Split
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I've been testing out a DMG ProSound set up that has a 3.5mm line-out connected to pre-pot and the built-in headphone jack connected to post-pot. Specifically the ASM v3 board.

I was curious how both lines would interract (as well as the speaker) so I tried different combinations of things and found something interesting: when a set of ear buds (or speakers) is connected to the line-out and another set of ear buds is connected to the internal jack the potentiometer (or perhaps more appropriately the post-pot headphones) affects the line-out headphones volume.

I recorded four different instances to a computer (with a line-in, not mic-in) using the line-out with just the internal speaker at max and min volume then another set with headphones plugged into the post-pot internal jack:

Line-out + Speaker Max: Full signal, good recording. Internal speaker audible, has hum
Line-out + Speaker Min: Full signal, good recording. Internal speaker muted, has hum
Line-out + Headphones Max: Low signal, poor recording.
Line-out + Headphones Min: Full signal, good recording.

My thinking is opening the potentiometer to the headphones is treating the original audio source like water, relieving the line-out full "pressure" and allowing the headphones to draw on it when opening the pot.

From what I understand both the pre and post sound mods bypass the internal amp while the speaker is still tied to an amp. I would think this explains why the speaker doesn't take away from the line-out signal since it takes the direct audio -> amp -> speaker. This also leads me to assume if the internal headphone jack is left stock and tied to the amp it would not affect the line-out signal, like the speaker.

Is this close to the correct thought process or is there more at work here? Or is this off completely?

I've seen it recommended (after experimenting) not to use headphones simultaneously with the line-out. Is this because it can be damaging to internals or peripherals or is it because of the signal loss to the line-out depending on heaphone volume?

Thanks for any insight!

nitro2k01
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Sweeeeeeden

The long and short of it is that you're tapping into the circuit between the audio output from the CPU chip and the amplifier, so anything you plug into can affect the signal. The internal speaker on the other hand doesn't affect the prosound output.

Split wrote:

Line-out + Speaker Max: Full signal, good recording. Internal speaker audible, has hum
Line-out + Speaker Min: Full signal, good recording. Internal speaker muted, has hum

This is the easiest case to explain. Nothing is affecting the circuit, so the recording is good. You hear hum in the speaker because the speaker always has hum that comes from the power supply line. The amp can be completely turned off when headphones are connected, and in this case the hum goes away because the amp is not driving the speaker.

Split wrote:

Line-out + Headphones Max: Low signal, poor recording.

In this case, the headphones are connected directly to the top (signal) part of the pot. This is the same as if you would have a pre-pot prosound, and then a passive splitter connected to both line-in recording and headphones. The headphones are loading down the line, and the signal becomes distorted. In addition, the headphones work as microphones in this case, so if you repeat the experiment and tap the headphones, this should come through in the recording. Another way to look at this is that the pot, which is maybe 100 kohm and the headphones which is maybe 8 ohms, are connected in parallel, both going to ground. The headphones are much lower impedance and effectively bypasses the pot.

Split wrote:

Line-out + Headphones Min: Full signal, good recording.

In this case, the headphones are connected directly to the bottom part of the pot (ground). The headphones are effectively separated from the circuit and make no difference to the audio quality.

Split
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Thanks for taking the time to write this out, it's a bit more clear now.

nitro2k01 wrote:

This is the same as if you would have a pre-pot prosound, and then a passive splitter connected to both line-in recording and headphones. The headphones are loading down the line, and the signal becomes distorted.

Plugging in a passive splitter with two headhones to the pre-pot was going to be my next test so good to know I was on the right track!

nitro2k01 wrote:

In addition, the headphones work as microphones in this case, so if you repeat the experiment and tap the headphones, this should come through in the recording. Another way to look at this is that the pot, which is maybe 100 kohm and the headphones which is maybe 8 ohms, are connected in parallel, both going to ground. The headphones are much lower impedance and effectively bypasses the pot.

This is super interesting, I did not know it would behave this way. If what I'm understanding is correct, before I play around with this any more, the sound signal direct from the CPU loses volume when powering headphones and introducing another set reduces the volume further. Since there's only so much signal pre-amp the more headphones introduced the lower the signal across all of them. Is the only downside to this a reduction in volume due to the signal loss or can anything get damaged?