Switching power supply and preamp.

[beppe61]

Dear Sirs,

I would like to know it anyone has experience with off-the-shelf switching power supplies used to power preamps .
I intend to use a 48V/1A SMPS to power my Brid of Zen preamp, down powered at 48V from the original 60V.
Is this a sensible idea ?
I like SMPSs because they are compact and come with a long umbilical power cord, so that it can be placed conveniently far from the circuits.

Thanks and regards,

beppe

 

[beppe61]

The selected PS should be the following:

http://www.mascot.no/pdf/9921.pdf

What intrigues me is the switching frequency of about 40kHz and a low enough ripple at 50mV p-p.
I wonder if it could be used with success also in a line preamp as the BOZ (that I intend to mod in order to use it with a 48V PS instead of the original 60V).

Thanks and regards,

beppe

 

[AndrewT]

50mVpp sounds enormous.
Long connection leads defeats the primary advantage of regulated supplies - low supply impedance.

 

[beppe61]

Originally posted by AndrewT
50mVpp sounds enormous
Thanks for your valuable reply Mr Andrew.
Not a sensible choice I understand.

Long connection leads defeats the primary advantage of regulated supplies - low supply impedance
Is this requirement mandatory also with class A designs where the current draw is constant ?
Actually I see a lot of high quality preamps coming with umbilical power cord, in order to place transformer away from the circuits.

Thanks again and kind regards,

beppe

 

[AndrewT]

class A designs where the current draw is constant

This is not true.

For push pull ClassA:
the rail currents rise and fall directly with the output current.
As the ClassA limit is reached the rails are flowing alternately from virtually zero current to twice Iq. i.e. Iq+-Iq=0 to 2*Iq

For single ended ClassA:
the supply rail current changes from Iq to Iq+-Iq as the hard limit for ClassA is reached, for the majority of topologies.
There is one topology, that I am aware of, that is rarely implemented that draws near constant current from the supply rail and sends near constant current into the ground return (the speaker is NOT connected to the ground return instead it is connected to the supply rail, this topology may not be applicable to pre-amp use since the ground return is also the reference for the next stage).

Don't repeat the mistakes of others, no matter how many times you read it.

 

[beppe61]

Originally posted by AndrewT
This is not true.
For push pull ClassA:
the rail currents rise and fall directly with the output current.
As the ClassA limit is reached the rails are flowing alternately from virtually zero current to twice Iq. i.e. Iq+-Iq=0 to 2*Iq

For single ended ClassA:
the supply rail current changes from Iq to Iq+-Iq as the hard limit for ClassA is reached, for the majority of topologies.

There is one topology, that I am aware of, that is rarely implemented that draws near constant current from the supply rail and sends near constant current into the ground return (the speaker is NOT connected to the ground return instead it is connected to the supply rail, this topology may not be applicable to pre-amp use since the ground return is also the reference for the next stage).
Don't repeat the mistakes of others, no matter how many times you read it.

Thank you very much Mr Andrew for your extremely helpful advice that changes completely my belief.
I thought that full class A were synonymous of constant current consumption, indipendently from the output.
This, as I say, changes things completely.
On this basis I understand that long umbilical power cords are bad by design, unless maybe they have a very big section ?

Thank you very much again.
Kind regards,

beppe

 

[AndrewT]

unless maybe they have a very big section

this only minimises the resistance.
The problem is inductance and capacitance.
As the load current changes in sympathy with the reproduced frequencies, the voltage available at the amp will modulate.

 

[beppe61]

Originally posted by AndrewT [/i]
this only minimises the resistance.
The problem is inductance and capacitance.
As the load current changes in sympathy with the reproduced frequencies, the voltage available at the amp will modulate

Please excuse me again, but placing additional caps very close to the amplification circuits would act reducing these problems ?
or the power supply must be placed close to the circuits in any case ?

Thanks again for your very kind and appreciated help.
Kind regards,

beppe

 

[AndrewT]

Hi,
a good regulator will be offering an output impedance around 1milliohm.
Most/many regulators do not like big or low esr caps on their outputs.

How can small, medium to high esr caps ever approach 100milliohms, never mind getting back down to 0.001 or less?

 

[beppe61]

Originally posted by AndrewT
Hi, a good regulator will be offering an output impedance around 1milliohm.
Most/many regulators do not like big or low esr caps on their outputs.
How can small, medium to high esr caps ever approach 100milliohms, never mind getting back down to 0.001 or less?

Thank you very much again for the interesting advice.
On the basis of your valuable advice I understand that a regulated linear power supply is indeed the best, or better, the only solution to power a preamp.

