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Old 5th November 2006, 09:01 PM   #1
Minion is offline Minion  Canada
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Default Low output power with LM3886??

Hi guys, I just finnished building a Guitar amp that uses the LM3886 as the Power amp and a simple Dual opamp preamp circuit...The LM3886 circuit was the basic circuit from the Datasheet.... I have it powered from a 100Va 18v,0v,18v, Torroid with 8000uF of filtering on each Rail or 16000uF total....
It has 2 speakers with an adjustable JBL crossover , One speaker is a 6in 20w speaker and the other is a 12in of unknown wattage....

The problem (it has a few) is that it just doesn"t go as loud as it should ,My 15w Practice amp goes louder......

I think there might be a few things that Could possibly be causeing this , But I don"t know if any of these reasons are valid or not so I figured I would throw them out there and maybe someone could tell me if any of these things could be causeing the problem....

1) Maybe the Inductor I used on the output is too big ,The schematic called for to use 22 gauge wire and wrap it arround a 10R 2w resistor about 12 times, all I had was 18 gauge and I might have mistakenly put 12 or maybe more wraps arround it...

Could that cause low power output??

2) Maybe my Torroid isn"t powerfull enough to push the full 50w or so?? It is also a rather small in size torroid ,about half as small as my other 100vA torroid but the torroid was custom made for me and it says that is is 100vA at 2.38A.....

Could that cause low power output??


3) I made the PCB myself useing a 1oz copper board and the Traces are about 3mm wide at there smallest and about 10mm wide at there widest....

Could it be possible that the 1oz board isn"t thick enough to carry the power needed to run the chip at full volume??

4) I don"t know if this makes a differance but I used a 10K pot as the Volume knob for the power amp, would a Larger value like a 50K be better?? also Some of the Pots do not have a connection to ground and I do get a bit of a Hum , Should I solder a wire to the back of these pots to ground??

All of the Pots are wired off board and some of the leads are a bit long ,would this make the signal weaker??

5) The last thing I can think of is maybe the Crossover is makeing the signal weak?? or the fact that I am useing 2 speakers and the power is being distributed to 2 speakers makeing it half as loud??

Is there an easy way of adding another LM3886 chip to the circuit without haveing to mod the PCB? ,like wireing the Chip in paralell to the board useing P2P wireing and just wireing the leads of the first LM3886 to the leads of the second Lm3886?? would that work for increaseing the ouput power?? if so would I have to change any of the values of the resistors and caps on the board??


Sorry for all of the Stupid questions but there are so many things that seem like they could be a possible cause for the low output problem and I didn"t know if any of these were valid and didn"t really know were to start in finding the problem....


Thanx Guys....


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Old 5th November 2006, 11:31 PM   #2
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Default low output

What's the gain on your LM3886 and your pre amp??
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Old 6th November 2006, 12:09 AM   #3
Minion is offline Minion  Canada
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I believe the Gain on the Preamp is about 50dB or higher but I have no idea what it is on the Power amp...Whatever the Gain is on the Basic implemtation of the LM3886 chip is what the gain is as I used the design directly from the Data sheet....

If I hook up a Distortion pedal and crank it up the Volume does get quite a bit louder .....


Cheers


I just checked the specs of the Preamp stage and it says that the Power amp stage has to have a Minimum Input sencitivity of at least 750mV to properly power the preamp stage and from looking through the LM3886 datasheet it says that the LM3886 has an input sencitivity of 894mV at a gain of 20 and 84mV at a gain of 200...What does this mean??....

Do you think the problem is that the Preamp isn"t putting out enough Signal for the Power amp??

If so do you know of a Guitar Preamp that would put out a very strong signal and also have a Good Crunchy Overdrive??

I built the PCB so I could easilly seperate the preamp and Power amp stages if one of them didn"t work out by simply cutting the PCB.....

Thanx
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Old 6th November 2006, 01:38 AM   #4
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Default low output

If you built the chip amp via LM circuit the gain should be 27db giveing you an input senstivity around 700mv, IMHO I would drive the chip a little harder add a buffer with about 2 to 3db of gain, because when you add the distortion pedal you probably add some more overall gain.
driveing the chip harder, more output.
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Old 6th November 2006, 02:18 AM   #5
Minion is offline Minion  Canada
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Hi, Ok ,Actually there is a 4.7k resistor I can change in the preamp to Boost the gain so I"ll try changeing it to a 2.2k and see if that helps but If I don"t get enough gain I will add a Booster stage either after or before the preamp or maybe both....

Maybe some sort of multiple cascadeing fet gain stages with a NPN buffer on the output to lower output impedance??

I"ll give it a shot...Thanx
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Old 6th November 2006, 06:24 PM   #6
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18V AC or DC ?
datasheet 3886 report :
+/- 28Vdc 68W @ 4Ohms 38W @ 8Ohms
+/- 35Vdc 50W @ 8Ohms
For an experiment you can also short-circuit the series
output inductor.

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Old 6th November 2006, 08:19 PM   #7
Minion is offline Minion  Canada
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Well the Transformer is a 18v,0v,18v AC transformer(it reads 21v AC without a Load) so after rectification and diode Voltage drop it is something like 24v DC....

So from going from the data sheet is should probably get a bit over 30w output power.....

Well I know for sure that I am not even getting 10w in loudness as my 15w Practice amp goes what seems twice as loud and my 15w Garnet tube amp (which is on the fritz right now) goes what seems like 4 times as loud.....

There are a couple things I am going to try to get more output power....

The first is changeing the gain set resistor from a 4.7k to a 2.2k which should increase the available gain....

I am also going to put a better opamp in the Preamp...I was going to use some OPA2134 that i had be I realized that I didn"t have them and only had some opa134 so I designed a small adapter PCB that converts two single opamps into a Dual opamp so I will make one and try a few Higher end Opamps that I have that are only singles like some OPA627 ,OPA602 and OPA134 and see how they sound in the circuit.....

I have also got a design for a Super Gain Booster/Overdrive circuit(It is basicly a high output distortion pedal) that Im going to install before the Preamp which should add that real crunchy loud distortion sound, It uses 4 Cascadeing low noise NPN transistor gain stages each with a Diode cliper which should add considerably to the signal the preamp gets and also have a low output impedance which means I get to use a bigger variety of Opamps for the preamp stage.....

Hopefully this will make it loud enough to at least jam and have a good crunchy distortion sound.....

I"ll let you know how it turns out.....

Cheers
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Old 7th November 2006, 08:26 PM   #8
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Do you not have a scope to just check the input/output signal levels at each stage? The LM3886 likes to have a gain of 10V/V or more up to about 50V/V.

-SL
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Old 7th November 2006, 08:30 PM   #9
Minion is offline Minion  Canada
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No i don"t have a scope...Wish i did....i will try the mods later today hopefully and get better results...

Thanx
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Old 7th November 2006, 08:56 PM   #10
CarlosT is offline CarlosT  United States
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Could it be that your speaker has a very high impedance?
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