Question on transformer

[Buzzy]

Toroidal transformers come in different secondary voltages. Lets say i buy a transformer that is rated at 300W with 28-0-28V, is it possible to change the voltage to something higher or lower being put out by the secondaries by using voltage regulators or something else ? The purpose of this question is to see if i can reuse a transformer for different amplifier projects which need different rail voltages. Thankz

[longthrow]

Originally posted by Buzzy
is it possible to change the voltage to something higher or lower being put out by the secondaries by using voltage regulators or something else ? The purpose of this question is to see if i can reuse a transformer for different amplifier projects which need different rail voltages.

using regulators,why not?....

[Buzzy]

So is it better to buy a transformer with higher voltages or lower voltages ? By the way are voltage regulators used to boost or drop voltages ?

[TheMG]

Depends on the type of regulator. If it's a switch-mode regulator it can boost the voltage, otherwise no.

It's always better to pick a transformer that supplies the correct voltage for your needs, as regulators decrease overall efficiency and add complexity. If it's a pre-amp, then using regulators is fine, even recommended, but on a power amp not really.

Keep in mind when selecting a transformer that the unloaded voltage you get once rectified will be the AC output from the transformer multiplied by the square root of 2, minus the voltage drop across the rectifier. Also since AC mains is never spot-on, do not pick a transformer that would come too close to the amp's max voltage rating, as an increase in AC mains could cause problems.

[djk]

Regulators are a PITA.

If linear they get hot and can only reduce the voltage. If switching they can boost, but are complex and have noise issues.

The most sensible way for a DIY amplifier builder to change voltages for different projects is to buy the highest voltage and VA he thinks he will need, and then the voltage can be lowered (with virtually no loss) by means of a small transformer (wired as an autoformer) on the primary side. A variac (variable autoformer) may also be used.

Use of a dual 12V CT at 3A transformer wired as an autoformer will allow adjustments of about +5%, 0, -5%, -10%, -15% and -20% of nominal. So with your 28-0-28 example the DC voltage available (no load) will be roughly ±42, ±40, ±38, ±36 and ±34V.

[Buzzy]

Originally posted by djk


The most sensible way for a DIY amplifier builder to change voltages for different projects is to buy the highest voltage and VA he thinks he will need, and then the voltage can be lowered (with virtually no loss) by means of a small transformer (wired as an autoformer) on the primary side. A variac (variable autoformer) may also be used.

Use of a dual 12V CT at 3A transformer wired as an autoformer will allow adjustments of about +5%, 0, -5%, -10%, -15% and -20% of nominal. So with your 28-0-28 example the DC voltage available (no load) will be roughly ±42, ±40, ±38, ±36 and ±34V.

Isn't this the same as buying another transformer ? So i guess for every new amp project i need to buy a new transformer and there is no way out.

[djk]

No, you don't get it.

You own a 28-0-28 300VA transformer that will put out ±40V DC

You buy a 72VA transformer.

Your existing transformer will now put out :
±42
±40
±38
±36
±34

So instead of buying four expensive 300VA transformers, you have bought one inexpensive 72VA transformer.

You can mount the 72VA transformer in either the amp chassis, or a seperate box (that would be used with many different projects).

[AndrewT]

Hi Djk,
help me.
I cannot see how to wire the autoformer to obtain -15% and -20%.
please explain, preferably with a diagram/s.

[djk]

Hi Andrew.

A and B are equivalent (boost mode).

C and D are equivalent (cut mode).

On a typical variac A is capable of about 10% boost, B is capable of about 5% (using dual 12V CT secondaries and 120V primary) with cuts of about 0%, -5%, -10%, and -15%.

C is a typical variac in cut mode, D is capable of about 0%, -5%, -10%, -15%, and -20%.

[AndrewT]

Hi Djk,
thanks for those diagrams.
I see where I misunderstood. Dual centre tapped 12Vac are a bit rare, unless one were to wind them oneself.

If one is in a 220/240Vac supply region, then dual 24Vac CT (1.5A on each winding) is required.