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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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13th May 2021, 6:23 am | #21 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,667
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
If you have one level your fine, then the meter is only showing peaks but if you take a signal that is amplified from 0v to whatever RMS then fitting this range onto the meter face can be problematic.My amps had a gain of over 26dB when on full whack the meter needles were slap up against the stops. That said how many folk play music at full whack? Most amps run at about 1w for normal listening.
If your intended ampis high gain a series resistor can be put before the meter with a little SW to shunt it out if the amp is ever used in party mode. Andy.
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13th May 2021, 9:56 am | #22 | ||
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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I think someone mentioned this earlier, but if there was a way that it could be calibrated to max out only if the volume is turned up, but with low levels it could still move the needle 20-50% on average it would be fine with me. |
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13th May 2021, 11:26 am | #23 | |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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I've used this circuit successfully in an amateur theater environment where there were multiple speakers (not all of them within my hearing) and I wanted to monitor that signals were being sent to the right ones. Hugh |
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13th May 2021, 5:54 pm | #24 |
Heptode
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 669
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
With a simple mod, the circuit in the link in post 23 does not need anything special for D1.
The mod is to swap the positions of D1 and R1, then connect a diode (lets call it D5) between the junction of R1-D1 and ground, anode to ground so that D5 conducts with negative inputs. For positive input swings, the circuit behaves exactly like the original, the added diode is reverse biassed, and current passes througn R1 and D1 to the meter. For negative input swings, current passes through R1 and D5, D1 is reverse biassed to block current to the meter, but only by the forward drop of D5, (0.7V for a small silicon diode). Thus D1 can be any small signal germanium diode, it doesn't have to withstand significant reverse voltages. Stuart |
13th May 2021, 6:06 pm | #25 |
Hexode
Join Date: Dec 2012
Location: Carmarthen, Carmarthenshire, UK.
Posts: 268
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
How about a stereo pot on the input of each channel of an amp, one as a standard volume control and the other to decrease the sensitivity of the meter circuit as the volume is increased.
Stuart.
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14th May 2021, 3:59 am | #26 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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I guess its just a matter of shunting the meter to suit? |
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14th May 2021, 8:07 am | #27 |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
Er no, shunting your milliamp meter will make it’s sensitivity worse (full scale will be more milliamps rather than less).
The combination of the meter and R4 (the series pot) forms a voltmeter that reads the voltage across C1 and the diodes. Setting R4 to a lower resistance should allow you to use a milliammeter rather than a microammeter but will probably make the whole circuit less sensitive as well as altering it’s response to changing volume levels. My advice would be to breadboard the circuit and test it so you can see whether the meters you want to use will be ok before you cut any metal. Hugh |
16th May 2021, 4:43 am | #28 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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Anyway, ive found a pair of voltmeters yesterday, i bought them both since they were identical and only cost $1. Can they easily be converted to operate as ammeters? From what I understand, ammeters essentially are an internally shunted volt meter (or the other way round, i cant remember) |
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16th May 2021, 8:48 am | #29 | |
Nonode
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 2,038
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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Andy G1HBE. |
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16th May 2021, 11:24 am | #30 | |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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Although those meters look great I'm afraid that whether or not they can be converted for use in this application rather depends on what's inside them. As voltmeters they should have an in internal series resistor (not a shunt - that's a parallel resistor). In addition, as the scale states that they're "AC DC" instruments, they've probably also got an internal rectifier. So to use them somebody is going to have to open the case, carefully withdraw the meter movement, identify and remove the unwanted components, connect the movement to the terminals on the back of the case and put it all back together. This is well within the bounds of possibility (and there are people on this forum who will have done it many times) but it isn't trivial and it needs great care, a steady hand and a workspace free of magnetic particles if you're to avoid ruining the meter. If you haven't done it before it'd be worth getting hold of another cheap meter of similar design and practicing on that first. On the other hand, at a cost of only $1 you're not going to lose much if it all goes horribly wrong. Good luck whatever you choose to do1 Hugh |
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16th May 2021, 12:18 pm | #31 | ||
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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I saw someone in another thread somewhere who modified his meters and disconnected the shunt resistor by snipping one end and simply putting it back together. Speaking of rectifiers, these are probably good to keep as spare ones for the meter in my valve tester. |
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16th May 2021, 2:24 pm | #32 |
Heptode
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 648
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
Well if you've been inside a panel meter before then you shouldn't have a problem.
One thing though. As these are voltmeters, there are no shunts. Instead they have multipliers. A shunt is a low-value resistance connected in parallel with a current meter, to enable it to read a higher FSD range of current than the basic meter does alone. A multiplier is a high-value resistance connected in series with a current meter, to enable it to read and be calibrated in terms of voltage. So you can indeed snip one end of the shunt in an ammeter and end up with a more sensitive ammeter but if you snip one end of the resistor in a voltmeter you will simply end up with a non-working voltmeter. |
17th May 2021, 2:19 am | #33 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: Suitable panel meters for use as a "VU" style meter on amplifier
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Doesnt look like that would work in my case, if anything, I need to add a shunt across these to convert them into a current meter? |
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