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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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"?
I'm purely subjective ; can tolerate non-soldered connections only on caps , nowhere else
as repair monkey , I saw too many filthy contacts (some behaving as lousy diode)
as repair monkey , I saw too many filthy contacts (some behaving as lousy diode)
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 just set up my Aleph J in a different system and am now getting a fairly loud crack sound when I shut it down. Previously the sound was a kind of dull thump that was annoying but nothing like the sharp crack. Can't figure out what is different.
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?
I've had this also happen with another class A amp. Best thing to do is leave the preamp on while your turn the power amp off first. Never had noise (loud bark) doing it this way.
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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Thanks Bob, I intend on doing that (reassembling) when I have some time. I just noticed that the old thump still comes out of the left side, the new "crack" is on the right channel.
I'll also try the shut down amp before the pre, Whaleman, and congrats and enjoy.
I just noticed that the LED on the side of the power supply board that feeds the right channel, the one with the "crack" sound, goes out immediately on turning the power off, while the other LED fades out over about 50 seconds.
Please take well-it, close-up, in-focus photos and post them here.
Also look closely at your bleeder resistors on the PSU.
Also look closely at your bleeder resistors on the PSU.