Simple Chip Amp for P to P wiring

[sandyhooker]

I would strongly advise against P to P construction of a chip amp. There are numerous PCB deals on Ebay for the LM3886 in fact I recently bought a set (2 amp boards -1 ps board ) for $7.99 plus $4.00 shipping.They are high quality and a proven design. Go to Ebay/Electronics/Amplifiers/and search LM3886. As stated in earlier post the big problem is layout and oscillation. The PCB solves that problem. Apex jr has Avel transformers (toroid) for $22 plus shipping . The plate Amp spoken of earlier is unbeatable in value .

[ted4412wilt]

Thanks JB74. Google must have been sleepy when they could not find that from Antek but now I have the correct address.

If I build a chipamp it will use this toroid
Antek - AN-6218
$75 shipped is a good price for 600 VA. That is the largest toroid with 18 volt output and will power about 6 3886 and do a really good job on less plus extra capacity for expansion. Extra capacity will keep the voltage high enough so you do not need regulation. Use a big bridge rectifier to keep things simple as recommended by several designers plus lots of capacitors. Most of Dogbreath's instructions can be followed with good success but make sure your input receives no DC or you must include input capacitor as shown on most chipamp designs. Linkwitz is probably the expert. I downloaded several popular schematics and put them all on a screen so I could compare. You need very big heatsink to keep the 3886 chips cool. Many diy worry about a pretty case more than the needed parts!
Ted

[wintermute]

Originally Posted by sandyhooker

I would strongly advise against P to P construction of a chip amp. There are numerous PCB deals on Ebay for the LM3886 in fact I recently bought a set (2 amp boards -1 ps board ) for $7.99 plus $4.00 shipping.They are high quality and a proven design. Go to Ebay/Electronics/Amplifiers/and search LM3886. As stated in earlier post the big problem is layout and oscillation. The PCB solves that problem. Apex jr has Avel transformers (toroid) for $22 plus shipping . The plate Amp spoken of earlier is unbeatable in value .

Hi Sandy, yes the risk of it not working is much higher than using if using a proven layout, but if done carefully it works really well I think coldcathodes decision is mostly a philosopical one

The reason I chose to do p2p was twofold one was for exactly the reason of problems caused by layout, the second was that at the time Hardly anyone seemed to be doing p2p 3886's, as it was considered to difficult (red flag to a bull)... Admittedly not everyone will want to make something as compact as what I did, but my way of dealing with layout problems was to make everything as close to the chip as possible I still kept separate returns fo critical earth return paths as per the data sheet. It is completely stable, and the quietest amp I have ever built!

Tony.

[coldcathode]

Originally Posted by wintermute

Hi Sandy, yes the risk of it not working is much higher than using if using a proven layout, but if done carefully it works really well I think coldcathodes decision is mostly a philosopical one

Tony.

Wintermute,

Spot ON! Sandy with no disrespect intended, anyone can purchase a PC Board and solder in some components per a datasheet or "proven design".

I come from the world of Vacuum Tube Audio where everything is "risky" and it may seem to most people a LOT of work for little results.

Example. I have a 45 Single Ended Triode amp I designed and built myself from scratch. It took 6 weeks to build, cost me over $350 for parts(not including $250 for tubes). It takes up a 14" x 18" area, weights 55 pounds! and puts out a whopping 1.5 Watts per channel.

The filaments (heaters) for just the 2 output tubes consume 7.5WATTS of power just to heat the tubes.

BUT...This is a BIG BUT, you will NEVER hear anything as smooth as silk and full rich sound as this amp!!

IMHO, 1 Watt of Tube Power = 10 Watts of the highest end Solid State Stuff you could put together.

Again, I mean no disrespect to the Solid State folks here, or else I would not be here looking for info on chip amps. To me everything has its place, and in this case I want to build chip amps for my subwoofers. To get 30-40WPC in a tube design could cost me in excess of $2,000! Not too mention the space it takes up. Since I feel that I would never be able to tell any audible difference in a tube amp or SS in the range of 25 to 100hz I think this is the most reliable, cost effective means for getting that level of power.

I intend for the chassis and overall aesthetics of the chip amps to follow the theme of my tube amps. So I won't be using torroids, EI's look more the part.

I am continuing to research and understand how the chip amps work. I am trying to get some understanding of how the different cap's and resistors affect the output.

I realize I could easily purchase two plate amps and be done with it, but I plan to have a system where every component (besides the sources) has been designed and hand built by myself.

[wintermute]

Hi coldcathode, I think the answer to your question on rg is in this thread

Note I assume the rg you are referring to is the one in the second link in Ted's post, as the first one has ri incorrectly labeled as rg (well incorrect from the standpoint of the datasheet )

At this point I'm thinking it would have been really smart to have made some notes about what I did I suspect I went with Brians schematic and used 22K for both rf and rg, I also suspect that I decided to just go with his gain calcs and used the 680 ohm for ri.

The datasheet has a pot on the input for the "typical application" so without it I guess rg is necessary, or you will get the problem Oliphant had

Tony.

[ted4412wilt]

Here are several 3886 versions. Pay particular attention to S.Linkwitz version in upper Left. Also follow carefully the assembly pics from Mic's 3 resistor 3886 assembly in my link I provided earlier in this discussion.

[coldcathode]

All,

A question I have about Power Supply.
I assume that the Positive and Negative supply voltages can be provide via separate EI transformers and bridges, correct?

When calculating current supply needed are we talking in terms of each supply or total.
IE, the calcs on the data sheets indicate a square root of 2xPo/Load is this amperage the peak positive or negative or the total of both?

I ask because I have a quantity of 25VCT 2Amp transformers and am wondering if I could use (2) per channel ?

Hypothetical 32Watts Po would be 64/8 = 8 sqrt = 2.8 amps? So 1.4 from each side or is it peak of 2.8 from either side?

[danielwritesbac]

Originally Posted by coldcathode

All,

A question I have about Power Supply.
I assume that the Positive and Negative supply voltages can be provide via separate EI transformers and bridges, correct?

When calculating current supply needed are we talking in terms of each supply or total.
IE, the calcs on the data sheets indicate a square root of 2xPo/Load is this amperage the peak positive or negative or the total of both?

I ask because I have a quantity of 25VCT 2Amp transformers and am wondering if I could use (2) per channel ?

Hypothetical 32Watts Po would be 64/8 = 8 sqrt = 2.8 amps? So 1.4 from each side or is it peak of 2.8 from either side?

About 26 watts before the results become unpleasant. This is insufficient for combining LM3886 with the described speakers.

There may be a way to do it. Do you have four identical Dual Bobbin transformers available?

[coldcathode]

Daniel,

What I have are 25VCT @ 2Amps I do have about 7 or 8 of them. My plan was to use one transformer for the positive and one for the negative supply.

[coldcathode]

What is confusing to me is this.

In TUBE design we primarily figure output as the power (watts) thru the Primary of an output transformer (OPT) = (the LOAD). This LOAD is based upon the impedance of the speaker reflected to the primary via the ratio of the windings squared.

Example an 8Ohm speaker attached to the secondarys of an OPT with 20:1 turns ratio imposes a load of 3200 ohms. A swing Peak to Peak of 300 Volts and a Current Swing of 50mA would yield a Pout of 15Watts into the load Divide by 8 for RMS and you get about 2 watts.

That is for class A operation where current is flowing all the time in the Primary.

The chip amps I guess operate in Class D this is where I get all confused.