Hypex nc400 Sifam AL19 integration

jqmce

Member
2009-08-22 9:22 am
Hello,
Looking for some advice on how to install Sifam AL19 vu meters to my NC400 power amp project.

From what I can gather the AL19 only requires a 3K6 resistor on the positive side to work. The AL19 is an AC unit.

Sifam Presentor AL19 | Analog VU Panel Meter | Hoyt Meter

Do I simply connect this in parallel to the speaker outputs of the power amp?

I am hesitant to just try it for fear of damaging something.

The amps are driving Klipsch forte iii speakers so it is an 8 ohm load.... ish.

Does anyone have driver circuit examples for the Sifam AL19?

Thanks for your time and advice,
Jules
 
Its a line-level monitor, not power. Connect it to the speaker terminals will burn it up.


You could make an attenuator resistor network to drive it - its specced to be driven from 600 ohm line with that 3k6 resistor, so divide down to 600 ohms or lower from the speaker, then feed the 3k6 from the divider.


Say your power amp has +/- 50V rails, its peak rms out is 30V or so which is about 32dBu


The meter goes up to 4dBu, so 28dB attenuation would be appropriate.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
The VU meter with series resistor is intended to be *bridged* across a *double terminated* 600 Ohm line (so 300 ohms seen).

This approximates 10:1 mismatch so line level hardly changes when meter is switched in/out.

If you truly have 600 Ohm lines, you DO want to switch the meter out when not metering because it throws about 0.5% THD onto the line. (Rectifiers, and today not even the intended copper-oxide softies.)

If you have modern zero-Z sources, THD is less and adding another 300r series makes it nominally "to spec", except the effect is on damping and these affordable meters are NOT damped to VU spec.

The bigger problem is that a VU can only dance the LOUD part, the TOP of your channel's dynamic range. There's a -20dB mark but it is really a 12dB-15dB scale. In real life you may play 98dB SPL on Friday night and 65dB SPL on Monday morning. (In the intended use, long lines for radio networks, program level was always high; the radio listener would set actual listening level to taste.)

Building a wide-range dancing needle is not a simple problem, though I know it was discussed nearby recently.
 

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