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detonator
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Just curious, how does it sound in comparison to any other ordinary opamp?
Discrete Dip 8 Opamp
- Thread starter abechap024
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abechap024
Well-known member
Well I'm in the process of rechippping the mix buses in my console. But I tried to do a subjective test as possible (having someone else flip the channels on off while I couldn't see, switching the discrete opamp to different mix buses, etc)
It became quite apparent the the discrete opamp sounds less thin than the IC chips. More 3d and real would be how I would describe it.
I was using opa227s as comparison cause thats what i have in right now and not amazing but dont sound bad by any means. I did RMAA and THD distortion was as Low if not lower than than the OPA227s (and they have low distortion!)
Obviously it would be better once we get some more opinions in here, but the circuit is pretty much unchanged from whats been floating around, so I'm sure electrons don't care what package they are in....
It became quite apparent the the discrete opamp sounds less thin than the IC chips. More 3d and real would be how I would describe it.
I was using opa227s as comparison cause thats what i have in right now and not amazing but dont sound bad by any means. I did RMAA and THD distortion was as Low if not lower than than the OPA227s (and they have low distortion!)
Obviously it would be better once we get some more opinions in here, but the circuit is pretty much unchanged from whats been floating around, so I'm sure electrons don't care what package they are in....
abechap024 said:
I'm afraid I can't map those subjective characterizations to circuit performance parameters. The sound of an opamp should be dominated by the negative feedback wrapped around it, until it runs out of loop gain or introduces some other error due to deviation from the proscribed transfer function.
Perhaps you could indulge this old man and perform one simple (still subjective) test for me. With two otherwise identical bus audio paths, one with and one without your new opamp. Feed both with the exact same audio signal (the more difficult the better, lots o'cymbal crashes and HF energy), then null (subtract) the two outputs from each other, by inverting one and summing again in another bus.
You can confirm the quality (depth) of your null by first nulling out two identical buses. This will establish your resolution floor. Then null the stock bus vs your new opamp. The null product or remainder will consist of either what is added or subtracted by your new opamp. The harder part is telling which it is, but the null test will help train your ears exactly "what" is different and you should listen for. Then individual, single bus listening should allow you to identify if it is information "added to" or "subtracted from" the original by the new opamp.
[edit] there are not just those two choices (add or subtract). The difference could be flat vs, subtract, or extra vs, flat. Move conventional bench tests help identify the what. [/edit]
Inquiring minds want to know.
JR
Have you checked the stability and sensitivity to the physical build? What is its electrostatic sensitivity?
As you move the top vertical boards distance/angle between each other the capacitance between components changes. This might introduce higher frequency issues with capacitance cross coupling and also being sensitive to physical build. If you insert a insulator between the boards that has a dielectric constant to think about(I was thinking if someone built this and used plastic etc between boards so not to short together). Each build might have a different gain bandwidth product.
IIRC C.J. posted about how the PCB of a solid state preamp had a cap designed in between traces.
Sometimes picofarads matter.
As you move the top vertical boards distance/angle between each other the capacitance between components changes. This might introduce higher frequency issues with capacitance cross coupling and also being sensitive to physical build. If you insert a insulator between the boards that has a dielectric constant to think about(I was thinking if someone built this and used plastic etc between boards so not to short together). Each build might have a different gain bandwidth product.
IIRC C.J. posted about how the PCB of a solid state preamp had a cap designed in between traces.
Sometimes picofarads matter.
abechap024
Well-known member
JR I will have to do that. Thanks for the testing ideas. If there is anything I've found its that listening tests are better left to people with the money and time to do so! so subjective! So reversing the polarity of one channel does sound like a more concrete way to analyze whats going on.
Gus - yes I was worried about how the different lengths and traces would interact. I think keeping input/output sections separated has hopefully helped. I need to dig a little deeper and do some more in depth tests. That said there are many many more people on this forum that are more experienced with the whole testing process. I have looked at it with my scope and so far so good... but I'm building a bunch if someone that has experience really putting an opamp through the rigors and has all the correct tools to do so I would be really grateful to send one and see how it preforms.
Gus - yes I was worried about how the different lengths and traces would interact. I think keeping input/output sections separated has hopefully helped. I need to dig a little deeper and do some more in depth tests. That said there are many many more people on this forum that are more experienced with the whole testing process. I have looked at it with my scope and so far so good... but I'm building a bunch if someone that has experience really putting an opamp through the rigors and has all the correct tools to do so I would be really grateful to send one and see how it preforms.
abechap024
Well-known member
Okay, I have a question... one of my opamps works great for a little bit, then after it heats up, it squeals off into distortion and stops passing signal.... let it cool down and it works for another 15-20 minutes and does it again.
