Happy Eastern to Everybody
On A gear I've repaired I've noticed a 47K resistor bridging the Hot & Cold input on a Typical 4*10K balanced op-amp input stage ?
What's the purpose of that resistor ?
What am I missing ?
Thank You for Your time
On A gear I've repaired I've noticed a 47K resistor bridging the Hot & Cold input on a Typical 4*10K balanced op-amp input stage ?
What's the purpose of that resistor ?
What am I missing ?
Thank You for Your time
While it would be nice to see a specific circuit in order to be more accurate, here's my best guess:
Answer#1:
High valued resistors, such as 47kΩ or higher, are often used on the AC coupled side of an input coupling capacitor in order to establish a DC value relative to the input circuit. They are high valued in order to not significantly affect the circuit's operation, but their value is low enough to establish a DC potential after an input coupling cap. Without this resistor to ground, the very small input bias currents from the amplifier would eventually cause the input voltages to the amplifier to saturate, since there will be no path to ground for these currents otherwise.
Answer #2:
Some circuits have high valued 'tackdown' resistors at their input jacks to prevent switching noises when switching among inputs. These resistors will establish the DC potential at each input to be equal to the audio ground, thus preventing switching noises when switching among inputs.
Consider these possible functions, but if this still makes little sense, post a specific circuit schematic!
Answer#1:
High valued resistors, such as 47kΩ or higher, are often used on the AC coupled side of an input coupling capacitor in order to establish a DC value relative to the input circuit. They are high valued in order to not significantly affect the circuit's operation, but their value is low enough to establish a DC potential after an input coupling cap. Without this resistor to ground, the very small input bias currents from the amplifier would eventually cause the input voltages to the amplifier to saturate, since there will be no path to ground for these currents otherwise.
Answer #2:
Some circuits have high valued 'tackdown' resistors at their input jacks to prevent switching noises when switching among inputs. These resistors will establish the DC potential at each input to be equal to the audio ground, thus preventing switching noises when switching among inputs.
Consider these possible functions, but if this still makes little sense, post a specific circuit schematic!
Monte McGuire
Thank You for Your input.
Your answers make sense. I've read something about it before.
The problem is, the resistors in question (1 for each channel) aren't connected to ground. They are connected at the back of the XLRs between Pin 2 and 3.
I don't have a schematic, although it is a Portuguese manufacturer with reasonable products.
Look at the middle bottom scan of the PCB, the pads where I've removed the resistors (blueish mark) are before the coupling caps.
If You trace the tracks ... on the bottom is the typical TL072 Op-amp input stage, from there it goes to a double volume pot (actually there are pads for two of them...) and then goes upstairs to the 3rd TL072 op-amp to two summing 470 Ohm resistors above the 0 Ohm bridge. Now the interesting thing... if You follow the track back to the right, there is a resistor (under pink cap) , a cap, 0R bridges and them on the leftmost middle side there are clamping diodes ... after the first stage ??? Power connector is on the leftmost middle side with one cap at each corner, was derived by two 5W 2.2K very hot resistors from the main amp rails, but I made a separate supply for it and shunted them.
So the first upper TL071 op-amp is for the left channel, the second for the right channel, the middle TL072 one for Sub summing buffer ?? and the rightmost TL074 one is for the Sub filter I believe. Crossover frequency stated in the back of the unit is 125Hz. L&R Outputs are on the upper side (3 pin) and sub out is above the second rightmost pot on the bottom. Notice the resistor connecting the sub pot to ground, preventing complete bass cutting.
Thank You for Your input.
Your answers make sense. I've read something about it before.
The problem is, the resistors in question (1 for each channel) aren't connected to ground. They are connected at the back of the XLRs between Pin 2 and 3.
I don't have a schematic, although it is a Portuguese manufacturer with reasonable products.
Look at the middle bottom scan of the PCB, the pads where I've removed the resistors (blueish mark) are before the coupling caps.
If You trace the tracks ... on the bottom is the typical TL072 Op-amp input stage, from there it goes to a double volume pot (actually there are pads for two of them...) and then goes upstairs to the 3rd TL072 op-amp to two summing 470 Ohm resistors above the 0 Ohm bridge. Now the interesting thing... if You follow the track back to the right, there is a resistor (under pink cap) , a cap, 0R bridges and them on the leftmost middle side there are clamping diodes ... after the first stage ??? Power connector is on the leftmost middle side with one cap at each corner, was derived by two 5W 2.2K very hot resistors from the main amp rails, but I made a separate supply for it and shunted them.
