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Old 10th February 2010, 09:06 PM   #861
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Seems to be true. I use 10ma red on 8mA oproc.
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Old 10th February 2010, 09:19 PM   #862
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Maybe exchanging some JFET types for the variable current LEDs CCS is in order now.
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Old 10th February 2010, 09:59 PM   #863
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I use a BF Fet with very high IDss so i think pushing it down to 8mA makes the current quite stabe. I could try a low noise one but i will not put much more work into that circuit at the moment. The sound is extremely pleasing right now. On par with much more expensive commercial stages and that is the essence of DIY i think. I am very bussy with the discreet INA, still working on stabilizing the feedback loop. I will try out Microcap soon to get a better handle on that kind of problems.
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Old 11th February 2010, 05:16 PM   #864
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The Balanced Discreet INA is working stabe. I had to find a current node to implement the loop compensation correctly so i used the node where the collector of the caskode transistor and the collector of the current source meet. R7,R8,C11,C11 produce a pole at ca. 1.5usec now. Quite a bit lower then before. They roll of the open loop gain over about 60kHz. R18, R4, C13, C14 form a second pole of about 15usec. That puts the open loop rolloff of the second stage at a bit under 10kHz. The workings of the compleet stage is a bit complex but i try to explain in my own way: The cascoded input stage amplifies the signal and the gain of that stage is set to 26dB by a lokal current feedback loop around R12, R12. The open loop gain of that stage is rolled of by R7, R8, C11, C12 over 60kHz.
The output impedance of that stage is about 490 ohm and forms a shunt feedback stage with OP1 and OP2. The amplification of that stage is 33000 / 490 = 67 = 36dB. R18, R4, C13, C14 roll of the open loop gain of that stage over ca.7kHz. R1, R2, R3 provide global feedback for a closed loop gain of 37dB. So we have about 25dB of feedback.
I expect that the upper frequency limit (-3dB) of the total INA should be around 120kHz because global feedback extends the frequency response of the open loop gain stage proportional to the feedback factor. This performance though good is a little slow for my taste so i will try less agressive compensation and see how far i can extent the speed without stabilty problems. One idea is to raise R4, R18 to 100kOhm and lower C13, C14 to 68pF for example. If that circuit is stable it whould go like this. 26dB gain in the first stage, 46dB in the second stage, open loop rollof over 15kHz. We whould then have a feedback factor of 35dB now. The upper frequency limit whould then be 750kHz (-3dB) provided the Opamps are fast enough. Let´s see where i can get.
See the working circuit and the planed modifications.
P.S. I had first put in other OPs ( NE5534) to look if the stability problem was caused by the LT1115 ( that i had perhaps already fried) but that was not the case, they got warm too until i found a solution. What was interesting with the NE5534 was that they had quite low and nearly similiar offset of 6 - 7mV at the output that canceled to under 2mV in the differential stage. D.Self who is an expert on NE5534 had pre warned me that they have surprising low offset, but stiil that was good to see.
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Old 11th February 2010, 05:24 PM   #865
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This circuit needs quite big 10uF coupling caps from the input to the shunt stage and i do not have much space on my proto PCB. So i build me a low distortion bipolar elcap. There is some controversy if bias on an elcap is a good thing but i trust the measurements of Bateman that found that for lowest distortion they like the least amount of bias posible. Bipolars faired better in his test too so i build my own version. It is made from four 10uF 63V elcaps. Two back to back positive and two back to back negative and then a good quality 1uF foil on top of that for high frequency bypass. I made one of my infamous little drawings and i hope you understand the concept. Some time ago i found a handy article about amplifier stability. I post it here for download.
Attached Images
File Type: gif Low Distortion Elcap.GIF (7.3 KB, 334 views)
Attached Files
File Type: pdf fb amp analysis tools sloa017a.pdf (135.5 KB, 46 views)
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Old 12th February 2010, 05:23 PM   #866
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I put more work into the Balanced Discreet INA headamp. The goal was to see how far i can extend the bandwidth without stability issues. I am stiil not able to work in my lab in the factory because we are moving into a new building and the lab has not been setup so far. So all i have is a 40,- € multimeter and pen and paper.
