What design element causes compression

[rutledj]

I'm curious as to what in the non-hrm design gives the amp a natural compression. I notice this much more with single coils than humbuckers.

Is it the 220k plate reisitors or maybe the feedback ckt?

Thanks,
Rut

[ayan]

I'm curious as to what in the non-hrm design gives the amp a natural compression. I notice this much more with single coils than humbuckers.

Is it the 220k plate reisitors or maybe the feedback ckt?

Thanks,
Rut

The V1B feedback certainly helps. The amp's midrangey character is a part of it too, and the smooth overdrive enhances the compression IMHO. The HRMs would have it as well, pluck out the HRM board and the compression will be there, but inserting the extra tone controls puts a notch in the frequency response and the texture changes big time. Of course, a properly tweaked HRM can sound smooth and compressed too, but the non HRM gives it up without any fuss.

Gil

[llemtt]

The V1B feedback certainly helps.

Sorry Gil but I don't think so, there's nothing in a negative-feedback-loop that enhances compression, but maybe I am just missing an easy explanation for such a behaviour..

However IMO the V1B loop add something to the "perception" of a more "compressed sound", but it's just a different harmonic content.

Compression in single-ended cathode-bypassed tube stages is mainly due to their transfer function that "rounds" the edges, but even more is caused by the inherent "bias shift" that can move the grid to a more negative dynamic "bias point" where the stage has less gain.

One can design stages to have more or less of this effect.

teo

[greiswig]

The V1B feedback certainly helps.

Sorry Gil but I don't think so, there's nothing in a negative-feedback-loop that enhances compression, but maybe I am just missing an easy explanation for such a behaviour..

However IMO the V1B loop add something to the "perception" of a more "compressed sound", but it's just a different harmonic content.

Compression in single-ended cathode-bypassed tube stages is mainly due to their transfer function that "rounds" the edges, but even more is caused by the inherent "bias shift" that can move the grid to a more negative dynamic "bias point" where the stage has less gain.

One can design stages to have more or less of this effect.

teo

I questioned this, too, but tried a test on my amp. I have the LNFB switchable between several different resistances, ranging from 44Mohms to 88Mohms. In my short and admittedly informal test, the same note on the same guitar seemed to sustain noticeably longer (by seconds) when the LNFB was at 44M compared to 88M. I had to turn the volume down at the higher resistance to compensate for loudness difference. Could a difference in harmonic content have caused that?

[odourboy]

The V1B feedback certainly helps.

Sorry Gil but I don't think so, there's nothing in a negative-feedback-loop that enhances compression, but maybe I am just missing an easy explanation for such a behaviour..

However IMO the V1B loop add something to the "perception" of a more "compressed sound", but it's just a different harmonic content.

Compression in single-ended cathode-bypassed tube stages is mainly due to their transfer function that "rounds" the edges, but even more is caused by the inherent "bias shift" that can move the grid to a more negative dynamic "bias point" where the stage has less gain.

One can design stages to have more or less of this effect.

teo

Hmmmm... Teo, how about this? Suppose the gain stage in question (V1B) is operating in it's 'compression' zone, before clipping, but at a point where the transfer frunction is not very linear - where we're flattening the tops of the wave. Wouldn't negative feedback of that signal, which is trying to 'correct' this non-linearity by increasing the signal of the compresed portion, cause that stage to maintain continue to compress and sustain the signal? I may be full of shite... just thinking out loud here.

[jelle]

Ok, who has a scope? This way we can test V1b with and without LNFB

[ayan]

The V1B feedback certainly helps.

Sorry Gil but I don't think so, there's nothing in a negative-feedback-loop that enhances compression, but maybe I am just missing an easy explanation for such a behaviour..

teo

No need to be sorry, Teo. I think you can hear this farily easily with a simple test, but maybe it's just my ears. The notion here being that I think we're using the terms compression associated with sustain. You can totally see how the V1B feedback loop reduces bass and treble, thus pushing the midrange and focusing the energy in those frequencies, enhancing the perception of sustain. I have been selling a little passive box for over a decade that nukes some bass off and on of the reasons people buy them is because they feel in enhances the guitar's sustain.

Gil

[larsmuller]

Tune your plate voltages....the lower, the more compression!

[llemtt]

Hmmmm... Teo, how about this? Suppose the gain stage in question (V1B) is operating in it's 'compression' zone, before clipping, but at a point where the transfer frunction is not very linear - where we're flattening the tops of the wave. Wouldn't negative feedback of that signal, which is trying to 'correct' this non-linearity by increasing the signal of the compresed portion, cause that stage to maintain continue to compress and sustain the signal? I may be full of shite... just thinking out loud here.

Interesting idea but no, negative feedback works at the waveform level instantly to maintain the out identical to the input, it doesn't work on the 'envelope' level, the 'envelope' of the signal stays unchanged. Think of those 70's hifi amps with tons of negative feedback (where the input stage would run into hard-hard-clipping without the global feedback loop), do they maintain the same out signal level regardless of the input level?

Gil is right

The notion here being that I think we're using the terms compression associated with sustain

Focussing the harmonic spectrum around the fundamental frequence let's your brain perceive a longer 'sustain', like the one out of a compressor, because that way the first part of the signal envelope is more similar to the tail. (the unprocessed first part of the envelope contains higher harmonics while the tail is mainly the fundamental).

But I don't tell you yet why negative feedback "in that stage" enhances the fundamental (while apparently it should linearize also the freq response), the "not so easy" answer can be found by hifi triode amps builders and I think it deserve a full article to describe. Maybe some day I will write it if Kevin O'Connor hasn't already in his new TUT6.

teo