Naim 522 MM phono boards re-visited

[coredump]

The Naim MM boards have always been somewhat bugging me as they seemed to be performing worse than their MC brethren; subjectively, they always sounded duller, somewhat lifeless, etc.

To my ears, even a lowly MF LPS has more life, space and dynamics (achieving this with the dreaded 5534 BTW). If you followed http://pinkfishmedia.net/forum/threads/nac92-second-life.199517/ then you know that the PSU is not to blame for this difference. The results are the same with a TeddyCap.

So, today I decided to measure the resistance and capacitance of both boards as seen by the cartridge. The multimeter said 61K5 and 61K6, which seems fine to me. Easy to change with a parallel resistor, if needed.

Then the capacity - 10.62nF and 7.74nF
I thought I did something wrong, double checked the connectors used for this, changed the cable to eliminate parasitic capacitance - but to no avail.

Tried the Benning MM3, used the same connectors - now the values were 97.3nF and 94.7nF.

I guess both devices use different frequencies for the measurement, both ones were reproducable.

I guess I need a function generator and a scope to really analyze what's going on here?

Best regards,
Oliver

 

[davidsrsb]

61R or 61k?

 

[coredump]

Hi David,
61K of course - corrected it in the original post.
Thanks,
Oliver

61R or 61k?

 

[davidsrsb]

Some Naim MM boards have 100pF on the input, others 470pF. The latter is way too high and will give a treble suck out followed by a big high treble peak.
A photo of your board would be handy.

 

[coredump]

Hi David,
it seems my boards fall into the latter category:

I guess I have to change the RC filter then?

Best regards,
Oliver

 

[coredump]

Taking http://s26.photobucket.com/componen...114/neiljadman/NJ322+Cards/NJ322Schematic.jpg <- this schematic as a reference, my general question would be how much of the capacitance is really part of the resonant circuit. Subjectively, the sound became better when a 100pF capacitor was attached to the board connectors - in parallel to the 61K resistor.

 

[davidsrsb]

I would change the 470pF to 100pF, the 47R resistor in series is low enough not to have much effect. There should be a second 330k resistor extreme bottom right in that photo. That resistor and the 330k top, second from right, are in parallel for AC and in parallel with the 62k, giving 45k for AC signals.

 

[coredump]

The missing resistor is implemented on the flip side of the board, as a bias filter (two resistors with one cap to ground „in the middle“).
Will change the cap, I‘m sure there are a few left in my stash of polystyrenes.
Thanks!

 

[davidsrsb]

What resistor and capacitor values? It will modify the input impedance

 

[BobMaximus]

I don’t understand what’s being discussed here. The 470pF capacitor is present on both the MM and the MC Naim boards. In fact it’s on all of the Naim boards. It’s there to filter out radio interference. It’s not seen by the cartridge, because it's behind the input coupling capacitor.

Only the MC boards have a loading capacitor, which is in parallel with the loading resistor. The MM boards do not have a loading capacitor.

 

[coredump]

Hi Bob,
my problem is the perceived loss of treble information with some cartridges - so I just wanted to find out the boards’ termination values (C/R).
As it seems, I am unable to measure these with my equipment.
If there is no capacitance visible for the cartridge, this would be an explanation for my findings.
The boards only received a recap und the bias filter mod.
Best regards,
Oliver

 

[davidsrsb]

...It’s not seen by the cartridge, because it's behind the input coupling capacitor.

Only the MC boards have a loading capacitor, which is in parallel with the loading resistor. The MM boards do not have a loading capacitor.

The circuit is something like this http://www.acoustica.org.uk/t/naim/preamp_pics/321_gain_board_schematic.jpg
The input resistor to ground is 62k
The resistor from the tantalum to the transistor base looks like yellow-violet-black-gold ie 47R, this about 1/10 of the coil resistance, so negligible. Therefore the 470pF IS across the cartridge

 

[BobMaximus]

OK sorry. What I said above isn't actually true! The MC boards do not see the 470pF capacitor, because it's on the next transistor stage. A cartridge on the MM boards, however, will see some of this capacitance. I'm not sure what effect this'll have though.

 

[coredump]

I‘ll give David‘s suggestion a try. Thank you both.

 

[MJS]

It's 470R not 47. That's what I see and the circuit diagram confirm.

 

[davidsrsb]

It's 470R not 47. That's what I see and the circuit diagram confirm.

It looks like it has been changed at some point, the soldering is not original.
That 3rd stripe looks more like black than brown. Not always easy to tell though with poor resistor marking and camera lighting.

 

[coredump]

Carts that from my POV suffer very much from treble loss are classic Goldrings (1042, 1022GX); their generator is specified by the manufacturer with 570mH/660R.

AT carts like the 140LC/440Mla/150MLX Seem to differ - I read somewhere on the web that the generator of these carts is more in the ballpark of 450mH and 2300R (at 1kHz).

Edit: 440Mla is stated to have 490mH/3200R (at 1kHz), dito the 140LC.
The resistance of the AT carts is 4-5 times higher than the Goldrings.

 

[coredump]

We have midrange definition!

Changed the cap as per David's recommendation to 100pF, superb result! The sound has gained definition, power - and colour. Still a bit "creamy" compared to the opamp based phono pres I know (with the exception of Loricrafts's "Missing Link"), but perfectly enjoyable.

Cheers, David!

For the record: Linn K-18 II with Jico ATN152ML sounding very nice on this fruit box.

Best regards,
Oliver