Pre Outs and peak-to-peak ? - diyAudio
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Old 8th December 2006, 06:37 AM   #1
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Question Pre Outs and peak-to-peak ?

I'm a new member - usually find me on VW or buggy forums - and am struggling with a car audio question. Hoping you can help me

I am in the UK, and am installling a light aircraft intercom into a kit car (a PS Engineering PM 3000 into a Volkswagen based beach buggy if you're interested !)

The intercom allows a car audio device to be connected, but contains this warning

Click the image to open in full size.

I'm not sure what 'peak to peak' means here - I've looked up peak-to-peak but that looks like its about AC ? The intercom is for a negative ground DC system (obviously) .

From stuff I found on this forum though it looks like the pre out from a car CD tuner head unit is AC ? Sound right ?

If I use a CD tuner with 1 x 1.8v pre out, is that OK ? I'm guessing not ?

If I've understood correctly when pre out figures of 1.8v or 2.2v are quoted, they are NOT peak to peak figures ? In which case, even 1.8v pre out is going to be too high as that'll be a peak to peak figure of 5v (based on RMS of 1.8 x 1.414 = peak, x 2 = peak-to-peak of 5v)

Does that sound right ? I'm way over my head here !!

Thanks in anticipation ...
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Old 8th December 2006, 09:19 AM   #2
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Location: Louisiana
3vp-p + 1.06v RMS

The head unit will produce 0 volts at 0 volume and ~1.8v RMS at maximum output (may be before the max volume setting is reached). It's very unlikely that the device would be damaged but to be sure, you could use a test tone recorded at 0dBfs and make note of the volume setting (never going beyond that level). You would have to leave the tone controls flat or perform this test with a 100Hz test tone and the bass at max. This will leave you with low output when the bass is not at the maximum level.

You could use a voltage divider to cut the level of the signal but this 'could' cause issues with sound quality if the input impedance of the device isn't flat across the audio spectrum.

Again, it's 'unlikely' that a 2 volt preamp level signal is going to damage the device. They are likely warning against connecting amplifiers to the inputs. You may be able to email them to confirm this... And you need to confirm this before connecting the head unit directly to the device.
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Old 8th December 2006, 10:18 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
you are not "way over your head" here.
You got this spot on
Quote:
If I've understood correctly when pre out figures of 1.8v or 2.2v are quoted, they are NOT peak to peak figures ? In which case, even 1.8v pre out is going to be too high as that'll be a peak to peak figure of 5v (based on RMS of 1.8 x 1.414 = peak, x 2 = peak-to-peak of 5v)
To be on the safe side I concurr with Perry, fit an attenuator.

If you use -6db (half voltage) then it guarantees that you never overdrive the input of your unit. If it's too quiet then reduce the attenuator value.
But try the -6db first.

And, Yes, the warning is to avoid blowing up the input by connecting speaker levels signals which can be upto 200Vpp. A tad higher than 3Vpp!!
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regards Andrew T.
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Old 8th December 2006, 04:45 PM   #4
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thank you both - can I ask some more questions please (as this is all totally new to me !). Apologies for the length of the post - I've highlighted the main points in bold


this is from the user manual for the Audio Link that is suggested in the extract from the manual that I pasted in above:

Click the image to open in full size.

and this looks like it would do the job , -6db, and be very easy to fit


but 3 follow up questions if I may....

1) when it says "clip resistors shown", what does it mean ?

(I've had a look at the resistors page (39) on bcae1.com, and I'm guessing that this is about connecting equal resistors in series to halve the voltage, but I'm not quite sure what I would physically do when it says "clip resistors" )

Do I just put a metal clip across the wire to join the top 2 and separately the bottom 2 ? I'm guessing so and that this creates the 'in series' link ?

2) is there a DIY version of this that I can make for what I need ? I've got a soldering iron and a local Maplin store, and I'd quite like the satisfaction of having found out what I need to do and stuck the bits together myself quite apart from having the sense that it may be a lot cheaper

I'm guessing that splitting the voltage is an easy thing to do, and from what I can see I just need to do this:

Click the image to open in full size.

Can I do this using (to start with, to achieve 6db) a 1K resistor for each of R1 and R2 ?

Do I just put such a divider into the + wire for the (say 2.2v) preout ?


According to what I've read, using 2 x 1K resistors for the attenuator would result in a max source impedance of 200 ohms and a min load impedance of 5K

Should that be OK ?

I'm struggling a bit here as I don't know what impedance is or where to find this source data for the head unit I'm looking at or the intercom..... but what I've read suggests that this is well within the likely range.

If not, what should I be using for R1 and R2 in this case ?

I don't know if this helps :

Click the image to open in full size.

Thanks again - I'm getting there, and will be pleased when I finally get this nailed and working !

3) The cop out option - do I just buy this from my local Maplin ?

Click the image to open in full size.

Is this the same thing as the Audio Link in Low Level Ground Loop Isolator mode (as per the extract pasted in above)

All I can find for specs on the Maplin site is:

Ground Loop Isolator Specifications:
Specifications:
Distortion: >0.001%
Frequency response: 40Hz-32Hz
Input/Output impedance: 10kŮ

I've no real idea what this is or does and only ask because it has the same name (Ground Loop Isolator) as the relevant bit of the Audio Link manual


Thanks so much for taking the time - its been an interesting hour or two's reading for me anyway !!!
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Old 8th December 2006, 05:53 PM   #5
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When it tells you to clip the resistors, it means that you need to cut them out of the circuit. You only need to cut one end of each free.

The divider with two 1k resistors should work well well enough. The audio frequency response is stated as having a 15kHz top end so it's not designed for audiophiles (but it should sound OK). The only way this won't work is if the input impedance is very low (600 ohms). Most audio equipment has at least 10kHz input impedance.

The line output converter is likely a 1:1 converter used to break ground loops. If you have noise problems due to ground loops, you could use the audio link piece. It will serve to cut the level as well as isolate the ground. Most audio equipment that has an input circuit also has special circuitry to prevent noise but I don't know if this device has such a circuit.
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Old 8th December 2006, 08:20 PM   #6
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thanks very much Perry

No need to worry too much about the audiophilic qualities as its to be fitted in this

Click the image to open in full size.

and will only be listened to over headphones via the intercom system

main concern was not to blow the (pricey !) intercom

thanks so much for all your help. Your web pages are fantastic and I'm working my way through them too

Cheers
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Old 12th December 2006, 07:39 AM   #7
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Nice Beach Buggy... Are those twin Delorto IDF carburettors mounted on a VW beatle engine?
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Old 12th December 2006, 08:30 AM   #8
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Dell DRLAs (same as Webber IDFs) yes, on a 2L type 4 engine with upright cooling conversion

lot of work to do yet though !

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