applying smps to a preamp with rectified power supply

[luc o]

Hi there,

I know very little technically but just bought some chinese cLass D amps (24V) and preamp (12V). Afer assembling these (delayed reaction) I noted that the preamp has 4 diodes, which I believe indicates a bridge rectifier ::: then of course, I realised that the SMPS being DC does not need to go through further rectification and extra smoothing -

Question
1: - how do I connect the SMPS and bypass the rectifiers?


oops... unable to make attachments, maybe because its my first posting

so, here is a link to pre amp on ebay:

2Pcs NE5532+servo circuit Preamplifier board-4 | eBay



Would be very grateful for any help

Luc o.

[!]

Does your class D amp really need a preamp? Powering the preamp with a SMPS, you might actually end up with inferior sound to using the power amp alone. SMPS (unless very high end/well-made/expensive/etc) are less than ideal for analog audio but especially just to power a preamp where signal gain isn't much, it seems a bad idea.

Anyway, your preamp board has a complete power supply circuit on it, except it needs a dual winding or center tapped transformer for power input. I mean it doesn't just have the bridge rectifier comprised of 4 diodes, it also has linear regulators on the heatsinks.

You can leave it as-is if you want, provided you give it a large enough input voltage. The regulators seem to be 7812 and (presumably) 7912 for +-12V rails. This means if you add about 2.5V minimum you have +-14.5V required input, then add the drop across the diode bridge (1.4V) and you'd need at least +-15.9V input.

So, you could remove the bridge rectifier diodes or just use a piece of wire to jumper across each/all of them and reduce your input voltage requirement by 1.4V. Similarly you could jumper across the regulators' input and output pins and reduce voltage requirement by an additional 2.5V.

Either way, is your SMPS dual rail, or only single rail (single output)? If the SMPS is only single rail it won't work with this preamp. Since the opamps use so little current, the regulators on this preamp could handle a much higher input voltage if so desired, EXCEPT that the larger capacitors right after the diode bridge are only 25V rated limiting you to at most +-25V input unless you remove or replace those capacitors. The remaining capacitors seem to be 50V rated except I can't see the voltage rating on the blue colored, 47uF capacitors.

[luc o]

1st off: Thanks for your quick reply:
!Does your class D amp really need a preamp? Powering the preamp with a SMPS, you might actually end up with inferior sound to using the power amp alone. SMPS (unless very high end/well-made/expensive/etc) are less than ideal for analog audio but especially just to power a preamp where signal gain isn't much, it seems a bad idea.

Anyway, your preamp board has a complete power supply circuit on it, except it needs a dual winding or center tapped transformer for power input. I mean it doesn't just have the bridge rectifier comprised of 4 diodes, it also has linear regulators on the heatsinks.

You can leave it as-is if you want, provided you give it a large enough input voltage. The regulators seem to be 7812 and (presumably) 7912 for +-12V rails. This means if you add about 2.5V minimum you have +-14.5V required input, then add the drop across the diode bridge (1.4V) and you'd need at least +-15.9V input.

So, you could remove the bridge rectifier diodes or just use a piece of wire to jumper across each/all of them and reduce your input voltage requirement by 1.4V. Similarly you could jumper across the regulators' input and output pins and reduce voltage requirement by an additional 2.5V.

Either way, is your SMPS dual rail, or only single rail (single output)? If the SMPS is only single rail it won't work with this preamp. Since the opamps use so little current, the regulators on this preamp could handle a much higher input voltage if so desired, EXCEPT that the larger capacitors right after the diode bridge are only 25V rated limiting you to at most +-25V input unless you remove or replace those capacitors. The remaining capacitors seem to be 50V rated except I can't see the voltage rating on the blue colored, 47uF capacitors.
!Does your class D amp really need a preamp? Powering the preamp with a SMPS, you might actually end up with inferior sound to using the power amp alone. SMPS (unless very high end/well-made/expensive/etc) are less than ideal for analog audio but especially just to power a preamp where signal gain isn't much, it seems a bad idea.

Anyway, your preamp board has a complete power supply circuit on it, except it needs a dual winding or center tapped transformer for power input. I mean it doesn't just have the bridge rectifier comprised of 4 diodes, it also has linear regulators on the heatsinks.

You can leave it as-is if you want, provided you give it a large enough input voltage. The regulators seem to be 7812 and (presumably) 7912 for +-12V rails. This means if you add about 2.5V minimum you have +-14.5V required input, then add the drop across the diode bridge (1.4V) and you'd need at least +-15.9V input.

So, you could remove the bridge rectifier diodes or just use a piece of wire to jumper across each/all of them and reduce your input voltage requirement by 1.4V. Similarly you could jumper across the regulators' input and output pins and reduce voltage requirement by an additional 2.5V.

Either way, is your SMPS dual rail, or only single rail (single output)? If the SMPS is only single rail it won't work with this preamp. Since the opamps use so little current, the regulators on this preamp could handle a much higher input voltage if so desired, EXCEPT that the larger capacitors right after the diode bridge are only 25V rated limiting you to at most +-25V input unless you remove or replace those capacitors. The remaining capacitors seem to be 50V rated except I can't see the voltage rating on the blue colored, 47uF capacitors.


Well, being hardcore audiophile for a few decades I would tend to go for a pre stage over passive pots and silver trafo's and other such things I sort of regard as snake oil-(ish!).

Also been impressed by the throwaway tpath type chinese offerings that use bottom shelf pre stages so - and remembering classic golden age monster Japanese intgd amps and the glorious sound of the better examples I hope that this device has every chance of doing a good job!?

I may be wrong - am open to advice ...

Unfortunately my SMPS is 12V and has a single output - drat!
The regulators are indeed 7912's.


