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Unconditional Stability
What tests should one employ for
unconditional stability when building or testing a valve/tube PP amp with NFB? Iain |
#2
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On Thu, 10 Mar 2005 12:33:44 +0200, "Iain M Churches"
wrote: What tests should one employ for unconditional stability when building or testing a valve/tube PP amp with NFB? Iain Output impedance must remain positive at all frequencies. An impedance below zero at any frequency (not just in the audio band) means that the amplifier will oscillate into some load impedances - usually capacitive. d Pearce Consulting http://www.pearce.uk.com |
#3
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Iain M Churches wrote: What tests should one employ for unconditional stability when building or testing a valve/tube PP amp with NFB? The amplifier must not oscillate at LF or HF or both without any load connected. The amp will not oscillate with any value of inductance connected The amp will not oscillate with any value of capacitance between 10 uF and 0.01 uf connected. The amp will not oscillate when a low level square wave signal signal is used in any of the above situtations. Testing with F below 20 Hz with a sine wave at 1/2 full output voltage will not produces bursts of HF oscillations when OPT has begun to saturtate at say 5 Hz. There is much else written about stability and *** margins for stability*** in RDH4 and other books. The open loop response for most tube amps with a resistive load looks like a section drawn through a mountain plateau. As NFB is applied in incrementally increasing amounts, the flat part of the plateau is lowered in levels, but at the slopes up to the plateau the response becomes peaked, since the reactive elements in the amplifier cause phase shift which prevent the feedback from being negative. Part of testing requires that you plot the response with some accuracy as the level of NFB is increased, at least at first with a pure resistive load. If you do not know how to plot a response in five minutes, then learn! Suppose that you are able to apply 35 dB of global NFB with phase tweaking networks in place before it is impossible to prevent LF or HF oscillations with any increase in applied NFB. Then suppose you settle for 20 dB of NFB with the phase tweaking networks left in place. You then have a 15 dB margin of stability, but only with a resistive load. 15 dB is a good margin of stability for a tube amp. But the amp trimmed like this may be unstable with a capacitor load, so the amp must have phase tweaking networks set up to reduce the open loop gain and phase shift with a capacitor load in place, usually a 0.22 uF for most power amps, and without any R load. The amplifier schematic at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm has LF and HF phase tweaking networks to improve the bass stability and the treble stablity. Usually, there is no need to worry about stability between 50 Hz and 5 kHz and its below and above these F that oscillations will always occur. In the above schemo, there is a network using 0.47 uF, 0.047uF, 1M, and 220k which attenuates the LF open loop gain by about 12 dB below 20 Hz, and reduces the ultimate phase shift below 10 Hz, and since applied NFB is dependant on open loop gain, and stability gets worse with more gain, the less gain at LF means there is greater stability. So in effect, although we may have 20 dB of applied NFB at 1 kHz, at 10 Hz there may be only 14 dB, and at 5 Hz about 10 dB, and our amp wants to then be stable. The bass response will still be very much improved overt he open loop response, even with the open loop LF response "artificially" reduced in the way shown in the schematic. In the schemo there is also a 330pF and 3.3k in series from the grid of the LTP input grid to ground. This acts to provide an RL **at HF** of ultimately of 3.3k to the input tube which would otherwise be 75k plus the 220k in parallel as shown. This is because at low RF, the 330 pF becomesa short circuit, so the RL = 3.3k, ( in parallel with 220k and 75k, which have little effect ). Therefore the HF pole caused by the miller effect of the LTP is transfered to a higher F than would be the case if the RC network were not present, and in addition the HF open loop gain is reduced in a frequency range to reduce the applied feedback as F is increased above 20 kHz, where there is little point in having the same amount of applied FB as one wants at 1 kHz. Other phase correction network tricks are employed to increase the margins of stability such as placing just the right size C across the global FB resistor, see the 0.001 uF across the 1 k FB resistor. Also, the FB network is a low impedance one, so the NFB is delivered via the 1k and 100 ohm divider, and the bias R of the input tube is bypoassed by a large value electro, see the 2.2k and 1,000 uF cap At really low F, the 1,000 uF becomes a high impedance, and the gain of V1 ia slightly