On 01/12/2012 11:45 AM, George Herold wrote:> On Jan 11, 11:55 pm, "Phil Allison"<phi...@tpg.com.au> wrote: >> "Bill Bowden" >> >> I see excessive crossover distortion with this audio amp not shown >> with LTspice. It simulates ok, but the wired circuit produces about a >> 50uS dead time step as the signal crosses the mid point. I tried >> increasing the bias with no help. Both inputs to the op-amp look good >> with no distortion. Any idea what is going on? >> >> ** You need to post a schematic - dickhead. >> >> Totally SMARTARSE of you to post only a Spice list on SEB. >> >> ... Phil > > I was going to say it's a push-pull.... but it's flipped about. > > Here's a pic > http://imageshack.us/photo/my-images/708/crossover.jpg/ > > Does this output have a name? > > George H.You could call it "Output stage that oscillates fiercely with no load, and melts transistors rapidly" ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Audio Amp Crossover Distortion?
Started by ●January 12, 2012
Reply by ●January 13, 20122012-01-13
Reply by ●January 13, 20122012-01-13
On 2012-01-12, George Herold <gherold@teachspin.com> wrote:> I was going to say it's a push-pull.... but it's flipped about. > > Here's a pic > http://imageshack.us/photo/my-images/708/crossover.jpg/This circuit is very poorly designed. I wouldn't even punch this circuit into a machine to simulate it, it is obviously so bad. One thing that stands out right away is that it runs off a single supply and generates reference voltages using unbypassed voltage dividers, which is a naive practice that begs for oscillation. The flipped-about push-pull arrangement seems not to be operating the transistors near cutoff like a conventional push-pull design, which is the main idea behind class AB. I haven't done the calculations, but at a glance, I can tell this thing will have a lot of current flowing through the transistors at quiescence. I wouldn't even bother thinking about cross-over distortion. That can't be a factor here, because it doesn't look like the transistors are not going to be cut off simultaneously. For instance if we look at Q2, you have half the supply voltage dropping through a 560/120 divider providing the base bias. This is likely enough to turn on the transistor, given enough supply voltage and the same thing is happening in the upper half, which is a PNP mirror-image. Resistor R11 provides only very modest feedback to limit the current because it is small. Let's assume a supply of 12, so the midpoint is 6V. 120/560 * 6 gives us 1.3V. This is way above VBE so transistor will turn on. We can estimate maximum collector current using (1.3V - 0.7V)/0.2 Ohms = 3A (!). There isn't nearly enough current-limiting by the tiny emitter resistors.
Reply by ●January 13, 20122012-01-13
On Sat, 14 Jan 2012 03:16:30 +1000, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> On 01/12/2012 11:45 AM, George Herold wrote: >> On Jan 11, 11:55 pm, "Phil Allison"<phi...@tpg.com.au> wrote: >>> "Bill Bowden" >>> >>> I see excessive crossover distortion with this audio amp not shown >>> with LTspice. It simulates ok, but the wired circuit produces about a >>> 50uS dead time step as the signal crosses the mid point. I tried >>> increasing the bias with no help. Both inputs to the op-amp look good >>> with no distortion. Any idea what is going on? >>> >>> ** You need to post a schematic - dickhead. >>> >>> Totally SMARTARSE of you to post only a Spice list on SEB. >>> >>> ... Phil >> >> I was going to say it's a push-pull.... but it's flipped about. >> >> Here's a pic >> http://imageshack.us/photo/my-images/708/crossover.jpg/ >> >> Does this output have a name? >> >> George H. > > You could call it "Output stage that oscillates fiercely with no load, > and melts transistors rapidly" ;) > > Cheers > > Phil Hobbs >OSOF or OSOF-MT ?
