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Old 5th August 2011, 07:36 PM   #1
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Join Date: Aug 2011
Location: North Carolina
Question First Audio Circuit (Gain Clone w/LM3886)

I am new to the site and really audio diy on the whole. Currently, I want to build a 2 channel system. However...I want to build using an active crossover and I was wondering the feasability or tips on this build. I would be using probably protoboard to nail everything down.

Power Supply:

36V, 10A center tap transformer rectified using
DIY LM3886 Chip Amplifier (Gainclone) Project


Currently my plan is to build using a quad op amp and just use a simple Sallan-Key topography with a two cap, two resistor system.

All cutoffs = 1/(2*pi*R*C)
Low pass = 338 Hz cutoff
High pass = about 3000 Hz cutoff
Bandpass = cascaded low and high pass with low pass at 300 Hz and high pass at about 3300 Hz.

Obviously, I would have a soldered 3.5 mm headphone jack for input to the system in stereo. Therefore, I would have a 3 way crossover for each channel and each crossover would lead to a different speaker (probably woofer, mid, tweet).

I know active crossovers are sort of out of practice but I really want to build this and I think it has the potential to sound great, but again I'm new.

For the amps themselves...

LM 3886 using the amp configuration from above but one seperately for each speaker. Hence, the huge power supply because 6 * each amp which is going to put out at most about 40 W on 8 ohm load @ 75% efficiency = total of about 300 VA including minimal power draw from op amps.

There would be slow blow about 3A fuses at power supply and I would use EMI filter at power input to transformer.

Thoughts???
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Old 5th August 2011, 08:45 PM   #2
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Hi Milsman2, good luck with your project. One issue you might consider is protection of your tweeters. Turn-on thump is very common when turning on the power amp. In passive crossovers, this transient gets passed to the big burly woofer, which can handle itself. If a nasty transient is generated by the tweeter amp I can see two obvious ways of dealing with it: a timed relay in the path, or a capacitor. The relay sounds complicated but really isn't, the capacitor could cost a few quid. For some, the capacitor quality is a massive deal.
Another issue is that crossovers should really be tunable so you can optimise the sound in your listening space. I use a miniDSP, others use filter components that are selectable with a pin header so they can tune pretty easily.
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Old 6th August 2011, 11:49 AM   #3
AndrewT is offline AndrewT  Scotland
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Don't build that version of a Gainclone. Too many "optional" components are omitted for good reliability.

Build an AC coupled Chipamp with all the optional components shown by National included.

Build one chipamp first. Test it. Assess it. Change if required.
Build four/six chipamps to your final specification.
Then decide how to package them along with the PSU and heatsink/s after you see how much space/volume you require.
Buy a chassis to suit.
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Old 7th August 2011, 05:38 AM   #4
Francec is offline Francec  Australia
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Quote:
I know active crossovers are sort of out of practice but I really want to build this and I think it has the potential to sound great, but again I'm new.
I don't believe this is the case.
Many DIYers use an active setup, as I do,m and it does sound great. I haven't tested the speakers yet; this is planned when the new 10" SB Acoustics woofers replace the 8" Peerless.
Depending on your woofer and if you like loud rock music (as I do), it may be worthwhile bridging two gainclones to provide plenty of power.

Frank
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Old 9th August 2011, 04:40 AM   #5
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Location: North Carolina
I plan on building the versions from ChipAmp for each driver. However, after some consideration and a little looks into partsexpress.com, I've come to the conclusion that I need to rethink my plan. That is to say I'm fairly unfamiliar with actual speaker design and I need some help picking out speakers with the right frequency response. I don't really know what the proper amount of crossover in terms of frequency should be among the three speakers per channel. After I get help with that, I could always adjust the active crossovers accordingly and proceed. Any help?
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Old 9th August 2011, 11:55 AM   #6
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Specfying a 3-way speaker and the cross-over frequencies without knowing which drivers you use is certainly making the second step before the first.

