opamp labs 425

[rafafredd]

recently I bought an opamp labs 425.

Someone said that this could be used anywhere a API 2520 or a 990 is used. Do you think itīs right?

Anyway. I know it can give me 60db of gain. Pretty good for a mic preamp. I just wanna make it into a mic preamp. With input transformers and transformerless outputs, if possible.

Here are the page for specs:

http://www.opamplabs.com/425.htm

[edanderson]

hey, i've got a few of these 425s as well, and was going to make something similar. scott dorsey said he thought that the built in transformer on the 360 module was a 1:5 triad, so i was thinking of maybe using the 1:6.45 oep for the input tranny. there's also an inexpensive edcor 8:1 that i was thinking about using in reverse, but i know it isn't shielded, and it may not have a faraday shield between the windings either. they're cheap enough i'll probably try both eventually.

the circuit on the opamplabs page is where i will start, but with the additional input transformer in there.

ed

[rafafredd]

Are you saying that you are going to start with the circuit example of the 425 or of the 360 module. As I understand, the 425 circuit example is a microphone capsule amplifier (mic body?) and not a mic preamp. So do you think a API312 circuit with an OEP 1:6.45 on the input would be fine? Or would you change some values on the 312 circuit?

If you have lots of this opamp, you could build a mic/line mixer! think about it...

[edanderson]

i'm not sure if you could use a 425 anywhere you could use a 2520 or a 990. the idea is the same, but i think the 425 is a lot simpler inside than either of the other two. but it might work. you could try the api 312 circuit as a place to start, but i wouldn't be suprised if it needed some changes. i'm going to start with the 425 mic amp circuit shown on the opamplabs website, which i think is for a mic preamp, not an in the mic amp.

my understanding is that there is a discrete opamp in both the 425 and 360 modules. the difference is that in the 360, there is a mic input transformer with a 1:5 ratio feeding in to the opamp. the 425 does not have this transformer, but it does have an output pair after the opamp, which helps it drive line outputs better.

so my thought is to add an input transformer to the 425, to make it sortof like a 360, but with better output drive capability.

if the original transformer in a 360 is a 1:5, then something like the oep 1:6.45 might be a good place to start. it might work even better with a higher ratio transformer- i can't find enough info to know without experimenting.

the original transformer in the api 312 circuit is a 1:8 transformer. when i converted my api 325 cards over to mic preamps i used a 1:10 lundahl, the ll1578xl. they work great. i've also seen recommendations for both the 1:8 jt-110k and the 1:10 jt-115k by Jensen.

ed