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Old 20th July 2008, 03:19 PM   #1
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Default help me with this opamp pre-amp

I am planning to build this, opamp will be opa627, but I would like to have an non-inverted configuration and class a pre-amp.
Can I just change the R1 R2 to non inverted configuration ??
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Old 20th July 2008, 05:04 PM   #2
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Hmmm .. have to think about this one.
To change to non-inverting wouldn't be difficult.
I leave this to someone else to post solution.

A couple of questions, that may help anyone posting reply:

1. You use very high power supply, 2x30-35 Volt, for being a preamp.
Why? You need very high voltage output?
Gain is x10.

2. I have myself used TO-220 lowprice BD241C/BD242C transistors.
They will work. But really, they are not for very good audio.
Can you think to use any other TO-220 transistors?

For example MJE15030/MJE15031 would be a considerable improvement.
They would much better match OP37 quality.
OP37 is a very good Audio Op-Amp

Regards, Lineup
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Old 20th July 2008, 05:13 PM   #3
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Default Re: help me with this opamp pre-amp

Quote:
Originally posted by fastvideo

Can I just change the R1 R2 to non inverted configuration ??
Yes!...
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Old 20th July 2008, 05:18 PM   #4
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I'm guessing ground Vin, put 1M or so in the other input line so it has a ground reference, then drive the other input. This is a variation on an old circuit. It uses the current draw of a heavily loaded op-amp to run the output stage. Though it could give good performance in its day, using few parts, I doubt it's a desirable way to build an audio circuit.
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Old 20th July 2008, 06:40 PM   #5
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yes, it looks like a method used to eliminate crossover notch in older op amps. it also bears some resemblance to a current feedback input stage for a power amp using a regular voltage feedback op amp.
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Old 20th July 2008, 08:09 PM   #6
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Quote:
Originally posted by lineup


1. You use very high power supply, 2x30-35 Volt, for being a preamp.
Why? You need very high voltage output?
Gain is x10.

Regards, Lineup

I am using opa627 non-inverting 10X to drive Nelson's F4 amp at the moment, but not enough voltage swing to drive to it's full output. Looking for some easy way to drive it before I decided which tube preamp to build.
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Old 20th July 2008, 08:21 PM   #7
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Quote:
Originally posted by fastvideo
I am using opa627 non-inverting 10X to drive Nelson's F4 amp at the moment, but not enough voltage swing to drive to it's full output. Looking for some easy way to drive it before I decided which tube preamp to build.
I'd just use one of the power chips, like an LM1875 or smaller. Gives you the swing you want in a super-low parts count.
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Old 20th July 2008, 09:45 PM   #8
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Quote:
Originally posted by fastvideo

I am using opa627 non-inverting 10X to drive Nelson's F4 amp at the moment,
but not enough voltage swing to drive to it's full output.
Looking for some easy way to drive it before I decided which tube preamp to build.
The F4 amplifier is a very easy load.
As there is a JFET input buffer.
Nelson: 48 kOhm input impedance
And we could probably use even higher input impedance, like 100 kohm or more

Voltage swing.
Power supply of F4 is nominally +- 22 VDC.
Max voltage swing at max power peaks would be something like:
+- 18 to 20 Volt peak

(not that I can understand what any speakers would need such much watt ???)
Quote:
Nelson Pass said:
First, you will find that the more sensitive drivers deliver good volume levels at quite low wattage.
My Lowther DX55ís (93 dB/watt) mounted in Alerion enclosures adequately fill
my 11,000 cubic foot listening room with about 5 volts
(the equivalent of a 4 watt amplifier).
I can achieve good levels with headroom to spare with my phono stage or DAC feeding an X1 preamp, which has a maximum of 14 dB gain.
If I lived in an apartment, I could get complaints from my neighbors.
Driving +- 18 Vpeak ( 13 Vrms ) into 48 kOhm is as easy load
so that even low wattage Tube amplifiers can do this!!!!
We need an output current of only 18 Volt/48000 Ohm
which is less than 0.4 mA ( 400uA )


A Class A output stage biased at 1 mA could do this ......

It would be nothing for a good op-amp, if it wasnt for the 18 Volt peak swing.
Most operational amplifiers does not provide such high output voltages.
And those that do will probably go into some distortion so close to rails.

======================================

I say, your idea in the first post is not bad:
Using a good op-amp assisted with output transistor at a higher voltage.

Also the LM1875 idea is probably a good way.
Normal supply voltage operation for LM1875 is +- 25 Volt.
But it can take max +- 30 Volt.

You may study the Burr-Brown OPA604 / OPA2604 datasheet.
This op-amp is one of the few that can have Max +- 24 Volt supply.
May not be impossible
that this JFET Op can deliver +-20 Volt swing into a light load ( like 47-100 kohm )
PDF: http://focus.ti.com/lit/ds/symlink/opa604.pdf
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Old 20th July 2008, 11:40 PM   #9
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Quote:
Originally posted by lineup


You may study the Burr-Brown OPA604 / OPA2604 datasheet.
This op-amp is one of the few that can have Max +- 24 Volt supply.
May not be impossible
that this JFET Op can deliver +-20 Volt swing into a light load ( like 47-100 kohm )
PDF: http://focus.ti.com/lit/ds/symlink/opa604.pdf
Yes, opa2604 is my first opamp to begin with, it does drive the amp to full swing at +-24v. But i replaced it with 2X opa627 and big difference in sound quality.
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Old 20th July 2008, 11:55 PM   #10
lineup is offline lineup  Sweden
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Yes.
It works at very high volt output, into 47 kohm.

If I am to believe one spice simulation I did with attached circuit,
the problems begin at ~ 21.5 Volt peak output.
For +- 24 V supply and a 47 kohm load. = F4 input.

This means OPA604 has got something like 2.5-3.0 Volt drop.
It corresponds well with the datasheet diagrams.
For +- 15 Volt, there is max +-12 Volt output swing. = 3 Volt drop.

As you can see, I have set the gain of my OPA604 circuit to like x15.
This gives max output at around 1 Volt RMS input.
I also added a 47 kohm Volume potentiometer at the input
and 1k gate resistors for the OPA604 input pins. (It wont hurt!!)

This circuit shows almost 'no distortion' below 20 Volt output.
Less than 0.001%.
I would think for a real circuit,
this preamp will NOT be the weak LINK in your audio system.

... knowing even very expensive high end speakers have
THD at the 0.500% level ..... at only 1 Watt RMS into 8 Ohm!!!
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