How low can I go? (Voltagewise with a Cambridge Audio 640p)

[admin_exported]

So I'm considering switching to a low output moving coil cartridge, but for the moment I'd like to keep my Cambridge Audio 640p phono stage (which I'm quite happy with for my current high output Denon DL-160 cartridge).

I've read somewhere on the net that the way to match a cartridge to a phono stage is

G = 20 log (V1/V2)

where G is the gain in dB, V1 is the output from phono stage, V2 is the output from the cartridge.

If I haven't forgotten all my maths then this means that

V2 = V1 / 10^(G/20)

Now the CA phono stage can do 55 dB gain for MC @ 1 kHz, and my Audiolab 8000a amp inputs take 100 mV.

So I think this means V2 = 0.177 mV. Does that mean that I can use any moving coil cartridge whose output is at least that? Would I be cutting it too close with a Denon DL-304, which has an output of 0.18 mV?

To double check, using the initial formula I find that I'd need a gain of 54.89 dB for this cartridge, where the CA amp says that it provides 55 dB (@1 kHz -- what exactly does this mean?)

Someone might also help me with this line which I found on the web for my Audiolab amp:
Inputs: Phono MM: 2.0 mV/47k/85pF; Phono MC:
110µV/100ohms/3.3nF; CD, video, tuner, tape 1 & 2: 100mV/20 kOhm; Power amp: 707
mV/50kOhm

That's where I got the 100 mV for my amp. Also -- does that mean I can use a 110 microvolt MC cart with the amp's phono stage? Seems very low!

Maybe someone can explain all this gain/voltage business to me properly in case I'm going about this the wrong way.

 

[chebby]

Try looking up the Denon AU-300LC step-up transformer. You can feed that straight into a MM phono stage.

 

[Anonymous]

I'm certainly aware that I can get a stepup transformer or a phono stage that will bring the DL-304 up to MM levels, but since I alread have a way (two in fact!) of using MC cartridges I'm more interested in how low I can go with what I've got.

Also, perhaps I'm being superstitious, but is it really a good idea to introduce a whole new stage to the signal path? Or is this what the MC stage of the 640P would do internally anyway?

 

[chebby]

The transformer will just plug straight into the MM input of your Audiolab. No need for the CA 640P.

(The Denon AU-300LC is very small and 'passive'. No need for any power supply and can be hidden away at the back of your hifi out of sight.)

 

[Anonymous]

Thanks Chebby but the MM stage of my Audiolab isn't as good as the MM stage of my 640p. And I'm not too keen to get a stepup at the moment as i assume they're not cheap (or are they?)

Have I really scared off all the vinylheads out there with my math? I thought the two types of esoterica would go hand in hand!

 

[chebby]

Don't care about the maths.

If you want to optimally match your moving coil (impedance, voltage output etc) to your phonostage AND you don't want to change the phonostage - then pick a moving coil cartridge that falls comfortably with the range of the 640P's 3.35mV / 0.5mV to achieve line output of 300mV as stated in the 640P's published specs.

The 0.18mV of the DL-304 is just over one third of the lowest recommended (0.5mV) by CA.

Unless you are wedded to the idea of the DL-304 there are plenty of other great MC cartridges.

Or you could get something like a Trichord Dino which has 'Very low output" selectable for 0.1mV - 0.25mV cartridges (74dB gain) ...

http://www.trichordresearch.com/dinospec.html

Ideal.

 

[Anonymous]

chebby:
If you want to optimally match your moving coil (impedance, voltage output etc) to your phonostage AND you don't want to change the phonostage - then pick a moving coil cartridge that falls comfortably with the range of the 640P's 3.35mV / 0.5mV to achieve line output of 300mV as stated in the 640P's published specs.

The 0.18mV of the DL-304 is just over one third of the lowest recommended (0.5mV) by CA.

Sounds like I've done the maths right then, since my Audiolab has inputs that are 1/3 the line output of 300 mV 😛

Anyway thanks for your input. I was just curious about how to figure it out.

Any MC carts you would recommend with slightly higher output? Under the 300 pound mark, or is that too low for MC?