAC? I have thought about having that done. RHB SounDezign is practically in my neighborhood and they do some nice work. They use V-Caps and custom circuitry in their mods. I suppose just a recap would give some goon bennies at the age of this pre, is that what you are saying?

I'm not ready to do away with the VAC and get an all passive unit but I have been following the threads on the subject.

 

[Shadowfax]

Found the Manual on line and this is what it says is happening. So it does sound like rolling the 12ax7s will change the sound in both modes.

Operation - Line Stage Section
The line stage can be operated in two basic modes, Active and Passive-Buffered. The active mode
may be of high gain or moderate gain. These three options are selected by a rotary switch on the
top of the chassis on the right side. Mute the preamp before operating this switch.

In the Passive-Buffered mode the amplifying triode (V4) is bypassed, and the controls are fed
directly to a Class A impedance translating Class A triode tube (V5). Since the output is buffered,
there are no gross cable interaction problems of the type encountered with passive control centers;
since there is no gain stage, this mode has ‘unity gain’. The VAC Passive-Buffered mode is
capable of driving long cable runs and the preamplifier may be located some distance from the
power amplifiers.

Since the Passive-Buffered mode has zero gain, you may need to turn the volume controls well
clockwise...the exact position depends on the output level from your source, and the sensitivity of
your power amplifiers and loudspeakers. Many audiophiles have the belief that it is never good to
run volume controls wide open. With a circuit of this type, however, it is actually preferable to do
so. Don't worry about where the control is set, just be concerned with how it sounds. The Passive-
Buffered mode does not invert absolute phase with respect to the line inputs.

The Active modes add a Class A amplifying triode tube to each channel, and causes the main
outputs to have inverted absolute polarity with respect to the line inputs. To correct for this you may
invert the polarity between your power amplifier and loudspeakers. In most systems, the Active
mode retrieves more ambient and reverb information.

 

[Face]

Not sure what the B1 is and correct me if wrong, you are suggesting a re-cap of the VAC? I have thought about having that done. RHB SounDezign is practically in my neighborhood and they do some nice work. They use V-Caps and custom circuitry in their mods. I suppose just a recap would give some goon bennies at the age of this pre, is that what you are saying?

I'm not ready to do away with the VAC and get an all passive unit but I have been following the threads on the subject.

No, I was making a suggestion to Ron(roncagg).