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Old 17th March 2009, 12:29 AM   #1
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Default Help with preamp please, new here

Hi, this is my first post here, I have been searching for information that would be useful for my project, but you guys have info for more advanced circuits, so here's the story.

I built a preamp, it goes between the HU and the amplifier. The HU gives me only .071 V at volume 30 using a 1khz sinewave @ 0dBfs. Given the fact that the HU has 62 volume steps, it takes a lot of time to turn the volume up or down, that's why I made this circuit. So that the preamp gives me 2 volts when the volume is set at 30. Also it would help to lessen the noise picked up by the RCAs due to the higher voltage.

Click the image to open in full size.

I tested it just after the HU and while it works and amplifies the signal, there is a lot of noise coming from it. If I set the preamp gain to 1, everything sounds ok, but as soon as I start to turn the gains up (POT resistance), there is this noise coming out too, the higher the gain, the louder the noise (regardless of HU volume). As you can see, the preamp is grounded using the RCA shield, as this was recommended by Perry Babin (bcae1.com I guess you know him).

NOTE: The quality of the sound is kept, but the noise is very loud. I replaced the TL084 but the noise is still there.

This is what I'm using:
Pioneer DEH-P6000UB
DLS Iridium 6.3i
SoundStream xta480.4

Any help would be greatly appreciated
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Old 17th March 2009, 02:01 AM   #2
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You may have an open shield ground on your head unit.

Even a pioneer head unit can handle the 0.010 amps that the TL084 will draw. Driven to the 2v that you require into a 1k ohm load (significantly lower than what you'll have), the current draw will likely be less than 0.040 amps. They use a 3 amp fuse in the shield.

The circuit is significantly different than the one you sent me and what I suggested.

You should have a capacitor across R2.

You should have a resistor between 12v and C2.

A voltage regulator may also help. A series resistor of 150 ohms and a 9v zener would work. The zener would be connected between the power supply input terminals of the op-amp.
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Old 17th March 2009, 02:41 AM   #3
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Is this using an electronic pot?
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Old 17th March 2009, 03:55 AM   #4
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I'm sorry I didn't write back, I didn't want to steal your time.

In the diagram I sent you, it was only one channel, this is for the 4 channels.

I replaced the input resistor for a 20k, as you said. I forgot the capacitor in the R2 (as I added the 1mF to block any AC current from alternator). My mistake.

By reading some information about EQs and Crossovers, I noticed that most of them have an output impedance, (Resistor in series with output?) of about 2k, that's why I added R11,R12,R13 and R14.

Then you said about the preamp ground, I did it. By open shield ground you mean that the RCA shield isn't connected to ground anymore? NOTE: The HU is grounded in the same place as the AMP.

I made the changes you just told me, the new image should appear instead of the old one.

Click the image to open in full size.
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Old 17th March 2009, 03:55 AM   #5
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Quote:
Originally posted by tsmith1315
Is this using an electronic pot?

No, I'm using a regular pot, a cheap one, as I didn't have a precision one around.
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Old 17th March 2009, 04:24 AM   #6
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The output impedance is either determined by the op-amp being used or by the series resistor used to provide a bit of protection to the op-amp. The resistor isn't really needed. If you decide to use one, it shouldn't be any larger than 100 ohms.

The 1k resistor on the input has to much resistance. It should be 100 ohms or less. 50-100 ohms would be OK if you weren't going to use a zener regulator. If you were going to use a zener (9-10v), I'd recommend a 100 ohm resistor.

If I remember correctly, I recommended a 2k input resistor and a 50k pot for this circuit. You wanted a gain of 28. That combination would give you a gain of 25. For C6, try a 0.1uf. You want it to charge as quickly as possible. A larger cap will charge more slowly so it will take longer for the op-amp to reach its final bias voltage.

For C2, I'd recommend 100uf.

The shield should be connected to chassis ground internally. Use the tests on the following page to check the shield if you don't know how.

http://bcae1.com/images/rca/temporar...eldrepair.html
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Old 17th March 2009, 08:00 PM   #7
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Quote:
Originally posted by Perry Babin
The 1k resistor on the input has to much resistance. It should be 100 ohms or less. 50-100 ohms would be OK if you weren't going to use a zener regulator. If you were going to use a zener (9-10v), I'd recommend a 100 ohm resistor.

For C2, I'd recommend 100uf.
It's little bit confusing following steps by reading and then checking, I believe I did it the way you suggested, but when I placed the Zener in the power supply inputs of the op-amps (after the voltage divider), the reference voltage from the op-amp (+) input wouldn't be half the total input voltage, it would be slightly higher, I know this shouldn't be a problem as the output would never reach a voltage such that it clips because of this, but I wanted to keep it symmetrical. That's why I placed it at the beginning, so that the whole circuit would have a 9v or 10v supply.

Following all your recommendations, this is what it looks like. I tried to be as accurate as possible. Luckily for me, I already had all components. I just need your aproval to test it, once in the car I'll check for the open shield ground.

Click the image to open in full size.
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Old 17th March 2009, 08:31 PM   #8
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The shield is the first thing you should check.

You may be able to use it without the regulator (zener). With a 10v zener and a TL074, the output was just over 2v at clipping. With the amp's gains set to the maximum, you won't need nearly 2v to drive the amp to clipping.

If the regulator is required for your system and you need more output voltage, you can use a rail-rail op-amp.
http://sigma.octopart.com/5443/datas...-TLC2272CD.pdf
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Old 18th March 2009, 01:39 AM   #9
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I just tested it and checked the shield following your directions. The HU has REAR, FRONT and SUB outputs, so I checked all of them. The three LEFT channels are OK (0.2 Ohms) but i can't get any reading for the RIGHT channels, even when trying ohms or kohms at the ohmmeter.

When testing the preamp, I was using the front right channel, then I did it again but using the front left channel instead, the problem still ocurred. There is this "airy" sound coming out and strange noise, similar to when there is a bad connection at the speaker terminal. This noise gets louder as the resistance in the pot increases. Again, at unity gain, the sound is clean.

I decided to keep the regulator as it is in the image because you suggested to use one and also because that way the voltage would be fixed to a given level (which is better).

Is the HU supposed to have continuity in the LEFT channels but not in the RIGHT ones? If it is, what should I be considering besides using a TL074 and/or a rail-rail op-amp?
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Old 18th March 2009, 01:54 AM   #10
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You should have ~0 ohms continuity between all of the RCA output shields and the case of the head unit. If you don't, connect all of the shields to the case of the head unit (preferably with a fused jumper).

Even with a perfect ground, a gain of 20+ will produce noise. If the noise is a hiss, the noise is mainly due to the amplification of noise being fed into the preamp. To determine how much noise the preamp is producing, short the center conductor to the shield on the input of the preamp (no RCA input - preamp grounded to he head unit chassis).

If the noise is engine noise (varies with engine speed), there may be a ground loop or a poor ground.

The TL084 is a decent op-amp. If the IC produces too much noise at high gain, you could look into some of the low-noise op-amps. Before you do that, you should solve all other problems.
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