I built this preamp to compare it with some of my other preamp builds.
http://diyaudioprojects.com/Forum/download/file.php?id=8195&mode=view
I built from the pot back and left out the phono part since I already have a good phono preamp. I used a perfboard and it only took a few hours to populate the board being it is so simple. Great sounding for such an easy cheap build. I have spent much more on more complicated builds that do not sound as good as this pre. The schematic is much the same from the datasheet. The PS section works like a charm. The OPA2134 is one fine chip.
http://diyaudioprojects.com/Forum/download/file.php?id=8195&mode=view
I built from the pot back and left out the phono part since I already have a good phono preamp. I used a perfboard and it only took a few hours to populate the board being it is so simple. Great sounding for such an easy cheap build. I have spent much more on more complicated builds that do not sound as good as this pre. The schematic is much the same from the datasheet. The PS section works like a charm. The OPA2134 is one fine chip.
It would be good if you could post the schematic image here so we don't have to register for another forum called diyaudioprojects.com just to see it.
Your gain is set quite high at 11times (+20.8dB)
Do you find that you have to keep the vol pot turned way down to avoid excessive loudness/distortion/clipping/noise ?
I would expect 2times (+6dB), or 4times (+12dB) to be plenty with most sources.
and many digital sources can manage with 1times (+0dB), i.e. a Buffer.
Do you find that you have to keep the vol pot turned way down to avoid excessive loudness/distortion/clipping/noise ?
I would expect 2times (+6dB), or 4times (+12dB) to be plenty with most sources.
and many digital sources can manage with 1times (+0dB), i.e. a Buffer.
A LC filter really wouldn't hurt after the switchers. Did you have a look with an oscilloscope at the output ?
But yes, the opa134 is a reliable audio workhorse.
But yes, the opa134 is a reliable audio workhorse.
The gain is somewhat high. Any suggestions for lowering the gain? I figured with some tweaking the circuit could be made much better.
No I have not looked at the signal with an oscilloscope. I do have one collecting dust somewhere.
No I have not looked at the signal with an oscilloscope. I do have one collecting dust somewhere.
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Yes I did notice too much noise on a cheap D board I have been playing with. With my FW amps it sounds great F6,M2, New V-fet.
Wouldn't it be better to use 10 times lower values for the volume pot and feedback resistors around the second op-amp to lower the noise? 100kOhm volume pot/feedback resistor seems excessive since OPA2134 and most other sources nowadays (AUX1/2) can easily drive 10kOhm loads.
Yes I thought using my standard 250K pot would be better. A 500K may even be better still.
try changing the feedback lower resistor to 2k
For a 2times gain set the upper feedback resistor to 2k
If you need extra gain change the lower resistor by paralleling another resistor.
Gain = Upper/Lower +1
2k/2k+1 = 2times
2k/1k+1 = 3times (By adding 2k||2k)
2k/680r+1 = 3.94times (or by adding 1k||2k for gain= 4times)
The extra parallel resistor could be selected with a dip switch containing open, 2k and 1k
What gains are your FW power amplifiers?
For a 2times gain set the upper feedback resistor to 2k
If you need extra gain change the lower resistor by paralleling another resistor.
Gain = Upper/Lower +1
2k/2k+1 = 2times
2k/1k+1 = 3times (By adding 2k||2k)
2k/680r+1 = 3.94times (or by adding 1k||2k for gain= 4times)
The extra parallel resistor could be selected with a dip switch containing open, 2k and 1k
What gains are your FW power amplifiers?
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They were suggesting a lower resistance vol pot. Try 20k or 10k. If you had a very powerful buffer feeding the interconnect cable you could even go down to 5k or 2k.
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Are you saying the first thing to try is change the lower feedback resistor to 2K and leave the upper 100K as is?
No,
I said try changing both to 2k
a 4pole dip switch allows two resistor values to be switched in for a stereo build. All 4 switches open = 2times gain,
all four switches closed = 5times gain. Using just two resistor values 2k & 1k
I said try changing both to 2k
a 4pole dip switch allows two resistor values to be switched in for a stereo build. All 4 switches open = 2times gain,
all four switches closed = 5times gain. Using just two resistor values 2k & 1k
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The F6 gain is 14db 5X. The gain is acceptable with the FW series. I also tried it with a SET tube amp that requires a pre with acceptable gain. In all cases you do not go over 9:00 on turning though. Some more range would be better.
If I understand you correctly what this does is increase the amount of feedback lowering gain.
yes more feedback equals less closed loop gain.
More feedback equals less distortion and less noise.
But the 2k feedback resistor is seen as the dominant load on the ouput of that opamp. I would not go any lower even though the datasheet shows as low as 600r as being acceptable.
The actual receiver impedance and the output cable capacitance are both in parallel to the feedback resistor. You need some current driving capability to drive all three loads.
that indicates at least 12dB of excess gain if you had a lin vol pot. But with a log/audio volpot it probably indicates an excess gain of ~20dB
Have you read?
http://www.diyaudio.com/forums/diyaudio-com-articles/186018-what-gain-structure.html
Maybe you should also allow for a Buffer for most of your listening and only switch in the gain block when you actually need more gain for the loudness you want during that listening session. There is nothing wrong with eliminating excess gain in the system and regularly running your vol pot at 90% to 99% of max rotation. Just apply the gain block when you find you need 101% to 150%.
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