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Old 29th April 2010, 11:17 AM   #1
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Default clipping

I have a question relating to voltage amp clipping, specifically in an srpp circuit, but I'm not sure its peculiar to srpp. I've been experimenting with different loads on an srpp circuit and discovered that with certain loads the output wave clips at the bottom and with others it clips at the top. At a certain load the clipping is synchronous at the top and the bottom of the wave.

At no point does the input signal wave exceed the bias point, so the input valve is neither cut-off or driven across zero bias.

Can anybody explain what is happening here(hopefully in amps-for-dummies style) or the significance, if any. Or, possibly point me at an easy read on the subject as I've searched with no success so far.

In regard to the srpp, I've also noticed that the gain(with respect to input level) becomes more even as the load decreases and actually approaches static at roughly the point where the wave clips evenly at top and bottom. The attached file is an excel file showing some will need renaming to .xls as I named it a doc to attach it.

I started looking at this because I discovered some papers on the net relating to balanced srpp and the formulas gave figures which differed from some published circuits.......the truth is out there, isn't it?


Attached Files
File Type: doc gain-chart.doc (25.5 KB, 16 views)
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Old 29th April 2010, 09:24 PM   #2
Merlinb is offline Merlinb  United Kingdom
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Aha! You have stumbled upon the peculiarities of the enigmatic SRPP!
Not sure how hard you searched but there's acres of stuff on the SRPP on e.g.,
The Tube CAD Journal: Mixed Mode Amplifiers
SRPP+ All-in-One & Impedance Multiplier Circuits
I have also written an article on the SRPP in this and next month's Audio Xpress.

The thing about the SRPP is that it is a small, push-pull power amplifier, (not just any old gain stage or line driver). Only under one particular load will the drive at the bottom valve equal the drive to the top valve, and only then is the circuit balanced and you get symmetrical clipping, and this is also where you get approximately the lowest distortion. At any other load one or the other will clip first.

Last edited by Merlinb; 29th April 2010 at 09:26 PM.
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Old 29th April 2010, 10:18 PM   #3
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Thanks Merlin

I thought I'd read most of JB's stuff on srpp, but I hadn't seen that last bit about srpp+. Nevertheless, most of it was over my head, and I hadn't seen anything about symmetric and asymmetric clipping. All the stuff I had found on the net was related to stomp boxes and effect pedals where assymetric clipping was being sought....

I'm still not sure if I'm on to a way of discovering the balance point in loading srpp though....

I have to say that the circuit does sound more crystalline with the load near the point where clipping is symmetric.
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