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Chip Amps Amplifiers based on integrated circuits

Fun with a cheap TDA2030A at only 6dB gain
Fun with a cheap TDA2030A at only 6dB gain
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Old 15th November 2019, 10:07 PM   #11
Elvee is offline Elvee  Belgium
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Knowing the exact measurement conditions would help: for example, did you measure the noise floor with the input shorted, or just the sound card muted?
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Old 16th November 2019, 02:08 PM   #12
Preamp is offline Preamp  Germany
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OK, I think I've found another mistake that I made. I'm using an ESI Juli@ sound card for the measurements and have it configured to use the symmetrical interface to avoid ground loops. Unfortunately the output impedance seems to be a little on the high side when using it on a single-ended input, thus the non-inverting input didn't have a sufficient path to ground, resulting in the rather high dc offsets. To fix this, I have now added an arbitrarily chosen 10k from the non-inverting input to ground.

The now fixed 10k input impedance however did nothing to increase the noise floor of the standard circuit. It still remains highest of them all. The output offset is still around 70mV, roughly halved to 1.25V for the second circuit and is basically zero for the third circuit.

The much better THD results that I reported yesterday actually came from the third circuit, which is about what PRR suggested. Instead of 47R and 1nF I used 47R and 4n7, because that was what I had handy. Notice that the noise floor of the third circuit is even lower than that of the second one, and the THD in this case increased by a whole order of magnitude!

The black trace shows the distortion measurement at 1kHz with ~3.3VRMS into 8R. The blue trace shows the noise floor with no soundcard output (ARTA generator set to External). The red trace shows the noise floor with the input shorted directly at the amplifier vero board, but the soundcard output still connected and muted (set to External again).
Attached Images
File Type: png V1 10dBV.png (53.7 KB, 157 views)
File Type: png V2 10dBV.png (48.3 KB, 153 views)
File Type: png V3 10dBV.png (49.7 KB, 153 views)
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Old 16th November 2019, 02:25 PM   #13
anti is offline anti  Slovakia
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On the 3rd circuit, you have the R1/C1 connected wrong. It sould go to the '-' input, not the '+'.


But hey, this should be in the CHIPAMP forum!
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Old 16th November 2019, 02:28 PM   #14
Preamp is offline Preamp  Germany
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Now with 10k present at the non-inverting input I have deliberately removed the 1n5 from the input lowpass. The only effect was a slightly worse THD result due to the higher bandwidth, but the output noise is the same.
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Old 16th November 2019, 02:35 PM   #15
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by anti View Post
On the 3rd circuit, you have the R1/C1 connected wrong. It sould go to the '-' input, not the '+'.


But hey, this should be in the CHIPAMP forum!
You're twice right!

Just had to check again that I didn't hook this up wrong again, but the circuit is correct; just messed up the schematic. Here's the correct one.
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Old 16th November 2019, 03:00 PM   #16
Preamp is offline Preamp  Germany
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Messed up the schematic for the first circuit, too : R7 should read 22k, not 1k!

I have made another test with reduced feedback resistor values. This is the standard circuit with a 22k + 1k feedback network and a 1k + (1k||47R) network for roughly the same gain. Overall performance is virtually the same; THD is a little worse with the lower resistance, but that's within the margin of error I'd say.
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File Type: png V1 10dBV.png (54.1 KB, 145 views)
File Type: png V4 10dBV.png (53.1 KB, 27 views)
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Old 16th November 2019, 03:51 PM   #17
Preamp is offline Preamp  Germany
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And another. Again the standard circuit with 22k + 1k, but this time with an additional 100R in parallel to the 10k at the non-inverting input to divide the input voltage down. Noise performance is still the same, but the THD skyrocketed due to the rather low input impedance.

In a second attempt I changed the input divider from 1k + (10k||100R) to 101k + 10k, which brought the overall performance back up to the level of the standard circuit.

So dividing down the input by 20dB and amplifying it up by 27dB afterwards doesn't bring any advantages in this case. The circuit as proposed by PRR on the other hand actually beats the standard circuit in terms of overall output noise level, THD, and output offset. The low gain of 6dB is of no problem, if a preceding preamp can put out 20Vpp with low noise and distortion, which would then be sufficient to saturate the TDA and drive it into its supply voltage limits.
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Old 16th November 2019, 04:23 PM   #18
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Preamp View Post
The circuit as proposed by PRR on the other hand actually beats the standard circuit in terms of overall output noise level, THD, and output offset.
PRR's circuit is the standard method when you want to stabilize an amplifier requiring >1 amplification to be stable.
Why would you ruin the DC stability and the LF performance with a direct shunt of the inputs, when adding a capacitor gets you the best of both worlds (mostly).
Up to the pole created by the additional cap, you benefit from the supplementary loop gain, which explains the reduced THD and lower noise
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Old 17th November 2019, 01:41 AM   #19
PRR is online now PRR  United States
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With 20dB less gain, we expect 20dB less self-hiss (in band). And 20dB gain diverted to NFB suggests 1/10th the THD.

Allowing for rounding (and repeated mis-wires and poor drive), this is exactly what you observed. Any other result would be funny.

The self-noise goes up above the corner point. In this case that may be 30kHz so there may be a peak of 100kHz hiss (above the range of your sound card). This is why we don't do it more often. (Also the fact that the lost 20dB would typically have to be made-up with more stages; though some applications don't really need 26dB gain.)
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Old 17th November 2019, 12:13 PM   #20
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by Elvee View Post
PRR's circuit is the standard method when you want to stabilize an amplifier requiring >1 amplification to be stable.
Learning something new every day . Never noticed this before.

Quote:
Originally Posted by Elvee View Post
Why would you ruin the DC stability and the LF performance with a direct shunt of the inputs, when adding a capacitor gets you the best of both worlds (mostly).
Now that I've done this test, I wouldn't!


Quote:
Originally Posted by PRR View Post
With 20dB less gain, we expect 20dB less self-hiss (in band). And 20dB gain diverted to NFB suggests 1/10th the THD.
Sounds reasonable now that you say it. I actually didn't expect this to work though...

Quote:
Originally Posted by PRR View Post
The self-noise goes up above the corner point. In this case that may be 30kHz so there may be a peak of 100kHz hiss (above the range of your sound card). This is why we don't do it more often.
How bad could that possibly be? Normal tweeters probably don't go that high; ribbon tweeters might, but I don't think that would be audible anyway. The LPF at the input should take care of higher frequencies going in and thus reduce the risk of creating intermodulation distortion which might again be audible.



I re-ran the measurement with the soundcard configured for the single-ended interface. The overall noise floor is a bit lower now, but so is the input voltage range. Had to back up by 5dBV, because 10dBV would clip the soundcard input.
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File Type: png V1 5,2dBV.png (52.3 KB, 19 views)
File Type: png V2 5,4dBV.png (45.6 KB, 14 views)
File Type: png V3 5,5dBV.png (49.8 KB, 14 views)
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