amp wont put out voltage without bass boost

[vario 10+ year member]

I was setting the gains on one of my cadence fxa1500 amps and noticed that it wont put out 31ish volts (recommended gain for 500 watts @ 2 ohms) without setting bass boost to the max first. is this common? I set bass boost to +18 and then the gain knob about 3/4 the way up and that got me a reading of 31.5 volts which should be perfect. I used sqrt(500*2) = 31.6. Sub is wired to 2 ohms and disconnected, I set the gains off of the empty terminals on the amplifier.

Only other thought is that maybe the preamp voltages are really really low. I have the pioneer headunit listed in my signature and the rca's from that go into the ultra linear (decent cheap eq) preamp/eq and then out of the subwoofer channel on that into the amplifier.

I guess I will also test the voltage of the preamp, what is an optimal range?

Any ideas on all this? I'd hate to use bass boost but it seems necessary.

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[MikeyB 5,000+ posts]

Seems like your really low on input voltage. Check all the hu settings and the eq. You shouldnt have to run bass boost up at all to get the right voltage unless the amplifier is damaged.

 

[dman4486 5,000+ posts]

try going strait from the hu to the amp

 

[vario 10+ year member]

With the headunit volume set to the maximum (62 is the number it displays) Voltage off of the rca's out of the Ultralinear: .8 volts. Voltage off the rca's directly .8 volts, voltage off the back of the head unit: .8 volts. And this is off the front channel. The rear channel (for the sub) is like .3 volts.

Think the head unit is blown? I reset gains for volume around 50, setting the sub's mono amp to 31ish and the fronts by ear to a listenable volume.

This sounded okay and it didnt require bass boost at all to get this voltage, the monoblock was set to about 80% of the way up as it was before, but the bass boost was turned off.

However, I turned the car off after about 15 minutes of cruising around and the amp was really hot to the touch, so I will not continue this approach. Any ideas?

Strange thing is the front channel rca's were plugged into the pioneer amp at .8 volts and the pioneer amp has a marking on it called normal (it has markings for .4 to 1.5? I think), so I set it near the bottom of course and it had output, while the sub required the gain to be 80% of the way up.

Should I repost in the head unit forum?

 

[vario 10+ year member]

With the headunit volume set to the maximum (62 is the number it displays) Voltage off of the rca's out of the Ultralinear: .8 volts. Voltage off the rca's directly .8 volts, voltage off the back of the head unit: .8 volts. And this is off the front channel. The rear channel (for the sub) is like .3 volts.
Think the head unit is blown? I reset gains for volume around 50, setting the sub's mono amp to 31ish and the fronts by ear to a listenable volume.

This sounded okay and it didnt require bass boost at all to get this voltage, the monoblock was set to about 80% of the way up as it was before, but the bass boost was turned off.

However, I turned the car off after about 15 minutes of cruising around and the amp was really hot to the touch, so I will not continue this approach. Any ideas?

Strange thing is the front channel rca's were plugged into the pioneer amp at .8 volts and the pioneer amp has a marking on it called normal (it has markings for .4 to 1.5? I think), so I set it near the bottom of course and it had output, while the sub required the gain to be 80% of the way up.

Should I repost in the head unit forum?

I should add that my multimeter is set to V~ and 200 and that gives me a display of .8, maybe 1 at the highest.

 

[helotaxi 5,000+ posts]

I'm guessing that the HU has 2v nominal outputs? That usually means that the HU will only make that voltage with volume and boos maxed, not that it really matters.

If the amp is making voltage but running hot, the preamp voltage has nothing to do with the problem. The amp is probably either overrated or broken. If you have to max the gain to get the amp to make the correct power that really isn't bad assuming that the amp can actually make the power that you expect. If it can't make that kind of power the problem is that you are clipping the crap out of it and that can easily explain why it's getting hot.

 

[dman4486 5,000+ posts]

maybe you wired to the wrong resistance....

just an idea I came up with this morning

 

[MikeyB 5,000+ posts]

Also if you have it faded up front than that could be why you are getting a higher voltage off the front preouts

 

[vario 10+ year member]

I'm guessing that the HU has 2v nominal outputs? That usually means that the HU will only make that voltage with volume and boos maxed, not that it really matters.
If the amp is making voltage but running hot, the preamp voltage has nothing to do with the problem. The amp is probably either overrated or broken. If you have to max the gain to get the amp to make the correct power that really isn't bad assuming that the amp can actually make the power that you expect. If it can't make that kind of power the problem is that you are clipping the crap out of it and that can easily explain why it's getting hot.

Full boost, full volume and still doesnt make anywhere over 1 volt output. I am measuring this directly off the back of the head unit, no rca's plugged in.

Wiring of sub resistance doesnt matter when setting gains because speaker is disconnected. Speaker is, however, set to 2 ohm as tested by multimeter. Balance/Fade/Crossover etc is not set to anything.

I ordered a pioneer deh p4900i, it claims to have 4v outputs. I hope that cures it. Im pretty sure its not the amp.

 

[DBfan187 5,000+ posts]

HU is Pioneer, set gains on vol 61/62 with all setting on flat.

 

[vario 10+ year member]

HU is Pioneer, set gains on vol 61/62 with all setting on flat.

Done. Thats what I did //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif.

Front channel amp has to have the gain **** near bottom and the monoblock has to have the gain near maxed so I think the rear rca is 90% dead and the front rca is like 50% dead b/c it doesnt produce near optimal voltage either.

 

[helotaxi 5,000+ posts]

What tone were you using when testing the HU preouts? Also a maxxed gain in an of itself won't cause the amp to run hot, so you have another problem somewhere as well.