I'm in the middle of designing a USB audio interface. However, one thing that has me confused is how a lot of mixing consoles/interfaces tend to use a single gain control for both XLR/TRS inputs for a single channel, with no explicit switch for line/mic in. I'm assuming each different input is buffered (to isolate their different input impedances/phantom power) and then summed before the gain control, but I haven't found any schematics to support my thinking.
Does anyone have any schematics of audio interfaces/mixers that use this arrangement or any suggestions?
Does anyone have any schematics of audio interfaces/mixers that use this arrangement or any suggestions?
Well maybe an audio interface/mixer is always line level and a preamp such as a Rane ML1 specifies "Each channel has a Mic / Line input switch". I just can't see an input working with both line level and mic level inputs without some gain changes.
That's why you have a gain control, as well as a level control 😀 It's trivial to have a gain pot that covers a HUGE range.
Often also the input uses different sockets for Mike (XLR) and Line (Jack), with the line input having an extra attenuator.
Another option is to have a switched attenuator on the input as well.
I am discussing this typical device: RANE ML1 preamp since we have Microphone inputs being used.
1. Switched gain level control for Mic or Line level input.
2. Nothing special about a XLR. It can accommodate both Mic and Line.
In fact this device also has "Euroblock" connectors for I/O.
3. Level control is used on the subsequent device that this preamp drives.
4. Best to have the fixed Gain control cover a Limited USEFUL range.
5. No Level control as such in sight.
Show me what device and information/ diagrams you are talking about ?
1. Switched gain level control for Mic or Line level input.
2. Nothing special about a XLR. It can accommodate both Mic and Line.
In fact this device also has "Euroblock" connectors for I/O.
3. Level control is used on the subsequent device that this preamp drives.
4. Best to have the fixed Gain control cover a Limited USEFUL range.
5. No Level control as such in sight.
Show me what device and information/ diagrams you are talking about ?
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I was thinking more along the lines of devices such as the Focusrite 2i2 and associated audio interfaces. They tend to have pretty poorly designed line inputs though. Looking at the Rane ML1 schematic, it just switches between Mic/Line impedances and pads the line input with the line/mic switch. I don't really have the front panel space for a selector switch, so I'm trying to keep the XLR as the mic input and the TRS as line in (hence each one is connected to the same gain device at all times). I'm looking for the optimal way to do this without a bajillion op-amps, or at least to cross reference my design with something proven.
Also, I noticed in the ML1 schematic, they convert from differential to single ended, when the ADC they're using has differential inputs. Doesn't this degrade the dynamic range, since it can't reach full scale input?
Also, I noticed in the ML1 schematic, they convert from differential to single ended, when the ADC they're using has differential inputs. Doesn't this degrade the dynamic range, since it can't reach full scale input?
That's how it's done sometimes.
The TRS connector sometimes switches the gain of the circuit.
Yet another method is to have about 20-25dB of gain on the mic input, with the rest of the gain common to both circuits and the attenuator only on the final output. So, two gain stages.
I think I might go with something like this. I included the input buffers for the line input because I wanted to have switchable input impedances (40k differential line and 1Meg+ for instrument input). It has a permanent -20dB pad on the line input as well. Just a rough idea, sans all the protection/gain circuitry.
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