Lunetta into guitar amp not working - why?

[ianbax]

Hi all,

This isn't so much of a problem just me trying to increase my knowledge. As I've been building I've had my lunetta plugged first into a set of old computer speakers then my studio monitors via a Behringer mini mixer (an old 'Eurorack' 8 channel thing).

Everything has been fine so this isn't a big deal. But randomly the other night I needed to test an old guitar amp (another story, I had to take the back off to reattach the control panel) and as I was at my workdesk I quickly reached for the lunetta output for an audio signal just to see if the amp was still working.

When plugged into the amp there is a faint signal at first then it slowly dies and then - weirdly - bits of the lunetta (a flashing led-ldr 'vactrol' gate for example) start to fail - the LEDs dimming. Nothing damaged, all is good if I reboot the lunetta and plug back into my mixer/monitor setup.

The lunetta output is going through a summing mixer built with 4049 (I know, I know, should've used an op amp). Roughly based on this article https://hackaday.com/2015/07/02/logic-noise-ping-pong-stereo-mixers-and-more/ and http://www.all-electric.com/schematic/simp_mix.htm - substituting the 4049 for the op amps.

I thought it might be just this old amp but I checked with another and the same thing.

It's like current flowing backward into other circuits or something? My output is about 1v peak to peak so way too hot for a guitar amp input? Impedance? Something to do with shared grounds or not shared grounds? The output cap on the mixer blocking? Unbuffered inputs on the guitar amp?

Any clues?

[PHOBoS]

I am no expert on guitar amps but I believe they have a very high input impedance.
If you have a capacitor in series with the output that might be causing the issue.
You could try adding a resistor from the output to GND (10K~47K) and see if that helps.

[ianbax]

Ah, yes, thanks PHOBOS - there is a big cap in series (100uf so bass is still passed) on the output to block DC (as per Elliot Williams circuit - https://hackaday.com/2015/07/02/logic-noise-ping-pong-stereo-mixers-and-more/)

That could be it. What's going on electrically? The capacitor is filling up and can't discharge so it blocks the signal and then ends up pushing current backward?

I'll have an experiment with the resistor.

[PHOBoS]

I don't know about pushing the current backwards or why your lunetta is starting to act weird but yes.
The capacitor gets charged, which increases its resistance, which blocks the signal flow.
Maybe it's something different in your case but what you described sound very much like that's the issue.

[Sven]

The 100µF cap that you use is calculated for 32 ohm headphones to pass everything above 50 Hz. Its explained on the link you postet. Your guitar amp should have an input impedance around 1 megaohm. Thats why it takes ages for the cap to charge and discharge. Instead you can use a 100nF cap and have a lower frequency of 15 Hz. With a 10 kiloohm input impedance a 1µF cap the lowest frequency will be 15 Hz.

[ianbax]

Thanks for the replies both - starting to make sense now!