Starting a student project, need some input (chip amp + speaker set)

[Sneasle]

Well, it really depends on the context. Sometimes it means that two amplifiers are used for each output channel in a BTL or paralleled configuration. In this case it means 'enough parts to build two mono amplifiers in separate cases'. The only difference is that the kit includes an extra power supply board so you can build two separate 'mono blocks' instead of a single stereo amplifier.

Cool, thanks. I figured it was something like that.


Alrighty, from my reading and what I have seen these are the most popular of the Chips:

LM1875, 20w
LM3875, 50w
LM3886, 68w

In all honesty, I think 20W would be fine for out application. That being said, Before I looked up the wattage ratings on these I called a buddy of mine and asked him and he said that Stereo @ 25W should be plenty for us, so it looks like the LM1875 is the winner.

Anyone here who has built all of these wanna give me a cost estimate for each? Unless one of the higher wattage chips can be built for a surprisingly good price, I think the LM1875 will be the chip to go with.


There is also the LM4780, but it looks to be a beast.

 

[Variac]

I think that almost the entire difference in cost between the various chip amps is the size of the transformer. What with the price of copper going up they are pretty pricey.

I suppose that the more powerful chips are a bit more too....

 

[error401]

Don't forget the LM4780 - the bonus there is that it's a stereo chip that contains two LM3886s. It's still going to cost you more than two LM1875s.

I haven't built the LM1875, but as a rough breakdown, ignoring the chassis:

-PCBs - you seem to be fabricating them, so free? $10 from chipamp.com
-Transformer - Variac has a point, this will be a large part of the cost. I think chipamp.com overspecifies (they say 250VA for a 2x20W amp). I would get the AN-1222 from Antek for $18.
-Heatsink - a 2.5" TO-220 sink per is probably sufficient, or you could use one large sink, or the side of a chassis or something. Probably about $5 if you go the TO-220 route. Don't forget you need isolation kits.
-Electronics - pretty cheap, 2xLM1875s and a power supply is going to be around $30. You could save another $5 by using cheap diodes instead of the MUR860 chipamp.com recommends
-Volume control, input jacks, power switch, binding posts - if you skimp, you can probably do all this for $10-15. Nice stuff costs though.

So assuming you can get the PCBs for free, I estimate $75-80 without a case, assuming shipping is negligible (combined ordering). You might be able to shave a few bucks off each amp with quantity discounts too. Don't forget to budget for parts getting fried, and make sure you have replacements on hand.

 

[Sneasle]

That sounds very doable.

We are hopping that by buying in bulk we can save some as well. We will also see if the dept. is willing to pitch in and help with the costs some.


I'm almost leaning more toward the LM3886 and just not powering it past 25W or so.


How difficult would it be to build a 5.1 amp with an LM3886? The only issues I could see would be isolating the inputs.... possibly issues with higher current areas in each amp being too close and causing some weird mutual coupling in another...


I get more and more excited about this the more I think about it.

 

[danielwritesbac]

Some Transformers for LM1875

Monoblocs / Dual Mono for LM1875 with full power supply:
1 amper 36vct (18-0-18)
http://www.alliedelec.com/Search/Pr...6K271HF&R=2272060&SEARCH=2272060&DESC=6K271HF Phone order prices may be had at $12.25 for projects like yours.

Monoblocs /Dual Mono for LM1875 with gainclone style power:
1.5 amper 36vct (18-0-18)
http://www.alliedelec.com/Search/Pr...MPN=F7-36&R=9677403&SEARCH=9677403&DESC=F7-36

Stereo:
4 amper 36vct (18-0-18)
http://www.alliedelec.com/Search/Pr...274VBR&R=2272065&SEARCH=2272065&DESC=6K274VBR

 

[danielwritesbac]

That sounds very doable.
How difficult would it be to build a 5.1 amp with an LM3886? The only issues I could see would be isolating the inputs.... possibly issues with higher current areas in each amp being too close and causing some weird mutual coupling in another...

Then you need a preamplifier and a delay circuit for the two rear channels. If the complexity of that distorts the natural, individual ambients present on each recording, then you'll also need a reverb; and, that's a poor substitute because it isn't unique for each recording.

I'd suggest creating a hi-fi stereo and then giving surround sound as an optional expansion project. In this way, you don't need to "fake it" with a reverb because you can avoid distorting the 2 main channels. Its a like "fake the room, not the music" approach. Either way, the 2 rear channels do run from a delay circuit.

As far as the .1 goes, its possible that you'll find woofers to be a more effective use of living space than wasting that same area with speaker stands and then wasting yet more living space with a subwoofer. Therefore. . .

