Battry powered skateboard mounted speaker box - charger and preamp questions

[jestah]

Hi all,

With the help of this forum I have been slowly reworking my little skateboard speaker system and have got to a point where I'm getting fairly happy...



So the build so far:

I have a ported enclosure that is tuned like so (green line):




The tuning was taking from the boominator thread as I use my system out doors most of the time.

I was using a single 25w amp with passive cross over but recently added a line out doc from my ipod to a attenuator and then an active crossover to try and gain more sound output as well as reducing the losses to the passive crossover. I can say the new system is louder but still not sure about the difference between the power the active uses and what the crossover was consuming.

I wanted to work out my current draw while the system was running by using a DMM but could not get a reading (still new to electronics - connected the ammeter in series to the positive line of the battery ).

I did a running test, fully charged battery running at half volume for 3.57 hours resulted in a 0.9v drop (13.57-12.67v). I have hunted for the theory that I could use to work out the consumption of my system but no luck yet. I get the feeling its going to be blindingly simple when i see the maths too

I have added a little battery panel meter that reads from 10-15v and have noticed under heavy load the needle jumps a bit with the bass, should I add some caps to to smooth the output?

From here the signal is feed to a 15w amp for the top end and the same 25w for the woofers. I have the 8ohm woffers paralleled to present a 4ohm load and the piezos in series to help reduce the output as they are far more sensitive than my woffers ( 90 / 96 db)


Mid wiring

The whole system is powered by a single 12v 7ah external SLA but I am sick of dealing with manually taking battery off the system and plugging into the charger.

I would also like to consider adding two small 1.2ah sla inside the speaker box so that the unit can work as a stand alone item when I am at home and near mains supply. I have to use two as the speaker box must have a central balance point or it will always want to pull left or right when riding!

I have done a bit of reading and this is the solution I am considering:

A float charger should be able to plug into the system and keep the battery topped off and some sources say that it is ok to leave this on even with the amp playing ( I am not looking for pure clean sound so not worried if a little noise is introduced but very worried about it being unsafe for the amps, battery or charger). I was going to use my current power pack style of float charger to do this job but noticed that on the instructions said to never have it plugged into the mains with out a battery connected and I can see this happening often. Is it safe to make a standard 13.4v psu (with a regulator) instead that could be constantly connected to mains?

Once every 6 months I should bring the battery up to 14-14.4v for a few hours to give the batteries a longer life.

I should also add a low voltage cut off switch that turns off any batteries when they drop below a set voltage ( around 50% discharge /12ish volts??) but leaves them connected to any possible income charging.

I am also using an Ipods line out and interested to know if this line level should be boosted before going to the active xover with something like this or is the Ipod line out strong enough to give me as much sound output as I can get from my amps? I ask this as I have been told to both use and not use that kit by a few members of different shops but no one really had decent arguments either way.

so to recap I would like to know:

• how to test current draw to help me select the right capacity batteries
• why is the voltage jumping around under load, is this a negative thing and if so what can i do to reduce this
• Is it safe to add a basic regulated psu in parallel with the batteries to act as a float charger that is also safe to leave on even if disconnected from the batteries.
• Do i need to take any special precautions if charging two different (but identical batteries) at the same time
• Do i need to take any special precautions if charging 3 different (2 identical smaller and one larger batteries) at the same time
• any links to a low consumption low voltage cut off circuit?
• should I try to boost the line out from my Ipod before the xover

Also free free to point out any other mistakes or improvement I could make as I am still fairly new to this sound system building and am keen to learn!

Thanks in advance for the help!
jestah

[gfiandy]

Hi,

See my answers to your questions below.

• how to test current draw to help me select the right capacity batteries

You need the meter is series as you described. However many meters have a seperate connection for current measurement so you may need to move the probes to this. Also you obviously need to put the meter on the current setting. Your amps average current (which this will measure) is probably very low so use the lowest current range you have.

• why is the voltage jumping around under load, is this a negative thing and if so what can i do to reduce this

The voltage jumps due to the internal resistance of your battery, this is completely normal. If you would like to reduce it very large capacitors can help, the car audio guys sell these you want something like 1F (this is a very large capacitor). It will mean the amp will play a very small amount louder and might have better bass performance.

