Opto isolator to couple valve stages ??

[Argus25]

I found when experimenting with optocouplers, the analog ones at least, they operate over a a certain dynamic range, meaning to get the best out of them it requires, for any type, a certain average current for the LED in the device, then a certain maximum peak to peak current modulation, and as noted that modulation current should be proportional to the incoming signal voltage. This no doubt relates to the properties of the phototransistor and not so much the LED but obviously there is a limit to how hard the LED can be pushed.

So in practice the easiest way to set this up is to put the LED in the collector of a transistor (with a small current limiting resistor in case of accident say 100R), Then place a resistor in the transistor's emitter of a value such that when the emitter voltage is about 1/3 the supply voltage, the emitter current is about right for the LED's average current. The transistor base is biased to just above 1/3 the supply rail.

The AC (audio or video) signal is then applied at the base (via a capacitor unless there is no DC offset from the signal source). So then you can vary the emitter resistor value (start with say 1k5) you vary the average emitter and therefore collector current, and you will find a point where you will have the circuit work with the largest possible AC signal swing (dynamic range) before distortions or clipping on the signal occurs on either peak or trough.

[Diabolical Artificer]

Thanks lads, just an idea, one day I'll have an idea that turns out to be pure genius : )

Andy.

[MrElectronicman]

Some interesting ideas here. I have been fooling around making an audio compressor circuit, and one idea was to use a home made Opto Isolator using LED and a Photoresistor. The whole thing is run using 4 opamps

The photoresistor in conjunction with a fixed series resistor acts like a volume control pot for the incoming audio. I coupled the photoresistor and LED together with heat shrink tubing and it sort of works, but the dynamic range is very critical due to I believe the drive of the LED.
The LED is driven by one of the opamps, however the idea of putting a LED in the collector of a transistor might solve the problem as it maybe more controllable.

[mhennessy]

Optical compressors have been around for decades, and are liked because of the sluggish time response of the lamp and LDR, which lets through the initial attack of the sound before the gain is brought down. Obviously, the attack time is a parameter that can be adjusted on a modern compressor that uses a VCA or similar, but it's also a parameter that can be incorrectly set...

If the output of your op-amp can travel over a fairly large voltage range, then that presumably means that you have a fairly high value resistor between it and the LED - in which case, then you're approximating current drive already. Adding a transistor to do that is likely to make things rather more complicated - if you think that a more linear relationship between a voltage and the LED current will help, then you can put the LED in the feedback loop of the op-amp - see figure 17 on this page: http://www.nutsvolts.com/magazine/ar...upler-circuits

This works because the op-amp attempts to keep the two inputs at the same voltage, so provided the op-amp doesn't hit a rail, the current through the LED will be equal to Vin divided by R3.

Of course, you could re-jig the topology of the compressor. Are you using a feed-forward compressor, where the input to the sidechain is from the input signal? If so, changing to a feedback type, where the input to the sidechain is from the output (after the gain control stage), means that feedback greatly reduces the need for the gain control element to be linear (in terms of gain vs. control voltage).

Compressors can be complex little beasts - but fascinating

[AJSmith625]

One of Currys early Matsui colour portables from about the early 80s which had a live chassis, (cannot remember the model number), used optocouplers to couple external audio and video in to the set. This was deemed not to come up to British electrical safety standards at the time and the few that were sold were recalled plus all the unsold stock were modified. We had to remove all the wiring from the input phono sockets to the chassis and a sticker was attached next to the sockets saying that this facility did not apply to this model. The same sticker was stuck in the instruction books. While we had one of these sets running on the bench isolation transformer we fed some external A/V from a video recorder showing test card F in the E to E mode. The results were not as good as the usual way via RF through the aerial socket.

Alan

[Tim]

I have used opto couplers for low speed digital and audio before, but only to drive Triac gates in a set of disco lights I built. It did work though, but I can't vouch for the audio quality you might get.