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LM301As and compensation capacitors
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Ricko



Joined: Dec 25, 2007
Posts: 251
Location: Sydney, Australia
Audio files: 27

PostPosted: Thu Jan 17, 2008 4:09 am    Post subject: LM301As and compensation capacitors Reply with quote  Mark this post and the followings unread

I've been pimping my in-progress ETI 4600. Fun!

It uses the old LM301s: a workable patch puts the signal through a minimum of 8 of these, which really result in a dulled sound: goodbye >10K. So I've replaced the LM301s in the output and mixer stages with TL-71s (cut out the compensation cap). Really great improvement in sound (though I think I may replace the final output with something with better drive.)

Anyway, along the way I looked at datasheets for LM301s. It seems that many people who used them just followed the instructions in the datasheet page 1 and used a 30pF compensation capacitor. But this reduces the bandwidth of the op-amp, down into audio levels. The ETI 4600 uses the 30pF values and so do many other old synths and designs.

But looking at the datasheet further, it gives the formula for getting the optimal compensation capacitor, and it is gain dependent. For an inverting amplified, C >= (R1 * 30pF)/(R1 + R2) which means that the compensation capacitor gets smaller with more gain, with 30pF being the maximum compensation that is ever needed (unless you want to overcompensate.)

So for unity gain, that means that a compensation capacitor of about 15pF can be used. This should give a faster slew rate, and maybe shift the frequency response curve up an 8ve, as far as I can see. I guess I'll try it and see...

So people who have vintage 301-using synths and like (?) the 301's distortion characteristics but would like better performance, it might be worth their while to calculate the optimal compensation cap and see if it improves the sound.
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