Power rails on opamp

[Noth]

Hi,

I'm mounting an MCI JH600 preamp and I have some questions/doubts about the power rails.
For those interested, here is the circuit:

The circuit is powered using +/- 18V. On the power supply section, we just have two 100 ohm resistors to lower this voltage a bit and a few filter capacitors.
When I power up the circuit with my lab power supply, I can see that initially the current is about 20mA on each power rail. After one minute, the current raises up to 180mA (especially the -18V) and the 100 ohm resistor starts to overheat.

1. Is this current growth "normal" ?
2. At 20mA, the power for 18V would be 0,36W so my 1W 100Ohm resistor is fine. But at 180mA this gives me 3,24W, hence the overheat. Now, I = U/R, thus 18/100 gives me those 180mA but to me it would mean that the opamp is shorted... Am I wrong somewhere?

Any advice would be appreciated!

 

[Khron]

Normal? Most definitely not.

Are either of the opamps breaking into (very high frequency) oscillation? How's the audio between power-up and burn-up?

 

[user 37518]

Hi,

I'm mounting an MCI JH600 preamp and I have some questions/doubts about the power rails.
For those interested, here is the circuit:View attachment 112857

The circuit is powered using +/- 18V. On the power supply section, we just have two 100 ohm resistors to lower this voltage a bit and a few filter capacitors.
When I power up the circuit with my lab power supply, I can see that initially the current is about 20mA on each power rail. After one minute, the current raises up to 180mA (especially the -18V) and the 100 ohm resistor starts to overheat.

1. Is this current growth "normal" ?
2. At 20mA, the power for 18V would be 0,36W so my 1W 100Ohm resistor is fine. But at 180mA this gives me 3,24W, hence the overheat. Now, I = U/R, thus 18/100 gives me those 180mA but I'm not really sure those opamps are supposed to behave like this.. Am I wrong somewhere?

Any advice would be appreciated!

Where exactly are you placing the resistors? in series with the voltage rails? That is as guerilla as it gets, you should use a proper voltage regulator instead. A good rule of thumb is that you must select the power rating of your resistor at least 50% higher than the power dissipation, so, if you are going to dissipate 3.24 W, then you need a resistor that is at least 4.86W, round that to 5W; your poor 1W resistor is taking much more than it can handle. MCI consoles used to include 5W fuse resistors in each of the modules in series with the power rails for protection, but they were usually around 4.7 ohms or so IIRC.

MCI consoles (especially the JH-500) are notorious for being barbeque grills, but 180 mA for a single preamp is just too much. Semiconductors tend to conduct more current as the temperature is increased, it could be that some transistor or opamp is faulty, or as Khron mentioned it may be oscillating, but without more information it is impossible to know.

Also, how much voltage are you using to power the preamp? a series resistor will only drop voltage when a current passes through it, if you add a 100 ohm resistor with 20mA through it, it will drop 2 V, if your input voltage is too high, then, the opamps might get damaged. Also, in general, using a resistor of such large value is a lousy way to lower the voltage regardless of the power dissipation. If a circuit requires a larger current, for instance, when an input signal increases, it will draw more current from the power supply, in turn, the resistor will drop a larger voltage, that is not good; you want a power supply to be as stiff as possible, not varying depending on the input signal level. Again, I am unsure of your exact circuit, and preamps do not draw too much current, but still...

 

[Khron]

Where exactly are you placing the resistors? in series with the voltage rails?

I'm assuming they're the ones visible in the lower-right of the schematic attached in the first post..? R510 / R511, despite each being "doubled"...

 

[user 37518]

I'm assuming they're the ones visible in the lower-right of the schematic attached in the first post..? R510 / R511, despite each being "doubled"...

I see, well, in that arrangement, the resistors form a time constant with the 15uF capacitors, creating a low pass filter with a cut-off frequency at roughly 100Hz, so that is ok, I thought only a resistor was being used without a cap, although I would probably raise those caps an order of magnitude just because, but I digress....

The MCI2003 opamps are equivalent to the NE5534, I can't seem to find many sources regarding the quiescent current spec right now, but an ON semi datasheet says it's around 4 mA (40 year old parts might draw more), the transistors colectors/emmiters draw 2 mA each. So, two transistors and 3 op-amps that is 16 mA, plus the other resistors and "stuff" ok, 20 mA seems like an ok figure to me, 180 mA, definitely not.

 

[RoadrunnerOZ]

Normal? Most definitely not.

Are either of the opamps breaking into (very high frequency) oscillation? How's the audio between power-up and burn-up?

It’s always a possibility with those IC’s. First console I worked on in the late ‘70s was a JH600 and it showed up 20 years later as a service with lots of heat problems.
Bit strange - according to the schematic it apparently has 2 separate power feeds - one to IC500+IC600 and one to IC700 - yet it appears both pairs of feed resistors have the same R510 and R511 numbers - are they dual or is that the schematic is showing them as separated for clarity but there are actually only 2 not 4 as drawn???
Don’t discount the electrolytic caps either - they may be old and leaky, but as per Khron’s post I’d be looking at the IC’s with a scope. Can’t remember if they’re socketed or not but if they use the old style red IC sockets those sockets break up badly over time and the pin receiving terminals tarnish badly as well as the chip pins - can make them do weird stuff including oscillating madly.

 

[RoadrunnerOZ]

The capacitors on those rails also are diagrammed as 25V which is a bit low for an 18V rail and also the amount of heat in the module space makes nice little kettles out of old caps. Worth scoping the power rails in DC mode see if the voltage goes down and what crud turns up on the rails - post 100Ω feeders, across the electrolytics. I’d be replacing with 35V ones anyway.
The JH600 and the JH24 tape machine were always prone to HF parasitic faults and bad channel card plug/socket connections - we used to bang the card drawers open and shut on the JH24 before starting a session or you’d get them dropping out randomly.

