Good evening guys, I have two resistors I need to replaced on an amp. It's most likely the typical class AB design with the supply resistors for the op amp (front end) input circuitry board.
I think the voltage somewhat is 33v+/- from the main power drive clamped with 15v zeners, I'm not getting any voltages beyond 2.1 positive and negative 1.3 for the op amp circuit wich should be 15+/- respectively after clearing those resistors. If the B+ and B- is removed from the TL074CN, I will get volts (16v).
I don't do enough tests as yet to know if the ics are faulty or if the voltage is under amped and dies out under load.
But regardless of my findings I still want to replace the resistors. I tried my V/IxR equations and ended up with somewhere 300ohms I don't know if I'm correct.
I've post a pic of the circuit if someone can tell me what resistors were used.
I think the voltage somewhat is 33v+/- from the main power drive clamped with 15v zeners, I'm not getting any voltages beyond 2.1 positive and negative 1.3 for the op amp circuit wich should be 15+/- respectively after clearing those resistors. If the B+ and B- is removed from the TL074CN, I will get volts (16v).
I don't do enough tests as yet to know if the ics are faulty or if the voltage is under amped and dies out under load.
But regardless of my findings I still want to replace the resistors. I tried my V/IxR equations and ended up with somewhere 300ohms I don't know if I'm correct.
I've post a pic of the circuit if someone can tell me what resistors were used.
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Sounds like the burnt resistor drops the voltage for the Zener, and the op amp shorted out,
overheating the resistor. Maybe around (18V resistor drop / 10mA current) = 1.8k.
If the current is 30mA, then the resistor is 600R. It's no more than a 1W Zener,
so its max current would be 66 mA, and then 30mA is reasonable. Your 300R sounds too low,
unless the circuit draws around 30mA also.
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There's not enough information so far to calculate the value.
Do you know the brand and model number of the amplifier?
The value isn't in any way critical. Most seem to be trying to drive the Zeners to 1w but that's not necessary. Even if the regulation didn't hold, it's unlikely that you would be able to tell (audibly). The amplifier and the op-amps have a very high power supply rejection ratios.
I've seen values from 1.2k down to 220 ohms. You could probably use something like 680 ohms and never lose regulation (which could be tested by varying the input voltage to the amp and watching for a significant deviation in the ±15v output).
I've seen values from 1.2k down to 220 ohms. You could probably use something like 680 ohms and never lose regulation (which could be tested by varying the input voltage to the amp and watching for a significant deviation in the ±15v output).
Hi everybody,
i do not know if it is correct to follow-up this thread after almost one year... anyway I have a similar problem on an 4ch ab amp, China made of course, as the 2 resistors stepping down the main rail voltage to op-amps -through the 15v zeners- are heating too much.... this was already the amp problem -along with a pair burnt power transistors causing also a fet blowing p. supply. So, after replaced blown parts and re-soldered many parts and rails quite bad, I've bench tested the amp and it turns on but only two channels had output... soon the upsaid resistors got hot and I turned off the amp for checking. This amp instead of number-code of parts carries just the original part code! so it's like having a schematic on pcb, not a bad idea sometimes... That is these resistors are shown as 180ohm/5W and related zener as 1w/15v of course...but pulled ot and measured -as colour codiing disappeared- they are 225ohm and i'm not sure they are 5w, so I replaced with 180ohm 7W ceramic ones, i also checked for signal feeding caps 10uF/50v and then crancked again amp, but also these resistors are getting very hot after a few minutes of working (now all channels have output).
I've measured voltage on zeners out and is a little high 17,5v with 14,2v supply, lowering it to 12,6v i see 16,6v lowering to 12v i have 16,5v that is still a bit higher than 15v... but other parts, fets and power outputs with drivers are not heating at all...
Any idea why those resistors heat so much? Would you Formas specify which ceramic disc cap have you replaced in that amp you worked? How is you, Perry, state that resistor value is not critical? If a photo is needed I will post at your request, but i'm much less than a beginner with cameras machine...
