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Old 29th Mar 2017, 12:11 am   #1
Apfelmus
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Default How does this circuit work?

Hi all,

I'm still working on my Stabilock 4040. It has a problem with the audio, it distorts when the volume setting is low and then cuts off.

Attached is the circuit diagram of the audio amplifier. Its basically an OP-Amp with a push-pull amplifier.

The audio signal arrives well at R137. The Zeners at this point seem to work as a limiter. At the output of the amp (pin 1 of the op-amp), I get the signal with a negative DC component (about as much as the signals RMS value). Strangely, I cannot see anything else than noise on the input at pin 2.

The amp is connected to the volume knob (potentiometer) and the loudspeaker, as indicated by the hand-drawing.

I've never seen anything like that, and until now can't figure out how this is intended to work. When I put up the volume, at one point it gets distorted and asymmetrical. When that happens, R37 and R40 get very hot, with R40 significantly hotter than R37.

Any ideas where I could look for the problem?

cheers
Martin
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Old 29th Mar 2017, 12:56 am   #2
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Default Re: How does this circuit work?

It's quite easy.

The op amp tries to drive the load through the resistor on its output pin.
In trying to do so, it pulls current from one supply rail or the other. This current demand from whichever supply pin of the opmp turns on one of the two added transistors. With its current gain, the transistor can provide a much higher current into the load than the opamp can. As the load voltage changes in the direction driven by the transistor, it moves towards the voltage the opamp is trying to drive it towards. As it gets there the opamp feedback reduces the amount of current the opamp output is trying to drive, so the power pin current falls, and the assisting transistor starts to turn off.

There are variants of this circuit where tho opamp drives a resistor to ground, and the two current-boost transistors do all the work.

Notice that the boost transistors are common emitter and could drive the output quite close to the rails (if the opamp driving ground trick is used) Making this circuit efficient in terms of power into the load given the supply rail voltages.

David
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Old 29th Mar 2017, 11:18 am   #3
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Default Re: How does this circuit work?

Probably the first thing to check is if the hand drawn section with the speaker is correct. It looks wrong to me. The speaker connection looks wrong. See below for what I think it is probably meant to look like.

Quote:
Strangely, I cannot see anything else than noise on the input at pin 2.
Probably the best way to analyse this opamp circuit is to view it as an error amplifier rather than an audio amplifier.

The mission in life of that opamp is to cancel external input errors to pin 2 (using negative feedback) and maintain the opamp virtual ground at pin 2. Ideally there should be no visible AF or noise here at all for signals within the loop bandwidth of the error correction, despite the AF input current to this amp via R137 etc. This is why you see nothing here.

Your audio input feeding through R137 into pin 2 via the switch is interpreted by the opamp as a pesky 'error' current in the form of an audio waveform.

So the opamp will swing its output pin1 in the opposite direction and this error correction signal feeds back current 'negatively' into the node at pin 2 via the resistance of the volume pot to negate the AF current via R137 to maintain the status quo of 'nothing' or no error at pin 2. If you look closely on their diagram you can see the arrows on the circuit traces that indicate the error cancelling feedback signal coming back via the volume pot into the node at pin 2.

Obviously, the bigger you set the red volume pot in the image below the bigger the error signal the opamp has to generate at pin 1 in order to feed back enough current via the volume pot to cancel the AF input signal at pin 2. So the signal to the speaker will get bigger.

The benefit of all this to you as a user is that opamp pin 1 (along with help from the driver transistors as David has described) will have enough grunt to drive a speaker at the same time it is busy housekeeping its errors to keep pin2 as a virtual ground devoid of any signals So we see (hear?) this as an audio amplifier rather than an error amplifier.

There is also a diode based level detector circuit at the output and maybe this acts as an overdrive trip if the level gets too high? Is the speaker connected correctly? I'm pretty sure it should be as my diagram below but I no experience of stabilock gear.

