Hello.

I installed a multi-room audio system for a client and it works very well. The only thing I'm not happy with is that the cable box, when played through the system, has low volume output compared to other sources. The sources it's compared to are iTunes, XM Radio, and Internet radio.

With-in the mulit-room system you can adjust the level of the analogue external sources (the cable box is maxed), but not the external network sources, other wise I would just turn them down a little. I tried the cable box with the output level both fixed and variable, turned up to the maximum level that sounded good.

I'm hoping to add an amplifier in between the cable box and it's input on the system. Prefferably something adjustable.

Maybe like this,


[Link: htd.com]

This isn't something I've done before. Is this something that is/isn't recommended? Anything I should watch out for?

Thanks, Craig

That'll be fine. In the past we used inexpensive PA mixers to boost the low cable box analog outputs.

Oh, darn. Another long post. Sorry.

Adding this amp brings in the concept of gain structure, something you have to think about when signals go through several devices with adjustable volume. I'll get to that in a minute.

That amp has two drawbacks.

First, the spec does not tell you what the signal to noise spec is, so you'll have to try out the thing to listen and see if it seems to add noise to your system.

Second, there are separate volume controls for each channel, pretty much guaranteeing you cannot set the two channels to identical gains without a diagnostic tool.

You can use a two-channel scope to do this. If you know what I mean, you don't need any more instruction.

Or you can use your trusty butt set. (Think of a butt set as an interface, in combination with your ears, between audio signals and your brain.) See, if the amp is clean and linear, a signal amplified by both channels to the same level will have identical output level on both channels. Here's how to use a butt set to set the levels:

1.First, read about gain structure.
2.Then adjust both level controls about the same amount, using the input from the cable box.
3.Then get a monaural input; if you read the Rane paper about Y connectors recently, you already know that it's okay to use a Y to split signals, just not to join them. So use a Y connector. Connect the monaural input to the little amp.
4.Connect your butt set from the hot of the left output to the hot of the right output. You will probably hear some sound. The butt set should be set to "monitor."
5.Gently tweak one of the volume controls up or down a tiny bit. If the sound goes up, tweak it in the other direction and find the adjustment that gives you the least sound. This is the point at which the output levels are the same, to the level of sensitivity of the butt set (which is way better than using the naked ear with speakers in a room).
6.Go back to using the cable box signal and see if it's set loud enough overall, as you've just messed with its volume level.

Now, gain structure.
First, being technical for a moment, that is a misnomer. We NEVER change the actual gain of ANY amp EVER. The gain is an internally set parameter. Instead, you attenuate the input signal to an amp, or in the case of speaker level room volume controls, you attenuate the output of an amp. Actually adjusting the gain means changing feedback components and doing other jiggerypokery within the amplifier circuit.

Now that we know it's not really gain structure, let's talk about gain structure.*

Every component, even wire, adds noise to any signal going through it. Our task is to minimize that noise and get the signal level we want.

Now, if you turned the cable box's output way down, then turned your room amp all the way up, you'd hear the cable box, but you'd hear a lot of noise, too. If you then turned the cable box up by 10 dB, the noise would stay at the same level, but you'd turn down your room amp by 10 dB so the noise that you hear would be 10 dB lower in relation to the cable box sound. That is, you'd have increased the signal to noise ratio by ten dB just by changing a volume setting. In this case, you've continued this process to the point where the cable box is all the way up, and noise hasn't been an issue yet but you don't have a loud enough signal.

Enter the little amp. If it's set to neither increase nor decrease the cable box signal, you'll have the same level of signal as before, but with the noise added by the little amp, which might not be audible; this is why specs are a good thing. As you increase the output level of the little amp, you get more output from the little amp and its internal noise is proportionally less of the output signal. So what the heck: turn the little amp up all the way!

But wait: now your signal is distorted. Everything sounds like a Nirvana chorus! That's way worse than noisy.

Gain structure is the concept of turning up all the components in a signal chain such that the noise they contribute is minimized, while at the same time keeping the output of each device in the chain low enough that it doesn't overload the other components in the chain and cause distortion. It's about noise, signal level (not gain, but we call it that) and distortion.