Like the amplifier, the speaker project can also be scalable in size. Look up and give a second look at Harbeth Monitor 40. See that if the central wideband driver were capable of significant bass response by itself (as is the case with a 2-way bookshelf speaker), then you could make 2 different size speakers (with or without the 12" woofer).

 

[danielwritesbac]

I'm almost leaning more toward the LM3886 and just not powering it past 25W or so.

Initally, that sounds like a great idea because it would decrease the price of heatsinks; however, in practice, much clipping can result from undervolting.

Another way to decrease the price of heatsinks is to find some old "slot" type (long and big) CPU coolers. Yet another source is broken or worn out equipment from Salvation Army and similar sources, as this equipment is likely to have generous heatsinks (visible through the vents). A third source is low-fidelity "ugly black box" mass-market electronics that appear to work, yet are inherently poor quality for listening--yet these can contribute many parts, including a heatsink. You can also find heatsinks nice, used, at lower prices, from amatuer radio supply.

 

[pacificblue]

Initally, that sounds like a great idea because it would decrease the price of heatsinks; however, in practice, much clipping can result from undervolting.

I would expect clipping to happen at very similar levels on the LM3886 and LM1875, if you use them with the same power supply.

Using the LM3886 has the advantage that you can still upgrade it with a bigger power supply later on. Next year's student project: Improve audio power supply and adapt heatsinks to the new situation. Analyze the sound improvements due to higher available output power. Compare your perception of SPL improvement in relation to the effective output power increase and your previous expectations.

 

[Sneasle]

Are the supply voltage ranges on both IC's similar? I can understand running into problems if you start to take it outside of it's specced range.

Or are you meaning that if your +/- rails aren't high enough that your input can possibly fluctuate enough that your output goes flat at some point?

Pacificblue: I'm really liking this idea, but I want to make sure any kinks are worked out before I start order pcb's, if possible .


I'll see if I can find some of my old board layouts and get them posted for you all. Not sure how I have the files saved, but we shall see.

 

[Variac]

Daniel, I don't think that he wanted to "Fake"the rear channels, just have the option of multiple channels. You have pointed out the complications of this- they will need a preamp with decoder. Usually the separate pre's are in the higher priced catagories I think..

 

[danielwritesbac]

. . . but I want to make sure any kinks are worked out before I start order pcb's, if possible

Interesting! You could try several chipamp possibilities before deciding on one. Although, a PCB is actually unnecessary. I think that having the students prototype first, and then design their own PCB, may be more educational than studying basic soldering skills on pre-made PCBs.

To answer the question, the voltage ranges for LM3886 and LM1875 do overlap, but they are not identical. See the PDF files provided at National Semiconductor's website.

Owner results from same-same powered LM3886 and LM1875 report the smaller as less likely to clip. Therefore, avoid trouble with LM3886, and aim for enough power-- from 40vct (20-0-20AC) to 48vct (24-0-24AC) transformer with generous amperage. That's low enough voltage to run slightly cooler, but high enough to insure performance. The meaning of this disparity is that the manufacturer's advertised power output rating of the larger chip is slightly less applicable--a very frequent thing for an amplifier document.
The LM3886 does make more output power than the LM1875; however, LM3886 requires more voltage to do so, and in a larger proportion than indicated by the documentation (if a significant difference is expected). Bear in mind that double power = 3db more output.

Tip: Use smaller voltage for 4 ohm speakers, but you can use higher voltage with 8 ohm speakers. Chipamps "centerline" spec at 6 ohm drivers, therefore use a larger heatsink than the document suggests (if the amp will be used with 4 ohm speakers and in the case of other electronics occupying the same enclosure).
When purchasing heatsinks, they may be advertised in "CT per 3 inches" rather than listing for the entire heatsink. Yeah!! I got some monstor heatsinks that way by accident. For a lot of student projects, it may be more economical to purchase an entire extrusion (like a really big stick) and then cut off the needed sizes. Tabor extrusions (makes aavid thermalloy) may be a good resource, because any factory seconds/blem not used by Aavid, could still work just as well as heatsinks.

Also, while "interviewing candidates" for your amplifier project, check out and see what a preamplifier can do for you. The LM4562, OPA134 series, and NE5534 op amps have been popular for this use.
The point to such a preamplifier is features of wider soundstage/euphonic in combination with the detail still intact.
That effect may provide all the surround sound you need, amplifying (instead of erasing) the natural ambients present on recordings, using just 2 speakers, with the additional advantage of not having to run wires helter-skelter about the home.
Well, I hope that illustrates the potential value of an optional preamplifier project.

