So, I'm working on a desktop headphone amp/preamp combination in a single box. In the past with similar devices, I've used an O2-style voltage doubler with a centre tap, powered off a single AC input.
Yesterday, I was browsing RS, and came across a really nice looking part that I thought could simplify and generally improve my design. 70051K | 2 Output Toroidal Transformer, 15VA, 2 x 9V ac | Nuvotem Talema I haven't decided on a particular power/voltage rating yet, but they all follow a similar footprint and size. My intention is to mount this transformer on the same PCB as the rest of the amp.
The case I was looking at using would fit the whole power supply internally rather than an external AC wall wart. my biggest concern is the potential for hum pickup or other interference from having AC current flowing so near to my amp circuitry. I feel the PSU could actually be better performing than the O2 style one, because it would use full-wave rectification rather than half wave - which also would reduce the physical size (and value) of the main power supply caps. This would lower cost, and be easier to physically work with inside the enclosure I want to use.
I guess my question is, with proper design, do I need to worry about interference/noise from having a transformer mounted on my main PCB? Are there any obvious pitfalls that I should try to avoid with this setup? Are there any tricks to reduce noise from a transformer in the same enclosure as the active parts of the amp? I thought maybe isolating the board using a gap in the ground plane, and perhaps some sort of metal shield between the PSU/amp sections could help with this.
Oh, one more thing, does anyone have a datasheet for these transformers? I'd like to get some more info than RS seems to have on the page, but there's no datasheet easily available as far as I can tell.
Yesterday, I was browsing RS, and came across a really nice looking part that I thought could simplify and generally improve my design. 70051K | 2 Output Toroidal Transformer, 15VA, 2 x 9V ac | Nuvotem Talema I haven't decided on a particular power/voltage rating yet, but they all follow a similar footprint and size. My intention is to mount this transformer on the same PCB as the rest of the amp.
The case I was looking at using would fit the whole power supply internally rather than an external AC wall wart. my biggest concern is the potential for hum pickup or other interference from having AC current flowing so near to my amp circuitry. I feel the PSU could actually be better performing than the O2 style one, because it would use full-wave rectification rather than half wave - which also would reduce the physical size (and value) of the main power supply caps. This would lower cost, and be easier to physically work with inside the enclosure I want to use.
I guess my question is, with proper design, do I need to worry about interference/noise from having a transformer mounted on my main PCB? Are there any obvious pitfalls that I should try to avoid with this setup? Are there any tricks to reduce noise from a transformer in the same enclosure as the active parts of the amp? I thought maybe isolating the board using a gap in the ground plane, and perhaps some sort of metal shield between the PSU/amp sections could help with this.
Oh, one more thing, does anyone have a datasheet for these transformers? I'd like to get some more info than RS seems to have on the page, but there's no datasheet easily available as far as I can tell.
Avoid any wiring returning to your star earth ground, encircling the transformer.
Yes shielding is a good idea.
Think about using 2x 15v AC instead giving + and - 21V DC, avoiding voltage doublers.
21v + and - is a nice margin of headroom if you intend using 15v + and - regulated rails.
Although I am using quite different circuitry, deploying current regulators
and leaving voltage higher would be my preference. The more and more I learn seems
to always lean on regulating current not voltage.... but that's me.
Cheers / Chris
Yes shielding is a good idea.
Think about using 2x 15v AC instead giving + and - 21V DC, avoiding voltage doublers.
21v + and - is a nice margin of headroom if you intend using 15v + and - regulated rails.
Although I am using quite different circuitry, deploying current regulators
and leaving voltage higher would be my preference. The more and more I learn seems
to always lean on regulating current not voltage.... but that's me.
Cheers / Chris
You'd do well to check out rjm's blog on here, he's done headphone amp designs and quite recently was improving his noise floor by screening his toroidal transformers both with copper and mumetal.
Based on his results I'd say definitely keep the trafo away from the electronics and audio wiring as far as possible.
Besides his blog, there's also this thread which is relevant - http://www.diyaudio.com/forums/head...dio-sapphire-desktop-headphone-amplifier.html
Based on his results I'd say definitely keep the trafo away from the electronics and audio wiring as far as possible.
Besides his blog, there's also this thread which is relevant - http://www.diyaudio.com/forums/head...dio-sapphire-desktop-headphone-amplifier.html
Nice! thank you both. I shall have a read of RJM's stuff, I have looked briefly at it in the past, but it looks like some nice designs.
My intention was to use a +/-15V transformer and hopefully regulate it down to +-15VDC. assuming those ratings given on the transformer are nominal RMS figures, 15VAC would leave me with just enough headroom to run a regulator, with very little excess power to dissiapate. I would love to get some information on those little transformers, they seem nearly ideal for what I want to do, but it's a bit pricey to get a couple of them just for testing if they don't end up being usable for the final product.
Assuming I can heat sink the reg's effectively, the +/-22V shouldn't be an issue either. It's a pity more op amps don't allow more than +/-15v input, or I'd use the 22v trafo with an 18v or 20v regulator. I shall have to look further into this possibility. More input/output voltage headroom is always nice to have.
My intention was to use a +/-15V transformer and hopefully regulate it down to +-15VDC. assuming those ratings given on the transformer are nominal RMS figures, 15VAC would leave me with just enough headroom to run a regulator, with very little excess power to dissiapate. I would love to get some information on those little transformers, they seem nearly ideal for what I want to do, but it's a bit pricey to get a couple of them just for testing if they don't end up being usable for the final product.
Assuming I can heat sink the reg's effectively, the +/-22V shouldn't be an issue either. It's a pity more op amps don't allow more than +/-15v input, or I'd use the 22v trafo with an 18v or 20v regulator. I shall have to look further into this possibility. More input/output voltage headroom is always nice to have.
There are a few choices from TI for opamps that work above 15V. OPA604 is a fairly good one for audio. If you want to go beyond that, there's OPA445, OPA452/3. And if you don't mind hunting around (its long out of production) there's my favourite which is LM144.
With small trafos don't forget to factor in the regulation - this can be as high as 15% to add for off-load voltages.
With small trafos don't forget to factor in the regulation - this can be as high as 15% to add for off-load voltages.
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