Hello.
Situation:
I am about to do my third TDA7293 amp. I want to compare it with the LM3886 I have already done. The TDA will have a clone of the LM PSU, +-24v. not loaded over 5 amp.
Bootstrap:
The first TDA7293 had a 22uf regular capacitor as bootstrap the second had a 47uf of high quality.
The second sounded better than the first. Also they had not the same power supply; on the first it was smaller, higher voltage but less current. I don't remember the specifics, it was almost 15 years ago and they don't exist anymore.
Is the quality or the size of the bootstrap capacitor make the difference for the sound?
Or should I suspect the power supply?
Gain:
If I look at the resistor for the gain on the schematic from ST it gives 33 of ratio and in the texte they say it's 30 and a min/max of 29 to 31db with a minimum of 26db on the footnote.
Does somebody have the formula to calculate the ratio to have the gain in db?
If I want to do a fair comparaison should I try to have the same gain on both the LM and the TDA? It's 23 (ratio) on the LM, too low for the TDA. Is the ratio should follow the voltage of the PSU (more voltage more gain) ?
What is the best gain for that chip?
Thank you for your time on this.
Situation:
I am about to do my third TDA7293 amp. I want to compare it with the LM3886 I have already done. The TDA will have a clone of the LM PSU, +-24v. not loaded over 5 amp.
Bootstrap:
The first TDA7293 had a 22uf regular capacitor as bootstrap the second had a 47uf of high quality.
The second sounded better than the first. Also they had not the same power supply; on the first it was smaller, higher voltage but less current. I don't remember the specifics, it was almost 15 years ago and they don't exist anymore.
Is the quality or the size of the bootstrap capacitor make the difference for the sound?
Or should I suspect the power supply?
Gain:
If I look at the resistor for the gain on the schematic from ST it gives 33 of ratio and in the texte they say it's 30 and a min/max of 29 to 31db with a minimum of 26db on the footnote.
Does somebody have the formula to calculate the ratio to have the gain in db?
If I want to do a fair comparaison should I try to have the same gain on both the LM and the TDA? It's 23 (ratio) on the LM, too low for the TDA. Is the ratio should follow the voltage of the PSU (more voltage more gain) ?
What is the best gain for that chip?
Thank you for your time on this.
Attachments
I´m quite certain that if used with same supply and speaker, both will sound very very much close to each other,
You have to be careful with gain on these chip amps as some have a minimum gain before they become unstable and oscillate. I got caught out with a hybrid valve/chip amp circuit I did. The chip amp made a good transmitter and sent signal back to valve via RF !
Had to read chip amp datasheet again before I realised my gain was too small.
Had to read chip amp datasheet again before I realised my gain was too small.
Nigel, I found the formula I was looking for and the 33 resistor ratio is 30dB so the datasheet is right and I will keep the ratio they recommend. I don't want trouble. 🙂
JM, yes they should sound close to each other, I don't expect much difference if any but I want to witness it by myself. After all DIY is about that right? Doing stuff, experimenting and comparing.
I keep looking for the best value for the bootstrap capacitor. The datasheet say 22uF minimum. Does it mean higher value is better? I saw 33uF on this website but cannot find it back (there is so much stuff here). I would like to know what it does to the sound when we change it.
Thank you both and have a good day.
JM, yes they should sound close to each other, I don't expect much difference if any but I want to witness it by myself. After all DIY is about that right? Doing stuff, experimenting and comparing.
I keep looking for the best value for the bootstrap capacitor. The datasheet say 22uF minimum. Does it mean higher value is better? I saw 33uF on this website but cannot find it back (there is so much stuff here). I would like to know what it does to the sound when we change it.
Thank you both and have a good day.
The bootstrap capacitor is used to raise the apparent power supply voltage for the VAS by a little bit (maybe a couple volts). Larger caps take longer to charge, but hold that charge for longer as well. For loud passages in music you can expect a larger cap to be better in most respects, as higher supply voltage reduces distortion. Conversations about capacitor quality can get very tetchy, so I'll desist.
I expect any differences to be visible on a measuring instrument quite easily, but not sure about the ear. For typical listening levels it shouldn't matter too much. A quick reference is the P3A by Rod Eliot, it's a discrete design that uses a 100uF bootstrap cap. There is a very long thread on the forum about it, which you can look at to see if it was discussed. It runs the same rail voltages as a typical chipamp, and the principles of bootstrap VAS being similar it can provide some guidance.
I expect any differences to be visible on a measuring instrument quite easily, but not sure about the ear. For typical listening levels it shouldn't matter too much. A quick reference is the P3A by Rod Eliot, it's a discrete design that uses a 100uF bootstrap cap. There is a very long thread on the forum about it, which you can look at to see if it was discussed. It runs the same rail voltages as a typical chipamp, and the principles of bootstrap VAS being similar it can provide some guidance.