Battery Powered Class-D Amp

[lefizz]

Hi,

This is my first post on here and hopefully i can ask for people help.
Just so as you knowI am not a musicain at all, however I am very into DIY audio and have build countless LM3886, LM 3875 chips amps and also a fair few Class-D Tripath based kits.

I am building a Amp for a friend who is a guitar based busker in London.
The main issue is this the busking pitches have no power so everyone needs to use battery powered amps.
The best of these are the Crate Taxi /Limo series.
All use some weird Class-H Philips chip which is very unreliable for day to day use.

I am mid way through building a amp setup but could use some advice.

For the power amp module to maximise battery life, and in my opion sounds quality i am using a kit from 41hz.com. Jan kits involve a lot of SMT bits so arent easy to build but sounds great and are reasonably priced. I also love the Tripath sound and think it would work very well with live music.

What i wanted to ask was advice on the preamp side of things. I have looked at loads and loads of sites and to me this is the hardest part particualrly when runnign of a single SLA battery.

What would be a good front end? My mate plays both electric and semi-acoustic guitar and would also needs a cd input for backing tracks from an iPod.

My idea was something like this an English Channel or Sans Amp GT-2 as a preamp, then an fx loop, and summing amp such as http://www.muzique.com/schem/mixer.gif to mix in the CD\ipod and finally a 41hz.com AMP 32.

Does this sound about right? Also has anyone got any experience with the English Channel or Sans Amp for use with an acoustic guitar?

I have no experience and would love some advice.

[teemuk]

In my opinion, acoustic guitars tend to sound quite bad with anything that is designed for electric guitars. You don't need to process their sound a lot (like at least the SansAmp GT2 does) so I would rather try something with moderately flat and wide frequency response - and preferably no distortion. I also think you might need a signal limiter and a feedback eliminator.

Anyway, I think that this eventually comes down to what kind of music your friend plays: Are we talking about folk-type acoustic stuff or grindcore?

[syndromet]

When it comes to amplifying acoustic guitars, I'm no expert, so I wont comment on that. Talking about electrics I think my oppinion might be interesting enough to share. My current setup consist of a stereo LM1875 amp, running different DIY stompboxes as preamps, most of the time with success. Most of the time I use a Behringer GDI-21 as my preamp, simply because it sounds great on the high gain settings. The GDI-21 is really just a Sans amp copy, and sounds very much like the original. I think the clean sounds are ok, but you'd probbably be better of with something else if your friend is playing using mostly clean or slightly distorted sounds. When I record clean sounds I use my trusted LM1875 with a Professor Tweed at low gain settings or any transarent booster. I also use the english channel when I need that clean/overdrive VOX-sound.

I would suggest something like this: give the amp two switchable preamps, one for acoustics and one for electrics. Depending on the pickups, a piezo equipped guitar usualy have a lot higher impedance than a guitar with magnetic pickups. For the acoustic preamp, I would suggest something as simple as a tl072 running close to its max power, followed by some sort of tree band eq. I have a piezo equipped strat, and it sounds very natural running through my microphone pre, wich is just a tl072 tunning on +/- 12V. Check http://www.syndromet.com/sounds/piezo.mp3 for proof. I use the piezo throuh the GDI-21 at times to, and it sound
ok too, but far from as good as the tl072 thing.

For the electric guitar-preamp, I  wouldn't bother to build the sans amp, as most buskers don't need high gain settings (I remember seeing this guy sitting in the middle of the streat in Oslo playing Sepultura and Pantera on one of those mini-marshalls. Sounded great!). If your friend plays country or bluesy stuff, I'd recomend the Professor tweed, and if he plays rock I would sugest the English channel. You might even consider the Eighteen, as it sounds great on clean settings as well. I have tried my amp with most of the ampsims over at runoffgroove.com, and they all sound a lot like what they are moddeled after. When it comes to powering the thing, I would suggest Gel Cell batteries, like they use on motor bikes. I use that for my stereo Ruby, and have been doing that for a year without recharging. Lightweight, reliable, quickly charged  and hold the power for ages.

[lefizz]

Brilliant!!!

This is exactly the advice I need, not being a guitarist makes the project that much harder since i don't know what sound I am after in a way. The guy mainly plays classic english rock from the 60's 70's (think Stones, Small Faces) but also some heavier stuff like Zepplin and Sabbath. Yeah he's got great taste!!.

Looks like my idea for the english channel would work for the electric side and the tl072 idea sounds perfect since i have got a few and also some tl071 as well. I had thought about dual preamp previously but didnt know if that was needed, so its good to sort that out. Do you have or knows of a circuit based on that opamp with tone controls?

I imagine that gel batteries are the same as SLA's. Sealed lead acid we call them over here and that is what i have to power the unit, a big 7 Ah one, should last a few hours considering the 90% efficency of the Class-D amp i want to use.

