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Old 22nd Oct 2020, 10:45 am   #1
DonaldStott
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Default Paul Stenning's audio output level indicator

My next lock-down project is the audio output level indicator designed by Paul Stenning - Audio output meter

Looks fairly straightforward but I'm having trouble tracking down a 250uA panel meter - the one referenced in Paul's design was from Maplin (Order Code LB80B) but no longer available.

Found a number of 200uA and 300uA meters online but don't know how critical the 250uA is?
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Old 22nd Oct 2020, 11:01 am   #2
Nuvistor
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Default Re: Paul Stenning audio output level indicator

200uA with a shunt?
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Old 22nd Oct 2020, 11:09 am   #3
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Default Re: Paul Stenning audio output level indicator

Simply increase R1 by 20% for the 200uA movement, so say 22k. It’s not a calibrated instrument but more a relative, indicative and useful aid!
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Old 22nd Oct 2020, 11:40 am   #4
paulsherwin
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Default Re: Paul Stenning audio output level indicator

It's just a rough AC voltmeter. You can use an analogue or digital multimeter instead, or even an oscilloscope if you have one. An analogue meter is arguably easier to read.

An even easier solution is to use a sound measurement app on a smartphone or tablet. Just put the phone near the speaker.
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Old 22nd Oct 2020, 7:36 pm   #5
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Default Re: Paul Stenning's audio output level indicator

Paul also gives alternative values for a 100uA meter but why not just put a small pre-set in series with the 18K...something like a 4.7K? Then you could fit a 200uA type and calibrate it for full scale using a known voltage input by adjusting the pre-set. Or just replace the 18K with a pre-set pot of 22K or 47K and adjust FSD as above?
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Old 22nd Oct 2020, 9:52 pm   #6
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Default Re: Paul Stenning's audio output level indicator

Remember that the meter isn’t used to measure their output - only as a peak level meter and isn’t critical. Apart from altering the value of R1 to suit the particular meter movement, In use, you can adjust the volume control to get the needle to mid-scale, then when for example you’re adjusting IFTs, as well as hearing the audio peak, you can observe the rise and fall of the peak on the meter dial.

Hope that helps.

Such a useful little tune-up aid which Paul designed many years ago.

I can’t imagine why I didn’t make one years ago.

Something like this 100uA meter would be fine:

https://cpc.farnell.com/unbranded/mc...-me-pd-mi-alte
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Old 22nd Oct 2020, 10:20 pm   #7
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Default Re: Paul Stenning's audio output level indicator

The more sensitive the meter movement the less the loading will be on the circuit be.
The exact sensitivity isnt that important.

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Old 23rd Oct 2020, 10:43 am   #8
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Default Re: Paul Stenning's audio output level indicator

If you wanted to use a less sensitive meter you could replace Paul's passive voltage doubler with a transistor, though it would then need a battery.

It's used across the speaker output, so the loading isn't significant.
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Old 23rd Oct 2020, 11:08 am   #9
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Default Re: Paul Stenning's audio output level indicator

Um, that circuit attaches a rectifier directly to the amplifier/speaker, with no series limiting resistance. That's nearly a short on the amplifier!
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Old 23rd Oct 2020, 12:32 pm   #10
DonaldStott
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Default Re: Paul Stenning's audio output level indicator

Thanks to everyone for their helpful Posts - I think I've assembled all the components and have a 200μA meter on order.

Just another quick lock-down project to keep me busy and out of trouble!

Has anyone found the need to address the issue raised by daviddeakin?
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Old 24th Oct 2020, 1:31 am   #11
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Default Re: Paul Stenning's audio output level indicator

Quote:
Originally Posted by DonaldStott View Post
Has anyone found the need to address the issue raised by daviddeakin?
C1 is a coupling capacitor against a DC short. Even then it wouldn't hurt a valve radio which you'll be using it on
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Old 25th Oct 2020, 2:02 pm   #12
Paul Stenning
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Default Re: Paul Stenning's audio output level indicator

As others have said, this is just intended as an indicator that is easier to see the peak on than a digital meter. If you have an analogue meter or a 'scope already then that would do the same job, more accurately.

Any reasonably sensitive meter could be used (maybe an old VU or tuning meter salvaged from scrap equipment) and the component values tweaked to get a reasonable sensitivity and speed of response. For 200uA instead of 250uA meter, I'd just increase R1 to 22k.

If anyone was concerned about the issue raised by David Deakin, a small resistor, say 100 ohms, could be connected in series with C1 and shouldn't have any significant effect on the results.
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