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Old 2nd February 2007, 11:06 AM   #1
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Default Gainclone LM4780 (poor bass ?)

Hello I look to buy GainClone with LM4780 (para) 1x 120w but many people said LM chip not great bass or no bass, blabla.......


what's the modifications to make to have BEST AMPLIFIER ?

What's the best +/- volt it's 30v or 35 v (for 120w 4 ohm load)

This amp have very little power supply Capacitor !
(no power reserve Capacitor for punch bass)

Do you add TANTALUM CAP for more great sound ?


WHAT CHANGES YOU HAVE MADE ??? (with good result only !

Thanks !!!

Nick
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Old 2nd February 2007, 11:35 AM   #2
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add a good bass filter or bass boost circuit and see the bass quality output.
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Old 2nd February 2007, 11:37 AM   #3
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On my last active system with 4780s, I used 1000uF on the highs, 2200uF on the mids and 10 000uF on the bass.
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Old 2nd February 2007, 12:09 PM   #4
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Quote:
Originally posted by pinkmouse
On my last active system with 4780s, I used 1000uF on the highs, 2200uF on the mids and 10 000uF on the bass.
I would say at least 10,000 uF on the bass.

The other thing, of course, is the input coupling capacitor and associated circuitry -- you can use a pair of electrolytics bypassed with a small polypropylene cap for this.
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Old 2nd February 2007, 02:02 PM   #5
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Put BIG caps on the power rails. Bypass them with smaller caps.

I_F
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Old 3rd February 2007, 12:53 AM   #6
Leolabs is offline Leolabs  Malaysia
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10,000uF+3.3uF+0.1uF
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Old 3rd February 2007, 09:48 AM   #7
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The only way to get good bass is to have big PSU caps. It's mathematical.

I'd suggest you to use 1000 or 2200 F on the chips + 2*10 000F on the PSU board.


You can calculate the needed capacity with this formula :

P is the power you need
R is the impedance of the load
U is the RMS voltage at the terminal of the load


P= U/R

Once you've got U, you calculate the equivalent voltage Ur of the supply rails :

Ur = U*root(2).

I'd suggest you to use the highest possible rail voltage.


Then you calculate C :

Uc is the voltage at the terminals of the cap
t is the period of the discharge cycle (time during which the capacitor is discharging)


Uc = Ur-Ur*exp(-t/(R*C))

With 50Hz mains, you have t = 1/100

Then you can calculate for a given C and a given Ur the real voltage Uc at then end of the cap's discharge cycle.

So you deduce the real uncompressed power you can get from your PSU :

Preal = Uc/R




From these formulas, you can see that your maximum output power is limited by the rail voltage and the caps of the PSU.


Don't forget that the amp also takes juice and has a voltage dropout, so increase the capacitance value by a 1,5 or 2 factor.


I have LM4780 amps in bridged mode and they work very well.
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Old 3rd February 2007, 10:13 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi You,
I would like to compare the numbers predicted by your formulae with other recommendations.
How does this work?
Quote:
Uc = Ur-Ur*exp(-t/(R*C))
I can do the (-t/(R*C)) for 8r and 10mF =-0.125 but what operator is exp applied to Vr?
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Old 3rd February 2007, 10:19 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi all,
I queried the recommendation of using <=+-2.2mF on the chipamp as the sole smoothing/decoupling cap for a wideband amplifier in another thread.
All to a man/woman insisted that these wideband amps sounded best when the caps are kept small even suggesting that +-1mF was optimum.

It appears our thread starters has been following this advice.

Now I see the completely opposite recomendation if one requires bass rather than the more onerous wideband response.

What goes?
Are there two camps in the Forum, [big smoothing+decoupling] and [big decoupling]?
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Old 3rd February 2007, 10:46 AM   #10
owen is offline owen  United Kingdom
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I'd avoid a large 'lytic... and rack up 20 or so 1000uF, with a little extra bypass.

I'd also include some small chokes (high amp, low DCR, inductance doesnt need to be colossal - you're using it to hammer RF grunge) in all 3 rails (+, - and Ground) - make the star earth point after the chokes.

I would also adopt a 'Pass' trick - run some of the capacitance across between the + and - rail.

Local bypassing of the chip is useful if you have marginal stability, or RF grunge present on the rails, to stop anything that might excite oscillations.


Have fun!


Owen
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