Constant Current Source
We simply love constant current source or constant current
sink (or sink) and used it widely across our products. It can be used as the load of a vacuum
tube, or the cathode bias of a vacuum tube, or even the bias of a push-pull stage
where the need of a splitter is eliminated!
Usually we use resistor, choke, interstage or output
transformer as the load for the tube in a single ended common cathode amplifier
stage. This load can be switched to a high voltage constant current source.
This will provide absolute linearity compared to the vacuum tube.
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| Type 10 plate characteristics and loadline |
What you can see above are 2 load lines for vacuum tube #10 –
black load line is for a normal resistor of 10,200 Ohm, the red load line is
for a constant current source set for 20mA.
- Linearity of the constant current source is much better compared to resistive load line.
- Headroom or range has actually increased from just to -70V to infinity, or as high as your B+ is.
- Better gain is achieved.
- Power supply rejection ratio is improved as the tube is shielded by the constant current source.
- With the constant current draw, less demand is exerted on the power supply and therefore gives better sound!
- Tube is protected from over current due to being over driven.
- Bias is easily adjusted by a twist of screwdriver to tune for your taste.
- And many more
These benefits not only look good on paper, it actually
sounds better compared to ordinary loads. You must experience it to believe the
significant change when converted.
 |
| Constant current sink & constant current source |
In above schematic, driver stage uses constant current source as load and power stage uses constant sink as cathode bias. On driver stage, you can use either the constant current source as load or constant current sink as cathode bias. In power stage, you can only use constant current sink as cathode bias in traditional single ended power stage with output transformer.
You can use constant current source as load too in power stage for parafeed topology, replacing the plate load choke. This is not common as too much power will be burnt in the constant current source circuit compared to a traditional plate load choke. Using just as cathode bias will bring enough benefits.
Apart from being a load, the constant current source can be
connected at the cathode as bias for common cathode, cathode follower and
push-pull designs. The benefits it brings are similar and more for push-pull designs
where the 2 tubes are forced into perfect collaboration once it is set right.
 |
| Constant current sink for push-pull topology |
The most common device to be used (to us), is the LM317 or the IXYS transistor. We find that it is not as great as the discrete version but it is very convenient and easy to implement, not forgetting the accuracy and protection the LM317 has. We can't complain to much! There are plenty of constant current source and constant current sink designs in the internet. You can Google for it. We won't go into details here.
With such benefits, we would employ the constant current
source in our designs, wouldn’t you? If you would like to employ constant
current source and do not know how to do so, please contact us for an
appointment or free slot for modifications. We can help you with a small fee.
J&K Audio Design
22/12/2013
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