SMPSU for heater supplies

[Ian Bell[_2_]]

I am building an audio mixer that uses 9 pairs of tubes and the heaters
take 12V at a total of 5.4 amps dc. I don't fancy building a single
6amp 12V linear regulator so as I see it I have a couple of choices.

1. Build nine separate 1 amp or 5 separate 2 amp regulators - a bit
cumbersome but workable.

2. Use an off the shelf 12V SMPSU. I have looked at a few examples. Most
seems to switch at around 25KHz and at rated current have about 200mV pp
ripple on the output (content unspecified but I bet there's some 25KHz
in there).


As a low cost, small size way of achieving what I want an SMPSU seems
the way to go but this 25KHz power circuit worries me. OK, the PSU will
be in a separate screened box some way from the mixer's sensitive
circuits but still am am a little concerned.

Anyone have any experience of using SMPSUs for heater supplies in low
level audio or any other comments or ideas??


Cheers

Ian

[Ian Iveson]

Ian Bell wrote:

> I am building an audio mixer that uses 9 pairs of tubes
> and the heaters take 12V at a total of 5.4 amps dc. I
> don't fancy building a single 6amp 12V linear regulator so
> as I see it I have a couple of choices.
>
> 1. Build nine separate 1 amp or 5 separate 2 amp
> regulators - a bit cumbersome but workable.
>
> 2. Use an off the shelf 12V SMPSU. I have looked at a few
> examples. Most seems to switch at around 25KHz and at
> rated current have about 200mV pp ripple on the output
> (content unspecified but I bet there's some 25KHz in
> there).
>
>
> As a low cost, small size way of achieving what I want an
> SMPSU seems the way to go but this 25KHz power circuit
> worries me. OK, the PSU will be in a separate screened box
> some way from the mixer's sensitive circuits but still am
> am a little concerned.
>
> Anyone have any experience of using SMPSUs for heater
> supplies in low level audio or any other comments or
> ideas??

Syntax could do with some remedial work.

I'm surprised that higher frequency SMPS are not available
to suit your application. In general, somewhere around
100kHz seems to be very common, perhaps because of the speed
of available semiconductors at the time when SMPS became
ubiquitous.

It's quite likely that the ripple fundamental is twice the
switching frequency, due to rectification. The difference in
your case could be significant.

It may also be worth checking that the frequency is fixed.
Some SMPS use a constant pulse width and variable frequency,
rather than vice-versa as appears to be most common.

Even with fixed frequency, the varying pulse width leads to
some ripple at frequencies unrelated to the fundamental.

If you are thinking of using a large input cap as part of a
filter, you might check that the SMPS can cope with that
kind of load.

I would consider using the innards of a battery charger, and
either modify or post-filter/regulate to get 12.6V.

Ian

[Ian Bell[_2_]]

Ian Iveson wrote:
> Ian Bell wrote:
>
>> I am building an audio mixer that uses 9 pairs of tubes
>> and the heaters take 12V at a total of 5.4 amps dc. I
>> don't fancy building a single 6amp 12V linear regulator so
>> as I see it I have a couple of choices.
>>
>> 1. Build nine separate 1 amp or 5 separate 2 amp
>> regulators - a bit cumbersome but workable.
>>
>> 2. Use an off the shelf 12V SMPSU. I have looked at a few
>> examples. Most seems to switch at around 25KHz and at
>> rated current have about 200mV pp ripple on the output
>> (content unspecified but I bet there's some 25KHz in
>> there).
>>
>>
>> As a low cost, small size way of achieving what I want an
>> SMPSU seems the way to go but this 25KHz power circuit
>> worries me. OK, the PSU will be in a separate screened box
>> some way from the mixer's sensitive circuits but still am
>> am a little concerned.
>>
>> Anyone have any experience of using SMPSUs for heater
>> supplies in low level audio or any other comments or
>> ideas??
>
> Syntax could do with some remedial work.
>
> I'm surprised that higher frequency SMPS are not available
> to suit your application. In general, somewhere around
> 100kHz seems to be very common, perhaps because of the speed
> of available semiconductors at the time when SMPS became
> ubiquitous.
>

I was surprised too, but the small survey of 60VA and over 12V OEM
supplies at Farnell and RS all cited a 25KHz switching frequency.

