LM3886 amp

[R.G.]

However the cost math works agains the modular approach as the number of modules increases. This is because the amp board itself is such a small part of the overall amplifier and chassis cost.

Each module has to have it's own power cord, fuse, switch wiring, jacks, metal chassis, heatsinks etc. etc. Each of these costs is replicated for each modual but often stay fixed for one bigger amp.

So cost likely will be cheaper for one right sized amp vs several in one box.

If everything else was equal... but it's not. The economy of scale - production volume in this case - will favor the mid- and small- sized box for a home constructor. This is because there are literally zillions of power cords, fuse holders, switches, jacks, and so on that work fine for a modest amp but are not usable for a MegaBlaster.  The modest sized device is piggybacked on the volumes of most of the electronics industry. The MegaBlaster has special requirements that just keep popping up. Many modest heat sinks can be had surplus at quite reasonable prices. Elegant, high power dissipation sinks tend to be expensive.

So no, I still disagree. While it may be that in custom manufacturing the pedestrian items run the cost up, the reverse is true for the constructor - the home builder needs parts that do the job that he can obtain. High power single amps are built largely out of unobtainium.

No. One large capacity power supply will be much cheaper than 4 power supplies of 1/4 capacity. Each of the 4 power supplies must have a rectifier, Power capacitors, and a toroid that cost nearly as much as a large capacity torroid. It is true that if you get up into a specialty/custom range things may look a bit different. But that is not what we are discussion.

No. While it may be true in general that a specific purpose designed power supply gets cheaper as the power goes up ( I used to design power supplies for living) it is not in general going to be true that the parts that a home builder can actually find will necessarily be cheaper. If a home builder was simply calling up Toroids, Inc, or Amveco, yep, you're right. But I can often find modest power toroids at a trivial price. Case in point: I can get a 42Vct 3A toroid on the surplus market for $12.50, about a penny a watt. You can get a 1kw toroid for about $100 to $150, just about the same price. Yes, if I had to buy the lower power toroid new, it would be more costly. But I don't. Power capacitors are energy storage devices. The energy storage for a number of small amps will be comparable to the monster, and possibly cheaper since the storage is a a lower voltage, and quite possibly hugely cheaper because of the surplus market.

I think my comments are accurate for the home builder.

Yes but are 4 speakers of the same frequency range and sensitivity really less expensive than 1 more capable speaker. (you might go to multiple speakers to drop the impedence thought) Again it will be more costly, especially when you consider the speaker cabinet, covering, connectors, connection wire and jacks, handles etc.

It depends a lot on the speaker, and even if the unit cost per watt is more, the incremental approach has the built in advantage that you don't have to buy 1000W worth at a time. You can buy one unit today. It's likely to be at the same price. Even if it costs modestly more per watt, you don't have to buy all the watts at once. That is not an inconsequential advantage to the home builder. Laying out $150 for a 50W amp every six months is not the same as laying out a couple of kilodollars for a Behemoth Mark 3 and making payments on it.

Flexibility, incremental purchase price, redundancy, piggybacking on volume production, and access to the parts on the surplus market are real advantages to the people in this particular niche.
Summarizing:

1) the modular approach will cost more

Not necessarily, and it's much easier to buy even if there is a premium in terms of $/W at the particular point in the market.

2) the modular approach will take up more space (multiple speaker cabinets and the like)

Guilty as charged. More speakers take up more room, and disperse sound better in real performances. You can even use one of the modules as a monitor and put more of the horsepower out with your audience.

Incremental amplification is not the only way to do it, never said it was. But there are some real advantages.

[moving_electrons]

Ah...Yes if you are comparing things to a 1000w unit then I see where you are coming from.  Those kind of amps have a lot of specialized components for the DIY (read expensive)

But if the problem is say a 100w or 200w then the economics is different.  Building 2 50w chip amp boxes  for $150 apiece when you can say build a 100w single amp for $200 is the situation I am pointing to.

The economy of scale - production volume in this case - will favor the mid- and small- sized box for a home constructor. This is because there are literally zillions of power cords, fuse holders, switches, jacks, and so on that work fine for a modest amp but are not usable for a MegaBlaster.  The modest sized device is piggybacked on the volumes of most of the electronics industry. The MegaBlaster has special requirements that just keep popping up. Many modest heat sinks can be had surplus at quite reasonable prices. Elegant, high power dissipation sinks tend to be expensive.

