Great video. I always learn something from them. I happen to be repairing a Marantz 2245 that came with a blown amp. I found the bad components and got it sounding good, but while in there, I changed the early model cylindrical bias transistor to the a newer TO-220 type. First try at setting bias, it won't go above 3.9 mV. I need to check the trimmer, but when I get back into it, I'm going to look at a low level 20kHz signal and see what is happening with the crossover distortion. Maybe I'll learn a little more. 😊
Hi, late to the game, but I am a noob. Asking: 1:10 How does one rig a test here? "20 KHz waveform > Amplifier > Digital oscilloscope" ? I mean, the connections. Where does the waveform comes from/generated? Where do you feed the waveform to the amp? For bias current adjustment, is 20 KHz a standard? Thanks!
OK, last question first. Bias is set under a no signal condition. I's also known as idling current, kind of like the idle of an internal combustion engine. A sine wave signal is usually obtained from a dedicated signal generator. But you can also get a sine wave from a computer, or burn one to a CD and play it. Or get a signal generator app for your phone. The signal is injected into an amplifier into an input; for a power amp, that is the left or right input. For an integrated amp, choose the aux input or tuner. Never use the phono input, as it has an RIAA equalization curve built in, and requires a very low signal. Your oscilloscope is placed across the dummy load, which is just a resistor or 4 or 8Ω, and is used to prevent deafening noise from inputting a sine wave and driving the amp to the edge of distortion.
Thanks Roeland! Yes it does. That's why audio amplifiers have some manner of thermal tracking, either a thermistor or transistor tied into the bias circuitry.
Ray, when you zoomed the sine wave on the scope and adjusted the meter read 13 mV. After you showed the math, the meter was reading 6.9mV. What happened?
As I mentioned (but failed to make clear) is that the DMM won't give an accurate reading of bias when signal is running into the amp. That's why service manuals state setting bias under no signal conditions.
Learned a lot, unfortunately, I don't have all the equipment yet, to analyze for one, scope is ok, lab supply is also ok, variac is also here, a safety transformer is also here, from 6 Amps, 230-230VAC, only one old multimeter. But still a good one. A glow lamp in series with the load is also here. The only thing I need is a signal generator, and an analyzer. But I get there. Thanks
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And the hits just keep on comin' -- another brilliant video wherein you discuss important fundamental concepts. Thanks Ray.
Ray, this is great information! I just setup a Sherwood receiver by using this tutorial, it came out great. Thanks again!
Very cool! Thanks for posting.
Thanks for making that really simple for me to understand. appreciate it
Just last week I was just trying to figure out how to bias an amp with no service notes. I feel like I can tackle that amp now. Thank you!
Glad to hear it, Brian!
Very enlightening video of a good point of controversy in power amplifiers.
Great video. I always learn something from them. I happen to be repairing a Marantz 2245 that came with a blown amp. I found the bad components and got it sounding good, but while in there, I changed the early model cylindrical bias transistor to the a newer TO-220 type. First try at setting bias, it won't go above 3.9 mV. I need to check the trimmer, but when I get back into it, I'm going to look at a low level 20kHz signal and see what is happening with the crossover distortion. Maybe I'll learn a little more. 😊
Thanks Steven, but right back atcha... I always read your posts on AK for the same reason, to learn something I didn't know.
Thanks for posting this and I really enjoy the way you explain things, straight from the hip! Subbed.
Hi, late to the game, but I am a noob. Asking:
1:10
How does one rig a test here? "20 KHz waveform > Amplifier > Digital oscilloscope" ? I mean, the connections. Where does the waveform comes from/generated? Where do you feed the waveform to the amp? For bias current adjustment, is 20 KHz a standard?
Thanks!
OK, last question first. Bias is set under a no signal condition. I's also known as idling current, kind of like the idle of an internal combustion engine.
A sine wave signal is usually obtained from a dedicated signal generator. But you can also get a sine wave from a computer, or burn one to a CD and play it. Or get a signal generator app for your phone.
The signal is injected into an amplifier into an input; for a power amp, that is the left or right input. For an integrated amp, choose the aux input or tuner. Never use the phono input, as it has an RIAA equalization curve built in, and requires a very low signal.
Your oscilloscope is placed across the dummy load, which is just a resistor or 4 or 8Ω, and is used to prevent deafening noise from inputting a sine wave and driving the amp to the edge of distortion.
@@raygianelli3612 Thank you.
Another excellent video
Nice video again. 🙂 Does temperature of the amplifier influence bias current much?
Thanks Roeland!
Yes it does. That's why audio amplifiers have some manner of thermal tracking, either a thermistor or transistor tied into the bias circuitry.
Ray, when you zoomed the sine wave on the scope and adjusted the meter read 13 mV. After you showed the math, the meter was reading 6.9mV. What happened?
As I mentioned (but failed to make clear) is that the DMM won't give an accurate reading of bias when signal is running into the amp. That's why service manuals state setting bias under no signal conditions.
Would have been interesting to see what the THD was when you set the bias current to 30mA.
Damn good thought. It would be interesting to see it in a real time spectrum too.
Learned a lot, unfortunately, I don't have all the equipment yet, to analyze for one, scope is ok, lab supply is also ok, variac is also here, a safety transformer is also here, from 6 Amps, 230-230VAC, only one old multimeter. But still a good one. A glow lamp in series with the load is also here. The only thing I need is a signal generator, and an analyzer. But I get there. Thanks