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Old 26th March 2004, 07:14 PM   #1
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Angry increasing current output

I have a Kenwood ka-127 amp and I want to find out how I can increase the current it puts out. The rails are +/- 80v, but I dont think that the amp puts out full current. It's driving two 3-ohm subs, one on each channel. When I ran the subs off a homemade amp, it was a lot louder with the same speaker excursion. What can I do to increase the current that the ka-127 puts out?
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Old 26th March 2004, 07:47 PM   #2
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If the speaker excursion is the same at all frequencies the loudness will be the same. I do admit that some set of circumstances might make this statement less than 100% accurate.

It's difficult to judge excursion by looking at cone movement.

Is it possible that the subs are wired out of phase? It's possible that one amp is inverting and the other isn't. This would mess things up at the crossover freq. Try reversing the phase of both subs.

It is also possible that the amp is clipping.

Anyway, you don't want to mess with the amp. With near 3 ohm loads it's pushing it's limits already.
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Old 26th March 2004, 07:58 PM   #3
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The subs are not out of phase, and the amp isn't clipping. At low volumes, I was getting louder sound with power from my homemade amp than from the ka-127 with about the same speaker excursion.

One more question: is there a way to control current with transistors, or just voltage?
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Old 26th March 2004, 08:36 PM   #4
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Well then, my next guess would be that it's just a matter of different amplifier sensitivities and you are mistaken about excursion.

It would help to have more information about your setup. You are bi-amping I presume?

The voltage output of an amp is limited by the supply voltage less various losses. The current output is limited by the same plus the current capabilities of the output transistors and the heat sinking. To get more current and assuming the power supply can deliver it, add output transistors in parallel. Of course, it's not THAT easy.
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Old 27th March 2004, 02:54 AM   #5
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I agree that it is hard to compare amp looking at speaker cone excursion.
But, I would look at the Power supply current capability (big Electrolytic caps) of your Kenwood amp. What are their value? what are the value of caps of the homemade amp?

The circuit topology used in an amp can make also a difference on the perceptive power.

Also, the DC blocking feedback capacitor (if present) or DC blocking input capacitor (if present) may have a too low value in respect with the resistors making the high pass filters. I found by experience that a lower low frequency cut-off contributes to a higher "loudness" at same input voltage level.
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Old 27th March 2004, 03:36 AM   #6
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How have you determined your amplifier isn't clipping? forget any indicators on the amp they're not showing you the real picture, I hate to be the one to tell you but if you want clean bass and lots of it , then look into an american made amp with some muscle, I dont like japanese audio gear, the mainstream receivers and amplifiers they produce are heavily current limited to protect their fragile output devices , check the specs to see what kind of power an amp will do into low impedances, if it comes close to double the power at half the impedance, then your on the right track, if the 4 ohm power is barely more than the 8 ohm rating what does that spell? low current capability! low because the amp was designed to stay together instead of sound good... If your good with amps then remove the current limiters from the driver stage in the thing, it will drive your speakers much much better, but at the expense of no overcurrent protection, and if you drive it past the soa , well ya know, good luck!
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Old 27th March 2004, 03:55 AM   #7
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For the vast majority of commercial products--and this almost certainly includes your Kenwood--the answer is no, you can't safely do anything to increase the current output.
Unless the amp is spec'ed down to a 3 ohm load (unlikely), it may actually be delivering less power than at higher impedances. How? Current limiting. Broadly speaking, the rail voltage determines how much wattage an amp can produce; the bias current determines how low an impedance it can work with. If you have sufficient bias, an amp will double its wattage every time the impedance halves. Without that bias, you're not going to get the power.
You can get tangled in semantics, but transistors will always deliver exactly the right amount of current for the voltage they develop into a given impedance. Always. It's one of those Ohm's Law things. That's not to say that it's as much power as you might desire, but that's another question entirely.

Grey
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Old 27th March 2004, 06:24 AM   #8
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Actually bias current in a class a/b amplifier has no impact what so ever on current capability. the bias is there to elimanate "crossover distortion" its a really simple fact, the amount of current being supplied to the bias device from the class a driver and current source (common to most audio power amplifiers) partially determines the current, but its the darlington output stage transistors hfe (gain) that will determine how low an impedance it can drive, triple darlington output stages are usually used for high current, Like I said before the current limiting is there to protect the amp, The amplifiers I build have no electronic limiting at all, I sense it through the emitter resistors but send the overcurrent command to the speaker protection relay that way it will just remove the load if the current gets crazy, not clip the tops off high energy peaks at all, regards
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