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RE: Mu Output of a CCS

Posted by Prospect (A) on March 9, 2016 at 10:28:50

In Reply to: RE: Mu Output of a CCS posted by KevinC on March 9, 2016 at 05:55:41:

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.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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