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Old 11th Aug 2018, 3:58 pm   #1
Diabolical Artificer
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Default Opto isolator to couple valve stages ??

Got to wondering as you do, and thought about using opto isolators to couple valve stages, whether this is practical I'm not sure, so thought I'd run it pass you...

I've not used them before but looking at a few datasheets's on the plus side they can block around 5kv. But what about phase? Do they have zero or near zero phase shift?

Obviously they'd have to have a low DC supply, around 30v, also there'd be a small positive DC potential on your grid, but this can be offset by setting your cathode R to have a bit higher voltage.

Would we gain anything in terms of impedance, IE high in low out? It occurs that the grid leak resistor of the following stage might be an issue.

If indeed there is no phase shift and the impedance situation is good that only leaves (there are probably other issue' I havn't thought of ) bandwidth; do they on the whole have a good frequency response? Or are they limited to a narrow bandwidth?

What do you reckon, a goer or a waste of time?

Andy.
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Old 11th Aug 2018, 5:48 pm   #2
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Default Re: Opto isolator to couple valve stages ??

At first look I can see that it would be possible, but I am not sure how worthwhile it would be. These opto islolators appear to be used digitally more often than not although can be used in analog mode. I presume you are talking audio frequencies at which the inherent capacity of the junctions would have little influence. In that way phase shift may well not be too much of an issue either. With impedances it is really a forward biased diode at input so low and a semiconductor output so low here too (relative to general valve figures). I am thinking generic though as there may be specific devices that are different). I would see them affecting the noise figures in early stages due to the semiconductor junctions.. It will be interesting seeing other input here
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Old 11th Aug 2018, 6:18 pm   #3
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Default Re: Opto isolator to couple valve stages ??

Back in the days of the live TV chassis, Television magazine published at least one project based on optocouplers that allowed you to get baseband audio and video into the TV set, allowing it to be used as a monitor.

So it can be done. But it's not straightforward and the results aren't brilliant.

You have to drive an LED, and bias the phototransistor correctly and amplify the output from that - the circuits I've seen end up having quite high levels of distortion, which really isn't all that surprising. There are clever variations on the theme that use a second, identical optocoupler in a feedback circuit strapped around an op-amp, the idea being to correct for some of the distortions present in the optocoupler that's carrying the signal across the barrier. Alternatively, some form of modulation scheme can be used. Or negative feedback, if the surrounding circuitry allows it. Either way, a lot of faffing around, really. A relatively modest audio transformer would have much better results and would be easier to implement.

In short, optocouplers are most useful (and easiest to use) for digital things. With analogue signals, you need a whole load of support circuitry, sadly.
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Old 11th Aug 2018, 6:19 pm   #4
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Default Re: Opto isolator to couple valve stages ??

There are a number of types of opto isolators.

Generally there is an LED as the input device.

There is a lot of variability in the output device, it can be a phototransistor or even a photovoltaic cell. Also the output device can be set up for a linear or a digital output, depending on the device.

Photovoltaic types generate a voltage proportional to the led current and their frequency response is slower than a phototransistor, but still adequate for audio.

Generally though, optical isolation is required when there are earthing or ground isolation issues.

Inside an amplifier with a common ground, there is no logical reason for optical isolation. It is required in SMPS supplies to isolate the mains circuitry from the low voltage side, especially for coupling feedback information.

Generally, the isolation inside a valve amp between stages is done with a coupling capacitor for the signal, the earth return is common as it is a common ground.

If you had two valve amps, plugged into different power points and you wanted to link them with signals, an opto isolator could help earth loop issues.

One interesting thing though; If you set up an optical isolator, and you short out the output of the isolator, there is no effect from that reflected to the input of the isolator. If you have most other kinds of isolation amplifier or coupling system, then there is often a small impedance change reflected to the input when the output gets loaded. For this reason an opto isolator is a good choice for buffering a VFO output in a radio, as loading that output then has no effect at all on the VFO frequency.
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Old 11th Aug 2018, 6:29 pm   #5
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Default Re: Opto isolator to couple valve stages ??

I've used an H11F1M from On Semiconductor in a few projects in the past. They have a FET as the coupled device and are quite linear - just as you would expect from a FET.
I've used them as gain control devices and remote faders but I'm not sure how fast they are for audio use. The spec sheet says it takes 45uS to switch on and off so they should be OK.
RS and Farnell sell them for a couple of quid a pop. I might have one or two lying around here if get a chance to have a look for them...

Ging
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Old 11th Aug 2018, 7:19 pm   #6
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Default Re: Opto isolator to couple valve stages ??

