Pre-out voltage amplifier

Actually this amp has no gain knob, it has a menu interface with LED display where I can set the gain, it is set to +9dB (possible settings: 0db, +6dB and +9dB). I know this amp can deliver more than it delivers now. It is rated 300W RMS @ 2 Ohms when supply is 14.4V. I have 13.5V, so RMS power should be around 260W (P=V*I). The current draw should be at least 20A, but I never got this amount when I was providing single frequency sine wave on the input. I assume input signal level is too small, so I'd like to amplify it. This amp can display supply voltage value and amount of current drawn, that's where above numbers are coming from.
 
It's Alpine MRD-M300
Specification

I made a mistake describing the amp, actually there is no gain setting, only input level selection. The gain I've told you about is on the deck, and its set to +6dB.

Input level on the amp is set to 0 dB and possible values are:
0 dB for 0.5V input level
-6 for 1.0V
-12 dB for 2.0V
-18 dB for 4.0V

I was running it on -12 dB for a while, but there was very little bass coming out of the sub.

Manual states there is a menu option for gain selection, but there is none on the amp. I'll go to the store to see what's up with that.

Anyway, I am going to amplify the input signal and I think I'll use this
 
So your amp is set to a .5V gain setting, your headunit has 4V preouts, and when playing a solid tone it's not drawing its rated current from the electrical system? Have you tried using a multimeter on the amp's outputs to see what kind of voltage it's spitting out? Are you sure that the tone you used was a 0dB tone and not say, a -12dB tone?
 
You won't be able to get readings at the outputs using multimeters, they won't autorange fast enough. You probably won't want amplify the preouts, but the Power input that you have set at 13.5v. If you can find a basic 741 OpAMP chip I can help you set it up. But you'll need a +/-16 to 17v power supply. This can be tough to get especially in a car. You almost need a transformer and a couple of half wave rectifiers set up right. Obviously quite a few barriers. Another thing you can do is just a basic amplifier circuit.

One question, what kind of sub are you powering with this amplifier? You might also be severely underpowering your sub depending on what you have.
 
sr20dem0n said:
So your amp is set to a .5V gain setting, your headunit has 4V preouts, and when playing a solid tone it's not drawing its rated current from the electrical system? Have you tried using a multimeter on the amp's outputs to see what kind of voltage it's spitting out? Are you sure that the tone you used was a 0dB tone and not say, a -12dB tone?

My head unit has 2.2V preouts and its Pioneer DEH-P300 by the way.

I haven't used a multimiter on the amp output yet. I've downloaded test tones off your home page and I'll check the voltage sometime this week (probably tomorrow).

I think the problem I am having is due to the missing (or hidden) gain on the amp. I am also going to measure the actual voltage on the deck's sub out when the volume is set at the max level I can listen to (which 23 out of 30). I do not like using deck's gain, it's making the sound less clean.
 
ChevS-10 said:
You won't be able to get readings at the outputs using multimeters, they won't autorange fast enough.

With music you can't, but if you use a solid tone you can


Sorry I don't know where I got that you had 4V preouts from, but even 2V should be more than enough to be able to send that amp into clipping


Stepping up the headunit's preout voltage would make it louder, but I have a strong feeling you're already at or beyond your amp's limits as it is, and increasing the preout signal even more would just send the amp farther into clipping, possibly damaging both the amp and the sub. Testing the voltage output from the amp should give you a general idea how much power you're getting out of it, it won't be exact but it should get you in the ballpark. That amp should put out somewhere around 20-27Vrms before clipping, depending on the impedance of the sub that's connected to it and the input voltage from the battery. If you're getting significantly less than this then you could try stepping up the preout voltage, if you're getting significantly more than this then you're already clipping the amp and you need to either get a more efficient sub, a larger amp, or simply learn to settle with less bass.
 
One question, what kind of sub are you powering with this amplifier? You might also be severely underpowering your sub depending on what you have.

Two 10" Infinity Reference 1030W in a sealed box (0.9ft^3 per driver). Single voice coil, connected in parallel, 250 RMS each. I know, amp is not a match, but I think it can deliver more. I had a 60W amp powering ported 10" Crunch sub once and my current setup is not that much louder when 0db gain on the deck is set. It's way louder with the gain set, but that's not the way it should work, I guess. Some CDs have low bass and I'd like to amplify it more, but with the deck gain already on max, it's not possible.
I got some MDF left, so If there is no solution, I'll make a ported box and see what happens.


But you'll need a +/-16 to 17v power supply. This can be tough to get especially in a car. You almost need a transformer and a couple of half wave rectifiers set up right. Obviously quite a few barriers. Another thing you can do is just a basic amplifier circuit.

I am planning on getting one of those DC-AC power converters and bi-amp other speakers on chip amps (got some LM1875s), just to see if there will be better sound quality. I've made a LM1875 based amp for home audio and it's quite decent, like to see if it can deliver the same in car, but that's another thread. I know DC-AC-DC is little strange, but its simpler than designing a DC-DC converter. This way I can obtain one or two way symmetric power supply for the opamp circuit.


I have a strong feeling you're already at or beyond your amp's limits as it is

This amp is supposed to have protection of some sort, it's not kicking in yet. On the other hand, it's supposed to have gain adjustment too....


simply learn to settle with less bass

Or learn synchronized swimming, while am at it (no disrespect meant to those who can, it's just impossible) ;)


Question: can one hear when amp is clipping?
Question: is there a way I can measure SPL with a generic microphone and a PC?
 
Time for a new amp?

Clipping, is when the AC wave can no longer be amplified because it exceeds the gain, the peak of the wave is just cut off. If you have really good ears and know what the music is supposed to sound like you might be able to tell.

The RMS value on your subs are 250. Where as the RMS value on your amp is 200 at 2 ohms mono with a power input of 11v. With the 13.5 you may be putting out 240-250w. from your amp, but these subs aren't even close to being pushed. I don't know what they sound like, but I would look into an amp that hits about 500 w. RMS. Then you will still be safe and the subs will make good sound.
 
michal said:
This amp is supposed to have protection of some sort, it's not kicking in yet. On the other hand, it's supposed to have gain adjustment too....

It probably has thermal protection, but it won't have protection against clipping. If it has a good enough heatsink then even while clipping it still might not be getting hot enough to trip the thermal protect.


michal said:
Question: can one hear when amp is clipping?

If the clipping is bad enough you can hear it pretty easily on mids/highs, on a sub it can be more difficult; slight clipping is very hard to hear on music. If you're listening to a solid tone then you can hear it pretty easily though, and at frequencies above ~200hz it's extremely easy to hear. If you want to hear what clipping sounds like on music then play a song on your computer (preferably one that's recorded pretty loud), and turn every volume setting you can find on your computer to max (make sure to turn down the volume on whatever speakers you're listening to or it would get obnoxiously loud). When it starts to sound...well, like crap, that's clipping. You can do the same thing with a solid tone if you want, when it starts to clip you'll hear the harmonics come in and make the tone sound "impure".