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Old 9th October 2006, 07:28 PM   #1
Minion is offline Minion  Canada
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Default A few questions about Building a LM3886 Chip amp???

Hi Folks, I am designing a Guitar amp arround the LM3886 Chip amp but I have a few questions That i could not find or calculate from the Info in the Datasheet.....

My first questions has to do with how big of a Transformer do I need to run a single Chip amp?? I have a 100vA 18v-0v-18v Toroidal transformer and the Power supply with be rectified by a 6 amp bridge Rectifier and with about 5000uF of Filtering on each power rail and will be Unregulated for the Poweramp stage but will be regulated to +/-15v for the preamp stage.....

will 100vA be enough?? if not what will be enough??

if the Transformer isn"t powerfull enough what symptoms will it produce?? Will it just not go as Loud or will it not work at all ? or will the Volume just go up to a certain level and then not get any louder as I turn it higher??

Also I understand that heatsinking the Chip is very important so I was thinking of just drilling a Hole in a Computer Heatsink and screweing the Chip to that and have the 12v Fan run off of the 15v rails through a couple 150r resistors....

With a Heatsink for a Socket A CPU be enough to cool the chip??

also does the heatsink need to be grounded or should it be Isolated from ground and Chassis??

I know a Lot of this info is probably in the Datasheet but for the most part I don"t understand most of what they are saying....


Thanx for any info you can give....

Cheers


PS: I did a search through this forum for the answer but like most forums there are poeple who have many seperate and totally differant oppinions which isn"t condusive to getting a correct answer...
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Old 10th October 2006, 07:57 AM   #2
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Minion

One 100VA trafo for one LM3886 is just fine and your "total-PSu" as describet should also fit well.

A socket A heatsink is also OK with this PSU and with the fan running you are absolutely OK .

If you use a LM3886TF you should be OK regarding isolation, but I recommment that you use a LM3886T (better heat transfer) and use some kind of heatconducting isolation between the chip and heatsink (silicon-Pad ext.).

Just using termal great will make your heatsink at V-, since the case of LM3886T is at V-. Avoid that.

Have fun

Thomas
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Old 10th October 2006, 08:45 AM   #3
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Default Re: A few questions about Building a LM3886 Chip amp???

Quote:
Originally posted by Minion
....will 100vA be enough?? if not what will be enough??
The transformer is made of much metal which means that the transformer should be rated for the average power need or twice the rating for max 10-20 minutes but this will hardly happen if you have normal music.

100 VA or 50 Watts in to the amp is rather much as an average. As Thomas say, 100 VA is quite sufficient if it only comes to not to overload the transformer.
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Old 10th October 2006, 03:48 PM   #4
Minion is offline Minion  Canada
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Thanx a Lot Guys I feel a Lot better now....I keps getting poeple on other forums saying that no you will need at least 160vA transformer for one Chip and other poeple saying I should be fine with 100vA.....

I believe the ones I ordered were the LM3886T model as that is the model shown in the e-bay auction I got them from....I paid $7.95 for 2 LM3886 Chips so about $5 each after shipping....

I was thinking of Building one Guitar amp with one chip and one Bass amp from the other chip, the problem right now is figureing out what speakers that are fairly cheap would work for a Guitar and Bass amp....I was thinking maybe a Big Woofer from a Car stereo for the Bass amp and a Big Midrange speaker for the Guitar amp.....

do you guys think I can get an OK sound that way useing big Car speakers???

Thanx a Lot!!!
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Old 10th October 2006, 04:26 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
18-0-18 sounds like you might be using a 4ohm speaker.
What are you planning to use?

If you run 50W into 8ohm then I suggest you increase your smoothing capacitance to +-6800uF.

If you run 50W into a 4ohm speaker then I think you should raise the smoothing to +-10mF.

These recommendations are based on +-2mF/Apk output current.