In terms of what else could be wrong, the feedback could be going unstable or the opamp could be faulty causing both output transistors to come on together (not a good thing to happen) and cause lots of heat in those circled resistors? if you have it connected as per your original circuit the speaker itself could be causing phase shifting around the loop that could lead to instability. But I doubt it will be wrong here in reality? Have you just drawn it incorrectly?
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Old 29th Mar 2017, 9:38 pm   #4
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Default Re: How does this circuit work?

Hi,

thanks for your insights. If I understand right, than there should not be any DC component on the output?

I re-checked the wiring, but it seems that my drawing is correct. Attached are some extracts of the related circuit diagrams, so you might check for yourself. The loudspeaker is on the control panel, from there it goes to the motherboard, on the mohterboard to another connector and from there to the AF-Detector boad where the amp is located.

The wiring as I see it:
from amp, PHONE -> 6a -> 14 -> middle of pot/cap
from amp, VOL -> 6b -> 15 -> othe side of pot

cheers
Martin
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Old 30th Mar 2017, 12:41 am   #5
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Default Re: How does this circuit work?

I think I may also have caused confusion with my red symbol for a variable resistor with the arrow through it. To clarify, I think you have drawn the speaker connection incorrectly. Your hand drawn circuit shows it connected to the input of the amp, not the output.

Quote:
thanks for your insights. If I understand right, than there should not be any DC component on the output?
It's hard to read the circuit and the component values clearly but the opamp also has a dc current fed into the node at pin 2. This looks to be from 5.5V via a 33.2k resistor? This would be treated as another error signal that needs to be cancelled by the feedback.

If this is so then when the variable 47k volume pot is set to a loudish setting (eg when it is 33k) then I'd expect the opamp to negate this input by shifting the DC point of the output to somewhere about -5.5V at this particular volume setting.

At lower volume settings the DC offset will be less at the output. Eg at low volume if the volume pot is down at 3.3k ohm then the DC offset will be lower by a factor of ten. eg around -0.5V? Presumably this is to make it quick to react and trip the mute at high volume settings? Maybe to save a big burst of stress to the speaker if the operator tries for a loud setting and there is lots of audio? It can trip and give a sharp mute response to the overload?

But this is a guess because I can't read the circuit or the component values very clearly. But the bit I think is wrong in your circuit is where the speaker attaches to the circuit? Maybe someone else can comment?
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Old 30th Mar 2017, 1:24 am   #6
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Default Re: How does this circuit work?

I'd assumed the drawing of the volume control and the speaker were the result of getting two lines on the diagram confused, so I only commented on the amplifier arrangement with the two current boosting transistors.. I couldn't comment on the original point of certain resistors getting hot as values and designators aren't legible on the posted image.

David
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Old 30th Mar 2017, 1:34 am   #7
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Default Re: How does this circuit work?

Yes, it's just too hard to decipher the circuit. Although this bit probably isn't relevant, I think the mute trip circuit may effectively track the volume pot because of the deliberate DC feed into the opamp pin 2 via the 33k2 resistor. So if you like a low volume control setting the audio mute threshold is automatically set lower. i.e. the drive will be cut off at a lower speaker level for low settings of the volume pot? Maybe this is to protect the operator from sudden increases in audio level way above their chosen volume setting? Maybe it isn't there to protect the speaker but the operators ears and pulse rate instead

But if you like loud volume settings the mute threshold ramps up? But the circuit is just too fuzzy to make much sense of it all really...
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Old 30th Mar 2017, 2:27 am   #8
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Default Re: How does this circuit work?

My last post makes more sense if the operator likes to wear headphones rather than use a speaker? I don't actually know what this Stabilock unit is or does but looking at the google images I assume it is an RF comms test set? There does seem to be a headphone socket at the front? So the trip circuit is probably an adaptive one depending on the volume control setting? Maybe to prevent the operator jumping out of their chair clutching their ears if there is a sudden burst of FM noise or something similar that takes the audio way above their chosen level? Different users will use headphones of differing efficiency?
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Old 30th Mar 2017, 12:24 pm   #9
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Default Re: How does this circuit work?

