CD4049UBE Headphone Amp

[blackcorvo]

So, ever since I learned that these inverters we use for some of our pedals can deliver 5 to 25mA output, I wondered if it'd be possible to make a headphone amp out of them.

And early today, after not being able to sleep, I decided to see it for myself.



After tinkering a bit with the circuit, I ended up with the following schematic:



The trimpot for adjusting the bias for the output stage is actually an idea from Tim Escobedo's Circuit Snippets, specifically from the Calavera circuit.

I think it could do with a filter at the output to kinda "simulate" a speaker cabinet, but I'm terrible with filter stages. Wouldn't even know where to start tbh.


Sound sample:

https://youtu.be/8VQJ2Yj2IJ8?si=pFdULSaG7aHjAICO

[saturated]

Awesome 

Watched the video and liked 

Love the tone at 00:52 

Nice and crunchy 

[blackcorvo]

v2



I figured I could use a 10k trimpot with a high value resistor from the taper to do the bias adjustment.
I also figured that I have like a half dozen of those 4M7 trimpots with the red knob, and that I'd like to use them on something, and they worked when I put them on the circuit, so I'm gonna use them in the final build.

Also V+ resistor reduced from 220R to 68R for even better driving output capabilities.

[J M Fahey]

Loved it, great idea 

[Tassieviking]

Love it, its going into my to do list, I might even make a tiny PCB up for it.
I might even make one in a stompbox and use it as an effect pedal.

[blackcorvo]

Love it, its going into my to do list, I might even make a tiny PCB up for it.
I might even make one in a stompbox and use it as an effect pedal.

This made me think of trying something else: using the chip only wired as the output stage, as a "clean" headphone amp to use with pedals. Like a really beefed up buffer that can drive earbuds. Hmm.

[Loudthud]

The output stage doesn't use any feedback to lower the gain. Check the old National Semi application note in post 4 of this thread:

https://music-electronics-forum.com/forum/amplification/solid-state-hybrid/922490-what-does-this-ic-do-in-the-circuit

[blackcorvo]

Just in case the link dies someday, I'm adding the file you mentioned as an attachment.

Now one thing I wanna figure out is having a better sound on headphones. On an amp (or a higher impedance speaker, as I've shown) it sounds great, but on earbuds it can sound pretty shrill.

I'd like to try adding a small filter to shape the sound on headphones, but I'm not sure where to start.

[Loudthud]

Operating the CMOS without feedback may emphasize higher frequencies where the impedance of the ear buds is higher. Try setting the gain of the output stage to something between 1 and 10.

[blackcorvo]

Operating the CMOS without feedback may emphasize higher frequencies where the impedance of the ear buds is higher. Try setting the gain of the output stage to something between 1 and 10.

That's a great suggestion, but I'm liking how the output stage is sounding as-is, and since there's volume to spare on headphones and earphones, I've decided to take an idea from the earliest Runoffgroove designs of a "cab sim" at the output of their many JFET amp emulator circuits, using a pair of RC low-pass filters. There's an alternate output (amp) bypassing the RC filters for use as a pedal.

I've also drawn a layout that I plan on fitting on a Mentos CleanBreath enclosure (the thin plastic ones).

[blackcorvo]

Finished build:



Used a tiny 12v bulb instead of a resistor (because it looks cool when the brightness changes with harder picking).

Tried a 10M resistor in parallel with the last stage (suggestion from someone on DIYStompboxes), barely noticed any changes tbh. Leaving it there cus I'm lazy.

Sample:

[blackcorvo]

And, finally, the finished build in a Barkley's tin:



Unfortunately I fried my only 4049, so I botched an adapter to a 4069 to use it for now (thanks Runoffgroove!)

[saturated]

Man that's crazy awesome 

I was gonna comment on your soldering skills which look hella awesome 

For real looks like point to point wiring on an old tube amp.   

But then seeing that last post I'm floored 

Looks like you clearly have the dexterity skillz to accompany the acumen 

I'm way beyond impressed 

[Loudthud]

What are the little circuit boards attached to the lid ?

[blackcorvo]

What are the little circuit boards attached to the lid ?

Battery and charging board + MT3608 voltage booster set at 15v.