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Old 23rd November 2014, 06:18 PM   #1
CJ900RR is offline CJ900RR  Sweden
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Default About the Jeff Rowland LM3886 amplifiers

No, this is not another thread asking about how to build a JR clone. It's more a clarification about how he did it, and a bit of reversed engineering.

In threads where people ask for schematics to a more powerful chip-based amplifier they are often referring to JR and they are often pointed to AN-1192 from National/Texas Instrument or to the website Shine7 Audio DIY Page where you can find 2 different ways on how to parallel more then 2 pcs of LM3886.

I don't want to discuss what method is best to use or is best sounding. I just want to show that Jeff Rowland did not use any of these configurations.

National/TI are using a bridge/parallel configuration with DC-servos to null the offset. 4 pcs of LM3886 that are paralleled 2 and 2 and then bridged together. One half of one channel is inverted, and the other half is non-inverting. This is a good sounding and reliable solution but it is expensive to build and frankly a bit to complex for many people.

Shine7 has taken a group of 3 paralleled LM3886 in an inverted configuration per PCB, where he null the DC-offset with a trim pot connected between the non inverted input and ground. He then bridges 2 identical PCB with a balanced line-driver or a trafo. This is working but, IMO, is not the preferred way to do it if you are going for a bridge/paralleled configuration, the way they do it in AN-1192 is more "correct".


Anyway. You can find more information about they do it in the links above.

Now, the Jeff Rowland amps differs a bit from AN-1192 and Shine7. JR uses 6 pcs of LM3886/channel but only paralleled, not bridged. Also he has connected the LM3886 in a non-inverted configuration and not inverted as many people claims on the Internet. At least at what you can tell from photos of his amps. I have not seen any of them in real life but there is some photos on the web like at the Shine7 website.

So how has he done it? Well, first and most, the LM3886 is nothing more than a basic Operational Amplifier with some extra addons. And if you know the basic of Operation Amplifiers then you know that you can easily trim the offset by just inserting a small amount of voltage to the inverted or non-inverted pin of the op-amp. Which one depends on how you have configured the Operational Amplifier.

I will not go in any deeper in the technical bits around this since how you do it is depending on your specs and requirements, but much information is to be found in MT-037: Op Amp Input Offset Voltage - Analog Devices and on page 6 and 7 in Application Note 31 Op Amp Circuit Collection among others.

Click the image to open in full size.

Above is a photo from one of his amps. You can not see all whats going on there but you can tell that the LM3886 is non-inverted.

On the next image I have made a schematic from what I can tell from the picture above.

Click the image to open in full size.

Im not sure of the value for R106 and C102. My guess is that C102 is a 100nF by it's small markings "A5". R106 should be set to give the gain in cooperation with R108. Below is a picture from where I have made a CAD of the schematic.

Click the image to open in full size.

Pretty corresponding to the JR uh?

Well I don't know if this is for good use to anyone but I just wanted to share how you can make a more powerful amplifier similar to the JR-amps, or how to combine it with one half that is inverted and make a bridge/paralleled amp that is cheaper than using DC-servos, but more "correct" than the Shine7-version. I have tested both the inverted and non inverted and it works well and you can easy adjust the DC-offset. But to be honest there is room for tweeking if you read the technical documents above.

As always, to have the best result, use 0,1% tolerance resistors and always adjust the DC before soldering the output resistors.

Best regards.
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Old 23rd November 2014, 06:33 PM   #2
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R106 is the gain setting resistor, R108 is the feedback resistor. A=1+Rf/Rg
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Old 23rd November 2014, 06:46 PM   #3
CJ900RR is offline CJ900RR  Sweden
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Quote:
Originally Posted by jackinnj View Post
R106 is the gain setting resistor, R108 is the feedback resistor. A=1+Rf/Rg
Yes I know

But since I don't know the marking on the R106 nor which gain JR uses I couldn't tell the value on R106. Maybe I was unclear but what I meant was that if anyone wanted to use the schematic then he/she has to calculate it him self to get the desired gain.

