# 400W Powered Amp for small signal input

1. May 7, 2006

### andrew

Hey guys,

I'd like to build a 400W powered amplifier for small signal input from a microphone source to amplify vocals out of PA speakers. I have found the following schematic and I have two questions for whomever is willing to help:

1) Is this circuit adequate for the application?
2) What type of power source should I be looking for (current output) for the +70V and -70V connections?

Thank you so much for your help.
Andrew

click to view schematic

2. May 9, 2006

### Staff: Mentor

The schematic GIF is kind of grainy and hard to read, but it looks like the power supply is around +/-70V (?). I would have expected the supplies to connect into the circuit at top and bottom between the fuse and diode in each case, but I only see that connection on the - rail. Where does the + supply come in?

400W at 140V is 2.9A, so you would need a balanced split +/-70V power supply that can supply around 3A. That's a pretty beefy lab supply, or you could maybe build your own if you know a bit about power supplies. The power supply is critical, if you want to keep hum out of the power amp.

BTW, +/-70V is high enough to be considered dangerous voltage levels. You need to be pretty careful in building and working with a circuit like this. You also need to follow UL-style design practices when building a device like this that has dangerous voltages present. Even the way that you connect your fuse holders is important to minimize shock hazards. Are you familiar with typical UL design practices?

3. May 9, 2006

### waht

There is 8 2SK1530 mosfets. If you look up their datasheet, it should tell you how much power they can dissipate. Then mulitply that by 8 and that will give the power of this amp.

The power supply will be difficult to make for +/- 70 V and 3 amps, thats pretty serious. You will need atleast a couple of 100,000 uf cans, otherwise your THD will be high.

4. May 9, 2006

### Averagesupernova

Wrong. Multiply by 4 since the output is never more than one side on at a time. That's STILL a very loose approximation. I think the OP already knows the power of the amp anyway.

Last edited: May 9, 2006
5. May 9, 2006

### Averagesupernova

I think it was just missed on the schematic.

Nope. You need more than 3 amps. You can't figure at 140 volts. Each 70 volt power supply supplies each half of the waveform. So you need to figure from 70 volts. You haven't figured in anything for inefficiencies either. The way I see it the OP needs about an 8 amp supply absolutely minimum. Here is how I come up with that: The amp has a 400 watt rating. It has 70 volt supply rails. This means that the peak of the sine wave can go no higher than 70 volts. This means that the RMS voltage of the maximum output will be 50 volts AC. 50 volts at 400 watts means 8 amps into the load. 8 amps from each supply. However, that is 8 amps only half the time from each supply but it is RMS current. Current will peak at 11.5 amps. So you could possibly cut back because of that. Filter the output of the power supply enough to smooth out the current demands into something more smooth. I am not a believer in under-building a power supply.

Personlly I would get a switching supply for this project.

Last edited: May 9, 2006
6. May 9, 2006

### Staff: Mentor

Good point. The linear amp is going to need extra current.

Yeah, my Carver audio power amp is sweet! A few hundred watts in less than a cubic foot.

7. May 14, 2006

### andrew

Thank you so much for your replies. I am not familiar with UL design practices and this is one of my first projects actually building and using my circuit. Currently I working towards getting my EE. As stated I am primarily looking for an amp that can take input from a vocal mic and output around 400W to a PA speaker. Would you be able to suggest anything easier and safer to build as well as where I might find schematics for the power supply you suggest I use?

Thanks you all so much again for your time.

Best

Andrew

Last edited: May 14, 2006
8. May 14, 2006

### Staff: Mentor

Who is your mentor or professor? Asking you to build a 400W amp with the associated high voltages is not a good thing, IMO. At the very least, he or her needs to teach you about UL design practices. I've been shocked more times than I can count, and I'm a very careful EE.

9. May 14, 2006

### andrew

I am working with a professor at the University of Calgary here in Alberta. This project is a personally motivated project that I'd like to undertake. It doesn't really matter what circuit I use but in the end I would like a single channel amplifier that can output 400W from a microphone input. It would be workable with 200W or 300W. How do you suggest I approach this?

