Setting Gains w/o Oscilloscope

[metanium]

Okay, here's a write-up based on the notes I took while Mark Eldridge was setting gains on ItalynStalyn's car. I'm sure there are some errors here and there. Please point them out, so i can edit this. I plan on adding pics when I can get them.

By properly adjusting gain setting in an audio system, the user can maximize the amount of clean power available from their equipment. This reduces noise, adds headroom, and prolongs the life of the equipment being used.

The first thing to do prior to setting gains is to set all pre-amp levels and EQ’s to flat (zero) on HU and processors. Secondly, any electronic crossover settings should be adjusted to include the frequency of the test tones that will be used to set gain. For example, my subwoofer output is usually set to low-pass at 63 or 80Hz. For setting gains on the sub channel, I will use a 100Hz test tone, so I will adjust the low-pass for my sub output to 200Hz.

Once the adjustments have been made, you’re ready to pop in a gain setting CD such as Autosound 2000 Disc 104. Gain settings should be adjusted from the beginning of the signal chain to the end. So, you’ll want to start at the HU(s) and work your way through processor(s), then to the amplifier(s). The gain for each channel (or channel pair) should be set using an appropriate test tone. By appropriate, I mean the test tone should be a sine wave at a frequency within the usable range of a particular channel (see note on crossover settings above).

I use a 3-way active HU, so I have High, Mid, and Sub outputs. I will set the gains of these outputs using 100 Hz, 1 kHz, and 4 kHz respectively. I will also be using a test tones recorded at -5dB. This will allow some minor clipping when driven to maximum levels, but any distortion should still be inaudible. The Autosound 2000 Disc 104 includes multiple tracks that will allow for overlap. Gain overlap will allow the user to extract every bit of undistorted power out of their system. The table below illustrates the pros/cons of using overlap when setting gains.

Note: The mini amp can’t play the 100Hz tone with much output at all, so it’s likely necessary to place the speaker of the mini amp closely to your ear before setting the gain. The change in pitch that occurs when the signal begins clipping should be audible.

0 dB Overlap No-clipping, but good amount of power unused +/- 0.1% THD
5 dB Overlap Minor, inaudible clipping (Good compromise) +/- 0.3% THD
10 dB Overlap Clipping is audible (Max. overlap to set gain) +/- 1.0% THD
15 dB Overlap Noticeable clipping is present +/- 10% THD

With all of the preliminary stuff out of the way, it’s time to determine at what volume level our HU starts to distort. The mini amp is connected to one of the mid channel(s). The min amp is to be powered on, but the volume should be kept to it’s minimum (turned all the way down) to avoid hearing the internal amp clip, as opposed to the incoming signal. With a 1kHz test tone playing, the volume of the HU is increased until the pitch of the tone from the mini amp changes. If the HU reaches full volume without this occurring, that HU doesn’t clip. If there’s a slight change in pitch of the tone from the mini amp, the volume of the HU should be reduced to the point just prior to the change in pitch of the test tone. This is the HU’s maximum, unclipped output (MUO) setting.

Note: The change in pitch heard from the mini-amp, indicates the point at which the signal begins to clip. The observed change in pitch comes from second, third, and fourth-order harmonics which occur with clipping. So instead of hearing a single-frequency tone, we hear multiple-frequency tones playing together.

This process is to be repeated, in order, down the signal chain. So any processors between the HU and amplifier(s) are to be tested with the mini-amp and test tones. Remember to use an appropriate test tone for each channel being used (i.e. 100Hz for sub, 1kHz for mid, and 4kHz for the high channel(s)). When setting gains on processors in the signal chain, the HU’s volume should be set to the MUO, as determined previously. The gain controls of the processor being set should be set for the MUO. If the output of the processor clips even when its gain is set to the minimum, the HU’s volume may need to be reduced below the MUO.

Once the gains have been set to MUO on the HU and any processors in the signal chain, the amplifier is ready to be adjusted. Since the output of the amplifier is much greater than the pre-amplified signal, its voltage must be reduced prior to feeding it into the mini amp. This is accomplished by using a voltage divider. I am using a 10:1 voltage divider to set gains for my amplifiers. Depending on the output of the amplifiers being adjusted, a larger voltage divider might be necessary.

The voltage divider is placed inline, prior to the mini-amp. The inputs of the voltage divider are connected directly to the speaker outputs of the amplifier. The HU is once again, set to its MUO. The amplifiers gains are adjusted using the same test tones as before. Once the gain pots on the amp have all been adjusted for MUO, you’re done.

NOW, you can go set you EQ, but remember that any boosts will cause the signal in that range to exceed MUO earlier. To avoid this scenario, only use “cuts” in EQ, as opposed “boosts”. Good luck!

[metanium]

From Mark Eldridge's response in the 12V Events section:

Originally Posted by Mark Eldridge

Hey everyone,

In as many years as I have been teaching the level setting and gain structure technique, I've never seen this much discussion on it. Glad to hear so many of you are interested in getting it right. It is the most important tuning step we can do after everything is installed.

I wihs we had been able to spend another hour explaining the process, because when you understand the theory behind it, everything makes more sense, and it's easier to approach without any confusion factor. But, that's what we do at the seminar, so come to one, and you'll learn a whole lot more than just gain structure...

