Hello everyone,
Noob here and its been a while since I did audio amps, but I recently got back into them and as usual, have run into a little problem. As i looked around for material, i came across this site Small audio amplifiers Very informative and the owner (Manfred) goes a little bit into detail describing the evolution of his project, but it's the last schematic that brought me to a grinding halt. In order to get rid of crossover distortion in the output stage, he introduces a Vbe multiplier and a current source to supply power to both the Vbe multiplier and the out put transistors. I know it works and its a common circuit because i have already seem a bunch of them already, but doesn't that bring about problems of its own. For instance, right off the top of my head, it seems to me that the voltage swing from the op amp can go as low as the op amp will go, but on the positive swing, it cant exceed 12v - (1.4v + 0.2v + 0.7v) = 9.7v. The 1.4v for the Vbe multiplier, 0.2v for the minimum Vce of Q4, and 0.7v for R6. The way i see it, if the output from the op amp exceeds 9.7v, Q4 would cut out because the collector voltage has risen beyond the emitter voltage and therefore current wouldn't flow. This would in turn cut supply to Q3, eliminating the output bias all together. I believe there would still be some leakage current through R6, and the emiter-base junction of Q4 would act as a diode, but thats irrelevant. From what I have gathered, the minimum output voltage the 4558 (which I'm using for this project) would achieve in this particular schematic is about 1.5v. So with that in mind, does that mean the we can only get about (9.7v - 1.5v) 8.3v swing out of the 4558 with this set up? I know that I would probably at best get a 9 volt (-1.5v from both rails at 12v) swing out of a cheap op amp like the 4558 and there isn't that much difference between 8.3v and 9v in practical terms, but I'd just like to confirm that my theory is correct so i can get a better understanding of the limitations of this design to avoid problems.
optransbias.gif
Click this image to show the full-size version.
If my theory is correct, that would mean R1 and R2 would have to be re-adjusted if i want to use the whole dynamic range of the circuit, because there's a greater loss on the positive swing than there is on the bottom swing. Probably change R1 and R2 to get somewhere about 5.6 volts out of the divider to avoid clipping the positive swing.
I'd appreciate anyone's helpful thoughts.
Noob here and its been a while since I did audio amps, but I recently got back into them and as usual, have run into a little problem. As i looked around for material, i came across this site Small audio amplifiers Very informative and the owner (Manfred) goes a little bit into detail describing the evolution of his project, but it's the last schematic that brought me to a grinding halt. In order to get rid of crossover distortion in the output stage, he introduces a Vbe multiplier and a current source to supply power to both the Vbe multiplier and the out put transistors. I know it works and its a common circuit because i have already seem a bunch of them already, but doesn't that bring about problems of its own. For instance, right off the top of my head, it seems to me that the voltage swing from the op amp can go as low as the op amp will go, but on the positive swing, it cant exceed 12v - (1.4v + 0.2v + 0.7v) = 9.7v. The 1.4v for the Vbe multiplier, 0.2v for the minimum Vce of Q4, and 0.7v for R6. The way i see it, if the output from the op amp exceeds 9.7v, Q4 would cut out because the collector voltage has risen beyond the emitter voltage and therefore current wouldn't flow. This would in turn cut supply to Q3, eliminating the output bias all together. I believe there would still be some leakage current through R6, and the emiter-base junction of Q4 would act as a diode, but thats irrelevant. From what I have gathered, the minimum output voltage the 4558 (which I'm using for this project) would achieve in this particular schematic is about 1.5v. So with that in mind, does that mean the we can only get about (9.7v - 1.5v) 8.3v swing out of the 4558 with this set up? I know that I would probably at best get a 9 volt (-1.5v from both rails at 12v) swing out of a cheap op amp like the 4558 and there isn't that much difference between 8.3v and 9v in practical terms, but I'd just like to confirm that my theory is correct so i can get a better understanding of the limitations of this design to avoid problems.
optransbias.gif
Click this image to show the full-size version.
If my theory is correct, that would mean R1 and R2 would have to be re-adjusted if i want to use the whole dynamic range of the circuit, because there's a greater loss on the positive swing than there is on the bottom swing. Probably change R1 and R2 to get somewhere about 5.6 volts out of the divider to avoid clipping the positive swing.
I'd appreciate anyone's helpful thoughts.
Attachments
It will make no difference to the gain as you are not adjusting the nfb components. R3, R4 and C2 set the gain. R6 and Q4 set the constant current source and R7 and Q3 set the quiescent current. Q3 must be thermally connected to the output transistors for optimum stability.
R1 & R2 set the 0volt or midway point.
R1 & R2 set the 0volt or midway point.
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You're correct about the maximum possible signal swing. The peak-to-peak voltage swing at the output is reduced by the current source and further reduced by the emitter follower Vbe drops. If this means your amplifier won't play loudly enough, you can make modifications.
