Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: Manfred on June 19, 2017, 03:26:22 AM

Title: LM 3886 current feedback poweramp
Post by: Manfred on June 19, 2017, 03:26:22 AM
I found this schematic in an old topic.
Who knows the value of the resistor which is covered by the lettering "STORE"?



Title: Re: LM 3886 current feedback poweramp
Post by: Enzo on June 19, 2017, 10:24:36 AM
Why not tell us what the schematic was for and which old topic you found it in, we might have the direct information you seek.

Meanwhile the datasheet and application notes for the part should have sample circuits.
Title: Re: LM 3886 current feedback poweramp
Post by: Manfred on June 19, 2017, 03:36:20 PM
Thats the old topic:
https://www.ssguitar.com/index.php?topic=3751.msg29306#msg29306 (https://www.ssguitar.com/index.php?topic=3751.msg29306#msg29306)
Title: Re: LM 3886 current feedback poweramp
Post by: J M Fahey on September 15, 2017, 09:20:28 PM
The 8085 circuits schematic makes not sense and the half covered one is unavailable, Roly too :(
Just look at the impedance correction networks in any practice TDA2030/2050 Guitar amp (Peavey/Fender/Marshall/Crate/Laney/etc.) and use the same with the LM3886 , they will work properly

All these chipamps are "powerful Op Amps" , just varying in supply and power output, same design rules apply to all.
Title: Re: LM 3886 current feedback poweramp
Post by: tarahall on September 19, 2017, 08:45:52 PM
Quote from: J M Fahey on September 15, 2017, 09:20:28 PM
All these chipamps are "powerful Op Amps" , just varying in supply and power output, same design rules apply to all.

JM can you please clarify this. When data sheets (and I quote the LM3886 here) says things like:

"68W cont-avg-output power into 4 ohms" or "38W cont-avg-output power into 8 ohms"

what does the term "cont-avg-output power" mean?
is that their way of saying 68w RMS or 38 RMS?

TIA Gavan.
Title: Re: LM 3886 current feedback poweramp
Post by: phatt on September 19, 2017, 09:49:14 PM
 (cont-avg-output power) = Continuous Output Power.

So a sine wave input would be "Continuous" but music is unlikely to ever be Continuous.

Regarding your Q of RMS output others here will know more,,
I just look at these as a ~40Watt chip.
Also remember that the whole power thing is governed by the power supplies ability to deliver that power so a larger transformer is the key if you want max Wattage. (no free power 8|)
Then you will need a lot of heat sink if you intend to run them hard.
Phil.
Title: Re: LM 3886 current feedback poweramp
Post by: Enzo on September 20, 2017, 12:03:36 AM
Just for marketing, somethimes they come up with some sort of spec where for one glorious nano-second the thing can be measured as putting out a thousand watts.  COntinuous output power means it can actually produce it.

A real world example.  I had an upright piano fall over on its back in our truck.  it pinned the toe of my boot.  Damned thing weighed 900 some pounds.  I could get my fingers under the edge.  I was able to jerk up on it just enough to free my boot.  Now I cannot lift 900 pounds, and certainly can't hold it up.  But I can apparently for just an instant peak my lift at 900 pounds.  SO in one sense, i can lift 900 pounds, but in any real sense I cannot.  Just so continuous versus peak power coming from an amplifier.
Title: Re: LM 3886 current feedback poweramp
Post by: J M Fahey on September 20, 2017, 10:58:55 PM
QuoteJM can you please clarify this. When data sheets (and I quote the LM3886 here) says things like:

"68W cont-avg-output power into 4 ohms" or "38W cont-avg-output power into 8 ohms"

what does the term "cont-avg-output power" mean?
is that their way of saying 68w RMS or 38 RMS?
They are speaking with the Dictionary in hand, which is fine with me.
Quotecont-avg-output power
is what average people on the street (and thousands of brohures, magazine articles, user manuals and probably even some books) calls "RMS power".

Strictly speaking there is not such a thing as RMS power, but RMS voltage or current.

That said, what everyday talking "RMS power" actually means (or should) is "power calculated using RMS voltage, sustained during a reasonable time"

personally if if I check an amp puts out 20V RMS, when fed some classic test tone, usually 1kHz or 400/440Hz , into a 4 ohms resistive load, without visible clipping and for at least 10 minutes, would prefer at least 1 hour, then *I* won´t argue at all if you say it is a "100W RMS amplifier".

There´s been tons of arguments lately arguing gramatics while the real problem lies in the monstrous fantasy claims posted everywhere using undefined, pure fantasy numbers.

