Hi @marigno
Obviously, L20 V9.2 needs modifications because, as delivered, it isn't really a great choice. Everyone who observed its square wave response should know that.
Before I ask a question, first refer to two schematics below that confuse me. One schematics shows just two diodes, another has four. I can see these two diodes on image L20V9.2untouched. JPG but not on the another schematics below. I suppose the schematics with four diodes is wrong.
Now, the question:
I want to apply your proposed modifications step-by-step, beginning with adding C251 and R251.
Now, correct me if I'm wrong: modification is a bridge between T224 and T225 emitters comprising of C251 and R251 in parallel.
Is that correct?
That seems logical to me and I'm sure this can improve the amplifier performance.
Obviously, L20 V9.2 needs modifications because, as delivered, it isn't really a great choice. Everyone who observed its square wave response should know that.
Before I ask a question, first refer to two schematics below that confuse me. One schematics shows just two diodes, another has four. I can see these two diodes on image L20V9.2untouched. JPG but not on the another schematics below. I suppose the schematics with four diodes is wrong.
Now, the question:
I want to apply your proposed modifications step-by-step, beginning with adding C251 and R251.
Now, correct me if I'm wrong: modification is a bridge between T224 and T225 emitters comprising of C251 and R251 in parallel.
Is that correct?
That seems logical to me and I'm sure this can improve the amplifier performance.
Hello Berlusconi!
Both schematics have a total of 4 diodes, 2 x 1N4148 for bias purposes, 2 x 1N4004/7 to (try to) damp opposite peaks. Take a look at the R243/L251 zone.
Forget (completely forget) the 1st schematic, it is old, same topology, but now the kit is delivered with some different values. The differential and the VAS have a bit more current, and there are some minor improvements.
You interpreted the modification correctly, but we have split your observation into two topics:
1) R251 makes the drivers work in Class-A (not properly, but a good approximation). This is a first improvement.
2) Rather than thinking of driver's emitters, which now operate more or less in Class-A, focus on the final pair's bases (which are the same electric points). The capacitor positively drives the bases to switch off immediately because the opposite drivers are in conduction toward the opposite polarity. It means the BJTs do not switch off on their own timing because of a lack of driving, but because they are driven to the off state. This is a second improvement.
Let's now focus on C216: the original is 1mF polarized. It means the positive semi-period of the signal returning via R233, partialized by R224, has a different treatment than the negative semi-period. We need a non-polarized capacitor that can treat the two semi-periods the same way. If you find a 1mF NP capacitor of good Audio quality, it is fine. Since I couldn't find it, I scaled the feedback network by a factor of (around) 5. Consequently, you replace R213 with the same value of R233 (thanks to @Kay Pirinha ). This was the third improvement.
The last improvement is to add a Zobel network and an inductor to the output. It would be better to add a double Zobel network, you'd better do it if you have room inside the cabinet.
My "official" schematic is the one at #57 post, like the 2nd one you posted.
Both schematics have a total of 4 diodes, 2 x 1N4148 for bias purposes, 2 x 1N4004/7 to (try to) damp opposite peaks. Take a look at the R243/L251 zone.
Forget (completely forget) the 1st schematic, it is old, same topology, but now the kit is delivered with some different values. The differential and the VAS have a bit more current, and there are some minor improvements.
You interpreted the modification correctly, but we have split your observation into two topics:
1) R251 makes the drivers work in Class-A (not properly, but a good approximation). This is a first improvement.
2) Rather than thinking of driver's emitters, which now operate more or less in Class-A, focus on the final pair's bases (which are the same electric points). The capacitor positively drives the bases to switch off immediately because the opposite drivers are in conduction toward the opposite polarity. It means the BJTs do not switch off on their own timing because of a lack of driving, but because they are driven to the off state. This is a second improvement.