reduced, but phase shift is reduced. In another schemo at http://www.turneraudio.com.au/htmlwe...00ulabmono.htm there is some RC networks in series across each 1/2 primary of the OPT to give the output tubes a resistive load at HF as the leakage inductance gradually decouples the tubes from the load at the output, but while it does, there is a serious phas shift and tendency to HF instability, and the series RC "zobel" networks add to the margin of stability. When an amp is stabilised to the list above, there will still be some overshoot on square waves with capacitor loads. The sine wave response is best measured with a cap load when output voltage = 1/10 of maximum for clipping with an R load at 1 kHz. Therefore the amp should be able to produce a true non saturated response for the cap load, since the cap load becomes a low impedance at HF, and no amp is able to put an infinite amount of current into a cap at HF in order to keep the output voltage equal to what it is at 1 kHz. If there is up to 6 dB of peaking in the sine wave response at say 40 kHz with a 0.22 uF, and other values of cap won't produce a higher peak, and you have 20 dB of applied NFB, then you have a nicely stable tube amp. Usually the addition of an R across the C load, or a small R in series with the C should improve the response. A test for ability to drive Quad ESL57 will have a 16 ohm R as a load with 2 uF and 1.6 ohms in series, also as a load. many amps will show a peaked response at between 16 and 40 kHz, maybe 6 dB, and 3 dB at 20 kHz, and this is a poor outcome, usually because tyhe OPT is poor, or the builder hasn't done his stabilising networks properly. Despite phase tweaking networks, it should be still possible to get a resistive load response with no peaking outside tha 20 Hz to 20 kHz band, and with -3 dB points at least from 5 Hz to 70 kHz at 1/10 full output voltage, and on the really good amps, between 16 Hz and 65 kHz at the full power clipping level at 1 kHz, even for where RL = 1/2 the rated RL for the amp. The idea of trimming the response is called critical damping, critical because the choice of RC correction components is a narrow one because we aim to have the maximum bandwidth with minimum phase shift with FB applied and still have stability into any kind of reactive load. The amplifier should be considered as a bandpass filter, and one around which we apply NFB, a loop which includes the phase delays in the transfer of signals. There is no theoretical way we can select damping components or exactly calculate the values, since the stray OPT C and leakage L are quantities not able to be quantified accurately enough to allow optimal damping, and its only after we have done a few amps that we get good at the cut and try approach to critical damping. I use the empirical notion that for zobels across the halves of the OPT, each R = 1/2 RLa-a, and C has Z = 1/2 RLa-a at about 100 kHz. For the RC across the output of V1, I use a variable radio tuning gang and a pot to find the least ring with a 5 kHz square wave wehn using 0.22 uF across the output. The C across the FB resistance is adjusted until HF oscillations at RF start, then the value halved. Its somewhat guesswork. But at the end of the procedure, no tube of the the amp should saturate, ie, be forced into grid current or cut off with an R load at full 1 kHz power between 20 Hz and 65 kHz. A pure cap load of 2 uF should produce a peak in the sine wave response at an F above 25 kHz, not below, and the peak no more than +3 dB, so that at 20 kHz, such a C should cause a lift in response of no more than 1.5 dB. Since such C loads are not encountered in real world speakers, and there is usually some series and parallel R also present, and so the response into ESL should be substantially flat if we have got it all right, and in fact the tube amp will often provide a better response than many SS amps. The secret to a good response is an OPT with a wide bandwidth. This means high primary L and low saturation F, low stray capacitance and low leakage inductance. ( and then such transfromers usually contribute a very small amount of HD to the THD of the amp ). This means all out phase correction tricks only affect frequencies well outside the audio band, where there is little energy in the recordings. Many amplifier makers pay only lipservice to the above concerns because its cheaper to make a crummy amp, and its easy to get away with it. There are other statements I have made on NFB and stability in the RAT archives. Patrick Turner. Iain |
#4
