Converting pro audio signal (1.23v) to consumer line level (0.31v)?

[steve71]

I need to convert the 1.23volt output of my DBX Drive Rack PA (Digital EQ and crossover) to 0.31 volts line level that my amps require.

Can I do this with just a simple voltage divider circuit?
As per this schematic http://en.wikipedia.org/wiki/Image:R...ve_divider.png

Vout = R2 / (R1/R2) * Vin

I want a Vout/Vin ratio of 0.31/1.23 = 0.25

So making R2=100ohms give a calculated value of R1=300ohm

Since it's the ratio that matters, can I select 1K ohm and 3K ohm resisters instead and get the same result?

[steve71]

No comments?

I understand that the above circuit will lower the line voltage by the correct amount. What I don't understand is how adding the resistors affects the input impedance of the power amps.

My understanding it that it'll be just fine, but I don't want to risk damaging my xover or amps. Sorry if this a dumb question, but it's been a long, long time since I studied electronics.

[Original Burnedfingers]

Quote:
I need to convert the 1.23volt output of my DBX Drive Rack PA (Digital EQ and crossover) to 0.31 volts line level that my amps require.


What amps are these?

[Iain McNeill]

No, it's a bit more complicated than that.

1st: it needs to be balanced. So a pi or T structure is needed. I like the double pi as it uses 1 less resistor (rolleyes: )

2nd: it needs to present the correct impedance each way. I guess you're 110ohms to 75ohms.

3rd: it needs attenuation

so there's a bunch of thevenin equivalents and parallel combinations to be computed.

[Original Burnedfingers]

I am real curious as to why you feel you need to pad down the signal to your amps. Don't you have the ability to lower the output from the other devices ahead of the amplifier?

Typically most Pro amplifiers usually require .775 volts to achieve full power. There are of course exceptions to this as some will need more or less. Most home stuff will fall in the 1-1.5volt range.

Most amplifiers of any quality will have an input gain adjustment for each channel. Proper setup would be to adjust the gain so that every piece in the chain would clip at the same time.



Quote from your DBX manual.

Once you have found the clip point of your amplifiers, you can mark this position and turn the
amplifiers back up to the point where they are clipping. You can now use the output limiters
in the DriveRack PA to protect the amplifier from clipping no matter what you do at the console.

Please list your Power amplifiers so that I can check their specifications. There is absolutely no need for a pad of any type because the DBX has ample adjustment to compensate.

[steve71]

Thanks for the response guys!

Ian, I'm not sure what a pi or T structure is... I'll do some reading but what is the name of the circuit that performs the voltage reduction?

Burnedfinger:

Amps are as follows:

Low's (80-500hz) Crown XLS 402D driving Altec 515G's in a DIY horn
Mid (500-7khz) Trends audio TA-10.1 Class D amp driving Altec 288 CD in Manta ray horns
High's (7-18Khz) Crown XLS402D Driving B&C DE-10/ME10 (IIRC) horn

Now I know what you're thinking... what a weird combination of amps... and why would anyone match up a 300watt/ch amp with high sensitivity, low power handling speakers.... well let just say that I'm working with what I have (amp wise) and it believe it or not works pretty well.

There is absolutely no need for a pad of any type because the DBX has ample adjustment to compensate.

The DBX has enough gain on the input side, but I'm having to internally (read digitally?) reduce the gain of the hi/med/low outputs in order to have an acceptable range of volume adjustment. Which means I'm not using much of the 24bit word length on the output D/A's, right? And yes the gain controls on the crown amps are set at their minimum.

BTW to be clear, this is a home theater system and not a PA application.

[Original Burnedfingers]

Lower the signal to the DBX. You can then raise the gain on the amps for a match. How are you time aligning the drivers?

I believe the input sens is 1.25V in for max power on the Crown according the their information. So this tells me the solution is to either lower the output of the DBX or lower the signal level on the input.

[steve71]

Originally posted by burnedfingers
Lower the signal to the DBX. You can then raise the gain on the amps for a match. How are you time aligning the drivers?

I believe the input sens is 1.25V in for max power on the Crown according the their information. So this tells me the solution is to either lower the output of the DBX or lower the signal level on the input.

I can lower the signal to the DBX via the volume control on the preamp, but I want both the input signal and subsequent A/D conversion to maximize the 24 bit word length. Same on the output on the DBX. At the moment the output LED's barley light up the -30dbu light. Doesn't it make more sense to attenuate the line level signal after the D/A conversion?

For time alignment I'm using the DBX for the B&C HF horn which is now sitting in the mouth of the Altec midrange horn. The Tapped Horn sub isn't going through DBX but it's time aligned (it has a 15ft internal path length) via my AV preamp.

[Original Burnedfingers]

How hard are you hitting the input on the DBX?
Quote:
I can lower the signal to the DBX via the volume control on the preamp, but I want both the input signal and subsequent A/D conversion to maximize the 24 bit word length. Same on the output on the DBX. At the moment the output LED's barley light up the -30dbu light. Doesn't it make more sense to attenuate the line level signal after the D/A conversion?

Lower the input to the DBX. Try to hit it at 0 db or maybe a little less. Raise the output level of the DBX enough so that you can level match your speakers.

No, it doesn't make any sense to attenuate the signal out of the DBX with a pad.

I have done a number of these in commercial applications in the past. I fully understand the sensitivity of the Altec drivers because I have worked with them in 100's of applications and I too have used overpowered amplifiers in a pinch.

[steve71]

Thanks for the reply burnedfingers! I'm heading out for dinner, but I'll reply in the morning.