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Old 22nd April 2005, 09:41 AM   #1
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Talking opamp driving tubes

I know i will get bashed if 'opamp' is mentioned in here.. I know you guys never listen to those plastic chips...

Anyway, I want to open a discussion about the possibility of using modern opamps to drive output tubes, especially in SE design. The advantages of opamp in ultra low distortion and noise is a strong advantage from technical aspect. Also, if the opamp is powered with +/-rails, no coupling cap is required. This is useful for people who cant live with the idea of having caps in their signal path. Furthermore, 'capless' designs tend to be expensive because more iron is need.
I am pretty sure this idea has been tested so I want to ask for opinions from those who has tried it.

I am thinking of using OP627 to drive 2a3. One of my concern is the output current requirement to drive output tube. I am not EE, but I think this should be easilly solved with a BUF634 buffer. Both of these chips has been used succesfully on SS preamp designs.

Suggestions?
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Old 22nd April 2005, 10:15 AM   #2
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Op-amps generally run off +/-15V rails. A 2A3 needs about 45Vpk to drive it, whereas your op-amp can probably swing to 13Vpk. It's not current that's the problem, it's voltage.
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Old 22nd April 2005, 10:17 AM   #3
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Sounds like a job for a chipamp!
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Old 22nd April 2005, 10:41 AM   #4
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Quote:
Originally posted by pinkmouse
Sounds like a job for a chipamp!
So then, what's the point of the output tubes and transformer after it?
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Old 22nd April 2005, 12:17 PM   #5
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Quote:
This is useful for people who cant live with the idea of having caps in their signal path.

Ah, yes, an opamp as a better alternative to caps, i like this

Once your design is complete you may further improve it by replacing the tubes with transistors and getting rid of the output iron.
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Old 22nd April 2005, 12:45 PM   #6
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Quote:
Originally posted by analog_sa
Once your design is complete you may further improve it by replacing the tubes with transistors and getting rid of the output iron.
Now that might just catch on.
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Old 22nd April 2005, 01:34 PM   #7
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Or you could use one of those hideously expensive Apex high voltage opamps. Compared to the usual tube driver stage, you'll reduce the distortion of the complete amplifier from 6% to 5.999%. And expensive is always better.

Sorry, don't mean to be facetious, but there's not much logic to doing this. Compared to the worst coupling capacitor ever made since the days of Leyden, the distortion of the SET output tube/transformer is massively larger.
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Old 22nd April 2005, 08:19 PM   #8
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Quote:
Originally posted by Brett

So then, what's the point of the output tubes and transformer after it?
Placeholders on the chassis.

So yeah if you must use SS to drive tubes... use a discrete driver that can handle the voltage required.

Note that this will result in the exact SS analog of the tube driver stage you are replacing.

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Old 22nd April 2005, 11:28 PM   #9
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Default opamp driving tubes

adjust the feedback values to obtain the desired gain and bandwidth.

the power supply rails at this example are +/- 32 volts, but depends of the opamp, you could carry them to +/- 36 volts.

if you only need 45 volts peak to peak, a good start is a power supply of the +/- 24 volts with a 5534 op amp. I think that the 5534 will work fine up to +/- 32 volts, but you need to check the voltage at pin 4 & 7.

I chose a single opamp because they have low crosstalk, remember the output voltage 45 volts peak to peak, but you can use a dual opamp with +/- 18 volts maximum supply rating

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Old 23rd April 2005, 08:40 AM   #10
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Op-amps generally run off +/-15V rails. A 2A3 needs about 45Vpk to drive it, whereas your op-amp can probably swing to 13Vpk. It's not current that's the problem, it's voltage.
Yes, that is the first limitation. I was thinking of interstage trans to solve this.. but this will limit the design to a good interstage trans, thus offset the cost reason.

[quote]Sounds like a job for a chipamp! [quote]
Yes, can be another alternative.

Quote:
Ah, yes, an opamp as a better alternative to caps, i like this.
Like I said, this thread is not for anyone who cant live with opamps. Personally I find nothing offending with opamps.

[quote]adjust the feedback values to obtain the desired gain and bandwidth.

the power supply rails at this example are +/- 32 volts, but depends of the opamp, you could carry them to +/- 36 volts.

if you only need 45 volts peak to peak, a good start is a power supply of the +/- 24 volts with a 5534 op amp. I think that the 5534 will work fine up to +/- 32 volts, but you need to check the voltage at pin 4 & 7. [quote]

Some opamps can go up to 34V p2p, but those opamps does not have the sonics I would like to integrate.
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