Setting Amplifier Gains When Retaining a Factory Harley Radio
Note: The steps in this article are also valid for aftermarket radios as well.
This article applies to most all bagger stereo systems, but is directly intended for users who are retaining the factory Harley Davidson radio on a '14 and up bagger.
Some Will Not Agree
Be prepared to be told that the following procedure is wrong. This is because for decades now the car audio industry has been doing it different. But a bike is not a car. Setting amplifier gains (amplifier input sensitivity) should not be done in exactly the same manner as it is done in car audio. There are a few very distinct differences in the equipment as well as the environment that a motorcycle stereo system is expected to perform in that dictate a difference in procedure.
The goal here is to maximize the amount of undistorted signal you can get out of the factory Harley radio and pass that clean signal on to the next audio device. The next device in the audio system is typically a digital sound processor like the Rockford Fosgate DSR1 and then to the amplifier(s), but in some cases might be direct to an amplifier.
The following section of this article goes into some depth of how and why this procedure is different than the one most car audio installers are taught to use. If this is not of interest to you, you can simply scroll down to the part labeled "The Procedure".
The current generation of factory Harley Davidson radios have known clipping (distortion) issues at high volume levels.
The Bigger Problem
Harley radios have speaker amplifiers built into them so that they can drive speakers directly without the need for an external amplifier. This amplifier is in play even when a bike is equipped with a factory Harley external amplifier. Harley has several trim levels and models that include one or more amplifiers that are external of the radio itself. The problem here is that the type of amplifier that is built into the Harley radio is particularly susceptible to clipping at high frequencies in such a manner that when passed through an aftermarket amplifier the result will quickly toast even robust speakers. Furthermore, this high frequency clipping is particularly damaging to tweeters.
Now Let's Make That Problem Bigger!
To compound the clipping problem, we pass this potentially clipped signal to a digital sound processor and a class D aftermarket amplifier (all modern amplifiers suitable for motorcycle use are class D amplifiers) that add in their own level of clipping when presented with a clipped signal themselves.
Now Let's Make It Even Worse!
Here is the real big kicker that the car audio guys always seem to miss. When the battery voltage drops (and it does) during high volume situations and long rides both, the clipping increases fast. So you used to be able to play your radio at a volume level of, let's say 80% on the radio volume without clipping. Now the radio is clipping at 60% on the radio volume while playing that same song.
Here Is Where the Car Audio Guys Come at Me with Pitchforks and Torches.
So this is often the way that this story plays out. A well intentioned audio installer who has done his homework and has invested in all of the cool gadgets like an oscilloscope or a distortion detector and has a legitimate high level of hard earned education, will set all of the gains on a bike with a clean sine wave recording made for testing distortion and all is good. Then that bike goes down the road and while the end user is jamming the new tunes the battery charge dips and the guy is likely not using the best recordings ever made. The radio clips, the rider does not hear the clipping (it might not even be audible clipping!) and speakers start to fail one by one. This by the way is also very hard on amplifiers and can also cause them to fail as well.
I point my finger at the car audio guy's way of doing things. I know all about this problem because, I myself am a car audio installer of 25 years. I have spent years as a certified IASCA competition sound and installation judge and I am an IASCA world finalist as a competitor. It's not that car audio guys are dim. It is simply a different application on a vehicle that many car audio guys (not all) are not as familiar with as they are with boats and cars with much larger charging systems. There are plenty of savvy car audio guys out there that do understand the clipping situation with modern Harley radios but it does not seem to be the norm yet. So now we move on to the solution.
Set the radio fader, bass, and treble to center positions.
Turn off the speed controlled volume feature on the radio. This is simply a feature that you have to give up in order to fulfill your ultimate thirst for sound. Sorry dude.
Dry set your crossovers to a conservative and known safe setting, and set all your amplifier gains to 50%. Be sure that your crossover settings are set and functioning by listening to each speaker individually at low volume levels. Once you are confident that you are not sending subwoofer signals to your tweeters you can proceed to turn your radio's volume level up until one of two things happen.
- The speakers show signs of reaching their physical limitations. This is relatively safe as long as you don't push the speakers for an extended period of time.
- You reach 90% radio volume. In most cases (but not all. It depends on your particular radio flash) you never want to run your radio's volume past 90% even if you can not hear distortion or even test for it with specialised equipment.
Now you need to either turn your gains up or down.
- If you can not reach 90% volume with your radio before the speakers reach their limits then the gains need to come down.
- If your radio volume reaches 90% volume and the speakers are not playing to their potential, then the gains need to go up.
Now turn the radio volume all of the way down. Listen for hissing coming from the speakers. If the hissing is excessive the gains need to be turned down. The goal is to reach a happy balance between how loud the system can play at 90% radio volume and how much hiss (noise floor) you get with the radio at low or zero volume.
Tip: you are better off having the gain set too high rather than too low. Gains that are too high produce a hiss at zero volume. Gains that are set too low fry speakers and amplifiers! Pick your poison. There is plenty of happy middle ground. Small changes in gain can make big changes in the level of noise floor.
That's pretty much it. Reality dictates that most bikes will have some amount of hiss at zero volume. This is because we generally over gain Harley audio systems in an effort to reach ultimate volume levels. Having said that, the noise floor at zero volume should not be alarmingly loud. If this is the case, then something has been missed. Read on to the next section that covers troubleshooting. What is important here is that we leave some room in the volume range on the radio so that we are far away from distortion. We know that the factory radio is particularly susceptible to high frequency distortion and we also know that small changes in charging system voltage leads to large changes in amplifier performance and ultimately amplifier distortion. The smart move is to give the entire gain structure of your audio system some room for these fluctuations in order to avoid damage to speakers and amplifiers.
- If you are using a DSR1 sound processor, be sure that the trim levels for all channels are at max (default value). Only use the trim levels to attenuate speakers slightly as needed once the gains are all properly set. For example: it is common to attenuate horn tweeters using the trim levels on the DSR1.
- An excessive noise floor can be caused by not grounding all amplifiers directly to the battery negative terminal. Never ground to any part of a Harley chassis. Also always get both power and ground for a DSR1 directly from an amplifier power terminals or run 14 gauge power cables directly to the battery (fused).
- An excessive noise floor can be caused by tweeters being over gained. All tweeters and especially horn loaded tweeters are much much more efficient than the mid woofers that they are paired with. You may ultimately decide to run your tweeters at a higher level than the rest of the speakers on your bike in order to overcome wind noise, but this will exacerbate noise floor issues. A horn tweeter for example that is wired in parallel with a mid woofer on the same channel will always play at an excessively higher volume level than the woofer. This might even be the volume that you want the tweeter to play at. Resistors can be used in line with the tweeter to lower the volume level. If though you run your tweeters in parallel with your mid drivers, a higher noise floor should be expected. Find that happy balance.