And, before any one else says it: this schematic is not finished and there should/could be more to it like zobel network, decoupling capacitors etc to ensure stability.

Last edited by CJ900RR; 23rd November 2014 at 06:51 PM.
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Old 26th November 2014, 03:58 PM   #4
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the manual states that absolute phase is inverting. however, since there is an input trafo the overall absolute phase may or may not be the same as the chipamp's circuit.
http://jeffrowlandgroup.com/Docs/M10Manual.pdf
to me the more interesting question is how it could be possible to deliver the claimed 150 w into 8 ohms without bridging given the LM3886 voltage ratings and the absence of an output trafo. pretty safe bet that it's a bridge-parallel design.
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Last edited by sfthurber; 26th November 2014 at 04:02 PM.
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Old 27th November 2014, 09:26 PM   #5
CJ900RR is offline CJ900RR  Sweden
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Well my thoughts is just a big guessing game (and some reverse engineering) since I have not found a schematic for his amplifiers based on the LM3886, nor any detailed photos of the input-board. The overall phase can certainly be inverting, but the I'm sure that the amplifier stage is built as a non-inverted stage only by looking at the photos of his amps.

Regarding the claimed power that the amp can deliver... well. I don't know from where he has gotten his numbers but if you enter 6 pcs of LM3886 paralleled into 8 ohms and with a power supply of +/- 24V into the Overture Design Guide then you get 160 W into 8 Ohm and 300 W into 4 Ohm. Paralleled should be able to drive more A than bridged? Or have I understood everything wrong?

And no offence, but if you study every photo of any LM3886-based JR amp you will see a "common rail" for the positive output to the speaker shared between all 6 pcs of LM3886. That would have been divided to 2 if the amps where in a B/P-config.

Best regards,
Carl
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Old 27th November 2014, 09:54 PM   #6
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24v rails give 36w RMS into 8ohms.
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Old 28th November 2014, 12:53 AM   #7
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The circuit look incomplete to me. I built a 4 channel 3886 poweramp after researching various 3886 circuits, and like it a lot. I don't recommend building poweramps for anybody who doesn't have a good understanding of phase margin, and a knowledge of how to verify it on the bench. If it blows up or only spuriously oscillates, there's a good chance it will destroy your speakers.
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Old 28th November 2014, 01:46 AM   #8
tomchr is offline tomchr  United States
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Rather than reverse engineering an existing circuit, why not just design a circuit that does what you want? If you want a bridged/parallel LM3886, National's BPA200 App Note (AN-1192) would be a good place to start. You'd probably also get quite a bit out of reading my Taming the LM3886 page.

Should you prefer to take a shortcut, you can assemble two of my Modulus-86 boards and connect them in a parallel or bridge configuration.

Not to knock Rowland's circuit, but I see several ways his layout could be optimized for better performance. At the very least, I'd move the local decoupling up to the IC, put the ground plane on the bottom of the board, and route the supply lines as pours rather than traces for lower supply inductance.

~Tom
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Last edited by tomchr; 28th November 2014 at 02:03 AM.
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Old 28th November 2014, 03:17 PM   #9
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the rowland power feed is through a tighly coupled parallel buss -- you can see a very small part of it and its shadow in the upper left of the pic (it's red-orange and it looks like a gigantic one-piece resistor network).
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Old 28th November 2014, 03:22 PM   #10
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Quote:
Originally Posted by CJ900RR View Post
Or have I understood everything wrong?

Best regards,
Carl
almost everything, yes.
see figure 5 in the application note in tomchr's post
parallel without bridging into 4 ohms the maximum output is about 120 watts with +/-35V -- at 24v and 8 ohms it would be much less.
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Last edited by sfthurber; 28th November 2014 at 03:25 PM.
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