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Tube line stage output impedance

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Tube line stageoutput impedance

Would it be reasonable to use a tube line output stage as a totem pole with an output impedance of 3500 ohms?

I know there are better options but I am trying to get away from the existing SRPP and stay in the confines of the PS.

The head amp followin this has an input impedance of 47k.

Are there other issues doing this beside the 8% drop in Vrms?
 
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I'm not totally sure I understand your question, what topologies do you define as a totem pole. (a term commonly used in ss technology, and generally not used with tubes.)

Stacked topologies (your totem pole) would include SRPP, Mu follower, and the White cathode follower, as well as a standard CF with a tube based CCS for the load. Of these only the white CF and the SRPP (for specific load conditions) would be considered pushpull and hence somewhat analogous to a ss totem pole output.

So, what exactly do you have in mind?

FWIW you would have to work pretty hard to get such a high source impedance out of any of these topologies, you could easily reach 1K or less with commonly available tubes.

You haven't provided any information on available supply voltage, what's driving this "totem pole" stage or any particular reason for rejecting the SRPP which is simple and works fairly well - where it may not work so well is with a load that presents a varying load impedance.
 
A totem pole is where the output is taken at the plate of the lower triode instead of the cathode of the upper triode (SRPP.)
Zout=Ra of the tube.

A totem pole is the same as the Aikido input stage.

Broskie goes into great depth as to why an SRPP is the wrong circuit for a line stage. It is push-pull and has a certain ideal load, which I calculated as only 7k with this operating point. Deviate from this load and distortion goes up which I measured to actually occur.

My best bet may be a Mu Follower, I think the 188V B+ with the 6DJ8's may be enough to pull off a mu-follower.
 
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regal said:
A totem pole is where the output is taken at the plate of the lower triode instead of the cathode of the upper triode (SRPP.)
Zout=Ra of the tube.

A totem pole is the same as the Aikido input stage.

Broskie goes into great depth as to why an SRPP is the wrong circuit for a line stage. It is push-pull and has a certain ideal load, which I calculated as only 7k with this operating point. Deviate from this load and distortion goes up which I measured to actually occur.

My best bet may be a Mu Follower, I think the 188V B+ with the 6DJ8's may be enough to pull off a mu-follower.

A mu follower should work given 6DJ8 and the supply voltage you mention. One thought would be to do a choke loaded mu follower. (Basically the SRPP topology with the upper cathode resistor replaced with a choke of both appropriate inductance and dcr - works well with 6SN7/7N7, but have not tried this with the 6DJ8 - the low dcr required might make this impractical.)

I don't read the tubecad blogs much so I should have suspected that the usage you gave for "totem pole" came from there, maybe it's just me, but I don't much like the term.. :D
 
With the Mu-Follower high much of B+ should I consume with the two resistors connecting the tubes (top cathode+bottom plate.)

I don't have enough B+ to dedicate the typical 1/3 B+. I was thinking 10V-15V's would be enough since I am only outputing 1.1 Vrms.
 
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regal said:
With the Mu-Follower high much of B+ should I consume with the two resistors connecting the tubes (top cathode+bottom plate.)

I don't have enough B+ to dedicate the typical 1/3 B+. I was thinking 10V-15V's would be enough since I am only outputing 1.1 Vrms.


Should work fine, remember it is a floating circuit node and the actual ac voltage across that resistor is very close to 0. Into your stated load I expect you could swing at least half the available supply voltage if not more..
 
CS-Load-Servo-Wavebourn.gif
 
revintage said:
This is the "ultimate" mu-follower that kevinkr suggests.
Zout ca 200ohm and gain over 30dB. It will surely put out more than 30Vrms into a HiZ load.

http://www.eflatjump.se/CHOKEMU.PNG


They must have changed the LL1668. It is now 680 ohm DCR and 100H. I don't see how you could run that with the bottom tube at 330 ohm Rk, would seem the balance would be off quite a bit.
 
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regal said:



They must have changed the LL1668. It is now 680 ohm DCR and 100H. I don't see how you could run that with the bottom tube at 330 ohm Rk, would seem the balance would be off quite a bit.


You might have to get something wound. Electra-Print could probably provide you with something suitable. I don't think you need more than 60H with a 6DJ8, but the choke would be bulky.

A regular mu follower based on the 6DJ8 would be the first thing to try.
 
Regal:
You can get the 1668 in a 200H/10mA version instead of 100H/25mA. Using only one of the coils give the values I indicated and below 5mA Ia.

You can also go for parallelled windings of the same and you will then have to adjust the cathode resistor to the nearest standard value, 180ohm. This will still give 50H inductance but now with a Iq in the ballpark of 8mA.

Did a quick sim with 50H and it indicates -0,1dB at 20Hz.

And as Kevinkr says, Jack at Electra-Print is the man to make a 60H/5mA pair for you at descent cost. He can also make them with shielded containers at a small additional cost.

He made me a pair 100H/10mA and the containers are not more than 2"X2".
 
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That ought to be ok, too bad though, I think you would be more than a little surprised at the performance it could bring to the table.

Try to find Siemens CCA version of 6DJ8 or old Mullard or Amperex examples, they do sound better than the JJ which is the only other one I find decent. Most NOS American types are not wonderful sounding IMO.

One other option you could consider would be a CCS loaded 5842, gain would be about 4dB higher, but a single stage would do the job. Less is more as they say..
 
I've been studying up on this 5842 tube.

I could use it to amplify a very low output from my DAC. The advantage is the lower passive I/V resistor one uses, the more linear the DAC chip behaves.

Would it be insane to feed a 5842 a 0.025V rms signal from the DAC? This would give me 1 Vrms output.

My concern would be low level detail (-96 dB down) would be feeding the 5842 in the sub-MicroVolt level.

Of course most masterings don't use the least significant bits, but would I be amplifying more noise than music? Or is this an accepted practice with the 5842?
 
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