Home |
Search |
Today's Posts |
#1
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
Hi,
In a typical class B transistor power amp with multiple output devices, what should be the acceptable variation of bias currents through each output device? In this example, 5 parallel NPN and 5 parallel PNP devices connected to output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47 ohm gives each device's bias current (not sure what tolerance the resistors are) which varies from device to device (and globally) quite a bit as temperature goes up and down. At what point would you consider a single device to be out of spec or suspect? This amp very occasionally goes briefly into protect, I'm wondering if there is a suspect output device and/or if I can find it. Cheers, Gareth. |
#2
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"Gareth Magennis" writes:
Hi, In a typical class B transistor power amp with multiple output devices, what should be the acceptable variation of bias currents through each output device? In this example, 5 parallel NPN and 5 parallel PNP devices connected to output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47 ohm gives each device's bias current (not sure what tolerance the resistors are) which varies from device to device (and globally) quite a bit as temperature goes up and down. At what point would you consider a single device to be out of spec or suspect? A general rule of thumb in electrical engineering is that accuracy should be maintained to 10 or 20 percent. If the currents are varying more than that, I'd be suspect. This amp very occasionally goes briefly into protect, I'm wondering if there is a suspect output device and/or if I can find it. I'm just guessing, but I would suspect the one that has a steeper slope of current vs. temperature. -- % Randy Yates % "With time with what you've learned, %% Fuquay-Varina, NC % they'll kiss the ground you walk %%% 919-577-9882 % upon." %%%% % '21st Century Man', *Time*, ELO http://www.digitalsignallabs.com |
#3
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"Randy Yates" wrote in message ... "Gareth Magennis" writes: Hi, In a typical class B transistor power amp with multiple output devices, what should be the acceptable variation of bias currents through each output device? In this example, 5 parallel NPN and 5 parallel PNP devices connected to output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47 ohm gives each device's bias current (not sure what tolerance the resistors are) which varies from device to device (and globally) quite a bit as temperature goes up and down. At what point would you consider a single device to be out of spec or suspect? A general rule of thumb in electrical engineering is that accuracy should be maintained to 10 or 20 percent. If the currents are varying more than that, I'd be suspect. This amp very occasionally goes briefly into protect, I'm wondering if there is a suspect output device and/or if I can find it. I'm just guessing, but I would suspect the one that has a steeper slope of current vs. temperature. -- % Randy Yates Thanks. this one's kind of tricky because all NPNs from both channels are on one heatsink the width of the rhe amp, and all PNP's on another the same. The ones in the middle of the heatsink run at a lower bias than the ones on the outside, presumably due to the difference in temperature which I haven't measured. As heatsink temp goes up and down after loading, the bias can vary from inner to outer up to 100%! (The top is off the amp for measuring, so heatflow will not be as designed, so its all a bit wibbly anyway). I've been stepping through the biases but I havent seen anything like a 20% anomaly in adjacent devices. The protect happened once yesterday for about a second, after 4 hours on the bench, reassembled with no load, and the fans weren't on so it wasn't hot. Nightmare. Gareth. |
#4
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"Gareth Magennis" wrote in
message news Hi, In a typical class B transistor power amp with multiple output devices, what should be the acceptable variation of bias currents through each output device? In this example, 5 parallel NPN and 5 parallel PNP devices connected to output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47 ohm gives each device's bias current (not sure what tolerance the resistors are) At worst, 20%. Could be as close as 1%. Why not measure them in-circuit (power removed)? which varies from device to device (and globally) quite a bit as temperature goes up and down. At what point would you consider a single device to be out of spec or suspect? I'd get worried if the variations are 20%. This amp very occasionally goes briefly into protect, I'm wondering if there is a suspect output device and/or if I can find it. Under what conditions? Usually, transistor failures of this magnitude are a one-way trip from good to permanently bad. |
