input of stomp boxes

[hpupo]

Hello

I have seen a lot of schematics for regular guitar pedals (mostly distortion, but apparently this applies for most of them), and they all have a Emitter follower as the first stage. I really understand why they do that, but what i dont understand is why not to use a regular opamp buffer instead. Wouldnt simplify the circuit? Is there any issue i am not aware of if one uses a opamp buffer instead?

thanks

[Enzo]

WHy would an IC be simpler than a single transistor?

[hpupo]

Well, for starters, there would be a lot less resistors and caps...But mostly importantly, is there a specific reason why use the transistors? Would the freq response and all other parameters be the same with a opapm?

Thanks

[nigelwright7557]

If there is a spare op amp in the package then you might as well use that.

[Enzo]

As Nigel suggests, if you have an unused op amp in a dual package, go ahead and use it. But if you have used all your op amps, then when you need an inut buffer and the choice is a transistor and three small parts, or adding another IC, I'd say the transistor is cheaper. The boards are stuffed and soldered by machine, so the difference is about parts cost, not labor.

Just looking through Mouser, a common 2N3904 sells for under 2 cents if you buy 2000 of them. 2000 is not a large order for a company cranking out pedals.

Common TL072 op amps will cost you about 20 cents in similar amounts - over 10 times as much.

Little resistors and caps cost under a penny.

SO the specific reason I would expect to be cost.

[jddoylevt]

I completely agree with Enzo. The only other reason I can think of is to save space - stompboxes being rather limited in that regard.

Also - it is important to remember, in comparison to what is seen in almost all circuits on this site, most stompbox circuits mangle the signal pretty significantly on their own, so the added 0.1% - 1% distortion from an emitter follower isn't worth worrying about.

The rules change completely with stompboxes. There is no such thing as an objective analysis of a circuit's sound, it simply HAS to be subjective as the circuit is inserted into the subject's rig and directly influences his/her tone/playing/technique. Basically, with hifi the user of a piece of equipment is 'outside the loop' and can only observe it's response within a known (hopefully) system; but an electric guitarist is most definitely 'inside the loop' as he/she is a component of the entire system and changes to that system change not only the system but the guitarist's response to it (i.e. the way they play).

That, and guitarists tend to act extremely odd when it comes to their tone...I would be willing to wager quite a large sum that 4 out of 5 guitarists would dismiss the opamp buffer as 'too sterile' or 'inorganic' or some other subjectified description and claim the emitter follower 'brings out more of the tone (of their rig)'. I'm fairly certain I would be one of those 4. No, it doesn't make sense. It never will. But hey - you hifi freaks have your $30,000 cables, 'audiophile grade' wooden knobs, directional wire, etc.; us guitarists have the 'mojo' laden JRC4558 chip, 'holy grail' germanium transistors for the Fuzz Face, and our own uber-talented kook in Eric Johnson who maintains a preference for the type of rubber bands used to hold his fuzz face together - once he determined that it sounded SO MUCH BETTER without the screws holding the bottom of the enclosure on.

Way more info than you asked for, but understanding that the rules change (in terms of 'good design', not the actual physics, obviously) with stompboxes can help limit the amount of time wondering 'what the @#$%?'

Regards,

Jay Doyle