Thanks again and kind regards,

beppe

 

[AndrewT]

Hi B,
I am not saying that SMPS will not work.
What I am saying is that the performance of a linear psu is very good when well designed and a pre-amp will benefit from attention to the details.

An SMPS may be able to do the job just as well, but the design and development could well be beyond many of us.

 

[beppe61]

Originally posted by AndrewT
Hi B,
I am not saying that SMPS will not work.
What I am saying is that the performance of a linear psu is very good when well designed and a pre-amp will benefit from attention to the details.
An SMPS may be able to do the job just as well, but the design and development could well be beyond many of us

I understand perfectly the point Mr Andrew.
Designing and building a SMPS is beyond the ability of the common DIYer.

Thanks again and kind regards,

beppe

 

[N-Channel]

Dear Sirs,

I would like to know it anyone has experience with off-the-shelf switching power supplies used to power preamps .
I intend to use a 48V/1A SMPS to power my Brid of Zen preamp, down powered at 48V from the original 60V.
Is this a sensible idea ?
I like SMPSs because they are compact and come with a long umbilical power cord, so that it can be placed conveniently far from the circuits.

Thanks and regards,

beppe

Beppe,

Does this COTS (Commercial Off-The-Shelf) SMPS put put +48V @1A, or +/-48V@1A?" Are you looking for a single-ended or bi-polar output SMPS? Are you interested in "rolling your own" or modifying the COTS unit you have?

I have not looked, in great detail, at the BoZ Amp, so I don't know what the supply voltage should be for the input stage. Designing an SMPS from near-scratch, while not an easy task (as if we DIYers shrink away from "hard" -v- "easy") 😉 , is not impossible. With some recommended reading, the average DIYer can make a *basic* SMPS. I'm not talking a multi-kW Active PFC'ed unit with all sorts of bells-n-whistles, but a basi9c DC-DC buck or boost converter.

With a little looking on line you can get schematics for AT/ATX pc supplies, DC-DC converters for car amps, and a few other applications. Any of these designs could be modified to suit your needs.

Start with a low-voltage 12V input DC-DC converter putting out, say, +/-48V at +/-1A (96W output). This should give you a good indication as to whether you would want to power a preamp with an SMPS. A well-designed and well-built SMPS should give noe problem to audio circuits. Otherwise, every high-powered car amplifier designed and sold since day-one would be of very inferior performance.

If you feel like going for it, you will find a wealth of info on these pages.

Just my $0.02 worth.

Steve

 

[AndrewT]

every high-powered car amplifier designed and sold since day-one would be of very inferior performance

sounds close to the truth.
And that's with the "experts" help out.

 

[h_a]

Hi,
a good regulator will be offering an output impedance around 1milliohm.
Most/many regulators do not like big or low esr caps on their outputs.

How can small, medium to high esr caps ever approach 100milliohms, never mind getting back down to 0.001 or less?

So what does that mean now?

Regulator close to circuit in the same box? Toroid external?

Is this a good idea to have the rectifyers in the same box together with a delicate phonoamp (in my case)?

I suppose no? So is there any solution to this other than just using a simple unregulated power supply?

Thanks for your great help!

Cheers, Hannes

 

[jan.didden]

So what does that mean now?

Regulator close to circuit in the same box? Toroid external?

Is this a good idea to have the rectifyers in the same box together with a delicate phonoamp (in my case)?

I suppose no? So is there any solution to this other than just using a simple unregulated power supply?

Thanks for your great help!

Cheers, Hannes

I agree with Andrew. My advice:

1 - keep transformer & rectifiers & first cap far away and give them their own ground return;

2 - Use a regulator as close as possible to the circuit to be supplied as possible. If you can, use remote sensing directly at the circuit;

3 - Use cap at the output of the supply to ensure stability, but it's useless to spend money on super caps after the reg. Supercaps (low esr) can actually promote instability and oscillations.
If different caps 'sound' different here, you have a layout or stability problem. Fix that.

Jan Didden

 

[h_a]

Thanks a lot for your reply!

Well I already had the same thoughts, but the problem is that the pcb does not allow a separation of the transformer and rectifiers...well of course one could do that nevertheless, however what's then the point in having a professional pcb.

Doing a pcb myself....well, I hoped my project would be finished at least in some point in the future ;-))

Cheers, Hannes

 

[AndrewT]

Hi,
I would not separate the tansformer from the rectifiers.

I would recommend that the transformer, bridge rectifier and first stage smoothing caps be fitted together. Do not take the 0v of this first stage smoothing to the input of the regs. Instead take it to the output of the regs or direct to audio ground.

Then you have a choice to either locate the amplifier with it's regulators next to the smoothing or locate the amp and regulators remote from the smoothing.