Inductor biased transistors in stompboxes?

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Inductor biased transistors in stompboxes?

Postby JakeAC5253 » 23 Dec 2016, 04:22

Is there any practical use for gain stages to be biased using inductors in stompboxes? I have never seen this configuration in a stompbox schematic, but I still remember the eye opening moment in college Engineering when we made an audio amplifier. After having disappointing results building an audio amplifier using a resistor to bias a common emitter gain stage, the professor passed around transformers and we used the primary in place of the collector resistor, and my eyes lit up when we heard the difference for ourselves. The result this time was much louder, cleaner, and much more musical. There has to be a practical use for this. Or maybe in order to get a good result, you need to use a part that is too large to be practical in a stompbox chassis? I mean as far as I can remember (this is close to 10 years ago) we used the primary of a 120v to 12v stepdown transformer, so the latter is understandable I guess. Likely would see no improvement on a stage that was limited by clipping diodes, but maybe a clean stage. Thoughts?
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Re: Inductor biased transistors in stompboxes?

Postby poiureza » 23 Dec 2016, 09:40

Mmmmh kinda hard to understand.

Inductive reactance is zero for DC ...
And inductors have next to zero DC resistance so I don't see how they could be used for DC biasing.
That big 120V transformer coil had probably less than 1K DC resistance ...
Do you remember the biasing scheme ? Was it base bias, voltage divider or feedback bias ?

However putting an inductor in the feedback loop could trigger interesting things when the transistor is fed with an AC source.
I have to think about that :D
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Re: Inductor biased transistors in stompboxes?

Postby JakeAC5253 » 23 Dec 2016, 14:02

poiureza wrote:Mmmmh kinda hard to understand.

Inductive reactance is zero for DC ...
And inductors have next to zero DC resistance so I don't see how they could be used for DC biasing.
That big 120V transformer coil had probably less than 1K DC resistance ...
Do you remember the biasing scheme ? Was it base bias, voltage divider or feedback bias ?

However putting an inductor in the feedback loop could trigger interesting things when the transistor is fed with an AC source.
I have to think about that :D


Yeah, exactly. Because transistors are current operated devices, not voltage operated like a tube, so the inductor would increase the clean headroom, which is exactly what happened in class. The fact that there is little to no voltage drop across the inductor combined with the increased availability of current at the collector is what causes this. Aww man, I don't remember how it was biased. I want to say it was maybe voltage divider, but it was too long ago. I don't recall the circuit specifics too well.
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Re: Inductor biased transistors in stompboxes?

Postby Dirk_Hendrik » 27 Dec 2016, 21:41

For an inductor used in an audio circuit means that what the inductor "does" is dependent on the frequency of the input signals. That in turn means I, and I think everyone else reading this, cannot say much about the matter unbtill it's clearer what exactly you were working on. As well, are you sure you're not in some way mixing things up with transformer coupled amplifier stages? That would make more sense. Especially since "louder" is not really a very good measure for amplifier quality comparison ;) (Cleaner is, "musical" is a very tricky one)
Sorry. Plain out of planes.

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Re: Inductor biased transistors in stompboxes?

Postby JakeAC5253 » 27 Dec 2016, 22:44

Dirk_Hendrik wrote:For an inductor used in an audio circuit means that what the inductor "does" is dependent on the frequency of the input signals. That in turn means I, and I think everyone else reading this, cannot say much about the matter unbtill it's clearer what exactly you were working on. As well, are you sure you're not in some way mixing things up with transformer coupled amplifier stages? That would make more sense. Especially since "louder" is not really a very good measure for amplifier quality comparison ;) (Cleaner is, "musical" is a very tricky one)


The circuit was an audio amplifier consisting of a single transistor stage to drive a single 12" driver. It was crude and far from ideal, but we made due with the parts we had to solve the problem. "Engineering" as it were. We plugged our iPods into the input of the amplifier, and plugged the output to the driver. No, not transformer coupled, we used the transformer primary instead of a collector resistor. Yeah cleaner doesn't necessarily mean better in audio, but in this case what I mean is with the collector biased using the resistor, the sound out of the driver was obviously lo-fi to the ear as well as noisy, and it was obvious that we were trying to squeeze wine from a stone. As it should be expected to sound with such a simple circuit trying to perform this task. With the inductor biasing the collector however, the signal was louder, less noisy, and seemed like it had a wider and flatter frequency response as well. It was obvious that we were able to squeeze a lot more juice out of this single stage circuit than we could before the inductor.

I know very well that this is not a gain stage that would be found in your average stompbox, but I do think maybe something could be gained from the concept. I remember tweaking the general idea of TS and Big Muff circuits to hell and back again, but I was very limited by voltage headroom for what I wanted to accomplish. With clippers that have forward voltages much higher than stock value, the signal would simply clip at the voltage rails of the gain stage leading to a terrible sound. If we build the gain stages out of transistors and feed the collector with an inductor, this should free up a ton of clean headroom to get a much wider voltage swing on the output I think. So the hope is that we can get away with using clippers with higher headroom and get a way more dynamic output tone.

Just something I'm thinking about. I'm probably wrong as usual :wink:
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