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"Patrick Turner" wrote in message ... Iain M Churches wrote: What tests should one employ for unconditional stability when building or testing a valve/tube PP amp with NFB? The amplifier must not oscillate at LF or HF or both without any load connected. The amp will not oscillate with any value of inductance connected The amp will not oscillate with any value of capacitance between 10 uF and 0.01 uf connected. The amp will not oscillate when a low level square wave signal signal is used in any of the above situtations. Testing with F below 20 Hz with a sine wave at 1/2 full output voltage will not produces bursts of HF oscillations when OPT has begun to saturtate at say 5 Hz. There is much else written about stability and *** margins for stability*** in RDH4 and other books. The open loop response for most tube amps with a resistive load looks like a section drawn through a mountain plateau. As NFB is applied in incrementally increasing amounts, the flat part of the plateau is lowered in levels, but at the slopes up to the plateau the response becomes peaked, since the reactive elements in the amplifier cause phase shift which prevent the feedback from being negative. Part of testing requires that you plot the response with some accuracy as the level of NFB is increased, at least at first with a pure resistive load. If you do not know how to plot a response in five minutes, then learn! Suppose that you are able to apply 35 dB of global NFB with phase tweaking networks in place before it is impossible to prevent LF or HF oscillations with any increase in applied NFB. Then suppose you settle for 20 dB of NFB with the phase tweaking networks left in place. You then have a 15 dB margin of stability, but only with a resistive load. 15 dB is a good margin of stability for a tube amp. But the amp trimmed like this may be unstable with a capacitor load, so the amp must have phase tweaking networks set up to reduce the open loop gain and phase shift with a capacitor load in place, usually a 0.22 uF for most power amps, and without any R load. The amplifier schematic at http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm has LF and HF phase tweaking networks to improve the bass stability and the treble stablity. Usually, there is no need to worry about stability between 50 Hz and 5 kHz and its below and above these F that oscillations will always occur. In the above schemo, there is a network using 0.47 uF, 0.047uF, 1M, and 220k which attenuates the LF open loop gain by about 12 dB below 20 Hz, and reduces the ultimate phase shift below 10 Hz, and since applied NFB is dependant on open loop gain, and stability gets worse with more gain, the less gain at LF means there is greater stability. So in effect, although we may have 20 dB of applied NFB at 1 kHz, at 10 Hz there may be only 14 dB, and at 5 Hz about 10 dB, and our amp wants to then be stable. The bass response will still be very much improved overt he open loop response, even with the open loop LF response "artificially" reduced in the way shown in the schematic. In the schemo there is also a 330pF and 3.3k in series from the grid of the LTP input grid to ground. This acts to provide an RL **at HF** of ultimately of 3.3k to the input tube which would otherwise be 75k plus the 220k in parallel as shown. This is because at low RF, the 330 pF becomesa short circuit, so the RL = 3.3k, ( in parallel with 220k and 75k, which have little effect ). Therefore the HF pole caused by the miller effect of the LTP is transfered to a higher F than would be the case if the RC network were not present, and in addition the HF open loop gain is reduced in a frequency range to reduce the applied feedback as F is increased above 20 kHz, where there is little point in having the same amount of applied FB as one wants at 1 kHz. Other phase correction network tricks are employed to increase the margins of stability such as placing just the right size C across the global FB resistor, see the 0.001 uF across the 1 k FB resistor. Also, the FB network is a low impedance one, so the NFB is delivered via the 1k and 100 ohm divider, and the bias R of the input tube is bypoassed by a large value electro, see the 2.2k and 1,000 uF cap At really low F, the 1,000 uF becomes a high impedance, and the gain of V1 ia slightly reduced, but phase shift is reduced. In another schemo at http://www.turneraudio.com.au/htmlwe...00ulabmono.htm there is some RC networks in series across each 1/2 primary of the OPT to give the output tubes a resistive load at HF as the leakage inductance gradually decouples the tubes from the load at the output, but while it does, there is a serious phas shift and tendency to HF instability, and the series RC "zobel" networks add to the margin of stability. When an amp is stabilised to the list above, there will still be some overshoot on square waves with capacitor loads. The sine wave response is best measured with a cap load when output voltage = 1/10 of maximum for clipping with an R load at 1 kHz. Therefore the amp should be able to produce a true non saturated response for the cap load, since the cap load becomes a low impedance at HF, and no amp is able to put an infinite amount of current into a cap at HF in order to keep the output voltage equal to what it is at 1 kHz. If there is up to 6 dB of peaking in the sine wave response at say 40 kHz with a 0.22 uF, and other values of