 

[error401]

24-0-24 is too much for the LM1875. That chip is specified for 60V only. Use 20-0-20.

 

[danielwritesbac]

Daniel, I don't think that he wanted to "Fake"the rear channels, just have the option of multiple channels. You have pointed out the complications of this- they will need a preamp with decoder. Usually the separate pre's are in the higher priced catagories I think..

My concern is that the typical digital decoder "fakes" the front 2 main channels, making high fidelity music reproduction impossible.

Instead of more of me, howabout this instead:
http://www.linkwitzlab.com/surround_system.htm

EDIT: At that link, Linkwitz illustrates how to replay hi-fi and surround, simultaneously.

 

[danielwritesbac]

24-0-24 is too much for the LM1875. That chip is specified for 60V only. Use 20-0-20.

Nobody said otherwise.

LM1875 max transformer:
20-0-20 with 8 ohm speakers
18-0-18 with 4 ohm speakers

 

[gtforme00]

I would recommend going with the higher power LM3886, even if you don't need all the power right now. You will have much more flexibility and universal appeal with the 3886. I have run them off voltages as low as +/- 15V and they do just fine. There will always be someone that wants a little more power, and the LM3886 can put out a remarkable amount of power for its size.

I agree with the recommendation for the Antec transformers, though Steve at www.apexjr.com has a good supply of suitable transformers as well.

The power supply board is the easy one! If you aren't going to buy the amp boards I wouldn't bother buying the PS boards either.

You might want to talk to a National rep about getting the program sponsored with free chips. They have a policy to provide 5 samples per student (with a valid email address) and you could order them individually through the website, but a call to National might get you more without the hassle of multiple people having to order them.

One advantage to ordering them individually is that the user can specify whether they want the insulated or non-insulated version. The insulated version is great for lower power designs, and saves the cost and inconvenience of providing and installing heat sink shims.

I have used heat sinks from the old slot style pentium processors and they work great. Some of them are attractive enough to even make decent sides to a case out of.

For the low power verisons, you don't even really need a dedicated heat sink, you can simply use the chassis itself as a heat sink, similar to Peter Daniel's famous designs.

Do you guys have regular meeting times? I don't really venture to Auburn anymore, but I would be willing to drive up for a meeting or two. I could even do a guest presentation of speaker design tools and software if you are interested (I use speaker workshop, a free measurement program).

Regards,
David Malphurs

 

[danielwritesbac]

Hey Sneasle!
I think you should take gtforme00 up on that offer. You asked for "applied" and that's it. Real live demos of how to do it, why to do it, and also how to use it. Priceless!!

I think this matters so much more than the exact nature and specifications of the equipment. Its using/applying the audio equipment that really counts (in my opinion).

 

[Sneasle]

David,

We would very much be interested in you coming up (at least I personally am). These are usually Monday night IEEE and SPARC meetings, chances are the IEEE meeting is the 'correct' one.


I like the idea of contacting National. I will keep that in mind as we progress.




Ya'll are gong a little over my head right now with all of this decoder stuff. Why is a preamp needed with 5.1 when not for 2 channel? My PC sound card has 3 separate outputs for my current 5.1 speakers, rear on one 1/8th plug, front sides on a second plug, and sub and center on another.

Soo.... why do I need preamps and decoders? Can't I just pull this inputs off and send them to the proper Amp circuits?



What do ya'll think about this for a case :

http://shop3.frys.com/product/5689841

I know its a comp case, but it can be modified.

 

[gtforme00]

I used a cheap metal project enclosure for my computer speaker chipamp. It was intended to fit in a cubby on my desk hutch so an external power supply was used with an umbilical cord to the amp. Attached are some pictures. I made a new front plate out of some 1/8" aluminum plate that I brushed with a belt sander.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

The local radioshack had the box and speaker terminals, I scavenged an old PII slot style processor heat sink which is mounted inside the amp.

Radioshack no longer carries the project box I purchased, but I found them online at webtronics. They even have larger boxes than the one I used; you wouldn't have to use a separate power supply box if you used the larger sizes. They are very inexpensive as well, and look good (to me).

11.9"W x 7.5"D x 2.9"H = $14.54

Monday nights are usually uneventful for me, so that should work out well if there is interest.

Regards,
David

 

[Sneasle]

that sounds great David.


Ok.. lets move forward using the LM3886 then.

I'm headed to the R/C field for most of the day tmrw so I'll do some reading when I get back and start trying to pick out parts and start getting some lists together.

 

[danielwritesbac]

. . . start trying to pick out parts and start getting some lists together.

Then, he needs to send you a parts list. Using the same parts is more likely successful. This relates to why he likes that amplifier so much, and the application of it.