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Old 23rd December 2012, 07:15 AM   #11
Calvin is offline Calvin  Germany
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Hi,

I didn't rule out SRPP/modulated CCS at all. Its just that the working into a complex load differs from that into a real load. A line level SRPP preamp is certainly a very different application than a ESL.
Both terms describe the same functioning, which has also been named SEPP. The term PushPull indicates a symmetrical behaviour regarding the currents through the output devices.
For the classic SRPP this holds true for a specific load impedance. The impedance of a Cap varies though over the full audio bandwidth by a factor of 1000. So apart from a certain range around the optimum impedance the SRPP ceases beeing a true PP. Of course it doesn't stop working alltogether.
As far as I understand SRPP I would design it for a optimum load impedance at say 5-10kHz. This way it could provide for twice the bias current into the load, hence maximum efficiency. For higher frequencies current capability would drop, meaning a reduction of full power bandwidth. Since at high frequencies there's not much music content -and if, then with low energy content- the loss in power bandwidth would be a rather academical than musical issue.
At lower frequencies the current demand of the load sinks due to its rising impedance nature. Similarly the SRPP gradually changes towards a CCS loaded grounded Cathode/Emitter/Source circuit, with a maximum load current of once the bias.
Stability could be an issue into the highly complex load, since the modulation-sensing resistors current is also a function of the complex load current. Then the 'lower' signal generating output master device and the 'upper' CS slave may not work in synchronousity any longer.
For larger high capacitance panels SRPP/modulated CS is probabely the only acceptable drive system, sonically as well as efficiency wise

jauu
Calvin

Last edited by Calvin; 23rd December 2012 at 07:24 AM.
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Old 24th December 2012, 12:08 AM   #12
jcx is offline jcx  United States
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I guess the issue is I really don't have much interest in tubes, especially triodes, triode strapped circuits - just use "SRPP derived" modulated current sources with SS parts that don't have mu, plate resistance, appreciable internal voltage feedback

you would proably have to cascode tubes, have a low Z buffer for screen grid with pentodes to get the sort of perfomance I see with Ixys depletion mode MOSFET modulated CS load
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Old 24th December 2012, 02:45 AM   #13
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I would stabilize SRPP by loading it with pure resistance and decouple it from capacitive load with follower, also some people just can't stand the follower stage per se.
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Old 24th December 2012, 08:21 AM   #14
Calvin is offline Calvin  Germany
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Hi,

SRPP or SEPP are circuit topologies and not restricted to Tubes.
The problem of stability remains with modulated CSs, regardless of Glass or Sand.
The pros of the SRPP are high gain, lowish output impedance and high load current (twice the bias current) into the right load. This may allow for a simple circuit with acceptable efficiency and heat loss.
The cons are stability of bias points and drift due to ageing and non-symmetrical behaviour in off-optimum load impedance.
Preloading the output resistively helps the impedance problem but is no real cure. Stability and drift due ageing may be cured with a feedback loop at the cost of gain/efficiency and probabely sonics too.
A dedicated Buffer may be a solution to the drive/impedance prob. In that case a SRPP i.e. the current drive capability may not be needed any more.
IŽd rather opt for a audio-signal-modulated CS cascoded by a HV-device -be it a tube or a Transistor- working into a highimpedance load, like a CCS, or a inductance. IXYS offers several veryhigh voltage/lowcurrent MOSFET devices that may be useable in a HV driver stage and HV-CCS (IXTA/IXTH/IXTV02N250, IXTF1N450, IXTF02N450, etc).

jauu
Calvin

ps. Thanks Alex for the Link to the Russian Power-JFET, a very interisting device for a highpower audio power amp. I follow these parts quite for a while, but availability and sourcing seems troublesome.
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