As a Class A newbie enthusiast, I'm looking for some designs simple to the last and found this at the link below. Is this a stable, class A audio amplifier design? The page is silent about the output though it says, the efficiency can be increased to 50% from a notorious 20% because of the coupled transformer at the output. Is there any modification with this basic design. Wise and experienced members, kindly respond.
Electronics 26
Electronics 26
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Yes, the transistor is a basic one, so are the output. Can it be replaced with some power transistor or FET for a fully functional 4-5W Class A. Is the circuit stable sans any oscillation? The design is very attractive.
Circuit topology is fine. Eventual oscillations due to inductive load are easily suppressed with a Zobel network in parallel to the transformer primary.
To obtain 4 W power in 8 ohm speaker with a 15:1 transformer, a 50 mA RMS emitter signal current is required. This requires a DC emitter bias current of around 80 mA.
The 12 V power supply needs to be increased to around 140 V to support the 90 V RMS primary excitation (50 mA x 1,800 ohm). The static power dissipation in the transistor will be around 11 W (80 mA x 140 V).
If you desire low distortion from this circuit at 4 W level, then you will have to raise both DC bias current and power supply voltage which significantly raises power dissipation in the transistor.
To obtain 4 W power in 8 ohm speaker with a 15:1 transformer, a 50 mA RMS emitter signal current is required. This requires a DC emitter bias current of around 80 mA.
The 12 V power supply needs to be increased to around 140 V to support the 90 V RMS primary excitation (50 mA x 1,800 ohm). The static power dissipation in the transistor will be around 11 W (80 mA x 140 V).
If you desire low distortion from this circuit at 4 W level, then you will have to raise both DC bias current and power supply voltage which significantly raises power dissipation in the transistor.
I think the 15:1 transformer is a distraction. It was necessary to drive the low impedance from a 2N2124 but at 4-5W and around 0.5A bias current, you will need to use a power transistor however you design the class A amplifier. Did you need the isolation of a transformer to drive a speaker or did you just assume there was some free power available using a transformer in the circuit?
If you do need the isolation, the same arrangement for any amplifier applies, noting that the primary winding in a single ended class A amp has to carry the full DC bias as well as the AC output currents, so the lower the primary:secondary winding ratio, the lower the losses.
For guidance, I would look at the low-power kit available for the JLH '69 amplifier. At least the JLH is a proven high quality audio design at the power level of interest - check the schematic details here: HOOD1969 Two Channel Stereo 2 Channel 2 CH Class A Amplifier Board Kit | eBay
If you do need the isolation, the same arrangement for any amplifier applies, noting that the primary winding in a single ended class A amp has to carry the full DC bias as well as the AC output currents, so the lower the primary:secondary winding ratio, the lower the losses.
For guidance, I would look at the low-power kit available for the JLH '69 amplifier. At least the JLH is a proven high quality audio design at the power level of interest - check the schematic details here: HOOD1969 Two Channel Stereo 2 Channel 2 CH Class A Amplifier Board Kit | eBay
There is a simplicity and beauty to this circuit that makes it irresistable to some. It is the solid state equivalent of a single ended tube amplifier and sounds like one too.
I have built several amplifiers this way using single depletion mosfet, two resistors, electron one ubt-1 transformer, and psu voltages upto 300 VDC. The sound is extremely natural and dynamic. The distortion is pure 2nd harmonic.
I have built several amplifiers this way using single depletion mosfet, two resistors, electron one ubt-1 transformer, and psu voltages upto 300 VDC. The sound is extremely natural and dynamic. The distortion is pure 2nd harmonic.
Yes, I could not miss to admire the design, and reading your precise but more exciting reply, can I ask for any design or image of your creation?
You might like what is being done here:
http://www.diyaudio.com/forums/solid-state/289300-simple-class-hybrid-mosfet-source-follower.html
An output transformer was mentioned as a possible output.
http://www.diyaudio.com/forums/solid-state/289300-simple-class-hybrid-mosfet-source-follower.html
An output transformer was mentioned as a possible output.
Here are some pictures of the 3 channnel amp that I built with the Triode Electronics SE 30W output transformers (17.67:1).
The DC bias is 120 mA and the supply voltage is 150 VDC. This amp is built with single CREE C2M1000170D mosfet (TO-247 under washer, the other TO-220 are resistors).
The input is transformer coupled so it can be easily driven with balanced signals. No additional amplification is needed as my source is DAC with 4VRMS output signal. The overall voltage gain is 1 thus delivering 2 W in 8 ohm load.
The power supply circuitry, including diode bridge and the large reservoir capacitor (2,200 uF) are all contained on the small PCB (2.5" x 3.8").
The DC bias is 120 mA and the supply voltage is 150 VDC. This amp is built with single CREE C2M1000170D mosfet (TO-247 under washer, the other TO-220 are resistors).
The input is transformer coupled so it can be easily driven with balanced signals. No additional amplification is needed as my source is DAC with 4VRMS output signal. The overall voltage gain is 1 thus delivering 2 W in 8 ohm load.
The power supply circuitry, including diode bridge and the large reservoir capacitor (2,200 uF) are all contained on the small PCB (2.5" x 3.8").
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Here's another Lateral Mosfet project: DIY Class-A 2SK1058 MOSFET Amplifier Project For a maximum 4.8W/8R, the power dissipated exceeds a constant 20W.
For the most basic, single device amplifiers, you will likely need a Mosfet design rather than BJT and the Pass Labs forum here is also a good place to look for information on these types of power amplifier. Here's a guide to all Pass DIY models : http://www.audiobanshee.com/reference/guide_draft.pdf
Also worth study, is the introduction to the ESP's DOZ amplifier which explains the design constraints often ignored by newbies. It doesn't help you build anything but it does help you understand where the problems may lie when things don't go to plan or you simply don't understand how they should work: Death of Zen - A new Class-A power amp
For the most basic, single device amplifiers, you will likely need a Mosfet design rather than BJT and the Pass Labs forum here is also a good place to look for information on these types of power amplifier. Here's a guide to all Pass DIY models : http://www.audiobanshee.com/reference/guide_draft.pdf
Also worth study, is the introduction to the ESP's DOZ amplifier which explains the design constraints often ignored by newbies. It doesn't help you build anything but it does help you understand where the problems may lie when things don't go to plan or you simply don't understand how they should work: Death of Zen - A new Class-A power amp
Thanks. The 2SK1058 design is in the line somewhat I was looking for. Simplicity and elegance.Yes, BJT have issues for single device design as I learned here. Can an introduction of an output transformer increase the efficiency as my posted images show?
Transformer improves efficiency because its primary swings above the supply voltage. For a given supply voltage it handles a peak to peak signal swing that is double versus a resistor load.
What I like about using an output transformer in a class A amplifier is the fact that the bias current can be so much smaller. Ideally it shouldn't make much difference but I just happen to prefer a high voltage, low current design over a low voltage, high current design.
Another benefit is that the secondary is isolated from ground; i.e. output signal flows through the speaker load only without electrical contact to any other part of the circuit.
What I like about using an output transformer in a class A amplifier is the fact that the bias current can be so much smaller. Ideally it shouldn't make much difference but I just happen to prefer a high voltage, low current design over a low voltage, high current design.
Another benefit is that the secondary is isolated from ground; i.e. output signal flows through the speaker load only without electrical contact to any other part of the circuit.
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