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petrol vendor 15th August 2008 05:48 PM

Preamp with limiter schematic, need some hints
Hi People,
This is my first post, iīm glad that iīm finaly part of this comunity!

This is a preamp design i did, nothing special, it is basically from the ina103 datasheet. Of course pin 10 and 11 must be connected, itīs forgotten in the schematic. Itīll use a twelve position switch to select the gain for the ina in 5db steps and a second gainstage offering 0 to 5 db gain using a pot.
Besides i am going to include a phase switch, 20db pad, an optical limiter and an aux bus for monitoring. Input impedance is 2440k.
All signals will be switched with relais, some logic will take care that 48v will not be applied when the input caps are bypassed (no idea if that is even audiable, but i like to check out if a cap free signal path is somewhat cleaner).
There are two sets of input caps, one resulting in a 65hz rolloff (maybe a little low), the other giving a 3hz lowpass.
The Limiter is quite basic, itīll have minimum attack and selectable release and can be switched into the signal path. The shematic is missing a load resitor going from the output of the ldr buffer to ground. Guess 100k is a good value. Also it is missing some trimmable gain to match the loss from the Resistor/LDR combination.
In my own design the threshold will be a trimpot set to ensure that my converters wonīt clip during peaks.

Now some questions remain:
1. Can i use cheap "break before make" rotary switches? If no Rg is connected the Ina103 gives a gain of 1. Maybe i should just try it, somehow i expect these gain jumps to pop.

2. The second stage has a cap going from the pot to ground, to set the gain for dc to one. How do i select the size of that cap? Is the type of cap critical?

3. In my schematic the Ina has an dc servo. Should the second gain stage get one as well? Or would it be a good idea to feed the servo from the point labled B in the schematic, rather than A? Or even from the output of the circuit?

4. Do i understand that for the servo low offset is the main selection criteria? And shouldnīt the non inverting input see a 100k resitor to ground, matching to the inverting in, to lower the offset?

Thanks in advance, any hints are very much appreciated! Hope i can present a complete shematic with values, opamp suggestions, logic, limiter controll and ledmeter soon.

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