Preventing clipping in head unit driving external amp? - diyAudio
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Old 25th August 2004, 02:12 PM   #1
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Default Preventing clipping in head unit driving external amp?

I have a factory head unit that:

- does not have preamp outputs (RCA/line level) only speaker outputs
- has a "digital" volume knob that never stops turning left or right
- has no visual indication of whether the volume is set to 0, 5, or, um, 11
- clips once the volume is turned over a certain level (duh)
- has no internal trimpots or adjustments of any kind (mostly IC's inside)

I would like to keep this head unit for a number of reasons, including that it has analog bass/treble controls, whereas most aftermarket head units I've looked at have none or only digitally-selected presets. I like to adjust the levels per-CD or per my preference of the moment. Plus it's free since I already have it.

How many things can be tried to reduce or eliminate clipping? I'm not going to drive speakers directly with it.

I have a LOC (line out converter) that I think has about 2M resistance on the inputs, but even with speaker wires connected through this I get clipping. I thought raising the resistance of the load would have helped.

If I can find the power transistors, can I either:

- tap their input signal and create a line-level out, or
- disable them, and send their input signal to a line level out, or
- limit their input (resistor in series with input signal?) to keep it below clipping level

My goal is to be able to turn the volume all the way up without any clipping, nevermind how much volume is lost, since it's going into another amp.

Any ideas?
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Old 25th August 2004, 05:54 PM   #2
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The no display of volume level is an irritating one.

You can add RCA outputs to a factory head unit, if you're brave (or foolish) enough to do so. The last stage of the pre-amp cct is the balance/fader controls, because it is easiest to do volume/bass/treble on a stereo signal, rather than have to do it on a quadrophonic one. Once you have located the last stage of the analogue output, the amplification circuit will start with a coupling capacitor. If you sever the connection at that point, you will have the cleanest signal that you're going to get from that head unit at that point. (Immediately after the capacitor.) You should add an ouput resistor to the line there, between 63 and 100 ohm typically and from there you ought to be able to go to RCA jacks.

This will only help in the event that it is the amplification stage which is clipping, and not the pre-amp stage.

There's another thread around here somewhere on adding pre-amp outputs in a factory head unit. I've done it with an older Toyota head unit, and got what I'm assuming are ~500mV outputs from it. I also cut the power to the amplification stage (really quite pathetic) in order to reduce heat. Doing this had no effect on sound.

As I'd mentioned in the other thread, I also added a line-level input by removing the AM Stereo decoder chip, so selecting "AM" got the aux input regardless of what (if any) station was tuned in. I figured that losing AM radio was a small price to pay.
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Old 26th August 2004, 02:02 AM   #3
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Quote:
Originally posted by SupraGuy
Once you have located the last stage of the analogue output, the amplification circuit will start with a coupling capacitor. If you sever the connection at that point, you will have the cleanest signal that you're going to get from that head unit at that point. (Immediately after the capacitor.) You should add an ouput resistor to the line there, between 63 and 100 ohm typically and from there you ought to be able to go to RCA jacks.
Assuming the distortion isn't happening in the preamp...

Instead of severing it, could I simply limit the signal at this point, so that what comes out the speaker wires won't clip? Would a simple resistor between the coupling capacitor and the power stage work?

In other words, make the volume knob on "10" act like "4", and "5" act like 2. Then I could still use the front/back faders with a LOC.

If I did it with two pots (or a stereo one) I could turn it all the way to one side for "factory gain" and the other way to reduce it.

If I were going to do this, I'd also add the RCA-outs while I was at it. In fact, that would be useful for a subwoofer amp not affected by the front/rear fade.

See anything wrong with that?
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Old 26th August 2004, 04:37 AM   #4
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Yes, we have to assume that the pre-amp isn't the source of the distortion, or else you're hooped no matter how you come at it, unless you want to start looking for the pre-amp selector IC, and attenuating its output.

You could simply attenuate the amp stage input, however, I'd submit that this is NOT the way to get the cleanest sound. The on-board amplifier adds noise. It has to in order to do its job. The line level converters add noise, again, they have to. In my Toyota head unit, there were 4 separate channels of amplification. This was done to make the most of the pathetic power supply voltage available and make it operate 4 channels. By tapping into the 4 separate amp stage inputs, and severing them, I was able to obtain very clean signal. Use of the factory fade and balance controls was absolutely unimpeded, and the tone controls also worked perfectly.

Chances are that the first thing after that coupling capacitor is already a resistor. Instead of getting multi-ganged pots, I would suggest a set of trimpots, which can be set once to limit output below the clipping level.

Be sure to test for clipping with both fade and balance in neutral settings, as this will draw maximum current in the amplifier stages, making the clipping start at the lowest levels.

I think that just routing pre-amp outputs is a more elegant solution, but I suppose no matter how you slice it, it's still a hack.
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