BA Input transformer ratio jumper switch

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ciscan.81
Posts: 24
Joined: Sat May 26, 2012 8:33 am

BA Input transformer ratio jumper switch

Post by 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?
Image

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

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Joe Malone
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Re: BA Input transformer ratio jumper switch

Post by Joe Malone »

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

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Joe :-)
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ciscan.81
Posts: 24
Joined: Sat May 26, 2012 8:33 am

Re: BA Input transformer ratio jumper switch

Post by ciscan.81 »

Thanks Joe.

Very simple. :D

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