If the sensitivity of the main amps is high why not go the passive route?Andersen said:
A remote controlled alps pot passive is not bad at all.Slightly better would be a DACT stepped attenuator type.Then for some more money a TVC can also give you 6db of gain.
If you must go active the Aikido is probably the most transparent tube pre out there.I have tested this vs passive pre´s (inserting it between a very low output impedance dac and a very sensitive high impedance amp) and it is very hard to choose between the passive and the aikido.
I don´t think higher praise can be given to an active pre.
To my mind almost all active stages subtract some transparency so unless I need the gain or there is a problem with impedance matching I usually prefer passive.The aikido may be the exception.
Shoog said:
Yeah, it's quite hard and costly to get a good paper in oil cap, esp. at that high value i.e. 20uf to 40uf.
Actually, my first idea is to build an ultrapath pre-amp. However, I have to wait for the ultra capacitors until this June when my friend in London brings them back for me. That's why I try to build another at this moment to gain more experience in this diy world.
The schematic below will deliver far better performance in my opinion than what you proposed. Spend the money on good caps,resistors, and stepped control. Loose the Tango and the GZ34. Use a common 5y3 or something similar that you may have on hand. The performance of this schematic by "Frank" is well above the run of the mill. Yes, I built one and its one of my favorite line stage. You can instal a switch and cathode bypass cap if you choose to do so. I put one in mine.
You could still use the GZ34 rectifier but there is no real need to do so as I asked Frank this question about cathode stripping in this instance and he told me "It will be ok just don't apply signal until its warmed up." This circuit will drive what you have in mind without a problem.
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Just for the sake of argument 
I'll respectfully disagree. I think the ultrapath is the much better circuit. I've never met a cathode follower I liked, and I'd prefer the one cap and one transformer in the signal path to the 4 caps (don't forget the last power supply cap) and gobs of NFB that this circuit uses. Plus, the gain here is too high. This circuit only makes sense if your power amp has an insufficient driver.
And what's the argument against the GZ34? My experience (with new production anyhow) is that they sound kind of flat, so I'd lose it for that reason, but new ones are $8, so why does it matter?
I'll respectfully disagree. I think the ultrapath is the much better circuit. I've never met a cathode follower I liked, and I'd prefer the one cap and one transformer in the signal path to the 4 caps (don't forget the last power supply cap) and gobs of NFB that this circuit uses. Plus, the gain here is too high. This circuit only makes sense if your power amp has an insufficient driver.And what's the argument against the GZ34? My experience (with new production anyhow) is that they sound kind of flat, so I'd lose it for that reason, but new ones are $8, so why does it matter?
hey-Hey!!!,
With a 6SN7 as the amplifier triode, something has to be done to bring the circuit's output Z to a reasonable level. There's a few ways to do this and many opinions surrounding each of them. Even a 12B4 can benefit from some sort of output buffer.
The compromise required of the rest of the amp if one runs straight off the plate of a 6SN7 are worse than the benefits from simplicity.
As to rectifier, try a pair of 6CB3's. They'll run circles around a good GZ34 and at an order of magnitude lower price. Then there's Mercury vapour to consider. It is good for an amp, and it is good for a pre. Takes a bit more doing than the others so the warm-up reuirement can be dealt with.
Most important is avoiding the linestage-sized power supply. It won't make much difference if you put a stellar circuit behind a wimpy PS.
As far as a best circuit...I will stick with my TV sweep pentode. Shunt-regulated g2 supply and a puny plate resistor( 750R ). It is making more heat than a SE 45 amp...
cheers,
Douglas
With a 6SN7 as the amplifier triode, something has to be done to bring the circuit's output Z to a reasonable level. There's a few ways to do this and many opinions surrounding each of them. Even a 12B4 can benefit from some sort of output buffer.
The compromise required of the rest of the amp if one runs straight off the plate of a 6SN7 are worse than the benefits from simplicity.