So no, I still disagree. While it may be that in custom manufacturing the pedestrian items run the cost up, the reverse is true for the constructor - the home builder needs parts that do the job that he can obtain. High power single amps are built largely out of unobtainium.

So yes I agree on the Megabaster type.  But even a 300w amp built for sub uses the same obtainable power cords, plugs, power caps etc as a 50w amp. So in the reasonable range I do think modular costs more.  But I do agree with your argument that at some point modular makes sense for the DIYer.

We probably can agree to disagree on where that point is.

By the way my DIY audio settup has multiple amps running multiple subs and I did it for distribution around the room reasons as you mentioned.  Did cost me more though.  

By the way there is a nice gain in efficiency if speakers are within one wavelength of each other.  This is pretty easy for low base.  I would have to look it up but as I remember you get about 3 dB of efficiency gain for the first 2.  It drops off so is not linear after that but going to 4 is still quite an additional gain.

[SirPoonga]

So how does running these chips in  series or parallel work?  what happens?  what's the difference?  any good website with info?

[moving_electrons]

So how does running these chips in  series or parallel work?  what happens?  what's the difference?  any good website with info?

Here is an application note from National

http://www.national.com/an/AN/AN-1192.pdf

Since amp discussions could reasonably be considered off topic in this stompbox forum.  Perhaps someone could suggest an appropriate bass amp forum.

DIYaudio.com in the chipamp section covers chips amps and occasionally has a guitar amp question  the chip amp forum.  But predominently it is an audio site with folks seeking pretty premium home audio sound.  Still you will likely get good chipamp help there, just not bass or guitar amp specific knowledge.  In that forum you will get a better reception there if you do some searches and research first.

Search for parallel and bridged.

[moving_electrons]

So how does running these chips in  series or parallel work?  what happens?  what's the difference?  any good website with info?

Here is an application note from National

http://www.national.com/an/AN/AN-1192.pdf

[Edit]You can skip the first part of the app note and jump to section 5 where the circuits are described.  Sections 5, 6 and 7 describe bridged, parallel and a combined bridged/parallel.  Don't get hung up on the graphs equations etc.  but go after the descriptions and circuit diagrams.

I think the bridged/parallel is what R.G. was refering to when he mentioned series/parallel when describing a high power version.

Bridged has one amp "pushing on one side" of the speaker while the other "pulls" with an inverted signal.  Each amp sees one half of the normal impedence.  So a 8 ohm speaker looks like 4 ohms.  So this can be a problem driving a 4 ohm speaker as the impedence wach amp sees is 2 ohms.  This likely will be to low and the amp will become unstable or the current draw through the amp will b eto high causing failure.

Parallel has two amps driving together.  The impedence each sees is double what one would see I believe.  A .1 ohm resistor is needed at the output of each so that slight differences in the amp chips does not cause one to take all the load.  A bridging of these is what a bridged/parallel is. [End edit]

Since amp discussions could reasonably be considered off topic in this stompbox forum, perhaps someone could suggest an appropriate bass amp forum.

The chipamp forum at DIYaudio.com covers chips amps and occasionally has a guitar amp question.  But predominently it is an audio site with folks seeking pretty premium DIY home audio sound.  Still you will likely get some help there.  As always in a forum you will get a better reception if you do some searches and research first.

Search for parallel and bridged.

[R.G.]

Ah...Yes if you are comparing things to a 1000w unit then I see where you are coming from. Those kind of amps have a lot of specialized components for the DIY (read expensive)

But if the problem is say a 100w or 200w then the economics is different. Building 2 50w chip amp boxes for $150 apiece when you can say build a 100w single amp for $200 is the situation I am pointing to.

OK. Seems we were in violent agreement. Yes, a 100W amp will be less than twice as expensive as a 50W amp.

Somewhere around 200W the economics start to shift, I think, largely because of the higher supply voltages. Even there, I would do a middle ground. I would build a power supply capable of providing the 200W output, but would put multiple independent 3886's in the box. That would keep the power supply voltages down, and then the only exotic technologies needed would be big cross section wires and good wire routing.

In fact, at some point I'm going to rewire one of my Thomas Vox Super Beatles for four 3886's, each powering one of the speakers in the cab. The original used four 2N3055 output transistors, and they are on the bleeding edge of dying all the time, accounting for the poor reliability rep of these amps. The same power supply will run four 3886 output stages.

