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Old 14th February 2010, 09:58 AM   #871
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Hi Joachim,
it'd be so cool if you could provide a photo of that discrete ina input stage with triple groundplane and deadbug-opamp style!

Rüdiger
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"I can feel what's going on inside a piece of electronic equipment. I have a sense that I know what's going on inside the transistors." Robert Moog
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Old 14th February 2010, 11:32 AM   #872
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It certainly looks quite jammed !
My digital camera was stolen and my son lost his in vacation.
Anyway, i wil buy a new one, and soon i will be back in the factory where we have professional posibilizies.
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Old 14th February 2010, 10:54 PM   #873
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Default In search of low noise

Today i DC coupled the other channel and everything worked fine.
Listening revealed that there was not much of a difference between the AC and DC version. I think the DC version is a little bit more transparent and the AC version a little bit more sweet and forgiving. No big deal. I can recomend both.
Yesterday i started to work on my subwoofer because at insane levels there was a slight hum comming out of the woofer only and interestingly not out the main speakers.
I have a quite sofisticated wall power outlet scheme and tried to trace down the hum problem by trying out different power connections. I have 3 independent lines. One line with particular low impedance and filtering. Wherever i took the power from there was no difference. Then i tryed to isolate the earth connection. Again no success.
I usually piggy bag my woofer high level after the poweramp, so i tried to connect the woofer low level from the preamp. I wanted to try that anyway because it is one amplifier less in the chain. Again no succes with the hum but the sound got more clear and transparent in the bass and also a bit lighter in tone. I readjusted the sub volume to my usual level by listening to familiar records. I do only simple acoustic measurements when i set up a woofer. Usually with pink noise. Records vary in amount of bass tremendoesly so even when i find a well measuring setup, i tune by ear afterwards.
Thinking about the problem for a while it struck me that i had used coaxial unbalanced cable from my tonearm so far. Technically it works because my arm has a seperate earth but much of the advantage of a balanced stage to suppress hum is lost that way.
So i made a truly balanced cable in the usual style. Twisted pair with seperate shield.
I made it very short (0.3m) and used Spiral Groove Litzwire with Eichmann Bullet Plugs.
After i inserted the cable and put the shield to the central mass point the hum in the sub was totally gone !
Also the hiss in the speakers sounded different. Before i had heard high frequency components that sounded a bit like muted fire crackers. Now it was just plain old white hiss with a much more benign character. The channel with the NE sounded even more soft so i decided not to compare the NE5534 with the OP27 any more but use NE5534 on both channels. I am running out of OPs, so i have to buy some next week.
The sound after that major change was better in any way. Especially dynamic range was increased and the bass much cleaner and faster.
I do not claim that the magic properties of the used wire and termination made the difference. No, this time it was only good engineering praxis.
In search of low noise:
The circuit i posted here has a voltage noise of ca. 1.45nV/qHz. I a balanced stage the voltage gain is higher but the input transistors are in series for a noise disadvantage of 3dB. A stage with an integrated INA163 would need a gain resistor of ca. 80 Ohms for the same gain of 36dB. Noise under this circumstances is around 2nV/qHz so some 3dB worse. A lot of expensive phonostages up to 3000,-€ use that chip and people are still not complaining about noise as far as i know.
1.45nV/qHz is not a world record and i started to think how that circuit could be improved.
The NE5534 can drive a 600 Ohm resistor to over 10V without much distortion so i lowered the feedback resistors to 340 Ohm.
50mV at the input whould drive the resistors to 1.7V and asuming that the NEs can put out 20mA of clean power there is still some headroom to 6.8V. The compleet stage doubles that at the output so there is still ample overload margin in theory.
That circuit has an equivalent voltage noise of 1.2nV/qHz. Not a huge improvement but i rebuild the stage with that modification to prove the concept. It works without detectable problem. The trimmers have to readjusted because the Fets draw a bit more current under tis conditions. Around 6mA each.
The next option is to double the input transistors. 4 plus 4 is the maximum i have seen so i will try that tomorrow. The current sources have to be slighly modified to be able to supply the expected 24mA of current.
That circuit should have around 1nV/qHz of noise.
The next step is to lower the values of the feedback resistors as far as posible.
One posibility is to use shunt feedback amps with higher current drive but i decided otherwise. I will use a second set of NE5534 as paralled buffers that will double the current drive. The circuit looks a bit exotic and unusual, especially with the aditional integrators but elemnts of it are well documented and it should work, I can now reduce the feedback resistor to 5 Ohm for a total of ca. 0.8nV/qHz.
From now on improvemenzs will be very small. Even if i whould reduce the resistors to 1 Ohm this circuit can not go under 0.6nV/qHz in theory. I praxis this amounts to less then 1dB.
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Old 15th February 2010, 09:54 PM   #874
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I am listening to the 5.0 version of the INA right now and doubling of he input transistors gave a bigger subjective improvement then going from 20 to 10 Ohm in the feedback resistors. If you allow me some speculation, i asume that noise from a plain resistor is less objectionable then noise from a FET. Equivalent input noise measured in nV/qHz does not tell about the quality of the noise. I have to make some measurements to sustain that notion of cause.
I am very pleased with the sound and dynamic range is now so high and distortion is so low that i can hear at very advanced volume without fatique.
The only problem is that my speakers give up before i give up.
I am using small 2-way monitors with 13cm woofers and 25mm dome tweeters plus an active subwoofer with 140W Hypex class D amp. That woofer stands in the middle between the main spaekers close to the frontwall and i am sitting close to the backwall.
Under theese circumstances the woofer (25cm long throw metalcone) has ample roomgain and no problems with subsonic because it summs the outputs of both channels. Disturbance from warped records cancel in that setup.
The problem is cone movement in the monitors and now that i supply the woofer from the preamp, i thought how i could releave the speakers a bit from heavy basswork.
I have lend my TDS subsonic filter to a friend, so i needed another quick solution.
The poweramp supplies the monitors only so i placed a little cap in front of the poweramp. It rolls of with 6dB octave under 100Hz and has 20dB damping at 10 Hz the aerea where the arm-cartridge resonance is. Really slow movements of the cones because of warped records are even more suppressed that way.
I had build little filters that way over 25 years ago with good succes.
There was a commercial version for TMR Standart speakers and Omtec class a monoblocks. The filter between woofer and midrange of the TMR was 6dB so i substituted the passive filter with little modules that could be inserted at the input of the poweramps. The gain in soundquality was quite substancial and the combination got popular in Germany.
The sound i am getting now after that modification took a big leap forward.
I can listen now extremely loud without bottoming the small woofers.
The sound gained in clarity and precission and was somehow "sweeter" without loss of information. I attribute that to lowering of mechanical distortion.
Whatever few losses that little cap may have ( i used a 33nF Röderstein MKP1937 to be specific) is swapped by the huge gain in quality because of less mechanical stress.
Somewhere in a chain of DC coupled high gain stages i recomend a safety cap anyway and this is the perfect place because it serves a double purpose. "Zwei Fliegen mit einer Klappe"
Just for fun i had disconnected one of the supply voltages of a DC coupled stage at full volume and you can imagine what nasty things happened next.
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Old 15th February 2010, 09:57 PM   #875
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Sorry, the Röderstein is a 1837 of cause.
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Old 16th February 2010, 12:57 PM   #876
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To reduce the noise of the Discreet INA further it is neccesary to reduce the feadback resistor to as low a value as posible. My solution is to add a buffer to double the current drive. I thought that putting it in the shunt feedback loop is not neccesarry and may create stability problems. Simply adding it at the output whould work as well i think. The buffer has 100% of feedback and is fast and low distortion. It is still in the global loop anyway. So i have redrawn the shematic slightly. I raised the decoupling resistors slightly and adjusten the 170 ohm resistors to 150 Ohm to compensate for the aditional impedance.
I will build this first and when it performes well, i will keep it that way.
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Old 16th February 2010, 01:48 PM   #877
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I had a funny idea. The input stage of the INA could be transfered into a Grounded Gate Low Z version. I have seen that done with tubes in an old VLT book by David Manley.
If it works here i do not know yet. I may try it later in the day.
Big elcaps are nessesary at the input but there is very low bias at that point and they work like a bipolar cap in this aplication so distortion from this source should be low.
They can be bypassed with a foil of say 1uF.
I have only 2200uF caps of good quality around so provided they work on a 10 Ohm feedback resistor they form a highpass with -1.8dB at 20 Hz. I can compensate for that in my sub. It has an equaliser that works in that low range.
The caps are Nichicon CL with very low ESR. They have an organic dielectric and have some leakage current so they may not be ideal here. I know at least one cap from Elna that is optimised for low leakage audio work, so that one should do a bit better. Anyway, leakage should cancel somewhat in that arrangement.
If you want to go low and have no provision to adjust here are some calculations what whould happen provided the feedback resistor is 10 Ohm. If a 5 Ohm is used the frequency doubles.
4700 uF : 20Hz -0.4dB 6.6Hz -3dB
10000 uF : 20Hz -0.1dB 3Hz -3dB
A measue of subsonic filtering could be intruduces this way and the sound can actually improve because of that.
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Old 16th February 2010, 02:03 PM   #878
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i prommised to be more specific in the future so i looked up the spec sheet of the Nichicon CL and compared it to the Elna RLB that is optimised for audio work:
Nichicon CL : Leakage current: 0.01CV or 3uA
Tan Delta: 0.26%
ERS: 0.06 Ohm @ 100kHz
Ripple current: 1190mA
Elna RLB: Leakage current: 0.006CV or 0.5uA
Ripple current: 1400mA

