Master oscillator for switching power supply AND Class D amp: Good or bad?

[earthtonesaudio]

My wild idea is to build an all-in-one effects and amp that is small enough to fit in a backpack, but powerful enough to be heard over a loud drummer.  This is just a part of the "big picture" plan, where I have a band that tours on bicycle...

Anyway,
For the heart of it, I'll need a decent amp and power supply.  Based on the size and weight requirements, I plan on a switching power supply for sure, and probably a Class D (PWM) power amp.

My plan is to use a single, master oscillator for both the switching supply and the PWM for the power amp.  Partly for simplicity, but also to avoid potential problems associated with multiple oscillators in close proximity.

So is this a good plan or fatally flawed?

Thanks,
Alex

[Sir H C]

Switching regulators have built in oscillators, most class D amplifiers have their own too.

http://focus.ti.com/docs/prod/folders/print/tpa3106d1.html is a cool 40 watt amp with high efficiency.

[alanlan]

It's a good question and I'm not sure how to answer it definitively but on one hand if they are synchronised, then removing one clock from the output should remove both.  On the other hand, if they are unrelated, you could argue that a low pass filter will simply average out any asynchronous noise.  If they are similar but not from the same source you could get beat frequencies appearing.

I'm not sure but my guess would be that if they can use the same source crystal then why have two anyway?  The final result may have more to do with careful PCB design and layout.

[sean k]

Hey Alex, I met a band of touring muso's and performers who did the bicycle thing. Well, I met the Tuba player and I like the ideas that revolve around such adventures. The guy who introduced us was into the same kinda lifestyle thing and I spent some time making him a portable amp for his break beats into a mic.

I'm just wondering why and how the drummer is going to carry his full kit on a bicycle?

I've found that these travelling minstrals either perform in towns, where they have access to power, or around campfires and things where the listeners will usually quieten down for the occasion. That said I definitely don't mean to rain on your parade but I'd be thinking about an efficient way of keeping those batteries charged and looking at finding a really efficient speaker so you don't need as much of the power in the first place.

Long time ago I built a sort of parabolic reflector for a band who were performing in a cafe that just sucked all their volume. Long thin room full of crowd and a high ceiling. We built the reflector and then they put it above them at about 45 degrees and it worked really well.

[SISKO]

I think it could be possible and would be interesting to see it implemented, but think:

  What are you going to win? Space? Make it double side pcb, some layers in beetween if you want, and smd. There youre gonna reach the smallest size holly grail
  Are you going to save some components? How much really? A precicion oscillator its no longer what it used to be, nowadays with a couple of components and some ICs you get a very stable oscillator full of protecction.
  Are you going to defeat pcb traking dealing with RF, EMI ? this need a high level of pcb designing to make it work well
  Are you going to save money  (the whole point in all of these IMO ) ? How much really? Does it worth the whole work mentioned before?

But if you want to just experiment, just go ahead! It would be very interesting to see it!

[earthtonesaudio]

Thanks for the replies!
First... this is awesome:

Switching regulators have built in oscillators, most class D amplifiers have their own too.
http://focus.ti.com/docs/prod/folders/print/tpa3106d1.html is a cool 40 watt amp with high efficiency.

It even has a clock input for syncing to another oscillator-just what I was looking for!  I'll have to look more into that.  40W from 25V with no heatsink sounds too good to be true.  But that's at 10% THD.  I don't really know what 10% THD would sound like from a class D amplifier... but I'm guessing it would be, uh, different than a 10% THD tube amp.

@alanlan: yep, the main thing I was worried about was beat frequencies due to separate but similar oscillators.  Sure you could tune oscillators far apart, but even then it could still be a problem, and one oscillator just seems much easier to deal with.

@sean: somehow I knew you'd chime in about the "band on bikes" bit...   Um, I guess the drummer would have to play electronic drums...

@SISKO: Space?  Sorta.  Space for silicon, resistors, and capacitors is not really an issue, it's space for transformers and BIG filter capacitors that I'm mainly worried about.   Components?  Well, using common components I already have on hand is appealing, but so is the "everything on 1 chip" solution. 
Dealing with the layout is the main reason I would want to build it from scratch.  That way I can make the master clock slow enough (but not too slow) that the PCB layout and component selection becomes less critical. 