Arguing against a time tested, **easily repeatable and consistent** system which has been used for decades and does not require hard to find equipment , difficult Math and on which most agree (or have agreed since forever until the latest Revolution)  sounds stupid and dangerous to me, because after dissing and putting out of the way a stable and repeatable test procedure, anything else goes.

FWIW the FTC itself has dropped **officially** any pretense of measuring or norms enforcement.  :grr
Title: Re: LM 3886 current feedback poweramp
Post by: tarahall on September 21, 2017, 07:37:00 AM
Quote from: J M Fahey on December 09, 2016, 07:45:20 AM
Agree on LM3886 opinion, but I guess I found the reason: LM3886 is simpler to use and it boasts 50 or 60W RMS ,
Now TDA7293/4 claim 100W RMS which simply is asking too much from the exact same package.

Thanks to y'all for your explanations.
If I want a 60W "RMS" into 8 ohms output stage which of these chips would be the most suitable:- LM3886, TDA7293 or TDA7294.

TIA
Title: Re: LM 3886 current feedback poweramp
Post by: phatt on September 22, 2017, 09:32:35 AM
The difference between 50 and 60Watts is hardly noticed in a live situation,, I doubt you would pick the diff in a blind test.
If you want louder use 2 speakers,, want more? use a quad box,, want to really go crazy? then use high SPL speakers.
A vox AC30 is louder than a 40 watt fender deville.  :o

Up to you?? but the LM3886 has 11 pins while the TDA7293/4 have 15pins.
The LM3886 is likely easier to route tracks on the PCB design.

the trick is to focus the tone shaping. so if you want it to cut through then design and test you preamp until you get the right balance and you will never miss the extra 10watts.

The aim of the game is to transfer electrical watts into acoustic SPL.
To get the full 100Watts from a 100Watt head you need 2 quad boxes.
IF you use the very same amplifier to drive 1x12 combo it will never come close to the same power output.
Again tone shaping is the key to great sound I've heard massive wattage rigs which are just dead pan boring and dull while others rip your ears off with ice pick fizz. you want to aim for balance.
Phil.
Title: Re: LM 3886 current feedback poweramp
Post by: Jazz P Bass on September 22, 2017, 10:09:41 PM
Good to keep in mind that the IC output potential is totally dependent on the power supply voltages into a load.

Within the limits defined in the datasheet.
Title: Re: LM 3886 current feedback poweramp
Post by: sajy_ho on September 28, 2017, 06:06:23 AM
 Hi, I've recently built a combo using 3886 in mixed feedback mode; here is the project file if anyone is interested:
https://www.dropbox.com/s/o8oe5be7xn0ycqc/LM3886%20Guitar%20Amplifer.pdf?dl=0 (https://www.dropbox.com/s/o8oe5be7xn0ycqc/LM3886%20Guitar%20Amplifer.pdf?dl=0)
Title: Re: LM 3886 current feedback poweramp
Post by: J M Fahey on September 30, 2017, 09:39:05 AM
Thanks Sajy  :dbtu:

Now we need some pictures of the completed build and the cherry on the cake would be some tasty guitar playing demo  :dbtu:
Title: Re: LM 3886 current feedback poweramp
Post by: sajy_ho on September 30, 2017, 07:45:15 PM
Quote from: J M Fahey on September 30, 2017, 09:39:05 AM
Thanks Sajy  :dbtu:

Now we need some pictures of the completed build and the cherry on the cake would be some tasty guitar playing demo  :dbtu:
Thanks man, here are some pics:
https://www.dropbox.com/s/7y0yxbi1y7fbp2z/20170912_215828.jpg?dl=0 (https://www.dropbox.com/s/7y0yxbi1y7fbp2z/20170912_215828.jpg?dl=0)
https://www.dropbox.com/s/7twizvnunrvuehx/20170912_215846.jpg?dl=0 (https://www.dropbox.com/s/7twizvnunrvuehx/20170912_215846.jpg?dl=0)
https://www.dropbox.com/s/27vcaq5ql3jfjw2/20170926_212818.jpg?dl=0 (https://www.dropbox.com/s/27vcaq5ql3jfjw2/20170926_212818.jpg?dl=0)
https://www.dropbox.com/s/o93el7x2wxknoc8/20170926_212848.jpg?dl=0 (https://www.dropbox.com/s/o93el7x2wxknoc8/20170926_212848.jpg?dl=0)
https://www.dropbox.com/s/amqx3744svgakjp/20170926_213010.jpg?dl=0 (https://www.dropbox.com/s/amqx3744svgakjp/20170926_213010.jpg?dl=0)

It's the so called 3886 power amp and before that I'm using ROG Thunderbird and Rebote 2.5 delay as preamp.
I wish I was a good player so I could post a video :'( but I will try my best soon :duh
Title: Re: LM 3886 current feedback poweramp
Post by: J M Fahey on October 01, 2017, 07:16:23 AM
Cool project, congratulations  :tu:

By the way, nice carpets, where are you from?

