the Vgs is negative. Id is 8.4mA and Idss is 12.4mA
There are indeed several models but I used only the ones named j2sk170 in the upper position and LSK170B as current source. They have respectively 12.4 and 8.4 IDSS. I did this to have two jfets with different idss and be able to see the offset.
Gate current is in the order of 0.4nA.
Here's the model including only the directives I actually use.
View attachment jFETposition.asc
To focus back on the DCB1 maybe it is better that I ask the question in another way. Is it correct to use the jfet with the lower idss as current source? If yes, is it correct to expect a positive voltage offset at the signal output relative to ground?
you should use the lower Idss device as the CCS.
This ensures that the Id of the upper device is less than Idss.
If these devices are very close in Idss then the Id is only a tiny bit less than Idss.
That means that when signal current (AC) is passing the Id varies both above and below Id and will exceed Idss.
This is the region where jFETs can be used with care. But higher temperatures (Tj) and higher voltage (Vds) will give higher leakage current.
When Id < Idss, then Vgs is -ve, i.e. Voltage on the Source pin is higher than voltage on the Gate pin.
This ensures that the Id of the upper device is less than Idss.
If these devices are very close in Idss then the Id is only a tiny bit less than Idss.
That means that when signal current (AC) is passing the Id varies both above and below Id and will exceed Idss.
This is the region where jFETs can be used with care. But higher temperatures (Tj) and higher voltage (Vds) will give higher leakage current.
When Id < Idss, then Vgs is -ve, i.e. Voltage on the Source pin is higher than voltage on the Gate pin.
If I understand correctly? Are you saying I can get away with, Idss only- Matched Quad, for Mesmerise?
I have stated explicitly quite a few times that the jFETs only need to be selected for Idss.
They do not need to be matched.
I had many buyers for the matched pairs of b grade lsk170 for their DCB1. I generally told them they were wasting their money. And gave them the choice to continue with the order, or select their own.
I would guess that more than half of my sales went to DCB1 builders.
I would not use the 5V relays, I recommend you don't use 5V either.
They draw much more current than a 12V relay. That puts more load on the transformer, it all comes off one winding, not equally.
It also increases the dissipation of the relay regulator and will probably require a heatsink. 15Vac transformer feeding a lm7815 to drive a set of 12V relays does not require a heatsink.
A 15Vac transformer feeding an lm7812 may require a very small heatsink, but I have never built this option. I used a relay current saver running off 15Vdc in both Hyp and Mez builds.
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A relay will switch over on a range of voltage. A 12V relay will typically switch with 5V to 9V across the coil. It will always switch at less than it's rated voltage. This is usally specified in the datasheet
That same relay will hold in after switching at a significantly lower voltage, typically 3V to 6V. This might be called "drop out voltage".
One can use this "holding" voltage as a way to reduce the power being dissipated in the relay coil.
Switch it at 12V and then reduce the voltage to something less, but above the datasheet drop out voltage.
I usually arrange my relay to switch with a short pulse of higher than rated voltage and then reduce to less than rated voltage.
eg.
using a 15V regulator feeding the relay via a resistor equal in value to the coil resistance will switch at 15V and reduce to 7.5V after the pulse has dissipated.
The short pulse comes from a small electrolytic that charges to nearly 15V and then discharges through the relay coil and switching transistor to that 7.5V, over a period of around 10ms.
22uF charging through 1100r is ~14V after ~5RC periods i.e. in about 120ms.
The 1100r coil will dissipate ~51mW @ 7.5V instead of 131mW
And it consumes 6.8mA instead of 11mA. That in a Mez saves >8mA of asymmetrical current draw.
That same relay will hold in after switching at a significantly lower voltage, typically 3V to 6V. This might be called "drop out voltage".
One can use this "holding" voltage as a way to reduce the power being dissipated in the relay coil.
Switch it at 12V and then reduce the voltage to something less, but above the datasheet drop out voltage.
I usually arrange my relay to switch with a short pulse of higher than rated voltage and then reduce to less than rated voltage.
eg.
using a 15V regulator feeding the relay via a resistor equal in value to the coil resistance will switch at 15V and reduce to 7.5V after the pulse has dissipated.
The short pulse comes from a small electrolytic that charges to nearly 15V and then discharges through the relay coil and switching transistor to that 7.5V, over a period of around 10ms.
22uF charging through 1100r is ~14V after ~5RC periods i.e. in about 120ms.
The 1100r coil will dissipate ~51mW @ 7.5V instead of 131mW
And it consumes 6.8mA instead of 11mA. That in a Mez saves >8mA of asymmetrical current draw.
Matched Pairs of SK170BL required?
This is my first B1 Buffer build, so I want to get it right. I have not read the entire thread yet, but I am working through the AudioCircle build thread. I've never built anything that required matched pairs of transistors, although I have several projects in the pipeline that will require them. I'm more of a solder monkey and kit builder, over the real theory of operation although I had rudimentary electronics training over ~35 years ago.
My question is: Are the SK170BL transistors required to be in matched pair configurations to fully appreciate everything the B1 Buffer offers, or is it simple a 'nice to have' for optimum performance? It appears from the BOM that it can be built without matched pairs, but I'd like to know what I'm giving up if I go that route.
Secondly, can you get matched pairs from a distributor when you buy them, and if not, I would like to know what and how do you measure transistors in order to match pairs. Do the distribution houses like Mouser, Farnell, etc. offer these as matched pairs and are they reliable. How much more $ usually for matched pairs?
I know this is a basic question for all you veterans of amp building, but I need to learn to go down one more level beyond my Heathkit building days of the 80's to understand the operation a bit better.
Rick
Edit: Apologizes if this has been covered elsewhere in the thread that I haven't gotten to yet.
This is my first B1 Buffer build, so I want to get it right. I have not read the entire thread yet, but I am working through the AudioCircle build thread. I've never built anything that required matched pairs of transistors, although I have several projects in the pipeline that will require them. I'm more of a solder monkey and kit builder, over the real theory of operation although I had rudimentary electronics training over ~35 years ago.
My question is: Are the SK170BL transistors required to be in matched pair configurations to fully appreciate everything the B1 Buffer offers, or is it simple a 'nice to have' for optimum performance? It appears from the BOM that it can be built without matched pairs, but I'd like to know what I'm giving up if I go that route.
Secondly, can you get matched pairs from a distributor when you buy them, and if not, I would like to know what and how do you measure transistors in order to match pairs. Do the distribution houses like Mouser, Farnell, etc. offer these as matched pairs and are they reliable. How much more $ usually for matched pairs?
I know this is a basic question for all you veterans of amp building, but I need to learn to go down one more level beyond my Heathkit building days of the 80's to understand the operation a bit better.
Rick
Edit: Apologizes if this has been covered elsewhere in the thread that I haven't gotten to yet.
None of the Mez jFETs need to be matched.
The two audio jFETs in each channel do need to be selected for Idss.
Read up on measuring Idss.You can do this yourself with very simple and very cheap resources.
As far as I know retailers do not offer a matching service.
You need to look at use DIYers, if you want device matching.