Reply by ●January 13, 20122012-01-13
On Jan 12, 9:47=A0pm, George Herold <gher...@teachspin.com> wrote:> On Jan 12, 9:30=A0pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My- > > > > > > Web-Site.com> wrote: > > On Thu, 12 Jan 2012 18:12:59 -0800 (PST), George Herold > > > <gher...@teachspin.com> wrote: > > >On Jan 12, 7:27=A0pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-=My-> > >Web-Site.com> wrote: > > >> On Thu, 12 Jan 2012 08:45:44 -0800 (PST), George Herold > > > >> <gher...@teachspin.com> wrote: > > >> >On Jan 11, 11:55=A0pm, "Phil Allison" <phi...@tpg.com.au> wrote: > > >> >> "Bill Bowden" > > > >> >> I see excessive crossover distortion with this audio amp not show=n> > >> >> with LTspice. It simulates ok, but the wired circuit produces abo=ut a> > >> >> 50uS dead time step as the signal crosses the mid point. I tried > > >> >> increasing the bias with no help. Both inputs to the op-amp look =good> > >> >> with no distortion. Any idea what is going on? > > > >> >> ** You need to post a schematic =A0- =A0dickhead. > > > >> >> Totally =A0SMARTARSE of you to post only a Spice list on SEB. > > > >> >> ... =A0Phil > > > >> >I was going to say it's a push-pull.... but it's flipped about. > > > >> >Here's a pic > > >> >http://imageshack.us/photo/my-images/708/crossover.jpg/ > > > >> >Does this output have a name? > > > >> >George H. > > > >> Unstable ?:-) > > > > What's the 10k to the noninverting input about? =A0There's an inversi=on> > >somehwere else in the circuit? > > > >Bill, Since it's a single sided power supply a class A would be > > >easy... And less parts. > > > >George H. > > > I like "sliding" class-A... 1/2 the normal standby power. > > Hmm, never heard of sliding class-A, a JT invention? > > So I'll guess some outer control loop that senses the voltage and > adjusts the bias. =A0A second pass element? > > But it's gotta be fast. =A0Is it load specific? > > George H.OK that was a silly idea. (Two pass elements is not going to save any power.) (It isn't any fun to google before posting... :^) So a sliding class A is also called class H? A variable (switch mode) power supply rail run a volt or two above the linear pass element. That's a pretty cool! So how fast can you oscillate a SMPS and how fast can you swing it's output voltage? George H.> > > > > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 ...Jim Thompson> > -- > > | James E.Thompson, CTO =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0| =A0 =A0mens =A0 =A0 |> > | Analog Innovations, Inc. =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ==A0 | =A0 =A0 et =A0 =A0 =A0|> > | Analog/Mixed-Signal ASIC's and Discrete Systems =A0| =A0 =A0manus =A0==A0|> > | Phoenix, Arizona =A085048 =A0 =A0Skype: Contacts Only =A0| =A0 =A0 ==A0 =A0 =A0 =A0 |> > | Voice:(480)460-2350 =A0Fax: Available upon request | =A0Brass Rat =A0=|> > | E-mail Icon athttp://www.analog-innovations.com|=A0 =A01962 =A0 =A0 | > > > I love to cook with wine. =A0 =A0 Sometimes I even put it in the food.-=Hide quoted text -> > > - Show quoted text -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -
Reply by ●January 13, 20122012-01-13
On Jan 13, 10:28 am, Kaz Kylheku <k...@kylheku.com> wrote:> On 2012-01-12, George Herold <gher...@teachspin.com> wrote: > > > I was going to say it's a push-pull.... but it's flipped about. > > > Here's a pic > >http://imageshack.us/photo/my-images/708/crossover.jpg/ > > This circuit is very poorly designed. > > I wouldn't even punch this circuit into a machine to simulate it, it is > obviously so bad. > > One thing that stands out right away is that it runs off a single supply and > generates reference voltages using unbypassed voltage dividers, which is > a naive practice that begs for oscillation. >The gain is set with the two 2K resistors and the 10K for a gain of 11. Bypass caps would upset the gain figure.> The flipped-about push-pull arrangement seems not to be operating > the transistors near cutoff like a conventional push-pull design, > which is the main idea behind class AB. > > I haven't done the calculations, but at a glance, I can tell this thing will > have a lot of current flowing through the transistors at quiescence. > > I wouldn't even bother thinking about cross-over distortion. That can't be a > factor here, because it doesn't look like the transistors are not going to be > cut off simultaneously. >There is cross-over distortion which doesn't seem to be related to the bias current since I ran the supply voltage up to increase the bias current with no effect on distortion.> For instance if we look at Q2, you have half the supply voltage > dropping through a 560/120 divider providing the base bias. This is likely > enough to turn on the transistor, given enough supply voltage and the same > thing is happening in the upper half, which is a PNP mirror-image. Resistor R11 > provides only very modest feedback to limit the current because it is small. > > Let's assume a supply of 12, so the midpoint is 6V. 120/560 * 6 gives us > 1.3V. This is way above VBE so transistor will turn on. We can estimate > maximum collector current using (1.3V - 0.7V)/0.2 Ohms = 3A (!). There isn't > nearly enough current-limiting by the tiny emitter resistors.The tiny resistors are only there to measure the bias current. Thermal stability is another problem. -Bill
Reply by ●January 13, 20122012-01-13
On 2012-01-13, Phil Allison <phil_a@tpg.com.au> wrote:> ** FFS - class A push-pull output stages are very common and technically > elegant.Indeed, that looks like what the circuit is shooting for. I found a this schematic which has a similar configuration of transistors as the OP's circuit, also single supply, but seems better: http://www.circuitstoday.com/wp-content/uploads/2011/03/class-A-headphone-amplifier-circuit.png Here, a pair of diodes and resistor R5 are used to set up the collector current through the transistors, instead of a resistive voltage divider bias. Since diodes are used, there is independence from fluctuations in the rails. R5 at 39 ohms is much larger than the 0.2 ohm emitter resistors in the circuit discussed in this thread, so the quiescent current is kept to about 18 mA. (Approximation: 1.4V diode stack minus 0.7 forward voltage, divided by 39 ohms). The OP's circuit could be improved by adopting some of these ideas, and also with the addition of some bypass caps on those voltage dividers around the op amp.
