Cogito wrote:
Ok, I understand how CCS impacts the linestage and its advantages. Since I am planning on a regulated B+ and extremely low ripple and noise power supply, CCS is not advantages in those categories.
As of now, I am not convinced CCS is the right topology for my application.
My requirements:
1. Low Distortion
2. Low Noise
3. Excellent Frequency Response
4. Micro details
5. Extremely low output impedance
6. Lots of headroom for transients
My DAC puts out 2.8V for PCM and 1.45V for DSD over RCA. That's 6dB difference. To bring DSD to the same level as PCM with headroom, 9-10dB would be plenty of gain for my system, I think.
Going down my list:
1. Low Distortion - CCS has lower distortion
2. Low Noise - CCS has higher SNR
3. Excellent Frequency Response - CCS has narrower bandwidth due to greater sensitivity to load capacitance
4. Micro details - CCS and Resistive load should be same
5. Extremely low output impedance - CCS impedance is about 1K (Rp). Not sure the impedance of resistive load topology. This is important to me because the preamp needs to feed 5 amps. Transformer coupling instead of CCS or tube buffer stage?
6. Lots of headroom for transients - CCS has limited headroom
May be it is my lack of understanding but CCS doesn't seem to provide the benefits I am looking for.
Shashi,
I am with Ray on the benefits of a CCS. I believe you are incorrect in a number of areas, misreading information in the article that Ray sent you the link for.
1. Low Distortion - correct - CCS loading has lower distortion because it loads the plate of the tube with an equivalent resistor of several megaohms. This results in a horizontal loadline across the tube curves where there is virtually no 2nd order distortion compared with conventional resistor loading, the value of which is limited by the power supply voltage. No other loading linearizes a tube more than CCS loading.
2. Low noise - correct, and with a regulated supply you will achieve vanishing noise levels.
3. Excellent Frequency Response - Incorrect. A CCS WILL have excellent frequency response. Any stage is going to be impacted by MIller capacitance of the following stage, but a CCS is much better in it's ability to drive into difficult loads...such as the input of a Mosfet in a source follower configuration (for driving A2). Look at TubeLab "Power Drive" circuit.
4. Micro detail - I would think the CCS would come out on top due to low noise floor that would otherwise mask low level signal.
5. Extremely Low Output Impedance - Not a problem - The output impedance is using a cathode bypass cap across the cathode resistor, or low equivalent resistance fixed bias arrangement, is the value of the plate load resistor in parallel with the Rp of the tube. Given a 10K plate resistor and an Rp of the 12B4 of around 750 ohms, this results in 10K||750 = 698 ohms. Using a CCS load of 2M ohm, the output impedance is 2M||750 = 750 ohms. This is negligible. Now if you an unbypassed cathode resistor of 500ohms, this multipled by the gain of the tube (say 6) adds to the Rp of the tube, 750+6*500 = 3750 ohms, in parallel with the CCS load of 2M results in a output impedance of 3743 ohms. Not so good. So you really want to use a bypass cap or something like LED or diode fixed bias with a low resistance.
6. Lots of headroom for transients - A CCS loaded 12B4 at 125V/20mA can swing +/-60V. That is an order of magnitude more swing than you need.
Suggest you hit the books some more.
David