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| Simple Audio Amplifier |
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| braddrew0:
Hi Guys, I'm trying to learn how to design simple discrete audio amplifiers. I've come up with the attached schematic, based heavily off Self's blameless amp but with a few calculations made by myself. Can anyone offer any feedback or see any glaring issues with it before I order the components and give it a go? Design Specs: * 16W into 8 ohm * Single Channel * SMD components My Calculations: * Supply current is 2A RMS * Voltage approx 12V RMS * Planned supply (+VDC and -VDC in the schematic) is +/- 20V * Long tail pair input stage - current source is a Wildar, aiming for approx 600uA * Cascode VAS, simple current mirror source, aiming for approx 100mA * I will be building this direct to PCB, running power from my benchtop supply (this is a learning circuit, not a practical one) * My big concern for this being SMD is the thermal capacity of the output transistors - initial plan is to use a big ground plane with lots of vias to see how much cooling I can get that way, but I will have a range of heatsinks (SMD and custom) to try and see how it goes in practical use Thanks very much for any feedback you have! :) |
| Calvin:
Hi, the schematic shows several issues. The most prominent one is the wrong ´polarity´ of the VAs stage (Q10). The Collector potential of Q1 would have to be positive, the Base potential of Q10 is negative. R2 should just be a fixed R of ~100R to 1k .... often it isn´t needed anyway. A value of 1k is often used as R-part of an RC-lowpass filter with C in a range of 220pF to 1nF. In that case place R2 ´in front of´, i.e to the left of C2. Instead of two current mirrors for the long-tailed pair CCS use simple 1-Transistor/1LED current sources with transistor and LED thermally coupled (the CM design/dimensioning appears questionable in itself) There´s no Miller compensation cap around Q10, nor any other compensation arrangement. There´s also no bandwidth limiting cap parallel to the feedback R11 ... typical a few dozen pF. Then there´s no resistor from the feedback point to gnd ... just C3. You have to add a resistor in series to C3, the value depending on the required gain (Av=1+R11/Rx). With R11 beeing 22k Rx would probabely fall between 470R and 1k (typical gains ~30dB). Finally You didn´t draw a V+ connection to the output transistors. So back to the design board ;) jauu Calvin |
| braddrew0:
To be honest I expected more to be wrong so this is a good outcome for me :) * I'll pull out the trim resistor R2 - in my head, this was going to be volume control. I'm planning to test by plugging into the headphone output of my computer and I wasn't sure if that output would be too high for the amp. * By 'polarity', do you mean Q10 should be a PNP? The design is based on VAS configuration D on the self article (http://www.douglas-self.com/ampins/dipa/dipa.htm) - that has both of the transistors as NPN, although what you're saying makes sense :) * Likewise for compensation of Q10 - is C4 sufficient? (Again that came from the same Self configuration) * I'll switch out the current mirror :) * I totally missed the voltage divider on the feedback loop! I'll add that in, along with a cap * Will also add the V+ line... that will help a lot :) Thanks a bunch for your help! |
| soldar:
This thread has several similar schematics you can look at: https://www.eevblog.com/forum/projects/classic-40w-audio-amp-with-discrete-components/ |
| Kleinstein:
--- Quote from: braddrew0 on August 22, 2019, 09:09:48 am ---...... * By 'polarity', do you mean Q10 should be a PNP? The design is based on VAS configuration D on the self article (http://www.douglas-self.com/ampins/dipa/dipa.htm) - that has both of the transistors as NPN, although what you're saying makes sense :) * Likewise for compensation of Q10 - is C4 sufficient? (Again that came from the same Self configuration) --- End quote --- Q10 should be PNP, or the input differential stage should be PNP. Usually the input stage and VAS stage use opposite polarity transistors. As the VAS stage tends to be more critical with speed, the preferred version is often PNP differential and NPN VAS stage. A value of 47 µF for C4 is way to large. The usual order of magnitude is more like 100 pF-1 nF. |
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