hi guys,
newbie here. been reading through the forums about Op-amp ugrades and i had my first try a couple of days ago. i used LM4562 as the upgrade.
i have an old 2-way active crossover lying around, when i opened it up i found 10 op-amps:
3 op-amps (2 ba4560, 1 ba4558) in the input section
2 op-amps (2 ba4558) in the filter section
2 op-amps (2 ba4558) in the highpass output section
2 op-amps (2 ba4558) in the lowpass output section
1 op-amp (1 ba4560) in between the powersupply and audio section
been aware of oscillation and dc offset issues, i decided to swap op-amps per section so i can concentrate on a small area should a problem arise.
i swapped out the lowpass section and found 1mv dc offset. i've read anything less than 20mv is acceptable??? then i played music low to high volume, no problem.
then i did the same thing to the highpass section, no problem.
then the one between the power supply and audio section, no problem again.
then the filter section, still no problem.
it is in the input section where the problem starts. since there are two kinds of op-amps, i decided to swap the ba4558 first. no problem there. but when i swap the ba4560, it still plays fine until a particular portion of a particular musical track (michael ruff's I will find you there) causes the entire crossover to 'lock up' and just oscillate.
the rest of the tracks on that cd won't cause the oscillation. i soldered the ba4560 back and the problem goes away.
the two 4560 op-amps are the first two op-amps just after the RCA inputs and they were both configured to have a corner frequency of 21kHz.
all the caps are still stock but i placed 0.1uf ceramics on the +/- pins and + to gnd and - to gnd, nothing helps the 4562.
i would really appreciate any help/suggestions offered
thanks!
newbie here. been reading through the forums about Op-amp ugrades and i had my first try a couple of days ago. i used LM4562 as the upgrade.
i have an old 2-way active crossover lying around, when i opened it up i found 10 op-amps:
3 op-amps (2 ba4560, 1 ba4558) in the input section
2 op-amps (2 ba4558) in the filter section
2 op-amps (2 ba4558) in the highpass output section
2 op-amps (2 ba4558) in the lowpass output section
1 op-amp (1 ba4560) in between the powersupply and audio section
been aware of oscillation and dc offset issues, i decided to swap op-amps per section so i can concentrate on a small area should a problem arise.
i swapped out the lowpass section and found 1mv dc offset. i've read anything less than 20mv is acceptable??? then i played music low to high volume, no problem.
then i did the same thing to the highpass section, no problem.
then the one between the power supply and audio section, no problem again.
then the filter section, still no problem.
it is in the input section where the problem starts. since there are two kinds of op-amps, i decided to swap the ba4558 first. no problem there. but when i swap the ba4560, it still plays fine until a particular portion of a particular musical track (michael ruff's I will find you there) causes the entire crossover to 'lock up' and just oscillate.
the rest of the tracks on that cd won't cause the oscillation. i soldered the ba4560 back and the problem goes away.
the two 4560 op-amps are the first two op-amps just after the RCA inputs and they were both configured to have a corner frequency of 21kHz.
all the caps are still stock but i placed 0.1uf ceramics on the +/- pins and + to gnd and - to gnd, nothing helps the 4562.
i would really appreciate any help/suggestions offered
thanks!
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btw, it doesn't oscillate 3 levels down the max unclipped volume of my Alpine 4v headunit. 32/35.
i did my own troubleshooting by myself but i don't know if i'm in the right path.
i threw in a sine-wave from my function generator and found out that a frequency with 3.2volts starting 5,996kHz to beyond causes the crossover to 'lock up' and oscillate.
once it locks up, it oscillates @ 2.5kHz peak
funny thing is i was probing the output leg of the 4562 opamp and accidentally shorted the output with the inverting input. the lock up went away momentarily. so i tried to purposely short the output with the inverting input and it plays normally again until i 'unshort' it.
i did my own troubleshooting by myself but i don't know if i'm in the right path.
i threw in a sine-wave from my function generator and found out that a frequency with 3.2volts starting 5,996kHz to beyond causes the crossover to 'lock up' and oscillate.
once it locks up, it oscillates @ 2.5kHz peak
funny thing is i was probing the output leg of the 4562 opamp and accidentally shorted the output with the inverting input. the lock up went away momentarily. so i tried to purposely short the output with the inverting input and it plays normally again until i 'unshort' it.
You say it's oscillating... I assume you are looking on a scope to see what is happening ?
2.5Khz is low frequency for opamp instability.
10 opamps ? the 4562 draws far more current than the 4558/60. Look at the supplies ! If any opamp is fed resistively or if they are all fed of just a 78L12 type supply then if the opamps drive a low impedance filter network "hard" it may just draw enough to pull the rails down or modulate them.
2.5Khz is low frequency for opamp instability.
