current amplifying phono stage

[Fred Dieckmann]

Which is it?

"The MC-cartridge is connected between +IN and -IN with 100 Ohm resistors to ground from the inputs. So the dartridge leads are totally floating with respect to ground."

100 ohms to each leg could hardly be called floating. I do think a fully differential input for phono is well the noise penalty. The extra noise is usually below the AC line harmonics that wind up mixed with the signal in a single ended input. Jocko and I have both built (different designs) phono preamps with differential inputs using the 2SK147s in the front end and were very happy with the results. Getting the grounding right on a tonearm and turntable can be a real pain. Electrical noise from the motor very often gets in the signal from the cartridge as well. It is a mystery why the phono input, the most affected by common mode noise, is so seldom done as a balanced input.

 

[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.

 

[scott wurcer]

Re: Which is it?

Jocko and I have both built (different designs) phono preamps with differential inputs using the 2SK147s in the front end and were very happy with the results.

Me too. I use a Grado where the 400 Ohm minimum source resistance helps, but the motor pickup is problematic.

 

[Elso Kwak]

Re: Which is it?

"The MC-cartridge is connected between +IN and -IN with 100 Ohm resistors to ground from the inputs. So the dartridge leads are totally floating with respect to ground."

100 ohms to each leg could hardly be called floating. I do think a fully differential input for phono is well the noise penalty. The extra noise is usually below the AC line harmonics that wind up mixed with the signal in a single ended input. Jocko and I have both built (different designs) phono preamps with differential inputs using the 2SK147s in the front end and were very happy with the results. Getting the grounding right on a tonearm and turntable can be a real pain. Electrical noise from the motor very often gets in the signal from the cartridge as well. It is a mystery why the phono input, the most affected by common mode noise, is so seldom done as a balanced input.

Fred, I meant floating with respect to the ground or chassis and arm of the turntable. Getting the ground right on tonearm and turntable does not seem to be a problem to me. On the other hand I do not understand: "is well the noise penalty". I absolutely don't have a noise problem.
I have a direct drive and the motor noise is not a problem either. The turntables transformer is banned to the outher world however.

 

[john curl]

Hi Elso, What feedback resistance value are you using? I now realize that it is probably higher than 33 ohms, but if you get low enough noise performance, then that is all you need. For the record, we were going to use a design similar to this in 1974 for a microphone preamp stage. It was first published in one of the electronic engineering mags, either 'EDN' or 'Electronics Design' back in 1974 or so. I think that it is a very elegant circuit.

 

[Elso Kwak]

Feedback Resistor

Hi Elso, What feedback resistance value are you using? I now realize that it is probably higher than 33 ohms, but if you get low enough noise performance, then that is all you need. For the record, we were going to use a design similar to this in 1974 for a microphone preamp stage. It was first published in one of the electronic engineering mags, either 'EDN' or 'Electronics Design' back in 1974 or so. I think that it is a very elegant circuit.

Hi John, Which resistor do you mean by feedback resistance Rg or R4? Interestingly the circuit has the lowest noise at the highest gain so I used the highest gain possible. This is also advantageous when cascading gain stages making the first stage having the highest gain.

 

[john curl]

Rg is the important resistor for noise contribution.

 

[Elso Kwak]

Rg is the important resistor for noise contribution.

Yes, I do understand that.

 

[Elso Kwak]

Cross Coupled Quad

Hi, the only reference I could find to the circuit was in the book of Douglas Self to: Feucht, Handbook of Analog Circuit Design, Academic Press, 1990, page 432.
Anyone has a copy of that for me?
Couldn't not find the EDN or Electronic Design article.

 

[john curl]

Elso, I lost almost everything in the big fire of 1991, BUT I might have a copy of this early design around somewhere. I don't know where at the moment. Relax, and enjoy your design. You reminded me that I actually have a working prototype at my warehouse, made in 1974. I might dig it out, just for fun.

 

[Elso Kwak]

The Levinson JC-1AC

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]

PRAT?????

You need to stop reading the Naim users group forum.

Jocko