Reffc
09-10-2013, 17:47
I've been meaning for a while now to build a buffer amplifier which I can place between any preamp and any power amp to ensure no more impedance mis-match issues. My current power amps have a low-ish input impedance of 5K1 Ohms which should still be fine for the croft pre (300 ohm output impedance) but I think that by using a buffer amp, it'll make the Croft's job easier.
The easiest way (so I was advised) to achieve this might be through the use of an op-amp based circuit using a unity gain op-amp, so I chose the TexasInstruments low noise dual NE5532 op amp chip for the circuit.
This needs DC power, so instead of going to the expense (for now) of a 240v to 9vDC power supply, I decided to power it with two 9V batteries.
Down-stream of the inputs, I need to ensure sufficient shunt impedance between signal and earth, so a 1mOhm resistor was picked. This was followed by a 1uF cap in series, a 1K resistor (in series) then the input bias resistor to set input impedance.
As it's the croft, 100K seems far more valve amp friendly than the 5k1 of the Albarrys, so that's what I've used. just prior to this as a precaution, there'll be shunt diodes to ensure that if slight over-voltage (DC) makes it onto the signal, it'll be dumped to the voltage rails and passed through the internal resistance of the batteries where hopefully no damage can be done.
That's the plan....here's the execution:
Bits all arrived this morning:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7315_zps0390c22a.jpg
Neat little extruded case which I figured would be just the right size although batteries will have to be sited externally and a power lead run through the front panel;
Simple 2-pole rocker switch for power:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7318_zpsdd5f0d21.jpg
Being wired up:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7317_zps3afdf401.jpg
front and rear panels drilled and RCA sockets fitted. Front panel cut out and drilled for rocker switch and power-on LED; Switch and LED fitted:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7319_zps926b44a0.jpg
Rear view:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7320_zps70ff7397.jpg
LED being tested with batteries connected:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7322_zpse6557f12.jpg
Rear view: resistor is there to prevent too much power reaching LED as it only requires a tiny voltage/current. I've used an 8K2 value:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7323_zps02bb39ea.jpg
Starting to populate the board. Left channel being figured out here:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7325_zpsc62a579e.jpg
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7327_zpse794174b.jpg
Both channels now populated after a few hours work. Top view:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7329_zps27716438.jpg
Underside showing solder connections and star-earth bare copper wire roughly central to the board. Anyone spot the two mistakes? :scratch: (needs careful examination of top view!)
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7331_zps976ffe33.jpg
I had shorted one of the series caps :doh: and hadn't connected one of the shunt resistor legs to the star earth...testing brought this up later.
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7332_zps30d309ae.jpg
Almost completed, minus the top, and ready for testing:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7333_zpsfe008171.jpg
Test measurements:
input impedance across signal to earth = 1MOhm (both channels)
DC offset voltage at output = 93mV one channel and 94mV t'uther channel.
This is a little higher than expected but is just the way the chip performs...a little DC is being seen relative to ground but it's still tiny and equal in both channels, so I'm happy. Another unity gain chip may have better performance but this one's well respected as a decent low noise chipset which wont add anything like significant signal distortion (perhaps not even measurable) so it's a good solution.
As gain is 1:1, all this amp does is to allow the plugging in of ANY preamp, even a passive or a standard gain pre with perhaps high output impedance, safe in the knowledge that it will drive any power amp with ease, and drive long interconnects without fear of rolling off frequency extremes. In theory it should restore an ideal balance...it's a sort of match-maker:lol:
Time to test it out :stalks:
The easiest way (so I was advised) to achieve this might be through the use of an op-amp based circuit using a unity gain op-amp, so I chose the TexasInstruments low noise dual NE5532 op amp chip for the circuit.
This needs DC power, so instead of going to the expense (for now) of a 240v to 9vDC power supply, I decided to power it with two 9V batteries.
Down-stream of the inputs, I need to ensure sufficient shunt impedance between signal and earth, so a 1mOhm resistor was picked. This was followed by a 1uF cap in series, a 1K resistor (in series) then the input bias resistor to set input impedance.
As it's the croft, 100K seems far more valve amp friendly than the 5k1 of the Albarrys, so that's what I've used. just prior to this as a precaution, there'll be shunt diodes to ensure that if slight over-voltage (DC) makes it onto the signal, it'll be dumped to the voltage rails and passed through the internal resistance of the batteries where hopefully no damage can be done.
That's the plan....here's the execution:
Bits all arrived this morning:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7315_zps0390c22a.jpg
Neat little extruded case which I figured would be just the right size although batteries will have to be sited externally and a power lead run through the front panel;
Simple 2-pole rocker switch for power:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7318_zpsdd5f0d21.jpg
Being wired up:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7317_zps3afdf401.jpg
front and rear panels drilled and RCA sockets fitted. Front panel cut out and drilled for rocker switch and power-on LED; Switch and LED fitted:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7319_zps926b44a0.jpg
Rear view:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7320_zps70ff7397.jpg
LED being tested with batteries connected:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7322_zpse6557f12.jpg
Rear view: resistor is there to prevent too much power reaching LED as it only requires a tiny voltage/current. I've used an 8K2 value:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7323_zps02bb39ea.jpg
Starting to populate the board. Left channel being figured out here:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7325_zpsc62a579e.jpg
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7327_zpse794174b.jpg
Both channels now populated after a few hours work. Top view:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7329_zps27716438.jpg
Underside showing solder connections and star-earth bare copper wire roughly central to the board. Anyone spot the two mistakes? :scratch: (needs careful examination of top view!)
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7331_zps976ffe33.jpg
I had shorted one of the series caps :doh: and hadn't connected one of the shunt resistor legs to the star earth...testing brought this up later.
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7332_zps30d309ae.jpg
Almost completed, minus the top, and ready for testing:
http://i1140.photobucket.com/albums/n577/24pacman/Hifi/_MG_7333_zpsfe008171.jpg
Test measurements:
input impedance across signal to earth = 1MOhm (both channels)
DC offset voltage at output = 93mV one channel and 94mV t'uther channel.
This is a little higher than expected but is just the way the chip performs...a little DC is being seen relative to ground but it's still tiny and equal in both channels, so I'm happy. Another unity gain chip may have better performance but this one's well respected as a decent low noise chipset which wont add anything like significant signal distortion (perhaps not even measurable) so it's a good solution.
As gain is 1:1, all this amp does is to allow the plugging in of ANY preamp, even a passive or a standard gain pre with perhaps high output impedance, safe in the knowledge that it will drive any power amp with ease, and drive long interconnects without fear of rolling off frequency extremes. In theory it should restore an ideal balance...it's a sort of match-maker:lol:
Time to test it out :stalks: