Switchable Output Transformer: Level Matching the Non-Trafo

[leigh]

I searched for info on this, but the concepts are still a bit fuzzy in my head and I may be missing some keywords necessary to find the right tidbits...

I'm finishing up a filter box project (full pics and details here when it's done), and one of the final sticking points is level matching the switchable output transformer. That is, I want the transformer output level to closely match the transformer-bypassed output level. Right now, the transformer output is quite a bit lower than the trafo-bypassed output.

For the output transformer, I'm using an Altec/Peerless 15095a can, which is wired for 15k/600. The filter box runs on +/- 15V, so at full output, it can be hitting the transformer really hard.

My current thought is to load the transformer-bypassed output with a resistor of about 15k. Is this thinking in the right direction?

The final opamp stage that would be loaded here is a 741 chip. (Not my design! The filterboard was pulled from a piece of old lab equipment. If I like the basic sounds of the unit, and it proves useful in the studio, I'll rechip the board... it uses 741's through-and-through, so there's about a dozen of them on there - hence I'm not going to rechip it until I know I care enough about it.) As I understand it, 15k is well high enough that the chip won't care about driving that vs. anything else.

cheers,
Leigh

 

[shabtek]

If my fuzzy maffs are correct; the max p-p signal out the tranny is 6V.

thats figuring 30v swing stepped down 5 to 1 turns ratio....

get a 1:1 xfmr.

 

[emrr]

With the transformer you have both step-down volume loss, plus transformer insertion loss. If you plug this into various inputs with different input impedances (say a 600 versus a 47K input) you will probably get a different variation between them no matter what you do. If you are always using the same impedance input after the filter, you can create an output pad for the transformerless tap that matches the loss. No option there makes me very happy.

 

[leigh]

[quote author="shabtek"]If my fuzzy maffs are correct; the max p-p signal out the tranny is 6V.

thats figuring 30v swing stepped down 5 to 1 turns ratio....

get a 1:1 xfmr.[/quote]

Doh! Of course! Not using a 1:1 trafo puts me off on the wrong foot for level matching.... somedays I feel so slow.

As emrr points out, whatever the difference might be could be leveled out by a pad. I would pretty much always be plugging the box into a 20K+ line level input, so that's not too hard to plan for. And while running a pad all the time on an output isn't ideal, on the other hand the rails are running at +/- 15V, which at full blast does get a bit hairy for regular line level (and some of the overdrive modes on this box do go pretty much rail-to-rail). So having a well-engineered perma-pad on the output doesn't seem like a terrible idea... unless it is, for some other reason I haven't considered yet.


Thanks for your help, people!

Leigh