Soundcraft Venue II sum amp problem

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syn

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Hi

I'm modding my Soundcraft Venue II sum amps. After blind testing various opamps I decided to go OPA1612 for mix sum, fader post and balanced output opamps - see the attached schematic please. However I run into a problem in regard to mix sum position.
Mix sum opamps are running quiet hot (50-60°C), compared to post fader and balanced output opamps, which run at about 10-15°C less. Various other opamps in the sum mix position run much cooler, again about 10-15°C less. All opamps are decoupled with .1uF NPO ceramics from +/- 17VDC supply rails to GND. 0.1uF caps are very close to OPA1612 supply pins (~5mm).
Any ideas what could cause such a problem? Maybe I need a small cap across some of those feedback resistors (R4, R8, R10)? If so what value?
I checked 'Small signal audio design' book by D.Self but don't have a clear idea on where to start.

Thanks
 

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JohnRoberts

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Low noise op amps generally run input long tail pair, at higher current density, likewise low impedance drive output stages may also draw more current. 60'C is warm but not catastrophic.... you can use glue on heatsinks for ICs. Looking at the data sheet you provided, they spec a max temp of 125'C max junction temp of 200'C (+85'C to meet specs.)

The nominal operating current of <4mA is only around 100mW dissipation. I have seen op amps run hot when they are oscillating but that amp is unity gain stable so should not sing along.

JR

PS; The old school test for components being too hot is to put some spit on your finger and touch the device. If the spit boils away you are dealing with 100'C or more.
 

abbey road d enfer

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Hi

I'm modding my Soundcraft Venue II sum amps. After blind testing various opamps I decided to go OPA1612 for mix sum, fader post and balanced output opamps - see the attached schematic please. However I run into a problem in regard to mix sum position.
Mix sum opamps are running quiet hot (50-60°C), compared to post fader and balanced output opamps, which run at about 10-15°C less. Various other opamps in the sum mix position run much cooler, again about 10-15°C less. All opamps are decoupled with .1uF NPO ceramics from +/- 17VDC supply rails to GND. 0.1uF caps are very close to OPA1612 supply pins (~5mm).
Any ideas what could cause such a problem? Maybe I need a small cap across some of those feedback resistors (R4, R8, R10)? If so what value?
OPA1612 is a SOIC package, that has much less thermal capability than the DIP packages of the other opamps in the mixer. They do run hotter.
How have you mounted them, adapters?
IMO there's not much improvement to expect, since the noise performance of the summing amps is governed by the transistors, which are unobtainium 2SB737's.
I'm not even sure you get an improvement for the post-fader voltage-follower.
After many customer complaints about the noise that happened when raising a fader, I replaced the opamp with a BiFET type (TL072), which has an instrinsic higher noise voltage, but the significantlty higher noise current of the 5532 resulted in noise modulation when moving a fader. In that respect, the OPA1612 is nearly 8 dB noisier than the 5532.
Today, I would use a lower noise OPA2134 or similar. I wished they existed at the time.
 
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JohnRoberts

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that data sheet says SOIC is 150'C/W thermal resistance junction to ambient. 1/5 watt (2x 3.6mA x30V) is only 30'C rise at the junction. Your measured 50-60' C at the case suggests junction could be marginal wrt meeting spec (85'C). Of course the case will not be at ambient temp, I don't see a thermal resistance spec for junction to case.

JR
 

Bo Deadly

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Have you recapped the supply? Maybe the supply resistance is just too high for local bypass to make a difference. Or maybe your ground resistance is compromised somehow. Use a meter to check ground resistance between the PS filter cap ground and bypass cap ground.

Otherwise you need to put a scope on it. It has to be oscillating. That's a very fast chip and probably quite cranky.

Note that "upgrading" op amps in a big desk like this is somewhat dubious. The 5532s are great amps. As Abbey already pointed out, there are other factors that might thwart your efforts. To get the noise benefit of an op amp like that you would have to consider all sorts of things. For example, the summing network impedance probably would have to be quite low to match that chip. Getting really low noise means using really low impedances which is something you sort of have to design for from the beginning.

