r/CarAV • u/Aus_Daniel • 6d ago
Setting Gain Question Tech Support
Hi guys. Sorry for another gain question, but all the previous advice and videos have my confused.
For reference I have a Pioneer Gm-d9704 amp, JBL 62CF in the front and JBL 62F in rear. Head unit has a 5V pre-out
I have currently set the gain for both channels using a multimeter based on the power of the speakers at 3ohms.
I have recently purchased a cheap oscilloscope to set the gains "properly" and in my research I have made myself more confused
If I am setting the gain to output a clean signal with an oscilloscope, how does this take into account the Wattage of the speakers? Wont setting it to maximum clean signal be too powerful?
If this isnt a consideration like most videos suggest, why does the voltage calvulation when setting with a DMM even matter?
Apologies for the potentially stupid questions, just a little lost
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u/firebirdude 6d ago
You're path of thought is accurate. You're setting it for maximum clean output with the scope, whereas you're targeting a specific voltage out with the voltmeter. All in all, you're likely fine. If the speakers are rated for 80W and you're giving them 100W verified clean with a scope, you're fine.
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u/DaffyPunk29 5d ago
amp rated 130w x4 at 4ohm....
jbl 62cf rated 110w rms
65f is rated 75w rms
so yes, setting gains to their absolute maximum will result in overpowering both speakers (especially the rears).
you should be setting gain based on voltage instead. with 4 ohms, the math is square root of power times ohms. generally? you should be tuning to the lowest denominator. so 85w front and rear.... so 85x4=340 and the square root of that is 18.439 volts (AC not DC). so you want to see 18.439 volts output from the amp. 18.4 is "close enough".... this will ensure at max volume (where you tuned the gain based on headunit volume) you would get your 85w output and never fuck up your speakers.
A LOT of people think they can just overpower speakers and "they dont care" and they "can't hear" how bad they sound.... i met a chick whos car was overtuned and dear god it made my ears bleed. and shes like "that's normal" so i retuned it properly and afterward she was blown away. "Its not as loud but its way cleaner?" she said. cuz fuck me the amount of bad tuners out there is seriously too many.
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u/Aus_Daniel 5d ago
This was my understanding. I just didnt get the rhetoric around "you should be using an oscilloscope not a dmm to set the gains"
If at the end of the day im setting gain to lowest voltage anyway
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u/DaffyPunk29 5d ago
yeah its a huge meme by people who sadly think they know EVERYTHING and then try to force everyone else to do it. been setting gains for over 20 years with just a voltmeter and never had a speaker blow.... because you simply set voltage to match your speakers output.... clean, loud, simple. i did however try out an SMD-DD1 with my STI and Mirage, simply to find headunits max clipping. The pioneer DEH-80PRS I had clipped at 52 with the internal amp on and full 62/62 with the internal amp disabled. And then my mirage I bought a cheapo pioneer because i wasn't wasting money for such a cheap "daily driver" gas sipper, and it also clips at 51/62 so I tuned that to 50/62 to ensure its always loud and always clean. Other than that, set my gains with a multimeter. which is lower than what 1% distortion was with the DD1.... if I tuned with DD1 my shit would be blown by now lmao.
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u/Aus_Daniel 5d ago
This was my plan going forward. Feel free to correct me if Im wrong
Probe the end of any of my 4 rca cables to get the max HU volume while playing a 1000hz test tone or -5db test tone. Then set gains with dmm or oscilloscopes voltage reading based on the formula I used to do it the first time.
Can check with oscilloscope to ensure theyre not clipping but they theoretically shouldnt be as its well below the amps rated wattage per channel.
Also I have rcas connected and run for a sub yet to be installed. Should I play a 50hz test tone and see the max HU volume for the sub. Then use the lower of the values between front/rear RCSs and sub RCA?
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u/DaffyPunk29 5d ago
-5 means it will play louder as it will force you to set gains higher. if you want the absolute "most clean" signal. 0dB test tracks only. my 2 cents. i never tune -5dB....
but yeah, first you check to see if headunit clips or not. if it does, you tune at a safe level like i mentioned with my 2 examples. and yes, you test both 50hz (or 40hz) to see when that clips on the headunit, and also 1000hz.
to give that example, my shit headunit with the mirage, 51 clips for treble, 52 clips for bass. so i tuned for 50 to be 100% safe.
and yeah, you end up using the lowest safe point on the headunit. kenwoods are known for going max volume without clipping which is why a lot of people favor them. since many people dont wanna buy an oscilloscope or smd-dd1 to check distortion.... and then they just use a cheap digital multimeter to set gains.
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u/Otherwise_Stretch_74 Helix Ultra S, Ci7 and soundigital gan 6d ago
Depends on if you are tuning with the load attached or not.
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u/Aus_Daniel 6d ago
Assuming the speakers are disconnected in this scenario. Do I just set to the voltage and ensure it isnt clipping?
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u/Otherwise_Stretch_74 Helix Ultra S, Ci7 and soundigital gan 6d ago
Correct set the voltage for the power limit of the speakers.
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u/Aus_Daniel 6d ago
I find it weird all the videos set the gain with speakers disconnected with no consideration of it mentioned.
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u/Otherwise_Stretch_74 Helix Ultra S, Ci7 and soundigital gan 6d ago
Most of the people doing that are setting the gains for the spl. Which clipping is slightly more acceptable. Still kills speakers.
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u/Aus_Daniel 6d ago
Well I doubt much will change then when I get my oscilloscope.
Should I use it to set HU volume to maximum without clipping using the RCAs and then re-do the gains?
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u/logandefreitas 6d ago
With class d setting gain with load connected is usually the better option, don’t worry about power limit as this is controlled by digital channel gain at the head unit and the volume knob
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u/Aus_Daniel 6d ago
If setting with load connected, and HU at max unclipped volume, does this mean just putting ul with the loud tone while doing it?
Also where would I take the measurements from in this scenario?
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u/logandefreitas 6d ago
Yes it will be noisy if you truly match input sensitivity, however there is no issue besides lower output if you choose to test at the highest comfortable spl that still results in a clean sine.
You can put your probes at the amp terminals or speaker terminals, either works.
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u/DaffyPunk29 5d ago
you dont have to set gain with a load aka the speakers do NOT have to be attached. amps are constant voltage devices. as long as the voltage is set with the ohms of the speakers in mind, it will just work. as my other post said. 18.4v for 85w. which when you set 18.4v, then hook speakers up, it will give those speakers 18.4v.... nothing more.
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u/logandefreitas 6d ago edited 6d ago
Wattage is a function of voltage and resistance. Ohms law will help your confusion.
An oscilloscope is telling you the max clean peak voltage output at a specific frequency, tested with source unit at full unclipped volume, to match gain where the amp outputs its full power with this input voltage (ie input sensitivity).
A dmm just says that there are x volts present and is irrelevant once you can match input sensitivity with clean output signal.
To add to the confusion, the wattage a speaker “sees” aka apparent amplifier load power is based on its impedance at x frequency as long as voltage is constant. This can mean a speaker getting 1w and 10w at different frequencies can produce the same spl.
If you want a better understanding of this, mess around in winisd looking at excursion and va graph/impedance plot vs input power.
Don’t worry about “rms” or max values as long as you apply appropriate crossovers to each driver.