Core Electronics Forum

Combining breakout Amplifier and Opto-Isolator

Hello,
I am trying to amplify a line level signal and feed it into, and isolate from, a slightly higher level (1.5V RMS) audio circuit.
Can I feed an audio signal output from your breakout amplifier SKU: BOB-09816 into your opto-isolator SKU: BOB-09118 ? And from the Opto-isolator into the higher level circuit above ?

Also, can you tell me what the transistors Q1 and Q2 are in the circuit for the opto-isolator please ?
John

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Hi John

They restore the signal to original polarity. If you were to take the signal directly from the Otto collector output it would be the inverse of the input. The whole second half of the circuit also level shifts to whatever you supply it with at the HV points.

NO. The Opto isolator is basically a digital thing and the LED in it is still a diode with a diodes non linear characteristics so the whole thing at audio signals would distort probably quite severely.
Having said that there are Otto isolators out there that are linear and some operate right up into video frequencies (analog video) but the average hobbyist would probably not normally come across them. I would think people like Element 14 would have them but I have not looked.

Why are you looking at Otto isolators and over complicating things . That audio breakout board is a basic 2 stage audio amplifier with a total gain of 100 with the pot full up. You probably (to obtain 1.5V RMS)) need a gain of about 2 to 5. Depends on what “line level” you are referring to. The term has several meanings. More info if you google “audio line level” and look at the wikipedia article. I think you are referring to the domestic use which would be about 300mV RMS.
Cheers Bob

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Thanks Bob.
The line that I am trying to inject the audio into has other devices on the line. Those other devices have an output spec to go onto that line as 1.5VRMS into a 33 ohm load. Basically there is a 33 ohm resistor in series with a 100 ohm pot across the line. The 100 ohm pot is for adjusting the line resistance as more devices are added.

I am using an MP3 Trigger to produce the line level audio. But I need a physical volume control (pot) and the volume when connecting the MP3 Trigger direct to the other line, the volume is a little too low. Therefore I decided to use the breakout amplifier to give it plenty of volume.

I had the feeling that the circuit I am feeding into was being effected or being dragged down and thought it would be a good idea to ensure the two circuits were isolated.
But as you have explained, the opto-isolator will not work.

Should the output from the breakout amplifier be sufficient to not cause any problems ?
ie high impedance connected to a much lower impedance circuit and therefore be just like another device on the second circuit
or
Should I use isolation and what can I use to isolation the two circuits ?

Hope you can help
Regards
John

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Hi John.
I am getting a little confused. Where does the MP3 fit in. Audio is analog and MP3 is digital, that is a bunch of numbers. An MP3 signal has to be decoded before it is much use.

You are probably right. You can’t really feed a heap of low impedance outputs onto the one line. You can feed a number of high impedance inputs FROM one line. The output resistance of any OPamp is quite low. That LM358 on that breakout board is quoted as 300Ω for open loop configuration which reduces somewhat with feedback. Probably about 75Ω or less. Easy enough to measure anyway. Doesn’t take many of these to get down to near zero.

Would not do much good. The best isolator for audio would be a transformer but impedances are reflected through anyway. You could transform the low impedance of the OPamp up to something like 10kΩ but when you connect the low impedance line (you say 33Ω) to this you will kill the signal anyway. No net improvement.

It might (read would) help if you could post a schematic showing what you are trying to do.
Cheers Bob

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The MP3 files are stored on an SD card in the MP3 trigger module that produces the audio output to go to headphones or another device as line level. Core sells these.

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Hi John.
OK. So it seems you are dealing with Audio, not digital. All my other comments still stand. I am still unsure how you are combining several low impedance audio outputs on one line. I understand having one low impedance line feeding several high impedance inputs but not the other way around. If that is indeed what you are doing. As I said some sort of schematic or diagram would help.
Cheers Bob

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Sorry for the delay Bob.
Here is part of the schematic of the many devices that are connected to the one audio line - all connected ion parallel.
The part highlighted in yellow is part of one comms station and there are many of these connected together in parallel. Note there is a 1K5 resistor in the yellow lines that go to the transformer.

What I am trying to do is inject the audio ontl this line that runs between all comms stations.

Considering you mentioned that the Op Amp may put something like 75 ohms across the line, and that we are only connecting one of these across the line, I would say that it will be OK because the base resistance across the line is 33 ohms (and up to 133 ohms) anyway. And I presume connecting 75 ohms will have negligable effect.

Note : there is only one 33 ohm resistor (with pot) in the whole setup, no matter how many Comms stations are connected.

