I do not find errors, at least with a voltmeter.
Resoldered the pins of the opamp.
I made a recording that demonstrates the effect,
https://filetransfer.io/data-package/iBMtq1RG#link
recorded with a feedback value of 19.5k, so I assume the gain must be around 30db?
Camera was set @-12dB.
First it seems to work normal, then, with a lout sound, it distorts and mutes (not totally,
you can hear me very faint and distorted for another one or two seconds).
Sometimes, a loud sound can trigger it back on!
I am relatively sure that the clipping point did not change going from 39k to 19.5k.
I wrote around -3dB for 39k (because of the coarse meters in the S5), but for sure it is -6dB with 19.5k.
Shouldn't the headroom not be higher and the shouldn't camera clip before the mic preamp clips?
The SOIC part is turned 90°, the traces for one non-inverting input and outbut run around the board.
While soldering the stacked SMD between the pins, maybe the protective varnish did melt
and the traces make a short ?
But I do not measure any shorts with the voltmeter...
But I remember I measured the bias voltages before soldering the op amp on the adapter board
and they were half of the supply voltage.
Resoldered the pins of the opamp.
I made a recording that demonstrates the effect,
https://filetransfer.io/data-package/iBMtq1RG#link
recorded with a feedback value of 19.5k, so I assume the gain must be around 30db?
Camera was set @-12dB.
First it seems to work normal, then, with a lout sound, it distorts and mutes (not totally,
you can hear me very faint and distorted for another one or two seconds).
Sometimes, a loud sound can trigger it back on!
I am relatively sure that the clipping point did not change going from 39k to 19.5k.
I wrote around -3dB for 39k (because of the coarse meters in the S5), but for sure it is -6dB with 19.5k.
Shouldn't the headroom not be higher and the shouldn't camera clip before the mic preamp clips?
The SOIC part is turned 90°, the traces for one non-inverting input and outbut run around the board.
While soldering the stacked SMD between the pins, maybe the protective varnish did melt
and the traces make a short ?
But I do not measure any shorts with the voltmeter...
Many thanks, I´ll do, but it might take some time, probably not before the late afternoon...
But I remember I measured the bias voltages before soldering the op amp on the adapter board
and they were half of the supply voltage.
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If you use thin-film SMD resistors, it is very well possible that their value has increased if they got too hot. I've had that several times.
Regarding resistance measurements, when you try to measure the resistance of a resistor that is mounted in a circuit, with everything switched off, you should normally either see the correct value, or a too low value if there is a parallel path that conducts some of the test current of the meter, or a near-infinite value when one of your test leads makes no contact. A too high value while the test leads make contact is suspicious.
Regarding resistance measurements, when you try to measure the resistance of a resistor that is mounted in a circuit, with everything switched off, you should normally either see the correct value, or a too low value if there is a parallel path that conducts some of the test current of the meter, or a near-infinite value when one of your test leads makes no contact. A too high value while the test leads make contact is suspicious.
Dear Marcel, got up earlier...
Supply Voltage: 3.29V
Non-Inverting Input Pin 3: 1.57
Non-Inverting Input Pin 5: 1.57
OutputA Pin 1: 1.47
OutputB Pin 7: 1.43
Seems normal... What about the clipping with 19.5k feedback resistors at -6dB?
Normal behaviour or too early?
I' ll get me some standard resistors today, I have no clue whether the SMD version is
thin film.... But if they were off, shouldn't be the DC bias at the output pins extremely different?
An when there is bias, why is only one output decoupling capacitor needed at the transformator?
All the best,
Salar
Supply Voltage: 3.29V
Non-Inverting Input Pin 3: 1.57
Non-Inverting Input Pin 5: 1.57
OutputA Pin 1: 1.47
OutputB Pin 7: 1.43
Seems normal... What about the clipping with 19.5k feedback resistors at -6dB?
Normal behaviour or too early?
