Modulus-86 build thread

From the LM3886 datasheet:

The performance of the LM3886, utilizing its Self Peak Instantaneous Temperature (°Ke) (SPiKe) and hybrid amplifiers by providing an inherently, Capability dynamically protected Safe Operating Area (SOA). SPiKe protection means that these parts are completely safeguarded at the output against overvoltage, undervoltage, overloads, including shorts
to the supplies, thermal runaway, and instantaneous to the Supplies via Internal Current Limiting temperature peaks.
 
Manufacturing of consumer electronics almost certainly has no patience for such testing and assembly, but we're DIY nerds, who take a bit longer to build an amplifier than Pioneer or BOSE does. So I think it's OK, or at least understandable, that we obsess over these details, if only to help us sleep at night ;-)

Obsess all you want. I just don't want someone who's new to this thread to get the impression that you have to orient the parts in certain ways in my circuits. That's all.

is there any drawback to boost converters with this amp was my question

Not to my knowledge. In theory, I guess you could get some beat products from the differences in switching frequencies. I've never seen that in practice, though. I'm not saying that it is impossible to see in practice. Just that I haven't seen it.

There's also not much benefit to the boost converters. The extra 4 V will give you 1.5 dB higher output power. Whether that's worth the cost and complexity of the boost converters is up to you.

What happens if the output is short circuited by accident?

The protection circuits in the LM3886 will kick in.

Tom
 
thanks for the reply tom i have boost converters that can up the voltage to 35 volts so i will run at 28v first to get a ruff idea of power needed and up it if necessary also at £2.50 each they re not expensive iv seen them used in the tpa3255 thread and with the mod 86 being far less picky about power supply than tpa3255 im thinking it would be a good candidate for use with these devices
 
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Film capacitor orientation

I've tried to measure the difference in capacitance between the outside and inside foil connections. No success yet. I'll ty a plate of metal next.

Back in the good old days, film capacitors were marked (properly) for the outside electrode. I don't know why this practice stopped. Using the outside foil as a shield was common practice for bypass capacitors.

-Chris
 
Application

I've tried to measure the difference in capacitance between the outside and inside foil connections. No success yet. I'll ty a plate of metal next.

Back in the good old days, film capacitors were marked (properly) for the outside electrode. I don't know why this practice stopped. Using the outside foil as a shield was common practice for bypass capacitors.

-Chris

Chris,
Was this for audio applications or microwave? For bypass I started using a Dale 0.10 or 0.15 ohm current sense resistor in series with the film cap to prevent hearing the rectifiers singing. The Stacked film PET caps I use help too.
The polarity of the wire, film caps, and resistors is below my radar.
It just in case I try to align the printing to read from source to load.
 
I've tried to measure the difference in capacitance between the outside and inside foil connections. No success yet.

There should not be any difference in capacitance if you measure from the inside to the outside or outside to inside foil. The area between the foil sheets has not changed by changing the meter connections. Neither has the dielectric.

You should see a difference in the parasitic capacitance between the innner/outer foil to ground (or external environment), however. The parasitic cap from the outer foil to ground will be greater. The parasitic cap from the inner foil to ground should be quite a bit smaller and drop to near zero if the outer foil is grounded.

The system can be modelled as two caps in series. Inner foil -||- outer foil -||- ground.

Tom
 
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Hi Tom,
Well, no. The outer foil shields in inner foil and there should be a difference in capacitance to an outside conductor. So I think that by using some sheet conductor, I can increase the capacitance and then it may show a difference. So far I'm getting close to the same readings.

Under these test conditions, ground wouldn't have anything to do with the readings. I'll let you know once I've done this experiment. I'm using a test frequency of 10 KHz. I can't use my long leads at 100 KHz. I need leads to manipulate an outer shield.

-Chris
 
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Hi Brian,
Not really. Generally speaking you would want the outer shield to be connected to the lowest impedance side in that circuit in order to drain whatever noise voltage was picked up. However, the leads may pick up more or on par with the outer shield.

