I found an old ipod charger that uses firewire and says it outputs 12v and 6.8 amp, so I had hoped that would be perfect for powering my new MAX9744 amplifier. Unfortunately when i cut off the 30 pin connector and checked the voltage on the wires before hooking it all up it was reading 17.5 volts. From what I read around the web that may be slightly higher than expected as it isn't under load but could it really drop 5.5 volts when hooked up? Or are they just a bit lax at Apple when sticking to specifications (also seems a bit unlikely, but that could just be me being naive...)?
For the amplifier I need 14 v max, and part of me wants to just hook it up and see if it's all ok, but another part of me doesn't want to watch £20 go up in blue smoke!
Does anyone have any experience with these chargers, and would expect that voltage to be ok once hooked up? Or should I be looking to drop the voltage somehow? And if the latter, should that be across diodes or using a voltage divider of some description? This is all pretty new to me, so any and all suggestions welcomed.
MAX9744 20W Amplifier Power Supply
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- jupiter22
- Posts: 3
- Joined: Tue Jan 19, 2016 10:53 am
Re: MAX9744 20W Amplifier Power Supply
Alas when I get the magnifying glass out it looks like I misread the ampage, and it is actually only .68, so guess this is woefully underpowered anyway...jupiter22 wrote:I found an old ipod charger that uses firewire and says it outputs 12v and 6.8 amp.
- adafruit_support_mike
- Posts: 67485
- Joined: Thu Feb 11, 2010 2:51 pm
Re: MAX9744 20W Amplifier Power Supply
Yeah, I'd suggest finding another power supply.
Is the one you have an Apple-branded device, or just something that's supposed to be compatible with the iPod?
Is the one you have an Apple-branded device, or just something that's supposed to be compatible with the iPod?
- jupiter22
- Posts: 3
- Joined: Tue Jan 19, 2016 10:53 am
Re: MAX9744 20W Amplifier Power Supply
It's a genuine apple one, which is why it surprised me that it seemed so high.
I have decided to go all out and build a sigma11 from amb.org, that should give me a good clean supply I believe. All this is going inside a vintage radio I gutted anyway so doesn't need to be a self contained psu.
I have decided to go all out and build a sigma11 from amb.org, that should give me a good clean supply I believe. All this is going inside a vintage radio I gutted anyway so doesn't need to be a self contained psu.
- adafruit_support_mike
- Posts: 67485
- Joined: Thu Feb 11, 2010 2:51 pm
Re: MAX9744 20W Amplifier Power Supply
Some switching regulators require at least a minimal load to work properly, but yeah, I'd put that one aside for use with non-critical circuits.
The sigma-11 looks like a fairly nice design.. the discrete op amp is interesting, and the parallel output mosfets are a fairly good design.
Personally, I'd add an NPN transistor to each output mosfet with its emitter connected below the resistor, its base connected above the resistor, and its collector tied to the mosfet's gate. That would provide basic current limiting in the event of overload. If the voltage across the current-sense resistor rises above 0.65v, the NPN would pull the mosfet's gate closer to its source, throttling the current.
There are as many opinions about how to design a power supply as there are designers and days of the week though. The existing design looks robust. ;-)
The sigma-11 looks like a fairly nice design.. the discrete op amp is interesting, and the parallel output mosfets are a fairly good design.
Personally, I'd add an NPN transistor to each output mosfet with its emitter connected below the resistor, its base connected above the resistor, and its collector tied to the mosfet's gate. That would provide basic current limiting in the event of overload. If the voltage across the current-sense resistor rises above 0.65v, the NPN would pull the mosfet's gate closer to its source, throttling the current.
There are as many opinions about how to design a power supply as there are designers and days of the week though. The existing design looks robust. ;-)
Please be positive and constructive with your questions and comments.