current amplifying phono stage - Page 5

Fred Dieckmann · 23rd April 2004, 07:26 PM

"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 · 23rd April 2004, 07:34 PM

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 · 23rd April 2004, 07:39 PM

Quote:

Originally posted by Fred Dieckmann
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 · 23rd April 2004, 08:41 PM

Quote:

Originally posted by Fred Dieckmann
"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 · 24th April 2004, 01:43 AM

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 · 24th April 2004, 07:55 AM

Quote:

Originally posted by 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.

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 · 24th April 2004, 09:03 AM

Rg is the important resistor for noise contribution.

Elso Kwak · 24th April 2004, 09:17 AM

Quote:

Originally posted by john curl
Rg is the important resistor for noise contribution.

Yes, I do understand that.

Elso Kwak · 24th April 2004, 10:34 AM

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 · 24th April 2004, 10:30 PM

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.

23rd April 2004, 07:26 PM	#41
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	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.

24th April 2004, 10:34 AM	#49
Elso Kwak Banned Join Date: Dec 2001 Location: Zamboanga, City of Flowers, Mindanao	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.

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23rd April 2004, 07:34 PM	#42
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	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.

24th April 2004, 01:43 AM	#45
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	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.

24th April 2004, 09:03 AM	#47
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Rg is the important resistor for noise contribution.

24th April 2004, 10:30 PM	#50
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	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.

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