VAC transformer box for pedal board

Help Support GroupDIY:

Potato Cakes

Well-known member
Joined
Jul 1, 2014
Messages
1,886
Location
Nashville, TN
Hello, everyone,

I am redoing my pedal board in hopes that cleaning it up and better organizing the layout that I will be more inspired to actually use it for recording instead of having it sit in storage. One of the things I want to sort out is to make a small enclosure with a 9VAC and 12VAC output to power a Whammy and a Hughes and Kettner Tubeman, both which require AC inputs. Finding a 115V - 12VAC transformer is pretty easy but locating a compact 9VAC that will do 2A is a bit harder to find. Some options are:

1 - Use a 18VAC transformer and wire the primaries for 230V, halving the secondary voltage (US power)

2 - Get another 12V and attenuate voltage on the secondaries to 9V

Now the Whammy has a regulated VDC supply using 7805/7905 regulators, which I'm sure are the fixed version and the spec sheet says that they have a max DC input of 35V, so I should just be able to use another 12V and not worry about it. But if I were to proceed with one of the two options, the questions that arose are if go with number 1, will the VA rating stay the same on the secondaries and if I choose number two, can the secondary voltage be adjust with just a potentiometer and a fixed resistor to prevent shorting them together?

Thanks!

Paul
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,036
Location
Marcelland
Potato Cakes said:
locating a compact 9VAC that will do 2A is a bit harder to find. Some options are:

1 - Use a 18VAC transformer and wire the primaries for 230V, halving the secondary voltage (US power)
hammond have several xfmrs, such as the 1182K9 that are spec'd as 18V CT but actually have two independant 9V secondaries that can be wired in parallels.

2 - Get another 12V and attenuate voltage on the secondaries to 9V

Now the Whammy has a regulated VDC supply using 7805/7905 regulators, which I'm sure are the fixed version and the spec sheet says that they have a max DC input of 35V, so I should just be able to use another 12V and not worry about it. 
Beware that the incraesed voltage may exceed the power dissipation of the regulators.

But if I were to proceed with one of the two options, the questions that arose are if go with number 1, will the VA rating stay the same on the secondaries
Yes .

and if I choose number two, can the secondary voltage be adjust with just a potentiometer and a fixed resistor to prevent shorting them together?
You would need power resistors and pot. Not a nice solution anyway.
 

Potato Cakes

Well-known member
Joined
Jul 1, 2014
Messages
1,886
Location
Nashville, TN
abbey road d enfer said:
hammond have several xfmrs, such as the 1182K9 that are spec'd as 18V CT but actually have two independant 9V secondaries that can be wired in parallels.
Beware that the incraesed voltage may exceed the power dissipation of the regulators.
Yes .
You would need power resistors and pot. Not a nice solution anyway.

Thanks for the info!

I did find those Hammond 18V transformers with the dual secondaries. When I initially didn't see any 9V options to suit my needs, my mind when to 18V transformers and wire the primaries for 230V but I skipped over the obvious solution for dual secondaries. I think sometimes part of my brain enjoys seeing the other side struggle by convincing it to do the complicated option first.

I did not think about the heat dissipation for the Whammy pedal's regulators. I'll just stick to 9VAC.

The potentiometer came to mind as the voltages on the secondaries which I am needing are about the same the as the max headroom (+18-23dBu) for most professional audio circuits, so the thought was it would be easy to find a potentiometer that could handle this amount of power. This is also the more complicated solution (see above) and definitely not as elegant as one would want.

Thanks!

Paul
 
Top