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Old 28th March 2012, 04:16 PM   #41
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Hi KSTR,

Thank you for your kind words. Indeed, it's a Sallen-Key LP filter.
The idea, using a 2nd order filter to limit dV/dt and protect the drivers, is from Andy_C,
while the implementation, the tiny cap tied to the output, I figured it out by myself.

Cheers,
E.
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Old 28th March 2012, 05:46 PM   #42
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Quote:
Cheers,
E.
BTW, what's your real name?
I would like to be called Keane. My wishful thinking is that my accomplishments will be known under that name...

I would be flattered to build your frontend too, unfortunately my bench is not that great (yet) and I'm of limited means. My soldering iron tip has corroded again, and who knows when I'll be able to get another one.

Adding a diode to the emitter of Q10 and Q11 would get the Baxandall drivers out of danger of quasi-saturation, but I don't know if this would bring any benefits to the circuit. Maybe better PSRR. Using a Widlar or other suitable current mirror to increase the Vce of the current mirrors may bring an improvement in current balance for the same reason.

I only started understanding most of this stuff in an organized way this year, and I've just graduated from HS, so be advised...

I found your capacitor trick to be entirely ineffective at curing oscillation in simulation, and it caused problems of it's own. This cascode has VERY specific needs. Specs are not as great after compensation, but this is a necessity.

Attached is what I came up with to stabilize the Baxandall cascode. This gets it stable, and with more extreme values it can be made unconditionally stable. I don't know if it can be stable any other way. The simulator suggests it has stability as-is, but I doubt it works like that in real life. Unfortunately this causes signal loss through the bias generator, but it's not too much at audio frequencies.

The diode is included to get the Baxandall driver out of quasi-saturation. Leakage between the base and collector (Early, w/e) decreases max output impedance (we control this specifically with the 47k resistor, which helps stability), so quasi-saturation is good to avoid. I'm not sure but I think it's necessary in real life. The 47k resistor is placed so there is no DC bias across it. May not matter, but why not?

Increasing the 1k resistor decreases the effectiveness of the cascode since it sort of degenerates the driver through the base. 1k was a good compromise, as far as cascode performance, but increasing it decreases signal loss through it and that may be more important.

If no one else has stabilized the Baxandall, let it be known that I was first!!!

THD20 is .000002% at 1V output.
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File Type: png Ed_Stuart_TIS_With_Kcomp.png (123.0 KB, 584 views)
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Old 28th March 2012, 05:56 PM   #43
Waly is offline Waly  United Kingdom
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Seems like an interesting design idea, I'll load this in the simulator ASAP.

Also interesting, you changed your opinion on the TMC input stage loading and the general TPC vs. TMC performance:

Quote:
Originally Posted by Your web site
TPC reduces distortion of IPS, TIS and OPS as well, though it puts a higher load on the TIS output.TMC only reduces the distortion of the TIS and OPS. However it decreases the load on the TIS output. Applied to a typical power amp topology, both compensation schemes appear to work about equally well.
This compared ompared to your previous hard stances about, e.g.:

Bob Cordell Interview: Negative Feedback
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Old 29th March 2012, 10:28 AM   #44
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Default Instability

Quote:
Originally Posted by keantoken View Post
I would like to be called Keane. My wishful thinking is that my accomplishments will be known under that name...

I would be flattered to build your frontend too, unfortunately my bench is not that great (yet) and I'm of limited means. My soldering iron tip has corroded again, and who knows when I'll be able to get another one.
Hi Keane,

Using solder with traces of copper helps to avoid that the tip dissolves in the solder.

Quote:
Adding a diode to the emitter of Q10 and Q11 would get the Baxandall drivers out of danger of quasi-saturation, but I don't know if this would bring any benefits to the circuit. Maybe better PSRR. Using a Widlar or other suitable current mirror to increase the Vce of the current mirrors may bring an improvement in current balance for the same reason.
To a certain extent I share your concern about quasi saturation (I wish I had better models). On the other hand, I've seen many circuits with transistors operating at Vce ~= 0.6V. But if it works, I prefer it this way, as one of objectives was a near rail to rail output swing. This is because I hate using an elevated and/or separate PSU for the front-end.

Quote:
I only started understanding most of this stuff in an organized way this year, and I've just graduated from HS, so be advised...

