Question on transformer

Buzzy

Member
2007-04-12 10:53 am
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
 
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

R.I.P
2001-02-04 4:23 am
USA
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

Member
2007-04-12 10:53 am
djk said:


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

R.I.P
2001-02-04 4:23 am
USA
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).
 

djk

R.I.P
2001-02-04 4:23 am
USA
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%.

transformer.jpg
 

djk

R.I.P
2001-02-04 4:23 am
USA
In the age of tubes, dual 12.6V CT was common for heaters (I still have one left).

I can buy new a 12C VT 3A for USD $8.50 each, a used variac in a housing for about $50, a loose variac (no case) for about half that.

The autoformer trick is nice to know for old tube gear rated at 110V. Nominal line is about 123V today, a 12V transformer will drop it down to 110V.