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Question about speaker impedance

Posted on June 17, 2020 at 22:28:42
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Posts: 40
Location: Central California
Joined: July 21, 2004
I am in the process of restoring a 78 rpm phonograph my father built in 1950. The original amp is a hot chassis amp. I want to replace the amp with something better. The speaker is an 8 inch Jensen. There is no indication of what its impedance is. How important is matching the impedance to the amp?


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RE: Question about speaker impedance, posted on June 18, 2020 at 07:01:37

Posts: 10430
Location: Arizona
Joined: February 4, 2003
You can't go too wrong with impedance. It's either 4,8 or 16. You can take a DMM set to resistance and measure across the speaker terminals to find out the impedance (resistance in this case).

The only real difference you may have is less bass if the impedance match is way off. You're not driving a hard load or large power outputs, so it shouldn't be much of an electrical issue.


RE: Question about speaker impedance, posted on June 18, 2020 at 07:01:57

Posts: 329
Location: Eastern US
Joined: September 6, 2005
In my experience, it's more an issue when mixing drivers since (again, in my experience) a mismatch there can affect the relative volume and therefore the overall performance. You'd definitely get the best performance with a match but it's not as if it won't work, and with a driver as you're using at that size you'll be getting mostly midrange anyway.

You can probably roughly estimate the impedance using a multimeter, although I'm sure others here will chime in that it's not accurate that way as there's no current going through the driver, but-yet again, in my experience-the rating you get just measuring off the terminals is usually correct.

I have a small portable from the 50s with a 7" driver of unknown origin that I also rebuilt (replaced the turntable with a different machine that I could install a more modern MM cartridge in) and I use the original tube amp, and love it. But I get why you might want to change it out.


RE: Question about speaker impedance, posted on June 19, 2020 at 01:40:33

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Location: Uppsala
Joined: March 6, 2015
Measuring with an Ohm-meter across the speaker terminals will give you the DC resistance of the voice coil. The total impedance is slightly higher, since it also has a reactive component. Around 6,5 Ohms DC for an 8 Ohm speaker would be typical. For a 16 Ohm speaker around 12 Ohms.


RE: Question about speaker impedance, posted on June 21, 2020 at 23:44:50
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Posts: 40
Location: Central California
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Thanks for your help, guys.


RE: Question about speaker impedance, posted on June 26, 2020 at 16:28:47

Posts: 1584
Location: North Ohio
Joined: May 29, 2016
I you canott restore the original amp the best bet is to throw together an LM3886 based ne, and I guess I mean one because it is mono no doubt. Except for having to pull the mute pin up with like a 200K, it operates like an OP AMP. set the gain with a couple resistors and you are done. It only has like nine pins or something I think so you can just screw it to something that resembles a heat sink and just wire, no PC board required. i have used them before. They are good up to a bit over fifty watts but you don't want that much. They got no problem with four ohms. And they are fairly cheap.

They are not high end, but neither are old record players. It probably doesn't even have a magnetic cartridge, in fact it might be crystal, not even ceramic. And if it is magnetic you can crank the gain of that chip enough, you just have to build the RIAA EQ.

I can design the thing in a few minutes. If you are down below like fifteen watts the thing will probably run in free air, but it is still better to mount it somehow. Something that old is probably not even fifteen watts.

At that low power level a wall wart could power it. Well a little bit bigger one. Still, they are cheap enough and they take care of the UL completely, that is why so much stuff uses them now.


RE: Question about speaker impedance, posted on June 29, 2020 at 14:28:18
Bold Eagle

Posts: 6935
Location: America's Heartland
Joined: May 27, 2001
If the original amp is a tube amp, the source impedance will be much higher than in an SS amp. That has the effect of boosting the bass output in the range around the speaker's bass resonance. To get some of that bass back when using a direct coupled solid state amp, you just need to wire a resistor in series with the speaker. Try a 10 watt, 1 ohm, non-inductive resistor to start.

You can find those resistors at Parts Express, and they are inexpensive and I'd recommend buying a 1 and a 2 ohm as 1 ohm in series may not be enough.

The first place I can recall seeing this fix was in an article in Electronics World by Edgar Vilchur in response to complaints from AR3 owners who noticed a loss of bass punch when switching from tubes to a SS amp.

My Dad loved music; but they didn't have a record player, so for Christmas that year I bought them a Webcor in a leather wrapped portable case. It had a single 8" speaker, a 5 watt tube amp, and a Webcor changer with a Sonotone ceramic cartridge. It looked wonderful and sounded pretty good. That was around 1958 or '59.



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