• Is it safe to add a basic regulated psu in parallel with the batteries to act as a float charger that is also safe to leave on even if disconnected from the batteries.

You need to read the datasheet for your battery to find out what trickle charge it can stand continually. Any power the battery cannot store ends up dissipated as heat so its not great for the battery. The ideal solution would be to buy one of the inteligent motorcycle charging systems as these check the charge of the battery and only charge it up if it needs it. However that are expensive. So yes it safe to use a regulated suppy but not ideal as it will probably age the battery.

• Do i need to take any special precautions if charging two different (but identical batteries) at the same time

No, the only difference is the charger will need to delive more current if the batterys are very flat, but the charger should be current limited anyway ( or simply big enough to deliver the current)

• Do i need to take any special precautions if charging 3 different (2 identical smaller and one larger batteries) at the same time

No so long as they are all the same voltage and construction type. i.e all SLA. (except as above charger must be capable of suppling current) It may be more of a problem as the batterys age if one fails then the other two may not get charged properly.

• any links to a low consumption low voltage cut off circuit?

Sorry, no.

• should I try to boost the line out from my Ipod before the xover

The maximum output of an iPod 0.9 V rms (Nano is 0.7 V). So it depends on the gain of your circuit. Normally you want to be able to amplify the signal at least 6dB into clipping for quiet tracks. To calculate the gain use Gain x 0.9 x 1.41 = 2 x battery voltage - amplifer loss

1.41 is sqrt of 2 and converts from RMS to peak voltage.
The amplifer loss is how close the amplifer output goes to the PSU rail (normally about 1v for digital amps but might be better or worse).
This equation assumes a bridge tied load which you amp is.

The gain of the total circuit is the gain of your power amp x gain of crossover in flat part of response. Solving the equation above for gain gives:-

Ideal gain = (2 x 12) -1)) /(0.9 x 1.41)

=> ideal gain of about 1:36 or 31dB (20 x log gain)

If your gain is much lower than this then you will probably find you can't get the amp loud enough. If its much higher then the amp will sound very distorted with the volume full up.

Regards,
Andrew

[gootee]

Just how big IS this skateboard, anyway?! <grin>

Just a quick comment: To measure the current consumption, if you can't get your meter's current measurement mode to work, you could instead insert a small resistance in series with the positive supply line and measure the voltage across it. Then just calculate I = V / R.

The only potential problems that I can see with that are: 1) The resistance would need to be small-enough that it wouldn't significantly affect the current consumption, i.e. the voltage across it should be much smaller than your supply voltage, and 2) As the resistance is made smaller, the voltages you would need to be able to read could become quite small, limiting the precision of your measurements. So you would probably then at least want to use the lowest mV voltage scale on your meter, and also make sure that you first (before each measurement) short the meter leads together and see if there's an offset that you'd need to subtract, to calibrate your measurements. Also, probably immediately after a measurement, you'd want to remove the resistor and measure its resistance while it's still at around whatever temperature it got to. And don't forget to short the meter leads in that case, too, and subtract out any resistance offset your meter might have. Also don't forget to at least calculate what the power dissipation in the resistor might be, and use an appropriately-rated one.

Cheers,

Tom Gootee

[r0b]

I would say, you're never gonna get a meaningful reading from your meter. Unless you input some sort of static signal (i.e. 120Hz straight) which is kind of unrelated to any music signal....

Why go so complex?

Your setup can output a max of about 50W, with a supply of about 12.5V and P=U*I that would mean

I=P/U=50W/12.5V =~4A

Figure in whatever losses, safeties, efficiency factors you think are feasible (or don't because, the 4A will probably be pretty much the max ever happening) and pick a battery suitable to deliver that current (which will probably rule out cheapo 7Ah SLAs )

This will still not give you any information about how long your setup is going to run. Nothing will do that however, except for using it and seeing when its empty.

Don't get me wrong, I don't want to be nihilistic or pessimistic, I just say, the calculations are not really applicable here, unless they get to some sort of academic niveau, so just try it out and you'll know.

Pick a battery that can deliver 3-4 A, and be happy

Edit: about the power caps: You could argue that these suck a lot of amps from the battery when recharging (pretty much what the battery is able to put out) which is kind of bad for the battery. Yes they might make the bass thumpier, but at expense of battery life. Nothing is for free in live

[Buckapound]

It's easy enough to hook up a switch so you can go back and forth between charge and play on your batteries, so there are no questions about the charger output going into the amp. I also installed a 10A switching power supply, so when the unit is plugged in and the battery is charging, all the power for the amps comes from the wall, not the battery.

