"SS Poweramp + Cab" Output setting and Class-D amps

GreatGreen

GreatGreen

Power User
In the Fractal Audio Blocks Guide, the section on setting the Output Mode to either "FRFR" or "SS Pwr Amp + Cab" has the following note:

NOTE: The SS PA +Cab Mode is NOT intended for use with “current drive” power amps, i.e. tube power amps,
Class-D current feedback amps, etc. This mode CAN be used, however, with FRFR monitors in high volume
applications where the monitor’s speakers are compressing, thereby achieving a more dynamic response.


Why are Class-D amps mentioned here? Are Class-D amps reactive to the load they see in some way? Do they not output a flat signal to all sources like a solid state Class-AB amp does?
 
GreatGreen said:
In the Fractal Audio Blocks Guide, the section on setting the Output Mode to either "FRFR" or "SS Pwr Amp + Cab" has the following note:

NOTE: The SS PA +Cab Mode is NOT intended for use with “current drive” power amps, i.e. tube power amps,
Class-D current feedback amps, etc. This mode CAN be used, however, with FRFR monitors in high volume
applications where the monitor’s speakers are compressing, thereby achieving a more dynamic response.


Why are Class-D amps mentioned here? Are Class-D amps reactive to the load they see in some way? Do they not output a flat signal to all sources like a solid state Class-AB amp does?
Click to expand...

Interesting question. I think the real crux of the matter resides in the fact that the topology is mentioned here i.e. current feedback. Without doing too much reading on the subject, I think that a Class D amplifier with a current feedback topology would attempt to emulate a tube-like characteristic, hence it not being desirable when used with the algorithms to model the same characteristic.

Would love to hear someone's take on it who is more knowledgeable on the subject!
 
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Yeah I would be interested in getting some higher resolution on this also. Using a Class D power amp like say one of the Duncan PowerStage amps into a classic guitar cab is really not a FRFR setup by any common definition. Yet it seems like we should use the FRFR output mode on the amp block in this case? It's really not clear from the Blocks Guide.
 
I used to run an FRFR 212 cab and now that u mention it, I'm not sure I ever selected the amp+cab setting in my AxeFx after I switched to a Matrix and a couple orange 212 cabs in stereo!
What do these settings actually do inside the AxeFx with respect to setup type?? What's really going on there??
 
I use this mode depending on the response I want overall. If it’s a cleaner tone where I want dynamics, I’ll use the SS+cab mode. If it’s a gain patch where I want smoothness, then FRFR. All my patches end in a dual path with one output to a PS170+cab and another output with a cab block going to FOH/pa speaker. The difference in settings is relatively small imo.
 
Ok so from what I've read, compared to FRFR mode, "SS Amp + Cab" mode simply removes the speaker compression emulation so your real cab can do the natural speaker compression thing.

Still doesn't clarify why Class D amps are not recommended for use with this mode though.
 
Hi guys.

As the new speaker compression algorithm of Cygnus is being rolled out, I got to doing some research to see if I managed to get a better grip on the whole sopeaker compression modeling and the various parameters around it do. I was under the impression that this parameter will not really be relevant for me, as I run a seymour duncan powerstage 170 (class D) on a Mesa Mini Recto straight (1x12) and always set the output mode to SS PA +Cab Mode. However, as it turns out, I seem to have been getting it the wrong way all this time, as I now realise that this mode is NOT intended for class D amps (which the SD Power stage is an example of).

Well, what does it mean? Is it that the FRFR mode would then be better suited for my setup? I was under the impression that when the SS+ cab mode was selected, then speaker compression was not modeled to allow the actual compression to come from the interaction between the SS amp and the real speaker. Now, seeing this, I am all confused.

1 basically, what output mode would you recommend Me?
2 will my setup benefit from the new Cygnus compression algorythm improvement?

thanks!
 
There has been a lot of talk about this lately. In particular the reference to Current-mode Class D amps at the end of this post by Cliff:
I understand why they can’t really suggest an amp by name, but may be some options?....
I for one would gladly try out whatever Fractal suggests.

FractalAudio said:
Just finished coding it. Sounds much better than the old algorithm. I put in a switch in the debug build to switch between the new and old algorithms and it's noticeably better.

What I realized with this epiphany is that tube amps are better for HiFi as well. I'm not a HiFi person but now I know why HiFi people prefer tube amps. They probably don't understand why it sounds better, they just know it sounds better.

This leads to a potential improvement for solid-state HiFi amplification: Current-mode Class-D. If you could make a Class-D amplifier that works in current mode instead of voltage mode you would get the same benefits as a tube amp for HiFi applications.
Click to expand...
 
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