Thoughts?
Thanks,
Abe
I didn't match the HFE on the input stage, this couldn't be the culprit could it?
Thoughts?
Thanks,
Abe
I didn't match the HFE on the input stage, this couldn't be the culprit could it?
abechap024 said:
There is probably class a/b biasing in output stage. If class A current is too high and/or inadequate degeneration it could get into thermal runaway.. If you copied some other design, check for correct values and parts, or look for a bad solder connection.
JR
abechap024
Well-known member
The schem I used for the design calls for 5R6 on output and all I have while I wait for my correct parts is 6R8, which I used on the output transistors. Do you think that would be enough to cause it?
EDIT: Changed to 5r6 and it seemed to help. But still it eventually keeps doing it..
Any ideas?
EDIT: Changed to 5r6 and it seemed to help. But still it eventually keeps doing it..
Any ideas?
Is D3 actually 2 diodes and not one like the schematic symbol? (one diode should run cool and perhaps exhibit crossover distortion, so I will guess there are two diodes there).
Are the D3 diodes thermally coupled to the output transistors? Are they physically close?
Using 6.8 instead of 5.6 should help rather than hurt over heating.
JR
Are the D3 diodes thermally coupled to the output transistors? Are they physically close?
Using 6.8 instead of 5.6 should help rather than hurt over heating.
JR
abechap024
Well-known member
Yes I have D3 is 2 diodes in series
The3 diodes are close to the output transistors, I super glued d1 and d2 to the respective transistors (q9,q8). But Should I be trying to thermally couple d3 to one of the transistors?
The3 diodes are close to the output transistors, I super glued d1 and d2 to the respective transistors (q9,q8). But Should I be trying to thermally couple d3 to one of the transistors?
D1 and D2 are only establishing the peak current limiting at roughly one diode drop divided by 5.6 or 6.8 ohms, so thermally not of consequence. D3a and D3b, are generating the two diode drop, base drive voltage to bias the output devices just barely on, so if the output devices heat up and D3 doesn't, the shrinking Vbe (which falls with higher temperature) means more voltage across the the 5.6(6.8) ohm emitter resistors and more class A current. This is positive feedback so more current, makes devices hotter, which makes more current, etc..
That said I still expect this to stabilize at some higher current, not melt, but yes, if any, D3 should be the diodes thermally coupled to the outputs, not d1 and d2.
JR
That said I still expect this to stabilize at some higher current, not melt, but yes, if any, D3 should be the diodes thermally coupled to the outputs, not d1 and d2.
JR
abechap024
Well-known member
Ahh okay that makes more sense. It also reminds of of a thread we were talking about this too, except with a power amp. Ever try to implement your transistor bais in an opamp? That might be kinda cool 8) Anyway back to the work bench, Thanks!
abechap024 said:
I haven't found any need to design a discrete opamp since about the '80s, when the noise and performance of integrated offerings eclipsed DOA (IMO). I did roll some custom DOA for noise sensitive sum bus amps, back in the day.
Now, I don't believe I could design a DOA that comes close to matching the performance of what I can buy off the shelf, even if I tried. Regarding my biasing scheme for opamp use, I wouldn't be surprised if they were already doing something even more complicated. Inside ICs small devices are very cheap, and device matching is almost free, so a different design palette is available to chip designers.
Back in the good old days they used to publish full schematics of the circuitry inside ICs, and it made good reading. These days not as often or as detailed.
JR
abechap024
Well-known member
Hi Dave,
Yes the wides you will either need a very skinny bent tipped soldering iron, or some small wire ( I recommend the small wire) and then solder the wire in first then the outsides.
I will be finishing up a much needed build manual here shortly.
Cheers,
Abe
Yes the wides you will either need a very skinny bent tipped soldering iron, or some small wire ( I recommend the small wire) and then solder the wire in first then the outsides.
I will be finishing up a much needed build manual here shortly.
Cheers,
Abe
abechap024
Well-known member
Ah okay good. Got them working better. Still need to experiment with them some more. But I had to switch some cap values around. Also you can mount the vertical PCBs kinda in an angle like a "V" for the wides without any effect on performance (that I can tell)
I will outline it more in the build manual.
