Figuring out sensitivity (and ? about sub amp/main amp)

ctviggen · « **on:** 19 Jan 2005, 08:47 pm »

I forgot how to figure out sensitivity. For a sub amp, the sensitivity is 1.7v = max power. For my main amp, it produces 26dB of gain. What's the sensitivity? Also how important is it to match the sensitivity between the sub amp and the main amp? (The sub amp also has a "volume" control.)

tkp · « **Reply #1 on:** 19 Jan 2005, 09:08 pm »

Bob,

If I remember correctly, I provided a way to calculate the sensitivity of amplifiers when you asked questions about matching JR and Bryston amps awhile back.

Typically, you want the subwoofer amp to have much higher sentivity than your main amp to make up for the differences in the sensitivity between large woofer and smaller ones. There are few cases where large woofer has higher sensitivity than small one. However, having a subwoofer with high gain and with a volume control, you can always adjust the volume down (cut the gain) of subwoofer amp to match your main amp. There is not much you can do if your main amp has higher sensitivity than the subwoofer amp.

ctviggen · « **Reply #2 on:** 20 Jan 2005, 04:47 pm »

You did, and I searched for your post but could not find it (without reading a ton of posts). I thought the formula was gain = 20 log (output voltage/input voltage), but this doesn't seem to work. That's what I thought about the sub amp. Another question is: why do people use active crossovers on their subs?

woodsyi · « **Reply #3 on:** 20 Jan 2005, 05:31 pm »

Bob,

The reason for active is to get a better control over the integration of sub and main speakers. Even if your sub has built in x-over it still can't do anything about the woofers on the main speakers. For example, your RM40 will go down to 20~30 Hz and the passive lowpass inside has them rolling off around 200Hz. If you biamp a sub and and it has a built in 80Hz rolloff, you've got 4 woofers (assuming the Larger) outputting at frequencies below 80 Hz. It can muddy up the sound quite a bit depending on sub placement. With an active crossover you can set both the highpass on the main speakers and the low pass on the sub for better integration.

ctviggen · « **Reply #4 on:** 20 Jan 2005, 06:48 pm »

I'll have to consider external crossovers in the future, then. (For now, I think I'm done upgrading -- it's a never ending process!)

tkp · « **Reply #5 on:** 21 Jan 2005, 03:38 am »

Bob,

Here it is.

"You can calculate the amp input sensitivity from the gain and power information:

Let take JR as an example:

20*log (Vout/Vin) = gain in db
Power = Vout x I = Vout^2 /R
Vout = square root (power * R)

The amp is rated at 150 Watts into 8 ohms so:

Vout = square root (150 watts * 8 ohms) = 34.64 Volts

So for 26 db gain we have:

20* log (Vout/Vin) = 26 db
Vout/Vin = 10 ^(26/20) = 10^1.3 = 19.95

Vin = Vout/19.95 = 34.64 Volt / 19.95 = 1.736 Volts

This means the amp will reach its max output when input voltage hits 1.736 Volts.

For 32 db gain we have:

20*log (Vout/Vin) = 32 db
Vout/Vin = 10 ^(32/20) = 10^1.6 = 39.81

Vin = Vout/19.95 = 34.64 Volt / 39.81 = 0.87 Volts
This means the amp will reach its max output when input voltage hits 0.87 Volts.

If you plan to use the JR amp for the mid/tweeter section and the Bryston for the woofer then you want to set the gain of the JR at 32 db and adjust the pot for the midrange down 3db to gain matched with the Bryson amp.

If you plan to use the JR amp for the woofer and the Bryston amp for the mid/high then you want to set the gain of the JR amp at 26 db and adjust the pot of the mid down 3db to gain match the Bryston amp with the JR. "