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 Post subject: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Sat 02, 2019 1:54 am 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 1407
Location: 77001
Hello all,
I found a rather pristine condition 1970's Sears AM/FM stereo/Phono/8Track/Cassette
unit.
It does not have speakers, and the rear panel gives no impedance information.
I haven't opened it up yet, to see if any info inside the chassis.

I don't have access to the Sams, which is an OEM print.

Sears model # 304.91943050 indexed in Sams.

How can I determine the speaker impedance, if at all ?

Thanks !
Hotwax


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 Post subject: Re: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Sat 02, 2019 2:28 am 
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Posts: 13608
Your speakers were in Denver.

https://www.shopgoodwill.com/Item/60733793

I don't think Sears sold any 304 sourced products before mid '80s.

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 Post subject: Re: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Sat 02, 2019 2:39 am 
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You may have already seen this:
https://www.samswebsite.com/en/photofac ... /id/151649

I did not find an actual Sam's folder #

You could try writing to Steve Johnson--his website here: https://stevenjohnson.com/manuals/schematics.htm

The typical S/S amplifier has an output impedance lower than the speaker impedance, which provides dampling and other benefits. I think that you can have a fairly wide range of speaker impedances, and the only big variable will be maximum power output. I would start by trying some 8-ohm speakers and see how it sounds.

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 Post subject: Re: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Sat 02, 2019 2:47 am 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 1407
Location: 77001
Hi Tom,
Boy, talk about missing out ! Ha ha !

Mark,
Yes, I saw that OEM in the Sams.

I had looked up info on matching speaker impedance
with SS amps, and found what you posted.
Thanks for the verification.
I have some 8 ohm same-era speakers that I'll try.

Thank you both,
Hotwax


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 Post subject: Re: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Sat 02, 2019 2:14 pm 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4292
Location: Sunnyvale CA
Hotwax wrote:
Hello all,
I found a rather pristine condition 1970's Sears AM/FM stereo/Phono/8Track/Cassette
unit.
It does not have speakers, and the rear panel gives no impedance information.
I haven't opened it up yet, to see if any info inside the chassis.

I don't have access to the Sams, which is an OEM print.

Sears model # 304.91943050 indexed in Sams.

How can I determine the speaker impedance, if at all ?


8 ohms nominal will be close enough.

Brett


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 Post subject: Re: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Wed 06, 2019 4:06 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2743
Location: Burbank, CA and Thailand
Agreed.

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 Post subject: Re: How to determine speaker impedance in SS stereo ?
PostPosted: Feb Wed 06, 2019 5:46 am 
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Joined: Nov Sat 26, 2011 4:09 am
Posts: 9364
Location: Texas. USA
Hotwax wrote:
Hi Tom,
Boy, talk about missing out ! Ha ha !

Mark,
Yes, I saw that OEM in the Sams.

I had looked up info on matching speaker impedance
with SS amps, and found what you posted.
Thanks for the verification.
I have some 8 ohm same-era speakers that I'll try.

Thank you both,
Hotwax
You don't really 'impedance match' speakers with solid state amps. That's a hold over from the tube days (and then not technically accurate either).

Solid state amps work differently to tube amps in that SS amps don't (generally) have an output transformer. That is where the impedance 'matters' (using an OPT) because the speaker Ohm is reflected to the primary and is the plate load of the output tubes. So if Ohm are off then the plate load will be wrong. So you needed to 'match' the Ohm tap to the speaker you were using. (True impedance matching is a power transfer formula and a different thing).

SS amps, however, don't have an OPT and, generally, don't have (the equivalent of) a 'plate load' either. The outputs are usually common emitter "current amplifiers'. As such, the 'load' doesn't matter in the same sense. Where it matters is that too low a speaker impedance will overload the current capability of the output transistors and can cause them to burn up. At the other extreme, too high an impedance simply reduces the power output. I.e. 16Vrms across 16 Ohm is less than 16Vrms across 8 Ohm. As a result, if in doubt, it's better to err on the high Ohm side than the low Ohm side. It might not produce the max power spec'd but it won't burn up.

Having said that, most 'home stereo' speakers of the era were 8 Ohm.


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