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Fun with a cheap TDA2030A at only 6dB gain
Fun with a cheap TDA2030A at only 6dB gain
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Old 21st November 2019, 08:20 PM   #51
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Preamp View Post
but it's a 2W wirewound type. Maybe the inductive part was just too much in that case?
Oh yes it was: if there is a single spot where any inductance can cause trouble, that's the one.

The Zobel makes sure the amplifier always sees an "acceptable" (meaning low and resistive) impedance throughout its full frequency range, and preferably a decade beyond to be completely safe, but if it becomes inductive at some point, things go seriously wrong
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Old 21st November 2019, 08:30 PM   #52
Preamp is offline Preamp  Germany
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Nice! I'll surely remember that one . Thanks again for pointing me in the right direction
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Old 22nd November 2019, 07:23 PM   #53
anti is offline anti  Slovakia
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Originally Posted by Preamp View Post
...
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.
...
This looks like the high-freq ground return path went thru the 4n7 caps and the 47R resistor, in effect substituting for the zobel that went "open" in high-freqs beacuse of the zobel resistor self-inductance.
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Old 27th November 2019, 08:32 PM   #54
Preamp is offline Preamp  Germany
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Okay, here's another (probably final) test.

To compare apples to apples, I have made all the following graphs this evening in one run and didn't re-use any of the previous graphs from the last days.

First off, here's the noise-gain compensated TDA2030A as proposed by PRR again, set to 6dB gain only. It is fed directly from the soundcard with -6dBFS output level (ca. 1.05V RMS), which is about the sweet spot for lowest distortion and noise.
Into 8 resistive Ohms I measured a THD of 0.0039% at 1kHz and 0.041% at 10kHz. The noise floor dips down to around -130dBV in the 10kHz range, which is considerably lower than what the standard circuit with a suggested minimum gain can achieve.

(The orange trace shows the measurement noise floor with everything hooked up and happily picking up stray noise, but with the DUT still powered off.)

Fun with a cheap TDA2030A at only 6dB gain-6dbtda-png

Next up is what I had in mind when I started this whole experiment. Now I've added my own preamp (with input selector and PGA attenuator) into the mix. I have left the soundcard output at the same level of -6dBFS and used the preamp's volume control to arrive at the same +6.5dBV output into the load.
As expected, the noise floor increased a bit, but only by a little 2 or 3 dBV, which is actually less than I had imagined. The THD at 1kHz is about the same with 0.0037%, but the value at 10kHz got almost cut in half to 0.021% now, which is a nice improvement!

Fun with a cheap TDA2030A at only 6dB gain-6dbtda_ultipre-png

As another means to compensate for the low gain of 6dB, I removed my preamp and added a simple non-inverting 20dB gain stage at the input instead, made up of a cheap TL074 jellybean opamp. The performance of this circuit is (within a small margin) about the same as the standard circuit set to a similar gain of 27dB. Because of that I didn't measure that one again, as the graphs would be almost the same as these.
Now the noise floor doesn't even reach -110dBV, which is about as bad as it gets. THD at 1kHz is an order of magnitude worse, which is on par with what I have measured before with the standard circuit. THD at 10kHz is also worse with 0.054%, but not by as much as at 1kHz.

Fun with a cheap TDA2030A at only 6dB gain-6dbtda_tl074-png

To arrive at this values, I had to lower the soundcard output to -29dBFS (73.5mV RMS) for the same output power. As this probably has some negative effect on the THD performance of the soundcard output, I went a little apeshit and added my preamp back into the mix .
Now we have the same -6dBFS soundcard output, attenuated down by an external preamp, upped by 20dB with a crappy TL074 and finally another 6dB by the TDA itself. Which is about the same as setting the TDA gain to 26dB in the first place and leaving out the '074, just a little more crazy. Let's see:
The noise floor again rose by 2 or 3 dBV, which is what the external preamp adds. THD didn't get any better though, it's even a little worse again.

Fun with a cheap TDA2030A at only 6dB gain-6dbtda_tl074_ultipre-png


What's your opinion?