I think this is all a bit "just because you can, doesn't mean you should".

Opto-isolators aren't usually designed to work linearly. They are usually used to couple logic signals, on or off. There is a linear region; but it's there by happy accident, not on purpose. Using the LED in an anode circuit, with the phototransistor pulling the grid down to chassis and a resistor pulling it up to cathode potential might work, over some range. Maybe even over enough of a range to give listenable audio.

But it probably wouldn't work any better than a simple capacitor. And unless you had completely separate HT supplies for the pre-amp and power stages, you'd be throwing away the opto-isolator's special advantage of having separate earths on each side of the package.

Of course, don't let that deter you from experimenting ..... A picture of a finished amplifier might make the above words taste a little better
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Old 11th Aug 2018, 7:58 pm   #7
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Default Re: Opto isolator to couple valve stages ??

Hi Gents, the TV article in the 70's was an opto coupled interface for a G8 so a BBC computer could be use with these popular second user sets. It even featured auto switching between PC and set.

It was one of my designs, so I'll see if I can dig the circuitry out.

Ed
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Old 11th Aug 2018, 8:31 pm   #8
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Default Re: Opto isolator to couple valve stages ??

So called 'linear optocouplers' aren't particularly rare - they have two accurately matched photosensors within the encapsulation, responding equally to the LED output, one on each side of the voltage barrier.

One is used to servo the LED current on the transmit side and the second one smply tracks the first on the receive side. e.g. the IL300, - specified at 0.01% linearity and 200kHz bandwidth and only £2.10 (!) from Farnell.

The basic idea really isn't new - I built a 'discrete' one as a student project 40 odd years ago !

Cheers

.
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Old 11th Aug 2018, 8:42 pm   #9
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Default Re: Opto isolator to couple valve stages ??

The BBC computer used simple digital RGB drives; in each pixel position, each electron gun could be either on or off, so giving black and seven colours. It would not have needed particularly linear opto-isolators; they just had to be able to switch fast enough to produce clean-edged pixels. The computer had its own speaker, so there would be no need to couple in audio.
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Old 11th Aug 2018, 8:46 pm   #10
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Default Re: Opto isolator to couple valve stages ??

IIRC Hitachi used such a system to couple the scart inputs on some of their TV’s in the early 1990’s I think it was the C2118/9 and the 25” equivalent.
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Old 11th Aug 2018, 9:16 pm   #11
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Default Re: Opto isolator to couple valve stages ??

LEDs have a rather good linear characteristic if you drive them with a controlled current rather than a controlled voltage. Their quantum efficiency says what proportion of electrons cause a photon to be emitted.

Similarly in a photo diode (or the c-b junction of a phototransistor) Another quantum efficiency effect says what proportion of photons smacking into the material liberate an electron. These two effects are usefully linear and have a useful dynamic range - just so long as you treat the input and output signals as currents. Beware that in a photo transistor, the c-to-b leakage current may be nicely linear to the intensity of illumination, but it then gets distorted by the current gain versus Ic and Vce characteristics of the transistor.

That said, I side with Julie. They don't solve any problems in audio amplifiers which aren't already soluble by easier means. They might give a bit less sensitivity to hum on the HT rail but that's pushing things a bit. Bandwidth can get into the 100's of MHz but so do capacitors.and with less noise.

If I saw any in valved audio amplifiers, I'd consider them to be marketing ploys "Look at me! I'm different!" and I'd walk on past.

Optical fibres might have a place linking preamps to remote power amps in noisy environments, but those too can be handled in other ways.

David
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Old 11th Aug 2018, 9:22 pm   #12
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Default Re: Opto isolator to couple valve stages ??

One handy application for photovoltaic opto couplers is to use their output to enhance the gate of a power mosfet for high side switching where another voltage source is outside the range of the power supply. It is slow because the fet's gate takes a while to charge from the relatively high internal resistance of the photovoltaic cell, but it works for low speed switching.

I was able to get an opto coupler to work for PAL colour video isolation many years ago (to solve an earth loop issue between two buildings) it needed frequency compensation in their drive to the photodiode and on the amplifier on its output, it managed just to about 5MHz bandwidth. I also got a similar result for fixing the earth loop problem with an opto coupler with a poor high frequency response, by cheating and bypassing the high frequencies from its input to its output with a low value high voltage ceramic capacitor. It still worked for video & to ignore 50Hz voltages because it's low frequency response was practically zero.