10 opamps ? the 4562 draws far more current than the 4558/60. Look at the supplies ! If any opamp is fed resistively or if they are all fed of just a 78L12 type supply then if the opamps drive a low impedance filter network "hard" it may just draw enough to pull the rails down or modulate them.
hi mooly,
here are the scope readings i took a while ago. maybe you can spend a little time helping me analyze it? that's the best wave resolution i can get out of my scope (kikusui 559a)
i'm using 8kHz tone (test tone cd) loaded from my alpine headunit. i'm getting my readings from the crossovers highpass RCA output gradually increasing volume level.
@ 0.281v
@0.671v
@1.724v
@2.307v
@ "locking volume", not headunit's max volume.
notice the frequency is now 2.5kHz even if i'm playing a 8kHz tone and the voltage is nearly 6volts when my headunit's max volume is around 4volts give or take.
here's the reading @ 1kHz. waveform is not very clean but i'm able to push max volume without "locking" the crossover.
and here's a photo of the power supply section. i'm not really good at this but i've been probing around it when i read your message.
readings with ba4560 installed at input stage
readings with lm4562 installed at input stage (photo still has 4558 and 4560)
the output at the diodes are strangely high with the 4562 (+/-18v) compared to +/-15.53v with the 4560.
thanks.
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Quite difficult to analyse without actually seeing it all for real.
In your first post you say it all seems OK until one particular opamp is replaced.
So we need to be methodical and measure (and scope) the power supply to that opamp and all the others as a first check. Lets see what that shows. The LT1072 is a low power switching PSU chip. It might be going into some form of limiting with the extra loading.
Connect the scope and measure from ground to Pin 8 of the opamps and then from ground to pin 4. The scope should show a straight line (DC) and if you AC couple the scope there should be minimal ripple on the rails. Anything over say 100 or 200 mv pk/pk may be enough to start causing problems. Ideally it should be way below that at only a few millivolts.
That has to be your first check.
Edit, you say the rails seem high with the LM4562 fitted. If that is being read on a DVM then it's possible that noise on the rails is giving a false reading. This is why we have to scope the rails to see what is going on.
In your first post you say it all seems OK until one particular opamp is replaced.
So we need to be methodical and measure (and scope) the power supply to that opamp and all the others as a first check. Lets see what that shows. The LT1072 is a low power switching PSU chip. It might be going into some form of limiting with the extra loading.
Connect the scope and measure from ground to Pin 8 of the opamps and then from ground to pin 4. The scope should show a straight line (DC) and if you AC couple the scope there should be minimal ripple on the rails. Anything over say 100 or 200 mv pk/pk may be enough to start causing problems. Ideally it should be way below that at only a few millivolts.
That has to be your first check.
Edit, you say the rails seem high with the LM4562 fitted. If that is being read on a DVM then it's possible that noise on the rails is giving a false reading. This is why we have to scope the rails to see what is going on.
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mooly,
when i read your message i already reinstalled the ba4560 @ the input section. i was checking the output and there was oscillation but not as severe as with the 4562. i'll check the rails as soon as i reinstall the 4562 but here's the data i got with the ba4560 at the input.
DC - straight line
AC ripple @ diode output (+/-) - 85.5mVp-p
AC ripple @ op-amp (+) - 37.05mVp-p
AC ripple @ op-amp (-) - 42.75mVp-p
here's a photo of the waveform magnified.
negative rail
positive rail
i'll try to reinstall the 4562 today and check ac ripple with them installed
thanks.
I also did this to an AC 2xs.However I did not swap out the low pass IC's.They were only being used from 333hz and down so I did see much sonic benifit from doing so.
After my first post I started thinking the same thing Mooly brought up.
The other ones were an AC Four.1 and an LA sound.
I had to rework the power supply on the LA sound because it was being pulled down to +/-5volts.However there were 17 LM4562 op amp's in the circuit.
I had no problems with the Audio Control units.
After my first post I started thinking the same thing Mooly brought up.
The other ones were an AC Four.1 and an LA sound.
I had to rework the power supply on the LA sound because it was being pulled down to +/-5volts.However there were 17 LM4562 op amp's in the circuit.
I had no problems with the Audio Control units.
Those rails are pretty noisy.
You could also try loading the rails (to save unsoldering opamps) by adding appropriate resistors across them although different opamps will react differently to the noise. It would allow you to see how the ripple increased with loading. Something to draw another 10 or 20 milliamps for example.
If this is the problem then all the advantages of better opamps are being wiped out by the noisy rails.
