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Old 26th January 2014, 07:47 PM   #1
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Default Op Amp Phase Splitter

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?
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Old 26th January 2014, 09:16 PM   #2
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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
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Old 26th January 2014, 09:56 PM   #3
00940 is offline 00940  Belgium
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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.
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Old 26th January 2014, 11:33 PM   #4
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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...

Last edited by Captn Dave; 26th January 2014 at 11:37 PM.
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Old 26th January 2014, 11:38 PM   #5
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Quote:
Originally Posted by 00940 View Post
Jfet inputs too, which often makes life easier.
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.

Last edited by Captn Dave; 26th January 2014 at 11:46 PM.
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Old 26th January 2014, 11:54 PM   #6
kevinkr is offline kevinkr  United States
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Old 27th January 2014, 01:52 AM   #7
00940 is offline 00940  Belgium
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Quote:
Originally Posted by Captn Dave View Post
I'm interested in that comment. Why do JFET inputs make life easier?
In summary: practically no input currents to take care of.
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Old 28th January 2014, 11:08 PM   #8
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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.
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Old 29th January 2014, 12:38 AM   #9
zelgall is offline zelgall  Canada
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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.
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Old 29th January 2014, 01:36 PM   #10
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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.

Last edited by Captn Dave; 29th January 2014 at 01:40 PM.
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