Amp building project

As a change from designing speakers and something I can work on in the warm in the evenings, I've started having a go at amp design. My last amp design project was valve based and I haven't touched it for over 5 years. This is my first real attempt at solid state amp design. Although I must admit that having repaired amps for the last few years gives a bit of a head start and plenty of schematics to steal circuit ideas from!!!

Here's a pic of the first prototype anyway. The chassis it's built in used to house a MOSFET amplifier designed by Andy (Matrix) and built by Anytronics.



The transformer is from the original amp and once rectified and smoothed the supply rails are +/- 65V with no load. I've only built one channel is it's very much a proof of concept prototype. Once I'm happy and I've learnt how to use the layout software, I'll get some PCB's built and do a proper 2 channel version. Currently the output power is about 225W/8R and 280W/4R continuous (proper continuous) 1kHz sine wave. The low power at 4R is due to the transformer being somewhat weedy and the supply rails sagging badly. It uses the new OnSemi ThermalTrak output devices which have thermally matched diodes for biasing included in the transistor package.

I could post a schematic but it's still changing as I battle issues with instability (at first it very much wanted to be a shortwave transmitter!) and bias stability (it's actually over-compensated, meaning the bias current drops with temperature).

I have plans for much more sophisticated designs in the future but I need to make sure I've got the basics nailed first. A question was asked on Speakerplans if my designs would be open-source. My policy in general with my designs, speakers or electronics is that I don't give away complete solutions because if for no other reason no-one would learn that way. I am however willing to provide enough information such that someone who has or is willing to research the subject can recreate what I have designed. I will most likely release a design on this site in due course for others who want to have a go at building an amplifier. You never know, I might even sell some PCB's to make it easy!

Edit: Forgot to check the 'Notify me of replies' box!

It's been a little while and progress has been slow but there have been a few developments on this project. I've fitted the new transformer, many thanks Owain, it's a huge improvement. I've also tidied up some of the wiring, no more mains fuses uninsulated in the case! Powercon inlet! Speakon outputs! I've made a few minor tweaks to the amp itself but nothing significant.

With the new transformer, it's now capable of a very respectable 500W into 4R! Not bad for something so small! Obviously it probably couldn't sustain that power continuously without overheating but since I'm never going to run it flat out, that's not a problem.



Incidentally the G clamp is to get round the fact I haven't sorted out the clamping of the output devices to the heatsink properly yet!

Hi, im interested in knowing about the transistors, whats the max supply voltage for them? Did you design the circuit yourself? or where did you det the basic design from?

Im sure i could design a powersupply, so that what im going to do first with my project, atm i have a 2x55v (2x9.1A) transformer and a 12000uf 200v cap, i am having trouble sourcing another one tho so i might not use it...

Whats the rating of your new transformer? and how complex is your power supply?

Hi Jake,

The power supply is really the easy part. When you're doing high end or commercial amps you have to do a bit more maths on it but for a one-off DIY amp you can just over-engineer it and it'll be fine!

My new transformer is about half the size of yours! It's 2x 50V@5A, 500VA total. Yours is a 1kVA transformer which means you could build a fairly serious amplifier with it. The capacitor you have isn't particularly useful, especially as it's only one. Unfortunately 55V secondaries just pushes you over the threshold for 80V caps (would be fine if you were in Europe with 220-230V supplies) so you're going to have to go for 100V caps. There are lots of choices but to go with your transformer you're going to need something like 8 of these uk.farnell.com/panasonic/ecos2ap472da/ca...00uf-100v/dp/1198728 Four in parallel for each supply rail of the amplifier. You'll need a bridge rectifier too. Again, plenty of choice: 35A, 400-600V devices are easy to come by. Because amplifiers like this run on unregulated supplies, that's all you need! Well, it's a good idea to maybe fit the odd fuse here and there! Just beware that the sort of power supply you're going to be building will make A BIG mess of itself and you if something goes wrong, so be careful! One of the first things you should build is the classic lightbulb in the mains device (wire a 60/100W bulb in series with the live, ideally with a bypass switch) and use it whenever you're powering up something you're unsure of for the first time. If it powers up ok you can hit the bypass switch and run as normal.

On to the amplifier itself, I'm using OnSemi NJL1302 and 3281 devices which are unusual in that they are 5 pin devices with a diode mounted in the same case for biassing purposes. I'm still working on how best to make use of the diode. Maximum voltage rating is 250V I think from memory, so they're good up to supply rails of +/- 125V. They do another device in the series which has a 350V rating but you have to be running serious supply rails to need that kind of rating.

I suggest you start reading the DIY Audio amplifier forum which is a mine of information, plus the articles and projects on ESP sound.westhost.com/

That should keep you busy for a bit!

