Ran into a weird problem.
the U100 LM833 was rail to rail shorted, and the regulators were blown up.
Replaced the regulators, and the LM833 with an NE5532 which is all i have.
High pass works, full range works, but NO lo pass at all.
I can see the low-signal on the outputs of the TL074 behind the FREQ pot.
However, U100 I have somewhat dirty audio signal on both op amp positive and negative inputs.
the negative inputs pin 2 and 5 are a +0.5v bias.
the positive inputs have no offset.
however BOTh op-amp outputs are -13vdc. hmmm.... strange....
any ideas?
the U100 LM833 was rail to rail shorted, and the regulators were blown up.
Replaced the regulators, and the LM833 with an NE5532 which is all i have.
High pass works, full range works, but NO lo pass at all.
I can see the low-signal on the outputs of the TL074 behind the FREQ pot.
However, U100 I have somewhat dirty audio signal on both op amp positive and negative inputs.
the negative inputs pin 2 and 5 are a +0.5v bias.
the positive inputs have no offset.
however BOTh op-amp outputs are -13vdc. hmmm.... strange....
any ideas?
It sounds like you have an open trace or possibly an open pot or switch.
The op-amp appears to be working correctly. If the inverting input voltage is greater than the non-inverting input voltage, the op-amp output will swing toward the negative supply rail. The gain (determined by circuit components, not the gain pot) determines how far it swings. Since the VCA (SIP IC, 2155?) is generally connected to the inverting input of the op-amp with no resistance, slight DC offset can cause very large swing.
Remove the VCA and connect a 1k resistor from it's input terminal to it's output terminals (the end pins are input and output). Does that eliminate the DC offset and allow the amp to produce audio when set to low pass?
The op-amp appears to be working correctly. If the inverting input voltage is greater than the non-inverting input voltage, the op-amp output will swing toward the negative supply rail. The gain (determined by circuit components, not the gain pot) determines how far it swings. Since the VCA (SIP IC, 2155?) is generally connected to the inverting input of the op-amp with no resistance, slight DC offset can cause very large swing.
Remove the VCA and connect a 1k resistor from it's input terminal to it's output terminals (the end pins are input and output). Does that eliminate the DC offset and allow the amp to produce audio when set to low pass?
the THAT2155 IC as no audio in its input or output if i scope it.
appears to be a single channel IC, but its a 2 channel amplifier.
appears to be a single channel IC, but its a 2 channel amplifier.
The preamp level is mixed in low pass, if I'm not mistaken.
You probably won't see audio on the input or the output.
Did you try bypassing it with the resistor?
You probably won't see audio on the input or the output.
Did you try bypassing it with the resistor?
i removed the IC with the old fashoned bend it until it falls off method. I got replacements kicking around just in case it was bad anyway....
Installed 1K resistor in place, no go. no sound on LPF, offset still present on U100 outputs
Installed 1K resistor in place, no go. no sound on LPF, offset still present on U100 outputs
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yes. theres still DC on the output of U100. There is -1.43v on the resistor DC.
no audio on resistor.
no audio on resistor.
Check all of the op-amps. Are there any op-amps that the voltage on the inverting input and the non-inverting inputs don't match within ~0.01v DC?
hard to take accurate measurements down to the milivolt but nothing out of the ordinary. all ok.
except for U100. its the only one thats wacked out.
except for U100. its the only one thats wacked out.
boy do i feel like an idiot.
I got lows now. I have the amp bridged on the test bench.
I had to turn the other gain knob up. then here comes the lows. which is strange. when LPF is on, it "inverses" the gain.
I got lows now. I have the amp bridged on the test bench.
I had to turn the other gain knob up. then here comes the lows. which is strange. when LPF is on, it "inverses" the gain.
Any meter will read down to 1mv. Place the probes directly on the adjacent inverting and non-inverting input pins for each op-amp.
With the resistor out of the circuit, what is the DC voltage on the input and output pins of the VCA?
With the resistor out of the circuit, what is the DC voltage on the input and output pins of the VCA?
The gain should be the same on low pass, as far as I know. Are you sure that both channels have output?
yes. both channels have output.
but when i flip over to low, the gains inverse. meaning when i turn up the left control, it brings up the right channel.
thats the weird part. Oh, about the meter, im using an old school fluke 8600A battery model with a fresh set of batteries.
Its not that accurate as the calibration is just a hair off.
but when i flip over to low, the gains inverse. meaning when i turn up the left control, it brings up the right channel.
thats the weird part. Oh, about the meter, im using an old school fluke 8600A battery model with a fresh set of batteries.
Its not that accurate as the calibration is just a hair off.
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