Reply by ●January 13, 20122012-01-13
On Jan 13, 12:16=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> On 01/12/2012 11:45 AM, George Herold wrote: > > > > > > > On Jan 11, 11:55 pm, "Phil Allison"<phi...@tpg.com.au> =A0wrote: > >> "Bill Bowden" > > >> I see excessive crossover distortion with this audio amp not shown > >> with LTspice. It simulates ok, but the wired circuit produces about a > >> 50uS dead time step as the signal crosses the mid point. I tried > >> increasing the bias with no help. Both inputs to the op-amp look good > >> with no distortion. Any idea what is going on? > > >> ** You need to post a schematic =A0- =A0dickhead. > > >> Totally =A0SMARTARSE of you to post only a Spice list on SEB. > > >> ... =A0Phil > > > I was going to say it's a push-pull.... but it's flipped about. > > > Here's a pic > >http://imageshack.us/photo/my-images/708/crossover.jpg/ > > > Does this output have a name? > > > George H. > > You could call it "Output stage that oscillates fiercely with no load, > and melts transistors rapidly" ;) > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > 845-480-2058 > > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted tex=t -> > - Show quoted text -Grin... maybe a good heater then. Bill, I hope you'll take this as good natured kidding. We all make mistakes. (I can only learn by making mistakes, so the faster I make 'em the more I know.) When I've got something new to design, I like to think of a bunch of ways to solve the problem. Then if you try a hair-brained one and it fails, you've got something to fall back on. George H.
Reply by ●January 13, 20122012-01-13
On Fri, 13 Jan 2012 15:36:20 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Jan 12, 9:47�pm, George Herold <gher...@teachspin.com> wrote: >> On Jan 12, 9:30�pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My- >> >> >> >> >> >> Web-Site.com> wrote: >> > On Thu, 12 Jan 2012 18:12:59 -0800 (PST), George Herold >> >> > <gher...@teachspin.com> wrote: >> > >On Jan 12, 7:27�pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My- >> > >Web-Site.com> wrote: >> > >> On Thu, 12 Jan 2012 08:45:44 -0800 (PST), George Herold >> >> > >> <gher...@teachspin.com> wrote: >> > >> >On Jan 11, 11:55�pm, "Phil Allison" <phi...@tpg.com.au> wrote: >> > >> >> "Bill Bowden" >> >> > >> >> I see excessive crossover distortion with this audio amp not shown >> > >> >> with LTspice. It simulates ok, but the wired circuit produces about a >> > >> >> 50uS dead time step as the signal crosses the mid point. I tried >> > >> >> increasing the bias with no help. Both inputs to the op-amp look good >> > >> >> with no distortion. Any idea what is going on? >> >> > >> >> ** You need to post a schematic �- �dickhead. >> >> > >> >> Totally �SMARTARSE of you to post only a Spice list on SEB. >> >> > >> >> ... �Phil >> >> > >> >I was going to say it's a push-pull.... but it's flipped about. >> >> > >> >Here's a pic >> > >> >http://imageshack.us/photo/my-images/708/crossover.jpg/ >> >> > >> >Does this output have a name? >> >> > >> >George H. >> >> > >> Unstable ?:-) >> >> > > What's the 10k to the noninverting input about? �There's an inversion >> > >somehwere else in the circuit? >> >> > >Bill, Since it's a single sided power supply a class A would be >> > >easy... And less parts. >> >> > >George H. >> >> > I like "sliding" class-A... 1/2 the normal standby power. >> >> Hmm, never heard of sliding class-A, a JT invention? >> >> So I'll guess some outer control loop that senses the voltage and >> adjusts the bias. �A second pass element? >> >> But it's gotta be fast. �Is it load specific? >> >> George H. > >OK that was a silly idea. (Two pass elements is not going to save any >power.) > >(It isn't any fun to google before posting... :^) >So a sliding class A is also called class H? A variable (switch mode) >power supply rail run a volt or two above the linear pass element. > >That's a pretty cool! > >So how fast can you oscillate a SMPS