[sburck]

So I am planning on building a cmoy to use with an ipod. I would like to run the amp off of the ipod battery. I was looking at these transformers:

78253/35C Murata Power Solutions Transformers

They have a 3.3v primary and a 5v secondary, they say that the "centre tapped secondary winding allows for full bridge, half bridge or voltage doubling." Now I am not quite sure how the voltage doubling works, but I was wondering could I wire the primary windings in parallel with the ipod battery as a source to supply each transformer with 3.3v, and then wire the secondarys (configured for voltage doubling) in series to get 20v?

 

[AndrewFischer]

That won't work. A transformer by itself can not change DC voltage or current. Batteries are DC. A battery connected to a transformer primary will give 0V at the secondary.

 

[chesterqw]

you need a boost circuit instead

 

[sburck]

Ah yeah induction, sometimes I think I need a refresher in high school physics. Anyway, I am checking out the boost circuit, since these things are switching devices isn't that going to give me a dirty dirty power source?

 

[AndrewFischer]

Quote:

Originally Posted by sburck /img/forum/go_quote.gif since these things are switching devices isn't that going to give me a dirty dirty power source?

It could. Scratch building a quiet one might give you trouble. There are commercial modules too. Noise is still a concern.

It also adds cost and complexity.
For a cmoy, I'd stick to batteries.

 

[sburck]

Plug-in Power Modules - Non-Isolated POL - PTN04050C - TI.com

What do you guys think about that? Check the data sheet. I really don't want the weight and bulk of 2 9v batteries. I do plan to excercise with this thing and my ipod on an arm band, so making is as small as possible is one of my biggest goals.

 

[TimmyMac]

tbh if you're just exercising I don't see why you need an amp. You're only half paying attention to the music anyway. To each his own, I guess.

 

[AndrewFischer]

Quote:

Originally Posted by sburck /img/forum/go_quote.gif PTN04050C - TI.com[/url] What do you guys think about that?

 

[Spacehead]

What is the voltage of ipod battery? There are many op amps that are capable of working from a 2.5V supply.

 

[chesterqw]

it is 3.3V out of pin 18

 

[qusp]

use one of the texas instruments Dc to Dc buck-boost convertors. they work quite well, total PITA to solder though, you really need an oven. seems like a lot of trouble to avoid just using a battery only to have your ipod battery run down much quicker

 

[sburck]

Quote:

Originally Posted by AndrewFischer /img/forum/go_quote.gif

I believe the ripple is in mhz. Would such a high freq ripple have an effect?



Quote:

Originally Posted by Spacehead /img/forum/go_quote.gif What is the voltage of ipod battery? There are many op amps that are capable of working from a 2.5V supply.

Would they have comparable sound quality though?

 

[AndrewFischer]

Quote:

Originally Posted by sburck /img/forum/go_quote.gif I believe the ripple is in mhz. Would such a high freq ripple have an effect?

The data sheet does not specify ripple frequency. F ripple is likely the switching frequency + the odd harmonics. Data sheet says the switching frequency is between 450kHz and 600kHz and varies with Input voltage and output current.

I'm far from an expert on this but maybe I can help a little. I can't give an answer to your question without knowing how you are going to use the device.


Do you have a schematic for a proposed power supply?

It would also help to know what your goals are for the amplifier.


I still think you are going down the wrong path, but I'm willing to take a look. In any case the design and review will be a good exercise.

 

[sburck]

Quote:

Originally Posted by AndrewFischer /img/forum/go_quote.gif The data sheet does not specify ripple frequency. F ripple is likely the switching frequency + the odd harmonics. Data sheet says the switching frequency is between 450kHz and 600kHz and varies with Input voltage and output current. I'm far from an expert on this but maybe I can help a little. I can't give an answer to your question without knowing how you are going to use the device. Do you have a schematic for a proposed power supply? It would also help to know what your goals are for the amplifier. I still think you are going down the wrong path, but I'm willing to take a look. In any case the design and review will be a good exercise.

I may very well be going down the wrong path here as I am pretty new to DIY hifi. I have a basic understanding of analog electronics and I have built 2 guitar amps. My goal for the amp is to have a very compact headphone amp, similar width and height as my ipod but no more then 1/2 an inch thick, something that I can slip behind the ipod in an arm band. The power supply itself would just be the cmoy power supply with the voltage booster in the place of the battery (plus the two caps and 1 resister that it needs to operate).



As for using a lower voltage op-amp, I am open to the idea and it would be preferable to me if I could get one that would have good sound quality running off of the 3.3v ipod battery. By the way, I'm going to be using this with a pair of shure SE530's.

 

[AndrewFischer]

I took a quick look at data sheets but didn't do any math or run a simulation.

I looked at using Panasonic FC's for C1 -- Mouser Part Number 647-UVR1V221MPA
and an OPA2132 for the op amp. This might just work.


The power supply caps have a .09 ohm impedance at 100Khz. Much of the noise in the audio band is going to be filtered out by C1.

The OPA2132 has a >50dB power supply rejection through the audio range.

Without doing any math, I'd guess that any power supply ripple in the audio frequency range will be below audible threshold.

High frequency noise, the 450KHz-600KHz kind, will go right through to your headphones. I don't know if this will make an audible change.
You will be jogging around with a low power AM transmitter. The pi-filter shown in data sheet figure 18 would knock down the switching frequency noise. You might end up needing it.