#5
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"Gareth Magennis" wrote in
message The protect happened once yesterday for about a second, after 4 hours on the bench, reassembled with no load, and the fans weren't on so it wasn't hot. That clarifies a lot. Obviously, there should be no protected condition. OTOH, what about the power? I recall that most protect circuits come on during the power-up cycle. A momentary loss of power could cause the symptom you observe. The next question would be about the logic that goes into detecting a protected state. I've seen some moderately complex circuits for turning on the "protect" light. The fault could be in a part in the protection circuit, not a fault of the circuit(s) being protected. |
#6
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"Arny Krueger" wrote in message . .. "Gareth Magennis" wrote in message The protect happened once yesterday for about a second, after 4 hours on the bench, reassembled with no load, and the fans weren't on so it wasn't hot. That clarifies a lot. Obviously, there should be no protected condition. OTOH, what about the power? I recall that most protect circuits come on during the power-up cycle. A momentary loss of power could cause the symptom you observe. The next question would be about the logic that goes into detecting a protected state. I've seen some moderately complex circuits for turning on the "protect" light. The fault could be in a part in the protection circuit, not a fault of the circuit(s) being protected. I spoke to a tech who fixes a lot of these, and he suggested an output device as the most likely culprit, so I am following up this line of enquiry first, as well as learning a few things here along the way. The next step will be to remove one power amp from circuit and wait (another whole day for God's sake) for it to go into protect or not, then try the other. If I could force this fault to happen it would make things a lot easier. As far as your power sugestion goes, I might look a lot more closely at the low voltage rails. A blip in one of those could well be the culprit. Cheers, Gareth. |
#7
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
Gareth Magennis wrote: Hi, In a typical class B transistor power amp with multiple output devices, what should be the acceptable variation of bias currents through each output device? It's not highly critical. In this example, 5 parallel NPN and 5 parallel PNP devices connected to output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47 ohm gives each device's bias current (not sure what tolerance the resistors are) Typically 5 or 10% which varies from device to device (and globally) quite a bit as temperature goes up and down. At what point would you consider a single device to be out of spec or suspect? This amp very occasionally goes briefly into protect, I'm wondering if there is a suspect output device and/or if I can find it. It certainly won't be that. Graham |
#8
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
Arny Krueger wrote: "Gareth Magennis" wrote: The protect happened once yesterday for about a second, after 4 hours on the bench, reassembled with no load, and the fans weren't on so it wasn't hot. That clarifies a lot. Obviously, there should be no protected condition. OTOH, what about the power? I recall that most protect circuits come on during the power-up cycle. A momentary loss of power could cause the symptom you observe. The next question would be about the logic that goes into detecting a protected state. I've seen some moderately complex circuits for turning on the "protect" light. The fault could be in a part in the protection circuit, not a fault of the circuit(s) being protected. I agree. Graham |
#9
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
Gareth Magennis wrote: I spoke to a tech who fixes a lot of these, and he suggested an output device as the most likely culprit Sounds like he's an idiot. Graham |
#10
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
On May 21, 4:16 pm, Eeyore
wrote: Gareth Magennis wrote: I spoke to a tech who fixes a lot of these, and he suggested an output device as the most likely culprit Sounds like he's an idiot. Graham And you are a Facist |
#11
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
gaby de wilde wrote:
On May 21, 4:16 pm, Eeyore wrote: Gareth Magennis wrote: I spoke to a tech who fixes a lot of these, and he suggested an output device as the most likely culprit Sounds like he's an idiot. Graham And you are a Facist I've heard that about him. He's absolutely obsessed with faces...looks at his own in the mirror at least once a day, as he shaves it. He's been known to study others' faces--both on the street and inside public facilities--and not just for purposes of recognition. In fact, I hear that he owns and employs a device known as a 'television' which displays peoples' faces on a screen inside of his home. Oh, the humanity! jak |