As to rectifier, try a pair of 6CB3's. They'll run circles around a good GZ34 and at an order of magnitude lower price. Then there's Mercury vapour to consider. It is good for an amp, and it is good for a pre. Takes a bit more doing than the others so the warm-up reuirement can be dealt with.
Most important is avoiding the linestage-sized power supply. It won't make much difference if you put a stellar circuit behind a wimpy PS.
As far as a best circuit...I will stick with my TV sweep pentode. Shunt-regulated g2 supply and a puny plate resistor( 750R ). It is making more heat than a SE 45 amp...
cheers,
Douglas
jeapel said:
I'm sure others are more qualified than me. I've built both and both worked well, but they had different tubes and different power supplies, so I can't offer a direct comparison. This might be helpful, though: http://www.nutshellhifi.com/library/ETF.html
-d
Ultrapath has bias current DC + signal AC going through the
transformer. Parafeed on has signal AC going through the transformer.
A DC blocking capacitor keeps the parafeed transformer free of DC.
Ultrapath requires a gapped transformer to account for the magnetization of the core consumed by the idling DC bias current
of the tube. Parafeed allows you to use an ungapped transformer.
The advantage to an ungapped transformer is that the core can be
smaller, and the winding volume thus reduced which minimizes
leakage capacitances and inductances. Further, with PF, because you
are not using a gap, it becomes affordable to use better core materials ; for example, Amorphous or Nickel or Cobalt rather than M6 steel.
Ultrapath substantially reduces any power supply noise. Ultrapath
doesnt let you effectively go after smaller/better transformers. Rather, its a technique to get some power supply ripple rejection.
For parafeed, its a matter of religious beliefs about how you feel
about your signal crossing a big oil blocking cap. In general,
parafeed sounds WAY good - with the right magnetics, in the
right circuit.
Finally, there is PP... Where if you do it right, not only do you get
very good ripple reduction, but you also effectively get AC signal riding the core and no DC.
-- Jim
transformer. Parafeed on has signal AC going through the transformer.
A DC blocking capacitor keeps the parafeed transformer free of DC.
Ultrapath requires a gapped transformer to account for the magnetization of the core consumed by the idling DC bias current
of the tube. Parafeed allows you to use an ungapped transformer.
The advantage to an ungapped transformer is that the core can be
smaller, and the winding volume thus reduced which minimizes
leakage capacitances and inductances. Further, with PF, because you
are not using a gap, it becomes affordable to use better core materials ; for example, Amorphous or Nickel or Cobalt rather than M6 steel.
Ultrapath substantially reduces any power supply noise. Ultrapath
doesnt let you effectively go after smaller/better transformers. Rather, its a technique to get some power supply ripple rejection.
For parafeed, its a matter of religious beliefs about how you feel
about your signal crossing a big oil blocking cap. In general,
parafeed sounds WAY good - with the right magnetics, in the
right circuit.
Finally, there is PP... Where if you do it right, not only do you get
very good ripple reduction, but you also effectively get AC signal riding the core and no DC.
-- Jim
I've just completed my pre-amp following the schematics that I posted with some changes as attached (note: I have not used any cathode capacitors). I'm quite satisfied with the results except that the bass is a bit hard and I need to make it softer.
During building this, I noted that the lower the B+ and the anode voltage at 1st stage (Va1) were, the more satisfied results I got. However, I stopped at B+ = 220V and Va1 = 95V and dare not to lower them. Can you guys please advise me what I need to do now e.g. keep/lower B+, keep/lower Va1, increase/decrease the value of cathod resistors and etc. One more question: what are the most appropriate voltages at cathods at each stage.
Thanks a lot.
During building this, I noted that the lower the B+ and the anode voltage at 1st stage (Va1) were, the more satisfied results I got. However, I stopped at B+ = 220V and Va1 = 95V and dare not to lower them. Can you guys please advise me what I need to do now e.g. keep/lower B+, keep/lower Va1, increase/decrease the value of cathod resistors and etc. One more question: what are the most appropriate voltages at cathods at each stage.
Thanks a lot.
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