Hello, I did some tests with my old clone of NAP 250. I found what produces the HF instabilities on it. In fact when I test the amp by putting my GBF on the amp the instabilities appear and if I take the GBF and move it away they disappear...
So my GBF actually radiates and the amp is sensitive to its radiation.
I then tested a pioneer SA and there on the other hand it is not sensitive to the radiation of the GBF!
Namely that indeed when I did not have this GBF and I was doing my tests with a PC I never had these instabilities.
I'm still surprised because I thought the aluminum case protected against this kind of problem.
I will try to redo the tests on my new clone and in function I will actually try to work on it because even if the amplifier works well it seems too sensitive to me.
So my GBF actually radiates and the amp is sensitive to its radiation.
I then tested a pioneer SA and there on the other hand it is not sensitive to the radiation of the GBF!
Namely that indeed when I did not have this GBF and I was doing my tests with a PC I never had these instabilities.
I'm still surprised because I thought the aluminum case protected against this kind of problem.
I will try to redo the tests on my new clone and in function I will actually try to work on it because even if the amplifier works well it seems too sensitive to me.
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Very good amp indeed. In terms of music at least! Music comes through emotionally coherent, the melodies do not fall apart, rhythm is bouncy, if it gets crouded you can easily follow the individual voices. But you will not be able to tell if the triangle player in a Mahler Symphony sits 40cm higher than the 3rd trumpet player...
Good evening, I have progressed in my little worries.
First my test PCB wasn't broken...it was a bad contact on the BNC lead!
So I redid a lot of tests, the problem of "interference HF" actually comes from the phase compensation. With MJ15003 there is no problem with the basic values (R25 2000/R26 470 /R27 150). With the MJ15024 it appears on the negative part of the important parasites. By modifying these values (R25 2200/ R26 680/ R27 560) the problem is solved.
be carefull when you change this value , the bias change too so you have to put correctly the bias.
I still have other tests to do but since I don't have a spare 2n6341 I should see the result directly on my new amp.
First my test PCB wasn't broken...it was a bad contact on the BNC lead!
So I redid a lot of tests, the problem of "interference HF" actually comes from the phase compensation. With MJ15003 there is no problem with the basic values (R25 2000/R26 470 /R27 150). With the MJ15024 it appears on the negative part of the important parasites. By modifying these values (R25 2200/ R26 680/ R27 560) the problem is solved.
be carefull when you change this value , the bias change too so you have to put correctly the bias.
I still have other tests to do but since I don't have a spare 2n6341 I should see the result directly on my new amp.
I used values that NAIM uses, the phase compensation may differ depending on the output transistors and circuit supply voltage.
I had fun replacing R26 and R27 with variable resistors to look at the oscilloscope what happens.
In fact by putting a square signal at high frequency (30,000 to 40,000 HZ) we can clearly visualize the "interference HF" they are stable and we can see that they seem to be harmonic sinusoids, and adjusting with the variable resistors that I I put we can see them vary and adjust so that they are minimum. Be careful to do it gradually and by measuring the bias constantly because it can go up depending on the values used!
As my laboratory power supply only goes up to 31 volts maximum, I cannot place myself in the real situation of the amplifier to make a precise adjustment, at least for the moment.
I had fun replacing R26 and R27 with variable resistors to look at the oscilloscope what happens.
In fact by putting a square signal at high frequency (30,000 to 40,000 HZ) we can clearly visualize the "interference HF" they are stable and we can see that they seem to be harmonic sinusoids, and adjusting with the variable resistors that I I put we can see them vary and adjust so that they are minimum. Be careful to do it gradually and by measuring the bias constantly because it can go up depending on the values used!
As my laboratory power supply only goes up to 31 volts maximum, I cannot place myself in the real situation of the amplifier to make a precise adjustment, at least for the moment.
In fact I started by doing a lot of tests I replaced the TR5 with a BC 547 which has a higher HFE closer to that of the original ZTX's, which seemed to give a slight improvement to the problem only.
I tried to put a 43pf styroflex instead of the MKP on the miller which didn't change anything. I tried to put a MKP 33 pf instead it aggravates the problem unsurprisingly. On the other hand, by going up to 66 pf the miller in MKP it still improves the problem.
I also tried to change the TR1 TR2 to switch to BC 546A which have a low HFE and that improves the problem a bit too.
In fact, once the miller went to 66pf, it removed some of the noise I think and by raising the frequency to a square signal I finally saw the noise signals clearly (I'll put pictures before and after changing the compensation of phase)
I also tried to decrease the capacitance value in the feedback loop to 47 uf which improves things slightly, nothing more.
Unfortunately I had wasted a lot of time before troubleshooting my test PCB not finding any faulty components I was perplexed...and it was just a bad contact problem on the frequency generator cord.
And Here you can see the result after changing resistors values in the phase compensation (it's 40 000 hz square wave) we can see a little oscillation but no HF parasites. I specify that here all the changed components have been returned to the original (miller MKP 47pf/ Retroaction Cap at 100uf, TR5 is a 2n2222A, TR1 and TR2 are BC550C)
I tried to put a 43pf styroflex instead of the MKP on the miller which didn't change anything. I tried to put a MKP 33 pf instead it aggravates the problem unsurprisingly. On the other hand, by going up to 66 pf the miller in MKP it still improves the problem.
I also tried to change the TR1 TR2 to switch to BC 546A which have a low HFE and that improves the problem a bit too.
In fact, once the miller went to 66pf, it removed some of the noise I think and by raising the frequency to a square signal I finally saw the noise signals clearly (I'll put pictures before and after changing the compensation of phase)
I also tried to decrease the capacitance value in the feedback loop to 47 uf which improves things slightly, nothing more.
Unfortunately I had wasted a lot of time before troubleshooting my test PCB not finding any faulty components I was perplexed...and it was just a bad contact problem on the frequency generator cord.
And Here you can see the result after changing resistors values in the phase compensation (it's 40 000 hz square wave) we can see a little oscillation but no HF parasites. I specify that here all the changed components have been returned to the original (miller MKP 47pf/ Retroaction Cap at 100uf, TR5 is a 2n2222A, TR1 and TR2 are BC550C)
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