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Class d amps

#22
ambient space said:
Hi. I am new here and have a question about buying one the these small light weight class d bass amps (400 to 600 watts, head or combo). What do you guys think about the sound quality and long term reliability of these amps as compared to the old boat anchors. I have always used the old heavy heads. Now in my 50's it's getting hard to move them. Any help would be appreciated.
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In 1980 I purchased my first Walter Woods MI-100-8, with an SMPS power supply, Analog Power Amp, and pre amp. That amplifier is still in service. I have two Walter Woods amps a MI-400-8 and an Walter Woods Ultra, The MI-400-8 (20 years old) has never
been serviced as of yet, (although I'll probably do that fairly soon) The Ultra had to be repaired because a few drops of water spilled into the top vents, and onto the circuit board. So, the cause of the repair was user error. (mine when I accidentally switched it on not
knowing that the moisture had not evaporated) My analog MI-400-8 sounds very tube like, the Ultra is more like a hybrid tube pre amp with a digital power amp.These amps are very pricey, so I wouldn't try to tell you that you needed one. All the newer digital amps
are pretty amazing, especially the Tecamp Puma 500, 900, and 1,000.

Tecamp Puma.png
walterwoods_mi400_00011.jpg




Ric
 
#24
I'll throw this idea in as another option - how about a pre/power setup using a Class D poweramp? For example, a Crest Prolite or Peavey IPR power amp is light (under 10 pounds), plenty of power, gives you lots of cab options, and fairly simple to setup. Then add whatever preamp - Sansamp, or VT Bass, to name a couple, for the "Ampeg" style tone.
 
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#25
In theory, I think the absence of a power transformer could make the new Class D's a little less resistant to voltage spikes and bad power situations. Those transformers had a side benefit of electrically isolating the input power from the amp circuit. Transformers store the incoming energy in a magnetic field and the energy is magnetically coupled to the power supply.

But I'm sold on the new Class D's. I have a Mark Bass LM II and a GK M800. Both have worked well. Just sold the 1970 SVT as well.
 
#26
after playing an svt2 (non pro) and an acoustic 370 for almost 30 years, I've gone class d. the main reason was weight with the svt (80 lbs for just the head!) and the 370 has some reliability issues (it's a 40 yr old amp!). i'm currently using a carvin bx500 and a carvin bx1500 (depending on band/gig). these heads sound great and have plenty of power. I found that adding a sansamp drri into the fold really opens up their potential. my rig sounds great whether its my riffy blues band or my full-on punk band. enjoy the search, and good hunting!
 
#27
GeoffB said:
In theory, I think the absence of a power transformer could make the new Class D's a little less resistant to voltage spikes and bad power situations. Those transformers had a side benefit of electrically isolating the input power from the amp circuit. Transformers store the incoming energy in a magnetic field and the energy is magnetically coupled to the power supply
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Ah... SMPS's have transformers too. What they do is first rectify the AC to DC, then convert it to high frequency (above 20KHz) AC and send that to the transformer. A transformer's size and weight is inversely proportional to the frequency it has to operate at so said transformer can be tiny and light :) .
 
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#29
All I can say is WOW! Thank you very, very much for all your help and suggestions. And thank you Linnin for the extra thread, gonna take me a while to wrap my head around that but I will. You've helped make my choice both easy and hard. Easy in that my next amp head will be a small class d amp (my back also thanks you) and hard for all the choices. Then again, searching and trying out new stuff is half the fun. Thanks again everyone for all your help. Bass players rule!!!
 
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#30
GeoffB said:
In theory, I think the absence of a power transformer could make the new Class D's a little less resistant to voltage spikes and bad power situations. [...]
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Switching power supplies are better at handling line voltage variations than the old (big/heavy/hot) linear power supplies. They generally have quite a wide range of input voltages and can deal just fine with saggy power, etc. without any real effect on the output voltage. Their DC output is, by nature, heavily lowpass filtered as well. Due to that and the high switching frequencies I expect any transient spikes from the input would be smoothed out or nonexistent on the output.

