Alpair 7.3 Impedance (for class d amp LP filter)? - diyAudio
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Old 8th May 2014, 04:18 PM   #1
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Default Alpair 7.3 Impedance (for class d amp LP filter)?

I'm probably missing something obvious, but what is the impedance of the Alpair 7.3? I'll be powering mine with a tpa3116-based class d amp. The amp has an onboard LP filter for removing HF noise introduced by the amp's switching.

I looked at the Alpair 7.3 data sheet from the MA website. It's not dumbed-down enough for me.

I guess it's a more general question, as most commercial off-the-shelf speakers give a single number for their impedance, but I realize that in reality, most speakers have a variable impedance across their frequency range. So what is the single number that is quoted? The average? Or the minimum? Or something else?

Looking at the A7.3 impedance vs frequency curve, it looks like it never goes below 6 ohms, and is around 10 ohms at 20 kHz... what value should I use for the best-matched class-d LP filter?

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Old 8th May 2014, 04:24 PM   #2
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Also, a related question: according to the tpa3116 datasheet, it's technically possible to run the amp without any LP filter, and use simple ferrite beads for removing/reducing EMI. In this scenario, the speaker itself becomes the filter for the high-frequency switching noise. In my reading I've seen suggestions that this might damage some speakers.

Just curious if the Alpair 7.3 could potentially be damaged by sending it an unfiltered signal that includes class-d HF switching noise?
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Old 8th May 2014, 04:46 PM   #3
chrisb is offline chrisb  Canada
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The nominal voice coil DC resistance (ReVC) = 5.4ohms , whereas as you note the graph shows minimum of just over 6. Impedance is reactive with frequency, but keeping it simple, probably plan on the 6ohm value.

Experts in DIY class D amps should pipe in here, but I'd take the conservative approach. I'd guess that while not likely to be damaged by anything but a severely misbehaving amp, the Alpairs have sifficient HF resolution that any artifacts from power supply noise would not be camouflaged.
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Old 8th May 2014, 04:56 PM   #4
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Quote:
Originally Posted by chrisb View Post
The nominal voice coil DC resistance (ReVC) = 5.4ohms , whereas as you note the graph shows minimum of just over 6. Impedance is reactive with frequency, but keeping it simple, probably plan on the 6ohm value.

Experts in DIY class D amps should pipe in here, but I'd take the conservative approach. I'd guess that while not likely to be damaged by anything but a severely misbehaving amp, the Alpairs have sifficient HF resolution that any artifacts from power supply noise would not be camouflaged.
Thanks Chris! By the way, these A7.3s are currently in transit to me from Victoria, BC, along with a CGR dMar-Ken7.3 flat pack that I believe was made by you.
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Old 8th May 2014, 05:10 PM   #5
chrisb is offline chrisb  Canada
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Originally Posted by matt_garman View Post
Thanks Chris! By the way, these A7.3s are currently in transit to me from Victoria, BC, along with a CGR dMar-Ken7.3 flat pack that I believe was made by you.
Matt: hope you enjoy them!
Dave should have some "suggested assembly" photos that might be helpful. While I never like to second guess any kit builder's skill set, and there are lots more experienced craftsman than I, these photos will show the process steps that I would use. Hint - if you plan on veneering or painting them, and have access to brad nailer, you'll need a lot fewer clamps.
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Old 8th May 2014, 06:23 PM   #6
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I am noexpert, but...

A Class D amplifier or switching amplifier has a very high frequency carrier that needs to be filtered out. The LP filter is < 20 kHz, so you'd like to use that frequency. Since most impedance curves pnlu go to 20 kHz, that is as close as you are going to get. So i'd use the 20 ohm number.

The nominal impedance single number given is just a general guideline that puts speakers into "bins". This is usually the DCR +a little bit for the reactive impedance so A7.3 would be called an 8 ohm speaker.

dave
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Old 8th May 2014, 08:55 PM   #7
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I'm not an expert either, but I do know this: The main difference in the sound of different class d amps is the output filter Dave is talking about. Cheap amps have cheap, sometimes almost nonexistent filters. More expensive amps are likely to have decent filters with decent parts. Sure, you can mod a $20 amp into something useful, but you will spend more on parts than the original cost of the amp, and of course your time is free.

The Tripath amps are now technically old. There are some nice implementations starting at ~$200 and up. I have a nice one using the TA2022 with an analog power supply and name brand parts that sounds pretty good. Texas Instruments has a line of class d amps that start with TDA. There are a large number of demonstration boards that start at $20, no case, no power supply. Full implementations are starting to appear.

