Wondering if anyone might be able to help me with some HF roll-off in a vari-mu build. Over the past 3-4 years I’ve been slowly building and modding a dual Sta-level style build. It’s at the point where several things are quite different from the original, including the metering and output section (post transformer), so I wouldn’t exactly call it a Sta-level, although the audio path is the same as the original. I’m using the Lundahl 1922 on the input and Hammond 1650G on the output. I’ve attached schematics of the output section design.
Basically, it’s an active output with a 600-ohm resistor strapped across the secondaries. There’s a switchable 10dB H-Pad via a relay. It switches between either the 600ohm load resistor or the h-pad. This is then unbalanced via a THAT1246, fed into an OPA1656 with a rotary in the feedback for 0.5dB steps. Then it’s balanced again via THAT1646 and then XLR out. My goal was to turn it into a much more subtly controllable compressor, with .5db steps on ins/outs.
There are a few reasons why I am using this output design, but the main one was an attempt to fix a frequency roll-off issue. Previously I was doing it passively with a 600ohm bridged T on a 2-pole rotary switch. I noticed when I was at unity, the response would flatten out more (but still a little HF roll-off), and at more attenuation via the switch, it would roll off more from about 8kHz. I thought maybe having a constant 600ohm load with an active attenuator might be another way to do it.
With it all hooked up, on the scope I am seeing a completely flat response at the secondaries, after the load resistor, and at the input of the THAT1246. After the THAT1246 I am seeing the roll-off. The amount of roll-off does not change at different levels of attenuation, so I am happy about that, but I can’t figure out why it’s there at all.
In the THAT1246 datasheet, I can see each input of the opamp has a 12k resistor, which I thought should set the impedance high enough to work.
Feeling a little out of my depths at this point. Any help would be amazing!
Basically, it’s an active output with a 600-ohm resistor strapped across the secondaries. There’s a switchable 10dB H-Pad via a relay. It switches between either the 600ohm load resistor or the h-pad. This is then unbalanced via a THAT1246, fed into an OPA1656 with a rotary in the feedback for 0.5dB steps. Then it’s balanced again via THAT1646 and then XLR out. My goal was to turn it into a much more subtly controllable compressor, with .5db steps on ins/outs.
There are a few reasons why I am using this output design, but the main one was an attempt to fix a frequency roll-off issue. Previously I was doing it passively with a 600ohm bridged T on a 2-pole rotary switch. I noticed when I was at unity, the response would flatten out more (but still a little HF roll-off), and at more attenuation via the switch, it would roll off more from about 8kHz. I thought maybe having a constant 600ohm load with an active attenuator might be another way to do it.
With it all hooked up, on the scope I am seeing a completely flat response at the secondaries, after the load resistor, and at the input of the THAT1246. After the THAT1246 I am seeing the roll-off. The amount of roll-off does not change at different levels of attenuation, so I am happy about that, but I can’t figure out why it’s there at all.
In the THAT1246 datasheet, I can see each input of the opamp has a 12k resistor, which I thought should set the impedance high enough to work.
Feeling a little out of my depths at this point. Any help would be amazing!