The CL is not that bad in any way and has a very small case.
The Elna is exeptional concerning low leakage but is somewhat bigger.
I compared the 6.3V versions.
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Old 16th February 2010, 02:21 PM   #879
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I ocured to me that noise could be lower in this arrangement because the cartridge impedance is now in parallel with the feedback resistor. The disadvantage is that the NE5534 have to drive (indirectly) a 3.75 Ohm resistor. That lowers dynamic range somewhat and makes the gain higher. The votage gain is now 182 x so 45dB. When i asume that the NEs can drive a 340 Ohm resistor to 5V for a total of 10V after the dif amp. 10 / 180 = 55mV so asuming that the cartridge has 0.5mV output at medium cutting level there is still 40dB overload margin. This headamp has huge dynamic
range !
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Old 16th February 2010, 02:28 PM   #880
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Actually the RIAA curve boosts the treble by 20dB so overload margin in the treble is somewaht reduced. Music has not much content here but scratches and blemishes could give high transient output. A clean and well preserved record is a good starting point or the gain could be reduced and the feedback driven with higher current.
The 75usec part of the RIAA could be implemented in the INA feedback loop to cure that issue too.
Anyway, having measured the voltage that develops over the 340 Ohm resistors while actually playing records i can asure you that this does not happen often in reality.
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