DIY digital PWM amp project

Yup - after all that umming and ahhing about which amp to upgrade to, whether it's pre/power or integrated etc and having mentally already "bought" a SimAudio i-5 integrated I've come to the conclusion that £2.5K (or even the £1800 s/h one that might still be at Audio Consultants) is way too much to pay for an amp so I'm going to go DIY

I'm going to build a 2ch. power amp based on LC Audio ZAP Pulse 2.2SE modules. Keeping things as simple as possible I'm going to get their PSU (which includes a DC filter and a soft start circuit) and 500VA toroidal xformer aswell. I'll also order all the wire, RCA and XLR sockets and WBT speaker binding posts from them since they seem to have it all.

The whole lot is going to cost me roughly €1000 which is roughly £700 + the cost of a case which I have no clue where to get (see other thread).

I'm going to build it with dual inputs - one RCA, one XLR. That way I can have the AV kit always plugged into one of them (RCA) and the preamp (when I get one) plugged into the balanced inputs and I won't need to have a pre with AV bypass nor will my AV front channels have to go through effectively two preamps.

Allthough, before I get a preamp I'll just use my Arcam A22 integrated as a pre into the RCA inputs.

I've also been mulling around with the idea of adding a volume pot or DACT attenuator to one of the inputs effectively getting rid of the need for a preamp. I wonder if you can get a DACT attenuator controlled by remote.....

Comments and suggestions welcome.... :MILD:

Michael.

 

Hope youve got a big hammer for the gentle tweaking a,and dont be suprise that you learn a whole new vocabulary of swear words.Is it a balanced input as your fitting XLRs or are youy adding a your own balanced to unbalanced stage?

 

The LC Audio PWM modules can be hooked up with a balanced or RCA input or even both at the same time which is what I'm going to do.

I'll have to be careful to make sure the two inputs don't receive signals at the same time...

Is adding a volume pot to a balanced input as straightforward as adding one to an unbalanced one? ie, can I just stick it in the path of the +ve and -ve wires? The passive volume control is the part of the project thats least likely to make it to fruition though

Michael.

 

yep but it will need to be a dual device,or a 4 pole device if you want stereo

 

I was thinking of a DACT attenuator modified with the IR remote unit and motor from Bent Audio.

Neither is cheap but they look like they'll do the job very well

Michael.

 

michael,
there is an article in this months (well last months i guess) hi-fi world about adding remote control to a motorised potentiometer. with part lists etc. you may want to take a look at that...
cheers


julian

 

Michael,

Have you seen this ?

 

I hadn't seen that allthough I knew that LC Audio were working on a pre. However, I've now ordered my "NOH" passive pre from Bent Audio

Michael.

 

michaelab Let us know how you get on. Also ifr you could take some photo's as you are doing the pre.

 

I just ordered the bits fot the power, pre will be along in a couple of weeks. However, I'm getting the pre ready-build as it looks like a right nightmare of a job doing all those wires - not to mention sticking the remote control gubbins in there with it.

Will do a "photo diary" of the power amp though

Michael.

 

Right, here are some pics of the power amp bits:

The 800VA :MILD: toroidal, the LC Audio PSU (which can actually supply up to 8 ZAPPulse modules so it's somewhat larger than I need), the 2 ZAPPulse 2.2SE modules and the input sockets (RCA and XLR):



The ZAPPulse modules close up - can you say surface mount ! Two of the ICs on each board (which is literally credit card sized) have been covered up with some black gunk presumably to disguise what they are. Underneath the big fat output filter coil are the 3 transistors screwed onto a 5mm thick aluminium block which is below the PCB which is all the heatsinking required:



Detail of the PSU showing the integrated mains DC filter and slow start circuit. LC Audio reckon it's some special design, more details here http://www.lcaudio.dk/com/v4p.htm :



With all that stuff I ordered a load of LC Audio's wire and some of their speaker wire for connecting up the terminals aswell.

I've got my WBT speaker binding posts on the way so all I still need is an IEC power socket (or plug as TMH would call it ), a switch and the blue LED gubbins. Oh....and a case.

That PSU is f$%^&*g big! I'm going to need a BIG case to fit everything in. Of the ones I had looked at none are remotely big enough so I may have to get one custom made if I can find someone to do that. It's a real bummer actually because without the case I can't really get started

Michael.

 

It really is quite remarkable how small these amp modules are and how much power they kick out. Lets hope it sounds as impressive as it looks!

 

must be a big bugger then,assume you've checked out the 19"rack mount cases?

 

Well the PSU is about 36cm wide and about 8cm tall so right away I need a case with internal dimensions of 36cm width and 8cm height - you'd be amazed at the number of cases that rules out

Then, laying it all out on the table for taking the pics it's going to need to be at least 40-45cm deep That 800VA toroidal isn't small either and the PSU being so wide means I can't have the PSU and transformer side by side.

19" rack mount cases, I had a look at some at Maplin and RS and some other places on the net but there wasn't immediately anything suitable. I don't want a case that's just a sheet of 1mm steel folded into a box - I'd like something with a bit more quality to it. Also, 19" is a little too wide for what I need.

The search goes on....

Michael.

 

michael,
just a thought, why not go 2 box? i.e. one for the amp modules and one for the psu gubbins or doesn;t that help?
looks like it;'s going to be a good un...
cheers

julian

 

The Naim effect has a strong hold on you Julian . Seriously though, I have thought about two box but apart from the extra hassles and sorting out some kind of "Burndy" lead the guidelines from LC Audio for use of their modules specifically states:

"Always use SHORT, heavy wire between PSU caps and the modules. Less than 4-5 inches." And on a subsequent diagram they emphasize the shortness of the power wires again. How much difference it would really make I don't know.

I'd prefer the neatness of having it all in one box aswell.

I've found these cases which are 435mm wide and are available in 80mm (internal height) and a depth of 400mm which, after measuring it up again should be just enough. They look OK too and aren't too pricey (36 Euros). I'll see if I can get one from them.

Michael.

 

you could always use thick internal cages within the chasis to help seperate the power supply?

just an idea...

Cheers
Chris

 

I'm not really worried about separating the PSU, the transformer will be far enough away from the ZAPPulse modules to not affect them.

I'm sure, if I need to isolate it a sheet of ERS dishcloth would do the trick

Michael.

 

Many people who believe that things sound different find steel an inappropriate material for amp cases. It's also a relative pita to work.

If you look at PFM DIY there are threads regarding the McBride NAP135 clones and a couple of people have fabricated cases from sheet aluminium. If you get it cut professionally construction boils down to glue and simple taps.

Anyway you could construct whatever size you wanted and have it blue anodised to match the power LED.

Paul

 

Yes, I was worried about that.

:chunder:

My father in-law might know somewhere where I can get an aluminium case made to order locally. I'll find something eventually.

Michael.