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Infinity Alpha speakers


Updating the Home Theatre system with new used speakers

  1. Introduction, why this old speakers?
  2. Overview of the Infinity Alpha System
  3. Infinity Alpha 50, main speakers
  4. Infinity Alpha 20, rear speakers
  5. Infinity Alpha Center, center speaker
  6. The crossovers and update of components
  7. Replacing bad tweeters

1: Introduction, why this old speakers?

Note:
I take no responsibility or liability for what are written here, you use the information at your own risk!

Earlier I had a lot of different speakers:
Sonab OA5, Sonab 2212 (four of them), Yamaha, Pioneer, self built with Vifa and Seas speakers elements. And the last 10 year with compact living just computer speakers. The computer speakers are not bad, 5.1 THX qualified active speakers, built by Altec Lansing, model AD995. But still a bit boring.

What to do if you don't want too spend to much money and still get something more exciting speakers? I searched the market for used speakers and found the Infinity. It's the older model Alpha series, maybe built 2002.

Here are some back ground information about Infinity:

Once they was very famous for their exclusive design and was very expensive. The ones I bought is of later design and not that exclusive.

Here you can read about the Infinity Vintage speakers:


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2: Overview of the Infinity Alpha System

The speakers looks nice in our living room with it's cherry cabinets. To be twin 8" main speakers they are very compact. Of course I should not have the rear speakers on top of the main speakers later, only now when doing test. On the back side of cabinet I can read that they all are of 8 ohm impedance and manufactured in Denmark. Included was also an active 12" subwoofer, that one was newer and from another manufacturer.

I bought this speakers cheap and never even listen to them before I bought them, not so wise. But I already when I saw them understood that it must be a lot of work to renovate them.

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3: Infinity Alpha 50, main speakers

Infinity Alpha 50 Infinity Alpha 50

The Infinity Alpha 50 main speakers are the biggest in the Alpha family. They have twin 8" bass range speaker elements. A 5" mid range and a 1" tweeter. They all have the special ceramic speaker elements which is a very stiff material.

Infinity Alpha 50 specifications:
Manufacturer: Harman
Year: 2002
Power rating: 10 - 200 watts
Nominal Impedance: 8 ohms
Bass reflex: vented box
Crossover: 3-way, bi-amp
Crossover Frequencies: 500 Hz and 3,000 Hz
Sensitivity (2.83V @ 1m): 91dB / 1W / 1m
Frequency response: 35 Hz - 22 kHz (+/- 3.0dB)

Drivers,
Tweeter, part number A1201B-H, 9747202:
25 mm ceramic dome

Midrange, part number A1205A-M, 9747204:
130mm CMMD (Ceramic Metal Matrix Diaphragm)

Woofers, part number A1208, 9747218:
2 x 200mm CMMD (Ceramic Metal Matrix Diaphragm) surface covered aluminium with 25mm voice coil.

Dimensions W x D x H: 250 x 360 x 1000 mm (front cover included)
Weight: 23.5 kg each speaker

Crossover drawing:

Infinity Alpha 50 crossover

15 year old speakers could had the membrane rubber edge broken. These ones looks to be good. The ceramic domes are very sensitive and one tweeter has a dent.

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4: Infinity Alpha 20, rear speakers

Infinity Alpha 20 Infinity Alpha 20

The Alpha 20 rear speakers are of similar design as the Alpha 50 but only 2-way speakers, a 6" bass / mid range and a 1" tweeter.

Infinity Alpha 20 specifications:
Manufacturer: Harman
Year: 2002
Power rating: 10 - 20 watts
Nominal Impedance: 8 ohms
Bass reflex: vented box
Crossover: 2-way, bi-wiring
Crossover Frequencies: 3000 Hz
Sensitivity (2.83V @ 1m): 89 dB / 1W / 1m
Frequency response: 55 Hz - 22 kHz (+/- 3.0dB)

Drivers,
Tweeter, part number A1201A-L, 9747201:
25mm ceramic dome

Woofers, part number A1206A-L, 9747213:
165 mm CMMD (Ceramic Metal Matrix Diaphragm)

Dimensions W x D x H: 220 x 290 x 385 mm
Weight: 9 kg each speaker

Crossover drawing:

Infinity Alpha 20 crossover Back to contents

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5: Infinity Alpha Center, center speaker

Infinity Alpha Center Infinity Alpha Center

The Alpha Center has twin 5" bass / mid speaker elements and a 1" tweeter.

