I took the easy way. I dug my old Joyo American Sound pedal out of the closet, and hooked that up to a class D amp and yet another pair of thrift-store Sony speakers.
The Joyo includes speaker emulation of some sort (maybe only a steep low-pass, I'm not sure). It seems like a good match to the class D board I have, which has a volume pot right at the input. Between that and the gain and drive controls on the Joyo, I had no trouble getting signal levels right.
I also have an old ART USB Dual Pre ( https://www.sweetwater.com/store/detail/USBDualPrePS--art-usb-dual-pre ) which I could press into service as the microphone preamp. Like the Joyo, it will also run on a 9V battery, or a 9 - 12 V DC external power supply.
If I can solve my battery dilemma, I can get a suitable DC-DC converter to step the battery voltage down to 9 V for the Joyo and ART preamp, and that would make for a very easy project putting it all together.
Good point. I have just relied on my own ears, but if anyone else is going to touch the volume knob, a limiter is definitely a good idea.
A pair of back-to-back red LEDs followed by a pot, maybe?
-Gnobuddy
I have a fairly large 4 X 7 inch board. It has 2 X 50 watt channels and 1 X 100 watt channel (two chips). There is a quad opamp for the crossover / LPF feeding the sub channel.
It has 5 caps across the supply lines. Two are large "Audio Grade" caps resembling black gates, they are 10,000 uF 35 volt. They are in parallel with three little 1000uF 25 volt Nichicons. I figured that I would swap or remove the Nichicons when they blew or showed signs of distress, but they haven't done anything bad yet and the idle current is near zero when the amp is in standby.
It has 5 caps across the supply lines. Two are large "Audio Grade" caps resembling black gates, they are 10,000 uF 35 volt. They are in parallel with three little 1000uF 25 volt Nichicons. I figured that I would swap or remove the Nichicons when they blew or showed signs of distress, but they haven't done anything bad yet and the idle current is near zero when the amp is in standby.
Are you sure you really need to step down the voltage for the pedal & preamp? I've heard that you can run 9 volt pedals from 12 volts with no problem.
Thanks for the links!! I think the voltage multiplier is a very good idea---not so much to get LOUDER, but (as gnobuddy says) to avoid the awful sound of Class-D clipping. I'm still leaning towards the TDA9492MV because it's mono only, which is all I need.
The ART preamp's manual actually says that 12 volt operation is fine. The Joyo came without a manual, fairly typical for a budget priced guitar pedal direct from China.
The catch is that I'm planning to use a battery voltage in the vicinity of 20 volts, for the reasons we've been discussing on this thread, and that is too much for the Joyo or ART.
That would have been a bit of a headache even ten or fifteen years ago (I'd have to put together some sort of linear voltage regulator, and waste lots of precious battery power in the process.) Today I think I can probably find an off the shelf ice-cube sized DC-DC converter to step 20V down to 9 V or so.
-Gnobuddy
Like this one.
DC 3.2V ~ 40V LM2596 Power Supply Buck Converter Step-down Module Adjustable | eBay
Dotneck335, I looked up the number TDA9492MV and nothing came up, could you check if it is right?
OOPS!!Typo error, my bad. TDA7492MV. Evidently, the MV is the mono version, and the P is the stereo one.
Seems both the TDA7492MV and the TPA3118D2 will put out 35W @ 1% in an 8 ohm load with 25V, both 13W at 15V. Didn't think they would be much different in power. I see that they are selling 'Digital Amplifier Speaker Protection Board Module', there is a thump when these things turn on. There is an input to mute the board, I'll have to see how it works.
No idea on the noise on the converters. I tend to put a small capacitor at the receiving circuit, I don't see why it would be excessively noisy that you couldn't cure it with a cap. You could always look at the Class D amps as noise generators themselves. But then again they do have inductors on their output. That is one thing that surprised me. Two mu-amp stages and a Class D amp powered by a switching PS and no isolation between them, no motorboating, nothing.
No idea on the noise on the converters. I tend to put a small capacitor at the receiving circuit, I don't see why it would be excessively noisy that you couldn't cure it with a cap. You could always look at the Class D amps as noise generators themselves. But then again they do have inductors on their output. That is one thing that surprised me. Two mu-amp stages and a Class D amp powered by a switching PS and no isolation between them, no motorboating, nothing.
I suspect your 9V circuitry draws less than 10mA from the battery.
Consider standby consumption of a simpler switcher and you possibly decide to return to some linear regulator.
I was worried about this a few years ago, but since then, I've used random thrift-store switching power supplies to power audio circuits (including guitar FX pedals) a few times now. Here's what I've found:
If I do nothing to filter the DC, some whistle audibly at a few kHz. (Newer ones tend to use higher switching frequencies, some above audibility.)