#12
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"gaby de wilde" wrote in message
On May 21, 4:16 pm, Eeyore wrote: Gareth Magennis wrote: I spoke to a tech who fixes a lot of these, and he suggested an output device as the most likely culprit Sounds like he's an idiot. Graham And you are a Facist I suspect you mean Fascist. It's always funny when someone engages in childish name-calling and then pees on themselves in the process. |
#13
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
gaby de wilde wrote: Eeyore wrote: Gareth Magennis wrote: I spoke to a tech who fixes a lot of these, and he suggested an output device as the most likely culprit Sounds like he's an idiot. Graham And you are a Facist And you are a MORON. |
#14
Posted to rec.audio.tech
|
|||
|
|||
Bias current in Power Amps
"Gareth Magennis" wrote in message ... "Randy Yates" wrote in message ... "Gareth Magennis" writes: Hi, In a typical class B transistor power amp with multiple output devices, what should be the acceptable variation of bias currents through each output device? In this example, 5 parallel NPN and 5 parallel PNP devices connected to output rail by 0.47 Ohm emitter resistors. The voltage across each 0.47 ohm gives each device's bias current (not sure what tolerance the resistors are) which varies from device to device (and globally) quite a bit as temperature goes up and down. At what point would you consider a single device to be out of spec or suspect? A general rule of thumb in electrical engineering is that accuracy should be maintained to 10 or 20 percent. If the currents are varying more than that, I'd be suspect. This amp very occasionally goes briefly into protect, I'm wondering if there is a suspect output device and/or if I can find it. I'm just guessing, but I would suspect the one that has a steeper slope of current vs. temperature. -- % Randy Yates Thanks. this one's kind of tricky because all NPNs from both channels are on one heatsink the width of the rhe amp, and all PNP's on another the same. The ones in the middle of the heatsink run at a lower bias than the ones on the outside, presumably due to the difference in temperature which I haven't measured. As heatsink temp goes up and down after loading, the bias can vary from inner to outer up to 100%! (The top is off the amp for measuring, so heatflow will not be as designed, so its all a bit wibbly anyway). I've been stepping through the biases but I havent seen anything like a 20% anomaly in adjacent devices. The protect happened once yesterday for about a second, after 4 hours on the bench, reassembled with no load, and the fans weren't on so it wasn't hot. Nightmare. Gareth. Well, we haven't been given the amp model, and have no schematic here, so generalizations are all that is possible but I have some experience troubleshooting problems like this. It is not a sin to defeat the protection circuit temporarily for the purpose of troubleshooting. For example, there could be a pull-up line to the protection circuit (or microprocessor as the case may be) which is pulling low momentarily. Opening this line up might help determine whether it's the amp or the protect circuit - ditto with the DC sensing resistor, it can be disconnected as well. Possibly a bad driver transistor clips the waveform briefly, triggering a DC protect. This would most often be a bad emitter-base junction on a TO-220 package driver. The DC voltage across this junction can be observed and of course if the voltage jumps from the normal .6 volts or so to more than a volt, or several volts obviously that transistor is bad. If the problem only occurs with a load connected this is a pretty likely scenario. Many protection circuits also monitor the power supplies as well; often the sum of the voltages, as seen through a voltage divider network, should be near zero volts. If for example a voltage regulator drops in output, the sum is no longer zero, and the circuit triggers. Rarely a resistor in the network could be bad or out of spec. Many protection circuits also feature a "fast shut-off" so that when the power is cut the speaker relay cuts off immediately, preventing the amp clipping or popping audibly at turnoff. This circuit usually consists of a small electrolytic lightly filtering a half-wave (single diode) line off the power transformer, going from there to the base of one of the transistors in the relay drive circuit. If the cap is totally bad, the amp would never un-mute, but it could be marginal and not totally open. Mark Z. |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
attn: Chris Hornbeck and Patrick Turner A question (6CA7 bias current) | Vacuum Tubes | |||
Measuring bias current | Vacuum Tubes | |||
current crop of affordable PA power amps | Pro Audio | |||
how do I bias these three amps? | Vacuum Tubes |