Now to answer your question, if you get a decent class d amp, you don't need to worry about the switching signal passing through to the audio range.

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Old 8th May 2014, 10:12 PM   #8
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Originally Posted by Bob Brines View Post
I'm not an expert either, but I do know this: The main difference in the sound of different class d amps is the output filter Dave is talking about. Cheap amps have cheap, sometimes almost nonexistent filters. More expensive amps are likely to have decent filters with decent parts. Sure, you can mod a $20 amp into something useful, but you will spend more on parts than the original cost of the amp, and of course your time is free.
I'm likewise no expert, but based on my limited hobbyist experience: I suspect the sound quality of different class-d amps is a function of both the output filter and the chip itself. Like a chain, it's only as good as the weakest component. But, to be fair, I'm just making assumptions.

Quote:
Originally Posted by Bob Brines View Post
The Tripath amps are now technically old. There are some nice implementations starting at ~$200 and up. I have a nice one using the TA2022 with an analog power supply and name brand parts that sounds pretty good. Texas Instruments has a line of class d amps that start with TDA. There are a large number of demonstration boards that start at $20, no case, no power supply. Full implementations are starting to appear.
Just to be pedantic: the Texas Instruments line starts with TPA. TDA is from another company.

In fact, take a peek of this tpa3116 thread here on diyAudio if you're interested. Or you can check out the Wiki that I've been trying to maintain as a consolidated version of the crazy-long discussion thread.

This A7.3 discussion revolves around my desktop/nearfield setup. SPL is low, so I don't need a lot of power, and space is at a premium, so I want a relatively small amp, so class D is a good fit. Plus the efficiency/low power appeals to me... and I'm cheap and I like to tinker.

Previously I was using a HiFiMeDIY T2 amp, which is based on the TK2050 chip. It's a fairly well-regarded implementation.

But over on AudioCircle, in the "Cheap and Cheerful" forum, someone was talking up this $10 Sure TPA3110 board. For $10, I figured it was worth a try. I immediately noticed a substantial improvement in sound quality. I happily ran with that for a while, until I stumbled on that tpa3116 thread. The tpa3110 and 3116 are very similar, with the latter having more output power capability. This Yuan-Jing came out with a tpa3116 board that bears an uncanny resemblance to a suggested PCB layout that one of the forum members (Danzz) floated. The board uses mostly through-hole components so it's easy to modify. (It was only $20 when introduced, can still be had for that much off ebay.)

The general consensus is that the biggest improvement comes from upgrading the output filter inductors... and that's what brought me to bring up this question, since the ideal inductor is "tuned" to speaker impedance.

To me, the lower-power 3110 still has a slight edge over the 3116. But, the 3116 stock output filter is designed with 4 ohm speakers in mind; my current desktop speakers are 8 ohm. And interestingly, the 3110 is a filterless design.

Also, FWIW, my living room system uses yet another class d amp: it's an IRS2092-based amp, model "SDS-470" from the company Class D Audio. I have it powering what I consider fairly decent speakers (Salk Songtower). This is a fairly expensive amp (relative to the tpa311x amps anyway), and in my limited hifi experience, I consider it to be excellent. (Way too overpowered for desktop use though.) I used my stock tpa3116 build in it's place for a while, and didn't note any obvious differences, although I haven't had time for much critical listening on that system.

Anyway, just rambling about the TI TPA amps, as I've been having a lot of fun lately hacking on them and reading about others experiences.
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Old 8th May 2014, 11:25 PM   #9
irribeo is offline irribeo  Netherlands
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There is a little something added after the LC filter most of times, not mentioned much, but it is parallel to speaker, TI calls a snubber.
Translated to single ended output parallel to speaker there is a 6.6 ohm resistor and a 5nF capacitor. How does impedance curve of alpair7.3 look with odd Zobellike parts of these values?
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Old 10th May 2014, 07:12 AM   #10
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Originally Posted by matt_garman View Post
Anyway, just rambling about the TI TPA amps, as I've been having a lot of fun lately hacking on them and reading about others experiences.
The TPA amps and links have been mentioned to Bob before but he blew them off and went for the TA2022, which is surprising given that the 2022 is considered by some Tripath aficionados as the worst sounding of the lot IIRC.

I am betting the TPA will play with the 7.3 just fine, however I would suggest having 2 Elna Silmic II caps on hand for the TPA Power supply as their warm, smooth presentation may be a helpful fit for the Alpairs especially during break-in. Just something to think about.

Last edited by wushuliu; 10th May 2014 at 07:14 AM.
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