The more sophisticated chargers are computerized, as has been mentioned, and do something called "desulfination," which extends the life of the plates in the cells.

If you are looking for a nice little preamp that works well on a single 12v supply (most don't), I can recommend the Arjen Helder tube buffer on eBay. Doesn't draw a lot of current, and in my unit, it does seem to produce a little more sound (I think I have the pot on the tube buffer turned up between half and two-thirds of the way to the max).

--Buckapound

[jestah]

You need the meter is series as you described. However many meters have a seperate connection for current measurement so you may need to move the probes to this. Also you obviously need to put the meter on the current setting. Your amps average current (which this will measure) is probably very low so use the lowest current range you have.

After a bit of fiddling I managed to get a reading but this would jump about with the music. I had peaks of 1.5-1.6A with a 0.9-1A average (roughly from my guess)

The voltage jumps due to the internal resistance of your battery, this is completely normal. If you would like to reduce it very large capacitors can help, the car audio guys sell these you want something like 1F (this is a very large capacitor). It will mean the amp will play a very small amount louder and might have better bass performance.

I see, should i start by adding 3-4 4700uf low esr caps to the system? What size bank should I aim to end up with and could i use many smaller non low esr caps as a substitute for a few higher priced esr caps?

You need to read the datasheet for your battery to find out what trickle charge it can stand continually. Any power the battery cannot store ends up dissipated as heat so its not great for the battery. The ideal solution would be to buy one of the inteligent motorcycle charging systems as these check the charge of the battery and only charge it up if it needs it. However that are expensive. So yes it safe to use a regulated suppy but not ideal as it will probably age the battery.

The battery has a the following printed on the side :

Cycle:
• Voltage 14.40V~15.00V @ 20°C

Standby:
• Voltage 13.5V~13.80V @ 20°C

So should I build a 13.8v regulated psu with a resistor in the charging path to limit the current? I am guessing this is required to stop any large surge current when a battery is first connected but also part of me thinks that once the battery is charged this could lead to too much current being passed to the battery.

No, the only difference is the charger will need to delive more current if the batterys are very flat, but the charger should be current limited anyway ( or simply big enough to deliver the current)

Could you give me some leads as to how I should add this current limiting feature?

I have also looked into just using a standard sla battery charger but have noticed that the instructions for the first unit i purchanced said DO NOT connect the charger to AC mains with out being connected to the battery and the second unit said to not pull the charging leads from the battery while the charger is connected to the mains. This is not a huge issue but I was hoping to add a DC line in on the side of the unit and can see that someone could trip over the charge cable and pull it out, basically resulting in the above conditions. One of the store assistance said I should not worry and it will be fine as with nothing connected there is no load but I would like some second opinions.

Surplustronics - Sealed Lead Acid Charger

12V 1 Amp Sealed Lead Acid Battery Charger - Jaycar Electronics

The maximum output of an iPod 0.9 V rms (Nano is 0.7 V). So it depends on the gain of your circuit. Normally you want to be able to amplify the signal at least 6dB into clipping for quiet tracks. To calculate the gain use Gain x 0.9 x 1.41 = 2 x battery voltage - amplifer loss

1.41 is sqrt of 2 and converts from RMS to peak voltage.
The amplifer loss is how close the amplifer output goes to the PSU rail (normally about 1v for digital amps but might be better or worse).
This equation assumes a bridge tied load which you amp is.

The gain of the total circuit is the gain of your power amp x gain of crossover in flat part of response. Solving the equation above for gain gives:-

Ideal gain = (2 x 12) -1)) /(0.9 x 1.41)

=> ideal gain of about 1:36 or 31dB (20 x log gain)

If your gain is much lower than this then you will probably find you can't get the amp loud enough. If its much higher then the amp will sound very distorted with the volume full up.

Ok, I am doing my best to follow along with this.... SO i take it that i need to find a pre amp that has the option of around 31db of gain? I know my active crossover has gain control of the high and lows but just assumed that this was just a reduction on the line level and no actually boost of the signal. If this is correct should I add my boost before my crossover?

here is a kit i have found that will work from a 12v supply:
Pre-Champ Versatile Preamplifier Kit - Jaycar Electronics

thank you Andrew for the in depth reply, I hope i have followed along correctly!