Chris

chaudio wrote: Hi Jake,

The power supply is really the easy part. When you're doing high end or commercial amps you have to do a bit more maths on it but for a one-off DIY amp you can just over-engineer it and it'll be fine!

My new transformer is about half the size of yours! It's 2x 50V@5A, 500VA total. Yours is a 1kVA transformer which means you could build a fairly serious amplifier with it. The capacitor you have isn't particularly useful, especially as it's only one. Unfortunately 55V secondaries just pushes you over the threshold for 80V caps (would be fine if you were in Europe with 220-230V supplies) so you're going to have to go for 100V caps. There are lots of choices but to go with your transformer you're going to need something like 8 of these uk.farnell.com/panasonic/ecos2ap472da/ca...00uf-100v/dp/1198728 Four in parallel for each supply rail of the amplifier. You'll need a bridge rectifier too. Again, plenty of choice: 35A, 400-600V devices are easy to come by. Because amplifiers like this run on unregulated supplies, that's all you need! Well, it's a good idea to maybe fit the odd fuse here and there! Just beware that the sort of power supply you're going to be building will make A BIG mess of itself and you if something goes wrong, so be careful! One of the first things you should build is the classic lightbulb in the mains device (wire a 60/100W bulb in series with the live, ideally with a bypass switch) and use it whenever you're powering up something you're unsure of for the first time. If it powers up ok you can hit the bypass switch and run as normal.

Is it really ok just to have a rectified signal and some big smoothing caps!?? i was going to try and get it more stable than that, although i suppose if the powersupply is meaty enough and the caps are big enough then ripple should be minimal. And handily enough i can buy anything from rs and farnell at the bottom line price (ie i can but one item for the 5000off price)

if i was making a big power supply i was thinking of having a couple of 20r 600w resistors in series with the caps, and a bypass switch in parrallel with the 20r resistors to swith on after a few seconds, so as not to strain the tranformer or any mains fuses... is that a sensible idea, or is it unnessicary?

There are hi-fi amplifier designs which use regulated supplies but almost all run on unregulated supplies. Regulation tends to introduce more issues than it solves. If nothing else, the regulation circuit could end up as big as the power amplifier itself!

A 'soft start' circuit is not a bad idea but the normal method is to implement it in the primary side of the transformer. You probably need to study the circuit diagrams of the big name amplifiers. If you go to www.chaudio.co.uk/ServiceInfo there is a reasonable selection to get you going. Most soft-start circuits involve a ~50R resistor in series with a PTC which is then bypassed by a relay.

Oh, yeah i didnt know that, well i'll consider the powersupply already done then!.

Thanks for that, theres lots of interesting stuff there. I have only had a breig look so far and the only soft start circuit i chave seen so far is the a6000, and to me that looks like too much hassel for me to worry about at this stage.

I wrote you quite a long response a couple of days ago, but it seems to have dissapeared, it was a few questions about how many transistors you are using and that kind of thing, i shal try to remember...

If your transistors can put out more than 100 volts, then thats gonna be over 10a into 8r. So how much current can one of your transistors handle?
Do you just add more transistors in parrallel to let them be able to drive lower loads, or to be more stable?

In terms of schematics to look at, I would recommend the Crest and C-Audio sections for starters. There are lots of articles on the web about amp design if you're looking for simplified circuits and descriptions.

My current amp uses two pairs of output transistors, so two NPN in parallel on the positive side, two PNP on the negative etc. They are rated at 16A each but the limit is usually heat dissipation rather than current limit. The more you have in parallel,the more heat you can transfer to the heatsink.

The no-load power supply voltage is +/- 75V. I actually get about +/-68V maximum output into 8R, less into 4R. The voltage the amplifier can deliver is determined by the power supply rails, minus how much they sag under load and how much you loose across the output devices, series resistances and other limitations.

You could also take a look at some Datasheets for transistors and their ratings.

I could write a whole essay on Amp design but really I'm only just learning it myself properly. There's a whole host of good articles written by much more knowledgeable people than me. The ESP site is really worth a visit or ten!

thanks for all the help chris! i had better get started then, speak to you later!

one last question, whats a safe gap between powersupply voltage and cap voltage? Would say, a +- 110v powersupply be safe with 126v caps (2x63 series)

You should be able to use caps right up to their rating. Allow enough gap such that variations in mains voltage won't cause you to exceed the working voltage. Mains here is normally up around the 245-250V region!!!

Don't forget that when you take your 55V secondaries on your transformer and rectify them, you'll get 55x1.414 = 77V supply rails! Now in theory you could use 80V caps but because with no load it'll probably creep up a bit I'd use 100V caps for safety. For a one-off amp the size/cost implications are not worth worrying about. For a budget design they'd probably risk 80V caps!