CCS question

[woody]

I have seen various post to the effect that solid state constant
current sources can sound as good or better than plate or
cathode resistors. Why does sand sound ok in this application
but not when called upon to be a voltage amp.

Thanks,

Woody

[Bas Horneman]

Hi Woody,

That's an excellent question but one I can't answer properly.

One thing though is that the tube is still doing the voltage amplification.

The other is that some find that it does cause a solid state type of sound and would rather use a resistor or plate choke (or a Mu-stage).

Another thing apparently is that the shunt capacitance if one of the most important variables when it comes to subjective appreciation of the gain stage. And the lower the better.

The added advantage of a ss ccs (assuming ofcourse that it works properly) is ofcourse the ripple rejection is so huge that the advantage of using one might offset any possible disadvantages..

I think the first point comes closest to an anwer.

[woody]

Bas,

Thanks for the reply and the fast shipping on the parts I
ordered from you.

Thanks,

Woody

[gingertube]

In a SS CCS the transistor is being used in constant current mode and therefore avoiding the worst SS issues of beta changing with collector current.

Of course you still have to cope with Early Effect issues which with a single transistor CCS will limit the effective load impedance to say 300K and will roll off highs at say 200 - 300kHz due to device capacitance. Often this is good enough. Another issue (well part of the same issue really) is that the capacitance (Ccb) changes with Vcb changes (i.e. the collector to base depletion region width and hence capacitance varies with signal voltage developed across the transistor). This introduces variable high frequency feedback in the CCS.

Both of the latter issues can be addressed by using a Cascode SS Current Source.

Bas has it right - the CCS is only providing a load (albiet a good high impedance load with good {linear} frequency characteristics) for the tube voltage amplifier and so sonically you are getting the signature of the tube itself, improved somewhat by the tube being operated in constant current mode also.

This maximizes gain and minimizes distortion from the tube itself.

Hope this makes sense to you.
Cheers,
Ian

[Bas Horneman]

Thanks for the reply and the fast shipping on the parts I

You are welcome Woody! Sorry I could not explain it better. Best advice as always is to just go ahead and try it for yourself.

Regards,
Buzz

[tubelab.com]

Whoa, this question could be the subject of a PHD dissertation. The answer would involve the discussion of why transistors sound the way that they do. There are many sides to this subject but two points are important here.

A transistor (fet or bipolar) is an inherently non-linear voltage amplifier when operated at large signal. Cascading many transistors to build an amplifier generally creates a non-linear amplifier.
To make up for the non-linearities, a large amount of negative feedback is applied. The feedback is largely responsible for the solid state sound, and most tube amplifiers do not use much feedback.

For a better explanation see:
http://www.tubelab.com/philosophy.htm section #7

The CCS does not operate the active device as a voltage amplifier. In fact it is not used to amplify the audio signal at all. An ideal CCS would pass a constant amount of current to the amplifying tube, while presenting an infinite impedance at ALL frequencies of importance to the amplifying stage. It should be noted that this is the ideal load for a triode. A CCS load should not be used for a tetrode or pentode.

As with everything else in life nothing is perfect. All CCS designs both tube and solid state have a finite impedance which changes with frequency. There are many good CCS designs available on the web.
This guy seems to have made a career out of them:

http://www.pacifier.com/~gpimm/

I have tried several different CCS's and have settled on an IC based current source that works excellent. In fact I have used it in every amp design that I have made since I found out about it.

http://www.tubelab.com/Simple45.htm

I have used this IC (IXYS 10M45) to build a driver that is flat out to 300 KHz while driving an 833A into A2. There are mosfets and IC's in the design, but no sandy sound.

http://www.tubelab.com/powerdrive.htm

I have even used one of these to load a 45 for chokeless parafeed. You need a 500 volt power supply to do this though.

[Brian Beck]

I agree with the above responses but just wanted to add that I think that a solid state CCS can sound like "sand" even if is "just a plate load". Nor should we take any false comfort from knowing that the tube is the voltage amplifying element. Amplification fidelity is a function of the environment too, plate loads, cathode loads, supplies, etc. A badly conceived SS plate load can make a tube gain stage sound plenty “sandy”. Only when a CCS has extremely high dynamic resistance, and extremely low capacitance (arguably <<1pF) then I think the tube parameters will dominate. The key, to me, is just how much shunt capacitance exists across this CCS, assuming that dynamic resistance is already very high (which is pretty easy). As mentioned above, capacitance associated with SS devices can be enormously variable with signal swing, creating a nasty non-linearity that causes phase intermodulation distortion. I think this is worse than Early Effect in aural destructiveness. Gary Pimm's work has driven the CCS dynamic resistance very high and also reduced the shunt C values, especially in some of his later versions, to vanishingly low levels. I see that he cascodes some of his designs with a pentode, which happily isolates the semiconductor capacitance even more. But too often I see well-intentioned, but overly simple, solid state current sources as plate loads. Sometimes you'll see just a dangling PNP handing off B+ with collector to plate, or a simple FET current source. These I would avoid! And MOSFET capacitances can be enormous and very distorting.

Here’s a thought: use a series choke with a CCS to further isolate it. It would have to be wound with very low shunt C itself though.

[tubelab.com]

I also agree that the capacitance of the CCS and the following stage is very important. I like to consider the DC and AC components of the triode's load seperately. Ideally they should both be infinite. Since this is impossible, they should be as high as possible. This is what the Mu - follower, SRPP and other similar circuits attempt to do.

I have tried most of those circuits with varying degrees of success. During the testing I found circuits that definitely sounded bad. I don't know if they sounded "sandy" but the "simple fet" circuits sounded dull and lifeless. A simple pentode (hard to find one with really constant current) CCS's didn't sound much better. As expected the complex circuits provided better sound AND measurements. Many of these circuits also provide buffering to increase the AC impedance that the triode sees. I have also seen the circuits with a fet CCS followed by a cathode follower.

While testing various CCS circuits, I found the IXYS 10M45 IC. This surprised me with good performance. Ok this solves the DC load problem, but what about AC. Just hanging the grid of an 845 off of the triode / CCS won't work. Cathode followers help, but I wasn't satisfied. I experimented with bipolar emitter followers but ran into SOA problems. I am currently using a mosfet source follower with good results. The gate to source capacitance may be high, but these two terminals are in phase at very similar voltages, so the actual capacitance seen by the triode is low. Still the choice of fet is important. I got some IGBT's but haven't tried them yet.

Another parameter that is often overlooked is the triode's current. Too often the designer chooses a 6SN7 or a 12A*7 and biases it to 1mA of current. I have found that more current is needed to charge and discharge the AC load capacitance. I found that the 5842 tube likes 10 to 14 mA of current. This is what I used in the TubelabSE, but since I designed that amp, those tubes have become trendy and the price has gone way up.

All of this science led to the development of the PowerDrive circuit. This circuit has ruler flat phase response out to 245KHz, while driving an 833A into A2. It sounds good too.

I see that your avatar is an 833A, have you built an 833A amp yet. If so, what do you use for an output transformer.

[Gluca]

Actually I have never listened to a CCS loaded triode, that why I am asking.

Ciao
Gianluca

[SY]

I am currently using a mosfet source follower with good results.

Same here. It drives the screens of a 6LF6 pair. I ended up with an IRF820 in that position- what have you used?