Amp4 battery customization ideas

[krilli]

Hi,

I'm planning the next Amp4 to build. I'll be running it on a 12.8V 3400mAh LiFePO4 pack from Batteryspace.com. I'll probably build a voltage doubler to get short-run high-power from the battery pack and more power in car usage.

About power saving and flexibility in the Amp4 ...

* I'll omit the relay, as it'd take some manouevering to get it to close on lower voltages. Might put it back in later, or perhaps have a bypass switch. Saves power, too.

* I may replace the diode bridge with a Schottky diode bridge to lower the voltage drop there.

* I'll replace the preregulator with a PS4, so we eat less idle current.

* I'll replace the 5V regulator with a low-dropout one. The LiFePO4 pack cuts out at 10V, and the PS4 must perform a minimum 40% voltage drop. I'll have reverse-polarity protection in, perhaps another 0.5-1V. This means the preregulator will at worst put out just under 6V. To squeeze the last drop of juice out of the batteries, an LDO regulator ought to put out a stable 5V until the last moment the battery cuts out. Also the LDO regulators have lower output impedance, which is never bad, eh?

Research questions (for me - though answers / ideas are ofcz most welcome):
- Is the PS4 a drop-in replacement for the TO-220 regulator in the Amp4?
- Do we gain anything by going to a Schottky bridge?
- Does it make sense to add an override/bypass switch for the relay? (The simplest form of this wouldn't make sense here - the relay drivers etc. still eat power even if the relay is bypassed. I'd need to take the transistors off-board or cut traces and disable power to them to really have a *full* relay bypass switch.)
- Does the preregulator only feed the 5V regulator, or is it used elsewhere? Will it bring any noise into any circuits?
- If there's any significant noise from the PS4, how well does the 5V regulator clean it up? Does the LDO perform differently in this regard?
- What changes does using an LDO regulator bring about? Is the max current the same? Is the noise spec the same? Does the lower output impedance have any effect? Do I need to take extra precaution at low supply rails to not disturb the 5V line? Does the LDO save any power over a normal one?

[krilli]

Modifications:
* I'll be replacing the output chip with a DDX2240 or STA517B for higher max current into 4 Ohm.

Further questions:
- How to best tune for 4 Ohm usage? Change the output filter? Keep the output filter and add an input PLLXO to tame the highs? A combination of both? Can a switch change between 8 and 4 Ohm mode?
- Is there any real chance of killing the chip with up to 28.8V rails and a 4 Ohm load with the upgraded output chips? For instance, can the user short the input while plugging something in or out, induce a large noise signal at the modulator inputs, big enough to put the full power of the amp through the load and exceed the maximum current?

[PedalPower]

The pre-regulator only supplies the 5V regulator - you can leave it out entirely if your 5V regulator can stand the input voltage and dissipate the power. You won't need an LDO one if your minimum supply voltage is 10V - you'd need to be able to supply it from before the voltage doubler.

You don't need a bridge at all as you're powering from batteries - but it gives you reverse-polarity protection (or rather polarity irrelevance).

The stock relay won't pull in at 10V, so you'd need one with a lower coil voltage if you are to continue to use it. You can reduce the power it uses considerably by putting a paralleled resistor and capacitor in series with it. I did this with my pedal-powered Amp4 and it also means the relay operates over a wider range of voltages. The relay is also a good way of stopping switch-on thumps if coil activation is delayed (which I also did with a single capacitor added to the chip's voltage sensing network).

If you're going to change the power supply voltage by a factor of two then you should change the input stage gain accordingly.

[V-bro]

Interesting thread!

Today I have built a DC up-converter that runs at 94% efficiency! So if you need any information to build one let me know, or wait until I have pressed some PCBs... This will be pretty soon...

I would try the PS4 and see what the noise level does, the beauty of the switch mode is that it will operate on a very wide input voltage range. Then omit the relay and rectifier completely, they're always lossy... Then the output filter, I don't have the AMP4 schematic files, but I reckon it is tailored for 8 ohms use. If not then changing the values to perform best at 4 ohms load would be preferable.

The STA517B is usable at voltages up to 60V and 8 ohms load, 4 ohm loads should be allowed at even higher voltages.

You could also omit the large bulk caps, or replace for some small ones.

Oh and the feedback resistors should also be tailored to the operating voltage. And the voltage sensing resistors also might need to be changed, certainly to run at higher voltages than 35 volts... It is possible to apply values that will eliminate the limits completely...

------For first power-up testing don't have the large power supply caps installed yet!!!------

[krilli]

Thanks for the excellent reply!

I forgot to make a part of the mission statement explicit -- I am actually aiming for easy and worry-free switching between ~13V long-play battery / ~25V high-power battery / transformer / SMPS.

quote:Originally posted by PedalPower

The pre-regulator only supplies the 5V regulator - you can leave it out entirely if your 5V regulator can stand the input voltage and dissipate the power. You won't need an LDO one if your minimum supply voltage is 10V - you'd need to be able to supply it from before the voltage doubler.

This is illuminating. My thinking here however is that if I'm running the amp at around 28V part of the time, a single-stage regulator dropping to 5V is at risk of toastage, and probably wasting more power than PS4 + small-reg ... I see I'll have to challenge this idea with some datasheets & math.

quote:The stock relay won't pull in at 10V, so you'd need one with a lower coil voltage if you are to continue to use it. You can reduce the power it uses considerably ...

OK you've sold the relay idea I'm rather interested in seeing if the PS4 prereg couln't handle a lower-coil-voltage relay.

quote:If you're going to change the power supply voltage by a factor of two then you should change the input stage gain

True ... the plan is currently to have the gain set correctly for around 24V use, and so have the gain suboptimal for 12V. I think this is OK here - AFAIK there's some added THD+N from the input stage operating on a low voltage signal. In this case I believe 12V battery use will probably be outside, partying.

I hope I'm reading you correctly here. BTW, I'm making a Wiki page for this, if you don't mind I'll put your answer/ideas there ... ?

Also, "polarity irrelevance" is great, thanks! [|)]

[PedalPower]

Pleasure - yes, lift all you like for your wiki page.

[krilli]

Note to self - here's a weird little regulator: LP2967. SMD-only, needs a board implementation like PS4.

http://www.national.com/mpf/LP/LP2967.html#Overview

[krilli]

LP2985 seems to be the same as the LP2967, except that it's a single reg, which makes more sense. Here: http://www.national.com/mpf/LP/LP2985.html

These are very small LDO regulators, with a noise level that seems quite acceptable to me as a layman.

The AMB Gamma DACs use these regulators: http://www.amb.org/audio/gamma1/

So with a 41Hz PS4 and an equivalent build of a board around an LP2985 or similar, we can have very good power efficiency and seemingly low noise in the 5V section of the Amp4 and other amps. The stock regulators work well, but I'm really keen on getting more overall efficiency. Modernization for modernization's sake! Ha!

edit: Funny, I completely missed V-Bro's answer up above until now. Sorry if I seemed oblivious to your post

[col_s]

I've been using a very similar chip which is ADP3336. Beware of the fairly poor ripple rejection at higher frequencies (look at the graphs in the datasheet). An LM317 is bad in this respect as well. If the PS4 is putting out any sort of high frequency noise, then you want to be filtering that before it enters the regulator as most of it will pass through it. You can use an LC filter or perhaps a gyrator circuit.

http://www.acoustica.org.uk/t/3pin_reg_notes2.html

[krilli]

Excellent info! Thanks, you just put my vague concerns / hunches into words. Thanks.