Drop a few AC volts ?

[DRONE7]

I have this dac board requiring between 15-0-15 to 18-0-18 ac volts for the I/V & lpf stage.
ES9028Q2M ES9028 I2S input decoder board mill board DAC balanced output | eBay

In the junk box I have a transformer that measures 20.6-0-20.6 ac volts out.

How best to drop around 4 volts ? Would 6 x1N4001 in series each side do it ?
Pros and cons ?
Alternatives ?

Thanks...

 

[Chris Daly]

A variable variac adjusted to lower 4V AC, is one way of doing this.

 

[rayma]

In the junk box I have a transformer that measures 20.6-0-20.6 ac volts out.

You could use multiple diode strings to drop the input AC voltage by 2.6V-5.6V.
Instead you could use a 7-12VDC wall wart.

There's a typo on the ebay page.

Board power supply: AC 15-18VX2 (3 lines) + AC 7-12V (2 lines)
instead should be
Board power supply: AC 15-18VX2 (3 lines) + DC 7-12V (2 lines)

 

[DRONE7]

A variable variac adjusted to lower 4V AC, is one way of doing this.

Lol...like I have a Variac in the junk box...! and even if I did it wouldn't be a long term solution...

 

[DRONE7]

You could use multiple diode strings to drop the input AC voltage by 2.6V-5.6V.
Instead you could use a 7-12VDC wall wart.

There's a typo on the ebay page.

Board power supply: AC 15-18VX2 (3 lines) + AC 7-12V (2 lines)
instead should be
Board power supply: AC 15-18VX2 (3 lines) + DC 7-12V (2 lines)

???
the board shows 2 inputs...both with a diode bridge....and I can confirm that feeding 9v AC to the input you think should be DC does work fine....

please check your info....

My request was for the I/V lpf side which is [15-18]-0-[15-18] AC input.... the 7-12v AC for the chip is all good thanks

 

[Mooly]

Why not just see what the 18-0-18 feeds. It looks like two standard reservoir caps and a couple of three terminal regs. If the caps are 35 volt rated and the regs run cool then I don't really see the problem. If needed you could bend the regs forward slightly and add a very small clip on heatsink to each.

 

[DRONE7]

@Mooly

caps are 35 volt rated and the regs run cool then I don't really see the problem. If needed you could bend the regs forward slightly and add a very small clip on heatsink to each.

Yes of course!.... thanks for that....just checked and the caps are 35v... I will bend the regs and add heatsinks....

 

[Audio1Man]

You could also consider adding a BUCKING TX to the primary to lower the input voltage , thus reducing the output voltage.
Duke.

 

[SyncTronX]

Duke,



How does that Buck Circuit look on the primary side?


cheers,

 

[Audio1Man]

The BOOST or BUCK circuit adds or reduces the voltage to the MAIN TX PRIMARY WINDING. The current rating of the boost or buck secondary transformer need only to have a larger current rating of the MAIN TX WINDING CURRENT. My example used 100 volts as the math is easy to follow.
Duke

 

[indianajo]

In the case the on-board regulators couldn't waste the extra 2 v, I'd use a triac series the transformer output, plus a resistor to the triac gate from input to turn it on. Drops 2 volts, AC rated, comes with a heat sink tab in the TO220 package. I do have these in the junk box and you can often find them in dead appliances like fans, A/C, heaters, power amps, etc.

 

[dangus]

Your 20 volt transformer may drop down to 18 volts when loaded.

 

[Alan4411]

If you know or can work out the current draw of the Dac why not use a couple of resistors? They work just as well in AC as they do in DC and Ohms law will get you close.

Your 'diode string' idea would work equally well by the way as long as they can handle the current and one string's anodes point to the transformer and the other away from it.