If you want the desk to work as well as it possibly can, you first need to know how well it's working now. For that, your number one priority should be to get very good high resolution spectrums of each channel at different outputs. If the noise is better then 90dB down (and it should be), I'm not sure what you can do to improve on that enough to justify the work and cost.
 

abbey road d enfer

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For example, the summing network impedance probably would have to be quite low to match that chip. Getting really low noise means using really low impedances which is something you sort of have to design for from the beginning.
The summing amps are already optimised for the bus impedance and super low noise. The noise performance is that of the super-low noise transistors 2SB737. The opamp in this circuit could be a 741, the noise performance would (almost) be the same.
 

Bo Deadly

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The summing amps are already optimised for the bus impedance and super low noise. The noise performance is that of the super-low noise transistors 2SB737. The opamp in this circuit could be a 741, the noise performance would (almost) be the same.

Ah, I didn't look closely enough. I thought that was the mic pre.
 

syn

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Thank you so much for the response
Low noise op amps generally run input long tail pair, at higher current density, likewise low impedance drive output stages may also draw more current. 60'C is warm but not catastrophic.... you can use glue on heatsinks for ICs. Looking at the data sheet you provided, they spec a max temp of 125'C max junction temp of 200'C (+85'C to meet specs.)

The nominal operating current of <4mA is only around 100mW dissipation. I have seen op amps run hot when they are oscillating but that amp is unity gain stable so should not sing along.

JR

PS; The old school test for components being too hot is to put some spit on your finger and touch the device. If the spit boils away you are dealing with 100'C or more.
Yes, I don't think it is singing along, once it gets to ~60°C it stays there, if it was oscillating it would get hotter and hotter over the time, I think. The main reason why I panicked is that I have 1612 in other equipment as well but I never seen it going this hot. Thanks for the glue/heatsink idea.
OPA1612 is a SOIC package, that has much less thermal capability than the DIP packages of the other opamps in the mixer. They do run hotter.
How have you mounted them, adapters?
IMO there's not much improvement to expect, since the noise performance of the summing amps is governed by the transistors, which are unobtainium 2SB737's.
I'm not even sure you get an improvement for the post-fader voltage-follower.
After many customer complaints about the noise that happened when raising a fader, I replaced the opamp with a BiFET type (TL072), which has an instrinsic higher noise voltage, but the significantlty higher noise current of the 5532 resulted in noise modulation when moving a fader. In that respect, the OPA1612 is nearly 8 dB noisier than the 5532.
Today, I would use a lower noise OPA2134 or similar. I wished they existed at the time.
Yes, I mounted them on adapters.

Great info thank you, yes, I noticed a strange modulated noise when I move the master faders with 1612 installed in the sum amp, I thought I forgot to clean them... My board is factory fitted with 072 in that position, so unlike me, they must have been be aware of the master fader noise problem you mentioned. Also C4 is 100p factory fitted not 220p as on the schematic. Thanks to your post I'm ditching 1612 in the sum amp position, I'll revert to FET opamp there. I'll go OPA1656 for now, as I have them at hand. Thank you so much.

Do you think I shall replace C4 with 220pF or keep it 100pF?


Have you recapped the supply?
Yes, I have recapped the whole board and the power supply.
 
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jensenmann

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As long as you don´t measure what´s going on you don´t have a clue what causes the problem. Feed a 1kHz rectangular wave into the summingamp and measure it´s output at C5 with a scope to see if there´s ringing/overshoot going on.

Having a Venue 2 here, too, I´d recommend what Abbey said to change the post fader amp to FET input which has a lower input bias current. I´d use an OPA2134, too. And change the 2u2 coupling cap at the fader´s wiper to a foil cap to get rid of the leakage. That´ll make your faders scratch a lot less.
 
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abbey road d enfer

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My board is factory fitted with 072 in that position, so unlike me, they must have been be aware of the master fader noise problem you mentioned.
This is an ECO I initiated at the time. It was resented by the designer as a step back in performance, but customers knew better.
Also C4 is 100p factory fitted not 220p as on the schematic. Thanks to your post I'm ditching 1612 in the sum amp position, I'll revert to FET opamp there. I'll go OPA1656 for now, as I have them at hand. Thank you so much.

