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Old 6th August 2009, 02:45 PM   #911
udailey is offline udailey  United States
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Trying things and learning is what DIY is all about, but if you ignore AndrewT's comments you will simply end up being frustrated. Sometimes its nice to try things and have them work well in your particular system. What you are planning will work... in the right system. What Andrew is telling you is that you will have high volume with the volume knob at zero.
Uriah
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Old 6th August 2009, 03:10 PM   #912
juluska is offline juluska  Ukraine
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Quote:
Originally posted by udailey
Trying things and learning is what DIY is all about, but if you ignore AndrewT's comments you will simply end up being frustrated. Sometimes its nice to try things and have them work well in your particular system. What you are planning will work... in the right system. What Andrew is telling you is that you will have high volume with the volume knob at zero.
Uriah
"What Andrew is telling you is that you will have high volume with the volume knob at zero."
Why? See the photo. I will use a 50k pot in the input.
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Old 6th August 2009, 04:06 PM   #913
AndrewT is offline AndrewT  Scotland
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Hi,
that new schematic is better.
You can improve it by adding filters at the input to attenuate RF interference and to block DC and attenuate ultra low frequencies from the source/s.

Look at the schematic. the gain of the first amp is 1times (+0dB) that gives 1.5Vac at the input and 1.5Vac at the +output.
The second amp takes the 1.5Vac input and inverts it to give 1.5Vac at the -output.
The difference between the two balanced outputs is 3Vac. A total gain of 2times (+6dB).

Look also at the resistances seen by the input pins of the two opamps.
The first amp sees zero ohms on the -IN pin.
The +IN pin sees a variable resistance from 2k2 to 27k7
This imbalance will create a small output offset from opamp1.

The righthand amp sees 5k on the -IN pin
The +IN pin sees zero ohms.
This will create a small output offset from opamp2.

Your amplifier must block or neutralise these output offsets.
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Old 6th August 2009, 04:12 PM   #914
juluska is offline juluska  Ukraine
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Quote:
Originally posted by AndrewT
Hi,
that new schematic is better.
You can improve it by adding filters at the input to attenuate RF interference and to block DC and attenuate ultra low frequencies from the source/s.

Look at the schematic. the gain of the first amp is 1times (+0dB) that gives 1.5Vac at the input and 1.5Vac at the +output.
The second amp takes the 1.5Vac input and inverts it to give 1.5Vac at the -output.
The difference between the two balanced outputs is 3Vac. A total gain of 2times (+6dB).

Look also at the resistances seen by the input pins of the two opamps.
The first amp sees zero ohms on the -IN pin.
The +IN pin sees a variable resistance from 2k2 to 27k7
This imbalance will create a small output offset from opamp1.

The righthand amp sees 5k on the -IN pin
The +IN pin sees zero ohms.
This will create a small output offset from opamp2.

Your amplifier must block or neutralise these output offsets.
I use the 10uf input caps in the pa150 boards, this will neutralise dc offset.
Sorry for my ignorance. You say it will be better using filters at the input of this schematic, another cap perhaps?which value?
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Old 6th August 2009, 04:27 PM   #915
AndrewT is offline AndrewT  Scotland
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Hi,
the cap value depends on your source impedance.

For a pre-amp I suggest you widen the passband by about an octave at each end.
If the power amp does F-1dB from 4Hz to 80kHz then the pre-amp should aim for F-1dB from 2Hz to 160kHz.

We cannot tell you everything about your system. At some stage you are going to have to learn how to measure output impedance and how to calculate your filter component values.
We can guess, you can do a lot better than guessing if you put the effort in.
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Old 7th August 2009, 09:32 AM   #916
TS Lo is offline TS Lo  Hong Kong
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Hi P-A,

Boards arrived. Thanks again
Wiki Updated.

Cheers
TS
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Old 7th August 2009, 04:47 PM   #917
logical is offline logical  Romania
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Andrew T, i think that if the opamp has FET inputs, and the input currents are very very small, it doesn't really matter if you have different resistances on the inputs...
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Old 8th August 2009, 11:34 PM   #918
juluska is offline juluska  Ukraine
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I go for this schematic. I'm now drawing the layout of the protoboard with one only quadruple opamp (TLE2074) for stereo configuration and SMD resistors 0,1% tolerance. The board will measure 40x40mm.
The 10uf input capacitor and the potentiometer will be out of the board.
The input capacitor will be this:
http://wduk.worldomain.net/acatalog/..._PXX_range.pdf
With this is not necesary a preamplifier.
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Old 8th August 2009, 11:42 PM   #919
juluska is offline juluska  Ukraine
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pcb
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Old 9th August 2009, 09:04 AM   #920
Electrons are yellow and more is better!
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Default I'm back!

Hi, guys!

I'm back from the vacation and I'm going to answer and process 20 emails. I really hope that we now have all Rhys' customers now. Some how I feel that I must draw a line somewhere which should be now.

I'll hope most people should have got there stuff from my last shipment.
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