
Home  Forums  Rules  Articles  diyAudio Store  Blogs  Gallery  Wiki  Register  Donations  FAQ  Calendar  Mark Forums Read 
Solid State Talk all about solid state amplification. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools 
29th November 2020, 04:50 PM  #11 
diyAudio Member
Join Date: Mar 2003
Location: Haarlem, the Netherlands

If you want the amplifier to be smallsignal stable with any passive load:
1. Determine the amplifier's output impedance without the L and R 2. Look for frequency regions where the real part of the output impedance goes negative. I'll assume there is a negative peak at frequency f_neg. 3. Choose an R that is greater than two times the opposite of the most negative real part of the output impedance, for example four times the opposite of the most negative real part of the output impedance 4. Connect an L in parallel with a reactance equal to R at f_neg 5. Determine the impedance of the amplifier with the L and R included and fine tune their values as needed to make the real part of the impedance positive at all frequencies I took a more pragmatic approach when I designed my amplifier: I came up with a target for the magnitude of the output impedance at 20 kHz, divided that by 2 pi * 20 kHz to find the inductance, put a resistor in parallel with a value close to the nominal load impedance (8.2 ohm) and shunted the output with a capacitor with value C ~= L/R^2 to turn the whole thing into a firstorder series filter. (Making it a firstorder series filter was recommended in an article of A. N. Thiele, as it helps to keep RF signals out of the amplifier.) I have no idea if it is stable with any passive load, but it certainly is with all loudspeakers and cables I ever connected to it. 
30th November 2020, 10:53 AM  #12 
diyAudio Member
Join Date: Dec 2015
Location: south east asia

Thank you for details. A standard amplifier exhibits resistive output impedance at low frequencies up to OLBW, than it becomes inductive. If a capacitive reactance comes on the output, the two opposite reactances in series provoke resonance. The serie LR will come in series with the output inductance to lower the resonance and damp it. The question is if I know my internal LR how to estimate the external one. Also to note that there is the Zobel which comes into account. If I understand, the Zobel, internal LR and the external LR must be link to each other, but how?

30th November 2020, 11:03 AM  #13 
diyAudio Member
Join Date: Dec 2009

As a result, get a compensated active circuit. At least at critical frequencies.
All together is called Zobel 
30th November 2020, 11:18 AM  #14 
diyAudio Member
Join Date: Mar 2003
Location: Haarlem, the Netherlands

True, but at yet higher frequencies, the real part of the output impedance will usually go negative, which may cause oscillations with the wrong load impedance. That can be solved with either an RC series circuit across the output or an LR parallel circuit in series with the output. I haven't a clue why designers often do both.

30th November 2020, 12:04 PM  #15 
diyAudio Member
Join Date: Dec 2015
Location: south east asia

Most Japanese, McIntosh, put the Zobel after the LR. The Zobel is mandatory for high impedance current outputs as the OLG depends on the load. If the load is inductive, the amp oscillates. All explanations I found about Zobel by google, relate to speaker impedance. An amp is universal impedance.

30th November 2020, 01:28 PM  #16  
diyAudio Member
Join Date: Mar 2009
Location: Buenos Aires  Argentina

Quote:
Too small, too few turns, too far from each other.
__________________
Design/make/service musical stuff in Buenos Aires, Argentina, since 1969. 

Thread Tools  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Output filter  Skorpio  Class D  5  8th August 2008 07:44 PM 
What output filter to choose  Groundloops  Digital Source  2  9th November 2007 02:06 PM 
Output filter  mtech  Class D  11  22nd September 2007 08:04 AM 
BTL output filter  ted12  Class D  16  26th April 2005 02:55 PM 
What matrial for output filter?  lumanauw  Class D  62  16th March 2005 06:15 AM 
New To Site?  Need Help? 