I once had a bad PSU cap that caused a similar issue. Well, I can’t say with absolute certainty that it was a bad cap, but changing the caps on the PSU took care of it. I tried reflowing first.
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The new transformer is physically quiet, unlike the old transformer which had a noticeable hum. I'll pursue a potential DC issue resolution but I will continue to look for other solutions until I get this squared away.
I have a variac I used to slowly bring up the PSU, so I don't have a light bulb tester setup. I'll work on that.
Rail Voltages:
PSU No Load: 25.3v + -
PSU + L Channel Only: 23.3v + -
PSU + R Channel Only: 23.5v + -
PSU Both Channels: 22.7v +-
Resistance from Signal 0V to chassis (CL-60 between the PSU Board and Chassis tied to mains ground pin) 10 ohms
I did notice that with only one channel powered up hum in the speaker connected is around half as loud as with both channels connected. This result was the same with either channel connected and I tried this test multiple times.
I still have hum in both speakers
LEDs, Resistors, and Zeners
PSU: LED Dropping Resistors R20, R21: 4.7K Ohm 1/4w 5%
PSU: LED TLHR6400 Red Diffused 5mm
Amp: LED Current Limiting Resistor R13: 10K Ohm 1/2w 1%
Amp: LED TLHB5400 Blue Tinted Diffused
Amp: Zener Z1, Z2: 1N4735A-TR 6.2v 1.3w
Amp: Resistor R7 feeding Z1: 10K Ohm 1/2w 1%
Thanks again for your patience!!
Glen
You're referring to Zen Mod's post #27? I read through that thread but the updated values for R7 and R8 didn't make it into the BOM and that's where I started. I'll get these 3K3 resistors on order. But if I have to change these, I might as well go ahead with 2 pico dumbs LED mod.
However, I wonder if anyone thinks with 6.2v for Z1 and Z2 and R7 and R8 being at 10K ohms as they are in my circuit would cause a hum through the speakers...
You have a ground problem. That the buzzing gets worse when you short the inputs is verification. Something is attached that shouldn’t be, or something that you think is attached to ground actually isnt.
I suspect a wonky joint at the PSU connecting the two sides together, and/or perhaps you have cold solder joint or two holding the PSU caps to the board. The photos you’ve posted have been good, but sadly no glaring error is obvious.
Your amp works fine - you have proper bias and no output offset.
I suspect a wonky joint at the PSU connecting the two sides together, and/or perhaps you have cold solder joint or two holding the PSU caps to the board. The photos you’ve posted have been good, but sadly no glaring error is obvious.
Your amp works fine - you have proper bias and no output offset.
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Replace both R7 and R8 with anything between 2.5k and 5k. A lower value is better.
This is most likely your problem. Either way it should be fixed.
10k is current starving your zener. You have zero regulation.
Fix this immediately.
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1) Yes. Make sure you install the leds in the reverse direction to zeners.
2) Yes, if you have high efficiency speakers. This should be fixed.
There is still the possibility you have an additional problem, but let's make this amp as good as it can be and eliminate all potential problems.
The leds work best around 8mA, so a 2k to 2.5K resistor for R7 and R8. They work better than zeners for all values of current, but at 8mA they are even better.
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Have you even tried mounting the Antek Flat on the base plate (not vertical as in your Pix) ?? Significant fields come out the axis of a Toroid
I have My leds mounted on the inside of the face plate of my Case.. shining thru 1.0 mm holes ..their teeny Cat 5 power wires travel far(ish), yet there are no noises as result. Look elsewhere ?
My F6 is totally quiet with my ears to the speakers.
However, i have the same buzzing hum issue as you when i build the other ClassA amp.
It started with both channels power thru a single toroidal with SLB crc board.
For weeks, i struggled with this humming. And i keep thinking there is some thing wrong with my boards. Then some helpful guy told me to isolate channel by channel individually.
Then i found out, when powered as a single mono bloc configuration, they are both totally quiet individually, without me doing anything to them.
So i know i have some kind of ground loop issues with this set of mine, till date i still cannot resolved this problem, so i run them as a dual mono bloc in s single chassis.
2 trafos, 2 SLB, 2 channels in 1 chassis.They are all quiet now.
However i can still replicate the same buzzing issue by just going back to single power supply.
Try running your single channel with the power supply first, and see what results you get.
However, i have the same buzzing hum issue as you when i build the other ClassA amp.
It started with both channels power thru a single toroidal with SLB crc board.
For weeks, i struggled with this humming. And i keep thinking there is some thing wrong with my boards. Then some helpful guy told me to isolate channel by channel individually.
Then i found out, when powered as a single mono bloc configuration, they are both totally quiet individually, without me doing anything to them.
So i know i have some kind of ground loop issues with this set of mine, till date i still cannot resolved this problem, so i run them as a dual mono bloc in s single chassis.
2 trafos, 2 SLB, 2 channels in 1 chassis.They are all quiet now.
However i can still replicate the same buzzing issue by just going back to single power supply.
Try running your single channel with the power supply first, and see what results you get.
Since you have installed the LED mod, I would now suggest to also using 0.1 Ohm resistors to further reduce any thermal effects to as low as possible.
If you're going to invest that kind of money you may as well maximise the performance as much as possible. Go with 0.1 ohm.
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I have been looking and looking for a ground problem but so far if that is the issue it has eluded me and defied all the other troubleshooting. I have good continuity between all of the ground points. I used liquid flux on all of my solder joints and the solder flowed quite well.
I hope Georgeged and 2 picoDumbs are on to something with changing R7 and R8 to a lower value. Someone on the The diyAudio Firstwatt F6 thread also posted something about using 10K Ohm resistors for R7,8. I need to find that post to ask if that person had hum in the speakers too...
Thanks for hanging in on this with me!!!
I should have the parts from Mouser by Wednesday. I will report back then...
I assume 3R3K resistors would be okay in this circuit too? Also, can the E-562 current regulating diodes replace R7 and R8?
I did - I removed the Toroid and sat it flat outside the chassis. No change!
Oooo - I think I'll drill the faceplate for LEDs too!