If you want tips on speaker choice, you will first have to specify what you expect the speakers to do. Make a list of your demands and then find out which speaker(s) can fulfill them all or what compromises you have to and are prepared to accept. Your list should contain at least the following items.
  • Price limit
  • How much of that money is for optics (enclosure material, paint, etc.), how much for sonics (drivers, cross-over, amplifier, cables, etc.)
  • Size limit
  • Set-up possibilities (free-standing, in shelf, in corner, desktop, 2.0, 2.1, etc.)
  • Listening distance
  • Listening room size
  • Desired sweet spot width
  • Desired low-end extension
  • Required SPL
  • Others
Active cross-overs are not at all out of practice. They are or were however only commercialized at high prices which somehow stood in the way of their widespread success. And commercial preamplifiers were only offered at high prices, too. At the moment there is actually a trend towards small and reasonably priced active speakers thanks to PCs, MP3-players and so on which have their pre-amps built in.
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Old 9th August 2011, 05:30 PM   #7
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Well, if you don't mind helping I'm actually dropping this down to a 2-way system.

As far as price is concerned, it's not really too much of a constraint, I'm really just trying to build this all for about under $500, but not too much of that would be visual appeal.

There is no size limit realistically.

I plan for this to either be a bookshelf speaker or in corner situation. I don't currently have plans for it to be 2.1 but it may.

This would be for a small room (i.e. apartment living room or large bedroom).

I would probably be about 15 - 30 feet away at any given time, but I don't mind the loudness a little.

The other constraints you posted are also very flexible.

Being that all of those things are the case, what would you suggest for using the LM 3886's? I have found on parts express several speakers which I *think* could meet my requirements. However, my biggest problem I am pondering at the moment is this...

If am running let's say at a PS voltage of about 35 and across 8 ohm the LM 3886 dumps about 40 W and across 4 ohm (i.e. woofer) about 90 W...with 75-80% efficiency that's something on the order of 175 W per channel. What kind of transformer (when rectified) could handle that kind of power x2?? Thanks for all the comments. Currently I am thinking woofer crossover at about 3000Hz and letting the tweeter sing from there.
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Old 10th August 2011, 08:49 AM   #8
AndrewT is offline AndrewT  Scotland
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You must use the National datasheet and application notes to determine a reliable voltage for use with 8ohm speaker and/or with 4ohm speaker.
National do not recommend +-35Vdc as a supply for 4ohm speakers.
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Old 10th August 2011, 12:07 PM   #9
Francec is offline Francec  Australia
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mils,
I had a passive two way which I converted to active and then to an active three way. Things just got better as I went along. I can suggest some options that do work very well with recognised brand, reasonable quality drivers that will really rock, if the need takes you. ;<} (As indeed it takes me regularly.)

If you want "the formula", let me know AND yes, it is/was all achieved with the LM3886. Don't worry about the last paragraph power calculations. Those are useful for passive crossovers when a lot of power goes into the crossover. When you are talking active, you won't think 20 to 30 W is underpowered. My neighbours don't think so either, nor will yours and I don't mean just next door.

Frank
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Old 10th August 2011, 12:37 PM   #10
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So you want an active 2-way system with no size limit that works in a small room at a distance of up to 30 feet either in a bookshelf or a corner. SPL is of no concern, neither are sweet spot and low-end extension and everything else is flexible. Are you sure, it must be a loudspeaker at all?

My recommendations
  • make a tour of a few shops that sell speakers and listen to several different ones. Then decide whether everything is really of no concern or flexible.
  • check out some studio monitors. They usually come with switches or potentiometers to adjust them to different positions. You will see that it is not the same to put speakers in a shelf or in a corner and that it will influence the enclosure choice and the cross-over.
  • read up on heatsinking. The LM3886 will not work for long into a 4 Ohm load from 35 V rails.
  • determine the amplifier according to the speakers, not the other way round.
  • determine the transformer size according to output power and ambient temperature, not according to heat dissipation.
  • determine the cross-over frequency after knowing the drivers, not before.
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