Hi all,

I'm currently in the process of trying to get my first product manufactured - it's basically a cooler with an audio system installed (we're currently hand-assembling them with off-the-shelf components and we've gotten great feedback from our customers). I'm here to ask for some guidance in designing the amplifier and pairing the right set of speakers to it. Hopefully this is the right community to help me out!

We're looking to use a 100W RMS amplifier and 2 6.5" speakers. Power supply is an 8 A-h lead-acid battery. Sound quality is important, but it must come at a good value.

The amplifier must have the heat sink, RCA/3.5mm inputs, and gain control on one face. The other 5 faces will essentially be surrounded by insulation (the amp will be recessed into a special compartment in the cooler). Will this cause overheating issues? Or is it not a problem, as long as the heat sink is exposed to airflow? Also, I doubt we'll be able to find an aesthetically-pleasing amp that meets our "one-face" requirement. So we might end up getting the housing custom-made, then buying the amplifier electronics. How would I go about finding amplifier electronics without a housing?

And what specs are most important for the amp? Here's what I have so far:

Power: 100W RMS/200W Peak at 4 ohms
Frequency response: 20-30k Hz
THD at rated power: <0.05% at 4 ohms
Damping factor: >500 at 4 ohms / 50 Hz
Signal-to-noise ratio: >95dB below rated power
Input impedance: 40 kohms
Output impedance: 2-8 ohms
Class: AB or D
Fused
CEA-2006 compliant

Anything I'm missing?

Sorry for all the questions, and thanks for the help!

Andrew

 

The spec for the output impedance of the amplifier is confusing.
An old vacuum tube amplifier had an output impedance of 2-8 ohms. But your solid state amplifier with an excellent damping factor of 500 at 4 ohms has an output impedance of 4/500= 0.008 ohms. Maybe you should say that the load for the amplifier can be 2-8 ohms (but then the max power will probably be different into each one).

Do the 6.5" woofers have attached tweeters?
Do the woofers have big enough enclosures for good bass?

 

I suppose the battery is 12V, I don´t see how you can get 100W into even a 2ohm load, unles you´re using a step-up converter which seems rather costly.

 

Also, watt is up with watts RMS? There is no such thing as "RMS" for power (watts).

 

ALL half decent amplifiers are spec'd with the power rated in "continuous RMS Watts into a certain resistance with a certain maximum distortion".

Cheap amplifiers are rated with "peak power, maximum power or music power". They cannot produce a continuous high output power and their horrible distortion is not spec'd at rated power.

 

Also, watt is up with watts RMS? There is no such thing as "RMS" for power (watts).

Where did you hear that? There is RMS voltage and current and yes, RMS watts. Tom, me thinks you better hit the books.

 

The TDA7560 amplifier IC typically produces 308 Whats into 2 ohms or 180 Whats into 4 ohms when its supply is 13.7V. This is the total of its 4 channels. The output is clipping like mad with 10% distortion and the input is pre-distorted.

It produces a total of 112 Watts continuous RMS into 2 ohms at low distortion (0.02% at 1khz) with a 13.7V supply.
It produces a total of 67 Watts continuous RMS into 4 ohms at low distortion (0.01% at 1khz) with a 13.7V supply.

 

I suppose the battery is 12V, I don´t see how you can get 100W into even a 2ohm load, unles you´re using a step-up converter which seems rather costly.

If the OP is really aiming for 100W RMS continuous(!), that battery had better be a lot more than 12V, especially if it's only 8 Ah capacity. 12V and 8Ah only comes to 96Wh, so it's not going to last long, without even thinking about the efficiency. Of course, it won't be full power all the time, but once you start draining too much current from a battery, its effective capacity drops miserably.

For 100W output, you are going to need a big energy store to work for long. Noting that this thing includes a cooler, how is that powered?

 

Noting that this thing includes a cooler, how is that powered?

I sure hope it's powered with ice

 

Like... a passive cool box with only ice (or dry ice?) to keep it cold , and someone wants to attach a 100W mega-boom-box to the side of it. Wow!

 

Dropping the power spec, which IMHO is too large for this application and those speakers, would make this a much more realistic project.