cap won't produce a higher peak, and you have 20 dB of applied NFB, then you have a nicely stable tube amp. Usually the addition of an R across the C load, or a small R in series with the C should improve the response. A test for ability to drive Quad ESL57 will have a 16 ohm R as a load with 2 uF and 1.6 ohms in series, also as a load. many amps will show a peaked response at between 16 and 40 kHz, maybe 6 dB, and 3 dB at 20 kHz, and this is a poor outcome, usually because tyhe OPT is poor, or the builder hasn't done his stabilising networks properly. Despite phase tweaking networks, it should be still possible to get a resistive load response with no peaking outside tha 20 Hz to 20 kHz band, and with -3 dB points at least from 5 Hz to 70 kHz at 1/10 full output voltage, and on the really good amps, between 16 Hz and 65 kHz at the full power clipping level at 1 kHz, even for where RL = 1/2 the rated RL for the amp. The idea of trimming the response is called critical damping, critical because the choice of RC correction components is a narrow one because we aim to have the maximum bandwidth with minimum phase shift with FB applied and still have stability into any kind of reactive load. The amplifier should be considered as a bandpass filter, and one around which we apply NFB, a loop which includes the phase delays in the transfer of signals. There is no theoretical way we can select damping components or exactly calculate the values, since the stray OPT C and leakage L are quantities not able to be quantified accurately enough to allow optimal damping, and its only after we have done a few amps that we get good at the cut and try approach to critical damping. I use the empirical notion that for zobels across the halves of the OPT, each R = 1/2 RLa-a, and C has Z = 1/2 RLa-a at about 100 kHz. For the RC across the output of V1, I use a variable radio tuning gang and a pot to find the least ring with a 5 kHz square wave wehn using 0.22 uF across the output. The C across the FB resistance is adjusted until HF oscillations at RF start, then the value halved. Its somewhat guesswork. But at the end of the procedure, no tube of the the amp should saturate, ie, be forced into grid current or cut off with an R load at full 1 kHz power between 20 Hz and 65 kHz. A pure cap load of 2 uF should produce a peak in the sine wave response at an F above 25 kHz, not below, and the peak no more than +3 dB, so that at 20 kHz, such a C should cause a lift in response of no more than 1.5 dB. Since such C loads are not encountered in real world speakers, and there is usually some series and parallel R also present, and so the response into ESL should be substantially flat if we have got it all right, and in fact the tube amp will often provide a better response than many SS amps. The secret to a good response is an OPT with a wide bandwidth. This means high primary L and low saturation F, low stray capacitance and low leakage inductance. ( and then such transfromers usually contribute a very small amount of HD to the THD of the amp ). This means all out phase correction tricks only affect frequencies well outside the audio band, where there is little energy in the recordings. Many amplifier makers pay only lipservice to the above concerns because its cheaper to make a crummy amp, and its easy to get away with it. There are other statements I have made on NFB and stability in the RAT archives. Patrick Turner. Thanks Patrick for taking the trouble to reply to my question so fully. I shall print your post for careful study. Iain |
#5
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The amplifier must not oscillate at LF or HF or both without any load connected. The amp will not oscillate with any value of inductance connected The amp will not oscillate with any value of capacitance between 10 uF and 0.01 uf connected. The amp will not oscillate when a low level square wave signal signal is used in any of the above situtations. Not in a box. Not with a fox. Not in a house. Not with a mouse. It will not oscillate here or there. It will not oscillate anywhere. I would not eat green eggs and ham. I do not like them, Sam-I-am. Sorry, couldn't resist ;-) |
#6
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Gilbert Bates wrote: The amplifier must not oscillate at LF or HF or both without any load connected. The amp will not oscillate with any value of inductance connected The amp will not oscillate with any value of capacitance between 10 uF and 0.01 uf connected. The amp will not oscillate when a low level square wave signal signal is used in any of the above situtations. Not in a box. Not with a fox. Not in a house. Not with a mouse. It will not oscillate here or there. It will not oscillate anywhere. I would not eat green eggs and ham. I do not like them, Sam-I-am. Sorry, couldn't resist ;-) Its good to see that in some situtations, some ppl have a sense of humerator that works :-) Patrick Turner. |
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