AMZ Super Buffer Headroom Question

[schrectacular]

Hi all,

I just completed the AMZ Super Buffer two-op-amp configuration. I have a couple pedals that buffer at the front of my chain that are usually on (Barber Tone Press > EP Booster clone (J201 version; I see there are a few versions of this circuit out there, a lot use 2N5457)) so I was planning on placing the Super Buffer at the end of the chain.

Perusing AMZ site, I came across a comment (http://www.muzique.com/news/buffers-and-pedal-order/) where "ucnick" notes that one should be careful about placing an op-amp-based buffer at the end of the chain as it can reduce headroom:

Buffers typically run on a 9V supply, so if buffer is opamp based with, say, TL07x family, will end up with roughly /- 2.5V to /- 3.5V swing at buffer output due to op amp output swing limitation (determined by op amp type and load).

Perusing this site I noticed that fellow pedal enthusiast KrazyKarl likes running his super buffer at 12v (https://www.diystompboxes.com/smfforum/index.php?topic=86362.msg723324#msg723324).

I am currently using TL072's in my version of the Super Buffer, but I also have some JRC4558s. I'm basically an electronics noob - I can read diagrams and solder and stuff but what is actually happening and how the circuit works ... I don't understand it well yet. So besides asking if there is a good book I could/should read that covers the electronics of guitar pedals in particular, my questions to the nice internet people on this forum are:

Am I in danger of reduced headroom if I run some loud pedals with my Super Buffer at the end? Am I correct that running it at 12v will increase the headroom? Could I go up to 18v? Will swapping out for JRC4558s change this equation at all?

Thank you for your attention!

[Vivek]

You could use any voltage which the active and passive components can handle. Generally, buffer circuits prefer higher voltages.

Voltage supply can have an effect on the slew rate of an Opamp.

That circuit seems to be built to drive loads down to 100 ohms. Do you need to drive such low loads ?

If you have an Amp with let's say even 100k impedance at the other end of a let's say 5 Mtr guitar cable, I cannot readily see the difference between one Opamp driving that line versus 4 Opamps (except maybe some noise reduction)

Have you seen Merlin Glassblower schematic ?


Here's another way to drive low impedance loads :

https://www.diystompboxes.com/smfforum/index.php?topic=127949.msg1230116#msg1230116

[idy]

Are the other pedals running on more than 9v? Are you running them with volume up all the way? With either or both of those cases you will want to use a charge pump or other power supply (maybe18v?) for your last-in-line booster. I used one between pedal board and hi-fi amp (line level) or between my rig and stereos at gigs.

But yes you have to look at your buffer circuit. What voltage are the electro caps?

Once you have a circuit that runs on 18v you can start looking into whether a "rail to rail" opamp would give you another few volts, but really you will probably just set it up so in your application it doesn't distort when the volume is maxed.(limiting the gain control to work within the safe clean zone.)

[schrectacular]

Thanks for the replies!

Generally, buffer circuits prefer higher voltages ... Voltage supply can have an effect on the slew rate of an Opamp.

That circuit seems to be built to drive loads down to 100 ohms. Do you need to drive such low loads ?
...
Have you seen Merlin Glassblower schematic ?

I seriously doubt I need to drive anything so low as 100 ohms... I was looking to build a buffer and this one seemed fun. I don't really even know what situations it would be useful... if I had a long pedal chain maybe putting it at the front would be a better place? And put my EP booster at the end for a little more oomf if needed?

Also, I built mine with only TWO op-amps in parallel, not four like the schematic shows. Still probably overkill. Thanks for the link and the suggestion of the Merlin Glassblower - it looks really cool.

Are the other pedals running on more than 9v? Are you running them with volume up all the way?  ...
But yes you have to look at your buffer circuit. What voltage are the electro caps?

The 100uf cap is 25v. The resistors are 1/4 watt. Would love to understand if 12/18v is ok for this circuit. I do have a couple of loudish pedals running on 18v that I sometimes run quite high, but not too often.

Cheers!

[antonis]

There are few pedals (mainly EQs) which "breath" more easily when fed with +18V (or +/- 9V)..

Power supply margins set headroom so, for a buffer, it's crucial to know input signal amplitude..
(for an undistorted output signal due to unity gain..)
Of course, guitar output amplitude shouldn't be any issue (unless you build a buffer of 2V or less headroom..)

Same stands for the vast majority of distortion pedals (especially for those who utilize hard clipping at the end..) and even for those with their own output buffer as long as they are fed with 9V.. 

If you decide to feed your buffer with 18V, it should worth a try for +/- 9V (provided you have or can build dual power supply) to save yourself space and items..
(no need neither for bias voltage configuration nor coupling caps..)

[idy]

The quick answer to all of this is, go ahead and try 12 or 18v for your buffer.

If your big electro is 25v you are safe with 18.
I've never seen film resistors that aren't rated for more than enough.

At these voltages we don't need to worry much about resistors: 1/4watt from 18v= .013amps (watts=volts x amps.) That means a resistor from +9 to ground of 1.384k would be close to the limit. A 1/4 watt resistor substantially larger, like 10k or 100k would be very safe. (people use small value series resistors from the power supply as part of an RC filter and as "fuses" that will burn out at over voltage. They are in series with a known load, so they can be small, under 100ohm.

And your opamps: a TL07x is rated ± 20v, or 40v for a single supply. The 4558 is rated ± 18, or 36v for single supply. So nothing to worry about there.

One of my previous answers was silly because I was talking about the gain knob on your buffer... most things called are set for (close to) unity gain. But most any (half way well designed) booster will have a buffering effect, it reduces unwanted interaction between stages.

[schrectacular]

The quick answer to all of this is, go ahead and try 12 or 18v for your buffer.

Done and it works fine. Thanks for your kind replies.

There are few pedals (mainly EQs) which "breath" more easily when fed with +18V (or +/- 9V)..
...
If you decide to feed your buffer with 18V, it should worth a try for +/- 9V (provided you have or can build dual power supply) to save yourself space and items..

I think you just gave me another project... I previously needed a +15V/-15V to build the General Guitar Gadgets TDA2030 amp, but I went ahead and purchased it so as not to zap myself. In retrospect I probably had a laptop power adapter somewhere I could have used but it's done now.