JLM AUDIO

Hi Joe,

I've got two Baby Animal preamps: One fitted with a Lundahl 1538 and works fine. However, the other one is fitted with a OEP262A3C wired low and although it has (as far as I am aware of) the correct RL/CL values in place it shows a skewed frequency response. As the attached image shows, the OEP transformer has less low end (normal, I guess), but also shows a steep rise in high end response above 10khz. Measurements were made going directly from a Lynx Aurora at low amplitude into the preamps. Different op amps (Gar1731, BB2521) didn't change the response.

Is there an obvious cause/remedy for this? Thanks!

living sounds wrote:Hi Joe,

I've got two Baby Animal preamps: One fitted with a Lundahl 1538 and works fine. However, the other one is fitted with a OEP262A3C wired low and although it has (as far as I am aware of) the correct RL/CL values in place it shows a skewed frequency response. As the attached image shows, the OEP transformer has less low end (normal, I guess), but also shows a steep rise in high end response above 10khz. Measurements were made going directly from a Lynx Aurora at low amplitude into the preamps. Different op amps (Gar1731, BB2521) didn't change the response.

Is there an obvious cause/remedy for this? Thanks!

That is the correct curve for a OEP with a fixed 27k RLoad and 220pF to 440pF CLoad WHEN driven by a low impedance line output.

But when the OEP is actually driven by a dynamic mic or mic with transformer output the high end bump is basically flat due to the fast rising impedance of the microphone output.

You can flatten the loop testing bump by removing completely or making Cload smaller. You can also add a CZobel and RZobel. Or Make RLoad smaller in value.

Or add variable impedance and when winding the impedance lower the transformer will flatten out.

We did lots of testing of this transformer when we first started using it over 20 years ago and in all testing in lab and with some of our biggest clients everyone preferred the sound with 27k Rload and 220pF Cload when compared to the Zobel flatten sound.

If you want super flat response use JLM14 transformer

Thanks Joe! It sounds good, but just a little too sparkly with my U47 clone (which has a higher amplitude and probably a lower impedance than the average condenser).

I'll leave it as it is.

It's very interesting how different the combinations sound. The 990/Lundahl channel comes over as midrange-forward in comparison, more API-ish than the channel with the actual API-type-op-amp and OEP transformer, which has more of a Neve-ish sound (lacking the fatter bottom).

How would you describe the sound with your input transformer? I used a BA with the JLM14 at a friends once and liked it a lot, but made no comparisons.

living sounds wrote:Thanks Joe! It sounds good, but just a little too sparkly with my U47 clone (which has a higher amplitude and probably a lower impedance than the average condenser).

I'll leave it as it is.

It's very interesting how different the combinations sound. The 990/Lundahl channel comes over as midrange-forward in comparison, more API-ish than the channel with the actual API-type-op-amp and OEP transformer, which has more of a Neve-ish sound (lacking the fatter bottom).

How would you describe the sound with your input transformer? I used a BA with the JLM14 at a friends once and liked it a lot, but made no comparisons.

JLM14 is very flat but due to its small size it adds a great 2nd harmonic to the signal which makes for fat solid low end and silky high end freq. It is great on drums and bass guitar and synths but also make a great alrounder mic pre.