Portable Amp/PA Power

[Mandobart]

A couple things to bear in mind - pure sine wave is the way to go to minimize noise and maximize the life of any component you choose to plug in to the inverter. A modified sine wave inverter is no bargain if I have to replace my amp every year. Carvin tech people have told me mod sine wave is ok, but I'm going for pure.

Inverters work best when sized for the load. A 1500 W inverter consistently loaded to only 100 W or less is not as efficient and can be damaged if continually operated this way.

Finally, the dark art of rating amplifiers is steeped in fiction, folklore and fantasy. The 250 watt rating just doesn't jive with the 100 VA input rating. Every mfr uses some creative advertising here - meaning my amp, in a reflective room, with hot mics/pickups, high quality power in, downhill with a tailwind could indicate close to 250 watts (sonically). For my amp to consume 250 W it would draw 2 - 2.3 A. Based on my actual current readings I don't see that happening.

[kjbllc]

Finally, the dark art of rating amplifiers is steeped in fiction, folklore and fantasy.

ain't that the truth , that's been going on forever

[foldedpath]

To be fair, sometimes the power ratings aren't entirely the result of the evil marketing department.


For example, the QSC K10 powered PA speakers I use are bi-amped, and rated at 500 watts for the woofer and 500 watts for the high frequency transducer. That's a nice amount of headroom for the bass frequencies that need the power, but it's an absurd amount of power for the highs. It would probably melt the transducer if all 500 watts at peak were allowed to hit it.

But it's not marketing fluff. QSC designed the dual amp to be modular; the same on both sides, so it's easier to manufacture parts and do repairs by just swapping out the dual 500 watt module. Class D amps are very lightweight, so there is no weight penalty for having an over-spec'd amp on the high frequency side.

Of course, it doesn't hurt that the marketing dept. can say it's a 1,000 watt powered speaker! And so they did. But it's not entirely deceptive advertising. More like a nice side effect for marketing, resulting from manufacturing efficiency.

[pops1]

500Watts at peak is almost double what the RMS rating is (root means squared) which is a more precise rating of the power.

[Mandobart]

Here are a couple interesting links I ran across on the subject of manufacturer's amplifier power ratings:

https://en.m.wikipedia.org/wiki/Audio_power

https://www.audioholics.com/audio-am...-power-ratings

This is why I directly measured the current draw from my amp rather than go by the stated audio output (250 watts) or even max input (100 VA). Both would result in oversizing the inverter and battery, resulting in more weight, cost and potentially shortened component life.

I also measured the current draw on my 1980's Peavey Special 130 with the bird plugged in and cranked up. Also less than 1 ampere.

I would be interested in the results of fellow cafe members willing and able to measure the current draw of their systems at normal performance levels. All you need is a clamp-on ammeter and an AC pigtail with the conductors separated (outer insulation removed) as you need to place the clamp-on around either the hot or neutral to get a reading.

[pops1]

At 250 watts RMS and 125 volts AC the max current draw will be 2 amps. That will seldom happen and normally will draw half, or more likely less than half, of that while running. Most amps running wide open won't reach maximum, tho there are those where the rated power is conservative.

[Mandobart]

250 watts may be RMS, may be peak power, who knows. However, the name plate by the AC IN plug clearly states "90-260 VAC 50/60Hz, 100 VA". For those that haven't spent their careers making electricity, VA stands for "volt-amperes." Power is a vector - it is the cross product of the across variable (voltage) and the through variable (current). Since it is a vector cross product it has three components that form a right triangle. When plotted the horizontal component is true power, expressed in Watts. This is consumed by the resistive elements of a load, usually dissipated as heat. The vertical component of power is reactive power, expressed as volt-amperes reactive or VAR. This is the power used by reactive components (inductive or capacitive). These loads are expressed in impedance, since the reactance varies with the frequency. Reactive load is a big deal on the grid and for a power station. The hypotenuse of this triangle is called apparent power and is expressed in VA. It is the largest component of the power triangle - there is no way a load can draw more true power (watts) than apparent power (VA). So Carvin has posted both 250 watts and 100 VA on my AG300. They cannot both be accurate.

I'm not calling out Carvin on anything - the AG300 is the best acoustic amp I've played through. It completely fits my needs as a musician. All amplifier manufacturers use obscure means to express the power output of their products. It doesn't matter to anyone until you start sizing a stand alone power supply for your amp.

[Mandobart]

After much searching I found this thread on the Acoustic Guitar Forum. It seems that others have done similar measurements with similar results - most amps, PA's and powered speakers don't draw that much power. So I ordered an inverter and small deep cycle 12 VDC battery. When it arrives and I set it up I'll report back with results.

