Demonstrating that TPC and so-called "TMC" are related - Page 2 - diyAudio
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Old 2nd October 2013, 01:47 PM   #11
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Quote:
Originally Posted by sreten View Post
Hi,
Doug Self got slew rate limiting with TPC due to the
VAS not be able to drive the resistor to ground.

rgds, sreten.

If I am not mistaken, D. Self did not get slew limiting with TPC "due to the VAS not be able to drive the resistor to ground". I think he said there was a slight increase in HF distortion due to such loading.

However, his findings were true only for the case where the capacitor connected to the TIS output is significantly larger than that connected to its input.
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Old 2nd October 2013, 07:53 PM   #12
mcd99uk is offline mcd99uk  United Kingdom
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Quote:
Originally Posted by wissting View Post
I think I agree, but a major difference in a real circuit would be that the TPC load the VAS significantly more than a TMC do?
The thoughts below may be completely wrong...

It does look like (to me) TPC loads the VAS output more but could this be negated by moving the TPC connection to the next stage (eg. drivers or pre drivers). But this may well require extra compensation as TPC compensation now encloses more transistors? And then the advantage of moving the TPC "take off point" may be reduced?
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Old 2nd October 2013, 08:04 PM   #13
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by michaelkiwanuka View Post
If I am not mistaken, D. Self did not get slew limiting with TPC "due to the VAS not be able to drive the resistor to ground". I think he said there was a slight increase in HF distortion due to such loading.

However, his findings were true only for the case where the capacitor connected to the TIS output is significantly larger than that connected to its input.
Hi,


Check your references. A significant increase in distortion at h.f.
due to assymetrical current clipping in the Vas with a resistor of
far higher value than you suggest, and similar capacitance ratio.

The point made is very valid, and is not to be dismissed IMO.

rgds, sreten.

Your model does not seem to include the very real effect.
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Last edited by sreten; 2nd October 2013 at 08:09 PM.
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Old 2nd October 2013, 09:08 PM   #14
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Quote:
Originally Posted by sreten View Post
Your model does not seem to include the very real effect.

I did not use the model to check the current draw of the various elements of the compensation networks on the TIS.

However, this is easily done by deleting the loop gain probes, replacing the SPICE directives with those for transient analysis at some high frequency of your choice and introducing an AC source at the input of each of the models.

Comparing the currents drawn by the compensation elements should enable you to compare the loading effects on the TIS of the two compensation networks.

For a more realistic simulation in this instance use a circuit using "real" transistors.

Last edited by michaelkiwanuka; 2nd October 2013 at 09:14 PM.
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Old 2nd October 2013, 09:57 PM   #15
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Quote:
Originally Posted by mcd99uk View Post
The thoughts below may be completely wrong...

It does look like (to me) TPC loads the VAS output more but could this be negated by moving the TPC connection to the next stage (eg. drivers or pre drivers). But this may well require extra compensation as TPC compensation now encloses more transistors? And then the advantage of moving the TPC "take off point" may be reduced?
To reduce the current draw of the TPC network on the TIS, simply swap the capacitors so that the larger capacitor is connected to the input of the TIS, while the larger of the two is connected to its output.

Including the output stage driver in the compensation loop is likely to require compensation of the minor loop by means of a shunt RC network at the output of the second stage. This, as you've observed, is generally the case where more than two transistors are enclosed within the minor loop.
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Old 2nd October 2013, 10:11 PM   #16
wahab is offline wahab  Algeria
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The Bang and Olufsen Beocenter 7000 compact stereo amp part
has a 47pF miller capacitor that goes from VAS input straightly to the
power stage output through a 270R and there s no added shunt compensation
for the second stage , only a lead compensation from output to inverting input
as well as a shunt at the IPS output ,so it depend ultimately of the practical
implementation ; i guess that there s more than a way to Rome..
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Old 2nd October 2013, 10:40 PM   #17
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by michaelkiwanuka View Post
I did not use the model to check the current draw of the various elements of the compensation networks on the TIS.

However, this is easily done by deleting the loop gain probes, replacing the SPICE directives with those for transient analysis at some high frequency of your choice and introducing an AC source at the input of each of the models.

Comparing the currents drawn by the compensation elements should enable you to compare the loading effects on the TIS of the two compensation networks.

For a more realistic simulation in this instance use a circuit using "real" transistors.
Hi,

That is IMO obfuscating something relatively simple.
VAS load current is not a complicated issue.

D. Self used about 10K for TPC, and still had problems
related to normal VAS current for driving the output.

rgds, sreten.

Use of not well known acronyms is just pointlessly tedious.
Is not remotely clever to not explain them when first used.
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Last edited by sreten; 2nd October 2013 at 11:01 PM.
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Old 3rd October 2013, 07:38 AM   #18
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Quote:
Originally Posted by mcd99uk View Post
The thoughts below may be completely wrong...

It does look like (to me) TPC loads the VAS output more but could this be negated by moving the TPC connection to the next stage (eg. drivers or pre drivers). But this may well require extra compensation as TPC compensation now encloses more transistors? And then the advantage of moving the TPC "take off point" may be reduced?
From my simulations on my last amp I`ve found that if VAS is a double BC557C (darlington-like) the Ccb is enough to stabilize this stage if there is an additional miller from the output. If the collector of the first VAS-transistor is connected to ground it will go unstable without a miller-cap.
My next amp (if VF) will most likely be using a very small VAS-miller (a few pF) and a TPC from the output to VAS input.

Of course, the VAS Ccb is hardly linear and it might be a bad idea to use it in any feedback-loop.
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Old 3rd October 2013, 10:28 AM   #19
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Quote:
Originally Posted by wahab View Post
...as well as a shunt at the IPS output...
Shunt compensation at the output of the input stage also helps stabilise the minor loop, but, I suspect, it compromises power supply rejection; further investigation is required in this respect.
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Old 3rd October 2013, 10:32 AM   #20
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Quote:
Originally Posted by sreten View Post
Use of not well known acronyms is just pointlessly tedious. Is not remotely clever to not explain them when first used.
Just run a search of the acronym at issue on the forum. I haven't used an acronym yet that I haven't previously exhaustively and repeatedly elucidated.
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