I got them running for long periods now. The first post of me saying that they work, was before I did some long term testing and realized they heat up and then stop working.
I believe it was from having the output transistors so close to each-other, maybe some oscillations making it heat up, and thermal run-away. But I think its sorted now. The one that is currently in, has a cap value changed and bent the transistors out to allow for more air flow and the D3 diodes are close to the output transistors. I'm going to see if that is really necessary though, As far as I can tell the originals and other clones don't seem to have the diodes thermally attached to the output transistors.
I will outline it more in the build manual.
I got them running for long periods now. The first post of me saying that they work, was before I did some long term testing and realized they heat up and then stop working.
I believe it was from having the output transistors so close to each-other, maybe some oscillations making it heat up, and thermal run-away. But I think its sorted now. The one that is currently in, has a cap value changed and bent the transistors out to allow for more air flow and the D3 diodes are close to the output transistors. I'm going to see if that is really necessary though, As far as I can tell the originals and other clones don't seem to have the diodes thermally attached to the output transistors.
jensenmann
Well-known member
I´ve built a bunch of clones and I did stick the diodes through the mounting holes of the output transistors and finally smeared a load of thermal compound between them to have them connected thermally. I dunno if that`s possible with your layout. On the original PCB that was already an adventurous lot of 3D leg-bending.
abechap024
Well-known member
okay good to know! Its easy on my layout. They are very close 
cheers
cheers
abechap024
Well-known member
okay for people that want to get started building here are the need to know stuffs:
www.acsoundstudio.com/MANUALS/page_1_jpeg.jpg
www.acsoundstudio.com/MANUALS/page_2_jpeg.jpg
www.acsoundstudio.com/DIY/discrete_opamp/SKEM_VERSION_A_smaller.jpg
Also r5 i use 3.9k
I've found works better with smaller value at c3 (15pf)
r16 possibly jumper-ed.
c2 is also 47pf, just cause thats all I have around at the moment...
Also be sure to clean all rosin from boards! Prior to placing them vertically. Haha the 10 megaohm input resistance don't like no rosin...
www.acsoundstudio.com/MANUALS/page_1_jpeg.jpg
www.acsoundstudio.com/MANUALS/page_2_jpeg.jpg
www.acsoundstudio.com/DIY/discrete_opamp/SKEM_VERSION_A_smaller.jpg
Also r5 i use 3.9k
I've found works better with smaller value at c3 (15pf)
r16 possibly jumper-ed.
c2 is also 47pf, just cause thats all I have around at the moment...
Also be sure to clean all rosin from boards! Prior to placing them vertically. Haha the 10 megaohm input resistance don't like no rosin...
There are several factors that influence the thermal stability, or sensitivity to thermal run-away in that output stage bias scheme.
The easiest to tweak is the emitter "degeneration" resistors. These 5.6 or 6.8 Ohm resistors are supposed to look large compared to the dynamic resistance of the Vbe junction as it changes with temperature and reduce the current increase with rising temperature. It's been years since I looked at the actual equations for this, and it is hard to apply completely because you need to know the actual thermal resistance from junction to air, for the output devices, but suffice it to say making the emitter degeneration resistors larger (say 10 Ohms) will reduce the likelihood of thermal run away. Obviously, perfect thermal tracking between the transistors and bias diodes will prevent the class A bias changing from initial conditions as they all heat up the same.
The trade off in making the degeneration resistors larger is lower class A current for a given DC operating point, and slightly higher losses when driving low impedance loads. Note: things like using different transistor packages that impact the thermal resistance to air and can impact thermal stability and different diodes/devices will have slightly different nominal Vbe/forward voltage.
In power amp output stages, that deal in much smaller value degeneration resistors (.1-.33 Ohm), the matching of the diode drops is more critical. At Peavey we had a dual diode in the system that was specified to meet a voltage @ current spec, and power devices were similarly graded for Vbe @ current.. A lot of work, but worth it to avoid factory trims that humans had to adjust correctly.
If you continue to see thermal problems I would look at increasing the value of the emitter degeneration resistors. If this starves the output stage for class A current you can tweak this some by increasing the current through the D3 diodes. Note: the fact that this is called D3 instead of Dx and Dy suggests to me that some original design used a proper dual diode. Note: Dedicated dual diodes are not always made by stacking two simple diodes. While this doesn't matter a lot, it looks like the design was dialed in for a dual. If the output stage isn't clean with larger degeneration resistors, you can add a small resistor in series with D3, but this will increase the nominal class A current, so DO NOT add this R without first increasing the emitter degeneration value or you are asking for smoke..
