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[Matt]

As part of my subwoofer project I'm looking for a simple way to amplify the analog signal in an RCA cable by 10-20 dB so that the subwoofer signal coming into the power amplifier will be louder than the other channels.

It's possible to make the correction instead using Media Center's Room Correction.  I currently need about a 20 dB difference between the subwoofer and other front speakers to achieve proper calibration.  This is a little severe for Room Correction, because it means all the other channels get turned down by 20 dB (since you can't turn a channel up past 100% or it'll clip).

I could use a headphone amplifier, but they're expensive, and I only care about frequencies below 200 Hz.

Does anyone know what a phono preamp would do to an RCA.  Something like this:
http://www.amazon.com/gp/product/B00025742A/ref=ox_sc_act_title_1?ie=UTF8&m=ATVPDKIKX0DER

Is there any other simple solution?

I could also give up and just use a plate amplifier for the subwoofer, but the power amplifier has more than enough power, so I'd really like to figure out how to use it.

Thanks for any tips.

[nwboater]

Hi Matt,

I don't think a phono preamp is at all what you want. This one takes a 50 millivolt signal and amplifies it up to 2 volts. You would most likely severely overdrive it. Also not mentioned in the specs is RIAA compensation which most phono preamps have. You for sure don't want that.

I'm not clear exactly what you are trying to do. Are you using extra amp channels in a receiver for your sub, or is this a separate power amp?

Is the 20 db gain you are looking for due to subwoofer inefficiency?

I think you should start the process by determining how much amp power you need for your sub to give an acoustic  match  to the mains when their amp is at full power, or at least as much as you or the mains can stand. Then add another 10 db to that for running the sub hot. Then decide if the drivers you have chosen for the sub can safely handle that power.

Now that you know how much amp power you actually need you will be able to determine if your existing amp can handle it. Then look at what voltage on the input of the amp is required to drive it to full power. Check to see if your source (soundcard?) can put out that voltage.

You may have been doing a somewhat similar process, but using db instead of voltage. The problem with that method is that you aren't taking into consideration the various voltage levels that devices put out, and those they require. I fear you could drive an amp to clipping by putting too much gain (voltage) at the wrong place.

Now jmone or others may have a better way of suggesting the process to you. I'll look forward to their posts.

Have fun.
Rod

[bob]

You could probably breadboard one without too much trouble.

Note that you don't want to use a phono pre-amp since it has eq to flatten out the curve of the phono cartridge which has far from flat frequency response.

[Matt]

I'm not clear exactly what you are trying to do. Are you using extra amp channels in a receiver for your sub, or is this a separate power amp?

Is the 20 db gain you are looking for due to subwoofer inefficiency?

I'm using a 7 channel power amp.  5 channels are going to speakers, and the other 2 channels to a dual-voice coil subwoofer.

The 20 dB is because:

1) A subwoofer should always be +10 dB relative to other speakers
2) My front mains are a lot more efficient than the sealed box subwoofer, so that adds about another +10 dB

I think you should start the process by determining how much amp power you need for your sub to give an acoustic  match  to the mains when their amp is at full power, or at least as much as you or the mains can stand. Then add another 10 db to that for running the sub hot. Then decide if the drivers you have chosen for the sub can safely handle that power.

Now that you know how much amp power you actually need you will be able to determine if your existing amp can handle it. Then look at what voltage on the input of the amp is required to drive it to full power. Check to see if your source (soundcard?) can put out that voltage.

The amp has plenty of power.  And I can actually get the voltage out of the soundcard I need, but it requires running the soundcard almost full-out if you want to watch a movie on the slightly uncomfortably loud side.

It's possible running the soundcard at 80 - 90% volume is a better choice than adding an extra preamplifier.  It's just that I've been used to running the soundcard at 30-40% before I needed this 20 dB boost to one channel.

[nwboater]

Hi Matt,

Thanks for the explanations. I now understand your situation.

