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Old 1st November 2010, 03:47 PM   #1
RobertE is offline RobertE  United States
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Default Preamp booster ?

First, sorry I'm a newbie so don't yell.... I've searched and read hundreds of articles but I'm still not certain what to do.

Basically I've created an audio controller as a replacement for the head unit in my car.

The problem is the controller is putting out line level but the special car amp wants more - like 10v peak to peak from what I've read. The car amp has software adjustable gain, so it only needs/wants fixed levels coming in.

How could I boost the 2v peak to peak to 10v with as simple a circuit as possible? A npn transistor circuit? An opamp?

I've gotten it to work using a power amp ic (tda2822) but it adds lots of hiss, and the circuit is a bit too large for the enclosure. I built an npn circuit but it didn't seem to work right - not enough or too much boost???

Thanks for any help!
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Old 2nd November 2010, 11:14 AM   #2
AndrewT is offline AndrewT  Scotland
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10Vpp is right at the maximum limit for a 12V supply.

An opamp using a feed of 13.8V should just about manage a maximum output of 10Vpp into a reasonably high load impedance.
Rail to Rail opamps will get closer to the supply rails, but to achieve this performance, some or many of the other parameters are compromised, sometimes severely.

BTW,
10Vpp is ~3Vac
2Vpp is ~700mVac
700mVac is an unusually low level for a maximum output.

The gain you need is 5times (=+14dB).
An opamp using a 12k & 3k feedback network will give that 5times gain.
If it then feeds a receiver with a 20k input impedance, the opamp sees the 20k//12k as it's total load. Getting 10Vpp into 7k5 from an opamp on 13.8V is getting into the high distortion regions where current output is starting to alter the opamp gain.

Have a look at the ESP and PASS sites for more info on gain stages.
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Old 2nd November 2010, 02:01 PM   #3
RobertE is offline RobertE  United States
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Thanks Andrew a few questions if you dont mind...

Isn't 2Vpp nominal line level? I.e. Line level audio signal voltage at mitat.tuu.fi

And isn't 10Vpp basically +-5v so can't a +-12v supply do this with ease???

I understand the V=IR aspects and how one side might not be able to produce the current to produce the required voltage level, but I have to assume the amp impedance is quite high.

I did some more listening and I think it is about 20% off the needed max on a linear scale, I base this on different songs on the iPod at different recording levels and how some of the higher level songs can flex the subwoofer cones to distortion at max volume.

I may try a PGA2311 preamp ic. I also may try the op amp route - simpler, although the PGA2311 or similar would allow a future removal of the specialized amp.

Is there a common ic people use the generate -5v from a +5v supply? So far vie been able do work with a single supply.

Thanks again.
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Old 2nd November 2010, 02:10 PM   #4
RobertE is offline RobertE  United States
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Just read my post... The iPod is 3Vpp at line out ???

Wiki Line level - Wikipedia, the free encyclopedia says line level is -10dbv which is only .447v peak, which is only about 1Vpp

I am really confused. What is correct ?
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Old 2nd November 2010, 02:25 PM   #5
Pano is offline Pano  United States
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Line level can be all over the place. Andrew has given you a good lead in is post. Your amp wants about 2-3V RMS which is the typical output level of a CD player. An iPod is about 1V RMS.

My question would be - "Does the amp really need 10 volts in?" That's more like speaker level than line level. And are you sure about the P-P versus RMS values? I'd check on that first before building a booster.

Sure, you can get the level you need with +/- 12V, but you have to generate that negative rail. Building an inverter can be tricky, but maybe someone here knows of a good ready-built unit. I've seen them, but not used them.
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Old 2nd November 2010, 03:07 PM   #6
RobertE is offline RobertE  United States
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Some other person did the analysis on the amp/headunit, and said it was a 5v balanced output with a .8v bias - I'm not sure what it means....

Anyway, everything sounds really good I just can't get the max volume I could get with the stock unit - but it's actually pretty close.

Could it just be that I don't have the .8v bias and that's the difference? If so, is there an easy way to add it?

I don't have access to an oscilliscope so testing audio is pretty difficult.
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Old 2nd November 2010, 05:41 PM   #7
RobertE is offline RobertE  United States
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One other thing, if the amp is designed to take basically line level signals, and I boost the line level using a headphone amp designed to drive 100ohm headphones, I shouldn't have a problem correct? As the amp impedance must be much higher an that. Typically, what would be the ramifications of this, vs. using an op amp circuit, e.g more distortion, etc. Would the headphone amp overdrive the signal. My understanding is that it's job is to produce a voltage potential signal on the line, so withe the higher impedance it just does less work.
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Old 3rd November 2010, 09:56 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by RobertE View Post
........ I boost the line level using a headphone amp designed to drive 100ohm headphones,
you must check the data sheet for the proposed chip/circuit for maximum output voltage into a line level load impedance.
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Old 3rd November 2010, 02:20 PM   #9
RobertE is offline RobertE  United States
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Hmmm... If a amp chip is rated to drive 8 ohm speakers, and V=IR, wouldn't a higher impedance require much less current? In all the amp chip data sheets I reviewed I only see 4,8,32 ohm ratings, and yes the power goes down as the R goes up, but isn't a preamp stage low-power?

If the audio switch IC with a buffer stage can drive the doesn't adding another headphone amp after the buffer make the job even easier?

Maybe you can point me to a dummies post I can read about this? This is where "newbie" gets lost...
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Old 3rd November 2010, 02:57 PM   #10
AndrewT is offline AndrewT  Scotland
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A headphone amp is designed for driving headphones.
Many headphone have an absolute max voltage limit of 5V.
You want >=10Vpp.
You need to check that the amp you select can meet your specification.
You need a line driver that can output >10Vpp with low distortion, feeding a load consisting of the amplifier input impedance in parallel with the cable capacitance you will install.
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