Have you ever looked through a Parts Express catalog? Seems like they have the kind of stuff that might give you ideas, if not a solution.

 

I've seen this fellas products being sold at auction and he has used automotive amps, which are rated at very high power outputs. That is probably why he is speaking of 100 watts rms. But automotive amps only produce that output into ultra low 1.6-2 ohm loads, often in bridge config to drive subwoofers.

I'm inclined to think that the 100watts he is referring to is far different than 100watts we are accustomed to from home audio gear. If you check out automotive gear, they are indeed frequently advertised with very high output specs.

 

Where did you hear that? There is RMS voltage and current and yes, RMS watts. Tom, me thinks you better hit the books.

Read this: http://www.hifi-writer.com/he/misc/rmspower.htm
http://www.google.com/url?sa=t&sour...FlpNtrl6A&sig2=xW0wjnvUk-1f_uUx9OItQA&cad=rja

 

As I said previously, cheap audio products have their output power rated in Whats, not Watts. The Whats is the instantaneous power into a resistance that is much too low times the age of the advertiser's grandmother.

Look at an aftermarket car radio that is advertised as producing 200 Whats or 220 Whats. Distortion at the rated power is not mentioned so it is probably clipping its head off at 10%. The battery voltage is not mentioned so it is probably over-charged at 14.4V. It does not say continuous power so it is instantaneous to avoid smoke.
It has four bridged channels and each produces 15 Watts into 4 ohms just before clipping when the battery is 13.7V so the total continuous RMS power is only 60 Watts.

Very high power car amplifiers use a 12VDC to 70VDC (or more) converter to power a normal high power amplifier.

 

Thanks for all the input, guys. To clarify - this is a passive cooler, with a 12v car audio system. Similar to this, but with a much bigger audio system. I've been using a 12v 8A-h sealed lead acid lawn mower battery, 2 6.5" co-axial speakers, and a 2-channel "200W" amp - advertised as 200W peak/100W RMS, which I know is not a technically-sound description, but that's how they describe their products.

Arm-chair science aside, this system is decently loud (loud enough for a BBQ or beach party) and last up to 8 hours at full volume (no clipping). My design intent is not high-fidelity, but mass-market appeal. It needs to be loud and clear enough for a party. Nothing more, nothing less.

Can anyone "fill in the blanks" for an appropriate set-up for the application I described?

Amplifier specs
Power: XXXW RMS at X ohms
Frequency response: XX-XXk Hz
THD at rated power: <X.XX% at X ohms
Damping factor: >XXX at X ohms / XX Hz
Signal-to-noise ratio: >XXdB below rated power
Input impedance: XX kohms
Output impedance: X ohms
Class:
Fuse rating : X amps
CEA-2006 compliant

Speaker specs
Sensitivity: XX dB
Peak Power Handling: XX Watts
Impedance: X Ohm
Frequency response: XXX-XXXXX Hz
Magnet structure: XX Oz.
Efficiency: XXdB (X Watt/1 Meter)

Speakers will be easy to find, but the amplifier has to be a custom-fit. Does anyone know where I could buy just the amplifier electronics? And how easy would it be to be to get them to interface with a custom housing?

 

As I explained, you cannot get 100W RMS into 4 ohms from a 12V battery.
the battery will be about 13.7V when charged.
A single-ended amplifier will have an output of about 11.7V p-p which is 4.1V RMS. Then the power in 4 ohms is only (4.1V squared)/4= 4.2 Watts. Stereo is a total of 8.4 Watts.

A bridged amplifier will produce about 22V p-p which is 7.8V RMS. Then the power in 4 ohms is only 15.2 Watts. Stereo is 30.4 Watts.

Where will you find space in a cooler for two huge enclosures for the speakers? Two enclosures will be the size of a cooler.

 

Since you're sticking an audio system into a cooler, I'd say your least important specs are THD and signal to noise. It has to compete with drunken barbequers/campers/tailgaters, so something must be sacrificed.

