Please help me adjust my MC pre gain to match my cartridge - diyAudio
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Old 11th February 2012, 12:39 PM   #1
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Default Please help me adjust my MC pre gain to match my cartridge

Hi,
I would like some advice on setting up my Classe Audio DR4 phono pre amp to match my Linn Troika cartridge.


The Classe Pre amp has changeable gain resistors in the MC phono circuit so I can adjust the gain to suit my cartridge. I can't really figure out what it should be set at and I thought some of you smart beans could have a think about it for me. I am quite a noob with it although I have had the system for some time I have never really known if it is set to optimum as the pre amp was bought 2nd hand and not matched to the cartridge previous.

-------------------------------------------
Here are the specs on the cartridge:

Specifications: Low-output moving-coil cartridge with unique three-point fixing arrangement. Recommended tracking force: 1.51.7gm. Stylus type: Nude Vital super-elliptical.
Output is about normal for a low-output 'coil (around 0.2mV at 5cm/s), and the tubed SP-10 had no noise problems. With the down force set at 1.7gm, the Troika just failed the 70m lateral modulation cut on the Ortofon Test Record at 78F (it was a hot day); with the down force increased to 1.8gm, it managed to clear the 70m cut, but 2.1gm was needed for the maximum 80m cut. It cleared the 50m vertical cut at 1.7gm down force. While not quite as surefooted as the Shure V15 V MR (others report that the Troika is less secure at lower temperatures), this is pretty good tracking for a moving-coil design.

Channel balance at 1kHz was spot on at around 0.05dB difference, while crosstalk was both superbly low and symmetrical, measuring -34dB at 1kHz, left-on-right, and -35dB, right-on-left. Obviously, the motor geometry is very well-aligned, both with the stylus orientation and the groove (as should be expected at this price level). The cartridge designers' ability to optimize the compliance for a dedicated tonearm design was shown by the positioning of LF tonearm resonance: ideally placed laterally around 10Hz in the Ittok, and vertically a little lower, around 9Hz, both with only a moderate Q and a moderate amplitude rise. The measured frequency response, using the third-octave noise bands on Denon XL-7007, showed the ubiquitous upper-midrange depression to be 23dB or so deep, with a 2dB rise at 20kHz.John Atkinson
-source: Stereopile magazine review.
---------------------------------------------------

Here are the specs and instructions from the Classe DR4 Pre Amp Owners Manual:

Schematic here:
Click the image to open in full size.

The Phono input can accommodate both magnetic or moving coil cartridges of varying output. In the MC Position, additional gain is factory set to 22dB. The gain can be adjusted from 20 to 40dB, in order to optimize system gain and low noise. (Note: The MC gain is in addition to the standard MM gain of 35dB.)
Changing the value of R6 and R48 (Left / Right) according to this chart:
Resistor Gain
1000F - 20dB
1270F - 22dB - Standard
1650F - 24dB
2150F - 26dB
2870F - 28dB
3830F - 30dB
5110F - 32dB
7680F - 43dB
1151F - 36dB
1961F - 38dB
4221F - 40dB
-------------------------------

I do not know what it is set to at the moment as I have not taken the lid off to see.

-----------------------

My System:
Linn Sondek LP12 - Ittok II - Valhalla - Troika cartridge
Classe Audio Pre Amp
Krell KSA150 Pure Class A amp
Duntech Princess speakers with newly rebuilt crossovers.

---------------

How this question came about:

I've had the above system foe about 10 years and always found the turntable to be somewhat noisy and very detailed, every pop and crackle and sometimes a bit of interference could come in with some buzzing etc. It just never really sounded Hi End to match the system. Just a bit too much background noise to sound great.

The Krell amp has been rebuilt completely and may have been the source of some of the above problems. I have never been really happy with the setup but I have no audio mates to nut out the problem so I just lived with it.

The Krell amp is in the shop again currently and I am using a cheap Yamaha integrated (RX-350) to get by. It has a MM phono pre and IT SOUNDS WONDERFUL! It sounds just like a CD but with a few tiny pop and crackles here and there. Totally listenable and enjoyable. Not much gain - I can run it full wick without clipping and it is at an easy level.

