THAT1512 / Transformer 'blind test' mic preamp

[dmp]

Another thing I don't understand is how the circuit is unbalanced in the SPICE simulation when the pot is set exactly half-way. My understanding was that it's there to adjust for tolerances in the resistors, but in the simulation, the resistors are all exact values.

I feel like I'm missing something really fundamental here.

The top opamp is inverting while the bottom opamp is non-inverting. The gain is different, Rf/Rin vs 1+Rf/Rin

 

[Newmarket]

The top opamp is inverting while the bottom opamp is non-inverting. The gain is different, Rf/Rin vs 1+Rf/Rin

The cross coupling (see R32, R33) takes care of that.

Below is as implemented in DDA AMR24

 

[amplexus]

I’ve used both the DRV and THAT chips with no output coupling caps with no issues. Just make sure you have the caps to the SENSE lines and you should be golden.

 

[dmp]

The cross coupling (see R32, R33) takes care of that.

Then why do the top and bottom have different pk-pk magnitudes?

 

[Newmarket]

Then why do the top and bottom have different pk-pk magnitudes?

idk. I might look to measure differentially with a floating "True RMS" DMM for confidence in the measured result.

 

[dmp]

It's in the spice simulation results

 

[Wavelength]

I have used both the DRV and the That parts. I would say hands down go That for the output. I direct coupled both but had issues with the DRV in protos so I talked the company I was designing for and the final design went with the That. I use the 1626 in a couple of 500 series modules I designed. The parts are great, another real nice sounding part is the That 1200 series for line inputs, stellar sounding. Used a set on an Audio Preamp board I am designing now.

 

[soulsonic]

I realise that I've made a mistake on the IC board and connected the input RF filter to signal ground. I can remove those capacitors from the board and wire it to chassis ground, directly on the input XLR instead. Is that correct?

Thanks,
James

Yes, I believe so. I made this same mistake on mine...

 

[jamesk]

The cross coupling (see R32, R33) takes care of that.

Below is as implemented in DDA AMR24

This is interesting. It's exactly the same circuit as in Self's book. I've just been having a look at the schematics for the desk and it's full of circuits from that book. I know he worked at Soundcraft but did he work at DDA when they made that, I wonder.

What's very interesting is that it says to calibrate the balance at 1kHz, which implies that the balance does vary with frequency, as I'm observing.

I have used both the DRV and the That parts. I would say hands down go That for the output. I direct coupled both but had issues with the DRV in protos so I talked the company I was designing for and the final design went with the That. I use the 1626 in a couple of 500 series modules I designed. The parts are great, another real nice sounding part is the That 1200 series for line inputs, stellar sounding. Used a set on an Audio Preamp board I am designing now.

Hmm, that's annoying to hear that about the DRV as it's much cheaper than the THAT. Maybe I'll end up going with the quasi-balanced circuit.

Yes, I believe so. I made this same mistake on mine...

Thanks, I'll do that!

I just want to check the overall grounding arrangement. I have the power supply in a separate steel enclosure. This will be connected to the preamp enclosure by a 5-pin XLR lead: +/-17V, +48V, signal ground and chassis ground. At the preamp end, signal and chassis ground will be connected to the two separate wires. These will then be joined at the power supply end to a star ground point on the enclosure, where the 0V from the power supply board also connects. Does this arrangement sound sensible or should I change something? I have also got separate ground connections on the PCBs for signal ground and the power ground from the decoupling electrolytics on the power input. Should this power ground be connected to the signal or chassis ground?

Thanks,
James