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Chip Amps Amplifiers based on integrated circuits

Fun with a cheap TDA2030A at only 6dB gain
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Old 19th November 2019, 07:22 PM   #41
Preamp is offline Preamp  Germany
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I exchanged the two 22uF caps with 470uF + 100nF X7R each, but to no avail. Still getting the same oscillations with the standard circuit and when adding 220pF or more to the output.

Playing around with the Zobel didn't help either. Adding 47nF in series to reduce the capacitance didn't show any real effect. Adding 2R2 in series to increase the resistance led to severe oscillations when the output cap was added. Adding both parts to increase the resistance and decrease the capacitance at the same time led to the same severe oscillations.

EDIT: Funny thing. The fully compensated 6dB circuit now is actually looking more stable than the standard circuit: Adding 220pF to the output doesn't result in oscillations anymore, and the 1nF oscillations look a little less severe.

Last edited by Preamp; 19th November 2019 at 07:34 PM.
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Old 19th November 2019, 10:40 PM   #42
sgrossklass is offline sgrossklass  Germany
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Originally Posted by Preamp View Post
Here's a photo of the test circuit, along with the EAGLE layout and corresponding schematic, in case you want to trace each route exactly.
Me no likey.

V+ / V- inductance should be decently low, but why would you route the output in between? Yeah, it does keep it away from the input, but...
V+ and V- to GND inductance is not that great.

Bypass caps in "Long-Legged Linda" mode with what seems like bends directly at the parts - (a) the latter may damage the caps and (b) the former does not exactly signify low inductance (why were these not underneath?). And where do they even go anyway? Not to ground by the looks of it.

No distinction between input ground and output ground. (Ground is ground, na na na-na-na...) This can really bite you in the rear end when it comes to ground loops. Try adding a preamp circuit fed from the same supply, and you'll probably see what I mean. (Countless DIY boomboxes suffer from problems because of this.) That said, since Tom Christiansen quite convincingly argues that this is the way to go, adapting everything else may be preferable. Still, you might consider inserting maybe 10 ohms into the ground between the 1n5 input cap and the 4n7 (maybe even ahead of the 1n5), with dedicated IGND and OGND connections.

Last edited by sgrossklass; 19th November 2019 at 10:43 PM.
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Old 20th November 2019, 07:18 AM   #43
Preamp is offline Preamp  Germany
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Originally Posted by sgrossklass View Post
Me no likey.
Me too, but I think I could have done worse. It's just a testbed after all, not a production model. Once I get rid of all those jumpers, the layout can get much more compact and optimized.

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Bypass caps in "Long-Legged Linda" mode with what seems like bends directly at the parts - (a) the latter may damage the caps and (b) the former does not exactly signify low inductance (why were these not underneath?).
I have never seen a commercial chip amp that put the caps underneath the board. Granted, in this case it would have been easy (and I'm gonna try it), but it must be possible to do it the 'normal' way. Compared to the generously spaced parts on the sample layout in the datasheet I'm already pretty compact here.

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That said, since Tom Christiansen quite convincingly argues that this is the way to go, adapting everything else may be preferable.
Yeah, Tom and his grounding mantra. In another thread I once argued with him about his impeccable simulation results, because when I replicated them and added some length of speaker wire into the mix, the results didn't look that great anymore. However, I think I'm not too far off from what he suggests actually. Nonetheless, I'll try to make it better. I can only learn along the way, nothing to lose here .
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Old 20th November 2019, 11:57 AM   #44
Preamp is offline Preamp  Germany
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Here's a second attempt at the supply decoupling. Same result. Tested it with another power supply and a different load resistor, connected with shorter leads directly to the screw terminals, but it's still breaking into oscillations with an additional 220pF.
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Old 20th November 2019, 02:12 PM   #45
Preamp is offline Preamp  Germany
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I have separated the input signal ground together with the 1n5 input filter cap from the rest of the ground and connected them through 10R directly to the power ground point between the two bypass caps. Still no change.
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Old 20th November 2019, 06:48 PM   #46
anti is offline anti  Slovakia
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Just a suggestion: try to add a couple-100pf cap between the + and - opamp inputs. This can stop some chipamps from oscillating. No guarantees though.
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Old 21st November 2019, 05:28 PM   #47
Elvee is offline Elvee  Belgium
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I smell a rat somewhere: the impedance of a 220pF cap is disproportionately high compared to the 1 ohm Zobel: the time constant is 220ps.
For such a chip, the Ft of the process must be 40Mhz or thereabout, and whatever happens in the GHz range should not matter, unless the Zobel is not really present or has an incorrect value: a dry solder, a 1 ohm which is in fact a 100 ohm, a capacitor internally open...
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Old 21st November 2019, 05:46 PM   #48
chermann is offline chermann  Austria
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Hi


i am for sure no expert but you use a TDA2030 - this could be fake chip and therefore they can do a lot of strange things.


i tried to put a 10k, 20k and 30k square wave signal to the input to "see" better the oscillation at the output. - this brought me to add a 100pF cap in parallel to the feedback resistor.

chris
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Old 21st November 2019, 05:49 PM   #49
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by anti View Post
Just a suggestion: try to add a couple-100pf cap between the + and - opamp inputs. This can stop some chipamps from oscillating. No guarantees though.
Tested with 110pF, 220pF, 440pF and 47nF (yes, nano) to no avail. THD increased a lot with 47nF, but the oscillations are not remedied.


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I smell a rat somewhere: the impedance of a 220pF cap is disproportionately high compared to the 1 ohm Zobel: the time constant is 220ps.
For such a chip, the Ft of the process must be 40Mhz or thereabout, and whatever happens in the GHz range should not matter, unless the Zobel is not really present or has an incorrect value: a dry solder, a 1 ohm which is in fact a 100 ohm, a capacitor internally open...
Okay, I'll toss them out and try some other parts. Let me see what I can find.
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Old 21st November 2019, 06:30 PM   #50
Preamp is offline Preamp  Germany
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i am for sure no expert but you use a TDA2030 - this could be fake chip and therefore they can do a lot of strange things.
Good point. Today they're faking everything, even something like what is probably the cheapest chip amp around today...


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(...) unless the Zobel is not really present or has an incorrect value: a dry solder, a 1 ohm which is in fact a 100 ohm, a capacitor internally open...
Elvee, you're a genius!! Looks like the resistor was the culprit! It measures 1.19 ohm on my UNI-T multimeter, so the value is about correct, but it's a 2W wirewound type. Maybe the inductive part was just too much in that case? I have replaced it with a 1R 1% metal film resistor (didn't know I had them, else I had used one of those in the first place), and now the picture looks a little different. I actually cut one of the resistors legs a few days ago already, to see how the circuits behave without the Zobel present. The result was a huge increase in oscillations when adding capacitance to the output, so I reconnected it and was convinced that the Zobel is doing its job.

The standard circuit is now immune to adding 220pF, 1nF or even 47nF to the output.
The circuit supposed by PRR is also fine.
The PRR circuit with the added capacitor across the feedback resistor is already oscillating without any capacitor added to the output now, just like you predicted a couple of posts back. Funny thing that it used to have the best THD values before, but is practically unusable now.
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