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Old 23rd Sep 2021, 9:28 am   #1
hayerjoe
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Default Speaker impedance confusion.

hello everyone

I have read many times that it's totally ok to drive higher impedance speakers from a lower rated amp. But today a person I know to be technically very proficient told me it was not a good idea and could/would blow resistors.

My specific scenario is I want to drive two 33ohm speakers from a 1972 solid-state HMV amplifier that specifies 8ohm speakers (actually 4 8ohm speakers). Can i safely do this? do I need to add any component such as an inline resistor to lower impedance? Do I need to wire them in parallel to lower impedance? do I need to worry at all?

Prior to today I thought the above would be ok, sound output would of course be lower but all safe to do

Let me know your thoughts

thanks
Joe
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Old 23rd Sep 2021, 9:35 am   #2
kalee20
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Default Re: speaker impedance confusion

Solid-state amplifiers are usually OK going higher rather than lower, though not always. The exception is for some types of transformer-coupled output.

Connecting your two speakers in parallel will have an effective impedance of 16.5Ω. If you connect a 15Ω resistor in parallel, you'll have a total load of 7.86Ω so that will be fine - it saves asking further questions!
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Old 23rd Sep 2021, 9:52 am   #3
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Default Re: Speaker impedance confusion.

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Originally Posted by hayerjoe View Post
hello everyone

I have read many times that it's totally ok to drive higher impedance speakers from a lower rated amp. But today a person I know to be technically very proficient told me it was not a good idea and could/would blow resistors.
To be fair, when this scenario is usually questioned, it invariably involved figures of 4 and 8 Ohms which are perfectly interchangeable under most situations.

33 Ohms is a bit different, however!
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Old 23rd Sep 2021, 10:28 am   #4
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Default Re: Speaker impedance confusion.

The only issue with a transistor amp is the lowest impedance that it will safely deliver power into. This is more often than not, 4 ohms. It's sometimes 6 or 8 ohms. So, 33 ohms should not pose a problem, but being such a high impedance, the total amount of power available from the amp will be limited as an amp's power output increases as load impedance decreases, but only down to that minimum load impedance as stated by the amp manufacturer when distortion or worse may occur.
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Old 23rd Sep 2021, 10:39 am   #5
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Default Re: Speaker impedance confusion.

Confusion may arise from valve amplifiers having serious problems feeding a too-high impedance.
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Old 24th Sep 2021, 5:11 am   #6
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Default Re: Speaker impedance confusion.

thanks for all your comments. I really should have said in my initial post that the 33ohm speakers are already attached and sound fantastic, and certainly NOT underpowered

Not quite sure what to do now, the general consensus would seem to be they should be ok left as-is, but kalee20's idea seems absolutely safe unless resistors in parallel really do lower the speaker volume too far

The speakers had no manufacturer mentioned at all, 33ohm is indeed a strange value, any ideas who made such things and what advantage they provided?
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Old 24th Sep 2021, 6:10 am   #7
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Default Re: Speaker impedance confusion.

HMV in the early seventies were rather behind the times. Their equipment was priced up the top end of the domestic market, but accountants had been busy on the innards.

So they clung on to germanium transistors quite late, and used small devices rather than proper power jobs, even when silicon was becoming common. These small devices tended to be more limited in current ratings than in voltage, by later standards, so the way to squeeze the most out of them was to use a higher impedance load. So they had high impedance speaker drivers made.

David
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Old 24th Sep 2021, 8:48 am   #8
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Default Re: Speaker impedance confusion.

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Originally Posted by Radio Wrangler View Post
HMV in the early seventies were rather behind the times. Their equipment was priced up the top end of the domestic market, but accountants had been busy on the innards.

So they clung on to germanium transistors quite late, and used small devices rather than proper power jobs, even when silicon was becoming common. These small devices tended to be more limited in current ratings than in voltage, by later standards, so the way to squeeze the most out of them was to use a higher impedance load. So they had high impedance speaker drivers made.

David
Just to clarify the speakers are definitely NOT the original speakers that came with this HMV radiogram
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Old 24th Sep 2021, 9:10 am   #9
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Default Re: Speaker impedance confusion.

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Originally Posted by Radio Wrangler View Post
HMV in the early seventies were rather behind the times. Their equipment was priced up the top end of the domestic market, but accountants had been busy on the innards.

So they clung on to germanium transistors quite late, and used small devices rather than proper power jobs, even when silicon was becoming common. These small devices tended to be more limited in current ratings than in voltage, by later standards, so the way to squeeze the most out of them was to use a higher impedance load. So they had high impedance speaker drivers made.

David
Interesting David, thanks. Makes 'sense'.
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Old 24th Sep 2021, 9:23 am   #10
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Default Re: Speaker impedance confusion.

If not original, then someone found properly suitable replacements. Late sixties radiograms from Ferguson etc used the same arrangement AC128 and complement running from close to 20v, coupling capacitor, high-Z speaker.

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Old 24th Sep 2021, 12:23 pm   #11
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Default Re: Speaker impedance confusion.

35 ohm speakers were quite common in early transistor equipment. As they were built in to the equipment they did not pose any problems. It’s only when you need replacements that will give you a problem trying to find some.
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Old 24th Sep 2021, 1:24 pm   #12
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Default Re: Speaker impedance confusion.

Why not measure the resistance of one speaker?
Or pictures of the drive units inside?
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Old 25th Sep 2021, 7:08 am   #13
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Default Re: Speaker impedance confusion.

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Originally Posted by roffe View Post
Why not measure the resistance of one speaker?
Or pictures of the drive units inside?
dammit, took the photos (attached) forgot to measure the resistance. Extra fun for me as the back is fixed on with 20+ screws!
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Old 25th Sep 2021, 8:14 am   #14
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Default Re: Speaker impedance confusion.

Don't take the back off - just measure on the input cable.
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Old 25th Sep 2021, 8:43 am   #15
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Default Re: Speaker impedance confusion.

Quote:
Originally Posted by vidjoman View Post
Don't take the back off - just measure on the input cable.
of course, I must be having a foggy brain day!
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Old 25th Sep 2021, 8:47 am   #16
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Default Re: Speaker impedance confusion.

That Magnavox is highly likely to be 8 ohms impedence. Is it a home made speaker enclosure?
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Old 25th Sep 2021, 8:48 am   #17
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Default Re: Speaker impedance confusion.

35ohm
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