Let's now focus on C216: the original is 1mF polarized. It means the positive semi-period of the signal returning via R233, partialized by R224, has a different treatment than the negative semi-period. We need a non-polarized capacitor that can treat the two semi-periods the same way. If you find a 1mF NP capacitor of good Audio quality, it is fine. Since I couldn't find it, I scaled the feedback network by a factor of (around) 5. Consequently, you replace R213 with the same value of R233 (thanks to @Kay Pirinha ). This was the third improvement.
The last improvement is to add a Zobel network and an inductor to the output. It would be better to add a double Zobel network, you'd better do it if you have room inside the cabinet.
My "official" schematic is the one at #57 post, like the 2nd one you posted.
Thank you very much for your extensive explanation.
Suggestion: please put the "official" schematics in the opening post #1 to eliminate the ambiguity. There is no schematics in #57. I would like very much to apply all your modifications safely.
Saluti
You're right, the schematic is at #43. I should stop doing more than one thing at the same time. I also edited the 1st post, adding the "official" schematic with all mods, and removed the first one.
@Radian : Please, are there any errors on the PCB showing the numbers of components at post #43? Is it correct?
@Radian : Please, are there any errors on the PCB showing the numbers of components at post #43? Is it correct?
I didn't measure the individual components, have only changed them per your instructions.
What escapes me is the the value of R234.
It's duty has not changed being in the place of the previously red marked 470 Ohms resistor. But now it's left at the stock value of 2k2 Ohms?
I didn't notice R234 at first, so I added it with a value of 470 Ohm. I later noticed that it was already on the original schematic, so I removed my modification. Follow the schematic at #43 or #1.
Is the picture with component numbers right?
Is the picture with component numbers right?
I use 16V Muse capacitors, but any voltage will be OK, since there is negligible DC or AC at their ends.
I actually don't know since I don't have the time to follow every trace and sketch it out.
All I know is that your instructions executed accordingly to the picture results in a beautiful sounding amp👍
Me too. 🛌😴
Finally I see the end of confusion, schematics in the OP (#1) is clear now and we all should forget all other variations published here. Now, I feel confident to proceed. A snippet of the essential part is below. I was confused with that
This is very important modification. I had just difficulty with various versions of schematics.
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Since we know that a single output pair sounds better in the mid and high range than many paralleled ones would it be feasible to just add a pair that is not in parallel with the others to drive the tweeter. My speakers can be biamped.
You can't do that because the feedback generates the correction signal, including the effect of anything in its loop (the 4 pairs too). The driving signal (containing, among the rest, the correction for the woofer) would end up driving the new pair you added for the tweeter, making the new pair useless. This happens if you apply the feedback to the new pair or you don't.
Nothing would change.
You need another complete board.
Nothing would change.
You need another complete board.
Thanks @marigno
I have applied this modification succesfully. I needed some time to understand how it works. I have reduced it ro the extreme of simplicity by connecting two output transistor bases leads to R251 II C251. Simple and works perfectly. I will apply all your modifications step-by-step.
I have applied this modification succesfully. I needed some time to understand how it works. I have reduced it ro the extreme of simplicity by connecting two output transistor bases leads to R251 II C251. Simple and works perfectly. I will apply all your modifications step-by-step.
But ofcourse, I will do that after every step to understand effects of every action.
I'm still not clear about position C217.
It is listed as a polarized electrolytic capacitor 10uF in the updated wiring diagram. In this place, a non-polarized capacitor with a value of 1uF is used in the original. Is a regular electrolytic model okay in this position, or would it be better to use a bipolar ES MUSE? On the original board, when making modifications, one struggles with a small space for many components, so I don't know if this position is also the case, and when choosing a component design, the small space for placement doesn't play a role.
It is listed as a polarized electrolytic capacitor 10uF in the updated wiring diagram. In this place, a non-polarized capacitor with a value of 1uF is used in the original. Is a regular electrolytic model okay in this position, or would it be better to use a bipolar ES MUSE? On the original board, when making modifications, one struggles with a small space for many components, so I don't know if this position is also the case, and when choosing a component design, the small space for placement doesn't play a role.