So the amp didn't hum when the jfets were removed? With the input stage disabled and it quiet means that 1) It's most likely not PSU hum, and is a wiring error, or bad devices themselves.
As all eBay K170/J74 are assumed to be fake, I think you may have figured it out...
fetaudio.com
He is essentially the only place with genuine devices.
Hi Whaleman
If finally you get new jfets at home
take care and handle them without ESD: electrostatic discharge.
Antistatic gloves is useful sometimes.
Best regards
http://www.superiorglove.com/industry-gloves/electronics-gloves/anti-static-electronics-gloves
http://www.navsea.navy.mil/nswc/crane/sd18/Public Documents/ResourcesElectDischargeCons.pdf
How to Test an N-Channel JFET - YouTube
EEVblog #247 - Anti Static Bag Myth Revisted | EEVblog - The Electronics Engineering Video Blog
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Hi Whaleman
If finally you get new jfets at home
take care and handle them without ESD: electrostatic discharge.
Antistatic gloves is useful sometimes.
Best regards
Anti-Static Electronics Gloves - ESD Electrostatic Discharge & Dissipative Static Free Glove
http://www.navsea.navy.mil/nswc/crane/sd18/Public Documents/ResourcesElectDischargeCons.pdf
How to Test an N-Channel JFET - YouTube
EEVblog #247 - Anti Static Bag Myth Revisted | EEVblog - The Electronics Engineering Video Blog
Thanks, I appreciate that video. Here is the P-channel version from the same gentlemen: How to Test a P-Channel JFET - YouTube
According to him, these (most likely miserable fake) Linear System J74s JFETS are bad and are from eBay. There is no conductivity as he suggests there should be. My meter is pushing 2.4 VDC for ohms testing at low resistance, and still does no come through the junction. See photo diagram, my meter does not show any conductivity when probes applied as shown.
If anyone has some real Linear Systems J74s out of a circuit, can you test this result?
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When I was having problems because of my toasted outputs, ZM commented "Please toss those horrible automotive connections". (see post 998)
I am starting to connect everything and have this same supply of Tyco Electronics connectors. I want to set up my amps so I can switch boards in and out if I want to and do it with some ease.
I can't see why these are so inferior. They are not gold, but do the job. Aesthetics, connection integrity, sonic effect? Why so "horrible"?
I am starting to connect everything and have this same supply of Tyco Electronics connectors. I want to set up my amps so I can switch boards in and out if I want to and do it with some ease.
I can't see why these are so inferior. They are not gold, but do the job. Aesthetics, connection integrity, sonic effect? Why so "horrible"?
As all eBay K170/J74 are assumed to be fake, I think you may have figured it out...
Not quite true. The J74 on ebay, yes, nothing but junk. The 2SK170...another story. I recently bought a lot of 200 BL's and they test quite well, with curve shapes near-identical to some BL's I bought from Mouser in 2008. Tested on the "locky-z" tester. Here's the test data:
Min = 6.67mA
Max = 11.44mA
Median = 8.04mA
Std Dev = 0.86mA
Now I have a bunch of 2SK170's I need to use somewhere...
I suspect the new system driving the amp is the culprit. Some preamps don't mute their outputs on shutdown, which could cause the crack (or pass through shutdown transients from sources). Can you reassemble the old system in the new location and replace one piece at a time to find the culprit?
So the amp didn't hum when the jfets were removed? With the input stage disabled and it quiet means that ... bad devices themselves.
As all eBay K170/J74 are assumed to be fake, I think you may have figured it out...
This is the conclusion of the whole problem, the noisy channel was due to a bad new pair of Linear Systems J74s from a US eBay seller. Fakes?
A pair of Fairchild J76 did the trick wonderfully, no noise whatsoever. So the other channel got them as well. They might be lower in transconductance, but not sure. All other specs seem to line up OK. I reset DC offset, BIAS seemed to not need much readjust.
This story has a happy ending, the amp is extremely quiet with no stray pickup or noisy transistors.
Thanks to you all, I ended up with a better wiring route and a lesson for future amps.
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