Hi,
One word of warning for the more technically inclined:
SRPP: SHUNT REGULATED PUSH-PULL is a monstruous mis-nomer.
To be kind, at best it's a circuit that can be of use for driving a lowish impedance source of say 10K or less (preferably less) fixed input impedance whilst sill pertaining some gain (mu/2) but not much else.
The worst part however is its extremely poor PSRR.
IOW, lest you understand the circuit and have a particular use for, avoid it. It's a bit akin to a hoax really....
Ciao,
I measure about -6dB PSRR for this circuit (in a simulator, for what it's worth). Not great, but not terrible either, particularly compared to a normal Class A gain stage with a resistor in the plate circuit. Most Class A circuits have less than stellar PSRR, but there are things that can be done about that.
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Of course, the PSRR for the triode grounded cathode is entirely a function of the size of the plate resistor. The bigger the resistor, the better the PSRR. The penalty is obviously the need for a higher B+ supply and greater dissipation in the plate resistor.
It's because a CCS gives the benefit of a gigantic resistor without the penalty of having a huge B+.
But, of course, choosing one or the other sometimes depends on other things, for example, cost and complexity. Most CCS are designed to go on circuit boards, for example, and if you're doing p-p wiring, it's a hassle to make a board and install it. A simple resistor is more appealing in that case.
Also, if low noise is the primary consideration, then maybe a CCS isn't the best choice.
Depends on the rp. For low rp, as I've demonstrated, CCS noise contribution is negligible.
All round means:
-best valve linearity from highest load possible
-lowest power supply noise, ie highest PSRR
-least power supply headroom for a given output swing (many CCS only need compliance of a few volts above the output signal)
-best ability to deliver current to the load
Overall a CCS load cannot be beaten by any other topography.
Shoog
How about sonics? The circuit will change its impedance during operation, causing some phase shift I guess.
I used electronics in the supply (regulator) and intentionally kept the circuit simple with either a big resistor or anode choke. My findings are that each electronic circuit has its sonic footprint, some more than others.
I used electronics in the supply (regulator) and intentionally kept the circuit simple with either a big resistor or anode choke. My findings are that each electronic circuit has its sonic footprint, some more than others.
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Although I haven't used it or heard it, I would want to compare a simple CCS with Wavebourn's gyrator circuit before concluding that the CCS can not be beaten. I realize that you may have intended your comment to apply equally to Wavebourns gyrator but thought I'd mention it for completeness.
Everything depends on what you want for your result. As I previously said valves make the best amplifying devices over transistors or FET's , and CCS allow them to show this quality to their best. An ideal CCS with low parasitic capacitance should contribute nothing to the sound that the valve doesn't intrinsically have to start with. In effect it is the ideal load.
However that is not what many people want from valves so for many people a CCS is a poor choice since it moves them away from the "valve sound" they want. In this case the Valve sound is pleasantly distorted.
If I was looking for a load type which brought its own quality to the preamplifier - it would have to be a transformer of choke load, and that is what my current preamplifier is running.
Of course there is nothing wrong with this since we all have to make choices about what we want from our projects.
Shoog
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A choke load of suitable inductance provides more gain (10-20dB, typically) than a CCS for identical operating conditions of the tube, and this increase in gain is provided without a noise penalty. Thus, the S/N ratio for choke load is higher than for CCS load.
The penalty for choke loading is higher cost, potential hum and other noise interference, and more distortion at lower frequencies.
The use of a CCS is not risk-free however, since they can oscillate or have other problems, apparently.
So, in places where the greatest S/N is needed, and the tube has low Rp and runs at reasonably high DC current, then a choke load can be of benefit.
The penalty for choke loading is higher cost, potential hum and other noise interference, and more distortion at lower frequencies.
The use of a CCS is not risk-free however, since they can oscillate or have other problems, apparently.
So, in places where the greatest S/N is needed, and the tube has low Rp and runs at reasonably high DC current, then a choke load can be of benefit.
SRPP each have an optimum load, i.e., a load that results in true p-p operation and minimum distortion. It is almost never a "light" load. The prolific Merlin Blencowe published some measurements on this showing a distinct null at a fairly low load for the tubes he used (ECC88 if memory serves).
edit: Here's his analysis:
http://valvewizard2.webs.com/SRPP_Blencowe.pdf
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