Building an amp

[Fran]

Hi all,


Some of you have asked for a simple description of what's involved in building an amplifier - with the finger pointed towards me as the one to do some explaining. So I've decided to take up the challenge, and the timing is particularly good as I've just completed a new amp in the past few days. I don't have pictures yet.... but I should get some going in a day or so.

So the first step is to sit down and decide on what you need in this amplifier. In my case, I needed something (well, I didn't actually need another amplifier, but hey, I didn't let that stop me!) with really high quality sound, great dynamics and clear clean tone with a touch of warmth not sterility. I use horns in my main system, so I didn't need much power (couple of watts is fine for me) and nor did I need much gain. The amp I will describe here will of course not suit many people - but it describes how I built it, and may provide some insight.

So to be completely honest, that's not quite how I came to decide on this amp, but still, these are the features of the amp I built. So, in my case what actually happened is that I had read about a new, very unusual amp that Nelson Pass (he of Pass Labs and First Watt) had designed and demonstrated at a DIY audio event in San Francisco. He also uses horns, and had tried out and experimented a bit with the amp and was happy with the result - a good start for me. Since then, the amp has been described as having a SET type sound, but a very good SET sound...... exactly what I would like.

The amp in question has now become known as the DeLite - and there are a few variations on the theme. One of the nice things about this amp is that the parts count is miniscule.... one of the bad things is that the mosfet used in it is unusual, and while available, is really only designed for industry and most sellers have a minimum buy of 50 or 100 pieces. I bought mine in a group buy....

So the first thing I suppose is to direct you towards the article Nelson wrote about the amp - you should really read this, as it helps a lot with understanding the various bits.

http://www.diyaudio.com/forums/diyaudio ... ifier.html

There is also a longish build thread on diyaudio about this amp - it details how others have fared building it, common pitfalls and of course serves as a place to ask questions.

http://www.diyaudio.com/forums/pass-lab ... p-baf.html

I should state straight up that I am no genius with electronics - I can build, put things in a case, solder well etc etc. but I am really a kits man, not the original inventor.

Correct on all counts. I think we're going to have some
DIY fun with this one, as it sounds far better than it has
any right to. - Nelson Pass

example of how one member put it together:



this is an example of the bare circuit (power supply not included)



Mr. Pass himself with the amp:



The amp at work:



So is anyone interested? Should I stay going and describe my progress?


Fran

[Ivor]

Well I'm intertested!

While it's unlikely that I'll ever actually build an amp myself all this information gives me a better understanbing of what amps actually do.

Of couse as I'm about to leap HEADLONG into the world of DIY (see thread on soldering one small jack plug onto existing headphones) I may well get bitten by the bug. They say minijacks are a gateway drug.
Of course it's more likely I'll solder my head to the kitchen table and have to be rescued by the Fire Brigade.

Carry on Fran... I'll watch from afar.

[tony]

I think carry on if only to here ivors funny interjections every now and then. I cant see myself building an amp either but want to understand how its done. Dac and maybe headphone amp interest me.When you complete the classes for the amp will you go straight into the dac and headphone amp modules?
I understand Ivors concerns re soldering as I could see myself melting a circuit board and burning the dog rather that getting a good joint.
Some of my lunar plumbing joints are a thing of strange beauty(to me)

[paulb]

that would be great....and the amp sure looks interesting(and not that complicated)!my skills at soldering wouldnt be anygood but i guess without practice wouldt get any better.

[Ciaran]

What do the lightbulbs do?

[fergus]

I also say go for it Fran. I am very far from any technical ability or even understanding but I am a great believer in education!

[Fran]

Ok, next installment so....


So the next thing is to have a look at what is actually required. So the basic circuit for this amp is very simple:



but we are going to go a little further and change it as per the instructions for a little higher voltage supply, bit lower distortion and a little bit higher wattage....



as you can figure out, both are pretty much the same, except that the mosfet used is different. This circuit also gets rid of the input capacitor. Personally, I have found that the fewer capacitors are in the signal path the better. Its not always possible, but in general, the fewer the better. So thats why I choose the second over the first circuit.

So next up, is a parts list. This is essentially your shopping list and for this amp:

Transformer
heatsinks
chassis,
connectors,
mosfet
resistors
light bulbs (the right kind)
rectifier bridge
power supply and output capacitors
wire, fuses, solder, flux etc etc

So the big money here is for the transformer (its pretty massive) and the heatsinks/chassis. Now very luckily I had these in stock already from a scrapped amp that couldn't be repaired so all I needed were the capacitors and mosfets. I also already had most of these from salvaged stuff..... so for me this time, this amp will cost small money, <€50 in this case.

Ciaran, the light bulb acts like a big resistor - it is a constant current source which provides the bias for the mosfet. The idea here is that a big power resistor can act as a CCS, but such resistors tend to be expensive and hard to find. So, the light bulb is a very good substitute, requires no heatsinking and is widely available. As you will see, it takes a bit of experimentation to get the right kind, but it does work!



more later,

Fran

[DaveF]

Great thread Fran. I've a reasonable understanding of how amps work but I'm a bit rusty on some of the finer points of analogue stuff. Digital design is my comfort zone.

[Fran]

mm, well I can tell yis now that Nelson was right when he said:

This amp has no right to sound as good as it does

[cybot]

Fair play to you Fran :-)