I was quite shocked at the difference in the two systems given my speakers are notorious for exposing the weakest link.

So this has led me to pose the potential mismatch level question.
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Old 11th February 2012, 12:49 PM   #2
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Old 11th February 2012, 02:08 PM   #3
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Hi null your mc has an output of 0.2mV and needs to be amplified up to the average mm output of 5mV at 5cms so 20log 5/0.2 = 28db .Hopefully your pre is set for a higher gain than this which might explain some of your problems.
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Old 11th February 2012, 05:20 PM   #4
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My friend who owns a Troika uses a total of 66 dBs, pre-pre mc(26dBs) + pre mm (40dBs).
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Old 11th February 2012, 05:48 PM   #5
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Hi,
I hope there is available an input impedance selection for your preamp. Most
MCC need a minimum low resistance (>100 Ohms) in parallel to a maximum large capacitance (< 2nF) as a load to match for optimum noise and frequency response performance. Your cartridge should list the RC spec.
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Old 12th February 2012, 07:20 PM   #6
vgeorge is offline vgeorge  Greece
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Quote:
Originally Posted by demeterart View Post
Hi,
I hope there is available an input impedance selection for your preamp. Most
MCC need a minimum low resistance (>100 Ohms) in parallel to a maximum large capacitance (< 2nF) as a load to match for optimum noise and frequency response performance. Your cartridge should list the RC spec.
No way MC' s need capacitance loading.
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Old 12th February 2012, 07:37 PM   #7
Nanook is offline Nanook  Canada
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Quote:
Originally Posted by epicyclic View Post
Hi null your mc has an output of 0.2mV and needs to be amplified up to the average mm output of 5mV at 5cms so 20log 5/0.2 = 28db .Hopefully your pre is set for a higher gain than this which might explain some of your problems.
To me this seems to be the most likely culprit. One thing you may consider is to create a nice little aux board using dip switches and a bank of resistors. The leads could be soldered to the spots on the board where the resistors sit.

Why not email Classé ?

As for not really enjoying or thinking your system lacks, using top tier products should give, good results, but there are no guarantees.
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Last edited by Nanook; 12th February 2012 at 07:46 PM. Reason: added email link
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Old 13th February 2012, 07:39 AM   #8
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Hello,

Please note that any RLC system can potentially present a high frequency peak given the appropriate component values. Not all MC heads are the same. The internal inductance varies from a few uH to several hundred uH and the loading of the system includes the large capacitance of the shielded cables of the turntable arm plus that of the interconnecting cables between the turntable and the preamp. A low preamp input resistance can usually avoid peaking but the optimum response is achieved by taking into account the whole chain from the MC head through cables and connectors to the RC of the preamp. The manufacturer of the cartridge usually suggests the recommended load in most cases that matter.
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Old 13th February 2012, 11:44 AM   #9
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Quote:
Originally Posted by demeterart View Post
Hello,

Please note that any RLC system can potentially present a high frequency peak given the appropriate component values. Not all MC heads are the same. The internal inductance varies from a few uH to several hundred uH and the loading of the system includes the large capacitance of the shielded cables of the turntable arm plus that of the interconnecting cables between the turntable and the preamp. A low preamp input resistance can usually avoid peaking but the optimum response is achieved by taking into account the whole chain from the MC head through cables and connectors to the RC of the preamp. The manufacturer of the cartridge usually suggests the recommended load in most cases that matter.
Hi demeterart,

What further information would one need in order to solve the problem you have suggested giving the info I have already supplied?
I will email the manufactures and return their response.
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Old 13th February 2012, 12:41 PM   #10
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Hello,

Well, what is inside the black box named Z1 at the front end? Or you could simple ask the manufacturer what is the equivalent input impedance ( R in parallel to a C) of the preamp and if that is configurable. You could also ask, the manufacturer of the cartridge this time, about the recommended load for their MC and see if you can match the two answers.
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