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the 4558 is readily available, why do you want to sub it ?
Dave
Thanks I'll give it a try.....see if radio shack have 1458 its the same chip different manufacturer
DAve
Starting at the op-amp, this has a gain of 23. The output is rectified and smoothed to give a positive feedback voltage.
The feed back voltage is fed to the second fet which is turned on when the gate is less negative. This shorts the signal to ground. So, the larger the signal, the more the attenuation. giving a more constant output.
The first fet is there to adjust the effectiveness of fet2
They're not the same. The main difference is the GBP (gain-bandwidth product) but this varies significantly between manufacturers. A clear example is STMicroelectronics, who manufacture both types. Their 4558 has a GBP three times higher than their 1458.see if radio shack have 1458 its the same chip different manufacturer