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Old 30th April 2013, 01:29 AM   #1
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Default Fishman Loudbox

So I have an original Fishman Loudbox from 2003. Really nice amp. I want to use it not just for guitar but also as a mono speaker. The frequency response is 100Hz - 20kHz 4dB. The unit is a triamped design and there is a crossover somewhere on the board before it goes to these amps. I attached the schematics. I know its a lot of surface mount components, but I can solder and desolder them. I want to bring the response down to 20Hz if possible, that way it can go from 20Hz-20KHz like a regular amp can. Also the woofer is a 2Ω impediance. Very low compared to traditional speakers. The specs claim that it can pump 160W into a 2Ω load. The main voltage rails seem to be -33V and +33V. I find it hard to swallow that only one pair of outputs for the woofer can deliver 160W. The outputs are 2SD1842 and 2SB1232. Although to be fair, they are rated at 40A and 100V, so I assume these outputs are very rugged.

The tweeters are driven by a LM3886, and the midrange driver has it's own LM3886 output IC. Filter caps are 10,000mfd for the record.
Attached Files
File Type: pdf LoudboxPowerAmpPCBrevBassemblydrawing.pdf (42.3 KB, 28 views)
File Type: pdf LoudboxPowerAmpschRevGP1.pdf (15.4 KB, 32 views)
File Type: pdf LoudboxPowerAmpschRevGP2.pdf (15.3 KB, 30 views)
File Type: pdf LoudboxPowerAmpschRevGP3.pdf (15.7 KB, 27 views)
File Type: pdf LoudboxPowerAmpschRevGP4.pdf (13.0 KB, 30 views)
File Type: pdf LoudboxPowerAmpschRevGP5.pdf (14.0 KB, 29 views)
File Type: pdf LoudboxPreampV2revAassemblydrawing.pdf (33.0 KB, 24 views)
File Type: pdf LoudboxPreampV2revAP1.pdf (17.9 KB, 32 views)
File Type: pdf LoudboxPreampV2revAP2.pdf (22.5 KB, 28 views)
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Old 30th April 2013, 04:20 AM   #2
Enzo is offline Enzo  United States
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I suspect your 160w rating is for music, not continuous sine waves.

Have you actually run signal through the amp at low freqs to see the amp response? I'd imagine a large part of the freq range is deteremined by the woofer, rather than the amp. After all, it is only an 8 inch speaker. How far down at 20Hz is the amp itself? And does adjusting the EQ get you there?

The crossover would set the cutoff for the woofer upper end, not its lower end. Page GP4 is the crossover though.
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Old 30th April 2013, 03:31 PM   #3
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It isn't the woofer. I connected a regular subwoofer to it, and same behavior. Granted it can play sub 100Hz frequencies, but not very well..
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Old 30th April 2013, 03:46 PM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
I want to bring the response down to 20Hz if possible, that way it can go from 20Hz-20KHz like a regular amp can.
Forget it.
As Enzo says, the limiting factor is not the electronics (they might go down to DC if needed ) but the speaker.
It's a ***very*** well designed amplifier+speaker combo, Loudbox people know their trade very well and it took the Music World by storm for very good reasons; you'll only mess it if you feed it lower freqs than expected.
And, quoting Enzo again ... it's an 8" woofer after all .
Feeding it below its natural response point will make it fart and flap, while still not providing true bass.
*If* you still want pants flapping bass, add an external proper crossover active subwoofer.

Or sell it to an Acoustic Guitar player who appreciates it for what it is and buy a Hi Fi system with the cash.

PS: THANKS for posting the schematic
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Old 30th April 2013, 04:16 PM   #5
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I do play acoustic guitar and thats why I have it. I just wanted to give it double duty.. I'll just leave it alone.
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