dugg
Apprentice Shielder
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Post by dugg on Feb 4, 2009 12:50:12 GMT -5
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Post by JohnH on Feb 4, 2009 14:51:44 GMT -5
I wonder how they work? Are they anything like Class D amps, which alos use very little extra power with low heat dissipation due to a switching power amp stage. My Powerblock is Class D and is the best SS amp I've every heard.
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Post by ChrisK on Feb 4, 2009 14:54:53 GMT -5
Yes, although any sufficiently advanced technology is indistinguishable from magic.  |
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dugg
Apprentice Shielder
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Post by dugg on Feb 4, 2009 16:24:06 GMT -5
I'm always searching around the backwaters for any kind of kung fu that is ignored by the mainstream. My mom taught me this; "If the majority of people believe in a thing, it is either not true, or it's barely worth knowing".
It's been a few years since I started reading about T class amps and thought to put one in an amplified guitar, so I figured others would have had a similar idea by now (you know, morphic resonance and all that). I have a few wide range efficient drivers kicking around, and building a custom body to house the parts is not a problem, but DIY plans for T class amps often use SMC's (surface mount components) which, for someone like me who's only rewired a couple dozen guitars might be impossible.
I guess I'm also wondering if the few kits that don't use SMC's would work as well? It's probably a moot point because sound quality is probably well into the 'good enough for an amplified guitar' range, that is unless we're planning to make an amplified nylon string classical.....hmm!
Another thing is that an internal battery could be used to take advantage of the fact that the current usage is so low. I had thought to adapt some sort of battery pack from a power tool, or?
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Post by ChrisK on Feb 4, 2009 18:26:32 GMT -5
Or, it actually IS true....... Most folk a'planet believe in gravity; that doesn't make it true, but empirical is..... As one of my T shirts sez, "Obey gravity, it's the law." Is this where if you hit the right note, the guitar turns into a driveway? SMT technology is used because smaller circuit geometries lead to better circuit parasitic control, because fewer plated thru holes are needed and the PCB is cheaper, and because the PCBs are smaller and cheaper. Most new ICs are not, and will never be, available in thru hole technology. Oh, and because it annoys the hobbyists.... These amps need good RF practices used in the supply decoupling and PCB layout. Thru-hole parts, perfboard, and wirewrap wire just won't cut it. The current wasteage is low. The quiescent/overhead current is much lower. However, work is work and power is power. If an amp is 50% efficient when outputting 20W RMS, it draws 40W of power. If an amp is 100% efficient when outputting 20W RMS, it still draws 20W of power. That's a big battery. |
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dugg
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Post by dugg on Feb 4, 2009 19:24:58 GMT -5
ChrisK, I'm keeping a close ear on my driveway.
Wouldn't the amp only drain the battery fast when I'm playing it loud, or is it a constant drain when its on? Also, 20w seems a bit much for an amplified guitar, I was thinking 5w-ish would be real loud with an efficient speaker.
Hang on, the asphalt is talking...or is it playing the guitar?
"that doesn't make it true, but empirical is....." I guess what my dear old mom meant was that there are truths and then there are truths beyond those, and at the higher levels some things almost, or completely do, contradict basic truths. You surely don't need to be a kung fu master to know this simple principal of learning. I mentioned it only because I thought would be interpreted as reflecting the true spirit of this forum, which is always innovative and forward thinking, and often down right OOB (out of box).
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Post by ChrisK on Feb 4, 2009 20:26:30 GMT -5
I posted my response with a tad of trepidation since I don't yet completely know your sense of humor. I find it to be excellent. See! No, it's not a constant drain since it's not a class A topology. It should draw current as a function of the output power and its efficiency thereat. Well, there is some constant current drain due to the (low) quiescent current, but this should be small. If you're going to use a battery, you'll need an inverting switching power supply since you'll need both a positive and negative (and ground) supply rail (methinks anyway). Or you could use two batteries. I would agree, unless you posses Marshall'mellowed ears. It would be real loud anyway. See! It's like an advanced IQ test; the layers of the onion change meaning depending how (well) one sees things. And a good politician too. So, You're on a game show. You get to choose one of three doors. Behind one door is the prize. Behind the other two there is nothing. If the host opens one of the other two doors, always choosing one without the prize, should you change your choice to the other closed door?  Prove it. |
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Post by newey on Feb 4, 2009 20:31:27 GMT -5