[MarcoMike]

I love the bike idea!! and as it seems everyone plays electric stuff you could send them all o the same amp, which may be carried by the singer
There's an italian bass amp manufacturer which uses class D and switching PS, it's called MarkBass. they make real good amps and they are incredibly light! on their site they claim "more than 100 watts per pound!"

and of course I'm not sayng you should buy one... but if they can, why shouldn't you??

[Sir H C]

Thanks for the replies!
First... this is awesome:

Switching regulators have built in oscillators, most class D amplifiers have their own too.
http://focus.ti.com/docs/prod/folders/print/tpa3106d1.html is a cool 40 watt amp with high efficiency.

It even has a clock input for syncing to another oscillator-just what I was looking for!  I'll have to look more into that.  40W from 25V with no heatsink sounds too good to be true.  But that's at 10% THD.  I don't really know what 10% THD would sound like from a class D amplifier... but I'm guessing it would be, uh, different than a 10% THD tube amp.

This chip actually uses the PCB as the heat sink, and class D doesn't need too much of a heat sink since you get a really efficient amplifier.  I think this one can be up to 90% efficient and has a cool switching scheme that means that you don't need the inductors that are often needed for these things.  These would be used for subwoofers in TVs and the like.  10 % THD is not too horrid, it is at least harmonics not random harshness.

[SISKO]

@SISKO: Space?  Sorta.  Space for silicon, resistors, and capacitors is not really an issue, it's space for transformers and BIG filter capacitors that I'm mainly worried about.   Components?  Well, using common components I already have on hand is appealing, but so is the "everything on 1 chip" solution. 
Dealing with the layout is the main reason I would want to build it from scratch.  That way I can make the master clock slow enough (but not too slow) that the PCB layout and component selection becomes less critical. 

Of course i did get your idea about the SMPS and the Class D amp about space  but i doint think it would be much of benefit having 1 master oscillator imo.
Maybe i got yor idea wrong. Still, go ahead with this interesting project!

[earthtonesaudio]

@ Chris: Yeah, I saw that bit about PCB heat-sinking.  That might be a little tricky for a DIY-er such as myself, but still not impossible.  The thing I like best about that chip is the "filterless output," which I believe is becoming more common with class D amps.  I'm just looking for something with a little more power, and maybe in an easier-to-deal-with package.

@ Franko: I think I mis-read you earlier, but now I think I get it.  I was talking about 2 separate things: class D and switching supply for it's small size, and using a single oscillator for keeping inter-oscillator modulations and noise to a minimum.  Sorry for the confusion!


As for going ahead with the project, I think I'll start with a low-power class D amp, since I've never built one before.  That way hopefully I'll get a feel for the topology and be able to apply it to a more powerful version later.  Maybe I'll try to fit it all in a 1590A. 

[earthtonesaudio]

I love the bike idea!! and as it seems everyone plays electric stuff you could send them all o the same amp, which may be carried by the singer

Haha, yes!

and of course I'm not sayng you should buy one... but if they can, why shouldn't you??

...Exactly.  That's the whole point of DIY, in my opinion.

[frank_p]

Alex, if you can find one of those army cab, they are all made from folded aluminium, and with an alnico speaker, they are very, very light.  I have two of them and that would be the ideal project to put  your stuff in.  The inside is compartimented for a reason that I don't know; but it's a good thing for isolation between the amp , the speaker cab and the power supply.  I'll post pictures of how it look, this week.

[sean k]

Hey Bro, I went to your site and I totally empathise with what youre about and at 46 I still get out regular on my bike and my car is an 850cc Diahatsu Mira which I totally love.
For me it's all about waste not/want not and a healthy respect for using up whats lying about before charging off into new stuff just 'cause it's there.

One thing I'd love to be able to do, but the knowledge ain't there yet is to join up a bunch of old computer power supplies and use the power to drive a high frequency Tig welder because I'd love to be able to weld aluminium.

And I love those wooden knobs you've got. That is such a good idea!

[Sir H C]

By the way, the band "Kottonmouth Kings" have the drum kit built into a bicycle when I saw them years back.  Not sure the bike was rideable, but it was complete.