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lost eden 15th July 2011 12:17 PM

power meter for amplifier (*not* VU/PPM)
I'm putting together a new amp based around the TDA8920BTH in bridge, which according to the datasheet produces 'typical' 210W into 6Ω at 10% THD. I'm using a 225VA toiroid (producing +-/27v which is what the 210W is measured at) & 40,000uF of tank caps, so I'm hoping it will happily achieve >100W into 8Ω (hopefully at substantially less than 10%).

I want to have a few LEDs on the front panel of the amp to show roughly how much power it is putting out - I know it won't be accurate. I already have 2x such meters on the front panel of my Sure TC2000/TK2050 amp which each have 5x LEDs, representing 20W per LED (at 8Ω I believe), but I bought these as kits from Quasar Electronics so I don't really know how they work w.r.t. what values of resistors are needed, etc.

Can anybody give me or link me to a simple schematic for a circuit that will do what I want, with some explanation about what values are needed for particular powers? I found the following circuit via Google, but it's for very low powers (4W max I think) & I don't know if I can substitute the components (eg much larger resistors) & use it for higher powers?

Ideally I think I'd probably want separate LEDs for 1W, 10W, 50W & 100W, assuming an 8Ω load.

Dr_EM 15th July 2011 12:35 PM

A set of comparators with the "-" input tied to a reference voltage which corresponds to a certain power level into an 8-ohm load and the "+" input of them all linked to the amplifier output, or, the amplifier input whereby you'll take into account the amplifier gain in your power equations.

I'd use a few more LEDs and focus most toward the low end of the power spectrum. You won't go over 1W that much with efficient speakers.

You may consider smoothing the output voltage from the comparators with a sample and hold type circuit (LED comes on immediately, but stays on for a bit) or 555 monostable circuits.

If you still have trouble I'll draw a quick schematic later for you.

Pano 15th July 2011 01:18 PM

And there are kits available. A quick search will find them. With Ohm's law you can adapt them to higher power levels.

Spiny 15th July 2011 09:17 PM

take a look at the LM3914 and LM3915 series of bargraph ICS, needs a few leds and a few other components.

LM3915 - Dot/Bar Display Driver

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