 

Gotta chime in on this....
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Car amp (receiver) rated at 200W Peak (100W RMS)????
You mean a receiver with a 22-Watt + 22 Watt built in amp.....
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These are usually advertised by shady Audio Resellers as 200-Watt Car Radios...
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Fact - You do not need 200Watts or even 100 Watts for automotive Hi Fidelity... 35+35 would be quite loud enough (wouldn't rattle the doors, but would be more than loud enough).
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Yes there is such a thing as RMS Audio Power... Been around since the early 60s when the Audio Industry Standardized. The need was driven by all the fly by night audio experts who claimed they were producing 100 Watts of pure audio power with a 9 Volt Battery powered radio. One where the battery only had to be replaced once a month (?????).
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Peak Power is often advertised as continuous power (but it isn't.... RMS means that it is .707 times the peak instantaneous power using a calibrated and verifiable sine wave signal source into a specified Inductive load - speaker). The reason RMS was so widely adopted was because it eliminated all the Hype. 10 Watts RMS into 8 Ohms is 10 Watts RMS. If you apply it to a 4-Ohm Load it is roughly 20 Watts RMS 4 Ohms... Dah.... So many 60s 100-watt Hi Fi amps were not as loud as some 35-Watt amps - why? Becauswe the 100-Watt Amps were Peak measured into a 2-Ohm load, and the 35-watt amps were measured using an 8 Ohm load....
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In the real world we reference to db's using a reference of 1 miliwatt across a 600 ohm load as 0dbmw. 0dbw is 100 times that 1milliwatt multiplied by 100 is 1 watt., 0drm is always referenced to a specific impedance load and power rating (1 milliwatt and 600 ohms), so 3dbrm0 is actually 2 milliwatts (every 3 db the signal doubles). The noise floor of pro audio is measured using a negative scale with 0dbrm0 being 0 (zero). -30 dbm is 1/1,024th the signal power. So naturally -60 dbm is really quiet.
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Trust me on this... In the late 60's and mid seventies I built numerous 500-Watt and even a 1.5KW amps system. I am partial to the RMS power factor calculations and do not understand why some one with more than average intelligence cannot realize that it is True Power - not hype...
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Incidentally My company in 1975 manufactured a 4-Channel Stereo Receiver with Quad 35-Watt RMS 8-Ohm amps... We only made and sold over 1,400 of them. They were plenty loud and we never got a single complaint. The power amps were simple designs an NE540 Driver OP amp feeding a pair of Complementary transistors.
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There were 10-times as many components in the preamps than the power amps. I still have a few, they are amazingly loud...
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Today the push is to class D amps.... class D amps are all about the instantaneous power, and require high efficiency speaker systems..... Please - don't get me started on "Digital Speakers"... ugggg....
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By the way - this is the key to Hi Fi... High Amp receivers and low efficiency speakers pushing the same sound pressure as a low power amp receiver driving high efficiency speakers... So the key is to match the speakers and amps... everything else is somewhat mute or a matter of personal preference...
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You want actual true Hi-Fi? Flatten your listening room response. This is fact - more power is wasted in an acoustically challenged room in over coming ambient noise, incidental and indirect sound reflections, just plain noisy reflections require lots of amp power to overcome the noise. Try playing a small transistor radi0o in a room full of chattering people.....,. Use acoustic isolators for your speakers (in Jr College we used two carpet squares stacked with the faces together, and put the speakers on them).. In broadcasting we put 150 lbs of sand in the turntable base cabinet and still used acoustic isolation at the cabinet base... Use base traps behind your speakers, and sound traps in the acoustically active corners and at key points in the room. In college we had a college radio station in the basement, we hung quilts on the walls (even did the egg crate thing - highly over rated, but it helped)... A couple of $35.00 Acoustic wedges in the corners (opposite the speakers) and a few acoustic traps on the walls opposite the speakers will improve your listening, in fact it would probably work so well you will find you don't need all that excess volume anymore... Dah....
