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#41 |
diyAudio Member
Join Date: Jul 2008
Location: Monkey Island
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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.
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Regards, Lasse LM317 under test | LTspice THD simulations | LTspice transistor parameter variation | Cascoded JFET current source Last edited by Preamp; 19th November 2019 at 07:34 PM. |
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#42 | |
diyAudio Member
Join Date: Nov 2004
Location: Germany
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Quote:
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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#43 | ||
diyAudio Member
Join Date: Jul 2008
Location: Monkey Island
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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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Regards, Lasse LM317 under test | LTspice THD simulations | LTspice transistor parameter variation | Cascoded JFET current source |
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#44 |
diyAudio Member
Join Date: Jul 2008
Location: Monkey Island
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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.
__________________
Regards, Lasse LM317 under test | LTspice THD simulations | LTspice transistor parameter variation | Cascoded JFET current source |
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#45 |
diyAudio Member
Join Date: Jul 2008
Location: Monkey Island
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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.
__________________
Regards, Lasse LM317 under test | LTspice THD simulations | LTspice transistor parameter variation | Cascoded JFET current source |
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#46 |
diyAudio Member
Join Date: Jun 2008
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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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#47 |
diyAudio Member
Join Date: Sep 2006
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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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#48 |
diyAudio Member
Join Date: Jan 2017
Location: Vienna
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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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#49 | ||
diyAudio Member
Join Date: Jul 2008
Location: Monkey Island
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__________________
Regards, Lasse LM317 under test | LTspice THD simulations | LTspice transistor parameter variation | Cascoded JFET current source |
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#50 | ||
diyAudio Member
Join Date: Jul 2008
Location: Monkey Island
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Quote:
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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.
__________________
Regards, Lasse LM317 under test | LTspice THD simulations | LTspice transistor parameter variation | Cascoded JFET current source |
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