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SRPP - determining load and balancing

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I have been trying to be a bit more scientific than usual when nailing the load for an srpp driver circuit. In previous amps I have taken published circuits as gospel but having discovered a few descriptions of balancing methods I have found 2 equations which disagree. The balancing revolving around correct load(Rl).

1. Rk = 2Rl + Ra / (mu - 1)

or

Ra + 2Rl
2. Rk = -----------------
µ - 1

by the rules of bodmas one of these cannot be correct.

Can anybody shed some light on which is correct or whether both are wrong or another way of determining the output load value for srpp.

Ed
 

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The hand-written formula you attached is correct for an SRPP with unbypassed cathode, and an additional resistor in the anode equal to 2Rk. However, it doesn't make a lot of difference if you leave out the extra resistor, except for really low-mu triodes.

Incidentally, both of the formulae you typed are correct! If you apply BODMAS to the first one you typed, you will end up with the second...
 
many thanks chaps

and many thanks merlinb for pointing out my bodmas error

the reason I ask is because I was looking at the old Angela 6SL7/2A3 circuit and that uses Rl of 470k......mmm?

using the equation above I get 6SL7 with 2k2 Rk to come out at Rl = 54k....well thats a long way from 470k so I was a bit intrigued....

any chance of verification of that result

Ed
 
the reason I ask is because I was looking at the old Angela 6SL7/2A3 circuit and that uses Rl of 470k......mmm?

using the equation above I get 6SL7 with 2k2 Rk to come out at Rl = 54k....well thats a long way from 470k so I was a bit intrigued....

any chance of verification of that result

Ed
There was a fad for SRPPs for a while, which seems to have dies down a bit now. However, most designers didn't/don't realise that the SRPP is not a generic driver stage, or even that it has an optimum load. Fortunately, if you use a load resistance which is too large the distortion only increases a little bit. If you use a load which is too low then the distortion increases very rapidly. Obviously the simplicity of the SRPP appealed to many hobbyists, even if they didn't appreciate what it is for (a small power amp for fixed loads).
 
I hate to see SRPP in line stages and DAC's, glad you guys posted this thread to shed a little light on why they never sound right in those applications.
Holy war?
The first step to get good sounded amplifier is to try to eliminate as many coupling capacitors as possible! SRPP stage (with the output taken at the cathode of the top tube) is the best solution. Low distortion, high gain and yet lower output impedance.
 
Hi, vitalstates!

The Rload should equal (Rk*mu - Ra)/2

As additional knowledge source I very recommended to read the article:
The Optimised SRPP Amp by Merlin Blencowe (From Audio Xpress, May 2010)

Regards, Konstantin

PS: Ooops!
I noticed an article on the interweb regarding balanced srpp(the original link is no longer valid, although an article by Merlin Blencowe is a good read on the subject)
Sorry, I did't know that you have already read it ;)
 
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What is good in such "optimization" if by adding an extra resistor in anode the stage intentionally adjusted for symmetrical distortions when overloaded? On lighter load it is worse, since we loose gain of a cathode follower/dynamic load. Also, it is even worse when it drives an output tube: it's control grid has asymmetrical dynamic resistance. John Broskie in his article made an assumption that SEPP means PP amp, and tries to adjust it such a way it better fits into this definition. But what do we need: cleaner sound reproduction, or better match to assumed by somebody definitions?
 
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The load line for the lower valve will not be straight as the upper valve is not a resistor. Even if the upper valve was completely linear (which it isn't) the value of the effective load resistor seen by the bottom valve depends on the output load applied to the cathode of the upper valve. You could then work out the Thevenin equivalent and use that for a load line. I don't think load lines are particularly helpful for SRPP.
 
the value of the effective load resistor seen by the bottom valve depends on the output load applied to the cathode of the upper valve

This is why a SRPP OTL for a headphone application is designed for a fixed load impedance? (i.e Designed for 300ohm headphones only)


If to replace cathode resistor of the upper tube by a variable one and call it "Balance" it is possible to dial-in between cathode follower and active plate load, without sacrificing of gain of the upper tube

Would this be applicable in a OTL headphone application?
32-300 ohm headphones. But wouldn't the lower cathode resistor need adjusting to match the upper also?

I also read Broskies ideas on splitting the upper resistor and taking the load in between the split, resulting in a lower Zo without any drawbacks.
(i.e 150 Rk, 100 Rk+50 Rk)
I tried this and the results were good.
 
Splitting the resistor means you get results partway between SRPP and active load. Whether you get the best of both or the worst of both I'm not sure. SRPP is good for what it was designed for: driving a particular signal down a particular load without distorting it too much or taking too much supply current. Its current popularity is simply a matter of fashion and misinformation.
 
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