A good stereo power amp to build

[Twisted85]

Hello,

I hope there hasn't been too many similiar threads like this before, but I'm looking for suggestions for an power amplifier to build. My own expertise is mainly speaker building, but I want to extend my hobby to amplifiers as well.

I'm not a total novice with electronics, so far I've built Millett Hybrid headphone amplifier, 41Hz AMP6 and Millett SoundCard Interface. All Of these work just fine and I've had very little problems building them. I've been reading about audio electronics from books and internet and I can build devices from good instructions, but I really can't design anything by myself, or even fully understand what happens in the circuits.

At the moment I'm using Pioneer VSX-1015 AV amplifier and naturally I would like to build something potentially higher quality for stereo listening. Later I would also like to make a pre-amplifier with remote controller and D/A converters, but I'd like to begin with the power amplifier as it is probably more simple to build.

Some requirements for the amp:

- It needs to be able to drive easily 4ohm load with no problems.

- Class AB. Class A sounds intriguing, but I guess it will be too expensive to build. A very good quality AB amp should be just fine in my case.

- Transistor, FET/MOSFET, OpAmp, I can't really say anything to this as I have very little knowledge about the differences. Tubes are out of question. Apparently my Pioneer uses MOSFETs.

- I'm living in a single, so I don't need that much power output, although my speakers aren't that sensitive (4ohm 85-86dB/2,83V). I believe 20-80W to 4ohm would be sufficient. It's really hard to estimate how much power is actually even used in my case, probably not much.

- I would also like to order a ready-made PCB and I need clear instructions and preferably pictures of a complete amp to ease the project. This doesn't mean that I'm looking for extremely simple circuit, a huge pile of components doesn't scare me away. Whole point of this project is to make something by my own and learn more. Too simplified circuits aren't that interesting.


I'm hoping for few good suggestions that meets my requirements, so I can take a better look for those designs and choose what pleases me the most. Thanks for anyone who can give suggestions with good arguments.



-Ari S

[nattawa]

The Leach Amp seems to be a good fit to your bill. It was well designed by Dr. Marshall Leach (R.I.P.) and was well documented. Many people DIY built it with satisfactory result in the past decades, and many of them are still in service. There are ready made PCB designs available, just checkout the group buy from time to time. Since some of the semiconductors used in the original design have become harder and harder to find, people have come up with PCB designs employing modern devices, and have yielded excellent builds, as far as I know. These designs are using more output transisters (at least 3 pairs per channel) than Dr. Leach's design aiming at most difficult possible speaker loads. Do a simple forum search with keyword Leach, and you'll get tons of info.

I built my own version of The Leach Amp myself and have driven a pair of PSB Stratus Gold-i with it for almost a year with nothing but excellent performance. These are 4-ohm floorstanders and have a low-ish sensitivity simmilar as that of yours. I designed my own PCBs and employed 5 pairs of output transistors per channel. I have never tried Dr. Leach's PCB design that employs 2 pairs of transisters per channel on my speakers, but I believe it would handle the speakers of this kind with ease.

[Twisted85]

Thank you for your reply.

The leach amp sounds interesting, but the power output seems excessive for my usage. 500VA transformer and huge heatsinks will cost quite a lot, especially here at Finland. I understood that the leach can be made with half the output power which could be an option. Does this modification have any other affects to the performance than the lowered power output?

How does the P3A compare to the leach amp? It seems to be very popular as well.

-Ari S

[Bigun]

This question does come up from time to time, the 'what should I build'. This forum has pretty much all the answers you need - but it really needs a 'sticky' thread or a wiki table of the options for people to refer to. It would help a lot of people.

I believe the P3A is a good choice of amp to build and rod elliot has a number of supporting projects like the power supply and speaker protection that you could also add as you enlarge your project.

If you are willing to make your own pcb and it isn't that hard, there are a lot of other good choices such as any of the DX amplifiers.