Infinity Alpha Center specifications:
Manufacturer: Harman
Year: 2002
Power rating: 10 - 20 watts
Nominal Impedance: 8 ohms
Bass reflex: vented box
Crossover: 2-way
Crossover Frequencies: 3000 Hz
Sensitivity (2.83V @ 1m): 89 dB / 1W / 1m
Frequency response: 55 Hz - 22 kHz (+/- 3.0dB)

Drivers,
Tweeter, part number A1201A-L, 9747201:
25mm ceramic dome

Woofers, part number A1205C-C, 9747211:
133 mm CMMD (Ceramic Metal Matrix Diaphragm)

Dimensions W x D x H: 420 x 220 x 185 mm
Weight: 8 kg each speaker

Crossover drawing:

Infinity Alpha Center crossover Back to contents

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6: The crossovers and update of components

Infintity Alpha speakers

The main speakers has bi-amp crossover filters. The rear speakers has bi-wiring crossovers, they have a common zero in the crossovers and will burn your amplifier if you try to bi-amp them.

Cheaper crossovers are built with bi-polar electrolytic capacitors, even these ones. I will replace them with non-polar capacitors.

Infintity Alpha 50 crossover

When I opened the cabinet and took out the filters I saw, it had been hot here! The resistor to the tweeter crossover was burned. The earlier owner liked to play loud! Here I have replaced the burnt resistor with a new one, I mounted it a bit higher to get better air cooling of it.

Infintity Alpha 50 crossover

Upper row, the old capacitor to the left and the new yellow one to the right. Lower row, the burnt 1 ohm resistor and the new one to the right.

Infintity Alpha 50 crossover

I bought new capacitors to all the speakers crossovers, but left the big 39uF and 56uF capacitors to be replaced later. I want to listen to the speakers and hear if they are worth spending more money on them first. Big non-polar capacitors are expensive.

Infintity Alpha 50 crossover

Heating up the solder machine and start to replace the old components.

Infintity Alpha 50 crossover

It's a bit tight around the new capacitors, but manageable.

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7: Replacing bad tweeters

Of course I later found that the tweeters has been burned in the Alpha 50 main speakers, that was the reason that the crossover resistor was burnt. I already know earlier that one tweeter had a dent on the dome so anyway I had to replace them.

Normally there is no problem to find replacement speakers. But this France made speakers was a bit tricky.

Infintity Alpha 50 tweeter

The tweeter part number is A1201B-H, 9747202. I found new ones but they were expensive, maybe this old speakers are not worth it. In the spare part list they say it's a 4 ohm impedance tweeter.

If I now rebuild the speakers to be better I think I shall spend some time to see if I can find ones with a better and a more modern design.

Infinity Alpha 50 crossover drawing

A simple drawing I made to help me understand what tweeters I can replace the original ones with.

For me it looks that the 1.2 ohm resistor and the 8.2 uF capacitor makes a Zobel circuit to the mid range speaker. By calculate backwards I found the the speakers DC resistance should be 0.96 Ohms, and the Inductance 7.6 mH. Good to know if there is a need to replace it.

For the bass speakers I did the same backward calculation and found that the DC resistance is 1.76 ohm and inductance is equivalent to 0.18 mH. Or 3.5 ohm and 0.36 mH for each because the two bass speakers are connected in parallel.

To make it easy for me I used the web calculator you find at the link below.

A Zobel circuit makes it easier to have control over the speakers impedance, then the filter works better and the load to the amplifier isn't that evil. With this we also see that the crossover to the tweeter is a 12 dB / octave filter, and the others only 6 dB filters. One good thing about that is that the phase shift isn't that high at the crossover frequency but it makes a heavier demand on the speaker elements.

Later, maybe I buy an instrument so I can measure the inductances of L1, L2 and L3.

After some research I found Dayton have one model that could fit both mechanically and electric as a replacement to the tweeter.

It says it's a 4 ohm impedance speaker, the Infinity system impedance is 8 ohm. But what I know the impedance of the original tweeter is 4 ohm and when calculate the crossovers frequency of 3000 Hz it looks to be correct. The Dayton tweeters have extra cooling fins, good.

Note:
Don't trust me of the above calculations and thoughts and buy a lot of expensive stuff, I'm not an expert in this subject.

If you want to get deeper in filter calculation you find it here:

I will come back later and tell how I solve this problems with the defective tweeters.

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