SMPS for RIAA

[Raka]

Has anyone tried to power a RIAA with a smps?
I'm working on a pcb, and wonder if this is a good solution or it has more problems than advantages.

[jackinnj]

the lowest noise SMPS controllers I have seen have noise circa 100 uV -- on the same order of magnitude of a MC cartridge -- so you have your work cut out for you -- consider using something like the LT3439 or LT1683 (Linear has a couple more ultra LN regs) with their ULN linear regulators operating in "post" fashion -- and a couple of micro-henries of inductance in the power leads.

[peranders]

I think the main reasons to use a SMPS are small size and/or high effeciency. Neither of them is important for a RIAA amp.

I think also noone here manage to do an ultra quiet SMPS. I mean high end here but if you only want to play some records I think it's quite possible to use some modern SMPS.

[Raka]

Size could be an issue, but not too much. I was thinking that if the smps has it's noise placed higher than 40Khz (for instance), all this ripple would be inaudible. I know psrr drops very fast, but as I've seen happy smps-gainclones owners, maybe this can be translated to a opamp based riaa.

[AndrewT]

Hi,
I would place some CLCLC or at worst CRCRC in the line feeds to the RIAA circuit to try to suppress most/some of the noise.
Try some small ceramic in parallel with the first and last Cs.

I would tend to think of an SMPS as a voltage adjuster rather than a quality supply to an analogue client.

You could try SMPS then low power Linear regulator to final voltage.

Good quality and effective (at all frequencies) decoupling at the opamp pins are normal. For SMPS I would say they become mandatory.

[peranders]

Originally posted by Raka
Size could be an issue, but not too much. I was thinking that if the smps has it's noise placed higher than 40Khz (for instance), all this ripple would be inaudible. I know psrr drops very fast, but as I've seen happy smps-gainclones owners, maybe this can be translated to a opamp based riaa.

There are two basic differences: A Gainclone is a high level device. A RIAA amp is a low level device.

HF energy can also be modeulated down to audiable frequencies.

A question: How many mic or vinyl amps have SMPS? Why is that?

[Raka]

I was told that one Linn riaa had a smps, but I'm not aware of other else.

[AndrewT]

Hi,
I don't know the LINN schematic and yes they are into some form of SMPS to keep thier power amps small and light.
Maybe this has filtered into the integrated and preamp as well.

BUT I guess they have VERY effective filtering before they send SMPS lines to the low level stages.

[jackinnj]

Pullllllleeaaseeee look at Linear Technology Application Note 70.

[hagtech]

Sure you can use a smps for phono. Just not easy to get it right. First, make sure the controller is PWM and not PFM. Nor hysteretical mode or other odd control algorithm. Design it for continuous conduction in the coil. Then slow the loop response down a little.

Ok, that's just for starters. Now make sure the coil is self shielded. Then add snubbers where needed. Add ferrite bead and capacitor filters on both inputs and outputs. Put distance between this circuit and the phono.

Now, add a bunch of RC or RLC filters to the outputs (not in feedback loop!) a couple of times. This will get your noise level down to whatever you want. At this point, it is really no different than your typical supply. What you won't achieve is direct regulation at the noise levels you seek. You have to burn power with the passive filters. And that sort of negates the usefulness of an smps in the first place.

jh