TRIODE Voltage amp + Power OP output follower - diyAudio
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Old 16th August 2010, 03:36 PM   #1
lineup is offline lineup  Sweden
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Default TRIODE Voltage amp + Power OP output follower

It is quite easy to build a good and simple 'tube amp'
if using a triode (like 6DJ8) as voltage amplifier
and add one Power Op-Amp as output stage, follower.

Getting 50 Watt RMS into 8 Ohm with less than 1% THD distortion is not difficult.
The dist that is there will be totally of TUBE characteristic
as the Op-Amp gives practically no dist at Unity Gain.

It is a bit like Tube-Buffered Gainclone.
Click the image to open in full size.
But problem with LM3875 and LM3886 is they can not work at Gain=1.
They are not true Op-Amps in this respect.

OPA541 can be used as a 'normal' opamp, at unity gain = 1.
And so easily work as output follower.
Power supply can be 70V for plastic models and 80V for metal package.
See images ...
Output current is 10 Ampere, so it drives 4 Ohm speaker, too.
Precision Amplifier - High Output Current - OPA541 - TI.com

My idea is very simple and works very good in SPICE.
I will post more detailed circuits later.
Here is the concept:
Attached Images
File Type: png 6DJ8_OPA541_1.png (4.0 KB, 273 views)
File Type: jpg opa541ap.jpg (23.4 KB, 270 views)
File Type: jpg opa541bm_2.jpg (5.1 KB, 261 views)
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Old 16th August 2010, 04:02 PM   #2
Bigun is offline Bigun  Canada
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I think this is a good looking approach, you could do a search to see what else might be helpful,

e.g.
Synthesis Magnum, commercial tube-chipamp hybrid...

Connecting tube gain stage to LM3886 for guitar amplifier??? How can this be done???

Tube with Power IC Output Stage - JLTi
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Old 16th August 2010, 05:28 PM   #3
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The JLTi is kind of the opposite of the OP. No gain from the tube, and all of the gain provided by the chip. The tube is pretty useless if a decent preamp is present in the system.

Lineup - could the OPA544 be used as a substitute, i.e., is it unity gain stable as well?
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Old 16th August 2010, 09:52 PM   #4
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Sorry mate, the distortion will not be tube b/c it is driving an easy load (the IC). The IC is driving the difficult load and the distortion will be like a SS amp.
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Old 16th August 2010, 09:56 PM   #5
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The distortion will be like a tube is driving SS amp, and SS amp is driving a complex load.
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Old 16th August 2010, 09:59 PM   #6
tinitus is offline tinitus  Europe
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isnt amplification from the tubes just about 20 or so
thus still needs some amplification from IC too

would be interesting to reverse it, IC driving tubes

or maybe IC driving IC, with tube interstage trafo
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Old 17th August 2010, 07:23 PM   #7
tinitus is offline tinitus  Europe
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would it have more interest in ChipAmp forum ?

I can move it over if you want ?
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Old 17th August 2010, 07:35 PM   #8
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Maybe. point is u're assumption is wrong, shouldn't matter which forum u chose. But that's the whole point about this site, discussing ideas.
The concept of a tube doing voltage gain and transistors doing current is fine, but the distortion will be just as Wavebourn said.
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Old 18th August 2010, 08:40 AM   #9
teemuk is offline teemuk  Finland
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