Thanks again

Andrew

10. May 14, 2006

### Staff: Mentor

Fair enough, Andrew. I just don't want you to get hurt because you and your advisor aren't used to working with line voltages. I'd suggest doing some Internet searches for info on moderate power audio amps, as well as power supply design rules to meet Underwriter's Labs (UL) safety standards. The UL design rules/guidelines don't have to be daunting -- it actually makes sense to put the HV side of the connection on the inside side of the fuse connection, for example. And the avoidance of shocks through the double-fault-required thing also makes sense.

For the audio part, you will obviously need to do several things to keep 50/60Hz hum out of your amp, and you will need to burn more power than you expect to keep the amp linear and sounding good. Plan on lots of heat sinking!

11. May 14, 2006

### andrew

With all that all in mind I think starting off with something a bit lower power and easier to build would be a much better choice. I've found this schematic:

http://www.geocities.com/pa_schematics/200W-poweramp.html

If you have a chance would you mind taking a look at this please?

Also, since my input will be coming from an XLR-connected balanced cable I would assume using a differential amplifier to obtain 1 input to the amp is going to be neccessary for anything I've looked at so far?

Thank you so much again

Andrew

Last edited: May 14, 2006
12. May 15, 2006

### Staff: Mentor

That looks more realistic for a first project, and you will still learn a lot. The source power supply is still pretty high (+/-55V), so you are going to have to be careful. If you can use a lab supply for this first project, that will help out (so you don't have to work with AC Mains voltages and make a quiet linear power supply with no hum). Hopefully that amp part in the middle of the schematic has some good application notes and a recommended PCB layout. Good luck!

13. May 16, 2006

### andrew`

Good to hear! Do you have any ideas on where I could find schematics for a +-55V power supply? Would I want to try and build a "switching power supply"? As this project will be used independently of the lab once it's finished I do in fact need to make a dedicated power source.

Thank you so much again

Andrew

14. May 16, 2006

### Staff: Mentor

That is problematic, and no, you ordinarily would not use a switching power supply for an audio application, unless you *really* know what you are doing in quiet switcher design.

The problem is that you need to take AC Mains power in (120Vrms in the US), and step that down to lower voltages (or split voltages in your case). The step-down stage is also used as a safety barrier normally, to give you electrical isolation between the wall power and your output power. You need to follow good design and construction practices when dealing with AC Mains input connections. There are very important rules for how you wire the input switch, fuse and ground connections, and how you make connections to your main input transformer. I've built a few home projects that use AC Mains input power, but I was *super* careful to follow the UL design rules and make my devices safe, even though I never submitted these personal projects to UL. I have also designed products in my EE work that have been qualified by UL for sale. It's not something that you can be taught in a web forum -- you'll need to get a good book at the very least to help you in starting to build audio projetcs like this that use AC Mains power input.

And then you have the added problem that your output voltages exceed the 42Vdc max (or I think 60Vdc max in Europe) that is consided SELV (safe extra low voltage), or safe for people to touch. Voltages higher than that need to use connections and encloures that prevent contact with humans, especially human fingers. There is actually a UL test finger that they use...LOL. There are some technologies that are older than this UL SELV rule, like the -48Vdc of POTS (plain old telephone sets) and phone lines, and those may be grandfathered by UL. But I honestly don't know how high power stereo-to-speaker connections are handled, if they involve enough power to exceed SELV. Hopefully again the good audio project design book will give you good info on this.

I'll google a bit to see if I can find a good starter hobby audio construction book....

EDIT -- Yeah, books like these.... I googled +stereo +power +electronics +project +design +book, and got to some good books on Amazon:

http://www.amazon.com/gp/product/0071341196/104-0129492-2932700?v=glance&n=283155

Last edited: May 16, 2006
15. May 18, 2006

### Staff: Mentor

16. Jun 27, 2006

### ..::D::..

Hi, i'm new here. can somebody put here an a schematic of Class AB, D,G or T. I need an audio amplifier that work with bjt and mosfet simultaneously, i think that kind of amplifier is class g, but i'm not sure,forexample i saw an 5000w amplifier with 2sc5200 and his pnp pair working with irfp250 with his pair, the power supply was 80V -0- 80V and 70V-0-70V.