I need serious help with microphone audio levels.

Thanks again Dave!

Would 1N4007s do? Maplin don't seem to have the 1N4008.

1N4007S 1A Silicon Rectifier | maplin

I've heard people talk about using Zener diodes but surely you just need ordinary types?

Yup! 'Sevens' are bigger and better! Not sure why they use zeners? Most IC inputs can stand the supply voltage but beware the TL071/2 series, they can be a bit feeble. That said. "our" HT pedal range used nothing else and although I replaced switches, pots and the odd valve in them, I never recall having a TL072 pop!

Dave.
 
Dave - would it work if I just put a diode from each input, just after the blocking capacitor on your original circuit to earth? Or is it much better to stick to the pair of diodes to the supply rails?
 
Dave - would it work if I just put a diode from each input, just after the blocking capacitor on your original circuit to earth? Or is it much better to stick to the pair of diodes to the supply rails?

Well you could but you are then limiting the input voltage to 0.6V pk-pk, 0.4V rms (about -7dBV) which is a pretty poor headroom for a mic pre! Also, the diodes will start to conduct slightly before 0.6V so distortion will start that much earlier.
You could use back to back 6.8V zeners giving a overload point of +13dBV (+15dBu) with a bit of earlier onset distortion but if you CAN use the supply biased off diodes, best and cheapest solution.

Dave.
 
Thanks again Dave. I will start now building the supply diode design. By the way, can I remove the 22k resistors to earth after this or should they stay?
 
Have just fitted the diodes to the supply rails and seems to be working well! Have put the LM4562 back in after initial trials with NE5532s. It is fabulous to have a tiny battery operated pre for condensers. I don't need the That 1510 in this amp as the signal from the condensers is so great. But I'm finding the S/N using the 1510 in my other amp is fantastic for dynamics. I think the EIN is 134dBu for this chip, so even with the 54dB or so gain I need with a Shure Unidyne III the S/N is around 80dB - this is much better than I get with the Tascam DP008 using condensers!
 
Thanks again Dave. I will start now building the supply diode design. By the way, can I remove the 22k resistors to earth after this or should they stay?

Yes, I don't remember exactly what they do, probably bias. Needed.

Dave.
 
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