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Old 4th April 2012, 02:57 PM   #2341
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Lazy Cat View Post
So NTC is working properly, glad about that. If it is still undercompensated with 220 ohm, than try a little lower value ie. 180 ohm. Without serial compensation resistor it is always overcompensated.
Thanks. With the 220 Ohm resister shorted, compensation seems very good. I adjusted the driver bias to about 10mA warm, and output offset within 5mV. I then shut off power and allowed everything to cool completely.

Upon cold start up, the driver current quickly ramps to about 7.5mA, then rises over the next 20 seconds or so to about 9.5mA. After a couple more minutes, it rises to 10.0mA and stays there. Output offset wanders a little but less than +/- 15mA. Square waves look very good - no overshoot, with a very small amount of hF noise visible in the leading edge, at 100kHz (may be due to all the probes I have connected, or may have to adjust the HF compensation a bit?).

The temporary heatsink that I'm using for the output devices is barely adequate, so it gets fairly hot, but not too hot to touch. Only the outputs are mounted on this sink. The board you see in the picture is about 1.5cm above the output sink. As the outputs heat up, the driver bias drops to about 9.5mA. So it looks like no resistor or a maybe a small one is all that is required for this version. Yes, I believe that with no series resistor it is overcompensated for the input bias drift, but that overcompensation is needed to take care of the driver thermal drift, which is otherwise not compensated for.

As I noted before, the drivers in this version are not cascaded, so there is significant power dissipated. And the driver bias is visibly sensitive to driver temperature. I have a small square of aluminum as heat radiators for the drivers (about 1mmx1.5cmx1.5cm). If I put a finger on the driver sink (they are warm to the touch, but not hot) the bias immediately begins to drop and the offset climbs.

My analysis could be completely wrong, but the result is nevertheless good.

Sheldon
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Old 5th April 2012, 12:52 PM   #2342
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Rat's nest.
I have seen bigger rats
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Old 7th April 2012, 07:21 AM   #2343
hahfran is offline hahfran  Germany
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The BJT version of SSA posted here earlier shows as expected very unfortunate sensitivity to complex loads.
The simulation of a 2 way speaker is accurate within 95% of the measured complex impedance of the actual speaker. A critical situation for amps sensitive to reactive distortion is around 330 Hz. There the thd of the amp jumps sharply from less than 0.03% to above 0.6% and such is audible.

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 6.000e+01 2.378e+01 1.000e+00 -0.01 0.00
2 1.200e+02 2.606e-03 1.096e-04 -100.19 -100.18
3 1.800e+02 1.418e-03 5.964e-05 13.05 13.06
4 2.400e+02 3.070e-04 1.291e-05 99.87 99.88
5 3.000e+02 1.042e-04 4.383e-06 33.05 33.06
6 3.600e+02 6.801e-05 2.860e-06 36.28 36.29
7 4.200e+02 1.450e-04 6.097e-06 -2.20 -2.19
8 4.800e+02 7.435e-05 3.127e-06 -48.58 -48.56
9 5.400e+02 6.682e-05 2.811e-06 -54.96 -54.95
Total Harmonic Distortion: 0.012578%
but
Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 3.300e+02 2.341e+01 1.000e+00 0.59 0.00
2 6.600e+02 4.036e-02 1.724e-03 116.46 115.87
3 9.900e+02 1.246e-01 5.323e-03 113.17 112.59
4 1.320e+03 2.305e-02 9.845e-04 43.14 42.55
5 1.650e+03 4.954e-02 2.116e-03 60.91 60.32
6 1.980e+03 2.034e-02 8.691e-04 -11.28 -11.87
7 2.310e+03 1.878e-02 8.023e-04 -18.24 -18.82
8 2.640e+03 1.905e-02 8.138e-04 -58.65 -59.24
9 2.970e+03 1.793e-02 7.659e-04 -128.37 -128.96
Total Harmonic Distortion: 0.627743%
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File Type: jpg SSA load.jpg (231.3 KB, 417 views)
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Old 7th April 2012, 12:11 PM   #2344
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It would be of interest to all of us, if you can verify in sim following tips to lower distortion in SSA:
- replace R26, R27 with CCS
- reduce R1, R2 to 100 ohm and R20, R21 proportionally, to maintain desired voltage gain
- reduce R9, R10 to few ohms to higher OLG
- cascode Q6, Q7

As partial solutions these tips were reported frequently as advancement but I don't remember if ever all of them simultaneously.

Regards Andrej
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Old 7th April 2012, 01:12 PM   #2345
mikelm is offline mikelm  England
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please give link for the diagram you refer to - thx
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Old 7th April 2012, 01:52 PM   #2346
mikelm is offline mikelm  England
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Actually ur last posted diagram does not have part numbers . . . .

Perhaps it's best to post a diagram with the question - if you want to ask questions about part numbers otherwise we can spend hours hunting through the thread and still may not find the diagram you had in mind . . . .
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Old 7th April 2012, 09:47 PM   #2347
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Lazy Cat View Post
It would be of interest to all of us, if you can verify in sim following tips to lower distortion in SSA:
- replace R26, R27 with CCS
- reduce R1, R2 to 100 ohm and R20, R21 proportionally, to maintain desired voltage gain
- reduce R9, R10 to few ohms to higher OLG
- cascode Q6, Q7

As partial solutions these tips were reported frequently as advancement but I don't remember if ever all of them simultaneously.

Regards Andrej
I have already reduced R1 R2 to 110 ohms and changed R20 R21 thus that full power is with 1.5 volts input, but that doesn't change the reactive distortion. I oticed further the op of the Vas is too low with roughly 4 mA
it should be 20 mA where /Ic linearity is good and ft is max.
I will also try to set the op of the predriver to 10 mA for the same reason because all planar BJTs of this size have their max ft at 10mA and also the max linearity /ic.
Will try CCS but am not convinced. We'll see.
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Old 7th April 2012, 10:32 PM   #2348
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Quote:
Originally Posted by hahfran View Post
Will try CCS but am not convinced. We'll see.
Quote:
Originally Posted by MiiB View Post
I think the basic topology is tremendous. You have attacked the output in a unique manner..where you aim to remove the P to N differences. I have fitted a current dumping error correction circuit on the version I have built. which by the way sounds rather ok...

Now to the house keeping. I do believe that the weakest part of this fine circuit and the very reason why it does not simulate particularly well is the poor relations between the impedance of the current drive and the feedback currents. The basic amp is basically fed from a 1Kohm current source..which is way too low to prevent the feedback currents from modulating the drive-current. This is the main source of error in the circuit and by fitting high (very high) impedance current-sources specs are terrific. I will not show my housekeeping here....But I'am quite sure you catch the direction.
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Old 7th April 2012, 10:53 PM   #2349
mikelm is offline mikelm  England
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WHAT DIAGRAM !
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Old 7th April 2012, 11:20 PM   #2350
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please give link for the diagram you refer to - thx
I am sorry mikelm, I apologize, I thought you were asking hahfran.

Well the parts I was comment are from hahfran's sch posted above in post #2343.
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