On the schematic the op-amp is shown as 1/2 of a package. That seems a bit wierd as the supply for BOTH op-amps will bias the power output stage (assuming both 1/2's share the same supply pins). Has this op-amp been changed and what does it's sibling do and does it look OK ?

dc

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Old 30th Mar 2017, 3:06 pm   #10
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Default Re: How does this circuit work?

Yes, that's a good point... however, it looks like they might get away with this because it looks like the other half of the package is the level trip comparator just to the right of it. This should be quite forgiving of the ripple on the opamp power pins caused by the audio amplifier. I don't think it can cause any issues back the other way because the comparator should just take a static current until it trips. Not sure I'd get that approach through a design review at work though (for the reasons you spotted).

But it's hard to make out the circuit clearly enough to be sure about any of this?

I also think you have a valid point that the performance of the circuit could be opamp specific so definitely worth checking if someone has swapped it for something else in the past?
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Old 30th Mar 2017, 8:08 pm   #11
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Default Re: How does this circuit work?

Thanks again for your thoughts.

First: the original PDF is from ArtekMedia, and its "secured", I seem unable to make a copy directly from the file. I have to print and scan it back, or make a screenshot. The latter is what I did, hoping the result is better.

The Stabilock is indeed a communication test set. It was about to be trashed by my company, yet working sets seem to be quite sought after. Guess its because of the upper frequency of 960MHz which is more or less DC by todays standards.

The user manual is mute on the volume knob and how to use it, so I have no idea what the intended functionality should be. To what I understand from the discussion, there seems to be some sort of "inverted squelch" here, i.e. that mutes when the level gets too high.

What do you think about the zener diodes prior to the switch, around R137? They look like a limiter to me. If so, why do I need such an "inverted squelch"?

I have checked the circuits, its the same type as on the drawing. I could try another one easily, they're all in sockets.

The resistors that get hot is R37 and R48: 68,1k and 27,4k respectively.

cheers
Martin
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Old 30th Mar 2017, 10:12 pm   #12
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Default Re: How does this circuit work?

Not sure what the best way to approach this will be...
I suspect that a typical service engineer (working against the clock) would check and double check the +15V, -15V and +5.5V power supplies first on a DVM and a scope looking for any ripple issues.

Then take advantage of the IC socket and try another opamp (but a new one, not from another part of the equipment because a direct swap across could damage another section if the original amp was faulty)

If this failed then change the transistors. I'd expect the transistors to be the weak link and an experienced service person might opt to change the opamp and both transistors regardless if only one device appears duff.
Could you remove both transistors and check them on a tester? eg a DMM with an HFE function?

Otherwise, it is going to be a case of probing around. The transistors should remain off until the opamp pulls about 7mA from its power pin as this will give 0.7V Vbe across the 100R resistor. Maybe that is something to look for? Or check that the opamp responds correctly to a sweep of the volume control in terms of the DC level at the output?

One other thing, I think the resistors in each transistor collector (that are getting hot?) will be quite low in value. Maybe 68R and 27R rather than 68k and 27k? The text is still a bit fuzzy but it might be 68.1 ohm and 27.4 ohm?
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Old 30th Mar 2017, 10:25 pm   #13
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Default Re: How does this circuit work?

R37and R48 look to be linking the collectors of the current boosting transistors to the output.

I wouldn't expect them to be as large values as 68.1k or 27.4k. Maybe 68Ohms and 27 Ohms would be more likely.

These transistors amplify the current taken from the supplies by both sections of the dual amplifier. The circuit design is not good. There are some terrible short-cuts and bodges. It's relying on the first section working hard to drive the load through that100 Ohm resistor, while the second section is much more lightly loaded.... in fact it looks to be a schmitt trigger with positive feedback through 1 megohm., and it drives the logic inputs of th analogue switch depending on the DC value at the output of the audio amplifier... switching if the long ter mean goes below about 300mV.

weird! what on earth were they doing when they designed this?

David
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Old 30th Mar 2017, 10:26 pm   #14
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Default Re: How does this circuit work?

