Mains Transformerless amplifier

Thread Starter

john1211

Joined Sep 17, 2018
12
Hi Thanks for reading.
I want to build a simple discrete amp with the absolutly lowest number of components possible, preferably a two stage with driver transistor and output transistor, as cheap as possible, i don't want to spend any serious money, just a few bucks.
As power supplies are expensive, do you have a design that runs off the mains ac 240volts, just a bridge rectifier and smoothing capacitors, no transformer.
A cheap and cheerful amp that works and is stable, not concerned about hi-fi sound or super low distortion
Thanks
 

LowQCab

Joined Nov 6, 2012
2,506
What You want is EXTREMELY DANGEROUS and
will also have very disappointing results.
( that is, if You can get it to work at all )
Stereo-Digital-Switching-Amplifiers are dirt-cheap from Parts-Express, save your Money and buy one.
.
.
.
 

Thread Starter

john1211

Joined Sep 17, 2018
12
That's true, i would rather build my own and learn from the experience, with help from forum members
I can buy chip amps, but again the problem is power supplies are expensive
If a transformless amplifier is not possible then that's fine. No offence intended to anyone
Thanks!
 
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dendad

Joined Feb 20, 2016
4,215
You absolutely MUST NOT go that way of a rectified mains direct driven amplifier. Just from your questions it is plain your knowledge base in not enough to do so. I second @LowQCab 's comments. And making an amplifier that runs on high voltage is very dangerous and will not be easy or cheap either.
Power supplied are not expensive, in fact, you can get some for nothing by using an old laptop 19V supply is a good start. If you want a bit higher voltage, a lot of HP printers use 32V supplies. Almost any old electronic devices can supply you parts to build a power supply and amplifier so it can cost you very little if you are able to resuse parts.
The 19V power supplies will enable you to build a quite good amp.
Why do you want...
I want to build a simple discrete amp with the absolutly lowest number of components possible
I would think an amplifier that works well would be better.
 

LowQCab

Joined Nov 6, 2012
2,506
Audio-Amplifiers are not a good "first-project",
they are actually much more "finicky" than they may appear to be in a Schematic-Drawing.
Every component must be carefully selected and matched to all of the other components.
.
.
.
 

dendad

Joined Feb 20, 2016
4,215
Yes, start with a very useful project like a variable power supply.
Once again, an old laptop 19V power supply driving an LM317 circuit can give you a real good one.
There are many internet posts to help.
 
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Ian0

Joined Aug 7, 2020
6,293
Are you trying to kill the vocalist? if your amplifier is directly connected to the mains, just think what happens when someone plugs in a microphone.

I suppose that an amplifier with a built in loudspeaker and input from a radio source (Bluetooth, for example) could be built safely just like the live chassis valve televisions, but 230V is not a convenient voltage for audio amplifiers. That loudspeaker with an 8kΩ coil is going to cost you a fortune to get specially made.
 

Ya’akov

Joined Jan 27, 2019
6,561
No need to repeat what others have said about safety and feasibility.

I would like to point out, though, that you have this thing upside down. You mixed up design and implementation. You reasonably state your design goals:

I want to build a simple discrete amp with the absolutly lowest number of components possible, preferably a two stage with driver transistor and output transistor, as cheap as possible, i don't want to spend any serious money, just a few bucks.
But then suddenly decide on an implementation detail by, without any other analysis, settling on the idea of making it use a transformerless power supply. You say “cheap as possible” but then ask if it is possible:

As power supplies are expensive, do you have a design that runs off the mains ac 240volts, just a bridge rectifier and smoothing capacitors, no transformer.
Do you have a design that does? Why would you assume this was the critical detail in meeting your design goals? This is also true of your “preference” for a two stage design. What is your actual goal here? Do you want:

Lowest part count?
Two stages?
Cheapest?

If we reformulate your goals, we might get something like:

I would like to build a low parts count, low cost discrete audio amplifier. The low cost is more important than the audio quality.

Input: ???dBV (lor line, mic, phono…)
Output: ???W into ???Ω
Budget: ???£/$/€/…

Those last things, critical to working out an implementation are missing or specified in a useless way. “Cheap as possible” isn’t really a thing when specifying something.

I am sure plenty of people here would be willing to help you work out a practical design that meets the design goals without the artificial constraints or potential contradictions.

You might feel like you‘ve been piled on, and if that‘s the case I apologize for at least my part. It’s not what is meant by these responses. Almost everything being said here is out of concern for safety mixed with a bit of weariness at your being the Nth person to appear with an unreasonable set of demands that we are expected to somehow fit into a working implementation.

If you would like to explore starting with a good set of requirements that doesn’t impossibly or dangerously constrain the final project, please reformulate your goals, absent implementation details, and with the additional information needed. It will actually be an interesting problem to solve in that case.

I hope you choose to continue.
 

AnalogKid

Joined Aug 1, 2013
10,125
As power supplies are expensive, do you have a design that runs off the mains ac 240volts, just a bridge rectifier and smoothing capacitors, no transformer.
False economy.

I have a friend who teaches grade school math in rural Georgia, so I checked with her. In the US, this is a sixth-grade problem.

1. Rectified and filtered mains delivers approx. 340 V. That means that the semiconductors must be able to withstand at least that, and preferably more for any kind of reliability. This will triple (or more) their cost compared to a more common 40 V or 60 V part in a 12 V circuit.

2. Output capacitor. A single-supply audiio amplifier requires an ontput coupling capacitor to prevent a huge direct current through the speaker that is independent of the amplitude of the audio signal. In your case that would be *** over 21 amps***. To block this DC you will need a very large electrolytic capacitor, something on the order of 220 uf at 600 V. There are 450 V parts, but that leaves you with very little operating margin against even a small AC line voltage surge.

