Is it safe to run line level synthesizer into a Teensy 4.0 & Audio Board?

Status
Not open for further replies.
P

pangolin

Member
I'm making a pedal for someone to use with a synth, but the circuit is designed for guitar. As I understand it, impedance matching is the only major issue when running synths into guitar pedals, which can normally be overcome by simply attenuating the synth output... but is there any chance that a synth could damage a Teensy?

This is the analog circuitry for the input section, I have PCBs for this design but i can change the components. Would it be worth using lower values for R2 and R3 for a more synth-friendly input impedance? Any suggestions for values?

Teensy_input.png

Thanks!
 
hi - I might have missed the point here but you should just be able to plug the synth jack straight into the audio board line in i.e. bypass the circuit you have above and just connect the jack input to the audio board line in.

cheers Paul
 
The only protection on the audio adapter audio inputs is the DC-blocking capacitors, so I'd recommend
having at around 1k of series resistance too to limit current to be on the safe side, especially if the
adapter might be powered down sometimes with audio in.
 
The input buffer is necessary as the audio signal splits after it, with the analog "clean" signal being mixed back in with the Teensy output later on (sorry i simplified the schematic).

Are there no simple mods to adjust this input buffer to better handle synths? I'm not too fussed about clipping, the synth can be turned down, and I plan to increase C1 for more bass. So impedance is my only worry i think.

From your suggestions I gather that a synth signal won't damage the Teensy at least?
 
Ok, see why you want the buffer.
The circuit you have is biasing the op amp at 4.5v which will be no use for the audio board - I’d remove r3 to remove that bias and it should be fine.

The other option is to just mix the ‘dry’ signal back in digitally (as part of the effect), it has advantages and dis-advantages depending on what you’re doing with the effect

Cheers Paul
 
houtson said:
@markT. The codec has 29k input impedance on the line in, would that not do ?
Click to expand...

Not very confident of that, its a CMOS chip so any internal voltages outside the rails will push current
through the anti-static diodes and risk CMOS latchup if those currents become large. This is why the
datasheet shows an absolute maximum input voltage range of GND-0.3V and Vdda+0.3V for the analog inputs.
 
Is it a good idea to leave the opamp unbiased like that? Won't I get weird assymetrical clipping from a floating/non-existing DC bias? People are generally keen on an analog dry signal so I'd like to keep it if poss.

Mark, when you say "1k of series resistance" would that be R4 or R5?

Thanks
 
pangolin said:
Is it a good idea to leave the opamp unbiased like that? Won't I get weird assymetrical clipping from a floating/non-existing DC bias? People are generally keen on an analog dry signal so I'd like to keep it if poss.
Click to expand...
Opamps always need DC bias, they are DC coupled.
Mark, when you say "1k of series resistance" would that be R4 or R5?

Thanks
Click to expand...
Neither, I mean some series resistance to the pin on the adaptor to protect from high currents (ac and transients),
whether or not an opamp is involved. 1k is a compromise value, too large and it will start to add noise, too small
and it won't protect very well, but its not very critical.
 
So the schematic as it is, with R4 at 10k, should give ample protection to the audio board from line level signals?
 
Status
Not open for further replies.
Back
Top