 

[Noth]

Hi,
Thanks you all for your messages, I will check is the opamps are oscillating (I use NE5534). There was a very small connection on the PCB between the +18 and -18 rails (a faulty PCB) that I did cut (but may be not well enough). I'm currently mounting a new one without this issue. I will make further tests with this cleaner basis and let you know!

 

[RoadrunnerOZ]

Spray freeze is a good way of isolating heat resultant issues but you really need extender cables to be able to work it live with freezer. I make up extenders for each new console type I work on rather than buying expensive extenders

 

[Newmarket]

As user 37518 says - series resistance is rather crude way to drop voltage (RC LPF noted).
But it's not uncommon to see a small series NFR resistor (non flammable resistor) there. Acting both as a "fuse" - so if the ic power pins go short circuit then they (hopefully) go open circuit and avoid damage to the power supply and other channels still work. And as a low pass filter in conjuntion with a bypass capacitance. IIRC DDA often used 10R and 22uF (probably a smaller 100n or thereabouts in there too).

 

[Walrus]

From what I rememnber, the mic amp used only Tantalum caps apart from the output cap.
Maybe one of the PSU decoupling caps is leaking as it warms up. As already suggested,
best to replace them with good electrolytics anway.

 

[RoadrunnerOZ]

This is the original type extender assembly but they are awkward:

 

[Newmarket]

There was a very small connection on the PCB between the +18 and -18 rails (a faulty PCB) that I did cut (but may be not well enough). I'm currently mounting a new one without this issue. I will make further tests with this cleaner basis and let you know!

Did the board get powered up when it had the +18/-18 short that you noticed ?

 

[RoadrunnerOZ]

I take it you maybe don’t have a whole console but just some cards? If you have the cards running on test and fully accessible the old finger test will tell you if those IC’s are overheating - they get hot fast. If the ones on that board are where + 18V got joined I’d just replace them but the caps may have fritzed as well so I’d run a supply test with the chips pulled. 5534’s are cheap enough and if C510, 511, 710 and 711 are tantalum I’d be definitely replacing them.

 

[living sounds]

The 5534 op amps from different manufacturers are not the same chips. One may be (marginally) stable, while the other may oscillate in a given situation.

And the decoupling caps are critical. The old electrolytics with their high ESR may actually provide the damping that prevents peaks in the PSU impedance curve in combination with the ceramic cypass caps that could cause op amps to oscillate. Bottom line: Don't use modern low-ESR caps to replace those old caps.

I now always follow ricardos advice of using one rail each of the highest ESR 10uf caps right at the op amps and have not had problems with oscillation since.

https://groupdiy.com/threads/opamps-and-local-decoupling-of-rails-some-questions.37307/

 

[scott2000]

I now always follow ricardos advice of using one rail each of the highest ESR 10uf caps right at the op amps and have not had problems with oscillation since.

Any tips on finding the best ground at the chips? Scared to read that thread for the 10th time to leave unsure again...

 

[living sounds]

Any tips on finding the best ground at the chips? Scared to read that thread for the 10th time to leave unsure again...

I tend to also follow ricardo's advice there and use a separate dirt ground. Then I often follow John's advice and decouple audio and dirt ground with a small cap.

 

[RoadrunnerOZ]

The Neve 51 series consoles used a soft ground system - all ground points connected to main ground bus by 100Ω resistors thereby isolating and decoupling each stage from each other, minimising noise but also managing to maintain a common ground potential for external equipment - this is all explained in a section of the manual set that came with each console.
Isolating the + rails of different sections of a channel card by use of 100Ω resistors may be a similar concept I guess.

 

[Noth]

Alright, so... !

I did use another bath of PCB without the two +/-18V power rails that were initially too close.
I had the same issue but this time on the +18V rail (instead of the -18V).

I tested without any NE5534 and planned to put them back one by one, or order to detect a faulty opamp.
Surprise : the issue also happens without any Opamp !
This lead me to a filter issue on the +18V rail, so I unsoldered the .1 capacitor and everything went back to normal!
For this .1 capacitor I used a 100nF ceramic (since there's no unit on the PCB, I guessed .1 was actually 100nF) but it appears that this one was leaking DC but unknown reason..

Thanks again to all of you for your advices!

To answer a few remarks/questions:

- I'm using NE5534 opamps
- I actually don't have a full console, I just have a pair of channelstrips in a rack
- I also noticed that 25V for the capacitors was a bit optimistic, thus I'm using 50V
- For the R510/R511 references that appears twice on the schematic, I was also a but confused. I looked to an original preamp and there's actually just one set of 100Ohm resistors.
- @Walrums : I just read your message, you were absolutely right with the leaking tantalum capacitor!

 

[Noth]

Shame on me!
I just noticed that these tantalum capacitors were polarized and this one was reversed... (I obviously should have checked). At least I've learned that property for this type of capacitors!

 

[living sounds]

Adding ceramic decoupling caps habitually can actually cause lot's of problems. I have far fewer of those since I don't do that anymore. ;-)

 

[RoadrunnerOZ]

They used tantalums for a reason in that section of the preamp filtering but to replace them I’d be using low esr electrolytics.

 

[user 37518]

Shame on me!
I just noticed that these tantalum capacitors were polarized and this one was reversed... (I obviously should have checked). At least I've learned that property for this type of capacitors!

You should replace them, tantalums are rabidly bipolar, I wouldn't take the chance of just flipping it around.