1. replace the current source by a 2R+1C bootstrap circuit, clawing back about 1.4V of output swing
2. use a higher voltage power supply
3. implement the output transistors Q1 and Q2 as "FETlingtons" which claws back at least 1.4V of output swing
4. replace the 8 ohm loudspeaker by a 4 ohm speaker, which doubles the output power
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1. replace the current source by a 2R+1C bootstrap circuit, clawing back about 1.4V of output swing
2. use a higher voltage power supply
3. implement the output transistors Q1 and Q2 as "FETlingtons" which claws back at least 1.4V of output swing
4. replace the 8 ohm loudspeaker by a 4 ohm speaker, which doubles the output power
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Thanks for the response, but i do understand the bit concerning voltage gain and which components set it. The only part i had trouble with was the constant current source and how it affects the output voltage swing.
Thank guys!
Now that my suspicions regarding the current source have been confirmed, i can proceed to the breadboard. I don't have any fancy noise measuring equipment like the author did, so ill just stick a DMM in the output stage and set the quiescent current to 10 mA and be done with it. I believe that should be enough to just put the output transistors in class AB operation. I also have some 1 watt 1 ohm power resistors on-hand which with only 10 mA in the output stage, should comfortably handle. The other thing i cant seem to get a fix on is the line RMS voltage i should design this amp for. Every source i consult mentions something different. Other people talk of between 1 and 2 volts! Ill be using a smartphone to play music through it, and i was thinking of downloading a tone generator to the phone and measuring the rms voltage produced by the phone using a DMM with volume at the max.
Otherwise, my biggest challenge has been resolved.
Thanks again.
Now that my suspicions regarding the current source have been confirmed, i can proceed to the breadboard. I don't have any fancy noise measuring equipment like the author did, so ill just stick a DMM in the output stage and set the quiescent current to 10 mA and be done with it. I believe that should be enough to just put the output transistors in class AB operation. I also have some 1 watt 1 ohm power resistors on-hand which with only 10 mA in the output stage, should comfortably handle. The other thing i cant seem to get a fix on is the line RMS voltage i should design this amp for. Every source i consult mentions something different. Other people talk of between 1 and 2 volts! Ill be using a smartphone to play music through it, and i was thinking of downloading a tone generator to the phone and measuring the rms voltage produced by the phone using a DMM with volume at the max.
Otherwise, my biggest challenge has been resolved.
Thanks again.
In a nutshell: it is not a very good circuit, don´t expect much from it.
That said, it´s a nice way to experiment, and if you are "getting back into amplifiers" consider it one step in the ladder: build it, enjoy it, and then continue to the next step.
And welcome back int the hobby
I understand, JMFahey. Its definitely a first step. I wouldn't want the first step to discourage me, so i went for the absolute bare bones amp i could find with descent output. I have plenty of drivers and speakers without amps lying around that I've been meaning to put to good use.
Glad to be back to the hobby!
You have computed the maximum output, I forget, but say 8V peak to peak.
You say you know how the voltage gain is set. 15K and 3K is about 6 on my abacus.
Therefore the clipping input is 8Vpp/6 or 1.3V peak to peak, 0.5V RMS.
Check my math!!
But why do you need to care? Start with your source turned WAY down. Turn up gradually. When it goes from clean to gritty, don't turn up more.
Now if that means running your preamp or iPad volume control in the crack between 0 and 1, you want less gain. Conversely if you turn to 99 and it still won't break-up (on a loud track), you want more gain. You already know where that is set.
Yeap! You read my mind. Almost all amps i have come across and the ones in my possession have a volume control, but to keep things simple, ill leave it out of this one. Besides, im not even sure how to implement volume control. Some circuit designers use a potentiometer in place of R3 and R4 to change the gain and in so doing, the volume as well. Others use a pot as a rheostat in the input, but the simplest i know of is using a pot on the input. The whole resistance of the pot in series with the input cap (forming an RC circuit) and the wiper going to the input resistor of the op amp.
Oh well, ill tweak the circuit to see what works best after bread-boarding it!
Alright, So i did the circuit on a breadboard. I decided to use a pot for the gain resistors. Hooked up a DMM in the output stage to measure current. Turned the power source on and adjusted for a 10 mA Q-current. After hitting that, connected the phone and cranked the volume on the phone up to the max while setting the pot that sets the gain on the amp to about the midpoint (approx gain = 1). Slowly turning it one way raised the volume and to my surprise, I was pleased with the result! I couldn't detect any noise what-so-ever! While listening to the output of course, couldn't measure anything coz i don't have any measuring instruments. The amp is connected to two 5 watt, 4 ohm speakers, and produces fairly high volumes without the output transistors getting too hot. I have to confess, I'm hooked! Planning to build a bigger amp for a bigger set of speakers!
Thanks for everyone's help. As for the original question, i will have to learn a popular circuit simulator to help me visualize how various parameters relate in a circuit. I was initially using NL5 but it seems to use ideal component characteristics, which brings about problems of its own! bottom line, its useless! LT Spice seems like the best bet.
Thanks for everyone's help. As for the original question, i will have to learn a popular circuit simulator to help me visualize how various parameters relate in a circuit. I was initially using NL5 but it seems to use ideal component characteristics, which brings about problems of its own! bottom line, its useless! LT Spice seems like the best bet.
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