3. Power dissipation. If the amp is delivering 8 W to an 8 ohm speaker, that is a peak current of 1.4 A. For that 1.4 A to get to the speaker, it has to go through the output transistor. There is 11.3 V across the speaker, which means there is 328 V across the output transistor. 328 x 1.4 = 464 W peak power dissipated in the output transistor. Forget fans, you will need liquid cooling to remove that much heat, assuming you can find a part that will function at all. Very few single transistors are rated for that level of power dissipation. Of course these numbers will change depending on the actual circuit topology. The actual power dissipation could be 1/2 or even 1/4 of that number, but that still is over 100 W being dissipated in a single transistor. And, not to pile on, you have neither the training nor the experience needed to build this type of circuit.

4. Fire. If the output transistor fails as a short circuit, that puts the entire 340 V across any other components in the output stage. If the output capacitor fails short, the speaker voice coil will literally burst into flames.

Watch out for grade-school arithmetic; it will bite you every time.

ak
 
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BobTPH

Joined Jun 5, 2013
5,759
Assuming this is a one-off project, you are not very resourceful if you can’t come up with a power supply for free. Most of my projects use re-purposed supplies.
 

MrChips

Joined Oct 2, 2009
27,117
Assuming this is a one-off project, you are not very resourceful if you can’t come up with a power supply for free. Most of my projects use re-purposed supplies.
I am an incurable hoarder. I collect things that other people throw away.
I built a guitar tube amplifier. The only item I did not have and had to buy new was the output transformer.

I also built a battery powered portable guitar amplifier. It runs on a 12V lead acid battery which can be replaced with a 12-15VDC wallwart. It uses an automotive audio amplifier designed to run on 12-15VDC.

You just have to be imaginative and resourceful.
 

Ya’akov

Joined Jan 27, 2019
6,561
False economy.
[...]

ak
This is precisely why one doesn't settle on an implementation path because of assumptions. It is normal and good to brainstorm possible solutions to your problem and test them against each dimension.

It is another thing entirely to make your "solution" become the problem when it isn't tested for feasibility, or, not cut your losses when you realize it's just not going to solve the problem the way you thought it would.
 

Ian0

Joined Aug 7, 2020
6,293
False economy.

I have a friend who teaches grade school math in rural Georgia, so I checked with her. In the US, this is a sixth-grade problem.

1. Rectified and filtered mains delivers approx. 340 V. That means that the semiconductors must be able to withstand at least that, and preferably more for any kind of reliability. This will triple (or more) their cost compared to a more common 40 V or 60 V part in a 12 V circuit.

2. Output capacitor. A single-supply audiio amplifier requires an ontput coupling capacitor to prevent a huge direct current through the speaker that is independent of the amplitude of the audio signal. In your case that would be ***42.5 amps***. To block this DC you will need a very large electrolytic capacitor, something on the order of 220 uf at 600 V. There are 450 V parts, but that leaves you with zero operating margin against even a small AC line voltage surge.

3. Power dissipation. If the amp is delivering 8 W to an 8 ohm speaker, that is a peak current of 1.4 A. For that 1.4 A to get to the speaker, it has to go through the output transistor. There is 11.3 V across the speaker, which means there is 328 V across the output transistor. 328 x 1.4 = 464 W peak power dissipated in the output transistor. Forget fans, you will need liquid cooling to remove that much heat, assuming you can find a part that will function at all. Very few single transistors are rated for that level of power dissipation. Of course these numbers will change depending on the actual circuit topology. The actual power dissipation could be 1/2 or even 1/4 of that number, but that still is over 100 W being dissipated in a single transistor. And, not to pile on, you have neither the training nor the experience needed to build this type of circuit.

4. Fire. If the output transistor fails as a short circuit, that puts the entire 340 V across any other components in the output stage. If the output capacitor fails short, the speaker voice coil will literally burst into flames.

Watch out for grade-school arithmetic; it will bite you every time.

ak
I know it’s great fun quantifying how ridiculous this is, but…
325V DC gives 115V rms maximum output from an amplifier, which would be 1650W into 8Ω. I’ve built amplifiers that produce 1600W.

The output capacitor depends ONLY on the frequency required and the speaker impedance, NOT on the power: 1000uF for 20Hz and an 8Ω speaker.

The supply would require 400V or 450V smoothing capacitors. The output capacitor only has to block half that, so a 450V device would be adequate.

400V MOSFETs are available in both N- and P-channel, and 400V small signal transistors are available. The Sempei & Ohashi circuit would do the job nicely.

But wait a minute. . .
115V rms output - sound like just the thing for a 100V-line public address amplifier, It could be made with quite sensible output power rating: 30 Watts would do; but I feel sorry for the receptionists. Whatever you do, DON’T TOUCH THE MICROPHONE!
 

AnalogKid

Joined Aug 1, 2013
10,125
I’ve built amplifiers that produce 1600W.
I've diagnosed and repaired them.

The output capacitor depends ONLY on the frequency required and the speaker impedance, NOT on the power
Never said the capacitance value did.

1000uF for 20Hz and an 8Ω speaker.
The TS said "not concerned about hi-fi sound ", so I winged a number for 100 Hz.

The supply would require 400V or 450V smoothing capacitors. The output capacitor only has to block half that, so a 450V device would be adequate.
Don't like running electrolytic caps at 75% of rated voltage. That is what could be across the output cap if an output transistor failed short (depending, of course, on the actual circuit topology).

400V MOSFETs are available in both N- and P-channel, and 400V small signal transistors are available.
Didn't say they weren't. I pointed out that they are way more expensive than "normal" parts, in a thread with "as cheap as possible" in the 2nd sentence.

ak
 
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