I' ll get me some standard resistors today, I have no clue whether the SMD version is
thin film.... But if they were off, shouldn't be the DC bias at the output pins extremely different?
An when there is bias, why is only one output decoupling capacitor needed at the transformator?
All the best,
Salar
Strange, the voltages are all a bit low, but not completely off. Could you also measure the voltage across the Zener diode? It is supposed to be well above half the supply voltage.
If one (or more) of the feedback resistors had a much too high value, it would not have a very large effect on the output DC voltages, so it is still very well possible that they are too high. Thin film resistors are usually advertised as thin film, as they are more expensive and more stable than standard thick film resistors (but much more vulnerable to too hot soldering).
There is a current path from one output via the transformer's primary to the other output. When you put a capacitor anywhere in that path, it ensures that no DC current can flow there. Hence, one capacitor suffices. There is no connection to ground anywhere in this path, so the voltage to ground has no effect on this.
If one (or more) of the feedback resistors had a much too high value, it would not have a very large effect on the output DC voltages, so it is still very well possible that they are too high. Thin film resistors are usually advertised as thin film, as they are more expensive and more stable than standard thick film resistors (but much more vulnerable to too hot soldering).
There is a current path from one output via the transformer's primary to the other output. When you put a capacitor anywhere in that path, it ensures that no DC current can flow there. Hence, one capacitor suffices. There is no connection to ground anywhere in this path, so the voltage to ground has no effect on this.
Ok, I´ll do this late afternoon.
As I will buy standard resistors for R1 and R5 on my way back home,
Any suggestions for the other parts. Should R7 be changed to another value than 560ohm?
Is the clipping normal or an error? (About 30dB boost with 19.5k, recorded at setting -12 on the camera, clipping at -6dB)
All the best, Salar
As I will buy standard resistors for R1 and R5 on my way back home,
Any suggestions for the other parts. Should R7 be changed to another value than 560ohm?
Is the clipping normal or an error? (About 30dB boost with 19.5k, recorded at setting -12 on the camera, clipping at -6dB)
All the best, Salar
Now, this is really strange! I did change the SMD resistors R1 and R5 to "normal" ones
(but precision resistors with 0.1% tolerance) and I did replace R7, with the same value and tolerance,
as it was heated many times. I measured the old resistors, though they were heated many times,
resistance was perfect, so no error there.
Switched the circuit back on, no change, the circuit would mute after clipping.
I did check voltages while the circuit was in "muting - state", and did also check the Xvives voltage
to rule out that the Xvives 3.3V supply was faulty.
Here it comes:
When I measured voltage at the junction of R5 and R7 vs GND, poof, the circuit came back to life.
And stays there, sometimes it still mutes after clipping, but not as regularely as before.
When it mutes, another loud peak brings it back to life.
So the effect is still there, but has "bettered"
So maybe a faulty op amp?
Annoyingly it still clips @ -6 db when the camera is set to -12db.
All the best, Salar
(but precision resistors with 0.1% tolerance) and I did replace R7, with the same value and tolerance,
as it was heated many times. I measured the old resistors, though they were heated many times,
resistance was perfect, so no error there.
Switched the circuit back on, no change, the circuit would mute after clipping.
I did check voltages while the circuit was in "muting - state", and did also check the Xvives voltage
to rule out that the Xvives 3.3V supply was faulty.
Here it comes:
When I measured voltage at the junction of R5 and R7 vs GND, poof, the circuit came back to life.
And stays there, sometimes it still mutes after clipping, but not as regularely as before.
When it mutes, another loud peak brings it back to life.
So the effect is still there, but has "bettered"
So maybe a faulty op amp?
Annoyingly it still clips @ -6 db when the camera is set to -12db.
So the clipping must be @ about 90db...?
All the best, Salar
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Have you got two equal-valued resistors with a value somewhere between 30 kohm and 300 kohm? If so, please connect one from the negative input of each op-amp to the positive plate of C6 to see if that helps.
No, need to buy two. 300k preferred? But the positive leg of C6 is almost unreacheable.
