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Old 19th November 2010, 09:25 PM   #1891
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I do not exactly understand. I stiil USE that coil at the entrance but do the bass-mid ACTIVE in the first stage too. I could of cause make the first stage simply a linear amp and then do the bass-mid in the second stage.
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Old 19th November 2010, 09:33 PM   #1892
Salas is online now Salas  Greece
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Misunderstood. Thought you do it vice versa. interesting to hear for your results soon!
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Old 19th November 2010, 09:38 PM   #1893
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Maybe we need a schematic, so that we all know what i am talking about. This is what i now build but please take it with a grain of salt. It is not PROVEN technology yet.
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File Type: pdf Simple Inductive RIAA.TSC - TINA.pdf (39.6 KB, 123 views)
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Old 19th November 2010, 09:41 PM   #1894
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Now I see! Thanks.
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Old 19th November 2010, 09:43 PM   #1895
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Ok, fine, i am releaved.
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Old 20th November 2010, 03:36 AM   #1896
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JG - did you by chance check out my thread on an inductive RIAA preamp that I posted about a year or so ago? It looked possible to do the preamp stages with a couple of jfets and a moderate value of inductance, attainable using gapped ferrite pot cores. I haven't had time to try it yet, but it may serve as food for thought. An advanced title search with "inductive RIAA" as the search parameter appears to be very specific....
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Old 20th November 2010, 03:59 AM   #1897
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Yes, i have gone over it and found it very interesting. I have to reread it now. Actually i have build an active inductive RIAA and posted it here. I got very low noise and the sound was particular "open" in the midrange. My friend Jürgen Ultee builds an commercial inductive RIAA called "The Grail" for Van den Hul that got very good reviews over here.
He uses Epcos open frame ferite inductors made from a special material. He told me that Van den Hul had to buy 20.000 cores because after he finished the design they stopped production and this was the minimum quantity for another roll. This much about an industry that has lost the interest in high quality audio a long time ago here. Ones Saba, Grundig, Telefunken etc. made world class products and the reason was, when i talk to older senior designers, that they had access to very good quality passive parts.
Now High End Audio is a boutique busyness.
Back to my Simple Passive Inductive RIAA, i ran into some offset problems. I am using the ADA4898 at the input because it likes to drive low value resistors. "Conventional" Phono Clone topologies use a feedback resistor not much smaller then 1kOhm but this whould give me too much gain in the input stage. The DC gain of my circuit is around 300 and the input offset voltage of the ADA4898 is specified as 20mV typical and somewhat over 100mV maximum. I landed at around -8V so this is in the ballpark. I will try a compensation resistor bypassed by a big Elcap. I could use a servo but time is short to ETF and this is only a prove of concept. If it humms i have some MU Metal sheets but i will not go into any length to win a new world record here in the few days that are left.
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Old 20th November 2010, 04:05 AM   #1898
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Well, actually i am talkig bull.....
The input offset is 20uV to 100uV so i may have overlooked something else.
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Old 20th November 2010, 04:28 AM   #1899
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I found a better solution. I put the bass-mid equalisation in the second stage. I did not like the rather big 1uF at that point anyway. I have it working without the bass-mid EQ and it looks good on the scope.
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Old 20th November 2010, 06:03 AM   #1900
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The idea to do the mid-bass EQ in the second stage works. The offset i get is 0.3V now and a good cap at the output is all i needed to manage that. The 1kHz square wave looks only half as bad as i expected. A very slight ultrasonic overshot and a response that leans just a bit to deeper regions. I can live with that without tweeking. See the schematic. I also will post a screenshot later.
Why is R13 so small you could ask ? First it does not contribute much noise that way and second the ADA4898 can swing 40mA clean so is able to do 6V into 150 Ohm and i think that this is more then enough considering that the signal is down ca.20dB @ 20kHz by action of the input coil. The second Opamp is an OPA134 i rescued from the waiste bin. It is not my absolute favorite but i think in this inverted configuration it is fine and being a Fet input amp, offset is low.
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File Type: pdf Simple Inductive RIAA 2.0.TSC - TINA.pdf (41.8 KB, 98 views)
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