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Hi John
Firstly

You assume very much incorrectly. 75Ω across 33Ω will reduce the 33Ω to 22.9Ω. Quite a significant reduction. Something like 30%. The 75Ω is only a guess, could approach zero. like I said, you would have to measure. I might measure a 258 (same thing) under these feedback conditions later if I get around to it.

That circuit is a start but only really shows a speaker amp and a couple of wires to it. Via some relay contacts which leads me to suspect that this is an intercom system of some sort where there are possibly only 2 stations connected together at any one time. Can you verify or supply a block diagram of interconnections.

Who introduced the 100Ω pot and 33Ω resistor combination across the line and could we assume that it is just connected across those 2 yellow marked wires. If it has been introduced as an add on the whole system may not be working properly.

If this is an intercom is this circuit typical of all stations. There is probably some complex relay switching going on. It is obvious how the audio gets to the local speaker but not so how the reply gets onto the line from a local source. Some intercom systems can be very complicated.

Where are you getting your audio from the mp3 device. From your constant reference to 33Ω I assume the headphone jack or the JP connected to it.

Actually what are you trying to do with it. Are you trying to inject it onto the line to play on all stations (I am assuming intercom here) or have it on the line so stations can select it.

A few questions here but need confirmation re intercom assumption and that connection or block diagram. Also a complete diagram of a remote station if available. I have an idea but I need to know these points.

As I said earlier connecting one low impedance output to lots of high impedance inputs is easy. Connecting lots of low impedance outputs on the one line is a bit of a no no but if this is an intercom system there may be a way.
Cheers Bob

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John
Add on. Give me a bit of time, Have had a closer look at that circuit and you say this is the same as the rest of the system. Just had a re read of your reply and sorry I missed that (senior moment, that’s my excuse). There may be a way to try but still answer my question re the 100Ω pot and 33Ω resistor. I think the impedance of each of these units is something like 3K.

Will post my idea when I look more closely at the MP3 thing and the OP amp breakout board. Please confirm also that both these units are the Sparkfun things.
Cheers Bob

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Hello Bob, let me answer in point form.

  • There can be as few as 2 of these stations in a system or more than 150
  • The stations are all the same
  • Yes this part of an intercom system, but only relates to the part where either I can inject audio onto that line OR one of the comms stations operates the relays shown towards the top left of the diagram, and voice coming from a microphone is then injected onto that line and therefore goes to all the comms stations’ speaker amplifiers in that intercom system
  • The 33 ohm resistor in series with a 100 ohm pot is NOT an add on. This is part of the system design and ONE is required per system. It is in a separate box, but wired across the line, somewhere in the middle of the system.
  • This relay state shown in the diagram is the idle state of the system. The items in the system are
  • two or more of those comms stations with the connection to the system highlighted in yellow
  • 33 ohm resistor in series with 100 ohm pot connected across the line
  • That’s it. For our purposes there is nothing else.
  • Most of the time, the relays are in this idle state and this is when I want to inject my audio onto that line that runs between all Comms stations and therefore broadcast it through al the comms stations’ speaker amplifier.

Does this help ?

image001.png

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P.S note the resistor, R50, in the yellow highlighted line in each comms station => 1.5K

image001.png

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And yes, both the MP3 trigger and the Op Amp breakout are Sparkfun items

image001.png

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Here is a typical system diagram. All comms stations and junction boxes are wired in parallel.

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Hi John

Yes. Things are becoming clearer.

This would appear to be the termination for the microphone amplifier in use. Coupled with X stations across the line in parallel I think it would be adjusted to the correct termination value.

This resistor would prevent a short circuit across the line when a station is “muted”. It appears the mute function shorts the transformer input.

The line and unit inputs:
This is a balanced line so you cannot just hang an OP amp output with one side grounded (unbalanced) across it. The input Z appears to be about 3kΩ, 1.5k resistor and 1.5k transformer. The mike amp output Z (Z= impedance) appears to be about 47Ω as there is a 47Ω terminating resistor across it in the idle state. Only guessing here.

This agrees somewhat with my earlier statement re connecting multiple higher Z loads across a low Z source. In your case 150 @ 3kΩ in parallel - 20Ω. That is assuming 3kΩ for the transformer. It could be much more depending on the value of the secondary termination at the time. The Z seen at the primary side is this termination value multiplied by the square of the turns ratio which in this case is marked as 4:1 or multiplied by 16.

What you are attempting is have 2 low Z sources connected. 1 (your MP3) connected at all times and a second (microphone amp) connected sometimes. Now I really don’t know what this will do as I have never attempted it before but whatever, this source has to be BALANCED.

I don’t know what level the mic amp is. There is a level marked on that circuit but I can’t make out if it is 2.0VAC or 20VAC. Could you confirm please.