[rockies]

If you are just amping a mandolin or guitar you might want to look at the Yamaha THR series amps. They run of 8 AA batteries or the A/C supply or a 12 volt battery to the DC input. They have the Yamaha boutique modeling electronics and USB control. I found the THR-10C was what I preferred for the mandolin as it has the cleanest effects, tons of output and sounds great. The 10X would be the one if you're into heavy metal stuff. Anyway really happy with mine (size of a lunch box) so worth a look.
Dave

[Mandobart]

If you are just amping a mandolin or guitar you might want to look at the Yamaha THR series amps. They run off 8 AA batteries or the A/C supply or a 12 volt battery to the DC input. Dave

Thanks for the suggestion Dave. However, for my needs I need an instrument and mic input. And though reasonably priced, the cost of making my own battery power supply capable of powering my existing amp is considerably less than the Yamaha.

Currently mfrs are making a full range of conventional AC powered acoustic amps and PA's and some battery powered acoustic amps and PA's. No one that I have found is making a lightweight portable AC supply to turn conventional AC powered acoustic amps and PA's into battery powered units. If my system works as designed (and a few others have made similar power supplies that do work) then this is a third option that doesn't require lugging a generator around to busk or play in remote areas. No I'm not going to start up a business but I do plan to share my experience with others that may be interested.

[Astro]

I would be very interested in how this turns out. We do a lot of gigs with a crazy long extension cord. We also hope to do some boat gigs with our 4 piece and we need our mixer and powered speaker set up to do it right.

[pops1]

I set up a boat with a true sine wave inverter a couple of years ago and we installed a small PA for the captain to give talks and for live music, it works great. Spent the money on really good ilnverter as it was going to be used 7 days a week and wanted the true sine wave, it was worth it. It just powers off the boats batteries and when the motor is running they are charging.

[Mandobart]

Here are the preliminary results of my inverter experiment:

I purchased a Cotek SK200-112 Pure Sine Wave inverter rated at 200 W continuous duty for about $127 from Amazon (they have since gone up about $20). It is powered by a Power Patrol 12 VDC, 12 amp-hr sealed AGM battery which cost about $40 with shipping.

The whole system fits in a shoebox and weighs about 8 lbs. Total cost so far about $170 USD. I haven't built a rack to carry it around in yet. I plan to add a voltmeter and maybe ammeter that can be switched on or off (no need to constantly draw power to check parameters). The inverter works great to power my Carvin AG300 amplifier. NOTE: current is measured in amperes, abbreviated "A" and frequently spoken as "amps." Not to be confused with the term "amp" as a contraction for "amplifier." An ampere-hour, A-H, amp-hr is a battery rating, meaning how many amperes it can supply for a given time. It is not a linear relationship. The higher the discharge rate, the less time the battery can supply useable current. A 100 amp-hr battery may supply 1 amp for 100 hours. It would not be able to supply 100 amps for 1 hour; it would be considerably less.

Lessons learned - the battery supplies (alternately the amplifier and inverter draws) more current than I calculated based on a 90% inverter efficiency. I calculated I would need to supply a little over 1 amp from the battery. I figured I needed about a 9 amp-hr battery to supply my amplifier for a 4 hour gig without discharging the battery more than 50%. So I ordered a 12 amp-hr deep cycle battery to have some margin but still be small and lightweight.

For anyone doing the same thing, remember that a clamp on AC ammeter may read current in a DC circuit but it will not be accurate. Since you cannot have transformer action with DC, the sensor for a DC clamp on ammeter is different than for one that measures AC. An in-line DC ammeter (requires you to break the circuit and insert the ammeter in series) will work IF your meter is rated for the current you wish to measure. Most bargain multimeters only go up to 200 mA.

I measured the following with an in-line DC ammeter:

Battery supplies 0.75 DC amps to power the inverter just idling (supplying no load, just running the inverter). When I plug in and turn on the amplifier the battery supplies 2.28 DC amps. If I use the USB charging port on my amplifier the battery current rises to 2.86 amps. If I then play music through the amplifier at pretty loud volume (not maxed out, my ears can't handle it) the total battery current goes up to 2.9 - 3.5 amps. Without using the amplifier's USB port for charging a phone/pad/etc. the current drops down to 2.3 - 2.5 amps. So a 4 hour gig at fairly high level will deplete 10 amp-hr which is a lot to drain out of a 12 amp-hr rated battery, even designed for deep cycles.

Options are add a second identical battery in parallel with the first to double the capacity (to 20 amp-hrs) OR buy a larger, heavier battery rated at 20 or more amp-hours. Or just see how long I can cycle the battery like this before it dies. There are pros and cons each way.

[pops1]

A physically larger battery would be heavier, but last much longer than a small lighter battery. It would also have considerably more AH.

[kjbllc]

I did not read through all of this so it may have been stated before.

I just looked at a friends van/camper. with solar recharge for kitchen battery, they are made for solar , and may do the job , holding a charge better?

