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Sets, Parts and Service Information Wanted (private buyers only, no swaps) If you need help obtaining components, sets or equipment, post a message here. Private buyers only - no traders. No swaps.

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Old 5th Jan 2019, 3:47 pm   #21
mhennessy
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Default Re: Wanted: Bookshelf speakers, pair of

Those ICs will happily drive 4 ohms provided the supply voltage is 9V or less. Should be capable of 1.4W when powered from 9V, or 0.5W (only 4.5dB less) from 6V.
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Old 5th Jan 2019, 11:33 pm   #22
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Arrow Re: Wanted: Bookshelf speakers, pair of

Mark - that's interesting (ultimate understatement ) - and gives the lie to everything I've said to date, re: "must be 8 Ohms".
So thank you - but if I may enquire, from where did you get that snippet of data? (Obviously I believe you!) All I have is a (dog-eared) copy of the SGS datasheet, which I acquired during the mid-1970s. A closer look at it does indeed refer to using a 4 Ohm speaker - but with a 6-v. supply and delivering 500 mW at the onset of clipping - which will be inadequate for my intended use. My test 'bird's nest' on the bench, using 8 Ohm resistors and a 12 v. supply, delivers just under 1.5 watts from approx. < 100 Hz to > 20 kHz.

Al.
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Old 6th Jan 2019, 12:54 pm   #23
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Default Re: Wanted: Bookshelf speakers, pair of

Here's a good quality scan of the datasheet: https://www.vintage-radio.net/forum/...91&postcount=2

I was looking at the graphs on page 5 (attached). The 1.4 watts I mentioned is at the onset of clipping, but the second graph shows that if you push slightly beyond that (10% THD), then it's more like 1.8 watts.

But before seeing the graphs, I noted that the IC can supply 1A peak, which is enough for 2 watts into 4 ohms, assuming no other limitations. In practice, a combination of the maximum dissipation of the device (1.35W) and resistive losses in the output stage will most likely be responsible for the upper limit.

It's probably a good idea to use an LM317T or similar to regulate the supply to 9V so that you're within the safe operating area. Apologies if that's stating the obvious, but the default assumption with more modern ICs is that it's fine to run them on smoothed but unregulated mains supplies. Separately, the datasheet doesn't appear to give a value for power-supply rejection ratio, so that is another good reason to use a regulated supply

Don't forget the sensitivity of the speakers. This varies a lot - smaller speakers tend to be relatively inefficient (perhaps 85dB per watt at 1 metre). Finding a pair that is 88dB/W/m would be equivalent to doubling the power output of the amplifier. I appreciate that you might not have that option here, but thought it was worth mentioning nonetheless.

Anyway, I hope this widens your options.

All the best,

Mark
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Old 6th Jan 2019, 1:20 pm   #24
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Default Re: Wanted: Bookshelf speakers, pair of

The importance of speaker impedance is all about power dissipation. All other things being equal, a transformerless amp can dissipate a lot more power driving a 4 ohm load than an 8 ohm one, and this can overrun components. You just need to limit the dissipation, either by reducing the supply voltage or simply not driving the amp close to clipping levels.
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Old 6th Jan 2019, 1:28 pm   #25
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Thank you, Mark.

Looking at the link for the datasheet that you have listed, obviously I am familiar with that - and that datasheet not only shows applications using an 8 Ohm load only, but all the text data in that sheet also refers to using an 8 Ohm load only. So I've always used that data as 'gospel'. However, I understand your analysis based on the performance graphs - I've never looked that deep - and will now think about a minor re-design to accommodate a 4 Ohm load - and then test it.

For me, using a 3-pin regulator with i.c.s such as this is simply de rigueur: the benefits obtained by incorporating suchlike into a PSU design far outweigh the effort and costs of so doing.

As regards speaker sensitivities and efficiencies, (electrical watts in vs. audio watts out) is something I've never really considered - until now. And that is probably because I'm not really an 'hi-fi / audio' enthusiast: more of an 'analogue R.F. man'. So thanks for that addition to your post.

Al.
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Old 6th Jan 2019, 1:33 pm   #26
mhennessy
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Default Re: Wanted: Bookshelf speakers, pair of

Quote:
Originally Posted by paulsherwin View Post
The importance of speaker impedance is all about power dissipation. All other things being equal, a transformerless amp can dissipate a lot more power driving a 4 ohm load than an 8 ohm one, and this can overrun components. You just need to limit the dissipation, either by reducing the supply voltage or simply not driving the amp close to clipping levels.
And of course, with real audio - as opposed to sine waves - the average power will be a lot less than the peak power. As it's the average power that determines how hot things get, driving 4 ohm loads is not as difficult in practice as it might first appear on paper
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Old 6th Jan 2019, 1:48 pm   #27
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Default Re: Wanted: Bookshelf speakers, pair of

I could use the Panasonics Rick if the OP has no need. They will go with my Panasonic SG4000 music centre splendidly.
Cheers.
Gaz.
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Old 8th Jan 2019, 7:10 pm   #28
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Default Re: Wanted: Bookshelf speakers, pair of

Gaz: post #27 - I have no need for the Panasonic speakers from Rick that you mention.

Al.
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Old 8th Jan 2019, 7:16 pm   #29
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Arrow Re: Wanted: Bookshelf speakers, pair of

Lloyd: you have a P.M., re-your post #9.

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Old 8th Jan 2019, 7:28 pm   #30
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Default Re: Wanted: Bookshelf speakers, pair of

Hi Al,

PM received and replied to

Regards
Lloyd
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