and how fast can you swing it's >output voltage? > >George H. > >>[snip] Google or Wiki are not always reliable sources of information. Check out... http://www.analog-innovations.com/SED/HearingAidSlidingClassA.pdf as one way I've done it. Normal class-A has one active signal device loaded with resistance or a fixed current mirror of value I. Say your supplies are +V and -V, load to ground. So the peak power that can be driven into the load is I*V. Quiescent power 2*I*V. Now imagine a current mirror load whose value decreases when the active signal device increases its current (and vice versa). Say the quiescent condition is the same as above. But now you can drive peak power of 2*I*V, twice what the normal class-A can do. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●January 13, 20122012-01-13
On Fri, 13 Jan 2012 23:54:46 +0000 (UTC), Kaz Kylheku <kaz@kylheku.com> wrote:>On 2012-01-13, Phil Allison <phil_a@tpg.com.au> wrote: >> ** FFS - class A push-pull output stages are very common and technically >> elegant. > >Indeed, that looks like what the circuit is shooting for. > >I found a this schematic which has a similar configuration of transistors >as the OP's circuit, also single supply, but seems better: > >http://www.circuitstoday.com/wp-content/uploads/2011/03/class-A-headphone-amplifier-circuit.png > >Here, a pair of diodes and resistor R5 are used to set up the collector current >through the transistors, instead of a resistive voltage divider bias. Since >diodes are used, there is independence from fluctuations in the rails. R5 at >39 ohms is much larger than the 0.2 ohm emitter resistors in the circuit >discussed in this thread, so the quiescent current is kept to about 18 mA. >(Approximation: 1.4V diode stack minus 0.7 forward voltage, divided by 39 >ohms). > >The OP's circuit could be improved by adopting some of these ideas, and also >with the addition of some bypass caps on those voltage dividers around the op >amp.While you are amusing yourself, what is it that determines the output Q-point... quiescent voltage at the + node of C3 ?:-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●January 13, 20122012-01-13
On 1/13/2012 7:13 PM, Jim Thompson wrote:> On Fri, 13 Jan 2012 23:54:46 +0000 (UTC), Kaz Kylheku > <kaz@kylheku.com> wrote: > >> On 2012-01-13, Phil Allison <phil_a@tpg.com.au> wrote: >>> ** FFS - class A push-pull output stages are very common and technically >>> elegant. >> >> Indeed, that looks like what the circuit is shooting for. >> >> I found a this schematic which has a similar configuration of transistors >> as the OP's circuit, also single supply, but seems better: >> >> http://www.circuitstoday.com/wp-content/uploads/2011/03/class-A-headphone-amplifier-circuit.png >> >> Here, a pair of diodes and resistor R5 are used to set up the collector current >> through the transistors, instead of a resistive voltage divider bias. Since >> diodes are used, there is independence from fluctuations in the rails. R5 at >> 39 ohms is much larger than the 0.2 ohm emitter resistors in the circuit >> discussed in this thread, so the quiescent current is kept to about 18 mA. >> (Approximation: 1.4V diode stack minus 0.7 forward voltage, divided by 39 >> ohms). >> >> The OP's circuit could be improved by adopting some of these ideas, and also >> with the addition of some bypass caps on those voltage dividers around the op >> amp. > > While you are amusing yourself, what is it that determines the output > Q-point... quiescent voltage at the + node of C3 ?:-) > > ...Jim ThompsonLooks like R3 should go to the output node. It doesn't make any sense being where it is now. Then DC feedback would make the output sit at about 5 volts, which would be about right for a symmetric swing given the compliance of the upper current source. I also don't know why R3 is connected where it is, it would make more sense connected to the top of R1. I think you'd then have a pretty conventional, but workable amplifier.