Of note and as mentioned by RW, it's best to control the led current making that proportional to the incoming or modulation voltage. To do this I had the incoming voltage converted to a current. This can easily be done with an unbypassed emitter resistor with the diode in the collector circuit. Also then a small HF compensation network can be added across the emitter resistor.

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Old 11th Aug 2018, 9:34 pm   #13
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Default Re: Opto isolator to couple valve stages ??

Quote:
Originally Posted by Michael Maurice View Post
IIRC Hitachi used such a system to couple the scart inputs on some of their TV’s in the early 1990’s I think it was the C2118/9 and the 25” equivalent.
I too was about to mention Hitachi TVs that used optoisolators for this very purpose.
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Old 11th Aug 2018, 9:53 pm   #14
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Default Re: Opto isolator to couple valve stages ??

One other option would be to use an isolation amplifier like the HCPL7800 though it would add quite a bit of complexity.
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Old 12th Aug 2018, 10:30 am   #15
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Default Re: Opto isolator to couple valve stages ??

Quote:
Originally Posted by Ed_Dinning View Post
Hi Gents, the TV article in the 70's was an opto coupled interface for a G8 so a BBC computer could be use with these popular second user sets. It even featured auto switching between PC and set.

It was one of my designs, so I'll see if I can dig the circuitry out.

Ed
Hi Ed,

That's not the article I had in mind - the one I was thinking of was definitely composite video and audio. The odds of me finding it now are pretty slim, as my (fairly complete) Television Magazine collection is relatively inaccessible at the moment. I seem to remember that it was on the front cover though? If so, that ought to make it easy to find if someone else has their collection in a better organised condition than mine.

Cheers,

Mark
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Old 12th Aug 2018, 2:49 pm   #16
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Default Re: Opto isolator to couple valve stages ??

Thanks chaps. "They don't solve any problems in audio amplifiers" I was thinking they might solve the phase shift issue you get with using a capacitor, so less phase shift = better stability = allows use of higher NFB.

I can see the extra complexity would be an issue but I have a couple in my bits drawer somewhere, I'll knock up a circuit and give them a go.

From a quick perusal online I see you can get Darlington types, might be useful for driving OP stages. I thought they might be useful here too if one wanted to bias the OP valves without a separate neg bias supply - just thinking out loud.

Andy.
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Old 12th Aug 2018, 3:44 pm   #17
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Default Re: Opto isolator to couple valve stages ??

The only problem with the plan is, in order to set up the correct average current in the photodiode by driving it with a transistor or OP amp, and have the correct dynamic range for the alternating component of the signal, generally would require AC coupling of the audio signal to that driver circuit, so you are back to where you started with low frequency phase shifts. Likewise, on the output side, likely you could end up having to have a coupling capacitor again as without dual supplies it might be difficult to get the DC axis of the signal correct.
If you want to get rid of most of the low frequency phase shifts as a result of RC coupling, simply use very large value coupling caps.
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Old 12th Aug 2018, 4:25 pm   #18
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Default Re: Opto isolator to couple valve stages ??

You can design a DC-coupled valve amp from one end to the other... If you really want to. No optoisolators needed, just appropriate circuit design. This sort of thing had to be done in classic oscilloscopes.

Lots of valves, and massive paralleling at the output.

Though if you're going to AC couple with an output transformer anyway, interstage capacitors are the least of your worries.

David
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Old 12th Aug 2018, 4:46 pm   #19
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Default Re: Opto isolator to couple valve stages ??

LED in anode, photo tranny in cathode of next stage?
 
Old 12th Aug 2018, 6:18 pm   #20
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Default Re: Opto isolator to couple valve stages ??

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Originally Posted by Diabolical Artificer View Post
Thanks chaps. "They don't solve any problems in audio amplifiers" I was thinking they might solve the phase shift issue you get with using a capacitor, so less phase shift = better stability = allows use of higher NFB.
Coupling capacitors only cause phase shift when they are starting to roll away at LF. If that causes a problem, simply increase the value of the capacitor

You need one capacitor to define the system LF point. Just one. This capacitor must be outside of the global negative feedback loop, so will often be the input cap, but other options might be possible. Having made that choice, all other capacitors should turn over at a much lower frequency this - at least an octave lower, but go even lower if it's economic to do so. If any of these capacitors happen to be electrolytic, then make them even larger again, as it's well-known that electrolytic capacitors acting as filters will introduce distortion (though in the context of a valve amp, that might well be academic).

If you'd like to experiment with the idea - and I'd definitely recommend giving it a go - I'd start by trying it in isolation (no pun intended!) from a big and scary valve amp. Just try a low-voltage breadboard circuit and see where that gets you...
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