[jestah]

Originally Posted by gootee

Just how big IS this skateboard, anyway?! <grin>

Just a quick comment: To measure the current consumption, if you can't get your meter's current measurement mode to work, you could instead insert a small resistance in series with the positive supply line and measure the voltage across it. Then just calculate I = V / R.

Cheers,

Tom Gootee

Hi Tom,

This is how big it is :

YouTube - Rad Stereo Surfin'

This was the test system that was just a few speakers i found on the side of the road and my cars head unit duct taped to a small skateboard. The system was very fun and had the required effect ( The groups of skaters i teach would stick together and the sound acted as an invisible leash. Also lets be honest there is something rather ridiculous about the concept). Like most things i start learning about i tend to keep going with the over the top theme and have really enjoyed learning about speaker design, amps, sound and most of all compromise!

Thanks for your idea bout using a resistor, very creative, but i have managed to get my DMM to give me readings!

[gfiandy]

I see, should i start by adding 3-4 4700uf low esr caps to the system? What size bank should I aim to end up with and could i use many smaller non low esr caps as a substitute for a few higher priced esr caps?

I wouldn't bother worrying about caps. Also the current will always fluctuate as it is the variable that determines the power in the speaker. It is the voltage that you want to remain stable. If you do want to add caps do it latter once the system is working.

The battery has a the following printed on the side :

Cycle:
• Voltage 14.40V~15.00V @ 20°C

Standby:
• Voltage 13.5V~13.80V @ 20°C

So should I build a 13.8v regulated psu with a resistor in the charging path to limit the current? I am guessing this is required to stop any large surge current when a battery is first connected but also part of me thinks that once the battery is charged this could lead to too much current being passed to the battery.



Could you give me some leads as to how I should add this current limiting feature?

I wouldn't try to design a PSU you yourself if you have bought a supply already.

I have also looked into just using a standard sla battery charger but have noticed that the instructions for the first unit i purchanced said DO NOT connect the charger to AC mains with out being connected to the battery and the second unit said to not pull the charging leads from the battery while the charger is connected to the mains. This is not a huge issue but I was hoping to add a DC line in on the side of the unit and can see that someone could trip over the charge cable and pull it out, basically resulting in the above conditions. One of the store assistance said I should not worry and it will be fine as with nothing connected there is no load but I would like some second opinions.

Surplustronics - Sealed Lead Acid Charger

12V 1 Amp Sealed Lead Acid Battery Charger - Jaycar Electronics

These supplies probably need a load all the time as they are cheap switch mode designs. This is OK we can supply that load.

Chargers of this type are normally arround 13.8V 350mA. I will assume that is the case for these.

If this is the case a charger needs a range of at least 10:1 so its minimum load should be no less than 35mA at 13.8V.

If we apply that load when the battery is removed it should be ok.

To calculate the dummy load.

R = V / I => R = 13.8 / 35 x 10-3 = 394R

It doesn't need to be exact any resistor from about 370R to 470R should be OK.

To calculate the required power for the resistor:

P = I x V => 35 x10-3 x 13.8 = 0.483 or about half a Watt. Since we want it to run cool to the touch I would go for something much larger than this, 2.5W or above should be ok.


You could use a 6.35mm jack plug like this that has a switch over when disconnected to conect the load if the cable is disconnected.

http://www.schurterinc.com/pdf/engli..._4803.2300.pdf

or you could just attach it at the output of the charger and accept the 0.5W of wasted power.

Ok, I am doing my best to follow along with this.... SO i take it that i need to find a pre amp that has the option of around 31db of gain? I know my active crossover has gain control of the high and lows but just assumed that this was just a reduction on the line level and no actually boost of the signal. If this is correct should I add my boost before my crossover?

31dB is the total gain required. I would look for an amplifer that has about this gain, many do, then you can just use the volume control in the iPod.

here is a kit i have found that will work from a 12v supply:
Pre-Champ Versatile Preamplifier Kit - Jaycar Electronics

See above, probably not required.

thank you Andrew for the in depth reply, I hope i have followed along correctly![/QUOTE]