Is this for the mute - behaviour? Could be something done about the early clipping?
Is this for the mute - behaviour? Could be something done about the early clipping?
Just to try it, you could connect one resistor between 15 kohm and 150 kohm (if you have one) from one of the negative op-amp inputs to the node that connects the positive side of C6 to R2, R4, R9 and R10. Does that change either the clipping or the muting behaviour or both?
Both quotes refer to the same modification. As I´ll have to buy resistors for the test Saturday morning,
please advice on a value.
I assume, resistance should be very high? Feedback resistors R1 and 5 are 22KΩ, R7 is 560Ω.
I assume, changing the value of R7 will not help?
All the best, Salar
Installed the 100kohm resistors to the node, which meant soldering "blind" as
the node is beneath C6 and the iron blocks view while soldering.
But the solder joints look ok, connections measure ok.
As I also had to put in new feedback resistors, I took 22k instead of 20k.
So hard clipping should start around 97dB. But I had to rush to work,
so testing will be done tonight, keep fingers crossed.
the node is beneath C6 and the iron blocks view while soldering.
But the solder joints look ok, connections measure ok.
As I also had to put in new feedback resistors, I took 22k instead of 20k.
So hard clipping should start around 97dB. But I had to rush to work,
so testing will be done tonight, keep fingers crossed.
So, half success! The muting is gone, clipping is unchanged.
https://filetransfer.io/data-package/tXtlNMTF#link
Recorded with the -12dB setting on the camera.
Feedback resistance is 22kΩ, "node resistance" 100kΩ.
As I measured 2.1V across the Zener diode -
does this maybe sink some voltage and gain?
I did not change the voltage divider yet, which is mounted
in an adapter box beneath the camera. Still 390Ω/240Ω
instead of 1000Ω/750Ω.
Does this add to the clipping behaviour?
The board is pretty stuffed now btw...
https://filetransfer.io/data-package/tXtlNMTF#link
Recorded with the -12dB setting on the camera.
Feedback resistance is 22kΩ, "node resistance" 100kΩ.
As I measured 2.1V across the Zener diode -
does this maybe sink some voltage and gain?
I did not change the voltage divider yet, which is mounted
in an adapter box beneath the camera. Still 390Ω/240Ω
instead of 1000Ω/750Ω.
Does this add to the clipping behaviour?
The board is pretty stuffed now btw...
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It is very well possible that the clipping behaviour gets better with a higher-impedance voltage divider. In fact, reading all the data in the datasheet, I don't see anything that shows that it works well at differential loads below 1200 ohm when the supply is less than 5 V (total voltage between the positive and negative supply pins). On top of that, you get extra losses due to transformer winding resistance when the load impedance is low.
Could you disconnect the voltage divider to see if that makes any difference? You can't make the impedance any higher than that.
Could you disconnect the voltage divider to see if that makes any difference? You can't make the impedance any higher than that.
I meant you can't get any higher while still connecting the camera.
With the voltage divider disconnected, it's 2.2 kohm, with the voltage divider connected, it's 475.36... ohm, and from the datasheet, I see that 1.2 kohm or higher should be no problem for the op-amps.
With the voltage divider disconnected, it's 2.2 kohm, with the voltage divider connected, it's 475.36... ohm, and from the datasheet, I see that 1.2 kohm or higher should be no problem for the op-amps.
912.5338142 ohm, so closer to but still less than 1200 ohm. Even with 190 ohm of winding resistance (sum of the primary and the secondary winding resistances of the NTM1, no data for the NTE1), it's still below 1200 ohm.
With 1800 ohm and 1600 ohm in the voltage divider (or 1000 ohm+750 ohm and 1600 ohm), it would end up just above 1200 ohm, even without transformer winding resistance.
With 1800 ohm and 1600 ohm in the voltage divider (or 1000 ohm+750 ohm and 1600 ohm), it would end up just above 1200 ohm, even without transformer winding resistance.