I raised the gain in the transimpedance stage by 10dB and set C13, C14 to 220pF for a time constant of 22usec. An NE5534 with a gain-badwidth of 22Mhz is just stabe under this circumstances. When i lower that timeconstant then first the offset drifts higher and then the chip gets warm and then hot when i lower that constant much. Chips like the LT1115 (GB = 40MHz) or the LT1468 (GB = 90MHz) are not stable under this condisions. I tried an OP27 ( GB = 8MHz) and that was rock solid of cause. Offset after the difference stage with the OP27 was under 1mV so a precission amp in that position is a good idea when you do not like to use a servo. With the NE5534 offset was around 5mV because of canceling in the dif stage. Instead of this old cestnuts you could use some more recent like the OPA227 that has lower distortion and settlig time. I do not expect much better measured data because the OPs are running in inverted mode and the transfer linearity used is much better then the common mode not inverted case.
A asume bandwidth of this circuit to be around 300kHz but i have to measure it to be sure.
I did also some weard experiments that shoud shock the OP swappers. I put NE5534 in one channel and OP27 in the other. The sound was just fine. I also experienced that cable differences got much smaller with this circuit ( i changed the cable from arm to headamp for that experiment). With the unbalalanced circuits the cable differences where quite obvious if not night and day. Thinking about it, it may be that shelding is an issue that did not get enough attention.
I run this high gain circuit wide in the open without case so i had some slight hum at first. It disappered when i connected one point of the groundplane over a 6.8uF cap to earth of the wall outlet. If that is a soluion that whould work in other contries i can not say but that circuit deserves a PCB and a nice case anyway.
You may ask me how that circuit sounds. Well i changed my information policy a little.
Ask me and then i reply. If you are not interested in subjective apraisal because it may influence your objective point of view, forget about that. It is just my experience and another person may have other priorities. For me the case if a circuit without global feedback sounds better then a circuit with ( at east in the case of a balanced INA) will soon settle. What i need to make now are some measurements and more listening.
P.S. : Noise is a bit higher in this circuit because of the balanced nature (3dB) and the 20 Ohm resistor. I will build a version with 4 + 4 fets and lower that resistor by using an OP with more current drive. That kind of amps have a GB of around 10Mhz so should not create any stability problems here.
Noise with the circuit right now is still low enough to be inaudible at the listening seat when i play the volume i like.
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Old 12th February 2010, 05:30 PM   #867
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Joachim,
I apologise if I sound like a broken record, and am very interested in how these circuits sound, but am disappointed that you didn't seem to do any loading resistor comparisons with one consistent circuit. It seems you put 47k in one circuit, then changed the circuit and then tried 1k.

Have you tried 47k compared to other values in one circuit? Comments?

Regards, Allen
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Old 12th February 2010, 06:27 PM   #868
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Allen, maybe you are not up to date but this thread get´s long so no wonder you missed some details. I may also not have comunicated everything i do here. I used the High Z MPP Fet Headamp consistently to listen to resistors. I have tested it with 47kOhm 1% Metalfilm, 1kOhm YAM2, 3kOhm Yam2, 1 KOhm nonmagnetic Tantal (Shinko ?), 1kOhm
Dale RS60 antimagnetic, 1kOhm Texas components Z foil, 1kOhm Caddock MK132.
Here is my ranking:
1. Z foil : best combination of detail and musicallity
2. Tantal : nearly as open as the Z foil, very elegant tonality
3. YAM2 : i could not hear a big difference between 1kOhm and 3kOhm. Very close to
Tanatal with a slightly more light tone
4: Caddock MK132: slighly bright. Came out best in an earlier version of the Fet
Headamp where it brought a wellcome opening of the sound.
5: Dale RS60 : kind of disappointing in this aplication. Loss of microdetail like tape
splices, preechos etc. Very "evarage" sound. Nothing spectacular and
moving.
6: 47kOhm Metalfilm: i only listened breefly to this combination because i prefered all
of the others.
So what i should do a comparison of the 1kOhm Z foil to a 47kOhm Z foil but i have non right now.
Sound of the Discreet INA : my finest yet. No loss of microdetail as far as i can tell. I listened to very critical records that i know in and out like "Return to Forever". The first time i heard a rattling noise in one of my speakers that proved to be of mechanical origin so low is the distortion and so high the dynamic range. I had to place that speaker slightly different on the stand to tune that problem away.
I use 2 x 47kOhm metal film for loading of this stage and use ages old OPs so improvement may be posible still.