Be warned - this hobby is addictive !

[jrockhead]

Check out the LM60 Mosfet amps from Ampslab. Everything you need in a kit, comes with pictures and assembly instructions. All you need is a power supply and a box. You can easily build the amp in about two hours and get it ready for mounting.

Sounds very nice and puts out about 60 watts into 4 ohms. Oh, do put the transistors on a LARGE heatsink, Mosfets dump a lot of heat as they are not as efficient as bipolars.

[ma_coule]

Originally Posted by Bigun

I believe the P3A is a good choice of amp to build and rod elliot has a number of supporting projects like the power supply and speaker protection that you could also add as you enlarge your project.

Also P101 amp from Rod Elliott (esp) sounds very good!

& for fun!

[niss_man]

I would recommend the Blame series amplifiers by "Destroyer". 50w 8ohm, 100w 4ohm. Very low distortion. Search for 'Blame ST' or 'Blame Supercharged' if you want twice the power.
*
Greg's Web Site

Regards

Niss_man

[Twisted85]

I've had some time to dig into these suggested amplifiers and any of these should fit my use just fine. Before creating this thread I looked at the earlier posts and Dx Amp, Leach and P3A was often discussed and recommended so I believe these are all fine choices. I just got to pick my own favorite And then look more into that.

What wonders me, is that the power supplys seem to be un-regulated. Is this typical? I was slightly confused about the PS schematics where output voltage is lower that I first reasoned, but if I understood right, the output voltage depends if the amp is driving a load or not.

For example this schematic:

http://sound.westhost.com/p3a-f2.gif

The transformer is connected to produce 50Vac, right? And the actual DC voltages from the rectifier is 35V when connected to a load? I am still slightly confused about this.

That suggested P3A PS seems very simplified to the comparable Dx PS:

http://www.nabucoeletronica.com.br/d...-schematic.pdf

Less is more?

How does the PS output voltage change at no load at all, normal load and heavy load? I understand that the DC voltage isn't flat if the capacitors can't handle the required power, but how does the transformer step into this.

[jaycee]

Unregulated power supplies are typical - the power amp circuits have enough PSRR to reject a good deal of the ripple. Instantaneous energy delivery is more important here.

There is *some* benefit to feeding the small signal stages of a power amp from a seperate, regulated supply, but it is a complex affair.

The Dx PSU you link to does include a number of fancy extras - it mainly comprises of a C-R-C filter to improve the supply quality.

[AndrewT]

Hi,
a single secondary with 25Vac when open circuit will charge a capacitor to [25 * sqrt(2) - rectifier voltage drop] at near zero current.
This will be the DC voltage measured at the capacitor when no load is connected.
As you increase the current demand, voltage ripple starts to develop on the capacitor.
The peak voltage stays fairly close to the no current voltage, but the rectifier voltage drop goes from ~1V to ~1.4V. The minimum of the ripple can be many volts below peak. This gives an average voltage at the smoothing cap of 25*sqrt(2) - rectifier voltage drop when delivering current = 50% * the Vripple at the capacitor.
The effect of this is that the 25Vac (opencircuit) does ~34.3Vdc. Increase the current to 100mA and the voltage will drop to ~33.5Vdc with a small ripple voltage. Increase the current to 1A and the voltage will fall to ~32Vdc with an even higher ripple voltage.

A dual secondary 25+25Vac transformer can give a dual polarity DC supply after it has passed through a parallel pair of rectifiers and smoothing capacitors.
That's where your confusion is beginning.
A centre tapped secondary of 50Vac is more often referred to a 25-0-25Vac (a total of 50Vac across the series connected windings). It is capable of creating a +ve 35Vdc supply and a -ve 35Vdc supply. This is referred as dual polarity when the two supplies are series connected to give +-35Vdc
The 25Vac transformer gives a 35Vdc supply
The CT 50Vac transformer gives two supplies each of 35Vdc.