current amplifying phono stage

[Elso Kwak]

Originally posted by john curl
The Levinson JC-1AC was developed 29 years ago as an AC version of the JC-1. However, I made a 'fatal' mistake. I used a LOW Z input by using a grounded base connection as the input. This overdamped the MC cartridges. I proved this by rewiring the design to be switchable to either 100 ohms input or grounded base (very low Z) and listened to the difference. 100 ohms input won the contest.

Hello John,
Don't be too modest. The Levinson JC-1AC was the best head amp I could find in that time.
I compared it with numerous head amps from Ortofon, Thorens, Van Den Hul a.o. and even more step up transformers. The JC-1 AC was always the clear winner.
My own JC-1AC was struck by lightning on the electricity grid of the street illumination. Also the cartridge and some modules in the JC-2 were gone. When this happened the model was discontinued so I decided to build my own with the SSM2017.

[john curl]

Elso, it might be that your phono cartridge preferred a summing type input or low Z. The JC-1 AC sounded 'overdamped' with my phono cartridges. I am not criticizing the rest of the design that much, even if I don't use it anymore.
Now, this thread has all sorts of people 'discovering' summing type inputs and while this is OK, it can also be a problem. For example, about 20 years ago, I designed my Vendetta Research SCP-1 (SCP-2 as well) input stage with a link that could be changed to have a summing type input. In future designs I have thought to eliminate this link because it didn't seem necessary for modern moving cartridges. However, JCarr and others have found low Z to be a good load for moving coil cartridges in some models. This is interesting to me, but I do wish that I had a specific model MC, or a design parameter to follow in order to determine what direction to go.

[BrianL]

So, John, to clarify what people have not stated clearly:

In the "old days" when the summing junction input (transimpedance
amp) was useful for phono cartridges, the cartridges themselves
had very low impedance and very low output. The advent of
MC's with medium/high outputs (and corresponding higher
output impedances <due to more windings of finer wire?>),
the transimpedance input is not the best match.
(the bottom line being that cartridge designers have been
trying to make their designs easier to interface with "normal"
electronics, improve s/n, etc.)

Is this a correct summary?

If so, it would seem that this would still be useful for cartridges
that are still designed using very low-Z.

[fdegrove]

Hi,

If so, it would seem that this would still be useful for cartridges

IMO, certainly.
The top quality MCs are still very low output and since these tend to be fitted to the best systems around, the capability of the preamp to convert that tiny signal into a usable voltage is still as important and difficult as ever.

Cheers,

[john curl]

Actually I don't know what the exact mechansim is, but I was once told that the magnetic assembly is important. Hi Z windings certainly would 'tend' to increase the optimum loading, all else being equal, but many MC's are still low Z and apparently, in one extreme case, best loaded with low Z. Go figure! Where are the phono cartridge designers, now that we need them?

[SY]

Where are the phono cartridge designers, now that we need them?

I'm telling you guys, give up this magnetic stuff. FM cartridges are the way to go. Where's Paul Weathers now that we need him?

[fdegrove]

Hi,

Not from the one you probably had in mind but here's the specs of one of the best I know:

J.A. MC 2 Finish:

Coil impedance 32 Ohm

Load impedance 845 Ohm

Load capacity 150 pF

Stylus pressure 1.8 Grams, (Max Tolerance 0.05 Gram)

Output Voltage 200 µV

Max tracing capacity 400 µmm

Channel separation 70 dB at 100 Hz

60 dB at 1 Khz

70 dB at 20 Khz

Frequency range 3 Hz to 100 Khz

Total THD % 0.1 %

Total unit weight 10 Grams ( incl. socket screws)

Cheers,

[john curl]

I am sorry that we got no feedback from MC cartridge designers. 845 ohms? Oh sure!

[SY]

Well, John, going to 847 ohms can ruin the soundstage and smear the transient purity. Or something. At 843 ohms, the timing, rhythm, and pace of the music is altered beyond recognition. Or something.

[Jocko Homo]

You need to stop reading the Naim users group forum.

Jocko