Could you post the bit of the circuit off the top left please. I think this is the bit you are trying to replicate and would be handy to see how they balance the output. A lot depends on that 2.0V or 20V result.

Do you have a spare unit somewhere. If one could be got into with your audio most everything is done. I am still unsure of the effects of having 2 at once but maybe you could try and use 2 microphones at once and see what happens.
Cheers Bob

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Yes this is correct. The mute function mutes the local speaker to this Comms station so that you don’t get feedback from the spearker to its own microphone. But it allows the voice to go to all the other comms stations.

Re : I don’t know what level the mic amp is. There is a level marked on that circuit but I can’t make out if it is 2.0VAC or 20VAC. Could you confirm please.

This is 20 Vac

Re : Could you post the bit of the circuit off the top left please.
Yes - see attached
voice part of circuit.pdf (1.5 MB)

Re : Do you have a spare unit somewhere. If one could be got into with your audio most everything is done. I am still unsure of the effects of having 2 at once but maybe you could try and use 2 microphones at once and see what happens.

Yes I have a spare. But consider this :
We have been talking about the “Page” line. There is also a “Talk” line in each comms station.

When the handset is lifted to talk, relay K1 changes state and connect the microphone voice part of the circuit to the “Talk Line”. Other people can also be connected to the talk line and you can have 2 or more people on the line from multiple comms sets. This is because the Comms sets are all connected in parallel and it is a “Party” line
So this is the same for the “Page” line that we have been dealing with.
You might get 2, 3, or 4 people at once on the talk line discussing something.
Generally, one person at a time uses the “Page” line, but it could be that something happens like an emergency and so you could get multiple people on comms station on the line at the same time, but that would be rare.

Re : This is a balanced line so you cannot just hang an OP amp output with one side grounded (unbalanced) across it.
Q1. How can I make the Op amp output balanced ?
Q2. So, is an isolation circuit required ? This is where I was heading (incorrectly) with the Opto-isolator board
Q3. Is the attached output stage of an ancient tone generator, a balanced output ?
This works and exists on a current system. When you measure the DC resistance across the “Page” line with this device connected in parallel with all the comms stations, it measures about 8 to 12 ohms plus any cable resistance between the device and where you are measuring
Ancient tone generator output stage circuit diagram.PDF (26.8 KB)

Hopefully, all this extra info helps.
I think it is better to receive your questions and provide info bit by bit so you get a better understanding than just dumping too much info all at once.

Regards
John

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Hi John.
I will try to deal with this a bit out of order.

Yes it does.

Totally disagree somewhat. A broad outline of what and how things happen is an enormous help. Along with a circuit if possible. Finer details can be requested if required. Remember you are asking a question of people who have no idea what the equipment is and does so it is likely to start off as a lot of guesswork.
This to me is always a problem and in the past I have likened it to pulling teeth.

Simplest, use a transformer. The Mic amp uses a transformer.

A transformer will do that

Yes. Note they also use a transformer. By the way, it is probably not that ancient. It uses OP amps. When I first started most of the test equipment (particularly the high end stuff) was full of valves. You know, those things that glowed in the dark.

That is a lot of signal. Equates to 56.6VAC peak to peak if the 20V is RMS.
By the way, are those signal levels P-P or RMS. Being mostly voice they could be P - P. It might say somewhere on the circuit or in the text but not on any of the bits that I can see. Can you confirm please.

Re tone generator.

Don’t take a lot of notice of this, it could be anything. DC Ω measurements are handy to determine say primary and secondary but we are concerned with the AC resistance which is something else again although a resistive load on a secondary will be reflected as the square of the turns ratio into the primary and become the effective AC load.

When that tone generator is connected into the system what is the level required for the sort of volume you want. Please advise if your measurement is RMS or P - P. Also output transformer T2 details if you have it
Cheers Bob

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Correction, I believe this is 2.0 Vac. I am trying to measure it, but the comms station I have, has just stopped outputting to the speaker. Grrr I am getting another one to see what the measurement is on that - and will advise.
In the meantime Here are some note from the drawing

Re : Totally disagree somewhat. A broad outline of what and how things happen is an enormous help. Along with a circuit if possible. Finer details can be requested if required. Remember you are asking a question of people who have no idea what the equipment is and does so it is likely to start off as a lot of guesswork.
This to me is always a problem and in the past I have likened it to pulling teeth.

OK. so you are one of the few that can handle this. Good to know.

Yes. Note they also use a transformer. By the way, it is probably not that ancient. It uses OP amps. When I first started most of the test equipment (particularly the high end stuff) was full of valves. You know, those things that glowed in the dark.

OK. This circuit is definitely obsolete and long gone - at least two or three models ago and these guys take years to change models :slight_smile:

Re : When that tone generator is connected into the system what is the level required for the sort of volume you want. Please advise if your measurement is RMS or P - P. Also output transformer T2 details if you have it.

