I thinking of something like the Boss Katana that uses a TDA7293 in the output stage and has a 0.5, 50, and 100 w power setting selector switch.
I could imagine that the gain is adjusted to switch from 50 to 100 W, but what about the 0.5w mode?
The TDA7293 has a minimum gain of 26 dB and +- 12v minimum supply voltage, therefore to get a guaranteed low power output I would of thought that you would need some kind of voltage or current limiting circuitry on the output, or is it using some other technique?
http://www.st.com/content/ccc/resource/technical/document/datasheet/4f/18/a6/c8/21/33/41/8d/CD00001887.pdf/files/CD00001887.pdf/jcr:content/translations/en.CD00001887.pdf
I could imagine that the gain is adjusted to switch from 50 to 100 W, but what about the 0.5w mode?
The TDA7293 has a minimum gain of 26 dB and +- 12v minimum supply voltage, therefore to get a guaranteed low power output I would of thought that you would need some kind of voltage or current limiting circuitry on the output, or is it using some other technique?
http://www.st.com/content/ccc/resource/technical/document/datasheet/4f/18/a6/c8/21/33/41/8d/CD00001887.pdf/files/CD00001887.pdf/jcr:content/translations/en.CD00001887.pdf
I'm speculating as I don't know this particular amplifier but could it just be a 3-position switched attenuator after the preamp and volume potentiometer? So the amp gain and supply rails remain unchanged, but the three positions of the attenuator correspond to:
1. No attenuation, a standard guitar input signal gets amplified to 100W RMS when the volume knob is at maximum.
2. -6dB attenuation, a standard guitar input signal gets amplified to 50W RMS when the volume knob is at maximum.
3. -36dB, a standard guitar input signal gets amplified to 0.5W RMS when the volume knob is at maximum.
If there's a zener limiter or soft clipping function in the preamp section before the attenuator and volume control, that would limit the final output power.
(Edit: I looked the amp up and altered my reply slightly based on it being a guitar amp)
1. No attenuation, a standard guitar input signal gets amplified to 100W RMS when the volume knob is at maximum.
2. -6dB attenuation, a standard guitar input signal gets amplified to 50W RMS when the volume knob is at maximum.
3. -36dB, a standard guitar input signal gets amplified to 0.5W RMS when the volume knob is at maximum.
If there's a zener limiter or soft clipping function in the preamp section before the attenuator and volume control, that would limit the final output power.
(Edit: I looked the amp up and altered my reply slightly based on it being a guitar amp)
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Does that imply that the power limiting is likely to be "approximately 0.5w" rather than an absolute "no more than 0.5w"?
Power and Gain are very different things!!
Get a hot Corvette. WOW! Now put a tennis-ball under the go-pedal. You just have to press harder, it still goes to the same WOW.
Reduce the gain of a 100W amp, strum harder or add a boost, it is still a 100W amp.
Classically you reduce supply voltage to reduce maximum power. London Amplifiers has gizmos for tube amps. The same could be done on tranny amps but rarely done. However this new amplifier may use a switching supply, which is typically regulated, which could be set to different values. Such a design is above my pay-grade.
It is fascinating that the "50W" power consumption is 47W; the "100W" consumes 77W. Either it has a magic power source, or (likely) the consumption is rated for "crazy guitar", not sustained Sine test-tone.
A lo-tech 100/50/0.5 rig could be a 4 Ohm and an 80 Ohm resistor switched in series with the speaker.
For a class D yes, however the TDA7293 used in the Boss Katana is a old school class AB.
Really? That is so 1990's.
Engineering and marking reasons.
Class AB is easier to get through EMI compliance testing and thanks to 15 years of aggressive marketing (to protect the worldwide cartel and status quo) musicians (and professional reviewers) have been indoctrinated that digital = bad.
Most pedal sized amps use class D amps. They have to because of the size and heat dissipation constraints. Reviewers don't complain because the marketing material for these products specifically omits this information because of the digital stigma.
For something like a 100w combo amp a large fraction of the cost goes into shipping 14kg (30lb) of cabinet and speaker. Moving to class D isn't going to reduce that much and dissipating 50w of heat isn't that much of a problem for a class AB amp when you have a physically large cabinet.
If you want to see the double standards at work checkout the "that pedal show" youtube channel. For those folks pedals containing DSP are fine, amps that contain DSPs are automatically excluded from their thoughts.
I love the high efficiency, high power output, compact size, light weight, and low cost of class D audio power amps.Really? That is so 1990's.
I don't love the way they sound when they clip.
So any guitar amp using class D power will need some sort of soft limiter before the input of the class D power amp, to keep it from ever clipping.
-Gnobuddy
I'll trade the added complexity of a soft clipper for the Class D benefits. Hey, how about a Class D tube amp?I love the high efficiency, high power output, compact size, light weight, and low cost of class D audio power amps.
I don't love the way they sound when they clip.
So any guitar amp using class D power will need some sort of soft limiter before the input of the class D power amp, to keep it from ever clipping.
-Gnobuddy
Got a link to the episode(s) that you're referring to?
There is DSP, and there is DSP. The clean channel of my Super Champ XD uses DSP for the voicing (EQ) and effects (reverb, etc). The voicing sounds exactly the same as if it was done with resistors and caps, and the effects are good enough to be usable. A well done FIR or IIR filter sounds no different than the same filter implemented in the analogue domain.
But the same Super Champ XD has a "dirt" channel. Here, the DSP attempts to generate tube-like nonlinear distortion (rather than linear EQ curves.) And it doesn't sound great - usable in a pinch, at best.
I think most non-techy guitar players don't know the difference between using DSP to generate an FIR filter, and using DSP to attempt to emulate a non-linear transfer function that intentionally generates harmonic distortion. All they know is that they've heard a lot of supposedly "tube like" DSP pedals and amp modellers that actually sound more like a kazoo than a tube amp.
-Gnobuddy
Amazingly enough, I've seen links to at least one DIY class D tube amp on the 'Net. It was done as a novelty, so the designer could say he had done something that nobody else had.
Unfortunately, I can't find it again now - it's been years since I stumbled across it.
I don't remember any details at all, but I would guess it was an OTL (output transformerless) design. I don't think you can make output transformers to cope with both the audio and switching frequency requirements of a class D tube amp!
-Gnobuddy
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