I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. Any ideas? -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
High Current Drive Output Stage
Started by ●February 15, 2023
Reply by ●February 15, 20232023-02-15
On 15/02/23 16:47, Ricky wrote:> I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > Any ideas? >Google" White's Follower", or find it in AoE. Clifford Heath
Reply by ●February 15, 20232023-02-15
On Wednesday, February 15, 2023 at 12:47:09 a.m. UTC-5, Ricky wrote:> I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > Any ideas? > > -- > > Rick C. > > - Get 1,000 miles of free Supercharging > - Tesla referral code - https://ts.la/richard11209These Rail-Rail CMOS OA's are harder to find replacements than sources of stock in China. https://www.digipart.com/part/LM8272 Tony EE since 1975
Reply by ●February 15, 20232023-02-15
onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky:> I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > Any ideas?I don't see how you can possible get anywhere near as small as what you have unless you find another dual opamp how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ?
Reply by ●February 16, 20232023-02-16
On Wednesday, February 15, 2023 at 5:47:32 PM UTC-4, Lasse Langwadt Christensen wrote:> onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky: > > I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > > > Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > > > I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > > > I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > > > I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > > > Any ideas? > I don't see how you can possible get anywhere near as small as what you have unless you find another dual opampThe two channels (think stereo) of differential output would map well to audio circuits, but I can't find a good chip to work with a 12V supply.> how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ?Looks good in some respects, but it is also not rated for 12V power. At ±5V it will swing ±4V, but that I can get from other parts... I think. Most of them are not well characterize in this regard. For whatever reason, they often list a minimum short circuit current. The absolute maximum power is 12.6V, so maybe it would run from 12V power. Funny, they didn't spec it, if it is intended for that. Thanks for the part number. -- Rick C. + Get 1,000 miles of free Supercharging + Tesla referral code - https://ts.la/richard11209
Reply by ●February 16, 20232023-02-16
On Thursday, 16 February 2023 at 08:26:45 UTC, Ricky wrote:> On Wednesday, February 15, 2023 at 5:47:32 PM UTC-4, Lasse Langwadt Christensen wrote: > > onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky: > > > I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > > > > > Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > > > > > I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > > > > > I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > > > > > I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > > > > > Any ideas? > > I don't see how you can possible get anywhere near as small as what you have unless you find another dual opamp > The two channels (think stereo) of differential output would map well to audio circuits, but I can't find a good chip to work with a 12V supply. > > how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ? > Looks good in some respects, but it is also not rated for 12V power. At ±5V it will swing ±4V, but that I can get from other parts... I think. Most of them are not well characterize in this regard. For whatever reason, they often list a minimum short circuit current. > > The absolute maximum power is 12.6V, so maybe it would run from 12V power. Funny, they didn't spec it, if it is intended for that. > > Thanks for the part number.Have you considered the OPA1656? John
Reply by ●February 16, 20232023-02-16
On 16/02/2023 08:26, Ricky wrote:> On Wednesday, February 15, 2023 at 5:47:32 PM UTC-4, Lasse Langwadt Christensen wrote: >> onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky: >>> I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. >>> >>> Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). >>> >>> I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. >>> >>> I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. >>> >>> I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. >>> >>> Any ideas? >> I don't see how you can possible get anywhere near as small as what you have unless you find another dual opamp > > The two channels (think stereo) of differential output would map well to audio circuits, but I can't find a good chip to work with a 12V supply. > > >> how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ? > > Looks good in some respects, but it is also not rated for 12V power. At ±5V it will swing ±4V, but that I can get from other parts... I think. Most of them are not well characterize in this regard. For whatever reason, they often list a minimum short circuit current. > > The absolute maximum power is 12.6V, so maybe it would run from 12V power. Funny, they didn't spec it, if it is intended for that. > > Thanks for the part number. >L272 and TCA0372 are well known dual power op amps. Maybe not HiFi audio but should be ok for telecoms audio. piglet
Reply by ●February 16, 20232023-02-16