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Me I like my Pioneer SX-880 Receiver driving my 2 pairs of Bose 502 Series two speakers. Yes I have two SG-9500 EQs, and two RG-1 Companders, and two SR202Ws - I have three 10.5" Reel/Reels, Two Cassette decks, and two CD Decks (one single CD Deck and one changer). I also have two SAE dynamic EQs and a pair of DBX-266's that I use with my mixer console. Five Guitar Preamps, Nine Vocal Preamps, Five SFX Processors, two Shure Mic/Line Mixers and two 16-Track DATs. Yes I have too much, and I have been doing pro Audio, Broadcast Engineering since the mid 60s..... Even weezled the time to spend 24 years in government service as a Satellite Systems, Microwave, Tropo Scatter Comm (You know the 800 MHz 10KW, 50KW and 100KW long range telecommunications radios that were pushed out by Cellular services....), Telephone switching systems, and Data Communications Technician/Engineer... Owned three Radio Stations, a recording Studio, A Custom Electronics Manufacturing Company, and still own the electronic design systems company (we designed the thingy called magic jack, and a few other USB interface devices).
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Oh yea, I also have two Studio Computers with Soundblaster sound cards and properly matched line preamps to feed into the Stereo and Mixer. Sound really good - but that is what I get paid the big bucks for - to do it right every time.
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But I am digressing... (Old age does that to you....).. It's just I cannot understand why the young uns don't pay attention to physics... I also understand this thing called the Internet, how anyone can make claims and then a million people will read it and believe it....
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Back to the subject at hand....
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The other thing is your speakers: 6 1/5" speakers (assuming dual cone, maybe triple cone) would take only 10-20 Watts to drive). In the sixties it was pointed out that only about 14-Watts was actually required for Hi Fidelity (assuming you had a properly matched system and quality amp), sure listening to a piano concert or any classical music would require a bit of sonic head room, but a 14-watt RMS amp driving it's rated speaker load, and containing power supply loading caps with sufficient reserve would be capable of reproducing those 19.5KHz highs at an instantaneous 42Watts without clipping...
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Speakers may say they are rated at say 60Watts or 120 Watts, but they really don't like that much power. Hint: The bigger the speaker coil (the more peak power they can handle) - the more power it will take to drive them efficiently. Match your speakers and amps PLEASE....If you use a 22w+22W Audio Amp IC made for automotive you will be more than happy with the results... (You can even use less exotic and less expensive speakers - will save you money) Using 3,300uf input power caps will more than give you enough instantaneous power for audio peaks.. Yes I said instantaneous power.... How do you think they make a good amp sound so good> Expensive Transistors? Get real... Most of all - Get with the program... That's what a good audio engineer gets paid to do - make it work right...
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Maybe you could share the model number and make of your receiver..... Would go a long way towards shedding some light on your predicament.
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Now for my main interest::::
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Who is doing your plastic work?
I am sure Coleman isn't making custom coolers for you (or are they? If they are, then I would assume you are in a partnership arrangement?)..
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So what exactly are you looking for? An out of the box Audio Amp solution, or somebody to do the manufacturing. Some one to press out the cooler boxes, or just some one to actually assemble the components in volume. (I have all the resources to do all of it at my disposal and I am interested....).
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I believe you can find my eMail in the members section.
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Dave
Phoenix, Arizona.

 

A few years ago there were many car radio amplifier ICs that are obsolete and not made today. Philips made a bridged amplifier IC with a voltage doubler built in and it produced a lot of power.

Today Philips Semiconductors is called NXP Semi and produce the TDA8594 quad amplifier IC. It is rated with a 14.4V supply and produces 22 Watts RMS into 4 ohms at low distorion or 37 RMS Watts into 2 ohms at low distortion per channel.

They also make the TDA8566 two channels amplifier IC that produces 30 RMS watts into 2 ohms at low distortion per channel.

 

mcgurkles- Your eMail doesn't work... I sent you a reply and it bounced back...
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Just how much capital are you and your group investing here?
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Are you going to continue modifying off the shelf components?
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Have you considered a peltier heat exchanger?
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The radio stuff is somewhat remedial (except when you start designing actual receiver/tuners. For that you can use plug-n-play modules. The LCD front screens are somewhat standard as well - so you can save some R&D bucks there too...
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eMail with a good address..
Dave
Phoenix, Arizona