The input high pass filter should be about one octave above the PSU RC time constant.
8r*6.8m=54.4mS, input filter ~=27mS.
4r*10m=40mS, input filter ~=20mS.
These input filters will restrict the F-3 frequency to 6Hz (8ohm) and 8Hz (4ohm) and will not cause a problem with a guitar amp.

For a bass amp you may want to reduce all these frequencies by at least an octave and preferably two octaves.

Your 100VA will support upto 70W of output power. So I agree with the other posters.
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Old 10th October 2006, 07:12 PM   #6
Minion is offline Minion  Canada
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Well the schematics I am going from says to use a 2 x18v Transformer and to use an 8 ohm speaker...

But for the data sheet it says I can use either an 4 or 8 ohm speaker....

I was told that with this type of power supply I should use a combined 10,000uF capacitance witch is 5000uF per rail but I can Increase that no problems accept that I only have 1000uF caps so I will be putting a Lot of them in paralell to come up with 10,000uF......


This stuff here:

.
Quote:
These recommendations are based on +-2mF/Apk output current.

The input high pass filter should be about one octave above the PSU RC time constant.
8r*6.8m=54.4mS, input filter ~=27mS.
4r*10m=40mS, input filter ~=20mS.
These input filters will restrict the F-3 frequency to 6Hz (8ohm) and 8Hz (4ohm) and will not cause a problem with a guitar amp

I have absolutly no Idea what you are talking about there...Maybe an explanation of what all of the abbreviations are and what you are talking about here will help me understand what you mean.....


Thanx
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Old 11th October 2006, 07:29 AM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
is it Apk, or mS, or RC time constant, or Hz, or octave or mF, or PSU, that needs expanding, or the general phiposophy of matching the high pass filters to optimise low frequency performance that needs clarification.
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Old 11th October 2006, 04:40 PM   #8
Minion is offline Minion  Canada
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Quote:
Originally posted by AndrewT
Hi,
is it Apk, or mS, or RC time constant, or Hz, or octave or mF, or PSU, that needs expanding, or the general phiposophy of matching the high pass filters to optimise low frequency performance that needs clarification.


I guess what I don"t understand is what does a PSU have to do with the Frequency performance of an Amp?? and what does a PSU have to do with high pass filters??

Most of my projects so far have been preamps and I do spend a Lot of Time trying to understand the basics of Electronics but this is the First I have heard of any mention of Implementing audio filters Via the PSU...

Unless I am totally misunderstanding what your trying to say??


Please explain....


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Old 11th October 2006, 05:55 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
some amps from way back suffered motorboating.
This is a form of low frequency instability that is fortunately rare these days.

To ensure that the low frequency instability cannot occur a few rules should be followed.

1. the input high pass filter should be set at least half an octave above the NFB high pass filter.

2. the NFB high pass filter should be at least half an octave above the PSU RC time constant.

Combining these two rules,
the PSU RC time constant must be at least one octave below the amplifier low frequency turn over point (F-3db).

If you set the input filter at 80mS then the NFB filter should be about 120mS and the PSU should be about 160mS.

It turns out that for an 8ohm load the PSU requires a smoothing capacitance of about +-20mF and this allows an output current of about 6Apk to 10Apk. Which will drive any 8ohm speaker to at least 150W and probably substantially more.

If you do not require 80mS at the input, then substituting 27mS for a guitar amp, one can scale all these values and the resulting power will better suit a chip amp.
PSU RC=27*2=54mS.
PSU C=54/8=6.75mF i.e.use 6800uF.
This supports 3.4Apk and P=50W.
That's where my suggestions come from.

As an aside, these values (160mS PSU) give a very extended bass response and it shows audibly in wideband speakers, even my tiny AE1s
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Old 11th October 2006, 06:37 PM   #10
Minion is offline Minion  Canada
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Lets just say for a second that I understand what you are talking about (Which I don"t ) how do I implement what you are saying into my amp design??


Thanx
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