Buffered Output Comments

[SlashLP97]

I am building my first pedal right now (Hot Harmonics) and I was thinking about adding a buffered output stage to it.  Would there be any benefit in doing this?  I was just going to run the output of the last CD4049 stage into a voltage follower configured RC4558 (maybe with a cap in the feedback, just to smooth the sound a little more and reduce some of the fuzz) and then into the output cap to the volume control pot.  Any comments on this idea?

Also, I was thinking about adding a tone control most likely a cap and pot in series from the output of the final gain stage to ground.  Any comments on this?

[petemoore]

  Impedance matching...
  whats the impedance of the HH output...and what's the load it's driving [input impedance of the next circuit in the chain.
  Note: long cables can load signals too.

[SlashLP97]

I am guessing that the output impedance of the HH is ~50k because that is what volume pot is called for.  I could use a 1M tone control pot (in parallel it wouldn't change the overall impedance from 50k very much, I could add a compensation resistor if needed) and then buffer that ouput into the op-amp, therefore creating a cable driver.  In line with the tone pot would be a capacitor (most likely .1uF as that is what I see in most tone controls), how would I calculate the effect of this element on the impedance? I know the impedance of a cap is 1/(jwc) but you have a wide range of frequencies from a guitar signal, so what do you use?  Or do you ignore the capacitance?

[SlashLP97]

I built the Hot Harmonics exactly per the schematic at http://www.guitar-pedals-effects.com/HotHarmonics.html (Current Schematic) and I thought it sounded very choked, after doing some research I found that the schematic has the output of the CD4049 buffer, stage A, driving a 50k volume pot.  Because the output impedance of the cd4049 is very high pretty much the bigger the load the better) I decided to add an output buffer, and the pedal sounds amazing now.  So I guess that answers my question about buffered outputs.  This would also solve the problem of adding a tone control (which I don't think it needs now, it is very bright and if I want more bass I can just roll the tone knob on my guitar back). 

[earthtonesaudio]

Yep.  These inverter-based circuits have high output Z.  Buffers allow them to do their thing without getting loaded down by whatever circuit follows.

For best results, put the buffer at the very end of everything, after tone/volume controls, but before the switch/output jack.

[SlashLP97]

If you put it before the tone/volume control then it wouldn't be doing it's job, the inverter would have a "small" load to drive, wouldn't it?  If I'm not mistaken you need to run the output of the inverter into the buffer, then into tone/volume controls.  Is this incorrect?

[earthtonesaudio]

You have the right idea about giving the inverter a "light load" so it retains it's output, which is an important part of a buffer. 

But there's another aspect to buffers that is useful and often overlooked, the low output impedance.  Good for driving long cables with no treble loss.

I don't know exactly what tone control you're using, but take for example the 50k volume pot at the end of the HH.  At any setting less than max volume, there is resistance added in series with the output.  This makes for unpredictable results with different things plugged into the output (different lengths of cable even) at different volume settings.
A buffer at the output (after the volume/tone controls) keeps the output impedance constant (and low) no matter where the controls are set.  This makes the performance much more predictable and consistent in the face of changing conditions at the effect's output.

This is a good article that explains it better than me:
http://www.muzique.com/news/boosters-are-not-buffers/

[Ben N]

IIRC, someone once mentioned here that you can parallel up inverters to make an output buffer, which would be handy as you have 4 to spare. Anybody know about this?

[SlashLP97]

Right now I don't have any load on the inverter (output through a 4.7uF cap into the opamp) and it sounds great.  I think the volume pot would load the inverter and decrease the gain of the last stage.  I don't have any tone control on it right now, but I have the 50k volume pot after the buffer.  I think any load << (I don't know how common that symbol is, but << means "much much less than") infinity would start decreasing the gain of the buffer giving more of a "fuzz" sound, but I was going for a more smooth, creamy distortion with a lot of added oomph in the positive harmonics, which is exactly what it gives now.  My next idea is to combine the volume control and the buffer, making an amp with a gain of ~0 to ~1, depending on the feedback resistor setting, which would be the volume pot.  Also, I want to try active tone control filters I think that could help increase the frequency response of a tone control because you wouldn't have to worry about the loading of a regular passive tone control.