class-a buffers and boosters

[Marco Pancaldi]

Hi all,

I'm testing the Radial ToneBone (loop controller, very smart and well-built device). In the manual the builder emphatize the class-a buffer (guitar input) and boosters, used for best tone, with usual discrete vs. opamp story ecc.

1 - Using a sinewave, I noted very appreciable second, third and fourth harmonics from the buffer. This is simply due to the discrete buffer, or is it a choice? In other words: is it possible to have discrete buffer WITHOUT so evident harmonic distortion?

2 - The switchable booster is variable from 0dB to about +16dB. The harmonic content is not varied appreciably between 0 and +6dB; after that the distortion increase gradually to a slighty fuzzy tone (at "full" the analyzer sees a lot of full range harmonics).
I suppose this is not the usual booster with fixed gain and a pot to control volume: have you an idea about how is it implemented.?

Thanks all
m.p.

[brett]

In other words: is it possible to have discrete buffer WITHOUT so evident harmonic distortion?

Yes, absolutely. 
As far as I'm aware, there's no reason why you can't get 12 to 14 dB of clean boost from a simple, discrete circuit.
cheers

[cd]

How hot of a signal were you running into the buffer?  Guitar level (<100mV) or higher?

[R.G.]

I'm testing the Radial ToneBone (loop controller, very smart and well-built device). In the manual the builder emphatize the class-a buffer (guitar input) and boosters, used for best tone, with usual discrete vs. opamp story ecc.

You have to be very careful when reading "technical" things in advertising (and user's manuals are at least partly advertising.) The blather about opamps ruining tone is especially blatently overused.

1 - Using a sinewave, I noted very appreciable second, third and fourth harmonics from the buffer. This is simply due to the discrete buffer, or is it a choice? In other words: is it possible to have discrete buffer WITHOUT so evident harmonic distortion?

It is entirely possible to have a discrete buffer without evident harmonic distortion. It may have also been a choice the builder made, as most guitarists really do prefer a bit of distortion in almost everything.

2 - The switchable booster is variable from 0dB to about +16dB. The harmonic content is not varied appreciably between 0 and +6dB; after that the distortion increase gradually to a slighty fuzzy tone (at "full" the analyzer sees a lot of full range harmonics). I suppose this is not the usual booster with fixed gain and a pot to control volume: have you an idea about how is it implemented.?

If that control truly varies the gain, that would entirely account for it. The fundamental issue is probably that the circuit is running out of power supply  room. If it's powered from a low voltage like 9V, then there's a limited amount of gain you can use before you limit at the power supply. This is such a common idea in EE that I'd never even thought it was remarkable enough to say. Craig Anderton was the first person that I read that wrote it down ( I think it was EPFM, over a decade ago). Others have replowed the field since.

Here's the rub. It's possible to get 12 to 14 db of clean gain from a simple, discrete circuit. It's much easier to get it from an opamp, and much cleaner. By the time the discrete circuit is anywhere near as clean as the opamp, it's no longer simple.

[Marco Pancaldi]

Hi all,

The power supply for this pedal is +15Vdc.

I have tested the input-output with a (about) -10dB 250Hz sine signal. I know very well this level is higher than the usual -20dB reference "guitar" or "instrument" level, but lot of guitar today have powerful PU. Simply, my Ibanez has similar loudness when direct injected (buffered with a bypassed BOSS) in the mixer with the same input gain. So, this is my usual reference to analyze a device without playing.

R.G.:  I know some of the single-fet (or transistor) buffers circuits. These have fixed gain and then the output is controlled with a pot, so I think that in this case the distortion is depending from the power rail and input level, but not from the output volume. If you have distortion, it is there BEFORE the pot.

I think this pedal works in a different way: it generates always some harmonics in bypass; activating the booster with "no gain" you have the same quality signal, then increasing gain you have more and more harmonics (no fuzzyness, only a appreciable increase in "complexity"). I think this is not a "fixed gain" with a pot, but a real "gain variation"... can you point me to some (simple if possible) circuit acting in similar way?

NOTE: maybe I'm completely wrong, and the gain (and harmonic/distortion behaviour) is fixed, with the pot is at the INPUT of the booster... But the hiss from the booster (even maxed to +16dB) is very little, and none at 0dB gain.

Many thanks
m.p.

[bioroids]

Hi!

Check out Jack Orman's Mosfet Booster http://www.muzique.com/schem/mosfet.htm

It works the way you say (the pot controls gain, not volume after some fixed gain).

I dont know about the one you have, but is posible that it works this same way.

Luck

Miguel