I found your capacitor trick to be entirely ineffective at curing oscillation in simulation, and it caused problems of it's own. This cascode has VERY specific needs. Specs are not as great after compensation, but this is a necessity.

Attached is what I came up with to stabilize the Baxandall cascode. This gets it stable, and with more extreme values it can be made unconditionally stable. I don't know if it can be stable any other way. The simulator suggests it has stability as-is, but I doubt it works like that in real life. Unfortunately this causes signal loss through the bias generator, but it's not too much at audio frequencies.

The diode is included to get the Baxandall driver out of quasi-saturation. Leakage between the base and collector (Early, w/e) decreases max output impedance (we control this specifically with the 47k resistor, which helps stability), so quasi-saturation is good to avoid. I'm not sure but I think it's necessary in real life. The 47k resistor is placed so there is no DC bias across it. May not matter, but why not?

Increasing the 1k resistor decreases the effectiveness of the cascode since it sort of degenerates the driver through the base. 1k was a good compromise, as far as cascode performance, but increasing it decreases signal loss through it and that may be more important.

If no one else has stabilized the Baxandall, let it be known that I was first!!!

THD20 is .000002% at 1V output.
Other people have reported instabilities as well, but I haven't encountered them yet. Dimitri Danyuk has built similar circuits (see his AES paper), but no mention about instability (BTW, the Baxandall transistors in his circuit operate at 0.6V). I have also simmed the circuit with other transistor models (2N5401, 2N5551, 2SA1407 & 2SC3601) but no sign of instability. So what's going on? Has it something to do with the spice models?

As for the capacitor trick (470pF), these caps do have an effect according the test circuit as shown below, fig.1.
Fig. 2: C1 = C2 = C3 = 0. It peaks at 19MHz.
Fig. 3: C1 = C2 = 470pF, C3 = 0
Fig. 4: C1 = C2 = 0, C3 = 10pF
As you see, no sign of instability with either of these caps.

Anyhow, can you give me the models of your transistors (in spice format, no LT format, please)? Maybe it will provide a clue.

BTW, the 100k bias resistors are wrong connected (not a fatal error, I think).

Cheers,
E.
Attached Images
File Type: png TIS-tst.png (15.0 KB, 518 views)
File Type: png TIS-C1=C2-C3=0.png (12.6 KB, 505 views)
File Type: png TIS-C1=C2=470p.png (12.5 KB, 485 views)
File Type: png TIS-C3=10p.png (12.5 KB, 337 views)
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goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 29th March 2012 at 10:30 AM.
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Old 29th March 2012, 11:38 AM   #45
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Edmond Stuart View Post
BTW, Some comparisons with a few other front-ends:
PGP: complexity: 45 trannies, THD20k = 53ppb
Super TIS: complexity: 18 trannies, THD20k = 74ppb
Bob's HEC: complexity: 19 trannies, THD20k = 1057ppb
My complete amp ThermalTrak+TMC amp 23 transistors all together with two output pairs simulate THD20k = 100ppb at 70W on 8 ohm.
dado
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Old 29th March 2012, 12:32 PM   #46
Bonsai is offline Bonsai  Taiwan
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Edmond re PM. Thanks - I understand.
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Old 29th March 2012, 01:08 PM   #47
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Default 100 ppb

Quote:
Originally Posted by dadod View Post
My complete amp ThermalTrak+TMC amp 23 transistors all together with two output pairs simulate THD20k = 100ppb at 70W on 8 ohm.
dado
Hi Dado,

Congratulations! For an amp of limited complexity, that is an extreme good figure.
BTW, did you check the phase margin of the internal Miller loop as well as the global NFB loop?

Cheers,
E.
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Old 29th March 2012, 01:30 PM   #48
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Hi Edmond,

Saw your thread only today. Looks interesting!
You asked about origin - I am not sure, and need to research this further, but it reminds me of an opamp simplified circuit I have seen.
Maybe I can find it, and maybe it is different. But it IS creative.

jan didden
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Old 29th March 2012, 01:59 PM   #49
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Thank you Jan.
Hope you can find a similar circuit or a precursor.
Cheers,
E.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 29th March 2012, 02:02 PM   #50
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@Keane,
I forgot to ask you: what were exactly the symptoms of instability?
Cheers,
E.
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