Impedance formula of a SRPP

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johnheath

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Hi all...

After searching the Internet trying to get a grip on how to calculate the output impedance of a SRPP I find some sites but they give different answers on how to calculate the output impedance and therefore I turn to this forum trying to find the answer.

I attach a small schematic of the basic topolgy... please feel free to correct me here if I missed some Components for the formula.

What I really want to know is how to calculate the output impedance (formula).


Best regards

/John
 

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> searching the Internet

Have you read all of  Broskie's related essays?

Very few others I would trust.

> the basic topolgy.

IIRC there are several different things informally called "SRPP", and details of connections can make a BIG difference.

IMHO the "SRPP" family promises much with few tubes, and often "works". Often not to promises, but then 97% of SRPP fans can't measure.

If my drug kicks in I will try to run a sim.
 
Oh, for Large Signal performance, the optimum load should be quite close to 2*Rp. Rp taken at the peak point on the loadline. Topology does not change basic conduction (unless your topology is massive parallel). This means for any likely triode, 600r will not put high levels in the load, a step-down is wanted.
 
> SRPP Teori.png 8.7 kB

12AU7, 300V supply, biased to 12mA: 4K Zout

This is nearly Rp/2 for 12AU7 at 150V 12mA. (6.6K @ 150V 12mA)
 

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Sim 2--

You can bypass the upper Rk also. This *reduces* gain (from 12 to 10). It also shaves Zout to around 3.3K. Image not worth showing.
 
You can shift the output to the top of the upper Rk (unbypassed). This is the "best" so far. Gv=12, Z(out) near 2.5K. This is significantly off from my knee-jerk "Rp/2"; my excuse is it is too steamy today to do derivation.
 

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Thank you sir… you are fast here…  I am reading some Broskie essays :)

So… If I understand you correctly you suggest me to have a much greater load than the given 600ohm? Is there a way to achieve this by adding a load resistor before the OT?

And you simulate with a 100uF coupling cap… that is a BIG one isn't it?
 
Model used for "12AU7":
Code:
.SUBCKT 12AU7-X 1 2 3  ; P G C; NEW MODEL
+ PARAMS: MU=21.5 EX=1.3 KG1=1180 KP=84 KVB=300 RGI=2000
+ CCG=2.3P  CGP=2.2P CCP=1.0P  ; ADD .7PF TO ADJACENT PINS; .5 TO OTHERS. 
E1 7 0 VALUE=
+{V(1,3)/KP*LOG(1+EXP(KP*(1/MU+V(2,3)/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G    ; TO AVOID FLOATING NODES IN MU-FOLLOWER
C1 2 3 {CCG}  ; CATHODE-GRID; WAS 1.6P
C2 2 1 {CGP}  ; GRID-PLATE; WAS 1.5P
C3 1 3 {CCP}  ; CATHODE-PLATE; WAS 0.5P
D3 5 3 DX     ; FOR GRID CURRENT
R1 2 5 {RGI}  ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS

This has "MU=21.5", not the 20 in the handbook or the 21 at the peak of GE's graphs. Some other model factors reduce MU for voltage and current. 12AU7 is pretty bent for a "linear" tube and even Mu varies widely with operating point.
 
Thank you PPR … it is an interesting simulation, but can you tell me if there is a given formula that I can use to calculate the output impedance of the circuit? I feel like I want to "know" how to calculate it on my own with different values of the components involved.

But also I was very interested in the different loads your simulation gave :)
 
I don't do detailed formula. My cave-man bet is "1/2 to 1/3 of Rp at op-point". I would rarely ask any more precision than that. Anyway tube parameters vary at least 20%. If bias varies 20%, and  Mu varies 20%, Rp may vary 40%. So we only need a ballpark, not a 8-digit answer.

> I am reading some Broskie essays

Can you throw us some of the more on-point links?

The high-level action of all these is dubious. THD may be low up to a point. But under load, the clipping is VERY asymmetrical.
 

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Yes, I know about the variation in measurements so I really accept the answer of the "cave-man-bet".

There are a lot of descriptions on SRPP's around the Internet but as I mentioned they come up with different formulas for calculating output impedances… or it might be a problem caused by my difficulty to read technical formal english… I don't really know… or the lack of coffee in my body at the moment.