The circuit is remarkable tolerant for PSU issues and component coices. For example i run it currently on two lead gel accus. One nearly empty at 9V and the other full at 12.6V. No problem with offset and noise or audible distortion under this circumstances.
It is really a kind of worste case szenario right now.
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Old 13th February 2010, 04:14 AM   #869
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Some may not like the coupling caps in the feedback chain so i thought about a way to DC couple. I think two simple integrators can do the job just fine. i did not build this, that has to wait until tomorrow.
While experimenting with my subwoofer i switched my poweramp on and off one time too much. When i can not get some fuses over the weekend i have to listen with my tube monoblocks. This is saturday the 13th and not fryday the 13th i thought.
Freudsche Fehlleistung included.
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Old 14th February 2010, 01:30 AM   #870
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Today i DC coupled one channel ( the one with the NE5534) to look if it works and if a run into any problems. The circuit worked fine exactly as posted before. No pumping at high level, no increase in noise.
I then did my wostcase experiment again, this time with two fully loaded 4AH lead accumulators from motorcircles. I played the DC coupled version on the left channel and the AC coupled version on the right channel. Again the sound was just fine. On stereo material it is hard to hear small differences like that anyway. Too much interesting things are going on. It certainly had no effect on the focus. Voices and instrument appeared sharply deleniated on the soundstage. Tomorrow i will DC couple the other channel and next week i will try out the same Opamps in both channels.
So when you hate servos put in caps and when you hate caps put in servos. I prefer the servo solution because an elcap can get faulty over time and a 10uF film cap is big, even a low voltage Polyester type not to mention PP or even teflon. A compact layout is then quite difficult. One theoretical advantage of the servo is that OP1 and OP2 are now running on a virtual ground and circulating ground currents can not enter PIN3 so easy any more. This beast is quite complex but sounds very transparent and works without problem. If you want the advantages of a discreet INA like good common mode rejection, very low distortion and good low level resolution i do not know which part i could spare. They all are neccessary in one or the other way.
I used Texas Instruments TL071C for servo purpose and the offset after the Diff Amp is under 2mV and rock stable. The TL051 has even better and garantied precission and other low offset OPs can be used here based on wallet and taste. I doupt if they bring a measurable benefit. This servo is very slow and should not worsen distortion in the audible range. I usually use OPA134 in that position to satisfy my soul to avoid conflict with the advanced user and even the very expensive OPA627 has been witnessed in this position. Anyway, the circuit erodes component coices and PSU issues extremely effective. Well i used paralled 1% PS-HX from LCR in the 75usec RIAA for good channel matching and stabilty. The new Röderstein 1% MKP´s work as well here. Both are easy to get at Farnell or RS Components and are not expensive for my taste. I have not tried Teflon in this position yet but have specified them in one of my commercial stages.
All the listening tests of the different Headamps where all done with the capacitive RIAA. I will post that circuit again for the compleet picture to what i am listenimg right now. Tha RIAA stage is run double momo on two LC Audio low noise regulators with 29mH coils in series and generous general and distributed decoupling. The INA runs wide in the open, dead bug style with my triple ground plane system and one point of the groundplane connected with a 6.8uF PP cap to earth of the wall outlet. That made a huge difference. I use 1m Wire World Silver Eclipse between tonearm and Headamp and 0.5m Platinum Eclipse between Headamp and RIAA. As i said before i could not hear much difference between my chaos fractal cable and the Wire World this time. I use the Wire World now because my cable is kind of fragile and i do not want to destroy it during stage swapping.
By the way, the Opamp in the capacitive RIAA is an LT1468 and the servo is an AD711 i think (maybe OPA134). I mounted the servo upside down so i can not see it. Maybe with a small mirror. The 100nF cap is a Vishey/Röderstein MKP1837 1%, the caps in the servo are NOS yellow Röderstein MKT. That simple circuit sounds very pleasing but is in no way the end of the story.
So what more can be done ?
I wait for the MU metal encapsulated precission coils from Souther.
When i have them i will try the inductive RIIA again and will see if i can erode the last trace of hum.
A fully ballanced RIAA with Balanced ouput could be developped. I have posted circuits like that before (without the balanced output).
That could make sense in my system because i have a 5m run of Koax Video cable between the RIAA stage and my preamp
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File Type: pdf Capacitive Active RIAA.TSC - TINA.pdf (49.8 KB, 79 views)
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