I would certainly vote for running the soundcard gain for the sub channel up to what you need as long as nothing is getting distorted. It doesn't make much sense to me to attenuate  the gain in the soundcard and then add an extra set of gain stages to make up for that attenuation. Just wasted money, extra complication and more chances of increased distortion and  noise. I like the KISS principle!

Consider yourself lucky that it worked out as well as it did!

Rod

[nwboater]

Hi Matt,

Wonder if you have made any decisions on this?

Have a good weekend.

Rod

[Matt]

I'm using the soundcard for now.

I can still get more volume than I really need this way, although I have to turn the volume up more.  Of course, my bragging rights about never turning the volume up past half way are gone

Once I finalize the subwoofer enclosure, which might change the levels, I'll reevaluate this.

[AAaF]

If you can do some electronics or have a friend who cn, you can make this by an opamp and resistors.

[nwboater]

Matt - mojave just mentioned this $35 Line level gain booster in another thread http://www.htd.com/LGB-1  If you still need more gain for your sub this might inexpensively do the trick.

Rod

[Matt]

Matt - mojave just mentioned this $35 Line level gain booster in another thread http://www.htd.com/LGB-1  If you still need more gain for your sub this might inexpensively do the trick.

Thanks for the tip.

I wonder if it's safe to send signals that exceed 1.0 volts to a power amp?  It might trigger the protection circuit, or worse.

However, all of the volume handling improvements in Media Center recently have made it so levels aren't really an issue for me.  I have a 20 dB swing between the center and subwoofer channels to get proper levels, and it works great now.

[mojave]

Most receivers will send signals much higher than 1 volt. For example, the Onkyo TX-NR709 has a maximum output of 4.6 volts. The miniDSP has an output of 2 volts.

[nwboater]

I think an interesting spec for Matt would be the voltage required on his sub amp to drive it to full power. This figure can vary from around .7 volts on some consumer amps to several volts on some pro amps.

Rod

[Matt]

Most receivers will send signals much higher than 1 volt. For example, the Onkyo TX-NR709 has a maximum output of 4.6 volts. The miniDSP has an output of 2 volts.

Interesting.  I didn't know that.

My X-Fi was at like 1.1V or 1.2V (I don't remember exactly) when I measured playing a sine wave with a multimeter.

I had read that the Asus cards can run a little hotter, and considered trying one just for that reason.

[mojave]

I think an interesting spec for Matt would be the voltage required on his sub amp to drive it to full power. This figure can vary from around .7 volts on some consumer amps to several volts on some pro amps.

The Emotiva XPA-5 requires 1.25 volts for maximum power but I can't find the input voltage for the UPA-7 which Matt is using for his subwoofer.

I had read that the Asus cards can run a little hotter, and considered trying one just for that reason.

Stereophile measured the output voltage as 2.16 volts from the Asus Xonar Essence ST and STX.

[hulkss]

For comparison, some pro-audio gear specifications:

The balanced outputs of my Lynx Pro audio DAC will output up to +24dBu= 12.27V rms = 17.36V peak.
The balanced input stage of my power amplifiers is specified to accept up to 18 volts peak.

Both the DAC and amplifiers have variable analog gain.

[Urbanito]

Hi Matt:
Have you consider to change you sub woofer for an active one? It may be more expensive, but you will solve all your problems and as if I think, your amplifier has a calibration device to correct adjust all the speakers and sub woofer, you will get the best of both worlds. On top of that, you could use the two remaining amplifier channels, to bi-amp the front speakers, with sensible results.
If I can recommend a subwoofer, choose one of the several Martin Logan. The bass is very tight and fast. Even the cheaper one is very good.
Best wishes and good decisions.

[Matt]

Have you consider to change you sub woofer for an active one?

You're right that using a separate higher gain amplifier for the subwoofer would simplify things.

But I recently went the other way:
http://yabb.jriver.com/interact/index.php?topic=66879.0

I'm really happy with the system.  Who wants simple anyway