My first DIY preamp (THAT 1512); questions? - diyAudio
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Old 6th December 2008, 12:48 PM   #1
diyAudio Member
Join Date: Dec 2008
Default My first DIY preamp (THAT 1512); questions?

So I decided to DIY a preamp based on the circuit in the THAT 1512 datasheet. Its so simple what could go wrong

First problem was getting +-15v (and +48v), I solved this with a DC-DC converter and some regulators. I then added all the other components to my breadboard and it works! woot!

Question is, both the THAT 1512 and the INA163 datasheets (very similar circuits) have a dc servo (which Im using) but neither datasheet shows the servo opamp's +-v inputs connected, just the +- & output signal ones. Should I connect up the +-v ones to my +-15v?

Second question, obviously I can use a potentiometer to adjust the gain the preamp creates. Im wondering about the possibility of using a digital potentiometer between RG1 and RG2 to get .5db gain steps. The problems as I see it are as follows:
* you need log taper with at least 128 steps at ~5kohm - nobody makes them
* it needs to handle the max voltage between RG1, RG2 (not sure but less than 10v)
* it needs to go all the way down to 5ohms, so wiper resistance may be an issue

My thinking (and please correct me if im wrong) is that I can solve the first problem by using a linear taper digital pot and connecting a taper resister to make it pseudo log. I can then use software to correct any other discrepancy. This way I can get 5kohms with 128 steps cheap

Max voltage may be a problem as most digi pots only do +5v. But there are some +15v ones so I should be ok, just might cost a bit more.

Lastly (and perhaps most annoyingly) the wiper resistance is normally about 50ohms, this is far too great to allow me to get anywhere near the 5ohms needed for 60db gain (bum).

Obviously you can get digital volume controls like the PGA2310 but as far as I can tell, they are intended to be used on the output rather than for controlling the gain in the preamp.

anyone have any ideas? How bad an idea would it be to always run the preamp at 60db and then attenuate down with a PGA2310?

Any help gratefully received.

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