Combatting volume drop when using a switch/mixer

[aaa]

I have a power amp (Topping PA3s) that I've been using with a bluetooth streamer (Moode on RPi with DAC Pro hat). There's a second sound source that I want to use with the amp, too. I purchased a Little Bear passive mixer so that I can switch between the two sources: channel 1 is Moode/RPi, channel 2 is the aux audio from my projector.

The problem is that the projector's volume is much lower than the RPi/Moode - so much lower than I have trouble hearing it! With the RPi/Moode, I rarely turn the amp's volume past 50% to get to a comfortable listening level. With the projector, the listening level is always too low, even with the amp's volume maxed out at 100%.

I used my oscilloscope to measure the Vp-p of the projector: it's 1V directly out of the device, which drops to ~700mA when passing through the passive mixer. For comparison, the RPi's Vp-p appears to be roughly 3.3V. (For testing the projector I used a pure sine wave signal. For the RPi I was less scientific and used music as the test signal.)

I want to boost the level of the projector's signal. I'm considering either A) swapping out the passive mixer for a different mixer or switch to reduce the voltage drop, or, B) acquire a preamp to insert before the projector's signal reaches the mixer.

I have some questions. Do all (entry level/affordable) passive mixers/switches cause the volume to drop? If not, please recommend other mixers or switches that cause a less significant drop in volume. Alternatively, I'm wondering if an active mixer would be a better solution. However I'm not sure if an active mixer is likely to introduce more noise compared to a preamp.

Thanks for any help/advice!

 

[DonH56]

If the projector's output is 1 Vpp and RPi's output is 3.3 Vpp then the RPi is simply going to be louder unless you amplify the projector's signal or attenuate the RPi's signal. The mixer may reduce the signal, but if the projector's output is already lower, then the only way to match them is to reduce the RPI's output level (in the RPi or the mixer). A switch should not affect the levels at all, and in this case while the mixer can reduce the signal level, the difference is from the two different sources and not the mixer/switch reducing the signal level. The disparity in levels happens before the mixer or switch, and a passive device cannot increase the signal, so if you want the projector's signal to be larger you will have to place an active gain circuit after it. That can either be a preamp into the mixer, or an active mixer that allows you to set the gain higher for the projector.

 

[aaa]

Thanks @DonH56. The disparity in volume level of the two sources isn't really important to me. What matters, and what I'm trying to remedy, is the fact that the projector is very quiet, and I want it to be louder than I'm able to achieve with my current setup.

It's surprising to me that the PA3s amplifier isn't capable of boosting the projector signal sufficiently. It's so quiet that I have trouble hearing and understanding soft dialogue when I'm watching films, even with the amp's volume cranked up all the way to the max.

 

[solderdude]

You either need to lower the volume of the loudest one by 10dB and turn up the volume of the gear following the switch so that the projector sound has the correct volume or use an amplifier to boost the lower level one (10dB so that's making it twice as loud).

You can do that (add gain) my adding a headphone amp that has a gain of at least 3x (10dB) for instance or using a small mixer for instance.
These don't need to cost much either.

 

[MaxwellsEq]

It's surprising to me that the PA3s amplifier isn't capable of boosting the projector signal sufficiently

It's not an issue to with the amplifier. The output of the projector is too low.

This is all about "gain structure". As digital became common, higher line-level voltages became common. This meant that amplifiers have steadily reduced their gain. A classic 1970s power amplifier might have a gain of over 30 and be designed for 1V input. It would be ideal for your projector. A modern power amplifier has a gain in the mid 20s and is prepared for 2 to 4V input. It's ideal for your RPI.

You either buy an old style amplifier or one with high gain and attenuate the RPI; OR you stick with your current power amplifier and add some gain to your projector with a preamplifier or a headphone amplifier etc.

 

[DonH56]

Thanks @DonH56. The disparity in volume level of the two sources isn't really important to me. What matters, and what I'm trying to remedy, is the fact that the projector is very quiet, and I want it to be louder than I'm able to achieve with my current setup.

It's surprising to me that the PA3s amplifier isn't capable of boosting the projector signal sufficiently. It's so quiet that I have trouble hearing and understanding soft dialogue when I'm watching films, even with the amp's volume cranked up all the way to the max.

What @solderdude and @MaxwellsEq said. The amplifier was designed to target a particular market and range of components; your projector's audio output is lower than their target group, so is a mismatch of application. Most projectors are connected through HDMI these days, with audio stripped by an AVR or processor, so chances are analog output from the projector was an afterthought. Not really anybody's fault nor a problem with the amplifier. You need to add a gain stage after the projector to boost its output. There are a bunch of them on Amazon for about $25 to $100 to as much as you want to spend. I haven't used any of them so cannot suggest one; you might want to edit the title in your first post to ask what people have used and like.