When a film capacitor is used as a bypass capacitor, one side is connected to ground. In this instance, you would be better off with the outside foil acting as a shield. As for audible sound quality differences I have never heard one, nor has anyone I know that works with components. If you consider what is going on from a physics standpoint, there is no difference at all. Besides, we are dealing with an AC current, that means it reverses periodically. It has to or the voltage across these components would increase until the part failed or the air ionized and discharged that way. If you place a DC bias across the capacitor, there is eventually very little current flow (leakage only). Then if you apply an AC current through the part, the net average still must be zero. The dynamic current reverses periodically just like the case where there isn't a DC bias.

-Chris
 
All bypass capacitors in the Modulus-86 are ceramic. The capacitor that cloud1972 asked about 30 posts ago is the cap for the Zobel network. In the older versions of the MOD86, I used a film cap there. The latest version uses a ceramic cap instead to lower BOM cost. There is absolutely no reason this cap needs to be mounted following any specific orientation. The output impedance of the LM3886 is in the low mΩ if not µΩ range with the amount of feedback applied. You will not move that node.

There is absolutely no reason to orient any non-polarized capacitor in the MOD86 in any special way. None.

What you seem to be arguing is that a 2-terminal impedance will have different impedance depending on which way it is measured. I'm pretty sure that violates Ohm's Law. If you include the ground plane around such an impedance as part of the circuit, you now have a 3-terminal device. That's different.
You are correct that some circuits benefit from having the outer foil of the cap connected to the lowest impedance node. In my view, that's a flawed design - or certainly one that is very costly to mass-produce. I have no plans of ever offering a circuit like that as modern parts make such trickery obsolete. C0G/NP0 ceramics outperform film caps in many applications.

Your observations are interesting and I wish you luck with your experiments. I'm curious to follow your findings and will be happy to continue this conversation in a separate thread, for example in the parts forum.

Tom
 
Chris, there's a fundamental flaw in the comments about film bypass caps. The impedance of the supply that you are bypassing should be as close to zero as practical. In fact, that is one of the reasons for bypass caps -- provide a low-impedance source at high frequencies for the device being bypassed. So the orientation thing is just another bit of fake news with negligible basis in physics.
 
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Hi Brian,
The impedance of the supply that you are bypassing should be as close to zero as practical.
Absolutely true, but it isn't at higher frequencies as we both know.
So the orientation thing is just another bit of fake news with negligible basis in physics.
True enough. But there are enough high impedance circuits out there where it may make a difference, especially if there are layout problems with the circuit. Let's just say with compromises that do not favor optimal power supply layout.

It is true that on a macro view, it absolutely doesn't matter. However, when you start looking at very high performance and very low noise, it can matter. If there is an easy way to determine which lead connects to the outer foil, no harm in going for it.

I think there is some confusion as to where I'm coming from here. For this amplifier, film capacitor orientation absolutely does not matter. Same for resistors or any other like component. I'm just giving a complete answer to a question, and I have previously stated clearly that I completely agree with Tom's views on this regarding this design.

There is zero point in worrying about capacitor orientation for most designs. The same goes for wire - in spades.

-Chris
 
first amp complete i was not going to post until build was complete but im going to have to agree about how good this sounds in mono how very quiet this amp is also what a joy this amp was to build after 2 surface mount builds it was nice not to have to use my usb scope.
kind i wish i had not asked about direction of caps as i seem to of opened a can of worms sorry tom
 
first amp complete i was not going to post until build was complete but im going to have to agree about how good this sounds in mono how very quiet this amp is also what a joy this amp was to build

I'm glad you enjoy the amp and the build experience. I actually spend quite a bit of time listening in mono at the end of a project. I generally don't have two channels built until fairly late in the project. You can tell quite a bit from even more reproduction if you mix left and right channels together.

kind i wish i had not asked about direction of caps as i seem to of opened a can of worms sorry tom

No worries.

Tom