Here are a few links to what I have been reading:

http://www.tubecad.com/articles_2002/SRPP_Deconstructed/index.html

http://www.valvewizard.co.uk/SRPP_Blencowe.pdf

 
This re-topology gives same tube-count, same supply current, same gain near 12, and a solid 300 Ohm Zout.

However for HIGH level output the happy load is above 2*Rp, say 13K.

Loaded in 10K and driven to 10V peak out, THD is 1% mostly 2nd.

There's no mystery math, and it don't stink.

For *low* THD into real loads, I do think you need a stage which is all about gain for NFB, separate from your hard-working stage. And probably three triodes: one hi-gain, one lo-gain, and one whatever.
 

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> difficulty to read technical formal english

Broskie is plain English though he does use humor and irony in ways that can puzzle non-natives.

Ah, but then he quotes Millman 1956. This is tedious pedantic English. Not as bad as German, but knowing German constructions won't help much with Millman.

I like this:

"...this circuit works best when it is not required to work. In other words,  the load steals the promised benefits of high gain and low output impedance."
http://www.tubecad.com/articles_2002/SRPP_Deconstructed/page9.html

Past that John wanders off into very complicated schemes which violate the SRPP's apparent simplicity. One of the first IS my gain stage plus dumb cathode follower. Then there are 76 ways to add tubes and cross-connect nodes looking for something "better". Since you show a transformer, by the time you get to 4 triodes there are probably much better ways to use them than any of Broskie's single-ended daydreams.
 
Yes, I do have more gain stages in my whole design… three in total, but the SRPP seems to be a popular driver for the output so I figured that I should learn as much as possible about it. I also like it and the sound it creates.

Your dc-coupled cathode follower, the "re-topolgy", is said to create quite high distortion according to some information on the Internet. Maybe that is not what I want for a mic preamp??? Or is it???
 
PRR said:
> SRPP Teori.png 8.7 kB

12AU7, 300V supply, biased to 12mA: 4K Zout

This is nearly Rp/2 for 12AU7 at 150V 12mA. (6.6K @ 150V 12mA)

Which is why I always  use a 6DJ8/6922 for SRPP stages.

The particular advantage of the SRPP is it is a true push pull circuit so for a given quiescent current it can  output twice the power of a single ended stage.

Cheers

Ian
 
Thanks for shining in Ian.

I also like the 6922 and it works just fine for SRPP as far as I know.

Can you tell me if the suggested transformer (600:600ohm) would work with the SRPP if the the cathode resistor(s) are 680R?

Regards

/John
 
Like PRR said earlier if you intend to drive 600 ohm loads with SRPP you need a step down transformer. White cathode follower would do somewhat better driving 600 ohm loads.
 
johnheath said:
Thanks for shining in Ian.

I also like the 6922 and it works just fine for SRPP as far as I know.

Can you tell me if the suggested transformer (600:600ohm) would work with the SRPP if the the cathode resistor(s) are 680R?

Regards

/John

You can use a (very good) 600:600 transformer but it will not drive a 600 ohm load - it will drive a 10K load without any problem, But you need a good transformer with a high primary inductance, one like the Sowter 5069.

However, you would do better to use a 2:1 step down transformer then you will be able to drive +20dBu into a 600 ohm load. You can use something like the Carnhill VTB2291 or the Edcor XSM-2.4L/600.

Cheers

Ian
 
> said to create quite high distortion according to some information on the Internet.

I have noted that for every opinion "on the internet", there is an opposing opinion somewhere else.

The single-ended stages will make even-order distortion, sure.

On paper (SPICE), the SRPP "can" be adjusted for "zero" distortion (below clipping). As Merlin's paper mentions, and Broskie implies, the optimum depends on load and specific tube. That's usually "bad" for studio gear where an output might be plugged to ANY thing.

Push-pull affairs will dump nearly twice idle current to a load, single-end things can only approach idle current, as Ian says. However he says even the mighty 6DJ8 in SRPP can't push 600 Ohms with authority. If you are driving 10K at normal studio levels you only need 1mA-2mA peak. Dropping 5mA or 10mA in a single-end CF may be wasteful, but is simple and not too dirty. It does become an issue when you stuff 10 or 64 such drivers in one desk, not so much for single preamps.
 

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