On Thursday, February 16, 2023 at 7:23:34 AM UTC-5, John Walliker wrote:> On Thursday, 16 February 2023 at 08:26:45 UTC, Ricky wrote: > > On Wednesday, February 15, 2023 at 5:47:32 PM UTC-4, Lasse Langwadt Christensen wrote: > > > onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky: > > > > I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > > > > > > > Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > > > > > > > I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > > > > > > > I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > > > > > > > I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > > > > > > > Any ideas? > > > I don't see how you can possible get anywhere near as small as what you have unless you find another dual opamp > > The two channels (think stereo) of differential output would map well to audio circuits, but I can't find a good chip to work with a 12V supply. > > > how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ? > > Looks good in some respects, but it is also not rated for 12V power. At ±5V it will swing ±4V, but that I can get from other parts... I think. Most of them are not well characterize in this regard. For whatever reason, they often list a minimum short circuit current. > > > > The absolute maximum power is 12.6V, so maybe it would run from 12V power. Funny, they didn't spec it, if it is intended for that. > > > > Thanks for the part number. > Have you considered the OPA1656?Yes, that part doesn't actually drive very much current at the output voltage I need. I'd like to get 80 mA from the output which drops almost 3V on the low drive and basically doesn't make it on the high drive. The typical graph ends at -6V off the rail (fig 6-32). When they talk about a temperature in data like this, I'm assuming they mean die temperature, rather than ambient, right? So I'm working with the 85 °C curves. -- Rick C. -- Get 1,000 miles of free Supercharging -- Tesla referral code - https://ts.la/richard11209
Reply by ●February 16, 20232023-02-16
On Thursday, February 16, 2023 at 9:04:24 AM UTC-5, piglet wrote:> On 16/02/2023 08:26, Ricky wrote: > > On Wednesday, February 15, 2023 at 5:47:32 PM UTC-4, Lasse Langwadt Christensen wrote: > >> onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky: > >>> I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > >>> > >>> Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > >>> > >>> I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > >>> > >>> I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > >>> > >>> I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > >>> > >>> Any ideas? > >> I don't see how you can possible get anywhere near as small as what you have unless you find another dual opamp > > > > The two channels (think stereo) of differential output would map well to audio circuits, but I can't find a good chip to work with a 12V supply. > > > > > >> how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ? > > > > Looks good in some respects, but it is also not rated for 12V power. At ±5V it will swing ±4V, but that I can get from other parts... I think. Most of them are not well characterize in this regard. For whatever reason, they often list a minimum short circuit current. > > > > The absolute maximum power is 12.6V, so maybe it would run from 12V power. Funny, they didn't spec it, if it is intended for that. > > > > Thanks for the part number. > > > L272 and TCA0372 are well known dual power op amps. Maybe not HiFi audio > but should be ok for telecoms audio.Thanks. The L272 has the drive capability for sure, but it's in far too large a package. Same for the TCA0372. These parts seem targeted to motor drive. -- Rick C. -+ Get 1,000 miles of free Supercharging -+ Tesla referral code - https://ts.la/richard11209
Reply by ●February 16, 20232023-02-16
On Thursday, 16 February 2023 at 18:08:05 UTC, Ricky wrote:> On Thursday, February 16, 2023 at 9:04:24 AM UTC-5, piglet wrote: > > On 16/02/2023 08:26, Ricky wrote: > > > On Wednesday, February 15, 2023 at 5:47:32 PM UTC-4, Lasse Langwadt Christensen wrote: > > >> onsdag den 15. februar 2023 kl. 06.47.09 UTC+1 skrev Ricky: > > >>> I have an audio amplifier circuit that uses an op amp I can't buy anymore because of the semiconductor shortage. It's not extremely special, but it drives highly capacitive loads (like a few hundred feet of cable) and will drive a 62.1 ohm load with 75 mA. I'd actually like a higher drive, but this was the best I could find in 2008 when I designed it and I've found nothing better since. > > >>> > > >>> Two main requirements are low power consumption (the LM8272 is around 1 mA per amp) and small size (8 pin MSOP dual). > > >>> > > >>> I'm wondering if I might replace these with a push-pull transistor output stage. The outputs are differential, so I would need an inverter to drive them. The current circuit uses positive feedback to allow a 12.1 ohm resistor to emulate a 50 ohm output impedance. In the new design this will have an analog switch to change between 300 ohm and 50 ohm. I can't see a useful way to do this same trick with a push-pull output. So this idea is not looking so great. > > >>> > > >>> I looked at audio drivers for headphones and speakers. The lower power devices all run on 5V power, while I need 12V power. The devices that use 12V power are pretty much all class D or similar and require output filtering which uses too much space. > > >>> > > >>> I've even looked for amps with differential outputs, but nothing workable. Price is not a huge issue, but some of these parts are $10 a pop and I need two channels of differential drivers. $40 is a bit steep. > > >>> > > >>> Any ideas? > > >> I don't see how you can possible get anywhere near as small as what you have unless you find another dual opamp > > > > > > The two channels (think stereo) of differential output would map well to audio circuits, but I can't find a good chip to work with a 12V supply. > > > > > > > > >> how about https://www.digikey.dk/en/products/detail/analog-devices-inc/AD8012ARMZ/620444 ? > > > > > > Looks good in some respects, but it is also not rated for 12V power. At ±5V it will swing ±4V, but that I can get from other parts... I think. Most of them are not well characterize in this regard. For whatever reason, they often list a minimum short circuit current. > > > > > > The absolute maximum power is 12.6V, so maybe it would run from 12V power. Funny, they didn't spec it, if it is intended for that. > > > > > > Thanks for the part number. > > > > > L272 and TCA0372 are well known dual power op amps. Maybe not HiFi audio > > but should be ok for telecoms audio. > Thanks. The L272 has the drive capability for sure, but it's in far too large a package. Same for the TCA0372. These parts seem targeted to motor drive.Another one that might be worth a look is the ALM2403-Q1. It isn't targeted at audio, but it does look interesting. The TPA6120A2DWP is a headphone driver which might do what you want. However, the data sheet doesn't make it easy to be sure. John
