n0npr0phet
New member
Hi All,
I am building a 6 channel preamp using THAT1512 and THAT1646. I didn't invent anything and I was mostly datasheet diving. It sounds and measures good but when nothing is connected it is obviously noisy (johnson?) when the gain is cranked up. I know.. I know.. turn down the gain when nothing is connected. :roll: but... I used the Nuetrik Combo XLRs that have switches that open when something is plugged in so I rigged up the following to mute the output of the THAT1512 when nothing is inserted into either jack.
The mute I think I found a link for in this forum to this edn soft mute article. I modified the suggested circuit as I don't need indicators etc.
So finally here are my questions. Is this the best place to do this (pin 6 of the 1512)? Should I provide a mute bypass for critical recording in case the JFET might be introducing distortion into the signal path? Using the mute I have used a series resistor that makes a voltage divider when the fet gate is not open. In this configuration with a 1K resistor the mute is -40db but it mutes more (-50db) with a 5K. Dare I use a higher value -60db would be nice? I was worried about using a 5K resistor here but I didn't see or hear any problems. What series resistor would you use?
Has anyone else solved this problem another way? I thought about somehow opening the circuit at pins 4 and 8 which would take the gain to -6db but I was concerned about introducing noise or other problems by messing with the gain loop. Obviously relays could be used in this loop or on the output but they are expensive and perhaps inelegant. My PCBs are made and I will have to lift traces to make the mute but my stepped gain selectors are not all made yet so suggestions on the latter approach if possible would also be great.
Thanks, This is my first post.
np.
I am building a 6 channel preamp using THAT1512 and THAT1646. I didn't invent anything and I was mostly datasheet diving. It sounds and measures good but when nothing is connected it is obviously noisy (johnson?) when the gain is cranked up. I know.. I know.. turn down the gain when nothing is connected. :roll: but... I used the Nuetrik Combo XLRs that have switches that open when something is plugged in so I rigged up the following to mute the output of the THAT1512 when nothing is inserted into either jack.
The mute I think I found a link for in this forum to this edn soft mute article. I modified the suggested circuit as I don't need indicators etc.
So finally here are my questions. Is this the best place to do this (pin 6 of the 1512)? Should I provide a mute bypass for critical recording in case the JFET might be introducing distortion into the signal path? Using the mute I have used a series resistor that makes a voltage divider when the fet gate is not open. In this configuration with a 1K resistor the mute is -40db but it mutes more (-50db) with a 5K. Dare I use a higher value -60db would be nice? I was worried about using a 5K resistor here but I didn't see or hear any problems. What series resistor would you use?
Has anyone else solved this problem another way? I thought about somehow opening the circuit at pins 4 and 8 which would take the gain to -6db but I was concerned about introducing noise or other problems by messing with the gain loop. Obviously relays could be used in this loop or on the output but they are expensive and perhaps inelegant. My PCBs are made and I will have to lift traces to make the mute but my stepped gain selectors are not all made yet so suggestions on the latter approach if possible would also be great.
Thanks, This is my first post.
np.