Good day gents,
I have a quick question/s about setting bias on what I believe is a 750s amplifier.
Amp came in with a badly burnt power supply and right channel. Both were rebuilt with new parts, amp is up and running thus far.
I’m setting the bias via the voltage across the source resistors. With the amp on start up, the voltage across the resistor is 5mV.
As the amp warms up it goes up and settles around 11mv. If I try to lower it however it remains at 11mv, if I go beyond the halfway setting on the pot, it increases.
For now I’ve set it at this (just under the halfway point in the pot) and have it running.
I compared it with the left channel, but the left channel remains constant at 0mv ( at start up and warm).
I belive this should be ok, but just wanted to see if anyone had similar experiences.
I have a quick question/s about setting bias on what I believe is a 750s amplifier.
Amp came in with a badly burnt power supply and right channel. Both were rebuilt with new parts, amp is up and running thus far.
I’m setting the bias via the voltage across the source resistors. With the amp on start up, the voltage across the resistor is 5mV.
As the amp warms up it goes up and settles around 11mv. If I try to lower it however it remains at 11mv, if I go beyond the halfway setting on the pot, it increases.
For now I’ve set it at this (just under the halfway point in the pot) and have it running.
I compared it with the left channel, but the left channel remains constant at 0mv ( at start up and warm).
I belive this should be ok, but just wanted to see if anyone had similar experiences.
Attachments
Are you sure that you have 11mv across all of the 0.1 ohm source resistors?
I would expect the idle current to be higher with that much bias.
I would expect the idle current to be higher with that much bias.
Yes sir.
I double checked. I placed my fluke 115 inline to confirm the current draw.
Only thing strange I’m seeing aside not allowing me to lower the bias is during the initial power up, within the first minute or two, the current draw fluctuates between 3-5amps, after which it stabilizes around 1.9amps.
I double checked. I placed my fluke 115 inline to confirm the current draw.
Only thing strange I’m seeing aside not allowing me to lower the bias is during the initial power up, within the first minute or two, the current draw fluctuates between 3-5amps, after which it stabilizes around 1.9amps.
Is the bias 11mv before and after the idle current changes?
I set the idle current as shown below. 1mv across the resistors is generally enough.
http://www.bcae1.com/temp/ausettingbias.swf
I set the idle current as shown below. 1mv across the resistors is generally enough.
http://www.bcae1.com/temp/ausettingbias.swf
At power up, the readings on the amp meter fluctuates between 3.5a to 5.8a, during this it’s difficult to get a volt reading across the source resistors.
After approximately 2 minutes or so, it stabilizes to around the 2.0a, and there I can read off the 11mv.
Turning the bias pot fully CCW does not lower the voltage across the source resistor.
After approximately 2 minutes or so, it stabilizes to around the 2.0a, and there I can read off the 11mv.
Turning the bias pot fully CCW does not lower the voltage across the source resistor.
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As a side note, it's not good to insert a multimeter in series with the power supply wiring. You can use a low value shunt resistor and measure the voltage across the resistor to determine the current draw.
Does the amp produce perfectly clean audio while the current draw is erratic and after the current raw stabilizes?
Are the heatsink-mounted FETs tightly clamped to the heatsink?
Does the amp produce perfectly clean audio while the current draw is erratic and after the current raw stabilizes?
Are the heatsink-mounted FETs tightly clamped to the heatsink?
Thanks for the note on the meter.
The amp doesn’t pass audio during the Erratic current draw. It passes clean audio only after the current settles.
I do have everything clamped/screwed in the heatsink. I thought I had it nailed, only to bias and final test before this.
Whilst doing these last checks I lost a couple of the FETs in the power supply, I’ll have to replace these and get back.
The amp doesn’t pass audio during the Erratic current draw. It passes clean audio only after the current settles.
I do have everything clamped/screwed in the heatsink. I thought I had it nailed, only to bias and final test before this.
Whilst doing these last checks I lost a couple of the FETs in the power supply, I’ll have to replace these and get back.
Thanks for the note on the meter.
The amp doesn’t pass audio during the Erratic current draw. It passes clean audio only after the current settles.
I do have everything clamped/screwed in the heatsink. I thought I had it nailed, only to bias and final test before this.
Whilst doing these last checks I lost a couple of the FETs in the power supply, I’ll have to replace these and get back.
The amp doesn’t pass audio during the Erratic current draw. It passes clean audio only after the current settles.
I do have everything clamped/screwed in the heatsink. I thought I had it nailed, only to bias and final test before this.
Whilst doing these last checks I lost a couple of the FETs in the power supply, I’ll have to replace these and get back.
After replacing the PS FETs, connect only remote and ground and confirm that the drive signal on the gate leg of every PS FET is OK.
Posting for the ones following.
I removed all power supply FETs, one was faulty, verified gate drive was ok, reinstalled all FETs.
I had a 5A in-line analog amp meter, I installed it on the B+ line and retried setting bias. Both channels were set and amp idles just about 1.8/1.9a.
Removed amp meter, and ran a signal with load just about 10/15mins, rechecked bias and it held.
Thanks for the help perry.
I removed all power supply FETs, one was faulty, verified gate drive was ok, reinstalled all FETs.
I had a 5A in-line analog amp meter, I installed it on the B+ line and retried setting bias. Both channels were set and amp idles just about 1.8/1.9a.
Removed amp meter, and ran a signal with load just about 10/15mins, rechecked bias and it held.
Thanks for the help perry.