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22nd Oct 2020, 10:45 am | #1 |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,848
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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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22nd Oct 2020, 11:01 am | #2 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
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Re: Paul Stenning audio output level indicator
200uA with a shunt?
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Frank |
22nd Oct 2020, 11:09 am | #3 |
Heptode
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 917
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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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22nd Oct 2020, 11:40 am | #4 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,966
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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. |
22nd Oct 2020, 7:36 pm | #5 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,577
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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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22nd Oct 2020, 9:52 pm | #6 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,763
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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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David. BVWS Member. G-QRP Club member 1339. Last edited by David G4EBT; 22nd Oct 2020 at 10:06 pm. |
22nd Oct 2020, 10:20 pm | #7 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
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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. Joe |
23rd Oct 2020, 10:43 am | #8 |
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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. |
23rd Oct 2020, 11:08 am | #9 |
Hexode
Join Date: Apr 2007
Location: W Yorks, UK.
Posts: 407
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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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23rd Oct 2020, 12:32 pm | #10 |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
Posts: 1,848
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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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24th Oct 2020, 1:31 am | #11 |
Dekatron
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Re: Paul Stenning's audio output level indicator
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25th Oct 2020, 2:02 pm | #12 |
Administrator
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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. |