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Old 12th April 2013, 03:37 AM   #1
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Default Need a little help with a L 20 amp

My 15 year old nephew built his own speakers so I figured it would be cool if we built an amp to power them with. I bought a L 20 amp kit from china and taught him how to solder it up. I picked up a broken amplifier off E-bay for the case and transformer and put the new L 20 amp boards inside. The broken amp was a Samson Servo 240 and was air cooled so there is a nice heat sink on the back to mount the new L 20 boards to. The transformer puts out 55 volts DC after the bridge rectifier which should match up nicely to the L 20 boards. I was also able to reuse two 10,000 uf 71 volt capacitors from the old amp.

The question is do we need a pre amp for this project? My nephew plans to run his I pod or a CD player threw this so they are at line level, but when I tested out the amp with my I pod I ran out of gain really fast and wasn't able to get the full volume out of the amp.

The amp sounds great with no hiss or hum, but I can't get enough volume out of it. I have it hooked up to a smaller PA monitor speaker which is good for 50 watts and can barely make it distort before running out of gain on the I pod volume. I hooked up a 100 watt sub woofer and easily maxed out the volume with out any distortion. The amp is no ware near clipping.

Please let me know if I need a pre amp for this amplifier and if ether one of these boards would be a good choice.

Thanks, Dale P.

http://www.ebay.com/itm/320915557448?ssPageName=STRK:MESINDXX:IT&_trksid=p 3984.m1436.l2649

http://www.ebay.com/itm/220866124397?ssPageName=STRK:MESINDXX:IT&_trksid=p 3984.m1436.l2649
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Old 12th April 2013, 04:35 AM   #2
phase is offline phase  United States
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Join Date: Oct 2004
That sounds like a fun project.
I want to try a pair of those boards at 95 volts.

I am using one of these as a headphone amp, and it works great as a pre amp.
Search "E5 headphone amp board weiliang"

Last edited by phase; 12th April 2013 at 04:36 AM. Reason: Spellin
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Old 12th April 2013, 05:37 AM   #3
rfj5106 is offline rfj5106  United States
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Location: Richmond, B.C CANADA
I have built several L20 amplifiers and run them without preamps using a CD player as a source and get huge amounts of volume. I use 50K potiemeters to attenuate the input which act as a volume control. I would try a regular CD player rather than an Ipod. I suspect there is some serious impedance imbalance for the Ipod. To use the Ipod you probably need to add some op amps to act as a buffer between Ipod and amp.
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Old 12th April 2013, 08:07 PM   #4
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I tried running a CD player through it and still don't get enough gain. The CD player was run straight to the amp with out a pot switch for volume control, just wide open.

I'm putting 55 volts DC to each side of the amp for a total of 110volts DC to the board. The transformer and capacitors are from the Samson Servo 240 amp which put out 120 watts per channel at 8 ohms. Right now I'm just powering up one of the L 20 boards so the transformer should have plenty of power to put out more Watts. I don't have any type of pre amp here or I would give that a try. Perhaps I will run down to radio shack and pick up a cheap audio mixer to hook in line with the CD player and amp as they have a pre amp inside that should boost the line level.

Adding a pre amp isn't too big of a deal if I do need one, except for the fact that I'm running out of room inside the case. The original transformer only has high power taps so I would have to add a second transformer to power up the pre amp and then also find room for the pre amp it's self.

Dale P.
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