I could have the four running series/parallel for the full output power, but I think that I'll just have each one run a separate speaker in the same cabinet. That gives me the redundancy advantage.

The Thomas Vox products are unique in my experience because they already include a soft limiter before the power amp. I think they did this with hopes of keeping the output stage alive, but it sounds good - much better than un-limiter-ed SS amps.

By the way there is a nice gain in efficiency if speakers are within one wavelength of each other. This is pretty easy for low base. I would have to look it up but as I remember you get about 3 dB of efficiency gain for the first 2. It drops off so is not linear after that but going to 4 is still quite an additional gain.

Yes, that is yet another advantage. In my (limited) stage experience, I was always frustrated by the limitations of a single mulitple speaker box. I could not get the sound where I wanted it. Even with multiples in single box, you get bad beaming of trebles, as well as sharply defined cancellation nodes in the audience. By distributing the speakers around, you also partially defeat the inverse-square law by moving the sound source closer to the listener; certainly it's at least pointed in more listeners' directions. That by itself improves the uniformity of your sound and cuts the need for massive power.

Bass is another item. For bass speakers, the Xmax of the driver is a key specification. Xmax - which you are probably familiar with, but the others here may not be - is the maximum displacement of the cone forward or backwards. It's one major determiner of how much bass power you can put out at low frequencies (other things being equal). For big bass, you have to move a lot of air a long way, and Xmax is one measure of that.

It turns out that Xmax is in direct opposition to electromotive efficiency.  To get a long throw on a big cone, you have to either have a long magnetic gap, lowering the magnetic flux per unit of gap, or a long voice coil, lowering the amount of voice coil in the effective gap. Both of these cut speaker motor efficiency.

This works to make single high power bass drivers either much less efficient (requiring huge amplifier powers to make up for it) or quite expensive.

IMHO, there is indeed an economic advantage in using multiple lower powered bass speakers because of the Xmax/efficiency thing. You still have to use big cones, but you can use relatively cheaper ones for the same SPL.

It's a theory at least.

So how does running these chips in series or parallel work? what happens? what's the difference? any good website with info?

The National App note moving quotes is the definitive one.

When I say series, I'm using National's mapropism for bridged. This is a setup where one power amp is fed the input signal, and another identical one is fed the inverse of the input signal. The speaker is tied between the two outputs, not to ground at all. What that does is to feed the speaker twice the voltage that a single amp could provide. Since power in a speaker is proportional to voltage squared, twice the signal voltage is four times the power for a constant speaker load, or twice the power for a doubled impedance load (i.e., 16 instead of 8 ohms).

The 3886 is not particularly good about truly huge currents, as you would expect by the limited area of heat rejecting surface. So it's not a good idea to run two of them and expect quadrupling of output powers. But you can run them in parallel by using some of the circuit tricks the National app note points out. This lets you put four 3886's together, two pushing and two pulling, and get four times a single 3886's power into a single speaker, about 200W.

There are ways to bridge the bridged amplifiers, again doubling the speaker voltage; Crown has done this. But it's not for beginners like me.

[SirPoonga]

So what would you suggest for a practice amp?  Since I am a beginner I just need something to play around with for now, exapansion in the future would be nice.

Thanks for the info guys.  There's more info in this thread than I have been able to find elsewhere.

One more thing.  Since I don't know much about bipolar DC supplies, how would I get +/- 28V for this?

What about heatsinks?  Where would be a good place to get a heatsink to cool these?  Actually, I should look where to get the LM3886's first


Edit:
Parts Express part number 129-080
I'd need to make that bipolar though, right?
So I'd need to do something like the power supply herem but I don;t know what values to use for the parts?
http://www3.sympatico.ca/gagnefr/index_fc.htm?/gagnefr/lm3886.htm

[R.G.]

So I'd need to do something like the power supply herem but I don;t know what values to use for the parts?

The only part he doesn't tell you the value of is the snubbing capacitors across the diode bridge, and those are optional.

The 10A fuse is ... um... optimistic. I'd use a 5A slow blow.
For the diode bridge - use a diode bridge. There are rectifier bridges as single parts. Get a 10A or 25A rectifier bridge, 200V. Mouser has them, and so do almost every electronics supplier and surplus supplier.

