I've enrolled in an online electronics course to broaden my understanding of the diy audio hobby. The last assignment is to complete and post a project of ones own design. I thought I'd combine equal measures of course learning (solid state) with some tube audio parts I have in abundance.
I plan to use an old Magnavox console pull EL-84 PP output stage together with op amps to replace the first stage of amplification and phase splitters.
There are thousands of op amps in production and of course the textbook and lectures treat the lot of them as ideal devices. I ask for some help with selection because I'll not have time for trial and error Mouser orders.
The Mouser catalog has a page of "Audio Operational Amplifiers" with a subsection "High Performance, High Fidelity, Low Noise". The one with the highest slew rate and bandwidth is the LME49722. Another is the LM4562.
Are either of these acceptable choices for an application such as this?
I plan to use an old Magnavox console pull EL-84 PP output stage together with op amps to replace the first stage of amplification and phase splitters.
There are thousands of op amps in production and of course the textbook and lectures treat the lot of them as ideal devices. I ask for some help with selection because I'll not have time for trial and error Mouser orders.
The Mouser catalog has a page of "Audio Operational Amplifiers" with a subsection "High Performance, High Fidelity, Low Noise". The one with the highest slew rate and bandwidth is the LME49722. Another is the LM4562.
Are either of these acceptable choices for an application such as this?
I am an solid state guy too, the EL-84 PP config requires 15v p-p to fully drive the tube.
so the chips should work. 1 inverts the other goes to grid with 2.2k grid stopper resistor.
I have the LM4562, it is a great op amp.
for my tube project I used a 2N5551 as the splitter
so the chips should work. 1 inverts the other goes to grid with 2.2k grid stopper resistor.
I have the LM4562, it is a great op amp.
for my tube project I used a 2N5551 as the splitter
Another opamp that might be of use is the opa604. Its good feature is that it can accept +/-24V rails and thus swing more voltage than other opamps.
Jfet inputs too, which often makes life easier.
Jfet inputs too, which often makes life easier.
Thanks for the ideas! I'll get a few of each and try them out.
And the_mantra, I'm averting my eyes from that schematic 😱 ;😛 This thing is for a class and the design should come from my my own hand. But thanks anyway. I'll post my circuit later for kicks. It's pretty simple stuff, but the point is to demonstrate the ability to use those concepts learned in class. Spec'ing components was not covered however, so I feel at ease seeking conformation for that here.
Ben, Those would be good for an EL34 or 6L6 application. What would be really cool is an opamp with the ability to accept +/- 240 rails. I have this 845 amp with a horrible driver circuit...
And the_mantra, I'm averting my eyes from that schematic 😱 ;😛 This thing is for a class and the design should come from my my own hand. But thanks anyway. I'll post my circuit later for kicks. It's pretty simple stuff, but the point is to demonstrate the ability to use those concepts learned in class. Spec'ing components was not covered however, so I feel at ease seeking conformation for that here.
Ben, Those would be good for an EL34 or 6L6 application. What would be really cool is an opamp with the ability to accept +/- 240 rails. I have this 845 amp with a horrible driver circuit...
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I'm interested in that comment. Why do JFET inputs make life easier?
At this level, besides saturation, we haven't discussed differences in construction or application other than to note that op amps can be built from MOSFETS or BJTs.
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Copyright infringing material removed. Links to copyrighted material are permitted provided the material is hosted by the copyright holder or is clearly marked as free for use. (copyleft etc.) Points next time..
Understood. The textbook presents an Op Amp Model that calls for infinite input resistance and impedance with i+ and i- equal to zero. Clearly, many (or perhaps most) op amps don't fit that model. I see the JFET types do approach the model.
As an original option for your course, you could use a vacuum tube opamp.
This model has an octal base and two 9 pin sockets: GAP/R K2-W VACUUM TUBE OP-AMP SPLITTER ADAPTER (do a search on that auction site)
I've seen adapters that use two 6SN7s.
They used these in computers.
This model has an octal base and two 9 pin sockets: GAP/R K2-W VACUUM TUBE OP-AMP SPLITTER ADAPTER (do a search on that auction site)
I've seen adapters that use two 6SN7s.
