Phono preamplifier load

[ALPUY]

Hi. I think I've already asked this question, but I can not find the link.
I have a Marantz PM6006 integrated amplifier, with integrated phono stage.
I have built several phono kits and I have seen several phono pre circuits from other manufacturers, but I have never seen this MM cartridge load configuration that the Marantz PM6006 has (the value of the choke is not specified).
My question is how this load configuration interacts with the cartridge (AT440MLb, 490mH and 3200 ohms).
I appreciate if someone who is knowledgeable in this topic can explain it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Of course I have disarmed the Marantz to get photos.

 

[analog_sa]

I think this is an RF filter and does not load the cartridge within the audible range.

 

[ALPUY]

Okay. Thanks, but I see a resistor of 330k after the choke, instead of the traditional 47k as charge.

 

[Nico Ras]

For all practical purposes the 330K and 56K is in parallel, that is 47Kthe reason for splitting it is to keep the cap discharged of any dc.

 

[analog_sa]

Okay. Thanks, but I see a resistor of 330k after the choke, instead of the traditional 47k as charge.

This is very common when the input stage is bipolar and it is essential to have a dc blocking cap - the 47k is split in two.

 

[ALPUY]

I see when simulating in LTSpice, an important fall between 10khz and 20khz (approximately to 11 dB).
Can it be related to the RF filter?

 

[lcsaszar]

Most cartridges have a mechanical resonance peak in this range. The resulting frequency response is supposed to be flat.

 

[analog_sa]

Cannot read values off the simulation. How did you choose the value for the choke?

Perhaps you should short the input filter and make sure the riaa itself works as intended.

 

[ALPUY]

The value of the choke was obtained from the circuit of the phono stage of the Marantz PM5004.
refers a value of 320 mH. It seemed a lot to me and in the circioto of the Marantz PM6006 I put 320uH.
Simulate by eliminating the RF filter.

 

[MarcelvdG]

320 mH is quite normal for a moving-magnet cartridge.

 

[analog_sa]

It is certainly 320µH, based on the pics, common sense and other similar circuits.

 

[PRR]

> It is certainly 320µH

Which is "nothing" compared to Alpuy's 490mH pickup.

And is only 40 Ohms at 20KHz, so no effect in a 47K circuit.

With the 200pFd present, 320uH resonates at 630KHz, and will fall 12dB through the broadcast band and beyond. As you say, an RF filter.

Anyway that doughnut is too darn small to do anything to a high impedance audio circuit.

 

[ALPUY]

analog_sa, I assumed it was 320uH.
That circuit I suppose is from a Marantz of the good guys.
I see that the phono circuit is designed on an HDAM.

 

[ALPUY]

AT440MLb

Coil impedance : 3,200 ohms at 1 kHz
DC resistance : 800
Load Capacitance : 100-200pF
Coil inductance (mH, 1 kHz) : 490
Dynamic compliance (x10-6 cm/dyne) : 10
Static compliance (x10-6 cm/dyne) : 40
Vertical tracking angle : 20-degree
Dimensions (mm) : H 17.3 x W 16.0 x D 28.2
Weight (g) : 6.5

Frequency Response 20-25,000 Hz
Channel Separation 28 (dB at 1 kHz)
Tracking Force Range 1.0-1.8 grams (1.4 optimal)
Stylus Construction 0.12 mil Nude
square Micro-Linear
stylus
Recommended Load Impedance 47,000 ohms
Output 4.0 (mV, 1 kHz, 5 cm/sec)
Channel Balance 1.0 (dB at 1 KHz)
Stylus Shape MicroLine®
Cantilever Aluminum Pipe
Mount Half-inch
Replacement Stylus ATN440M

 

[MarcelvdG]

Sorry, I was mixing up the RF filter coil and the cartridge. 320 mH would be a normal inductance for a cartridge, but not for an RF filter coil.

 

[madis64]

I see when simulating in LTSpice, an important fall between 10khz and 20khz (approximately to 11 dB).

I would say that more than 400 pF of capacitive load is a more likely reason.

You should try removing C4001 from the simulation as a start - and also from the amplifier, to check if you hear the change in sound.

From my personal experience - capacitive load 200 pF vs 400 pF is audible.

 

[ALPUY]

I have thought about that exactly. the AT440MLb charges with 100-200 pF. Only the turntable cable has 125 pF. Probably simulate without the load capacitors and then remove them from the circuit. I have several DIY phono pre and. inguno has charge capacitance

 

[madis64]

... and then remove them from the circuit.

Probably it would be better to leave at least 100 pF on the phono input - you can add capacitance later on e.g. with (diy) capacitance load plugs.