Thanks in advance for any help from yours
i do not know if it is correct to follow-up this thread after almost one year... anyway I have a similar problem on an 4ch ab amp, China made of course, as the 2 resistors stepping down the main rail voltage to op-amps -through the 15v zeners- are heating too much.... this was already the amp problem -along with a pair burnt power transistors causing also a fet blowing p. supply. So, after replaced blown parts and re-soldered many parts and rails quite bad, I've bench tested the amp and it turns on but only two channels had output... soon the upsaid resistors got hot and I turned off the amp for checking. This amp instead of number-code of parts carries just the original part code! so it's like having a schematic on pcb, not a bad idea sometimes... That is these resistors are shown as 180ohm/5W and related zener as 1w/15v of course...but pulled ot and measured -as colour codiing disappeared- they are 225ohm and i'm not sure they are 5w, so I replaced with 180ohm 7W ceramic ones, i also checked for signal feeding caps 10uF/50v and then crancked again amp, but also these resistors are getting very hot after a few minutes of working (now all channels have output).
I've measured voltage on zeners out and is a little high 17,5v with 14,2v supply, lowering it to 12,6v i see 16,6v lowering to 12v i have 16,5v that is still a bit higher than 15v... but other parts, fets and power outputs with drivers are not heating at all...
Any idea why those resistors heat so much? Would you Formas specify which ceramic disc cap have you replaced in that amp you worked? How is you, Perry, state that resistor value is not critical? If a photo is needed I will post at your request, but i'm much less than a beginner with cameras machine...
Thanks in advance for any help from yours
In the future, start a new thread unless you're trying to help the OP. I wouldn't normally answer here but it applies to the OP's problem.
For simple shunt regulators, the resistor (ideally) has to be a low enough value to keep the zener at its breakdown (rated) voltage. If the supply voltage feeding the resistor/zener chain has a value that's too high, the resistor won't pass enough current to supply the circuit being fed by the regulator to remain at the rated voltage so the regulated voltage won't remain regulated.
If the resistor value is too low, the zener will run too hot and will fail prematurely. Many times, the board will be damaged from heat stress. The solder connections will likely be damaged n time as well.
You want a value of resistor that's low enough to maintain regulated voltage under the worst operating conditions but not so low to cause the circuit to be unreliable.
The reason that the value isn't all that critical is because virtually all of the circuits being fed by the regulator have a very high PSRR (power supply rejection ratio). This means that they are mostly immune to noise that's on the power supply voltage feeding them.
The resistors run hot because they are dropping a lot of voltage (large voltage across them) and because current is flowing through them. Simple Ohm's Law P=I*E. Amps with relatively high rail voltage are not good candidates for this type of circuit. An active circuit or a current boost shunt regulator is generally better. The current boost type regulator includes a transistor and since the transistor can be mounted to a heatsink to dissipate the heat, it's generally more reliable.
If you have any other questions, please start a new thread. You can include a link to the thread or any individual post in a thread (right-click the post number (right side of individual posts) and select copy link).
For simple shunt regulators, the resistor (ideally) has to be a low enough value to keep the zener at its breakdown (rated) voltage. If the supply voltage feeding the resistor/zener chain has a value that's too high, the resistor won't pass enough current to supply the circuit being fed by the regulator to remain at the rated voltage so the regulated voltage won't remain regulated.
If the resistor value is too low, the zener will run too hot and will fail prematurely. Many times, the board will be damaged from heat stress. The solder connections will likely be damaged n time as well.
You want a value of resistor that's low enough to maintain regulated voltage under the worst operating conditions but not so low to cause the circuit to be unreliable.
The reason that the value isn't all that critical is because virtually all of the circuits being fed by the regulator have a very high PSRR (power supply rejection ratio). This means that they are mostly immune to noise that's on the power supply voltage feeding them.
The resistors run hot because they are dropping a lot of voltage (large voltage across them) and because current is flowing through them. Simple Ohm's Law P=I*E. Amps with relatively high rail voltage are not good candidates for this type of circuit. An active circuit or a current boost shunt regulator is generally better. The current boost type regulator includes a transistor and since the transistor can be mounted to a heatsink to dissipate the heat, it's generally more reliable.
If you have any other questions, please start a new thread. You can include a link to the thread or any individual post in a thread (right-click the post number (right side of individual posts) and select copy link).
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