Since switching PSUs are mostly the same as DC-DC converters (the only major difference being rectifying the incoming 120V AC to DC first), I assume DC-DC converters might have similar issues.
The good news is that it is a heck of a lot easier to filter out noise at, say, 6 kHz, than at 60 Hz. You only need one-hundredth the capacitance for the same amount of filtering! Sticking something like a 100 ohm resistor in series, and a 100 uF electrolytic cap to ground, has done the trick for me in every case so far - there is no audible power supply noise after that.
If you are concerned about voltage drop in the 100 ohm resistor, you should get the same filtering action from 10 ohms and 1000 uF, still a very reasonable cap value these days.
I also have a One Spot ( 1 SPOT(R) – Truetone ) power supply on my pedalboard. From its small size, light weight, and beefy rating, its definitely a switcher. It's also dead quiet, presumably the manufacturer added extra internal filtering since it was intended to be used with guitar FX pedals.
-Gnobuddy
????? My 2x 9 volt Boss pedals draw ~75 mA of current. Are you saying that using a linear regulator (i.e., LM317) will be MORE efficient---draw LESS current--- than a DC-to-DC converter?
A linear regulator will never be more efficient, but it will be quieter.
All switchers generate some noise. Most of it is well above the audio range and easily filtered. The issue arises when multiple switchers are used in the same design, each with a different switching frequency, or worse varying frequencies (a common trend today).
Without precautions two switching frequencies can mix to create a tone in the audible range which can be hard to eliminate. If one or more of these switchers uses a moving, dithered, or load dependent switching frequencies, there may not be a tone, just noise, hash, or that vague swishing sound that makes you want to smash the whole design.
I spent years chasing these effects in cell phone designs until we got "power management" chips that put all the switchers and sometimes even the audio circuits inside one chip with all the math modeled on a computer before the design was done to avoid this stuff.
All switchers generate some noise. Most of it is well above the audio range and easily filtered. The issue arises when multiple switchers are used in the same design, each with a different switching frequency, or worse varying frequencies (a common trend today).
Without precautions two switching frequencies can mix to create a tone in the audible range which can be hard to eliminate. If one or more of these switchers uses a moving, dithered, or load dependent switching frequencies, there may not be a tone, just noise, hash, or that vague swishing sound that makes you want to smash the whole design.
I spent years chasing these effects in cell phone designs until we got "power management" chips that put all the switchers and sometimes even the audio circuits inside one chip with all the math modeled on a computer before the design was done to avoid this stuff.
Not at 75 mA of current consumption. (But yes at very low power consumption, when the DC-DC converter is barely idling, but has to draw power to magnetize it's internal transformers, and run its internal electronics.)
Voltwide's point was that if you happen to have pedals that have very low power consumption (many analog pedals only draw a few mA), then the switching DC-DC converter may be no better, or even worse, in terms of efficiency. If a linear regulator draws 10 mA to supply the pedals, and a switcher draws 10 mA idle current, plus another 5 mA to power the pedals, there is no advantage to the switcher. In fact, it is considerably less efficient at this very low output current.
The story would be very different at 75 mA, where the linear regulator still has to draw the full 75 mA from the 20 V battery, wasting a lot of power. Meantime, if the switcher has 85% efficiency when supplying 75 mA to the load, it will draw only (9V x 75 mA)/( 20 V*0.85) or 39.7 mA from the 20 V battery.
In this case the switcher is drawing only 40 mA to supply the pedals, where the linear regulator would have to draw a bit over 75 mA for the same job. A huge win for the switcher.
If the Joyo American Sound is very nearly a Sansamp/Tech21 Blonde clone, as I've heard, then it's all analog, but rather complex, with quite a few op-amps in the circuit.
The ART mic pre I'm thinking of using can run on an internal 9V battery for recording in the field, so it probably has pretty low power consumption as well.
I'm curious how the power consumption of the pedals will compare to the average power drawn by the class D power amp. One single watt into a 95 dB/W guitar speaker can be very loud, so it may turn out that the class D board is only drawing a watt or less on average. If that is true, then a pedal that draws 75 mA @ 9V (that's 0.675 watts) will be drawing a sizeable proportion of the total power from the battery.
The discrete JFET preamps Printer2 has been discussing typically only draw a milliamp per JFET, so if they sound anywhere near as good as your BOSS pedals, they may be worth considering.
Spare time has been in short supply for me, so I think I'll take the quickest route for my version of this project. If I can use something off-the-shelf, I probably will.