Always fill Country/City information in your user profile ... sometimes we are surprised :)
Title: Re: LM 3886 current feedback poweramp
Post by: phatt on October 01, 2017, 08:53:53 AM
Agree,,, a Very good effort, thanks for the pictures. :dbtu: :dbtu:
Obviously a lot of thought went into that project.
I take it you cut and bent the metal out by hand?
I know how hard that is,, because I've done it many times. :duh

Just note, You might need a bigger heat sink if you run it at full power for a long time. ;)
Phil.
Title: Re: LM 3886 current feedback poweramp
Post by: sajy_ho on October 01, 2017, 09:20:38 AM
Quote from: J M Fahey on October 01, 2017, 07:16:23 AM
Cool project, congratulations  :tu:

By the way, nice carpets, where are you from?

Always fill Country/City information in your user profile ... sometimes we are surprised :)
Thanks man, glad you liked it.
I'm actually from Iran; here carpets are pretty common and can be found on every house's ceiling... :cheesy:
Title: Re: LM 3886 current feedback poweramp
Post by: sajy_ho on October 01, 2017, 09:28:04 AM
 :duh
Quote from: phatt on October 01, 2017, 08:53:53 AM
Agree,,, a Very good effort, thanks for the pictures. :dbtu: :dbtu:
Obviously a lot of thought went into that project.
I take it you cut and bent the metal out by hand?
I know how hard that is,, because I've done it many times. :duh

Just note, You might need a bigger heat sink if you run it at full power for a long time. ;)
Phil.
Thank you Phil, Unfortunately that's the biggest heatsink I could find in stores here! Anyway I have a CPU heatsink laying around and I might give it a shot.Untill then I have to keep the volume down :cheesy:
Title: Re: LM 3886 current feedback poweramp
Post by: J M Fahey on October 01, 2017, 02:14:28 PM
You have 2 options:

1) get a nice thick aluminum extrusion, some which is available normally anywwhere for "normal"  uses by "normal"  people ... not us crazy experimenters  :duh
You can always find somebody who makes aluminum doors and windows who will have surplus odds and ends who he will sell cheap "by the kilo" or even free.
Some will make a nice, (if odd) heatsink.
(https://tricityextrusion.com/uploads/images/20090603945671.jpg)

2) add a small computer type fan blowing into the current heatsink.
Best is a standard 80x80mm PC supply fan, followed by two mini CPU fans in series.

Both will need a series resistor to drop from your supply (25/28V?) to 12V or 24V needed there.

Measure *actual*  fan motor current consumption using a 12V supply, then calculate needed resistor using Ohm´s Law.
Also calculate resistor dissipation, you might need a 2W to 5W resistor.
Bypass fans with a 1000uF capacitor, both to improve running speed and to minimize motor noise.

Title: Re: LM 3886 current feedback poweramp
Post by: sajy_ho on October 01, 2017, 04:17:52 PM
Thank you so much J M Fahey, the aluminium extrusion seems a pretty cool idea :dbtu: I'm heading to local aluminium maker next weekend.

Thanks again...
Sajad
Title: Re: LM 3886 current feedback poweramp
Post by: phatt on October 04, 2017, 09:54:23 AM
Even a thick angle of alloy will be better than nothing. The chip needs to dump heat instantly under heavy load and *Mass* is the key. Obviously many fins on the edge of a large mass is ideal (look at CPU heat sinks for clues) but in a pinch 6mm alloy angle is better than flimsy thin bits with no mass.
Here is a pic of a biamp monitor from Behringer,, 3 LM3886 chips on one chunk of alloy angle. The thick angle is also using the outer case to help dissipate heat. Working on the limit I'd say but hey it ran for ~10 years before the tweeter drive blew.
Phil.
Title: Re: LM 3886 current feedback poweramp
Post by: J M Fahey on October 04, 2017, 12:24:47 PM
Fully agree.

If you go the surplus/leftover route, you can bolt 2 L or U aluminum profiles back to back to increase thickness, at least where the chipamp sits.
You can (should) add a little thermal grease between mating surfaces.