Increase output of Audio Amplifier. Very simple amp.

[fastline]

I have a desire to try and upgrade a factory subwoofer amp in a car. This is a Bose design with two subs integrated in an enclosure. The amp is one channel. The whole amp is 3x3" and not much to it.

I dabble in electronics but never tried to boost the output of an amp. Not sure if this is basically a complete amp redesign or if replacing a few components can make all the difference. Does anyone have enough board level experience to discuss this?

 

[ronsimpson]

I don't think you can increase the output. If the designer could get one more watt out without add much cost he would have.

The limiting factor in a car amplifier is voltage. With only 12 volts (14 volts if the engine is on) you can not get many volts across the output.

You don't have a schematic so modifying circuits (randomly) will not end well.

 

[fastline]

I probably have an amp worth about 0 dollars in the used market so any gamble to improve this system might be considered. I have to wonder how you get 1000w amps if the voltage is so limited? From the looks of this, my rail voltage would likely be from the drain of each of the two mosfets? One is positive and the other is negative?

 

[MikeMl]

Oem car stereo systems tend to use low impedance speakers, like 2 or 3.2 Ohms, to get more power starting from only ~4Vrms (12/√2).

Another trick is to use two identical amps driven out of phase where the speaker is bridged between the two amp outputs. This effectively quadruples the audio power.

 

[Brevor]

I have to wonder how you get 1000w amps if the voltage is so limited?

A lot of those specs the manufacturers claim for automotive sound equipment are not worth the paper they are printed on.

 

[picbits]

Actually a lot of the higher powered car amplifiers have switch mode boost converters inside which boost the input voltage to +/-60v and above for the amps to use.

 

[fastline]

Well, that might be something to consider Mike. Seeing as how these amps are next to no cost, I could possibly get another one like it and either bridge each amp to each speaker or try the configuration you are talking about.

I believe I did determine this is a two channel, not a mono amp. there is continuity between the pos of one speaker term and neg of the other term. I would expect to see two pair of matched terminals for a mono design.

If that is the case, it might be possible to bridge this amp and get another one like it for the other speaker. Of course this could fail miserable....

 

[unclejed613]

the way they get so much wattage is they use a switching supply (dc-dc converter) with the required output voltages. for instance, for a 200W (into 4 ohms) amp, the converter takes 12V DC, chops it at 50khz or so, and runs the high frequency square wave through a transformer which steps it up, and then through a rectifier and filter, and you get +/-65V out of the rectifier. this becomes the power supply for the power amp. additional secondary windings and rectifiers provide the +/-12V or +/-15V for the preamp and bass filters. be aware that although only about 7.25A is provided to the speaker (at 200W), the input current to the switching supply is over 16A. it's actually a bit higher because the power supply's efficiency is usually 85-90%. such a switching supply is not something for beginners, and will not work well with the amp you already have. you would be better off replacing the subwoofer with one that has a higher sensitivity than the one you currently have. i'm guessing that a Bose sub probably is rated about 80 to 85db@1W@1 meter. if you replace the sub with one that has a sensitivity of 90 to 95db@1W@1 meter, you have just increased the amount of acoustic power by a factor of 10, so if your amp is a 10W amp, it would be equivalent to replacing a 10W amp with a 100W amp.

 

[audioguru]

The amplifier is probably already two bridged amplifiers producing about 15W at clipping into 4 ohms when the battery is charging at 13.8V. A bridged amplifier IC is inexpensive. You cannot bridge it again.
The speakers are probably 2 ohms each and are in series then make a "one-note-bass" sound at resonance (like a bongo drum) since in series the extremely low output impedance of the amplifier cannot damp their resonance.

 

[unclejed613]

actually, i think it may be worse than that.... in a bridged amp, the output Z's of the two amplifiers are in series and floating, which reduces the damping factor to 1/2 of a single amp. a typical TDA amp chip has an output impedance around 500-800 milliohms. bridged, this would be 1 to 1.6 ohms. DF is Zl/Zout, so with a 4 ohm load DF is between 3 and 4... very poor, and the sub's cabinet resonance will dominate the sound, and that's why there's so much "one-note-bass". class D amps also have generally poor damping factors.

 

[audioguru]

actually, i think it may be worse than that.... in a bridged amp, the output Z's of the two amplifiers are in series and floating, which reduces the damping factor to 1/2 of a single amp. a typical TDA amp chip has an output impedance around 500-800 milliohms. bridged, this would be 1 to 1.6 ohms.

Why do you think the output impedance is so high? The amplifiers have a very high open-loop gain and TONNES of negative feedback.