I must type faster.

David

At least we agree Jeremy!
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Old 30th Mar 2017, 11:13 pm   #15
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Default Re: How does this circuit work?

It is definitely an odd looking circuit. I think that (in theory at least) the output voltage at the final output at the circled test point TP9 should be a faithful but scaled/inverted copy of the voltage at R137. This is because of the negative feedback that feeds back from TP9 through the 47k volume pot to the inverting input of the opamp at pin2. So the clunkiness of the signal sharing between the opamp output and the driver transistors becomes less relevant as long as it can get the required waveform voltage at TP9 somehow on time. So TP9 will resemble a voltage source (that inversely tracks the voltage at R137) as long as the overall circuit is fast enough to stay in closed loop. So the idea would be that it can easily drive a low impedance speaker at TP9 whilst also remaining quite linear at TP9.

C12 330pF is also in the feedback path and maybe this is there as a means to limit the loop gain at high frequencies to keep a sensible phase margin across all volume pot (gain) settings? i.e. this cap would limit the bandwidth and may be needed to maintain system stability?

How well it achieves all this over various drive levels and DC offsets remains to be seen because I have no practical experience of this particular circuit. I think they also sum in the DC via the 33.2k resistor to provide some form of adaptive trip or AF squelch according to the user setting of the volume pot? I think the second section is just a comparator/switch that does the trip/mute function.
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Old 31st Mar 2017, 3:48 am   #16
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Default Re: How does this circuit work?

Here is the schema a littlebit more explizit.
The Monitor Amplifier gain the AF-Signal from Mod Gen, Rx Discriminator, IF-Demodulator, TX-Offset, AC/ACDC through switch D3, Op-Amp A2,
to the speaker/phone jack.

Measuring point is at A2 Pin 6 -250mV ±10mV (with R51).

Peter
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Old 31st Mar 2017, 5:23 am   #17
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Default Re: How does this circuit work?

Here the same schema with Vol poti drawn.

Maybe the 2 transistors are only for stabilizing the center (Bridge)
because the bases are fixed at DC level.

This 'Monitor Amp' has a Main Amplifier with filters and attenuators in front
(Input to D3)

Peter
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Old 31st Mar 2017, 8:35 am   #18
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Default Re: How does this circuit work?

David, of course you and Jeremy are right. I wonder where I got the "k" from... didn't drink this much.

Quote:
Originally Posted by Radio Wrangler View Post
I wouldn't expect them to be as large values as 68.1k or 27.4k. Maybe 68Ohms and 27 Ohms would be more likely.

David
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Old 31st Mar 2017, 9:06 am   #19
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Default Re: How does this circuit work?

Thanks a lot all, this gives me some ideas for further measurements. I already checked supplies, they're all OK.

What do you think of C23 (on pin5 of the OpAmp) being leaky? Its the same type of black electrolyte miniature of which I had two failed (see my other thread).

Actually, the reason why I started looking at the circuit is not the distortion at high volume but its behaviour at low volumes.

When I had the unit repaired and tested, I wanted to lower the volume since the 1kHz tone was quite irritating to my ears and the family. But below a certain threshold the sound just disappeared. Some loud crackling can be heard just at the position where the volume suddenly drops to zero.

First I thought this was a problem with the volume pot, like in many equipments and old radios. I discarded this option for a set of reasons:
- there is 0 volume on the first 40° or such of travel. Usually, a crackling pot being left at some position (i.e. 20°) does not crackle anymore and let the signal pass, more or less.
- Quality of mechanical components is very high in this unit
- crackling only occurs at this 40° position, not before and not after.

Thats why I finished up tracing the signal towards the output amp, but could not get any further clue.

Cheers
Martin
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Old 31st Mar 2017, 9:55 am   #20
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Default Re: How does this circuit work?

probably the amp is trying to oscillate at certain settings of the control, a storage scope on the output may reveal the truth!

I did wonder about having a variable resistor in the feedback loop but thought Martin must had made a mistake drawing it Seems like it was the designers mistake !

dc
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