So the first upper TL071 op-amp is for the left channel, the second for the right channel, the middle TL072 one for Sub summing buffer ?? and the rightmost TL074 one is for the Sub filter I believe. Crossover frequency stated in the back of the unit is 125Hz. L&R Outputs are on the upper side (3 pin) and sub out is above the second rightmost pot on the bottom. Notice the resistor connecting the sub pot to ground, preventing complete bass cutting.
That makes little sense. I can't think of any reason why a 47kΩ resistor across pin 2 and pin 3 should be used. They won't do much harm, but they don't seem to be useful either. And, it's odd that a diode clamp bridge is placed anywhere but near an IO connector.
It could be a less than well designed device? Not everything is 100% ideal, and I know I have stared at a design for hours wondering why something was done. Sometimes, there is no good answer for 'why'!
You seem to have a good handle on things, so trust your instincts and consider removing spurious components. I'd worry about that diode bridge too - they can add significant distortion, and there's no need to tolerate it if there's no purpose for the bridge.
Best of luck!
It could be a measure to keep the inputs from floating away from each other with disconnected input connector. That would make sense.
Jan
Jan
A 4 resistor diff stage has the op amp inputs tacked down though, through the FB resistor and the input shunt, so they aren't completely floating as-is. Will remain a mystery!
Now another mystery. The amps have 3 audio input pins. One connects to a small input cap, and is connected also to a limiter circuit (I think it is an NLS32 because it has the same shape and 32 written on it) , the middle pin connects to rail ground, and the third pin connects to a 100 uF cap that goes to two transistors. What is strange is, the 3rd pin and the ground are shorted in the preamp. Fist I've thought the amp had balanced inputs. Another strange thing, the sub board has 4 transistors and 4 high wattage resistors (perfectly normal). The satellite amps boards are the same but have only two transistors, but three resistors. I thought removing the third one, but it is connected to that two transistors I mentioned before ??? Perhaps the amp is deriving feedback from the emitter resistors...
I have a problem with this unit, the right channel stops playing, and if you put your ear near the Sat speaker, You hear nothing, no noise, no hum, NADA !!! as if the wire was cut. And to make things worse, it doesn't matter how hard I push the thing, Full Volume (It has limiters), Mixer clipping I can't replicate it at home in order to test if it is the preamp or the amps. It only fails at certain venues, I swapped cables, did even replace one of the protection relays (common to L&R). And if I power-cycle the think it works again. So it isn't a heating problem. There are no AVRs or Pics, DSPs or LCDs in it. Pure analog and heavy toroid supply. Sometimes it fails after a few minutes, sometimes after a few hours. I refreshed some solder pads replaced the molex connectors and connecting cables, replaced the spade connectors for the supplies, Let's hope it doesn't fail again. Upon assembling, I swapped the L & R audio connectors. If it fails again on the same channel I know it is the Amp. I can think of only one cause for this, low mains voltage. Nominal is 230V, but I've found venues with low as 200V. Why does only the right channel fail ? Components tolerance ?
It is playing in my garage for two days now at moderate volume.
Regarding the 47K resistors ...I've found this, but left them out anyway.
Balanced Line Driver & Receiver figure 2
I have a problem with this unit, the right channel stops playing, and if you put your ear near the Sat speaker, You hear nothing, no noise, no hum, NADA !!! as if the wire was cut. And to make things worse, it doesn't matter how hard I push the thing, Full Volume (It has limiters), Mixer clipping I can't replicate it at home in order to test if it is the preamp or the amps. It only fails at certain venues, I swapped cables, did even replace one of the protection relays (common to L&R). And if I power-cycle the think it works again. So it isn't a heating problem. There are no AVRs or Pics, DSPs or LCDs in it. Pure analog and heavy toroid supply. Sometimes it fails after a few minutes, sometimes after a few hours. I refreshed some solder pads replaced the molex connectors and connecting cables, replaced the spade connectors for the supplies, Let's hope it doesn't fail again. Upon assembling, I swapped the L & R audio connectors. If it fails again on the same channel I know it is the Amp. I can think of only one cause for this, low mains voltage. Nominal is 230V, but I've found venues with low as 200V. Why does only the right channel fail ? Components tolerance ?
It is playing in my garage for two days now at moderate volume.
Regarding the 47K resistors ...I've found this, but left them out anyway.
Balanced Line Driver & Receiver figure 2
Last edited:
Unlikely that low mains voltage would cause an amp channel to completely fail. Much more likely that you have an intermittent fault.
- Status
- Not open for further replies.