[Tom Wright]

Thanks for the detailed report. I remember thinking 18 amp/hrs would be a safe bet.

If you're willing to spend more, I found a 22 amp/hr 12V lithium-ion battery for about $300. 6lbs, max continuous discharge current 40 amps.

[Barry Wilson]

I have a small solar panel I use to top my motorcycle battery. It was cheap and helps replace power. Solar panels have gotten so cheap. The little one I have is 2.5 watts and cost me $15 cdn. For a little more I was looking at a 20 watt but you are supposed to use a charge controller. I woulodn't have to leave it plugged in. I think with the 20 watt (1.34 amps) could run some serious power draw. The little one I have fits in my case with cables, mic, fx etc

http://www.canadiantire.ca/en/pdp/coleman-solar-battery-charger-20-watt-12-volt-0112023p.html#.VxrJPvkrKM8

[Mandobart]

I think with the 20 watt (1.34 amps) could run some serious power draw...

Yep, probably could supply serious current of.....1.34 A....

A solar charger cannot "hold a charge better." A battery takes a charge. The way to take or "hold" more charge is to increase the plate size = a bigger, heavier battery. Also, just as a battery lasts longer when it supplies less current, it charges more deeply if trickle charged (very low current in). You get a better charge if you charge at 2 A vs at 10 A.

For this application a "deep cycle" battery is needed as opposed to a starting battery as it won't need to supply a large current but it will need to survive several charge/discharge cycles. I could run my old Coleman popup trailer for a few weeks in the boondocks (12 VDC system) with two 6 VDC golf cart batteries in series. It more than doubled the amp-hrs over a single 12 VDC battery.

[Barry Wilson]

I understand that. I use a deep cycle myself. I have played for almost a year now with no power sources. The Traynor Travelmate 50 has it's own battery with a mic in and a 1/4" with gain (plus rca). I just use the eliminator for my vocal harmonizer but wondered about using it for my yorkville pa. I just like the idea of solar putting some power back in to extend "runtime".

I just soldered up plugs on a couple 3s lipos for my harmonizer. for cctv setups and 6800 mah. even less to pack. I am testing that this weekend, though I still have the 20ah setup. I'd like to be able to connect midi guitar, fx and such, even the drum machine and mixing board.

the solar was kind of like trickling water back in as I drained it out... I am experimenting with this stuff too as I like not diddling around with power. my setup is fast and I generally play 2-5 hr shows.

[almeriastrings]

Quite some years ago I built a custom rig for this kind of job for a friend. I wanted to avoid the efficiency losses of inverters, so we found a used (but defective) acoustic guitar amp going cheap on Ebay and essentially just ended up using the speakers and the cabinet itself. All the other electronics were removed. I found a 40W power amp module that would run from a single line +24 v supply, and added a mic pre and a couple of Hi-z FET based inputs with an EQ module between these and the power amp module. The batteries were 2X 12V sealed lead-acid types as used in burglar alarms linked in series (I can't remember their exact rating). It worked really well. Total cost was just over $130 as I recall. I know he could easily get two hours of use out of it without needing to charge again.

[pops1]

The Carvin stage mate that I have is a 4 channel 100 watt amp with a 10" speaker and with a motorcycle sized battery can go (when new) 3+ hours. Add another battery and you have a gig easy. I have the batteries out as they don't hold the charge any more and it's lighter to carry without them so I just plug it in. I think the newer class D power amps draw less current than the older power amps for the same rated power. They are definitely lighter. I am not sure how long their new stage mate will play on batteries, smaller speakers, but more of them and more power, more channels.

[Barry Wilson]

I can get 15 hrs off a single charge with the travelmate. That's a whole weekend. I rented one with a gift certificate I had won before I bought it. $20 rented it for 2 months. During a really bad wind storm here last year, many places were without power. I played 2 markets that weekend. I love this off the grid stuff now.

I have learned my vocal harmonizer/fx unit will run on as low as 9v as I tested before charging the one battery. Had to be careful and resolder the plugs because for some reason tc helicon put the positive on the ring.

[Mandobart]

It ain't pretty....

Here is my first prototype inverter supply for powering my Carvin AG300. Overall dimensions 8" wide x 8.5" deep x 14.5" tall. Total weight 29.8 lbs, nearly all of that the battery, a 12 VDC 35 amp-hr deep cycle I got from Harbor Freight for ~$73. The black switch is a DPST to isolate the battery from the inverter. The inverter also has its own ON/OFF/REMOTE toggle.






Total approximate cost (so far) about $250.

[Barry Wilson]

It will be interesting to know how it works out.

[Mandobart]

It should be able to power my amp for 5 hours or more. I'll give it a real world test on that soon. For now it runs my amp for an hour practice session with no drop in terminal voltage.