I think the results look pretty good! Since my preamp will always be in front of every amp I'll be using anyhow, and can put out enough juice to drive a little TDA with +-22V supplies and only 6dB gain into saturation easily, I'll probably build two of these little buggers just for the fun of it and give them a listen.
In case I should actually manage to fry my tweeters along the way, I won't hesitate to report back here !
Attached Images
File Type: png 6dbtda.png (111.5 KB, 238 views)
File Type: png 6dbtda_ultipre.png (113.0 KB, 142 views)
File Type: png 6dbtda_tl074.png (116.9 KB, 141 views)
File Type: png 6dbtda_tl074_ultipre.png (112.4 KB, 140 views)
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Old 28th November 2019, 03:55 AM   #55
PRR is online now PRR  United States
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Originally Posted by Elvee View Post
..the impedance of a 220pF cap is disproportionately high compared to the 1 ohm Zobel....
I see 220nFd there? 0.2uFd is reasonable. It damps nominal 8 Ohm load at 200kHz, which is where device hFE starts to fall; is down to 1 Ohm at 1.6MHz where output devices are about dead.
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Old 28th November 2019, 06:18 AM   #56
Preamp is offline Preamp  Germany
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Elvee was referring to the 220pF that I added to the output for testing, and was sufficient already to cause oscillation. The zobel uses 220nF like the datasheet suggests.
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Old 28th November 2019, 12:11 PM   #57
anti is offline anti  Slovakia
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Originally Posted by PRR View Post
...
at 1.6MHz where output devices are about dead.
...
TDA2030 are well known to die much sooner than that...
Say a wrong word, curse a bit - and out comes the magick smoke!
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Old 28th November 2019, 03:41 PM   #58
Elvee is offline Elvee  Belgium
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Originally Posted by Preamp View Post
Okay, here's another (probably final) test.

To compare apples to apples, I have made all the following graphs this evening in one run and didn't re-use any of the previous graphs from the last days.

.....
What's your opinion?
It looks logical: the 4n7/1K pole is at 33kHz, meaning the 3rd harmonic (which is probably dominant) will be fully attenuated by the increased loop gain.
The effect will die out at higher frequencies, but a benefit will still be felt for the 5th and 7Th
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Old 30th November 2019, 03:48 PM   #59
Preamp is offline Preamp  Germany
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Fun with a cheap TDA2030A at only 6dB gain-chiquita_01-jpg

Here it is, a fully functional Stereo version of the 6dB TDA2030A. Banana for scale. Or for name? Why not call it "Chiquita"... sounds better than pesky little bugger .

Fun with a cheap TDA2030A at only 6dB gain-chiquita_02-jpg

Looks a little messy inside. Case is about 21 x 10 x 3.5 cm in size. Toroid is 2 x 12V 30VA for +-18.5V when idle. All of the parts were lying around and/or recycled.

Fun with a cheap TDA2030A at only 6dB gain-6-6dbv-png

This is the performance of one channel under the same conditions as with the last test (-6dBFS soundcard output, external preamp, 8R load). Measured mains noise got a little worse due to the PSU now being in close proximity to the amp itself, but that is really only measurable and totally inaudible! This thing is dead quiet, even with the ear against the speaker.
Total harmonic distortion once again got better over the last iteration, at 1kHz from 0.0037% to 0.0021% and at 10kHz from 0.021% to 0.0079%. Not bad.

Fun with a cheap TDA2030A at only 6dB gain-full_throttle-png

This is maximum output into 8 Ohm, just before the onset of clipping. The scope showed an output swing of about 24Vpp at the load. The measurement output was taken at the 20% tap of the load resistor as to not overdrive the soundcard input into clipping.


I really like this little amp! Now on for some listening tests
Attached Images
File Type: jpg chiquita_01.jpg (315.5 KB, 89 views)
File Type: jpg chiquita_02.jpg (628.0 KB, 97 views)
File Type: png 6,6dBV.png (86.1 KB, 101 views)
File Type: png full_throttle.png (92.8 KB, 96 views)
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Old 1st December 2019, 12:09 PM   #60
anti is offline anti  Slovakia
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Did you do anything different (layout, component values, wiring) to get the lower THD rating?

I see you use a simple wire loop as soldering posts on the perfboard. If you twist the wire (on the component side), this becomes a much sturdier post. When soldered, it isn't that much inferior to commercial soldering posts - dosen't look as neat of course, but hey ....

P.s. IIRC Meier Corda Rock also used TDA2030. Perhaps they also use some clever engineering tricks?
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