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Chip Amps Amplifiers based on integrated circuits

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Old 12th February 2012, 03:12 AM   #1
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Join Date: Apr 2009
Default Beginning crazy project, looking for advice and whatnot!

Hi all! So I've been kicking around the idea of building a new pair of 3-way tri-amped speakers. The speakers would be based on the HiVi F8, the HiVi F5 and the Vifa XT25TG30-04; specs being:
F8: Impedence: 8Ω, Re: 6.5Ω, 60Wrms, 120Wmax
F5: Impedence: 8Ω, Re: 6.5Ω, 35Wrms, 70Wmax
XT25: Impedence: 4Ω, Re:2.9Ω, 90Wrms, 140Wmax

Thorsten and I have already traded a handful of messages on facebook and came to this conclusion:

2 * LM3886 Bridged for LF (20Hz - 320Hz) with a 230VA or bigger 22V + 22V transformer and as much capacitance as sensible
1 * LM3875 SE for MF (320Hz to 1.3KHz) with a 60VA or bigger 22V + 22V transformer and 4 * 1,000uF
1 * LM3875 SE for HF (1.3Khz to 20KHz) with a 60VA or bigger 22V + 22V transformer and 4 * 1,000uF

Transformers types being:
Torroid for the LF. Thinking about using this one here.

I've read about a lot of guys using the Plitron torroids for their builds with good results and I really like the price point.

For the MF and HF, Thorsten suggested "EI, R-Core or double C-Core preferably with electrostatic screens."

I haven't the slightest as to which transformers to use for MF/HF, so I could use suggestions there.

I plan on building each power supply using CarlosFM's snubberized schematic, I had a reference by can't find it at the moment.


I'm planning on building these amps in a fully balanced configuration, as the active crossover, and DAC I'm planning on using, as well as the pre-amp I'll be building are all going to be fully balanced. If my understanding is correct, I can simply use a stereo chip but wire it so that one channel is the positive side of the signal, and the other channel is the negative side.

I'm still trying to work out all the details and would love any and all input you guys have.

TL;DR: 6 channels of amplification, balanced input, LM3886 based amp for F8, LM3875 based amps for F5 and XT25TG30-04. In need of MF/HF transformers as well as more details about balanced input.

-Ben
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Old 12th February 2012, 07:24 AM   #2
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Join Date: May 2008
Attack the project step by step. Use one transformer for all three amps. When you have the speakers running, you can still experiment with more transformers, but will probably come to the conclusion that you don't need to.

It doesn't make much sense to fix the crossover frequencies before you have the speaker built up and measured.

You don't write, how and why you decided for the amps and transformers. It is usually not recommendable to use more then 2x18 V on those chip amps into 4 Ohm loads, i.e. your woofer and tweeter amps are likely to give trouble with thermal protection. The tweeter amp can be driven into its current limit, too.

Maybe you really need that BTL amp for the woofer. Hard to judge without knowing, whether it will be a closed box, bass-reflex, open baffle or whatever, and how loud you usually listen, but what makes you think you need more power for the tweeter than for the mid range? And why use different chipamps for the woofer than for midrange and tweeter?

Don't worry too much about which transformer to use. They all turn out AC voltage at mains frequency, so buy just any.

Yes, you can use a stereo chip as BTL amp. You would use the LM4780 instead of two LM3886, need a more sophisticated double-sided PCB and it will increase the heatsinking problem even further.

Looking forward to see your fully balanced crossover schematic.
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Old 12th February 2012, 08:27 PM   #3
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Quote:
Originally Posted by pacificblue View Post
It doesn't make much sense to fix the crossover frequencies before you have the speaker built up and measured.
The frequencies listed are simply reference points. The actual crossover frequency will be set once the speakers are built and measured.

Quote:
Originally Posted by pacificblue View Post
You don't write, how and why you decided for the amps and transformers. It is usually not recommendable to use more then 2x18 V on those chip amps into 4 Ohm loads, i.e. your woofer and tweeter amps are likely to give trouble with thermal protection. The tweeter amp can be driven into its current limit, too.
The amps were suggested by Thorsten (I believe he goes by "Thor" on here.) I was originally looking at building something like the Nelson Pass F5s, which would have ended up costing more, taking up more space and generating more heat (which is a big issue in Texas in the middle of July.) He suggested the Gainclones and after looking into them a bit, I decided the ones he had suggested looked like they would fit the bill quite well.
The tweeter would be driven with a lower voltage transformer, such as 2x18v in order to keep the wattage lower. It was mentioned that the Vifa guys are a bit optimistic about the power handling of that tweeter.

If anything, I could put a 100w 4Ω power resistor in series with the tweeter and run the amp at 2x22v. This is definitely a good point to bring up however!

Quote:
Originally Posted by pacificblue View Post
Maybe you really need that BTL amp for the woofer. Hard to judge without knowing, whether it will be a closed box, bass-reflex, open baffle or whatever, and how loud you usually listen, but what makes you think you need more power for the tweeter than for the mid range? And why use different chipamps for the woofer than for midrange and tweeter?
The woofers will definitely be built in a bass-reflex configuration. I haven't decided on ported or passive radiator just yet, but I'll make that decision eventually.

As far as how loud I listen: On average I listen at what I was taught was the high side of "mixing" volume, so ~88-92dB. That said, I do occasionally enjoy really cranking it up when a song I really love comes on.

Quote:
Originally Posted by pacificblue View Post
Don't worry too much about which transformer to use. They all turn out AC voltage at mains frequency, so buy just any.
I've read arguments both for and against that statement, for that reason I will not be spending a crazy amount of money on transformers (other than the fact I'll need quite a few by the end of all this). But I also don't want to pick up a super cheapo transformer that compromises something such as safety or is overrated.

Quote:
Originally Posted by pacificblue View Post
Yes, you can use a stereo chip as BTL amp. You would use the LM4780 instead of two LM3886, need a more sophisticated double-sided PCB and it will increase the heatsinking problem even further.
I plan on using all chips as mono amps, so I'd need at least 6 chips to get the amplification I desire. I plan on building these amps in a pseudo-monoblock configuration; left speaker's LF, MF, and HF amps will go in one chassis, right speaker's in another. The chassis will either be a 3u or 4u enclosure (depending on the height of my Pre-amp), with the PSU at the bottom of the unit separated from the amp section with a sheet of metal.

More than likely I'll end up building my amps on perfboard or maybe p2p, if I'm feeling confident enough in my soldering.

Quote:
Originally Posted by pacificblue View Post
Looking forward to see your fully balanced crossover schematic.
Sadly, I feel a fully balanced, digital crossover with time align, phase adjust and EQ is a bit out of my league. I plan on using an "Ashly Audio Protea 4.24C Speaker Processor" to handle all my crossover duties. I plan on using a subwoofer I built years ago to supplement low end when watching movies or playing games, so I needed an active crossover that would be able to give me the option for either.


As far as testing equipment goes, I've got a Fluke 110 DMM, a Hitachi V-1065 100MHz oscilloscope and a B&K Precision Model 4003A 4MHz function generator. I plan on purchasing a calibrated measurement microphone and an interface for testing/measure purposes once I begin work on the speakers.

-Ben
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