Your idea, at its most basic level, is an onboard amp and speaker, battery-powered. This has been done by many folks, I remember seeing such things in the '60s. You propose using a different style of amp in such an endeavor. When this has been done, it has not been particularly successful. There are 2 basic reasons why it has not succeeded. First, it was/is something of a novelty item, and so cheaper components were used, as the appeal was limited to a lower price range. This could be remedied in a project guitar, just as you propose a (presumably) upgraded amp for your project. The second reason it hasn't succeeded is that an onboard speaker can only be so big- and not very big at that. You stated: How big? If they are much more than about 4", you won't get one into any reasonably-sized guitar. And no matter how efficient, a 4" speaker is a 4" speaker.  In other words, the amp technology isn't the limiting factor, and never was. You can take any of the little battery amps like a Pignose and run it through a small cab, it'll sound much better than the on-board 4" speaker does. Please understand I'm not trying to discourage you from building this if you want to do so. It sounds like a fun project. But "innovation and forward-thinking" happen only when one knows what has been done before, and builds upon that knowledge. I might suggest that if you do decide to do this, you wire the speaker with the option to go out of the guitar to an external cab, disconnecting the internal speaker. Then you can see whether the onboard set-up was really worth the effort. |
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Post by ChrisK on Feb 4, 2009 21:12:24 GMT -5
Physics is. If you mount the speaker real close to the strings, you might get a Sustainer for free. |
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dugg
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Post by dugg on Feb 5, 2009 11:49:58 GMT -5
ChrisK, you're too kind. But seriously, don't you think it's your sense of humor we should be worrying about? An inverting-switching power supply or two batteries. If I used two batteries, would they both have to be 18v, or whatever the power requirement is for those T's, or will the power be halved? newey, the wide range drivers I have are fostex's, like you find in auratones, I'm pretty sure they're 4". I've done quite a bit of research on amplified guitars over the years, I guess I didn't indicate that specifically, but I figured it would be implied by my 'tone' Though your right newey, in that most amplified guitars are 'kids toys', not all are. There are the Fernandes Nomads, which I'm sure you've seen before;http://www.fernandesguitars.com/nms.html I disagree with the idea that the speaker is going to be the 'weakest link' in an amplified guitar design. I think the amplifier is equally weak in this particular application. For an amplified guitar to be a serious instrument, it's going to need certain things that I can think of. First, it can't have any cord so, battery. It has to have an efficient amp, not only so the battery lasts, but also to keep heat inside the axe to a minimum. Hang on, my friend's at the door, I'll finish this in a couple hours..... |
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Post by ChrisK on Feb 5, 2009 12:29:32 GMT -5
Assuming a symmetrical supply (both a positive and negative rail), the output power will be determined as follows:
PH+ Positive Headroom. This is the positive rail dropout voltage in the amp and limits the maximum positive output voltage level.
PO+ Positive Output Voltage Level. This is the positive supply (at its lowest level) minus the Positive Headroom. This is the maximum positive output drive voltage.
NH- Negative Headroom. This is the negative rail dropout voltage in the amp and limits the maximum negative output voltage level.
NO- Negative Output Voltage Level. This is the negative supply (at its lowest level) minus the Negative Headroom. This is the maximum negative output drive voltage.
Both the PO+ and NO- define the maximum output levels and hence the maximum peak output voltage for the output. Using a sine wave as a reference output waveform (most RMS calculations are based on such), the peaks of the wave are PO+ and NO-.
Let's assume that we have two 12 VDC NiCad battery packs. Let's assume that we have an 8 Ohm speaker. Let's assume that the headrooms are 2 VDC.
Our maximum PO+ and NO- are 10 VDC each. The peak output current for each half of the sine wave is 10/8 or 1.25 ADC.
Now, that sine wave thingy. The RMS current of a sine wave will be 1.25 ADC/sqrt(2) or 0.884 Amps. Power = I^2 * R.
The RMS voltage of a sine wave will be 10 VDC/sqrt(2) or 7.07 volts. Power = V^2 / R.
This is 6.25 Watts no matter which way that you calculate your way to China.
So, for a given output power required, one can easily calculate the required supply rails, since one can calculate the output power given the supply rails and load impedance.
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dugg
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Post by dugg on Feb 6, 2009 13:23:19 GMT -5
ChrisK, you're a real prince. I can't tell you how much I appreciate your attention on this because I know it's not really an area of your interest. As you're probably guessing, this amplified guitar build is not exactly back burner for me, but it does have 5 half builts ahead of it. Never the less, I always study, plan and think extensively before working, and I knew this would be the best place to pick brains. I'm also hoping maybe someone else with less pending projects and equal interest in an amplified guitar might drop by. As far as I know, these T class amps haven't been pressed into this, most obvious to me, use yet, and I wonder why. It seems to me that, if amplified guitar were your goal, the T class amp would be perfect with it's audiophile quality sound, low current usage and cool operation. Heck, with the sound quality being so close to SET, why aren't there more guitar players trying them as external amps? Or, can the T amp be 'overdriven' to produce a tube like distortion? With the prices being only a fraction of an SET amp, I'd think these would be more pressing questions? ChrisK, I'm copying everything you posted for me here for future ref so don't think yer wastin' yer breath on a fencepost, or was that the driveway? |
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