I have never used these but they might be worth considering, the "Excalibur TLE2072" which is a much improved TLO72. It has a low quiescent current and very high drive capability. I always liked the "old" TL072 sonically when used withing their limits.
http://www.datasheetcatalog.org/datasheet/texasinstruments/tle2072.pdf
You could also try loading the rails (to save unsoldering opamps) by adding appropriate resistors across them although different opamps will react differently to the noise. It would allow you to see how the ripple increased with loading. Something to draw another 10 or 20 milliamps for example.
If this is the problem then all the advantages of better opamps are being wiped out by the noisy rails.
I have never used these but they might be worth considering, the "Excalibur TLE2072" which is a much improved TLO72. It has a low quiescent current and very high drive capability. I always liked the "old" TL072 sonically when used withing their limits.
http://www.datasheetcatalog.org/datasheet/texasinstruments/tle2072.pdf
Randy,
did you also replace the input and filter stage op-amps? and yes, i checked a while back, the other op-amp is connected to the LEDs.
Mooly,
i'll give it a try. anyway, i reinstalled all 4558/4560 back to try and scope/measure each time i replace one. i just tried to replace one op-amp at the highpass output and this is what i got.
here's a photo of the highpass section and the op-amp i replaced.
there's a pot connected to the non-inverting input of the op-amp referenced to RCA ground. i can get a clean waveform when i turn the pot counterclockwise but the resulting output voltage would then only be @1.2volts
and by the way, you mentioned that the LT1072 is a low power switching PSU chip and that it might be going into some form of limiting with the extra loading. is there any way i can check this or we can conclude that the noisy rails' the culprit of all these? i can still give about a week more of tinkering before i might go get new op-amps.
thanks
did you also replace the input and filter stage op-amps? and yes, i checked a while back, the other op-amp is connected to the LEDs.
Mooly,
i'll give it a try. anyway, i reinstalled all 4558/4560 back to try and scope/measure each time i replace one. i just tried to replace one op-amp at the highpass output and this is what i got.
here's a photo of the highpass section and the op-amp i replaced.
there's a pot connected to the non-inverting input of the op-amp referenced to RCA ground. i can get a clean waveform when i turn the pot counterclockwise but the resulting output voltage would then only be @1.2volts
and by the way, you mentioned that the LT1072 is a low power switching PSU chip and that it might be going into some form of limiting with the extra loading. is there any way i can check this or we can conclude that the noisy rails' the culprit of all these? i can still give about a week more of tinkering before i might go get new op-amps.
thanks
unfortunately AC doesn't give out schematics for their products, at least that's what i got from them as a reply in my email.
i did however sketch it on a couple of pieces of paper. i'll try to see if i can scan it. will the power supply section be ok? i'll try to redraw that part only.
thanks randy. i would really appreciate it if you could show or pm me a photo of both sides of the board.
I wouldn't draw it out tbh... I think we are on to the problem with the extra loading of the opamps.
The pot you have shown appears to be a level control. If pins 1 and 2 are connected together and pin 3 goes to the wiper of the pot then it's a buffer with the signal level determined by pot setting.
Your scope shot shows quite a distorted signal but without measuring and following the signal through from input to output its impossible to say. Even slight distortion on an input signal can be greatly exagerated if any boost or cut (hp and lp filters) are applied.
I'm not quite sure where you are up too. Have you fitted all the original IC's back ?
mooly,
as of now the only 4562 installed are the ones in the output section.
so far, the distorted signal only shows at the highpass output using 8khz signal. using 1khz in the lowpass section yielded as clean waveform. but it starts to show distortion @4khz.
now back on to the highpass output, the waveform @ pin 3 of the 4562 has a clean signal, measuring @ pin 1 shows the distorted signal in the photo i posted above.
as of now the only 4562 installed are the ones in the output section.
so far, the distorted signal only shows at the highpass output using 8khz signal. using 1khz in the lowpass section yielded as clean waveform. but it starts to show distortion @4khz.
now back on to the highpass output, the waveform @ pin 3 of the 4562 has a clean signal, measuring @ pin 1 shows the distorted signal in the photo i posted above.
Another thought. Maybe we have two issues at work here.
The BA4560 (I had to look up the RC4560 opamp to get this) has a high drive capability, twice that of the LM4562. I wonder if the impedance of the filter network is just loading the opamp output stage too much. That distortion looks like an output stage loaded to heavily. So you may have to use the 4560 where originally fitted.
The other issue is the PSU loading with all those LM4562's. They do seem to be loading the rails and causing a lot of high frequency hash on the supplies.
The BA4560 (I had to look up the RC4560 opamp to get this) has a high drive capability, twice that of the LM4562. I wonder if the impedance of the filter network is just loading the opamp output stage too much. That distortion looks like an output stage loaded to heavily. So you may have to use the 4560 where originally fitted.
The other issue is the PSU loading with all those LM4562's. They do seem to be loading the rails and causing a lot of high frequency hash on the supplies.
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