Try the amp. There's not enough information to know if it will work well for you, but it probably will.




*A chef from India says, in the same vein, "there's no such thing as curry. Here are several recipes for curry."

Thanks for the replies every one. Ernie I don't have a scope (I have used one), or a but set. But with your detailed instructions I'm tempted to get one.

I know exactly what you are talking about with gain structures and signal to noise ratio, from years of matching components in car audio. I am tempted to get this piece but I'm also wary because of the lack of documentation. I found a bunch of other cheap ones but the S/N was around 85 dB.

At this price I think I'll just grab one but I don't want to wait for shipping. Is anyone aware of a similar piece that may be had from A&! or A/@& ?

For this application, I'd be happy with 85 dB S/N. Maybe even 70!


Yeah, but I had to say it.......................

Funny. This post reminded me of the horribly low RF audio output on Rogers' free cable box model, even when it's maxed out... and sure enough, you seem to be in Rogers territory too. The big R never seems to get it right!

If you set it and can't hear a problem with the channel balance, it won't matter. If it becomes a problem, change it when you have a chance.

85dB S/N ratio is nothing to sneeze at. Most CDs don't do much better than that and they make up for this by setting the max output at, well, max and not allowing for any dynamic range. If the cable provider's S/N is better than 70dB, I'd be surprised. The cable box may be, but I doubt the signal is.

Ernie will love this. He most likely has a way to measure level of each channel with a super duper mobile phone appilizer. And there are some, or use to be some decent level boostilizers from the 12 volt side I have used in the past. Is there not a way to turn down the inputs of the DA system inputs connected to the sources with more output?

We use an inexpensice PA mixer all the time. Look at Berringer mixers. They have a few around $100

I tell people I'm a professional installer and I don't see anything wrong with whipping out my test telephone and, in three or so minutes, setting it better than nearly right.


Did you read what I wrote? I use a butt set, which is a telephone on a hook with a switch that in the telephone world, lets you monitor the phone line or in the audio world, listen to an audio signal. Nothing more complicated than that is needed.


Probably true, but I bet most of us who used to do car audio don't any more.


The problem inputs, as explained in the first post, are the digital inputs to the system. They never exist as analog audio where volume trim is possible.

A mixer seems inappropriate because whatever adjusts the volume level must allow selection of only one component at a time. A $100 mixer won't let you remotely kill two channels of XM in favor of two channels of Internet radio. And, as said, those inputs are digital signals.

Craig, you were wise to ask about the cheap analog amp. Otherwise folks would have told you how you could spend a thousand dollars getting three D-A converters!

He also did not mention the brand of DA system. Perhaps it does have the capability to adjust sources. There is another post i believe you are involved with regarding a Marantz SR 5008 where the person is asking for similar help and in the manual is the info. I thought I might bring it up.

Perhaps we should require model, serial number, and firmware versions of all equipment involved before advice can be given. There are forums for other industries that require this.

No, but he did give some information that has been ignored more than once in this discussion. He says he cannot adjust the volume of the digital sources, being iTunes (I think), XM Radio, and Internet radio. I don't know what good it will do specifically in this case to know the model of the equipment involved, unless to show the original poster that he's wrong. And maybe there's some value in that. Otherwise:


Let's go back to that starting point, shall we? That's why I brought up numerous D to A converters as a possible suggestion but a bad idea economically.

I think we've pretty much answered the question by agreeing that Craig should try the little amp he proposes.

Thanks for the dialogue everyone.

The system is a Casatunes CT8. I am 100% sure that you cannot adjust the digital network source levels, per a phone call with Casatunes.

I didn't say it was Casatunes because I didn't think it was relevant. And I didn't want to start a, you should have used a x system, or no one else here uses Casatunes so you shouldn't either blah blah blah, tengent. I'm getting a little tired of the key board warriors. Ernie I'm surprised you answer questions at all but I'm glad everyone does.

Casatunes is awesome. The support is incredible ( I had an issue. Casatunes support remotely logged into the server and did an upgrade for me, tweaked a couple of other things, no problem) and best of all... The client loves it.

I'll use the amp.

Thanks again for the input.

Craig.