Increase Pre-Amp Zone Level

Hardware
1

Hi,

I’m using one of the pre-amp zone outputs to feed a QSC two channel amp i used for two outdoor subwoofers. The QSC performs the low pass filtering (no EQ on Amplifi yet) and uses the L to feed one channel/sub on the amp, and the R to feed the other.

The issue i’m running into is the pre-amp output level is too low for the QSC and i have to effectively max out the output of the amp which leave a lot of subwoofer amplification from the sub left on the table. Is there a way to boost the output level for a certain pre-amp zone?

Thanks!

1 Like
3

Wait - there’s no EQ on Amplipi? Anyone know if this is on the horizon in a future software update? This is a pretty big omission.

5

No EQ on AmpliPi?

AmpliPi’s hardware does not support per zone EQ settings, however it can support EQ settings for each digital stream.

One user has added per source EQ settings by changing the ALSA configuration, check it out here: Alsa Equalizer HOWTO · micro-nova/AmpliPi · Discussion #308 · GitHub

6

Along with what @linknum23 mentioned, I’ll add the following (note this is a hardware mod and may be a bit advanced for some):

We currently have all our line level signals in AmpliPi to be basically consumer line-level. I say basically since the level changes based on the amount of attenuation you add via the volume controller. As far as how to boost the output level for any pre-amp zone - YES, you can totally do this.

Have a look at the preout board schematic here: https://github.com/micro-nova/AmpliPi/blob/main/hw/sch/preout_board/preout_board_1a.pdf

Each pre amp output has a buffer on it (U1-U6). The buffer is as NE5532 op-amp configured as a voltage follower. Basically, as configured, it adds no extra gain to the signal, it just passes it through. If you change the two resistors on each buffer, you can adjust the gain. Here is a page that can help you determine what resistors to select to change the gain:

https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/non-inverting-amplifier.php

So for example, if you want to double the output the preamp outputs for zone 6, you could replace resistors R1, R2, R3 and R4 with 10k resistors. You want 0805 SMT resistors, these would probably work fine: https://www.mouser.com/ProductDetail/YAGEO/RC0805FR-0710KL?qs=7bY8808IzHq6vrU3BeiiYw%3D%3D

If your looking in the box, the circuit is where I have drawn a red circle:

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image1582×1124 290 KB

7

Just an FYI, we haven’t tried this and will likely lead to clipping. The negative supply for the NE5532 op-amps in AmpliPi is GND, so the op-amps only pass signals down to ~1.4V cleanly. The audio signals in AmpliPi are biased to 2.5V, so the max pk-pk voltage is 2*(2.5-1.4) = 2.2 Vpk-pk = -2.2 dBV. We’ve scaled all the internal AmpliPi signals to this level, and that is what is passed out of the pre-out. For reference standard consumer audio line-level is -10 dBV = 0.894 Vpk-pk.

There is a chance the adding a gain to the pre-out board’s NE5532 will be okay and it’s only the inputs that are limiting to 1.4V on the low side, but I’d suspect there will also be problems trying to drive the output lower as well.