Adding Input Ratio Jumper Switch to JLM BA...?

[ciscan.81]

Hi

I've got a couple of 312-Style JLM BA pres with JLM2520HV FET opamps and EA2622 from classic api.

Both are working great.

I am planning to mount these into a 1U chassis with 2 new 312-Style (slightly different) builds to make a nice _3124 Style pre with two slightly different flavoured pairs.

I have tired the pres with the input transformer ratio set to HIGH and LOW.

I think I prefer the more mid-driven aggressive sound of the HIGH setting (as recommended by Ed Anderson the transformer designer and Jeff at Classic API). However, I think the fuller sound on the LOW setting could be useful too.

I'm thinking of trying to implement a switch so I can flip between the two settings to give the pres some extra versatility.

To slightly complicate this I have variable Z on all channels. I used the normal JLM Z pot wired to 100k with a 15k resistor in series.

My questions are:

1. Is wiring a DP4T switch like this, the best way to get the job done? Or is there a better way?

2. Will the variable Z range be workable on both settings? or should I try to implement some sort of Z range switch also?

Using (Input Z = Rload/ratio squared) with 100k pot + 15k resistor, I get:

HIGH ratio Input Z range = 300 to 2350
LOW ratio Input Z range = 1225 to 9400

I'm not sure if this formula is correct or an over simplification, I'm not great at transformers.

3. Are there any other unforeseen problems with switching the ratios as described?

Any thoughts or advice would be greatly appreciated

Regards

Wayne

 

[ciscan.81]

for anyone else who might find this helpful someday, I received this reply from Joe at JLM Audio over on his forum:

You can wire a high / low impedance switch using a DPDT switch and switching while powered is fine. Z control will work fine over both ranges