Be very very very very very very very very very very careful with your AC wiring and if you've never done it, get someone experienced to help you.  AC line voltage can kill you dead, and it can do it months or years later when you touch it wrong.

[R.G.]

Forgot - go to Power Supply Basics at GEO for some of the how, why, and how much-es of power supply design.

[SirPoonga]

Ok, guess I have to figure out what transformer I need.   The one thing I can't figure out is what A the transformer should be.

Another question.  Lets say I have a lighted toggle switch that I want lit when I turn this on.  What would be the best way to wire that?

Edit: http://www.national.com/an/AN/AN-1192.pdf
So I looked at that.  It would be tempting to make the 200W circuit.  However that company they list for the transformer doesn't seem to have that part anymore  Edit3: I found it, 2x31.2V,interesting...  

Edit2:  I went through the calcs.  ok, I can do the math.  But where the heck do I find a transformer that I need?  I've searched mouser and digikey.  I don't see anything that would work.  Or maybe I am not understanding something.

Other than the transformer listed in the above pdf, what other options are there?

[R.G.]

Ok, guess I have to figure out what transformer I need. The one thing I can't figure out is what A the transformer should be.

We need 21+21Vac (or 42Vct)  to 25+25Vac (50Vct) at 2 to 3A.

Look at the transformers at Parts Express (http://www.partsexpress.com) for a few choices. There's a toroid on ebay now for about $25 that will run one or two.

One of the shortcomings of Mouser and Digikey is that they don't stock any thing suitable for amps.

[SirPoonga]

Thanks.  What about if I wanted to do the 200W circuit?  I showed a friend this project idea and if I can figure out how to do this he wants to do the 200W version.

100W should be fine for bass for practive.  Yeah, I've seen a 100W ampeg head at the local music store.

Are you talking about ebay item 3873347429 or 3873057137?
Just want to make sure I understand what to look for.  Other possibilities:
3873326077
3872871277
3874275242
3874276358
3874129918
3873455040
3874282709
3874254619

ok, that's enough to get an idea if i am looking for the right thing.

Though many of them don't seem to give amperage.  That's what A stands for, right?  That's sorta important, isn't it?

[moving_electrons]

Thanks.  What about if I wanted to do the 200W circuit?  I showed a friend this project idea and if I can figure out how to do this he wants to do the 200W version.

100W should be fine for bass for practive.  Yeah, I've seen a 100W ampeg head at the local music store.

You could implement a separate power supply to feed the second set of amps.   But it would be more economical (assuming it is all in the same box) to just get a transformer with twice the VA rating.  

www.apexjr.has a lot of surplus parts right now that you likely will find useful.  

Be sure to check out the "new" page as there are some great buys in 10,000uF and 12,000uF caps.  Remember a power supply ends up being about 1.4 times the transformer secondary voltage. So a 25v secondaries produces about +/- 35v DC power.  You generally want to use at least 50v power supply caps.  You can always use larger voltage values (within reason) but not smaller.  

There is a lot of evidence that unless snubber diodes are chosen with a lot of research and calculations they can do more harm than good, so don't feel bad about leaving them off.

Something that sounds similar on the DIY audio threads is the "snubberized" power supply.  This is a progression smaller value capacitors mixed with resistors that reduce the ringing etc. as well as present lower inductance to the chip power pins.  This appears to help the mid range and highs as far as clarity for high end audio.  If you want to pursue this you can find interesting threads at DIYaudio.  The www.chipamp.com LM3886 amp implements these.  

No guitar amp (as far as I know) currently implements these and you will likely have a fine bass amp without these.  Or add the additional components later to compare or make yourself feel better later down the road.  Some pretty interesting network analyser work was posted showing what was going on.

But again if you don;t research that and implemement a simple power supply you will have a very clean solid amp.

[moving_electrons]

One last thing.

It is useful to put a 220pF monolythic (multilayer) or standard (not quite as good at high frequencies) between the input+ and input - on the chipamp chip.  Note these are the inputs to the amps not the V+ and V- power ins.

Doing this will eliminate any noise coming into the circuit coming from RF sources such as radio stations etc.   Many folks do fine without this but I and others have to do it.  I live near some radio towers and you may have your amp near them at somepoint as well.  it is a simple precaution.

