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#1 SRPP stage useing a 5670

Posted: Sat Oct 06, 2018 3:38 pm
by acorn
Good day all

What is the advantage of useing a SRPP input stage as apposed to a normal input stage
would it be a better frequency response and lower distortion figures ? or are there any better
circuits out there I could use.

Acorn

#2 Re: SRPP stage useing a 5670

Posted: Sat Oct 06, 2018 5:40 pm
by Mike H
I'd forgotten what SRPP is, so Googled - ah yes ...

This may, or may not, help:

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

I would have used it for the upper valve being more like a constant current source, rather than a resistor,so therefore should be more linear, but having read a bit of the above I'm not sure now :shock:

#3 Re: SRPP stage useing a 5670

Posted: Tue May 25, 2021 11:16 am
by ruffrecords
The SRPP is more of an output stage designed to deliver power to a load rather than an input stage. They also suffer from relatively high distortion. Using a pair of triodes in a mu follower stage would provide more gain and lower distortion. I use an ECC88 based mu follower in one of my microphone preamp designs.

Cheers

Ian

#4 Re: SRPP stage useing a 5670

Posted: Tue May 25, 2021 4:10 pm
by Wolfgang
What is the advantage of useing a SRPP input stage as apposed to a normal input stage
None, if you don’t need more than let's say 30% of the idle current of a simple SE input stage into the load of the next stage. Simplicity of the circuit and the best suited tube for the job is more in this case IMO.

#5 Re: SRPP stage useing a 5670

Posted: Tue May 25, 2021 4:36 pm
by Nick
They were fashionable for some time, due in part I think because some Japanese designers used them

#6 Re: SRPP stage useing a 5670

Posted: Tue May 25, 2021 7:24 pm
by JamesD
Tubecad explaining what the SRPP stage is, how to design it and why not to bother except for some limited use cases :D

https://www.tubecad.com/may2000/

ciao

James

#7 Re: SRPP stage useing a 5670

Posted: Wed May 26, 2021 11:30 am
by ruffrecords
You need to take the opinions of TubeCAD with a healthy does of salt. His analysis is excellent but his applications (use cases -ugh what a horrible term) are equally limited. The SRPP is a very useful tool in the tube circuit designers arsenal but like all topologies, considered in isolation it has its limitations and advantages.

Cheers

Ian

#8 Re: SRPP stage useing a 5670

Posted: Wed May 26, 2021 12:45 pm
by Paul Barker
There was a Glass Audio article which came out in favour, with the condition use different values cathode resistors for each stage. Thats all I recall.

#9 Re: SRPP stage useing a 5670

Posted: Wed May 26, 2021 2:03 pm
by ed
I messed with srpp quite a bit back in the day and found an interesting article about having the correct load for the correct valve. This messing about showed me that some of the accepted circuits on the internet were not producing the best sq.

I wrote a bit on my web page after I'd lost the original article, but included another article by Merlin Blencowe:

https://vitalstates.org/vsl/2a3-single-ended/

I've since found the original article, which has a new web site:

https://ttradio.net/srpp-amplifier-theory-pictures/

following the equation for Rl I got the best front end(IMO) for the 2A3. I usually miss the point, but fwiw I like srpp.

#10 Re: SRPP stage useing a 5670

Posted: Wed May 26, 2021 2:17 pm
by JamesD
Hi Ian,

I find I agree with JRB far more than I disagree with him... but that's just me and for an engineering subject audio circuit and system design is certainly a broad church.. His SRPP article has always drawn a lot of comment and criticism..

Welcome here, btw, its great to have another experienced engineer here :D

Given your experience, I'm curious as to when you think using SRPP is the optimum choice and when it might be the expedient choice?

ciao

James

#11 Re: SRPP stage useing a 5670

Posted: Thu May 27, 2021 12:32 pm
by ruffrecords
I just typed a big long reply to this and then lost the lot. I hate Chromebooks where the control key is just below the shift key! Accidentally pressing Ctrl W closes the current tab and everything you types is lost.

Anyway, very short version. My interest in valves is for use in professional audio, particularly mixers. Often amplifier stages in a mixer are required to be able to drive quite low loads (2K5 is not unusual) to high levels (10V rms is not unusual). Bottom line is an 6922 based SRPP will do both without requiring more than about 6mA quiescent from the HT - total HT current is important in a mixer containing many amplifiers. Unfortunately, the SRPP stage on its own produces a lot of distortion at high levels so I use it with one half of a 12AX7 with about 20dB of NFB. This combination creates a very useful general purpose amplifier with up to 40dB gain.

Cheers

Ian

#12 Re: SRPP stage useing a 5670

Posted: Thu May 27, 2021 1:07 pm
by Nick
Unfortunately, the SRPP stage on its own produces a lot of distortion at high levels so I use it with one half of a 12AX7 with about 20dB of NFB. This combination creates a very useful general purpose amplifier with up to 40dB gain.
Similar idea to the Macdonald Augmented Cathode Followers

https://jrossmacdonald.com/jrm/wp-conte ... llower.pdf

#13 Re: SRPP stage useing a 5670

Posted: Thu May 27, 2021 2:02 pm
by JamesD
Ian,

Many thanks for the reply - a shame your more detailed one was lost as I have enjoyed your longer posts on DiyAudio.

I'm a Broadcast Engineer myself and have worked both Video and Sound - on the Sound side from valve EMI mixers, etc. forward so I appreciate the control of power draw in valve circuits!

I did wonder how you drove 10Vrms or 14 volts peak into 2500R with just 6mA in Class A - I would expect high distortion with that using any triode based circuit... glad that you found a way around it and other than the MacDonald circuits that Nick referenced not sure what I would use at close to 6mA draw...

#14 Re: SRPP stage useing a 5670

Posted: Thu May 27, 2021 2:07 pm
by Nick
He did say "6mA quiescent from the HT"

#15 Re: SRPP stage useing a 5670

Posted: Thu May 27, 2021 2:48 pm
by JamesD
Indeed - am I missing something? I think if you drive 14.14 V across 2500R you need 5.6mA that leaves 400uA for the triode - look at that on a 6922 plate curve and its well into the lower curvature region yielding high distortion - I know the SRPP is effectively PP but the valve is still pulled into the heavily non linear part of the curve so distortion will increase ... isn't it the global feedback loop that keeps distortion under control on a conventional PP amplifier when its operating in this region and SRPP doesn't have that on its own - its only present in the augmented versions?

ah or do you think it draws more than 6mA under load - how is that CLASS A then - I thought SRPP only operated in Class A?