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Bob Cordell Interview: Negative Feedback
Bob Cordell Interview: Negative Feedback
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Old 26th December 2010, 01:29 PM   #3301
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by richiem View Post
I'm not being totally facetious when I suggest using some kind of clipping/overload detector/indicator, and advise one and all to just NOT CLIP THE AMP! I used to just connect a scope across the output terminals, which was quite instructive and caused no sonic ill effects. Much of the discussion I see around amplifier design is about having amps with sufficient clean power output to *never* have clipping. It's obnoxious, it's dangerous, and most of all, it's not good for your ears.

Now if you are folks who are not listening at home and are doing monitoring in studios, etc, and the head-bangers who want to hear what it sounds like when it's really loud are paying the bills, then some form of unobtrusive hearing protection is a really good idea. Having been there, I recommend it highly.

However, if you;re listening at home, perhaps your solution will be to use speakers with decently higher voltage sensitivity/efficiency, so that your megawatt monsters just don't ever have to work that hard.

Really. End of rant.
Hi Richiem,

You're right, its best if amps don't clip - and many don't realise how often they clip when playing music at realistic levels, especially if the speaker efficiency is only modest. Mitigation of clipping effects is important, like the use of Baker clamps. A soft clip circuit in front of the amplifier that adapts to available rail voltage is also very effective, since it prevents the amplifier-proper from ever clipping.

Cheers,
Bob
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Old 26th December 2010, 02:53 PM   #3302
Upupa Epops is offline Upupa Epops  Czech Republic
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By well applied antisaturation diodes at VAS aren't any problems with clipping and recovery...1 kHz:
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Last edited by Upupa Epops; 26th December 2010 at 03:05 PM.
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Old 26th December 2010, 02:55 PM   #3303
Upupa Epops is offline Upupa Epops  Czech Republic
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At 10 kHz also:
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Old 26th December 2010, 02:56 PM   #3304
Upupa Epops is offline Upupa Epops  Czech Republic
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Detail of 1 kHz:
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Old 26th December 2010, 02:57 PM   #3305
Upupa Epops is offline Upupa Epops  Czech Republic
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And detail of 10 kHz:
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Old 4th January 2011, 11:51 AM   #3306
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default Mystifying TMC, correction

Regarding a previous post of mine, I have to confess that I made an stupid error. Due to a wrong sign (+ instead of -) the reported currents are NOT correct. My sincere apologies for this embarrassing blunder.
Here are the new data, which I believe are correct.
This time, I also have simmed the IPS output current (Io) under slightly different conditions: first with a sine wave of 20kHz (instead of 100kHz) and 1Vpk, next with a square wave with the same frequency and amplitude. Also the VAS output current (Iv) is given, peak values.
Sine wave:
CMC: Io = 168uA; Iv = 183uA
TPC: Io = 8uA; Iv = 1.77mA
TMC: Io = 179uA; Iv = 74uA

Indeed, as stated by Ophelia, the IPS has to deliver the highest current in case of TMC, though compared to conventional Miller compensation (CMC) the difference is negligible. Regarding the VAS output current however, the picture is totally different: the highest for TPC.

But we are still not done. As it is common practice to torture amps with a square wave, let's look at the currents under this condition:
Square wave:
CMC: Io = 4.37mA; Iv = 4.76mA
TPC: Io = 4.76mA; Iv = 47.9mA
TMC: Io = 4.35mA; Iv = 4.53mA

As can be seen, now IPS and VAS currents are lowest in case of TMC. So, Ophelia's conclusion that:
> On the flip side, TMC extra loads the input stage, with the equivalent lead-lag compensation network Miller reflection. For identical global amp
> results, this may require a beefier input stage (more current, more degeneration, etc...) compared to simple Miller or TPC compensation.

still doesn't hold water.

Cheers,
E.

PS: Compensation component values: C1 = 68pF C2 = 680pF, R1 = 330R
The square wave has been 'conditioned' by means of a LP input filter (330R, 1nF) in order to limit the SR to a modest 54V/us.
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Old 4th January 2011, 11:57 AM   #3307
HarryDymond is offline HarryDymond  United Kingdom
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Quote:
Originally Posted by Edmond Stuart View Post
PS: Compensation component values: C1 = 68pF C2 = 680pF, R1 = 330R
Don't forget you can swap C1 and C2 in the case of TPC to give significantly lower Iv numbers should you so desire. Unity Loop Gain frequency, the zero frequency and phase margin remain the same.
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Old 4th January 2011, 01:01 PM   #3308
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default C1 > C2 instead of C1 < C2

Quote:
Originally Posted by HarryDymond View Post
Don't forget you can swap C1 and C2 in the case of TPC to give significantly lower Iv numbers should you so desire. Unity Loop Gain frequency, the zero frequency and phase margin remain the same.
Hi Harry,

Good point! Indeed, figures are much better now. When subjected to a (filtered) square wave, I got Io = 4.04mA and Iv = 6.64mA (instead of a crazy 47.9mA).
Thanks for your hint.

Cheers,
E.
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Old 10th January 2011, 06:37 PM   #3309
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by HarryDymond View Post
Don't forget you can swap C1 and C2 in the case of TPC to give significantly lower Iv numbers should you so desire. Unity Loop Gain frequency, the zero frequency and phase margin remain the same.
Unfortunately, doing that loads the input of the VAS heavily, reducing it's open-loop gain. The final result is likely to be much higher distortion.

Some sort of compromise might be best e.g. C1 = C2 = 120pF, R1 = 1K.
The idea is that it may be better to worsen two different aspects of performance by a factor of 2, rather than worsening one by a factor of 10 in order to achieve minimal degradation of the other.
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Old 10th January 2011, 07:08 PM   #3310
ostripper is offline ostripper  United States
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Quote:
Originally Posted by Bonsai View Post
I just ran a simulation on a balanced VAS amp and put it into clipping. Result:- No collapse of the VAS standing current (I run at 30mA).

Another good reason for balanced drive topologies in my book.
Another plus to the fully comp. is the almost "soft clipping" characteristic it exhibits (at least on my soon to be actually completed CLX - below 1)
I can't wait to actually clip this voltage stage on my 40V rail OPS to see whether LT "lies" ...



Quote:
Originally Posted by Bob Cordell View Post
Hi Richiem,

You're right, its best if amps don't clip - and many don't realise how often they clip when playing music at realistic levels, especially if the speaker efficiency is only modest. Mitigation of clipping effects is important, like the use of Baker clamps. A soft clip circuit in front of the amplifier that adapts to available rail voltage is also very effective, since it prevents the amplifier-proper from ever clipping.

Cheers,
Bob
But the end user is not always prudent , so higher than needed rails in addition to "crippled" gain ( less than would be needed to create clipping at 2V inputs). I have a input switch on the rear of the amp to halve the input signal , keeps the kids from blowing woofers with their dang movies... With the amp below , I would not be as concerned except for the obscene amount of power you would actually be AT with 140V p-p !!


OS
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