Thanks so much for all your advice,

Phil

[teemuk]

Do you have or knows of a circuit based on that opamp with tone controls?

I know too many of them to answer thoroughly. What you have are pretty generic opamps that work in just about any imaginable opamp application. Anyway, a lot of active tone controls are presented at this page:

http://www.headwize.com/projects/equal_prj.htm

For acoustic pre, I might consider something like the three-band Baxandall; it has a flat response when controls are in the middle so it's rather transparent. If you are adventurous you could even build a graphic equalizer with many controllable bands. This can be done by adding more bands to the Baxandall circuit or by the means of resonant circuits like:

http://www.ethanwiner.com/spect-5.jpg

However, I have a doubt that adding all those gyrators (simulated inductors) will increase the current consumption a tad too much for a battery-powered circuit.

An electric guitar will benefit from some midrange notch since the pickup's genuine sound is rather middle-range dominant. If you google for "tone stack" you should find a (passive) schematic similar to one used in thousands of amplifiers rather fast. Heck, I do it for you...

http://www.pentodepress.com/graphs/tonestack.gif

The component values that provide proper impedance matching and desirable "voicing" are so subjective issue that I won't bother to comment on that. There are numerous variations of that circuit - and like just about any tone control circuit it can be converted active as well (refer to how "James" stack was converted to active one by Baxandall). Duncan Munro's Tone Stack Calculator (TSC) software has been pretty popular tool for simulating the frequency response of various tone control circuits. I prefer SPICE since it provides a lot more realistic results.

You may also wish to read

http://amps.zugster.net/articles/tone-stacks

You see, there are too many of these to keep track... I hope that helped.

[lefizz]

Thanks very much, I am building an English Channel today on some breadboard but i will take a look at all those options tomorrow. Thanks a lot guys you have really got me back into this project.

[teemuk]

In the English Channel: If you make the 10kohm resistor, that connects from bass potentiometer's wiper to ground, variable you have yourself a middle range control.

[syndromet]

From the bands you mention, I think both the english channel and the eighteen would sound great. My guess would be that the Eighteen fits a littlebit better. You should breadboard them both, and let you buscker decide.

[lefizz]

To be honest i went with the English channel for the decent tone controls which i think would be important for my mate.

Teemuk what value would i need for the replacement pot, i know the resistor is 10k but would that mean i should go for a 10k pot or a 20k lin so it is 10k in the middle?

The breadboarding is going pretty well, almost done.

[teemuk]

Teemuk what value would i need for the replacement pot, i know the resistor is 10k but would that mean i should go for a 10k pot or a 20k lin so it is 10k in the middle?

That's up to you to listen and decide: Practically you face the issue how sensitive you wish the control to be versus how linear you wish the frequency range to be at the end of the dial where mid-range is maxed. The following attachment may help - it is a screencap of a SPICE simulation plotting the tonestack part of the English Channel (with proper source and input impedances considered). Legend is: Green - mid-range potentiometer at maximum resistance, Blue - half resistance and Red - lowest resistance. It is obvious that the higher the potentiometer resistance the less sensitive it gets. You might be able to compensate this with a logarithmic resistance or something. Anyway, it's your choice: Potentiometers don't cost much so you can A-B the options.

[lefizz]

Hi,

Right well i have breadboarded the English channel and am waiting for the guy from runoffgrrove to supply me with biasing value for 13.8v. I am now back to the idea of the acoustic  pre amp an dand getting a little confused. i guess the main issue is that 90% of the preamp circuits i find are for split power supplies and so i get worked up about how to run them, i realise with a couple fo resitors i could create a +/- 6.8v setup but then how does this work connecting to other parts in setup, this them my ground is actually 6.9v? that stuff always confuses me.

This seemed like a good starting point :-

http://www.headwize.com/images/eq3.gif

Any thoughts?

[teemuk]

I think you should try an alternative "virtual ground" method: With the same two resistors (and a capacitor) you can create a bias voltage (which is half the supply voltage) for the opamps, this allows you to power up the opamps using an unipolar supply instead of dual rail.

See this application note for more details:

http://focus.ti.com/lit/an/sloa058/sloa058.pdf

[lefizz]

Has anyone tried the Little Rebel design, just cos it has a basiclly eveythign i need on the preamp i.e. a tl072 based preamp, full tone controls and a runs off a single supply.
I imagine for an aucostic guitar i would need to change gain controls and for my application remove the LM386 power section but i seem like a pretty good starting point.
Any thoughts

http://rebel.massberg.org/source/layout.pdf

http://rebel.massberg.org/source/schematic.pdf

Phil

[lefizz]

OK i have had a go at modifying the 'Guitar Handbook' preamp with reference to the TI single supply handbook to try and improve it.

I was thinking that this followed by a passive baxendale tone stack would be a pretty decent setup.

Could anyone have a look and check that it all seem in order?

I would use a TL071 or better for my version.