They used these in computers.
YES! I was aware of vacuum tube op amps (it was actually mentioned in the text) and even considered that idea. Ultimately, I decided that classmates would relate better to a circuit that combined modern devices that they now understand with some magic from 55 year ago. I think the contrast will be striking.
And I've recalibrated some. I have an old Conn organ amp chassis in the pile. Push pull 6L6 with good iron and twin diode rectification. I'm going to use the push pull power tube circuit, everyting downstream from the coupling caps, together with my big Lambda 500v / 500 ma power supply and some coke bottle Chinese 6L6s I got real cheap to build a guitar amp using the op amp driver circuit. With the power supply I can easily adjust and demonstrate bias and red plate the tubes as a grand finale. Humm, it'd be nice if I could get Neil to come by and play the guitar.
And I've recalibrated some. I have an old Conn organ amp chassis in the pile. Push pull 6L6 with good iron and twin diode rectification. I'm going to use the push pull power tube circuit, everyting downstream from the coupling caps, together with my big Lambda 500v / 500 ma power supply and some coke bottle Chinese 6L6s I got real cheap to build a guitar amp using the op amp driver circuit. With the power supply I can easily adjust and demonstrate bias and red plate the tubes as a grand finale. Humm, it'd be nice if I could get Neil to come by and play the guitar.
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Got access to local library?
Electronics World, March 2000 "Hybrid Audio Power" Wim de Haan
Electronics World, Feb 2003 "Hybrid Amplifier Improved" Wim de Yager
I'm fairly sure these were tube Output SS front end and phase splitter designs.
EL84 are pretty easy to drive. Op Amp Splitter Circuit will be simple if the EL84 are cathode biased. If fixed biased things will be a little more complicated.
If I remeber right those designs noted above were for fixed bias and EL34 but should be adaptable.
Cheers,
Ian
Electronics World, March 2000 "Hybrid Audio Power" Wim de Haan
Electronics World, Feb 2003 "Hybrid Amplifier Improved" Wim de Yager
I'm fairly sure these were tube Output SS front end and phase splitter designs.
EL84 are pretty easy to drive. Op Amp Splitter Circuit will be simple if the EL84 are cathode biased. If fixed biased things will be a little more complicated.
If I remeber right those designs noted above were for fixed bias and EL34 but should be adaptable.
Cheers,
Ian
Below is the article that describes one possibility to get high signal levels with ordinary op-amps.
http://joebrown.org.uk/images/DualPSU/BootstrappingOpAmps.pdf
http://joebrown.org.uk/images/DualPSU/BootstrappingOpAmps.pdf
That is a very clever circuit. That might work for that 845 driver. Thanks for the link.
Here is the circuit I used.
I was very satisfied with it. I used it with a push pull pair of 6L6GC with a 360v, 62ma, -16.5 bias and 270 g2. That required a little more than 25volt pp drive and the TI LM4562 did quite well. I was stunned by the absolute silence with no source signal. The most quiet amp I've ever built. Interesting exercise.
I was very satisfied with it. I used it with a push pull pair of 6L6GC with a 360v, 62ma, -16.5 bias and 270 g2. That required a little more than 25volt pp drive and the TI LM4562 did quite well. I was stunned by the absolute silence with no source signal. The most quiet amp I've ever built. Interesting exercise.
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You can play around LME49810/LME49830. Not exatly an opamp but these can work until ±100V rails.
Not really but I get the comparison. The first op amp inverts and amplifies. I ended up with a 33k and a 2.2K resistor around that op amp. The ratio is the gain- about 15. That is enough to allow a 1v rms signal (2.8 pp) to drive the output to saturation with a 38 volt differential power supply. An adjustment in gain is just a resistor away. The second op amp just inverts that signal without gain if the resistors are closely matched.
The other source represents the feedback. I'm summing in negative feedback at the first op amp so it appears equally in both outputs.
One chip contains two op amps so it happens with just one device as you can see in the photo. The other device is a rail splitter for the power supply.
There are plenty of other ways to do this but this one seemed simple and foolproof.
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If you do, I recommend you start with a 2K2 and a 33K resistor on the first op amp. As drawn its too just much gain.
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