As far as making two voltage dividers, why not just put a switch in the 10:1 divider you have, with an additional resistor in to switch between 10:1 and 20:1. You could use a DPDT switch, and add a third setting for 5:1 for very low powered amplifiers. You can get even crazier and add outputs to connect your volt meter to check the actual voltage, an o-scope, and some of those really cool LED lights that dance with the music...

Following are the steps for setting gains that are in the Advanced SQ Seminar notebook. There's not a lot of explaination with them, just the basic steps listed so it'e eeeasier to follow when working on a car.

System Level Setting and Gain Structure – The Most Important Tuning Step!!!

What is it?

Adjusting the input and/or output level controls of each component in the audio system so that the output performance of each component is matched to the input of the following component

All system component input and output levels are set to clip at the same point

Done properly, it will ensure that the system’s overall signal-to-noise ratio and dynamic range are maximized


Why is it so important?

It allows for peak system performance with minimal distortion and unwanted noise. It also allows full use of system controls (i.e. the volume control will not be limited to only the first three clicks before onset of distortion and magic smoke)


The Important Factors

Dynamic Range – The ratio between the highest and lowest audio
signal levels, measured in decibels

Noise Floor – The noise power generated internally within the system
components in the absence of any input signal, usually measured
in decibels referenced to a specific power level

Signal-to-Noise – The ratio of the signal power to the noise power in decibels

Max Output Level – The maximum output level attainable from a device
with no distortion, measured in decibels or as voltage

Distortion – In theory, it is the addition to or modification of a signal caused by a
piece of equipment. In our reality, it is the undesirable effect of
clipping, causing harmonic distortion.

Tools Needed

Test CD – Autosound 2000 CD #104
Radio Shack Mini-amplifier w/ RCA input adapter
Oscilloscope (optional)
10:1 high-power voltage divider (20:1 if amplifier outputs are over 60V)


The Steps on How to Do It

1. Ensure access to inputs and outputs of all components in the signal chain
(you can, for example, get head unit output access at input of first processor). Also ensure all speakers are disconnected from the amplifiers.

2. Set all tone and equalizer controls to flat if possible

3. Insert CD in head unit, set track to Track 11 (1 kHz @ 0 dB down) on repeat

4. Connect mini-amp and/or o-scope to head unit output

5. Turn up head unit volume to maximum undistorted output level (watch o-scope
and/or listen to mini-amp)

6. Leave the head unit set there

7. Connect head unit output signal to next component input and connect mini-amp to
it’s output

8. If a component has both input and output level controls, set the component’s output level below 1/4 max output temporarily

9. Adjust its input level to max undistorted level

10. Now adjust the output level for max undistorted output

11. Connect its output to next component’s input, and adjust the next components input
and output levels accordingly

12. Continue through all pre-amp level components

13. For crossovers, choose appropriate 0 dB down frequency tracks for the crossover output band frequency range being adjusted (At very low frequency ranges, lower than 200 Hz, and unclipped signal will not be audible through the mini-amp. However, clipping of the fundamental will be audible if the amp is held close to your ear. So you will hear nothing with the gain set for no clipping, and as you turn it up and start to clip, the distortion will be heard on the amp. Looking at a scope makes it easy to see visually)

14. At the amplifier, connect the scope and/or the mini-amp with voltage divider to the amplifier output

15. Choose an appropriate frequency and recorded level track on the CD, recommend -5dB or -10 dB for a 3:1 voltage gain overlap

16. Adjust amplifier gain for maximum undistorted output

17. Now, adjust relative gains for each frequency range (i.e. pull down any channels that are too loud relative to the others, using the amp gains first, and the crossover outputs second.

18. THAT”S IT!!!



Hope this helps.

[metanium]

Bump for the most impoertant step in tuning......setting gain(s)!!!!!!

[BeatsDownLow]

nice write up, i will have to try it coming up with the DIY scope I got last month

[EVcelica]

Very good writeup.

All my gains are all the way down on every amp, I guess that is one way to set them, get a headunit with crazy strong rca's (8V) and amps with lots of headroom, and turn them all the way down.

[chad]

Originally Posted by EVcelica

Very good writeup.

All my gains are all the way down on every amp, I guess that is one way to set them, get a headunit with crazy strong rca's (8V) and amps with lots of headroom, and turn them all the way down.

I swear to god the whole gain control thing is gonna drive me bonkers.

You are just setting up the sensitivity of the amp.. that's it. All the way down on one amp is completely different than all the way down on another model/make.....

[jbholsters]

There is a similar tutorial using the Radio Shack mini amplified speaker and disc 4 in one of the Autosound 2000 tech briefs. If I remember correctly, it shows how what parts you need to attach it to the speaker leads with everything contained in a small project box. Unfortunately I lent my briefs to someone many moons ago and never got them back.

[shagdrum]

So...where does one begin in making (or getting) a "voltage divider"? Don't know what that is or where to begin in making one.

[chad]

Let me google that for you

[shagdrum]

All those links look like gibberish to me. I don't know the first thing about reading a wiring diagram. can you by these anywhere?

[chad]

Nope, you picked the wrong advanced hobby, can I interest you in crochet?

[shagdrum]

What size resistors would be needed for a alpine PDX 4.150 and a PDX 1.600? Do I need to go with a 10:1 ratio or a 20:1 ratio?

[shagdrum]

Anyone?

[dohcser]

nicely done

[Nico156]

interesting!