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An inter-stage coupling cap re-biases the signal for the following stage. It forms a low-cut filter. It works well for signals possessing little nearly DC content, and removes content at very low frequencies. It needs to pass 20Hz signals (or somewhat higher for guitar).
What happens when the driving stage is clipping one edge of it's input waveform and flipping it over? We see (in our mind in this case) that the DC level of the waveform shifts away from the clipped edge, somewhat proportional to the amount of clipping, and this shifting is provided at a low-frequency rate.
The shift is good for a distortion generator. By shifting the bias, the inter-stage coupling cap's circuit creates duty-cycle asymmetry, providing "nice" even harmonics.
It all works great for constant amplitude audio frequency sine waves and their sums, but consider signal dynamics. If the driving stage clips one edge, and the receiving stage clips the other, you might think that you would get a constant amplitude output when both triodes are clipping. You don't. Once the driving stage starts clipping, increasing it's input level increases the change in DC level of the driving stage's output, shifting the level of the previously clipped edge that the receiving triode sees. This edge passes through the receiving stage unmolested, maintaining around half the dynamics of the original signal. The receiving stage's haircut clipping and following filter similarly reduces but does not eliminate the signal dynamics. You get around 1/4 of the dynamics, not none. And the even harmonics increase, even beyond the point where both stages are clipping.
I've not seen discussion of this. RDH4 and the original designers would simply recommend that you turn down the amp. I recently read a comment in an article quoting John L. Murphy, former Carvin wizard, and he mentioned the even harmonic effect of bias shift, which got me thinking. It raises lots of questions. Does any amp modelling software take this into account? Can and does a Kemper-type amp capture it? Surely a few people here have played with different values of the cap in question, the associated grid leak, etc. What does the effect on pick-attack sound like?
I discovered the concept of prosody when I looked it up in Wikipedia, and I want to increase the prosody of guitar amps - make them better communicators. Heavy clipping kills dynamics as it increases sustain and harmonic content. Making the dynamics live on as changes in harmonic content sounds like a good thing (or maybe it sounds like an auto-wah). It lets how hard you pick control a variable of musical prosody.
It should be possible to remove the shift in signal offset, at least up to a certain frequency, for clean distortion, whatever that is. It could even have a knob.
Of couse, if one string is played hard enough to clip the amp, the effect on the signals from other strings playing softly is dramatic. The weaker signals aren't just modified - They're unrecoverable. It's the sound of rawk, but I think the metal ERG guys are doing it just to annoy me. I'm already annoyed with the way chords and arpeggios sound when I adjust my amp for singing over-driven sustain on lead lines. Polyphonic distortion would help a great deal, but digital modelling the amp chains by brute force needs maybe four filters and three triode emulations time 8 strings, just for the preamp. Add more for the power amp, which shares similar problems. That's 32 filters and 24 look-up tables. I can't see any easy way to combine the filters or triodes due to the non-linearity of the triodes. I don't think a program can keep up in real-time. I can imagine an FPGA running fast enough to process eight channels through one filter structure and one look-up table structure, but it's a big job. Twelve 12AX7s is pretty unappealing, from a reliability standpoint alone. 24-40 JFETs starts to look good.
Understand that I'm not interested in modelling exactly. If you want something that sounds just like a Plexi, you should probably buy or build a Plexi. Models should be judged by how they sound, not what they sound like.
What happens when the driving stage is clipping one edge of it's input waveform and flipping it over? We see (in our mind in this case) that the DC level of the waveform shifts away from the clipped edge, somewhat proportional to the amount of clipping, and this shifting is provided at a low-frequency rate.
The shift is good for a distortion generator. By shifting the bias, the inter-stage coupling cap's circuit creates duty-cycle asymmetry, providing "nice" even harmonics.
It all works great for constant amplitude audio frequency sine waves and their sums, but consider signal dynamics. If the driving stage clips one edge, and the receiving stage clips the other, you might think that you would get a constant amplitude output when both triodes are clipping. You don't. Once the driving stage starts clipping, increasing it's input level increases the change in DC level of the driving stage's output, shifting the level of the previously clipped edge that the receiving triode sees. This edge passes through the receiving stage unmolested, maintaining around half the dynamics of the original signal. The receiving stage's haircut clipping and following filter similarly reduces but does not eliminate the signal dynamics. You get around 1/4 of the dynamics, not none. And the even harmonics increase, even beyond the point where both stages are clipping.
I've not seen discussion of this. RDH4 and the original designers would simply recommend that you turn down the amp. I recently read a comment in an article quoting John L. Murphy, former Carvin wizard, and he mentioned the even harmonic effect of bias shift, which got me thinking. It raises lots of questions. Does any amp modelling software take this into account? Can and does a Kemper-type amp capture it? Surely a few people here have played with different values of the cap in question, the associated grid leak, etc. What does the effect on pick-attack sound like?
I discovered the concept of prosody when I looked it up in Wikipedia, and I want to increase the prosody of guitar amps - make them better communicators. Heavy clipping kills dynamics as it increases sustain and harmonic content. Making the dynamics live on as changes in harmonic content sounds like a good thing (or maybe it sounds like an auto-wah). It lets how hard you pick control a variable of musical prosody.
It should be possible to remove the shift in signal offset, at least up to a certain frequency, for clean distortion, whatever that is. It could even have a knob.
Of couse, if one string is played hard enough to clip the amp, the effect on the signals from other strings playing softly is dramatic. The weaker signals aren't just modified - They're unrecoverable. It's the sound of rawk, but I think the metal ERG guys are doing it just to annoy me. I'm already annoyed with the way chords and arpeggios sound when I adjust my amp for singing over-driven sustain on lead lines. Polyphonic distortion would help a great deal, but digital modelling the amp chains by brute force needs maybe four filters and three triode emulations time 8 strings, just for the preamp. Add more for the power amp, which shares similar problems. That's 32 filters and 24 look-up tables. I can't see any easy way to combine the filters or triodes due to the non-linearity of the triodes. I don't think a program can keep up in real-time. I can imagine an FPGA running fast enough to process eight channels through one filter structure and one look-up table structure, but it's a big job. Twelve 12AX7s is pretty unappealing, from a reliability standpoint alone. 24-40 JFETs starts to look good.
Understand that I'm not interested in modelling exactly. If you want something that sounds just like a Plexi, you should probably buy or build a Plexi. Models should be judged by how they sound, not what they sound like.
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