How many decibels should I give to the power output?

[Ema79]

Good morning everyone. I'm not good at math, so I'm asking for your help.
I have two audio amplifiers that operate at 27 dB. A DAC outputs at 5.1 volts (XLR), and a preamp that can boost the signal up to +14 decibels. How can I figure out how many decibels I need to give the preamp to provide the right amount of voltage to the amplifiers? Thanks for your help.

 

[RandomEar]

The numbers for the amp and preamp are called gain and are relative - essentially, each device can "add" that much dB to the signal. Absolute signal levels would be given in dBu with 0 dBu being equivalent to 0.775 VRMS. You need to find out the input sensitivity / max input voltages of your preamp and amp to define which level your DAC should be set to if it has volume control. If your DAC lacks volume control, your preamp has to be able to accept an input of 5.1 V without saturating. Check the specs of your devices.

As a rule of thumb: You want your DAC output to be as high as possible while keeping the gains in the preamp and the amp as low as possible to avoid amplifying the noise.

 

[Ema79]

Hi. The DAC is set as per Puro without volume and is a Gusgard x26III. The preamplifier is a Topping Pre 90. Unfortunately, I didn't find much information in the technical data sheet for this device. The power amps are 2. 1 Purifi Audiophonics 1ET400A, and an IcePower 1200AS2.

 

[DVDdoug]

Typically, a "preamp" is a "control center" operating as an attenuator rather than an amplifier, except MAYBE sometimes adding gain when you turn it up loud. (That's ignoring phono preamps or microphone preamps which are high-gain.)

The outputs from a DAC, preamp, DC player, TV, etc. are all line level which is rather loosely defined and not calibrated in home systems. It depends on the equipment, the "loudness" of the recording, and sometimes it's volume controlled and some line outputs have volume controls and some don't. But most line-outputs have enough signal and most line-inputs have enough gain that everything usually works fine together. The one complication is that pro line level (usually balanced SLR connections) is higher than consumer level, so sometimes you may not have enought signal for a "pro" amplifier.

 

[bmc0]

Let's do some math, shall we? Given the maximum output power, P, the load impedance, R, and the gain, G, the input voltage in dBV (dB relative to 1V RMS) required to drive the amplifier to full output power is given by: 20×log(sqrt(P×R))-G, where log() is the base-10 logarithm.

From the manufacturer's specs, the Audiophonics 1ET400A-based amp appears to have a default gain of 26dB and a maximum output of 425W into 4Ω, so the required input voltage should be: 20×log(sqrt(425W×4Ω))-26dB = 6.3dBV. The ICEpower 1200AS2 datasheet specifies a voltage gain of 25.8dB, so using the single-channel 1250W into 4Ω power rating, it needs: 20×log(sqrt(1250W×4Ω))-25.8dB = 11.2dBV.

The DAC's 5.1V RMS output is 20×log(5.1V) = 14.2dBV, and from the Pre90 review we know that "0dB" is actually +6dB when using the XLR outputs. So, with the Pre90 set to "0dB", full scale from the DAC is actually 20.2dBV. Now we can finally calculate the volume setting at which each amp would reach maximum output with a full-scale sine wave from the DAC. For the Audiophonics amp, we have: 6.3dBV-20.2dBV = -13.9dB. For the ICEpower amp: 11.2dBV-20.2dBv = -9dB.

 

[Ema79]

So if I understand correctly, I should set the preamp to 0 volume. And despite the similar gains between the two power amps, would I still have a difference in SPL between the high and low range?

 

[Sokel]

So if I understand correctly, I should set the preamp to 0 volume. And despite the similar gains between the two power amps, would I still have a difference in SPL between the high and low range?

High and low range, how?
There must be something between then setting gains, like a el.x-over.

You probably have to describe your setup better.

 

[Ema79]

High and low range, how?
There must be something between then setting gains, like a el.x-over.

You probably have to describe your setup better.

between the preamplifier and the power amps there is an Xilica XP 8080.

 

[Sokel]

between the preamplifier and the power amps there is an Xilica XP 8080.

So there's your (any, you want) balance.
You can set the desired gain for each way and get the result you like.

Traditionally we give lows a little more gain so we can knock-it down later during the RC without losses.

 

[bmc0]

So if I understand correctly, I should set the preamp to 0 volume.

Without the DSP unit, -9dB is enough for full output from either amp. With the DSP unit, it would depend on the settings of said unit.

And despite the similar gains between the two power amps, would I still have a difference in SPL between the high and low range?

Since the gain of both power amps is approximately the same, you'll get very close to the same output voltage given the same input voltage as long as you're below the clipping thresholds. The ICEpower amp can put out more voltage (and thus, power), so it needs more input voltage in order to reach maximum power.

 

[Ema79]

I've had the Topping preamp for much longer than the Xilica. I'm used to the classic setup. But as I was reading, a thought occurred to me: Since the Xilica has a gain of about +15 dB, and is able to adjust the levels for each channel, the preamp must have become a useless accessory by now, right? Or would it be better to keep it? If so, why?

 

[DVDdoug]

the preamp must have become a useless accessory by now, right?

Don't you need it as a volume control? You don't need any additional amplification with your current setup but I assume you're using it as an attenuator/volume control?

Like I said, a "preamp" is usually a "control center" rather than an amplifier, with multiple selectable input sources, a volume control, tone controls, and other usually other controls. But they do have amplification if/when you need it.

And integrated amplifier is a preamp and power amp all in one box. A receiver adds a tuner. Audio Video Receivers add a DAC (allowing digital inputs) and more channels. Some modern stereo receivers also have DACs but not all of them can decode DVD & Blu-Ray formats.

 

[Ema79]

Hi, my DAC has a volume control. So it works even without a preamp. Let's say that when I added the preamp, everything came to life more. But I only had one power amp and I didn't have a DSP. But now I have an Xilica that gives me 8 inputs; I can adjust the levels, gain, and much more since it's a DSP. Functionally, I don't think I need a preamp. The question is whether it does anything good for the signal. Or rather, whether it has better volume, whether it's capable of raising levels with better quality.

 

[DVDdoug]

Functionally, I don't think I need a preamp.

Correct. You don't need a preamp unless you need a source selector or tone controls, or a headphone jack, or something else additional.

Let's say that when I added the preamp, everything came to life more.

It was probably louder. You didn't do a careful level matched Controlled Blind Listening Test. A little louder is often perceived as "more dynamic" or various other "mysterious" characteristics. And when you listen louder, because of the Equal Loudness Curves it sounds like you've turned-up the bass even more.

The question is whether it does anything good for the signal. Or rather, whether it has better volume, whether it's capable of raising levels with better quality.

No. Technically it can only degrade the signal by adding noise and distortion, especially since it doesn't have tone controls/EQ.

Practically, it's not going to change the audio quality/character in any way (other than volume) since the noise and distortion are below audibility.