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Old 9th March 2003, 12:29 PM   #1
Bricolo is offline Bricolo  France
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Default Opamp based preamp, for a gainclone

I want to build a preamp for my gainclone, since my soundcard and portable cd player's output level are low compared to my cd player's one. (I have to turn the knob near 75%)

So a basic opamp based preamp would be nice, IMO. To continue with the simple design of the gainclone.
This could be a cool "all IC amplifier"

An inverting opamp would also revert the phase correctly, without having to invert the outputs + and -

Does anyone have a shematic for this?
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Old 9th March 2003, 01:19 PM   #2
mltaunt is offline mltaunt  United States
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Try Rod Elliot's site he has several
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Old 9th March 2003, 02:38 PM   #3
Bricolo is offline Bricolo  France
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Rod's ones have bass, treble and balance control

I don't need all of this.

And they also are non inverting, I'd prefer an inverting configuration.
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Old 9th March 2003, 03:18 PM   #4
joensd is offline joensd  Germany
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Well, with the BOZ it wouldn´t be an IC-amplifier .
What about a simple and single opamp circuit?
Use inverting operation and that´s it.
Something like an inverting CMOY which you probably know.
If you want an additional stage acting as buffer and the pot in between those stages I could send you a schematic with PCB layout.

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Old 9th March 2003, 04:04 PM   #5
Bricolo is offline Bricolo  France
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I've made a first shematic for the gainclone preamp + gainclone amp

The preamp's gain has been calculated to have a full output power on a TDA7294 with 40V supply and 8R load (90W RMS)

90W RMS in 8R means 27V RMS, and 1.23V RMS at the TDA's input

My soundcard outputs 0.3V RMS (can anyone confirm this? I've tested with my digital multimeter, that's not the best way to go but it's the only one I have)
and it's the weakest source I have

So, to give 1.23V after the preamp, I fixed the gain to 4 (12dB)

Things I am not sure:

- the +in is directly connected to ground (like in the minimised gainclone shematic). I've heard that having the same impedance to +in and -in is optimal for the opamp
- there's no cap between the preamp and the amp, so if the preamp's opamp has a dc offset, it will be amplified
- should I put a 50K resistor to grouns before the amp's 10k input resistor?
- the input and feedback resistor's values are "arbitrary fixed". Theyr ratio ir 4, but I'm not sure for the values. I've learnt at school that only the ratio fixes the gain, but some values are certainly better that others. I'd like some advices on that

What do you think about it? (X1 is the preamp chip, X2 the gainclone chip)
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Old 9th March 2003, 05:36 PM   #6
Asen is offline Asen  Bulgaria
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Think about a cap between the ICs. In case there is any DC at the output of X1 it would be multiplied by the gain of the GC.
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