DIY digital PWM amp project

Sorry to disappoint .

I think it will be worth the wait though. On the PSU voltage front I did get a new multimeter this morning since I had to go to the mall anyway to pick up some cat food. It's a big yellow one with all the bells and whistles and much better than the piece of crap I was using until now.

Checked the voltages with it and everything is fine. 46V a/c on the xformer output windings and +/-63V dc on the PSU outputs. 227V a/c at the mains...that's much better

Now I've just got 2 weeks in London and as soon as I get back (7th May) I'll finish it off. Only about another hour or two's work left to do.

Michael.

 

What was the total cost then Michael?

It seemed to be getting on for £1,000 which seems a tad strong, given that you can buy similar amps built up for not much more than that. Or was it substantially less?

 

I don't have all the figures with me here in London but I think the total cost was roughly 1000 Euros which is about £670 - at most it was a bit over - say £750. That doesn't of course include the cost of the tools I had to buy which I didn't have (cone drills, small hacksaw, set of HSS drill bits etc) as that's got nothing to do with the cost of the amp per se.

Of course I haven't heard it yet but I think you'd struggle to get a decent PWM power amp for less than £1000 new. I could also have economised by not buying the big LC Audio PSU and just rolling my own with a couple of rectifier bridges and some caps, maybe adding in just the the LC Audio soft-start circuit which the sell separately.

Lastly, there's all the fun and pride of doing it all yourself

Michael.

 

Come on michaelab is it as good as you hoped it would be.

GAZZ

 

I'll be finishing it off this weekend and if I haven't blown my speakers up I'll let you know how it sounds

Is there a way to test an amp with just a decent multimeter (ie, without hooking it up to some speakers) ? It would be nice to check that it's doing roughly what it's supposed to before connecting up the Dynaudios.....

Michael.

 

Only in the most limited ways.

First check there is no DC offset - if you find more than 50-100mV go recheck everything, esp. the signal and power grounding.*

The only way with a multimeter of checking that the amplifier is doing its thang would be to run a 50Hz test tone (use Cooledit or similar, burn a cd or link to soundcard and run the file) and check that you get an AC voltage reading on your multimeter that varies 0- approx. 0.7*rail voltage from the output of the amplifier as you vary the volume control across its range.

NB this last test is crude; multimeters (unless you've spent the equivalent of a small amplifer on it) are average-responding, not true-RMS. It's a good approximation on a 50-60Hz signals (since such meters are most likely to be used on the mains) and fairly close over the range 50-400Hz, presuming a true sine output. (* Because of the averaging it may be that the multimeter reports a small DC offset as a result of aliasing the HF switching noise on the output of a Class D amp - I don't know, 'cos I haven't tried it; but less than 100mV and you should be safe to test it with a real speaker). The multimeter can tell you nothing about quality though

Now hurry up and finish the project, we all want to know what it sounds like

 

get some 10 ohm (4.7 if its a 4 ohm output) 7 watt resistors and use them as dummy loads,check the current on each of the power supply rails ( possible by replaceing the psu rail fuse with a multimeter set to amps) without an input signal,replace the fuses and measure the output voltage of the psu,if there both within the desired range check for any d.c voltage at the speaker terminals,if everythings good so far replace the resistors with some speakers,perferable some that dont matter,plug in a test signal,monos better as both channels will be getting the same thing and give it a listen,let it play for an hour or 2 and recheck the above tests.
or hippys method,plug it up and turn it on ,if it works let it play for a few minutes then have a sniff around the circuit,if you dont smell burning or see smoke check the output for any d.c if its ok put the lid on and enjoy

 

I think that's going to be my method too . I've got my cheapo Tannoy mx2 speakers in my office system which can be the guinea pigs - I'm not too fussed if I blow them up.

Michael.

 

Here it is - up and running . Didn't realise each module had it's own little blue LED. Must be good



What does it sound like? Ahh - small snag there. Just after I took that picture and before hooking it up to anything I made a final check of everything and noticed I'd mixed up the -ve and GND connections on the inputs :cry: .

Now I've got to un-solder and re-solder those buggers again

Shouldn't take too long though.......

Michael.

 

OK - sorted now. Case on, all the screws done up and just it's now sitting nicely on the bottom shelf of my rack. That blue LED is a bit bright .

It's too late in the evening now to do any worthwhile testing but it works which is the main thing

Have noticed a snag though: it's such a powerful amp (about 230W per channel into 8ohms - double into 4ohms :MILD: ) that it even when the volume control on my passive pre is at "mute" (which is actually just very high attenuation) I get significant volume coming through the speakers and the meaningful range of my volume control is now about 3 or 4 positions . It's made worse I because the DAC64 has a very high output and I'm using balanced connections all the way so there's an extra 6db there.

So - is there an easy way to attenuate the input to the amp without significantly affecting sound quality? AFAIK Rothewell attenuators only exist in RCA form. Is it as simple as sticking a resistor on the +ve wire?

Will give it a proper test tommorrow

Michael.

 

Rothwell now make XLR attenuators, but I'm sure it's straightforward enough to DIY something.

http://www.rothwell.omnia.co.uk/

-- Ian

 

Cheers Ian. Just ordered myself a pair of XLR attenuators. Fairly pricey (£69 for an XLR pair, free shipping) but almost certainly will yield a better result than any DIY attempt and TBH I'm pretty much DIYed out for the time being . Hopefully they'll arrive sometime this week.

Michael.

 

how much all ya need is 4 resistors per channel,

 

I was going to suggest a preamp mod, but you use a transformer-based pre...

As TMH says, no difficulty in putting some attenuation directly in the input of the power amp. And for even the highest quality resistors you might care to use, it will be a hell of a lot cheaper (and better quality than the attenuators) than £69!

 

Cheers hippy but I can't be arsed now. Anyway, might be better if I don't hardwire the attenuation into the amp in case I want to use it anywhere else where it's not required. Also, if I just bought any old resistors I'd probably end up with a channel imbalance and....I'm making excuses - basically, can't be arsed

Michael.

 

know the feeling well,got half a dozen projects on the work bench in a simaler state,

 

A few people have been asking me about the cost involved and TBH I wasn't really sure of the total myself so I knocked up a little spreadsheet with the info. I converted it to HTML using Excel so I hope this works:

Cost breakdown

So, the total was 1106.97 Euros, roughly £740. I could have saved a lot of money by building my own PSU (the massive V4P LC Audio one is overkill really) and by ordering the quantity of wire & cable from them that I really needed. I've got enough of their special "Teflar 2" speaker wire (used to connect the modules to the speaker terminals) left to make a decent run of speaker cable! I ordered 10m and actually needed about 1m

Still haven't had a chance for a decent listening session yet

Michael.

 

I'm curious. I assume you must have ordered the bits before the cable-sceptic epiphany, because €200+ is a lot to spend on wire...

 

Michael

Just to say looking good

 

Sure, but it was just easier to get as much as possible from the same source (LC Audio) rather than faffing about getting bits and pieces here and there so I used the wire that LC Audio recommend. I did order way, WAY too much of it. Ended up using about 50cm of each of the red, black and blue "stripline" wire and about perhaps a tad more of their special "Teflar 2" speaker wire. In terms of the cost of the wire actually used it's probably more like €20.

Still - won't need to buy any more wire if I decide to go for any more DIY projects

Michael.