The volume that the old generator has would be good. I have just found some specs on that
Its “Output Level” was / is "Adjustable 0.1 to 2.0Vrms max across 33 ohm load
Does the volume control in the SparkFun Op amp Amplifier take care of the volume adjustment ?

Transformer T2 in the Old circuit is most likely to be the same as T2 in the Comms stations Voice part of the circuit as in the PDF provided previously
PRI : SEC : TERT
4.5 : 3.2 : 1
D.C.R. 12 ohm : 10 ohm : 3 ohm MAX

Given that T2 was probably made specifically for those stations (or that company), what transformer could I use that is readily available in Aus, say from Altronics, Element 14, RS etc ?
Regards
John

p.s. still working on confirming what I think is the 2.0 Vac level - will advise

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Hello Bob,

Firstly, considering I found the output level of that old tone generator to be adjustable between 0.1 and 2 Vrms, and the output you asked about was either 2 or 20 Vac, plus your advise that 20V would be particularly high, I will run with the figure being 2.0 Vac and that the decimal point was missing on the drawing.

I have been thinking about and researching transformers. Would I be correct with the following scenario ?

We have the line level audio from the Sparkfun MP3 Trigger feeding into the Sparkfun dual op amp amplifier and the amplifier is providing the boost in the signal and includes a volume control. Would I now feed the output of the Sparkfun amplifier into a 1:1 transformer and then from the transformer to the Page Line in the comms station ?

If this is correct, which impedance transformer should I use ?
600R to 600R
3K to 3K
10K to 10K

Regards
John

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Hi John
I have to go out for a little while. Leave it with me and I will have a think. I don’t think you can use a 1:1 transformer. Think about what I said before. The secondary terminating impedance is reflected back into the primary multiplied by the square of the turns ratio. In this case the primary would look like about 30Ω. A bit low. That is the reason a 4:1 transformer is used.
Cheers Bob

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Hi John

I think this is correct. I think there are a few other decimals missing too. What I thought was 135VDC is probably 13.5VDC But I haven’t got a circuit of the power supply so only a guess.

I also think this is correct. I have come to the conclusion that this tone generator was made by the same company as the rest of the gear and was supplied as some sort of test / set up aid. Only a guess mind you. In that case it would make sense to use the same transformer.

Based on the Max resistance figures I think the turns ratios would be:
Primary to secondary 4.5:3.2 or 1.4:1
Primary to tertiary 4.5:1
This is close to the same ratio as the resistance figures, given that these are max values there will be a bit of leeway. Also there is a signal level at the collector of Q3 of 3.7V with a reference to note 4 (which I am not privy to) which is probably close to what is across the primary. 3.7 / 2 = 1.85:1 which is in the ball park.

You are I think quite correct here.

I had a quick look and could not see anything that looks like those resistance figures (ignore the tertiary winding, it would not be used), they are all up in the 100s of Ω

Statement. You are going to need a transformer. You are going to need to do some experimenting. A 600 / 600 1:1 transformer would be a start. Be aware though some of these are telephone isolating transformers and only have a bandwidth of 200Hz - 4kHz, voice frequency. As the whole set up is for voice frequencies anyway don’t expect HiFi. There are probably other filters in the system (R40/C17 in the speaker amp for instance) to make sure of this. Keep in mind also most of these units are “Bridging” transformers used to “bridge” across a line for measurement or monitoring purposes.

Yes it would if you can use that device. You need to feed the transformer via a capacitor to get rid of the DC component, the output sits at half rail voltage so the cap can be a polarised electrolytic, probably at least 4.7µF. Connect the transformer from this cap to ground.

This OP amp may not have enough power handling capability for this job and you may have to interface something else. I would use a 12V supply anyway as there is a danger of clipping before you reach a useful level. This OP amp only goes down to about Gnd + 1.5V and up to about VCC - 1.5V, or for a 5V supply means from ≈ 1.5V to ≈3.5V (2V P-P).
This board also has a tremendous amount of gain, 100. 10 in the ist stage and 10 in the second. You may have to reduce this. Better in the first stage I think. If you are adept with a soldering iron you can do this by reducing the value of R1 (100k). Connect a resistor across it, 100k will reduce the gain by half while 10k will reduce the gain to a little below1 (about 0.9) and anything in between. This is irrespective of supply voltage (VCC).

At what point to you intend sourcing your audio on the MP3 device, at the headphone socket / terminal block or the speaker output. There will be a couple of things you will need to do depending on what your answer is here. Please advise. Also at what level, stipulate RMS or P - P. Thanks
Cheers Bob

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