> It's quite likely that the ripple fundamental is twice the
> switching frequency, due to rectification. The difference in
> your case could be significant.
>
> It may also be worth checking that the frequency is fixed.
> Some SMPS use a constant pulse width and variable frequency,
> rather than vice-versa as appears to be most common.
>
> Even with fixed frequency, the varying pulse width leads to
> some ripple at frequencies unrelated to the fundamental.
>
> If you are thinking of using a large input cap as part of a
> filter, you might check that the SMPS can cope with that
> kind of load.
>

Yes I noticed that too. Quite a few specified a maximum capacitive load
of just a few thousand uF.


> I would consider using the innards of a battery charger, and
> either modify or post-filter/regulate to get 12.6V.
>
> Ian
>
>

Someone else suggested that but I am not quite sure why unless the idea
was for a source of a cheap high current 12V transformer. Given the
light duty cycle of a charger and the low cost I am not that keen on one
as a source of a heater transformer. Toroids are cheap enough.


Cheers

Ian

[Matron]

On 11 July, 18:13, Ian Bell > wrote:
> I am building an audio mixer that uses 9 pairs of tubes and the heaters
> take 12V at a total of 5.4 amps dc. *I don't fancy building a single
> 6amp 12V linear regulator so as I see it I have a couple of choices.
>
> 1. Build nine separate 1 amp or 5 separate 2 amp regulators - a bit
> cumbersome but workable.
>
> 2. Use an off the shelf 12V SMPSU. I have looked at a few examples. Most
> seems to switch at around 25KHz and at rated current have about 200mV pp
> ripple on the output (content unspecified but I bet there's some 25KHz
> in there).
>
> As a low cost, small size way of achieving what I want an SMPSU seems
> the way to go but this 25KHz power circuit worries me. OK, the PSU will
> be in a separate screened box some way from the mixer's sensitive
> circuits but still am am a little concerned.
>
> Anyone have any experience of using SMPSUs for heater supplies in low
> level audio or any other comments or ideas??
>
> Cheers
>
> Ian

Hi
Check out the amateur radio suppliers. Several 20A SM supplies with
adjustable voltage and switching frequency are available.

Regards
Todd

[Ian Iveson]

Ian wrote:

>> I would consider using the innards of a battery charger,
>> and either modify or post-filter/regulate to get 12.6V.

> Someone else suggested that but I am not quite sure why
> unless the idea was for a source of a cheap high current
> 12V transformer. Given the light duty cycle of a charger
> and the low cost I am not that keen on one as a source of
> a heater transformer. Toroids are cheap enough.

I had in mind an SM lead-acid battery charger able to
deliver at least 6A continuously, as would be required for a
medium-sized car battery for example. At board level, these
are manufactured in large enough numbers to be relatively
cheap. You may be able to adjust the voltage, or you can
lose the excess 1.2V (charger V is usually 13.8) in
post-regulation or RC filtering.

Now I've looked, the price looks quite steep to me, and
typical efficiency of around 75% isn't great. Neither can I
find boards available retail. Complete chargers have fancy
features to justify the high price compared to the
traditional linear examples.

Todd's suggestion may be better.

Ian

[Matron]

On 17 July, 18:48, "Ian Iveson" >
wrote:
> Ian wrote:
> >> I would consider using the innards of a battery charger,
> >> and either modify or post-filter/regulate to get 12.6V.
> > Someone else suggested that but I am not quite sure why
> > unless the idea was for a source of a cheap high current
> > 12V transformer. Given the light duty cycle of a charger
> > and the low cost I am not that keen on one as a source of
> > a heater transformer. Toroids are cheap enough.
>
> I had in mind an SM lead-acid battery charger able to
> deliver at least 6A continuously, as would be required for a
> medium-sized car battery for example. At board level, these
> are manufactured in large enough numbers to be relatively
> cheap. You may be able to adjust the voltage, or you can
> lose the excess 1.2V (charger V is usually 13.8) in
> post-regulation or RC filtering.
>
> Now I've looked, the price looks quite steep to me, and
> typical efficiency of around 75% isn't great. Neither can I
> find boards available retail. Complete chargers have fancy
> features to justify the high price compared to the
> traditional linear examples.
>
> Todd's suggestion may be better.
>
> Ian

Hi

The amateur radio SMPs are designed for use adjacent to sensitive
radio equipment so they should do a fine job. If you do consider using
one you can find reviews etc on the various ham radio forums, Just
google the model etc.

Hope this helps
Todd