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Mu Output of a CCS

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Posted on March 7, 2016 at 12:53:17
Prospect
Audiophile

Posts: 38
Joined: September 7, 2002
 I'm designing a new phono stage and have a few questions about the K&K CCS boards. Perhaps someone can provide an assist.

1) Are these boards quiet enough for use in the first stage of a phono preamp (either as plate loads or in the tail of a differential pair)?

2) If I use a CCS as a plate load for a tube, can I take the amplified signal from a "Mu-out" position on the board (at the junction between the lower MOSFET and the top of the current-set resistors) to obtain a super-low output impedance?

3) What is the approximate Z-out, if taken from this position? Is it stable, or would it vary with frequency (or other factors)?

4) If no one knows the Z-out, is there a way I could measure it empirically if I breadboard the circuit?

Thanks for any help you can provide!

 

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RE: Mu Output of a CCS, posted on March 9, 2016 at 05:55:41
KevinC
Manufacturer

Posts: 2984
Location: NC
Joined: April 19, 2001
 I use these current sources in my Trio phono preamp in the input stage, so I can write from experience about them being quiet enough.

It is possible to take the output as you describe, but I have no idea what sort of output impedance this produces or what the frequency dependence might be. You can measure output impedance easily by capacitively coupling a 5K potentiometer between the output and ground. First, with the potentiometer disconnected and a signal on the grid of the input stage you are testing, measure the output voltage. Then connect the potentiometer between the output capacitor and ground with it set for maximum resistance and adjust the resistance downward until the output signal is half of what the open circuit output signal that you measured earlier was. Then take the potentiometer out of the circuit without disturbing the setting and measure the resistance of the potentiometer. This value is the output impedance of the circuit being measured.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Mu Output of a CCS, posted on March 9, 2016 at 09:52:19
drsx
Audiophile

Posts: 369
Location: around
Joined: September 11, 2005
 Here's a method to estimate the mu output: http://www.audioasylum.com/forums/tubediy/messages/18/182709.html

 

RE: Mu Output of a CCS, posted on March 9, 2016 at 10:28:50
Prospect
Audiophile

Posts: 38
Joined: September 7, 2002
 Thanks Kevin and DRSX for responding to my querry!!

Last night, after many hours of searching, I came up with somewhat similar results. Kevin's description of testing for Z is simple and practical. I love it.

I eventually found a couple of sources for calculating the Mu-out Z. An example is presented below:

The output impedance of the Mu-out depends on the value of the current-set resistor and on the plate resistance of the tube you are loading. Here is the way to calculate it (according to what I found in other posts):

First, find the transconductance of the CCS MOSFET. For Kevin's CCS boards, I looked up the IXTPO8N100D2 datasheet and found a transconductance chart. It indicates a value of about 0.1 Siemens (or 100 mA/V) for a current of 30mA, although this is very near the origin of the chart where all the values are collapsing. As a cross-check, the specifications give a minimum Gfs value of 300 mA/V when the MOSFET is at 30 volts and 0.4 amps, so my chart reading seemed reasonable.

Second, define your operating data. For this example, assume a 6H30p tube at 30 mA and with a plate resistance of 1300 Ohms. I'm also assuming that the combined value of the two current-set resistors is 150 Ohms to pass 30 mA. This is just a guess, since I haven't ordered any boards yet. Then proceed as follows:

Calculate a Factor
Multiply Gfs x Current-Set resistance: 0.1 Gfs x 150R = 15

Calculate the contribution of the tube to the output-Z
Divide the Tube Rp by the Factor you just calc'd: 1300 R / 15 = 87

Add the contribution of the CCS to the output-Z
Add about 50 Ohms: 87 + 50 = 137 Ohms
This is the final result.

I don't really know where the 50 Ohms from the CCS comes from, but it was proposed and accepted by more than one poster.

Doing all the searching was educational in many respects. I learned a lot about how people are using CCS loads in various circuits. I also found a kind of discrimination present regarding the Mu-out position of CCS boards in general. There were two camps: First there were those who were dismissive of this output and either raised objections, expected the sonics to be poor, or who appeared mistrustful based on their prior experience with Mu-follower tube circuits. Then, there were those who had actually tried the Mu-out, and generally thought it was great.

For now, I'm going to keep it in my design because it solves a problem I'm having with RIAA filter values. I'll try it out and see how it works. Unfortunately it won't lend itself to any kind of "before/after" testing for sonics in my application. I would need to radically change the filter to account for the impedance change.

It also appears that empirical testing will be more practical than estimating through calculation, because the IXYS datasheet doesn't provide sufficient granularity at low current levels.

Thanks again for your help. I will be ordering some boards soon.

 

RE: Mu Output of a CCS, posted on March 9, 2016 at 20:07:01
drsx
Audiophile

Posts: 369
Location: around
Joined: September 11, 2005
 For now, I'm going to keep it in my design because it solves a problem I'm having with RIAA filter values.

I am agnostic as to the wisdom of using the Mu output, but note that you can solve the same problem with a source follower after the tube.

 

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