The caps are all rated @50V.


Can you suggest the simplest (most elegant) solution?

Thanks in advance,

Luc o

[luc o]

PS. the power amp modules are: TDA7293 X2 70W+70W Amplifier kit-4 on the saame site : thx

[!]

The simplest solution would be to buy a 12-0-12 transformer, rated for 2VA or higher. Higher would give better regulation but since the preamp has 2200uF for such a small current then linear regulators, it wouldn't make much difference. Toroidal type would be preferred if it's in the same amp or preamp casing to reduce EMI.

On the other hand, less elegant but cheapest would be a couple of surplus/spare 12VDC wall warts (AC-DC power bricks), I would "guess" you would want 500mA or higher rating so the voltage stays high enough. If the preamp didn't have regulation it would be important that they are a matched pair but since it does have regulation that won't matter. They'd be wired as shown in figure 2 of this PDF http://www.audioxpress.com/magsdirx/...tamler2874.pdf which would allow using the configuration already present for the preamp's diode bridge and connectors, simply tying one wart's positive to the other's negative and using that as ground (the wall warts cannot be the earth grounded type), with one wart's positive going to preamp positive rail and the other wart's negative going to the preamp negative rail.

[luc o]

Could a dual 24V smps be applied, I suspect its a matter of shorting the diodes?
What would I do with the regulators?

Luc o

[Minion]

You seriously should just drop the SMPS Idea .... You would need a +/- SMPS and most off the shelf SMPS are single voltage which would mean you would need 2 SMPS but that might not work either because some SMPS"s don"t work correctly when wired as a dual ..... Pluss a badly designed SMPS or a SMPS not designed for audio is going to sound bad .....

You are also going to need a seperate Dual PSU for the Poweramp (the TDA7293 isn"t class D so you are going to need a dual PSU that can put out quite a bit of power) ......


Look for a Power transformer (or two seperate transformers) that has 28v 0v 28v at 300 VA and has 12v 0v 12v at 10vA .......

You are just asking for trouble if you start jerry rigging a bunch of SMPS"s and bypassing and moding parts of the curcuit , you are going to fry something if you don"t know what you are doing and maybe even fry yourself ......

good luck and stay thirsty my friend .....

[!]

I agree with Minion, but in theory yes a dual 24V SMPS could be used if it's +-24V, not +24V and a 2nd +24V output, or two fairly identical output voltage, separate SMPS for each power rail so long as one of the two, the one supplying the negative power rail, has a floating ground not tied to electrical ground or neutral.

Even if the SMPS had (theoretically) perfectly clean output, you would still need a way to drop their voltage because NE5532 has a max input voltage of +-22V per its datasheet, it would still make sense to keep regulators in the circuit to drop the 24V down to (any value +-22V or lower but in this case, since they are fixed voltage regulators you can just leave them in the circuit to get...) +-12V as the opamp current is low enough that a ~ 12V drop won't create too much heat for the heatsinks the board already has (120mW or less per regulator).

On the other hand, another bad way to design it would be to leave the diode bridge in, remove the regulators and jumper the regulator locations with another diode each so your total forward drop across the diodes was roughly 2V, bringing the +-24V down to about +-22V.

If the preamp is going to be in the same casing as the power amp, as Minion mentioned the dual secondary PSU you'd use for the power amp could be used as the input for the preamp board too. That is what I would do, when I mentioned buying another transformer previously I had overlooked that the power amp has it's PSU (or had assumed the power amp used just the 24V single rail PSU as I didn't look up its specs).

[luc o]

You guys have been so helpful. Tomorrow I'm going to take a pic of what I've done so far including layout, with the 120W 24V supplies, (BTW, I think they adjust down somewhat, I'll see what I can get them down to and let you know).

I had (it seems incorrectly) figured that since I never need to use the 70WPC of the power amp I could use the 120W smps - please advise. Originally I had planned to use 1 smps per aamp module.

Anyway, I think you guys know all possible permutations so I'm keen on your views.

Sound quality is prime, but my experience with batteries and many tests with friendly engineers does not stack up with Trafo v SMPS arguments so far, even if the scope indicates otherwise (just thrown this into the mix for info exchange).

Thanks again ... you'll get more post tomorrow as outlined.

Luc o

[!]

There is no need to adjust the SMPS to a lower voltage, it will just reduce the peak output power of your amp(s). Yes the two SMPS should be enough for two of those amp boards, TDA7293 only puts out ~42W @ 10% THD or ~ 32W @ 0.5% THD (8 ohm load) with +-24V rails. I would go the other direction adjusting the SMPS and raise the voltage some if you can though the amp kit seller spec'd up to +-35V so maybe it has 35V capacitors making that the upper limit on voltage it can use.

You cannot use 1 x, single rail 24V SMPS per amp. EACH amp board needs +24V, ground, and -24V power. IF your SMPS have a floating ground (check continuity between ground and the PSU metal casing and AC electric ground, there should be no continuity) then you can power both boards from the two SMPS by wiring as suggested previously for wall warts, take positive output from one SMPS to both amp boards' + input, take that SMPS's negative and connect to the other SMPS's positive and then take those to both amps' ground connector, then take the negative on the 2nd SMPS and connect to both amp boards' - input.

That is the only way it will work with two single output SMPS but make sure the SMPS's ground is not earth ground or you may short out the 2nd SMPS.

Also, the amp boards do not have much capacitance on them. While you don't want to overdo it and use TONS of capacitance when it's powered by a SMPS, normally a minimal amount of capacitance on the amp is complimented by a far larger amount on a linear PSU board. I think I would put at least 2200uF capacitors on + to ground, and ground to - power rails, between the SMPS output and the amps' power input.