I think each of the two amplifiers has an output impedance of 40 milliohms or less. Then a speaker's or its cabinet's resonance is very well damped even if the total of two bridged amplifiers is 80 milliohms.

Remember the poor damping of speakers by an old vacuum tube amplifier? Some guitar speakers have two speakers connected in series and are driven by a vacuum tube amplifier for a very boomy sound. Maybe on purpose since the speakers did not produce bass sounds anyway.

 

[unclejed613]

you're right.... i calculated the output resistance, not the output impedance.... the output impedance is the output resistance divided by the gain margin. still the damping factor won't be all that great. cheap amp chips don't have a large open-loop gain (about the same as a 741), so their distortion is relatively high and damping factor relatively low.

 

[Nigel Goodwin]

you're right.... i calculated the output resistance, not the output impedance.... the output impedance is the output resistance divided by the gain margin. still the damping factor won't be all that great. cheap amp chips don't have a large open-loop gain (about the same as a 741), so their distortion is relatively high and damping factor relatively low.

The keyword there is 'relatively'

Distortion is low and damping factor high, both will far exceed what's needed.

The importance of damping factor is also greatly exaggerated here, with series/parallel connection of multiple speakers making no audible difference.

However, this doesn't help the OP - he needs a completely new amplifier, there's nothing he can do to 'upgrade' the existing one. It's also far cheaper to buy one than try and build one, although he does need to be aware of the imaginary power output claims for in-car amplifiers.

 

[unclejed613]

i repair a lot of Kenwoods and Alpines.... Kenwood's published RMS power specs seem to be a few percent on the conservative side (their 300Wrms amps usually measure about 325W with a 13.8V supply).... Alpine amps on the other hand are usually pretty close to the edge of their published specs (usually a few watts shy of spec at 13.8V), but both companies specify their wattage at 14.4V. as for "off brand" amps, i agree, their "whattage" ratings are exaggerated (AG, i hope you don't mind i borrowed your term for bogus specs?). any manufacturer that says something like "300 watts @ 10% THD", avoid them like the plague... what that really means is more like "270 watts @1% THD with a 40 MPH tailwind, on a tuesday"...

 

[Nigel Goodwin]

i repair a lot of Kenwoods and Alpines.... Kenwood's published RMS power specs seem to be a few percent on the conservative side (their 300Wrms amps usually measure about 325W with a 13.8V supply).

I've underlined the relevant part - if it doesn't say RMS then it's totally meaningless and wildly exaggerated.

 

[audioguru]

Peak Power or Maximum Power are terms that are used by cheap amplifiers and speakers. They are simply double the true RMS power because the amplifiers are playing square-waves at full blast (Acid Rock "music").

Frequently the spec's for a cheap amplifier or speaker do not say "continuous power" because full power output is only momentary before the amplifier or speaker overheats and/or before the power supply voltage sags.

 

[WTP Pepper]

I once saw a car kitted out with a 1000W RMS system at a Motor Show. The wiring was impressively huge with several 1 Farad Caps to smooth the input supply from an extra huge battery and a seriously uprated alternator. All combined with a switchmode PSU about the size of....erm another smaller car in the boot. It had and probably needed a 4 litre engine with about 2BHP once the stereo was switched on.

How anyone would/could listen to anything over 1watt in a car is beyond me.

I agree with previously posters and often seen on PC speakers systems. 1000W Peak Music Output Power from a plastic speaker and a power supply no larger than a tin of beans. Usually equates to about 10W RMS depending on the advertising and their choice of maths. Meaningless.

 

[audioguru]

I bought this powered speaker system for $25.00 a few months ago. It is advertised at 150W but inside its little power transformer is labelled 9VAC/1.1A which is 9.9W. Its amplifiers are about 50% efficient so its full output power is probably only 5W.

But it sounds great and it looks good.

 

[Nigel Goodwin]

How anyone would/could listen to anything over 1watt in a car is beyond me.

You must have a seriously expensive car then do you drive a Rolls?.

A standard bridged car amplifier (without inverter) can provide 16W RMS in to a 4 ohm load, four channels give you 64W RMS, and it's not difficult to clip the amplifiers, particularly on peaks.

 

[audioguru]

My first job was with Philips repairing car radios on the production line. Then I was quickly promoted to engineering new car radios.
The first car radio had no ICs and the output transistors were complimentary power germanium types. The radio had two small output transformers to stepup the output voltage to provide 4W into 8 ohms. The tape player was 8-track.

I played with Philips' first LED and their first compact cassette tape recorder/player. I think we had the first car radio that used a compact cassette tape recorder/player.