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In Reply to: RE: Level 5 2A3 SE Design posted by tubejack on October 28, 2015 at 08:09:22
The other posters have hinted at a fundamental question. Do you want a HIFI amp or an effects processor? The 2A3 is linear. IMO, you should exploit that linearity.
That design can be a starting point, in obtaining HIFI behavior. After all, "simple" RC coupled circuitry is being used.
"Full wave" voltage doubling the O/P of a Triad N-77U isolation trafo, as was done in the original "El Cheapo", will inexpensively yield a 200+ mA. B+ rail that will not sag, when playing bass. :>) Use high PIV Schottky diodes for a switching noise free rail.
As previously indicated, 2.5 Kohm primary O/P "iron" is appropriate in combination with a "300" V. B+ rail.
A single high mu/high RP twin triode (6SL7, 12AX7, 5751, 7F7 ...) will handle voltage amplification for both channels, if DC coupled ZVN0545A source follower buffers are employed. Constant current (CCS) loading can be employed to maximize both gain and linearity.
Eli D.
Follow Ups:
"A single high mu...."
Just wondering why one would need a high mu tube? High gain?
If the 2a3 were operated at text book operating points, biased at about -45 volts, then a gain of 20 (6sn7 CCS loaded) would give 56.4 volts peak at the grid of the 2a3 from a typical CD player with 2vrms output.
This means the musical peaks on the CD would more than drive the output tube to full power.
Is there any reason to do more than that?
2vrms is 2.82 volts peak, 2.82 times 20 is 56.4 volts peak.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It all pencils out well until someone buys some weird HOMC cartridge that's like 2.5mV and runs into a MM phono stage, then has like 900mV of signal to play around with.
Or there's the other problem where one pairs a little 2A3 amp with highly inefficient speakers, then the amp "doesn't have enough gain" because it won't play loudly from the headphone jack on a cell phone (which is 300mV).
I do agree, however, that 2.5V input sensitivity on a power amp seems perfectly reasonable, but isn't really so reasonable for an integrated amp. The OP hasn't said enough specifically to know if this will be a problem.
I was not addressing the correctness of the gain structure. I was addressing the (IMO) excessive bottle count and mediocre drive capability.
Even those wimpy twin triodes easily drive the tiny capacitances of the ZVN0545A. The little FET can yield acceptable transconductance and the very light load it presents makes for voltage gain block linearity.
Eli D.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
There's more to it than just gain.
What other factors apart from gain? Do you mean the current passed in the driver tube for instance?
Thinking of another example: 4P1L into PSE 4P1L. To quote the same calculations:
"If the PSE 4P1L were biased at about -18 volts, then a gain of 9 (4P1L CCS loaded) would give 25.4 volts peak at the grid of the PSE 4P1L from a typical CD player with 2vrms output. This means the musical peaks on the CD would more than drive the output tube to full power. 2vrms is 2.82 volts peak, 2.82 times 9 is 25.4 volts peak."
The difference here would be that the 4P1L could operate at 25mA - much more than a 6SN7. Theoretical gain of 2a3 x 6SN7 is 4.2 x 20 = 90.
Theoretical gain of 4P1L x PSE 4P1L is 9 x 9 = 81
Anode dissipation of 2a3 and PSE 4P1L is very similar.
Yes, but still there is more. See my post below.
Of course there is.
Just to list a couple of the other things a driver stage must do.
1. The driver stage has to be able to fully drive the Miler capacitance of the output tube so the output impedance and the drive current need to meet specs.
2. The driver stage should be very linear, after all the output stage is very linear.
I suppose I could go on but what did you have in mind?
Is there something specific, that needs to be done, that a CCS loaded 6sn7 drive stage would not accomplish?
Thanks.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I have really enjoyed the sound of my 6sn7s loaded with CCS. To my ears the the high end improves. If the 6sn7 has a negative, it can provide too much second order in a conventional loading configuration. This valve will always be a favorite. I have another 6sn7 plate follower line stage. It out performs my 5687 based line stage CCS loaded in most cases. I think I can fix that...though
Stuben
HI Stuben.
which CCS did you use on your 6SN7 plate ?
cheers
LT
Hey Lovetube..
I used the Coscoded mosfet kit that K&K offers...probably should work out a home grown, but these guys are decent quality and cheap..hard to resist.
Stuben
thanks Stuben
i will try some of them in my next project
LT
Don't forget to try the 'mu' output. Delivers very low output impedance( drive anything, nearly ).
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Yes, there are those things. And, transconductance...not enough jump for me.
The 6SN7 sounds linear but kind of boring, if you will.
So are you saying that the gain from a high transconductance tube is somehow different than the gain from a low transconductance tube (apart from the gain amount itself)?If you don't like the way a 6sn7 sounds then pick a different tube but you shouldn't need more gain.
I would like to hear a full, technical description of "jump factor", not a subjective description.
BTW I have my own technical description of so called "jump factor" but I don't think you will like it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/28/15 10/28/15
Tre'
Gain is gain; but, the the sound can be different even though the gain is the same.
I do pick something other than the 6SN7. Been there done that.
I'm just saying there's more to consider than gain. Heck, if you think about it, people "prefer" different 6SN7s - their gain is the same, no? Taking this a bit further, if it were just gain, one could picky many different solutions to achieve the same gain, yet I seriously doubt they would all sound the same. Surely, you don't think that if the gain is the same the sound is the same...
I never said you need more gain. I said there is more to it, meaning the sound of the amplifier, than simply the gain of what's in front of the 2A3.
I further suggest to the original poster to consider transconductance. He's looking for some help, no?
Stick with your technical descriptions. I won't offer you mine. I'm pretty indifferent and could care less about yours, frankly. I've been comfortable in my own audio skin for a long time now...
Not sure what axe you have to grind here??
I'm not interested in specifically commenting on the circuit at hand other than to suggest some consideration for the original poster. The topic is their schematic in question.
"Gain is gain; but, the the sound can be different even though the gain is the same."Yes.
"I never said you need more gain. I said there is more to it, meaning the sound of the amplifier, than simply the gain of what's in front of the 2A3."
Again, yes.
"Not sure what axe you have to grind here??"
Sorry, I thought you were going somewhere else.
"Heck, if you think about it, people "prefer" different 6SN7s- their gain is the same, no?"
I agree.
Please remember that my first post was a response to Eli's comment that "A single high mu, high plate resistance tube will work IF a SS source follower buffer is added" (paraphrased) and I was just trying to point out that if a low mu, low plate resistance tube is used then the SS source follower isn't needed and that the extra gain also isn't needed.
Maybe that wasn't clear.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/29/15
Since more gm is needed, and mu of 20 is sufficient there are a few other choices... The 'get a bigger hammer' train of thought comes to mind. Try an HY40....:) Or perhaps an EL84. Rigged as triode, gain is there, and rigged as pentode, the plate load can be quite low, delivering very good drive to the power tube grid.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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