So where do you guys get your JFETs from? The DiyAudio store has matched quads availble in 6~8 mA Idss flavor but they're $40 plus ten for shipping. That seems like a lot of money fors mall signal JFETs. Am I crazy or is that just the going rate?
I got mine at DIY Store but for me is not so much the cost of the JFETs (don't get me wrong, it's the second most expensive thing on the BOM), but the shipping that I find a bit too high.
Yeah. Four FETs could go ina na envelopoe for 49 cents but instead they put it in a box and charge $10 ! Oh well.
I guess they arer 2 or 3 bucks less from Digikey so we are paying for the matching. Not a huge issue, I agree, I guess I'm lucky compared to other scenarios. Also. DIY is selling the Toshiba part numbers, not the Linears, in the 170/74 quads. Are they really the coveted Toshibas?
If you are really brave, you can build the original ACA (not this Mini) using these PCB mounted heatsinks instead of chassis mounted heatsinks. The original ACA ran at a class A bias current of 1.45 amperes while this ACA Mini runs at a class A bias current of 0.46 amps. That is a factor of 3 times less current (and 3 times less power).
However if you are really brave, you can juuuuust barely prove that the original ACA on these heatsinks, operates its MOSFETs juuuuuust barely below the absolute max allowed junction temperature (150 degrees C). The heatsinks will burn your finger (or your toddler's tongue) instantly, but the MOSFETs will survive.
The great thing about this is: original ACA can be built with no Toshiba JFETs. You save big money AND you save the aggravation of paying for separate, costly, transistor shipping.
All you need to do is (a) create a PCB layout of the original ACA using this form factor and these heatsinks; (b) perform circuit design calculations which show you how to replace the single Toshiba JFET source follower, with a $0.65 Fairchild JFET source follower, and get the same behavior. Original ACA doesn't have both N-JFETs and P-JFETs, so there's no requirement for matched characteristics between N and P. Just an NJFET and a tiny number of resistors. Plop in the new Fairchild NJFET, plop in your newly calculated resistor values, and Bob's your uncle.
Here's a figure from the engineering datasheet of the ACA Mini's heatsinks. The Original ACA's transistors dissipate about 18 watts, which means these heatsinks will be about 57 degrees C hotter than the ambient air. If the air is 27C then the heatsinks will be at 84C. Ouch. But the MOSFETs will survive, juuuuust barely. And if you're not all that brave you can rig up a couple of DC fans to cool the heatsinks and create additional safety margin. Fans sell for under $10 so you still save big bux.
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However if you are really brave, you can juuuuust barely prove that the original ACA on these heatsinks, operates its MOSFETs juuuuuust barely below the absolute max allowed junction temperature (150 degrees C). The heatsinks will burn your finger (or your toddler's tongue) instantly, but the MOSFETs will survive.
The great thing about this is: original ACA can be built with no Toshiba JFETs. You save big money AND you save the aggravation of paying for separate, costly, transistor shipping.
All you need to do is (a) create a PCB layout of the original ACA using this form factor and these heatsinks; (b) perform circuit design calculations which show you how to replace the single Toshiba JFET source follower, with a $0.65 Fairchild JFET source follower, and get the same behavior. Original ACA doesn't have both N-JFETs and P-JFETs, so there's no requirement for matched characteristics between N and P. Just an NJFET and a tiny number of resistors. Plop in the new Fairchild NJFET, plop in your newly calculated resistor values, and Bob's your uncle.
Here's a figure from the engineering datasheet of the ACA Mini's heatsinks. The Original ACA's transistors dissipate about 18 watts, which means these heatsinks will be about 57 degrees C hotter than the ambient air. If the air is 27C then the heatsinks will be at 84C. Ouch. But the MOSFETs will survive, juuuuust barely. And if you're not all that brave you can rig up a couple of DC fans to cool the heatsinks and create additional safety margin. Fans sell for under $10 so you still save big bux.
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Attachments
Tomorrow I'll be sending out messages to all members of this group buy. You'll be asked this question: Would you like to receive your boards asap (meaning mailed in the next couple days) OR would you like to receive your boards at a later date with a matched set of genuine Harris 9520/520. Please don't answer in this thread, it will just get messy and I won't be able to keep track.
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That's a great idea. Getting the Harris devices would really ease getting parts!
Everything is ready to go to the Post Office tomorrow! I’m to the point where I just want these things out of my life😂
IMPORTANT:
You will receive two baggies with each board. Each baggy contains a MATCHED PAIR!
You will receive two baggies with each board. Each baggy contains a MATCHED PAIR!