Speaking of which, I've been wanting to re-assess the sound of the Joyo American Sound. So, at this week's jam, I plugged my guitar into a Danelectro Fish-n-Chips graphic EQ pedal, ran that into the Joyo American Sound, and ran the Joyo straight to the mixer. I set the Fish-n-Chips for an acceptable clean rhythm guitar sound, then turned on the Joyo and set it to give a little volume boost, a little compression, and just the tiniest tinge of grit for almost-clean solos. The P.A. already has reverb + delay in it for the vocals, so I didn't use any other guitar pedals.
It worked quite well, all I had to do was step on the Joyo's footswitch for solos. The Joyo overloads less gracefully than a real Fender valve amp, but is also about 35 lbs lighter and a heck of a lot smaller. Not a bad compromise considering how much stuff I'm hauling to these jams.
I don't have an A/B switch yet, otherwise using one might be even better - select either Fish-n-Chips or Joyo, so the eq dialed into the Fish-n-Chips for cleans doesn't alter the signal into the Joyo. Each one's EQ can be optimized solely for it's function.
-Gnobuddy
I am trying to gauge power consumption in a portable battery-powered guitar amplifier. So I looked up the commercial offerings for a comparison. A Roland MicroCube runs on 6 x AA batteries; it is quoted on their website as putting out 2 watts into its 5" 4Ω speaker, and draws 185 mA. How is that possible? From my figures, 2 watts into 4Ω is 2.83 volts and 707 mA......??????? Is Roland lying?
Serious power specifications nowadays are the exception, not the rule.
Doing the math 6xAA=9V powering a single-ended amp may deliver 2W into 4Ohms
Whenever I measured battery power consumption, it was surprisingly low.
One reason is you do not draw full power continuously, another the impedance of 4 Ohm speaker
is a bit bigger than 4 Ohms in most cases.
And btw - DC-current consumption always is a fraction of loudspeaker rms current.
All in all - these specs should come close to reality.
Doing the math 6xAA=9V powering a single-ended amp may deliver 2W into 4Ohms
Whenever I measured battery power consumption, it was surprisingly low.
One reason is you do not draw full power continuously, another the impedance of 4 Ohm speaker
is a bit bigger than 4 Ohms in most cases.
And btw - DC-current consumption always is a fraction of loudspeaker rms current.
All in all - these specs should come close to reality.
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One of the less than stellar things about the Micro Cube is that it has almost no bass response at all in the usual sense of the word. IIRC, bass rolloff begins around 500 Hz or 1 kHz, (!!), and everything below that is rolled off at 6 dB/octave or so. No wonder it sounds like a tiny computer speaker that's been dropped into a plastic bucket!
Since the most of the meat and potatoes in an electric guitar's output is down around 300 Hz, and the Microcube doesn't actually reproduce those frequencies, but only the midrange frequencies that the human ear is more sensitive to, the cube uses much less power than a better guitar amp would for the same subjective loudness.
For old-fashioned class B output stages, average current draw from the power supply at full output is 2*Ip/pi; since pi is around 3, that's only about 2/3 of the peak current in the speaker.
For class D, there is even less current draw. Figure output power to the speaker equals about 80% - 85% of the power drawn from the power supply, and that will let you estimate current draw from the power supply.
Like Voltwide, whenever I have attempted to measure average power delivered to a loudspeaker, I have been surprised by how little power was actually being used.
-Gnobuddy
Since the most of the meat and potatoes in an electric guitar's output is down around 300 Hz, and the Microcube doesn't actually reproduce those frequencies, but only the midrange frequencies that the human ear is more sensitive to, the cube uses much less power than a better guitar amp would for the same subjective loudness.
For old-fashioned class B output stages, average current draw from the power supply at full output is 2*Ip/pi; since pi is around 3, that's only about 2/3 of the peak current in the speaker.
For class D, there is even less current draw. Figure output power to the speaker equals about 80% - 85% of the power drawn from the power supply, and that will let you estimate current draw from the power supply.
Like Voltwide, whenever I have attempted to measure average power delivered to a loudspeaker, I have been surprised by how little power was actually being used.
-Gnobuddy
Looks near-right to me. On steady Sine the DC current would be ~~320mA. (Remember two current paths working alternately through a Totem Pole.) Even heavy guitar is not as severe, over a full set, as a steady Sine. Apparently they like a 50% fudge factor between test and playing.
This gives several hours on primary AA cells, which seems tolerable.
Is this an aural observation, or have you measured it? I wonder if they do it electronically in the power stage, or if it is just an artifact of the crummy little 5" speaker? I would think that the line level output is not rolled off.......it even says in the manual that The MicroCube sounds better through headphones! My application (C6 lap steel) requires less bass, as the lower strings are tuned ~ a fifth above a regular guitar.
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