[V-DiV]

I finally got around to casing my "Pre-STEPS" AC power supply (pics below). However, I am a little concerned that the voltage is a bit over that called for by my amp.

My power supply is the AC front end of a STEPS, up to the rectifier. I am using a Triad VPP28-720 transformer (Digi-Key - 237-1072-ND (Triad Magnetics - VPP28-720)) wired parallel to get 14 VAC/1.44 amps. It supplies a bit over 17 VAC (no load). One forum member said that was normal for a transformer under no load. However, when I plug the amp into my PSU and run it, I still measure just slightly over 17 VAC at the PSU output.

Questions:

1) Why doesn't the voltage come down more than a couple tenths of a volt?

2) Is it dangerous to run an amp rated at 14 VAC, 3 volts over?

3) If so, is there a simple/clean way to bring the AC voltage down a little?

Thanks,
Vic

 

[tangent]

Quote:

Originally Posted by V-DiV /img/forum/go_quote.gif Why doesn't the voltage come down more than a couple tenths of a volt?

Voltage drops linearly (first approximation) with respect to current draw. You only get down to 14 VAC at 1.44 A. Back of the envelope calculation says your load circuit is drawing somewhere around 90 mA, or about a sixteenth of 1.44 A. In other words, you only drop about one sixteenth the difference between 17.whatever and 14.0 VAC.

Quote:

Is it dangerous to run an amp rated at 14 VAC, 3 volts over?

Depends on the amp circuit and what parts you used in building it. Care to tell?

Quote:

If so, is there a simple/clean way to bring the AC voltage down a little?

If the downstream circuit really wants AC, you can use a second transformer, or change out the transformer you have now. If it wants DC, the general solution is regulation.

 

[V-DiV]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif Voltage drops linearly (first approximation) with respect to current draw. You only get down to 14 VAC at 1.44 A. Back of the envelope calculation says your load circuit is drawing somewhere around 90 mA, or about a sixteenth of 1.44 A. In other words, you only drop about one sixteenth the difference between 17.whatever and 14.0 VAC. Depends on the amp circuit and what parts you used in building it. Care to tell? If the downstream circuit really wants AC, you can use a second transformer, or change out the transformer you have now. If it wants DC, the general solution is regulation.

Thanks Tangent for the information (and all the other great stuff on your website!). This power supply is for a CI Audio VHP-2 amp. The wall wart that come with it is 14 VAC, 850 mA. It puts out about 15.5 VAC with no load. (I presume the amp must have rectification and regulation on-board?) The recommended upgrade power supply, VAC-1, is rated 14 VAC @ 1.44 A. Based on Rusty Vawter's comments on it in a Stereophile interview there is not a lot in the way of filtering in the VAC-1 and I thought I could improve on it with this project using the pre-rectifier filtering in your STEPS (Thanks!).

From what you said should I guess that the VAC-1 is also likely to run at a higher voltage as well? So maybe I don't need to worry.

By the way, this power supply makes an audible improvement over the standard 850 mA PSU with my K01s. Instruments have a bit more body. The bass is fuller. The amp is able to more cleanly play complicated, high & loud passages (I listen to a lot of string quartets).

 

[FallenAngel]

Have you considered simply using another PSU instead of VAC1? Take a photo of the inside of that VAC1.