Do you think I shall replace C4 with 220pF or keep it 100pF?
This cap is here for stability. I wouldn't replace it blindly. You need to measure the response, distortion, IMD, phase margin, and choose a value that works with not only one sample, but a batch of opamps.
If you don't have access to an Audio Precision or equivalent, just leave that alone. That is unless you see the summing amp oscillating, of course.
 

JohnRoberts

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Thank you so much for the response

Yes, I don't think it is singing along, once it gets to ~60°C it stays there, if it was oscillating it would get hotter and hotter over the time, I think. The main reason why I panicked is that I have 1612 in other equipment as well but I never seen it going this hot. Thanks for the glue/heatsink idea.
Excess heat from oscillation is related to capacitance loads on output and perhaps mutual conduction in output drivers. It can be realtively steady state if it doesn't heat the junction above 200'C (silicon melts).
Yes, I mounted them on adapters.

Great info thank you, yes, I noticed a strange modulated noise when I move the master faders with 1612 installed in the sum amp, I thought I forgot to clean them... My board is factory fitted with 072 in that position, so unlike me, they must have been be aware of the master fader noise problem you mentioned. Also C4 is 100p factory fitted not 220p as on the schematic. Thanks to your post I'm ditching 1612 in the sum amp position, I'll revert to FET opamp there. I'll go OPA1656 for now, as I have them at hand. Thank you so much.
modulated noise is more evidence of possible HF oscillation
Do you think I shall replace C4 with 220pF or keep it 100pF?
I'd trust Abbey's advice

JR
Yes, I have recapped the whole board and the power supply.
 

Matt Syson

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From the schematic posted previously it strikes me that U1 (shown as NE5532) is already quite busy. It has a possible variable load on it's output as it feeds the insert point and in conjunction with the transistors it is coping with quite a high capacitive 'loading' of the virtual earth summing bus which may be altered by routing channels to the bus. OK a small choke is shown so you can at least tune your radio! Expecting one amplifier to cope with multiple and varying situations is likely to cause some issues. A 5532 is pretty tame, as would a MC33078 which I quite like but putting particularly 'fast' op amps into a stage that can see varying parameters is not always a good plan.
yes to replacing C6 (2u2) for a polysomething or swapping for Fet op amp.
Matt S
 

Matt Syson

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In a nutshell, fast opamps can be a liability when installed into boards/systems that were not designed for them. Correctly positioned ground planes and local RF bypassing are necessary and indeed some chips have 'tech notes' to say they shouldn't be socketed which would I expect mean that 'adaptors' could cause problems. Both for heat removal and extra stray capacitance / inductance.
Matt S
 

abbey road d enfer

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From the schematic posted previously it strikes me that U1 (shown as NE5532) is already quite busy. It has a possible variable load on it's output as it feeds the insert point and in conjunction with the transistors it is coping with quite a high capacitive 'loading' of the virtual earth summing bus which may be altered by routing channels to the bus.
That is the case in many mixers, where, with a large number of channels routed, the opamp runs out of open-loop gain at high frequencies. About 48 channel count sems to be the acceptable limit of stems. Dealing with the capacitive bus impedance is usually satisfactorily done with the help of a small "lead "compensation cap.
Actually, the summing amp is a transconductance amp, where the OLG increases with the number of channels routed, resulting in an almost constant gain and phase margin.
 

syn

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Thank you very much, all great stuff to learn.

I'm running OPA1656 in the mix sum amp with no additional problems, no noise, no overheating. I'll replace those post fader (C6) caps with some sort of poly, looking for something nice that can fit, at the moment.
 

jensenmann

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Thank you very much, all great stuff to learn.

I'm running OPA1656 in the mix sum amp with no additional problems, no noise, no overheating. I'll replace those post fader (C6) caps with some sort of poly, looking for something nice that can fit, at the moment.
Check Wima MKS2 caps for that purpose.
 

jensenmann

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Just for the record, I pulled the master of my Venue today and found in the left summing amp position a NE5532 and in the right position a TL072. Didn´t notice it for a decade....
 

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