Sounds like the GGG site has some good wiring info.  Check out the articles at the Elliot Sound Products site on wiring the transformer and AC and don't hesitate to have someone check your work before you power up.

[SirPoonga]

Sounds like the GGG site has some good wiring info.  Check out the articles at the Elliot Sound Products site on wiring the transformer and AC and don't hesitate to have someone check your work before you power up.

Yes, that is why I am asking all my questions here to get rid of any doubt.  I know this is not power to toy with.

I understand the basics of the electronics behind stompboxed and the preamps I am looking at.  however these transformers are going to take some learning before I do anything.

And I am more of a hands on person.  I tend to remember stuff if I do some action or project with it.

[moving_electrons]

You might find the discussion in the link below useful when you start considering the speaker driver and box as well.  Robert is a very strong believer in vented enclosures and I have a tendency to disagree from time to time on some speaker conclusions for home audio. (I really like sealed woofers despite the lack of efficiency and need to brute force them.)  He does not particularly value reduced group delay but really values efficiency. Just the thing for a bass cabinet design I should think!

His calculations and conclusions on venting are always helpful and well thought out.  The bass enclosure discussion in the link below seems quite useful.  He typically has been very willing to verify designs or propose alternatives.  You shold feel very comfortable posting there.

http://www.instantboard.com/users/rode/viewtopic.php?t=1595

[SirPoonga]

http://www.chipamp.com/lm3886.shtml

Whoa.  What is this?  There isn't much for docs.  Is this what I am looking for?  I would rather buy a kit if I could.
Is this everything needed except the transformer?

[SirPoonga]

You might find the discussion in the link below useful when you start considering the speaker driver and box as well.  Robert is a very strong believer in vented enclosures and I have a tendency to disagree from time to time on some speaker conclusions for home audio. (I really like sealed woofers despite the lack of efficiency and need to brute force them.)  He does not particularly value reduced group delay but really values efficiency. Just the thing for a bass cabinet design I should think!

His calculations and conclusions on venting are always helpful and well thought out.  The bass enclosure discussion in the link below seems quite useful.  He typically has been very willing to verify designs or propose alternatives.  You shold feel very comfortable posting there.

http://www.instantboard.com/users/rode/viewtopic.php?t=1595

This is one thing I know quite abit about.  But it is always good to read more.  I built my home theater system speakers.

[moving_electrons]

Sounds like the GGG site has some good wiring info.  Check out the articles at the Elliot Sound Products site on wiring the transformer and AC and don't hesitate to have someone check your work before you power up.

Yes, that is why I am asking all my questions here to get rid of any doubt.  I know this is not power to toy with.

I feel comfortable discussion it here but it is pretty out of place in a stombox forum.  Perhaps it would be best to post the discussion in another forum so others would benefit and R.G and I can follow you over there.  Perhaps start it as a "power supply for bass amp" discussion.

You can start a "Bridged LM3886 bass amp discussion" to pursue that as well.

But I am not sure where. The Elliot Sound Products forums for Power supplies is really provided to discuss power supplies for the designs and boards on that site.  The chipamp forum at DIYaudio will suffer from the problem that no one there will recommend doing a high capacitance supply (which you need) without the "snubberized" supply adds and MUR860 diodes due to their experiences trying to get the most out of the 3886 for home audio.  Their thoughts are not bull as the ideas discussed there really get some outsanding results for audio.  But for a bass amp these may be minor considerations as far as you are concerned.

Perhaps it should be in the "Musical Instruments" forum over there.  By focusing on the need (bass amp) we can get a good discussion recorded there.  I will be happy to follw it through the design and implementation.  

Perhaps someone could suggest a guitar amp forum.  In absense of that, how about the "Musical Instruments" forum at DIYaudio.  I don't want to step on any sensibilities here.  If there is a better alternative, I hope somone will suggest it.

[SirPoonga]

Same thing I was thinking.  We kinda took over this thread and it isn't stompbox related.  I am open to moving the discussion elsewhere.  Just point me (link) in the right direction.
I am enjoying this conversation.  Pretty cool stuff.  Like I said, I am learning more in this thread then elsewhere on the net.  Not many people seem to be talking about building a bass amp.  Lots of guitar amp stuff out there...
And this (power amp) is the only piece of the puzzle I am not comfortable building yet.  We are getting close though

/me looks at the clock.  Whoa,  past bed time.