I haven't seen schematics on these things but what I wonder is whether they even bother having a stable switching DC output stage at all (e.g. a split between the power supply and the amp), or if the switching power supply is essentially turned into the power amplifier by fooling with the duty cycle / feedback loop to create the amplified waveform and going from rectified line voltages to the speaker output?
 
#32
johnpbass said:
I'll throw this idea in as another option - how about a pre/power setup using a Class D poweramp? For example, a Crest Prolite or Peavey IPR power amp is light (under 10 pounds), plenty of power, gives you lots of cab options, and fairly simple to setup. Then add whatever preamp - Sansamp, or VT Bass, to name a couple, for the "Ampeg" style tone.
Click to expand...

This is what I did, but I'm now thinking I'll probably pickup another micro-head as a backup anyways.

Well assuming that my GAS stops whispering "Wounded Paw Black Sheep Pre-amp" long enough for me to take the responsible option instead of the hedonistic one...
 
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#34
I haven't seen schematics on these things but what I wonder is whether they even bother having a stable switching DC output stage at all (e.g. a split between the power supply and the amp), or if the switching power supply is essentially turned into the power amplifier by fooling with the duty cycle / feedback loop to create the amplified waveform and going from rectified line voltages to the speaker output?
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All in one class D amp / switching supply has been done, but it's done by eliminating the rectifiers and filter caps. The power supply switching is locked at near 50% duty cycle, so the output of the transformer secondary is also at near 50% duty cycle. MOSFETs are used in place of diode rectifiers and are switched at the same frequency as the supply switching frequency, also at 50% duty cycle, but 90 degrees out of phase. The resulting output with zero audio signal input is a 50% duty cycle square wave that is twice the frequency of the power supply. The audio modulates the phase of the secondary side switching above and below 90 degrees to change the duty cycle. The MOSFET-gated PWM output is simply passed through the output filter directly to the speaker. The Peavey DPC power amps operated that way. Other companies have done it too. It's a really good idea on paper, but in practice there are a lot of inherent issues that over complicate it to the point of not being worth it.

Some have tried using the audio to modulate the feedback loop, and it works to a point, but there are a lot of issues with that. One issue is that the crossover frequency of the feedback loop is usually less than 10% of the power supply switching frequency, and the gain of the loop is non linear. A power supply feedback loop is meant to have tons of DC gain, but not much AC gain. Another issue is that the power transformer core will saturate at audio frequencies because it's meant to operate at higher frequencies. There would likely be lots of ripple on the power supply output because you'd have to eliminate the filter caps so that they wouldn't filter the audio output, and the feedback loop would be tied up dealing with audio instead of doing one of it's jobs--cancelling ripple.
 
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#35
ambient space said:
All I can say is WOW! Thank you very, very much for all your help and suggestions. And thank you Linnin for the extra thread, gonna take me a while to wrap my head around that but I will. You've helped make my choice both easy and hard. Easy in that my next amp head will be a small class d amp (my back also thanks you) and hard for all the choices. Then again, searching and trying out new stuff is half the fun. Thanks again everyone for all your help. Bass players rule!!!
Click to expand...
Super Cool My Bass Bro.
 
#36
Nowadays, the Class D heads are just like asking about Tube or SS heads in general...everyone has their favorites ;)

I prefer an amp with some character to it, thus I have found the GK MB200 to suit my needs. Honestly though, you would be hard pressed to find a standard-features Class D head over $500 these days....and those < $500 are still EXCELLENT quality in most cases. Literally no way other than to try them out and see which you like, no easy answer here buddy. Like I said though, I dig the GKs and plenty of other people feel the same way :)
 
#38
Class D is very simple in concept, but it's complicated to execute well. That's probably why power modules are a popular way of putting the technology into finished products. It's probably also why there have been some products that come up short in performance or reliability.
 
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