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Old 8th March 2010, 03:17 AM   #991
Salas is online now Salas  Greece
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I am always getting good results at 10X cart's Zo for some reason. On many carts.
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Old 8th March 2010, 07:32 AM   #992
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Originally Posted by Joachim Gerhard View Post
[snip] I remember he told me that there are relays that are optimised for the uA range so they should have good resolution with tiny signals.
Yes, I noticed that many relays have a spec on *minimum* current through the contacts. Has to do something whith what they cal punch-through IIRC, for the signal to get through the surface (oxide?) layer on the contacts.

Then there are small-signal relays that are specifically designed for very low signal currents. As I understand, there is still the need for a minimum current but it is extremely small. Anybody has more info on this?

jd
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Old 8th March 2010, 01:22 PM   #993
brianco is offline brianco  Ireland
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................I have a pair of YAM2 ready for that.
I will send you some YAM 300 Ohm in the morning!
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Old 8th March 2010, 06:16 PM   #994
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Thanks Brianco. Resistor swapping is fun but also hard work !
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Old 8th March 2010, 06:23 PM   #995
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Why don't you get a copy of Holm's book Jan?
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Old 8th March 2010, 06:25 PM   #996
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Hi Joachim,
I'm a bit lost: which is your current High Z version?

Rüdiger
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Old 8th March 2010, 10:15 PM   #997
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Hi Rüdiger. I will post my current setup soon. The latest High Z MPP is trimable in terms of distortion and DC offset. The gain in quality is suprising considering the small change in topology.
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Old 8th March 2010, 11:13 PM   #998
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Here is the circuit i curently use. On first look it is not much different but it is optimised in many ways. First i raised the gain in the input stage and lowered the gain in the output stage. I raised the base resistors in the cascodes to 470 Ohm. I trimmed the input stage with the 20 Ohm trimmer. To make the low Z Fet version work i needed that trimmer to make the input offset low. I whould not recomment that version for DIY because a lot of nasty things can happen when the powersupply voltages are not stable or when only one voltage is working. That can happen on switch on or when you use batteries that can have different fill.
After having adjusted the 20 Ohm pot for lowest DC offset at the input i wired back into High Z and could not beleave my ears. That circuit now sounds as good as the Discreet INA, so low is the distortion. I was really stupid using 10 Ohm fixed resisrtors here all the time.
I connected the "new" High Z Fet MPP to the inductive RIAA. I am now using the coil that Sowter made for me. Hum is not existing with this coil, even with the coil in open air without earth connection. This MU meatal case is very effective. It should be because 100´s of transformers and inductors are used in professional mixing boards without hum problems.
I could bring the offset from the High Z MPP down to + - 5mV. This is still too high for the inductive RIAA because i have to use a 100kOhm resistor after the servo. A lower value whould have an effect on the frequency response of that stage. So i am using an output cap.
I have an idea to stabilise the offset even further and will post it later.
Attached Files
File Type: pdf High Z MPP Inductive, DC and Dist. Trim.TSC - TINA.pdf (93.0 KB, 103 views)
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Old 8th March 2010, 11:39 PM   #999
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This version has even more stable bias because the current sources are referenced to ground. The trimmer allows for + - 40mV adjustment. i think this could be DC coupled into anything.
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File Type: pdf High Z MPP Fet DC offset fineadjust.TSC - TINA.pdf (69.9 KB, 97 views)
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Old 8th March 2010, 11:51 PM   #1000
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High Jan and John !
I checked the relays my friend is using.
They are NAIS TQ2 12V.
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