To dial in the output stage class A current, you can measure it by looking at the DC voltage drop across the emitter resistors, but the real Goldilocks test for too hot, too cold, or just right class A current is to look at HF crossover distortion. If you don't have a distortion analyzer handy you can see severe crossover distortion with a scope. Using a clean sine wave source at 20kHz look closely at the transition region between + to - voltage and back again. This will be most visible for modest output voltages driving a healthy load (600 Ohm?). With a distortion analyzer I set the class A current so the crossover distortion components were well below the other distortion products. With just a scope, the best you can do is see it go away, and maybe add a little margin.
I would be tempted to use all transistors and diodes from the same batch (so they will be from the same production lots and similar), and dial in the operating point, by first starving the output stage until the crossover distortion is visible (or measurable), then increase that class A current until it cleans up nicely. If you can't clean up the output stage "and" still have thermal stability, it's time to tweak the design. Note: You need to tweak the bias so it is clean at minimum rated supply (+/-12V), while still thermally stable at max supply (+/-20v?). The nominal current in D3 increases with this rail voltage so if it is clean at +/-12v it will will be cleaner (and hotter) at +/-20v.
I suspect this design and values were dialed in for slightly different parts and different layout.. In such designs the details can matter.
Good luck and have fun.
JR
The easiest to tweak is the emitter "degeneration" resistors. These 5.6 or 6.8 Ohm resistors are supposed to look large compared to the dynamic resistance of the Vbe junction as it changes with temperature and reduce the current increase with rising temperature. It's been years since I looked at the actual equations for this, and it is hard to apply completely because you need to know the actual thermal resistance from junction to air, for the output devices, but suffice it to say making the emitter degeneration resistors larger (say 10 Ohms) will reduce the likelihood of thermal run away. Obviously, perfect thermal tracking between the transistors and bias diodes will prevent the class A bias changing from initial conditions as they all heat up the same.
The trade off in making the degeneration resistors larger is lower class A current for a given DC operating point, and slightly higher losses when driving low impedance loads. Note: things like using different transistor packages that impact the thermal resistance to air and can impact thermal stability and different diodes/devices will have slightly different nominal Vbe/forward voltage.
In power amp output stages, that deal in much smaller value degeneration resistors (.1-.33 Ohm), the matching of the diode drops is more critical. At Peavey we had a dual diode in the system that was specified to meet a voltage @ current spec, and power devices were similarly graded for Vbe @ current.. A lot of work, but worth it to avoid factory trims that humans had to adjust correctly.
If you continue to see thermal problems I would look at increasing the value of the emitter degeneration resistors. If this starves the output stage for class A current you can tweak this some by increasing the current through the D3 diodes. Note: the fact that this is called D3 instead of Dx and Dy suggests to me that some original design used a proper dual diode. Note: Dedicated dual diodes are not always made by stacking two simple diodes. While this doesn't matter a lot, it looks like the design was dialed in for a dual. If the output stage isn't clean with larger degeneration resistors, you can add a small resistor in series with D3, but this will increase the nominal class A current, so DO NOT add this R without first increasing the emitter degeneration value or you are asking for smoke..
To dial in the output stage class A current, you can measure it by looking at the DC voltage drop across the emitter resistors, but the real Goldilocks test for too hot, too cold, or just right class A current is to look at HF crossover distortion. If you don't have a distortion analyzer handy you can see severe crossover distortion with a scope. Using a clean sine wave source at 20kHz look closely at the transition region between + to - voltage and back again. This will be most visible for modest output voltages driving a healthy load (600 Ohm?). With a distortion analyzer I set the class A current so the crossover distortion components were well below the other distortion products. With just a scope, the best you can do is see it go away, and maybe add a little margin.
I would be tempted to use all transistors and diodes from the same batch (so they will be from the same production lots and similar), and dial in the operating point, by first starving the output stage until the crossover distortion is visible (or measurable), then increase that class A current until it cleans up nicely. If you can't clean up the output stage "and" still have thermal stability, it's time to tweak the design. Note: You need to tweak the bias so it is clean at minimum rated supply (+/-12V), while still thermally stable at max supply (+/-20v?). The nominal current in D3 increases with this rail voltage so if it is clean at +/-12v it will will be cleaner (and hotter) at +/-20v.
I suspect this design and values were dialed in for slightly different parts and different layout.. In such designs the details can matter.
Good luck and have fun.
JR
