Yes you precisely understood my reasoning.
I've decided to go lower Idss than your 11mA/13mA pair because I am intending to go with 28V rails (running 23V at the moment) when the new transformers arrive.
I have some heatsinks arriving for the jfets that are yet to be experimeted on, so we will see whether I can go higher.
I'm also using TO264 laterals biased around 1.7A, so my circuit values are bit different than other peoples.
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Yeah, but NP tried higher power and went back to lower powered version. Was it due to standard FW case and heatsinks?
That wasn't high powered with PCF.
That was high powered with cascodes and negative feedback. ie Conventional high powered amp with laterals, but it is very difficult to get the open loop gain required to get the feedback to generate decent damping factor using laterals in the selected amp topology, hence the reason to use PCF with the laterals.
Read over the F7 manual again, Papa explains the design process.
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I know it's a pain in the ars3 chasing that up so, here it is:
"Most of your “better” amplifiers have higher damping factors like 20 or 100, and this amplifier was only about 5 or so, typical of a simple Common-Source topology and low negative feedback.
The damping factor, which is the inverse of the output impedance, determines the flatness of the amplifier response when the load impedance varies and is important to the transient response of reactive loudspeakers, which is just about all of them. You could argue that single-ended tube amplifiers sound good and don't have much damping factor, and you would be right, but I'm not trying to duplicate single-ended tube amplifiers here – the First Watt SIT amplifiers, doing a decent solid-state emulation of Triodes, represent that genre nicely. I'm shooting more for the sound of The Beast With a Thousand JFETs but with obtainable parts.
I tried other Fets, varied the the feedback network values and tweaked the gain symmetry. These efforts delivered a little more warmth and dimensionality, and about a year ago I decided to go ahead and build a small pilot run in anticipation of a product release – after all, it was a perfectly nice amplifier. Still, I was not really satisfied, so I created new printed circuit board artwork, adding cascode operation to the input stage and doubling up the output devices.This version had a little more control, but still fell short.
So...
I went back to the original simple design. On the wall in my lab is a little box with a glass window labeled:
DANGER - POSITIVE FEEDBACK
I broke the glass. Inside was a single resistor."
"Most of your “better” amplifiers have higher damping factors like 20 or 100, and this amplifier was only about 5 or so, typical of a simple Common-Source topology and low negative feedback.
The damping factor, which is the inverse of the output impedance, determines the flatness of the amplifier response when the load impedance varies and is important to the transient response of reactive loudspeakers, which is just about all of them. You could argue that single-ended tube amplifiers sound good and don't have much damping factor, and you would be right, but I'm not trying to duplicate single-ended tube amplifiers here – the First Watt SIT amplifiers, doing a decent solid-state emulation of Triodes, represent that genre nicely. I'm shooting more for the sound of The Beast With a Thousand JFETs but with obtainable parts.
I tried other Fets, varied the the feedback network values and tweaked the gain symmetry. These efforts delivered a little more warmth and dimensionality, and about a year ago I decided to go ahead and build a small pilot run in anticipation of a product release – after all, it was a perfectly nice amplifier. Still, I was not really satisfied, so I created new printed circuit board artwork, adding cascode operation to the input stage and doubling up the output devices.This version had a little more control, but still fell short.
So...
I went back to the original simple design. On the wall in my lab is a little box with a glass window labeled:
DANGER - POSITIVE FEEDBACK
I broke the glass. Inside was a single resistor."
Have a look at Nelson's article on testing mosfets (not matching). Make his little test circuit. Mine is a simplified version.
http://www.firstwatt.com/pdf/art_mos_test.pdf
Are you trying to match Vgs threshold, Vgs at an operating point or Id vs Vgs curves?
Are you trying to match N to N or N to P?
If you're try to match N to N and P to P, I start by sorting them into Vgs threshold values. I do that with a component tester you can get them on ebay very cheap.
Then I start producing Id vs Vgs curves. Refer to Nelson's article. All you really need is a trimpot with wiper fed across a 1 Meg resistor into the gate of the mosfet.
I found on my parts that they match so closely (ie N to N, P to P), that matching for Vgs threshold ended up giving you perfectly matched parts.
You don't need critical matching for this amp though.
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Only trying to match same types n to n; p to p. Just 1 pair. I think trying to an operating point. I can only get to 30v max on the lab supply. Was just hoping to see a match at 15v, maybe at 30v. Repairing amp with 42v, class AB. I'm using the exicon "s" but just want to see how close they are. I have the red for n-ch and green for p -ch.
I thought they were the same articles in different places.
Thanks, I'll check it out.
I thought they were the same articles in different places.
Thanks, I'll check it out.
I wouldn't worry too much about matching at 40V Vds.
Anything above 20V Vds is probably ok. Then take Vgs measurements at 50mA 100mA 200mA 500mA and 1A (mounted on a heatsink). I start at 1A and go backwards after letting heatsink hit equilibrium, repeat 2 more times. One day I'll get some sought of thermostat for the heatsink.
Label mosfet with a small piece of masking tape, record values in excel, and produce Id vs Vgs curves.
After you've done this you'll probably realise all you needed to do is match for Vgs threshold. Haha. Invention is 99% perspiration 1% inspiration. In my case that always seems to happen taking a dump on the toilet.
If they're all from the same manufacturing batch these devices match extremely well.
Anything above 20V Vds is probably ok. Then take Vgs measurements at 50mA 100mA 200mA 500mA and 1A (mounted on a heatsink). I start at 1A and go backwards after letting heatsink hit equilibrium, repeat 2 more times. One day I'll get some sought of thermostat for the heatsink.
Label mosfet with a small piece of masking tape, record values in excel, and produce Id vs Vgs curves.
After you've done this you'll probably realise all you needed to do is match for Vgs threshold. Haha. Invention is 99% perspiration 1% inspiration. In my case that always seems to happen taking a dump on the toilet.
If they're all from the same manufacturing batch these devices match extremely well.
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Hahaha
They're laterals. This is perfectly normal.
Vgs threshold is extremely low on laterals
Years ago I worked as a mechanical design engineer for Beltone electronics at a time when the company designed their own amplification circuits for the hearing aids they manufactured and sold to people through a network of hearing aid dealers. I was involved with designing the cases the electronics were packaged in along with the battery doors/enclosures and the hook shaped part that attached to the case and hung on the users ear....not too high tech but essential. The really high tech guys that were electrical engineers. Designing miniature amplifiers plus the equalization circuits. As part of their job they designed amplifiers and preamps for full sized listening rooms that they used as as a reference for miniaturization. It was pretty cool comparing the two in the anichoic chambers. I should have realized I would have been happier as an electronics engineer.