Balanced MM phono preamp with SSL9k Front End

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Gold

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It recently occurred to me that it would be better to use a balanced configuration as a phono input than the standard unbalanced arrangement. The CMRR advantage would be particularly useful when interfacing a cartridge with a cutting lathe. Sometimes a long line between the cartridge and the preamp is unavoidable. The grounding can also be difficult to get right with the tonearm on the lathe chassis and the preamp down the wall.

The step up transformer for an MC cartridge takes care of this naturally but I'd like this advantage with an MM cartridge. I would run twisted pair to take full advantage of the topology. I think I would need a 50k:50k transformer to interface an MM cart to a more classic topology. That's not an easy to find or inexpensive proposition.

I thought the front end of the SSL 9k would be a good candidate. Here is a link to Keith's SSL 9K project. http://www.studio21.ch/diy/neeno/ssl9k/DIY_9k_Preamp.pdf
I don't know how to separate out the schematic on page 2 and post it, sorry.

I think only the components before the MAT02 would need to be changed. Any insights would be appreciated. Is the concept valid? Is this a good starting point?
 
The input devices on the 9k are optimized for low impedance sources; you would find that noise is inadequate with a typical MM cartridge.
As long as the cartridge connections are floating, you don't need balanced inputs. You just need to make sure that the cold point of each side of the cartridge is connected to the preamp's input ground.
Although it is possible (and relatively easy) to design a balanced input stage optimized for an MM cartridge, the noise factor would be 3dB worse than unbalanced, because of the two input devices introducing noise.
Although it takes some expertise, it is perfectly possible to reduce hum to a strict minimum with unbalanced connections.
 
abbey road d enfer said:
The input devices on the 9k are optimized for low impedance sources; you would find that noise is inadequate with a typical MM cartridge.

That's what I thought. Would it be more like a total redesign to optimize for an MM?

As long as the cartridge connections are floating, you don't need balanced inputs. You just need to make sure that the cold point of each side of the cartridge is connected to the preamp's input ground.
Although it is possible (and relatively easy) to design a balanced input stage optimized for an MM cartridge, the noise factor would be 3dB worse than unbalanced, because of the two input devices introducing noise.
Although it takes some expertise, it is perfectly possible to reduce hum to a strict minimum with unbalanced connections.

Sometimes in large piecemeal setups like cutting studios it is near impossible to get the cartridge quiet enough. Believe me, I have spent many hours over many years banging my head against the wall. I would like to pursue the balanced input arrangement. Any hints on where to look for a suitable schematic and/or application note?
 
yup, mic preamps will be nominally 2k input Z, MM carts want to see more like 47kOhm input Z..

P10.gif


Here is a MM phono pre design from a kit I published in popular electronics back in the early 80s. The input was actually floating, but some guy in texas actually got a patent issued for a balanced input phono preamps (I'm not kidding about that), and he threatened me with legal action, so I shorted one input to ground with a wire jumper and in my literature, while the PCB was still laid out for floating inputs. FWIW I have seen transformer input phono preamps in old tube manuals that would surely provide prior art to knock out his patent but it's ancient history now. Just more patent nonsense to ignore.

BTW my recollection is that some carts grounded one terminal of the 4 in the cartridge to a shield or perhaps the tone arm, but this seemed to vary between cart makers and not be a standard.  Look out for capacitance with very long cables, the higher Z mm carts can be sensitive to termination capacitance and cumulative cable capacitance counts. Also make sure you're using 2 conductor shielded, or else balanced input is waste of time.  Star-Quad might be interesting if it wasn't higher capacitance.

JR
 
There are newer low noise JFETs than the ones I used in that old kit...  I used some sweet 1nV rt/Hz 2sk117s in a later (unbalanced) preamp design. Those early ones in that kit were around 4.5nV...

I still have some of the old JFETS laying around that I selected for Vgs, that you can have for the cost of postage (PM me if interested)... While i would still be tempted to recommend buying some newer (much) quieter parts. 

There are even some modern dual JFETs that may have good enough Vgs tracking as is.  They don't have to be matched super close, but if way different pinch off, one device will steal quiescent current from the other through the gain resistor.  Maybe buy 10 pcs of a decent low noise JFET and use the two closest matched pairs.

To select just ground the gate, with drain connected through 3k to +15V, and source connected through 3k to ground. Measure the voltage from source to ground, and use the two closest matches as the first pair, next two closest as second pair for the two inputs.. I had the luxury of selecting from hundreds of pieces, which is why I still have some left.   

JR
 
John,

I have a couple of questions about the schematic. I'm not sure what's going on with R6, R7. I can't find a C15 and can't make out the note for R7. I think either R6 or R7 connects to the opamp that feeds SW2 to bypass the EQ network.

How is power applied to the circuit? Is it  +30VDC supply? What pin does the power do to on the input transistors?

Is it R4 that biases the input transistors and optimizes the noise for a high source impedance?
 
Gold said:
John,

I have a couple of questions about the schematic. I'm not sure what's going on with R6, R7. I can't find a C15 and can't make out the note for R7. I think either R6 or R7 connects to the opamp that feeds SW2 to bypass the EQ network.
R6 and R7 go to +15V regulated. which uses a 10 ohm in series after the 3 term regulator and 1,000 uF to ground, with a handful of .1uF discs at the ICs. 
How is power applied to the circuit? Is it  +30VDC supply? What pin does the power do to on the input transistors?
Circuit runs from +/- 15v

The JFET source current is supplied through the 3k resistors going to the opamp outputs.
Is it R4 that biases the input transistors and optimizes the noise for a high source impedance?
R4, actually sets the gain of the first stage  1+3k/200 ohm x2  since there is a + and - output swing summed in the following differential.

========

Coincidentally Wayne is in the middle of revisiting this front end for a slightly different application (click and pop removal).  But his discussion is 
here http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=6&t=423

He is using a nice dual JFET 2sk389 that is lower noise than the old soldiers I used and looks like it doesn't need hand matching.

JR
 
I suspect the issue is not just the distance but also the relatively noisy environment around the lathe.

I raised the concern of distance in the context of cable capacitance and it's effect on the cart termination.

JR
 
tv said:
2sk389 are old soldiers ...
(I've got my tiny stash)

I guess "old" soldiers is a relative term.. i designed this preamp in the late '70s and the 2sk389 data sheet is dated 1997 almost 20 years later.

I recall messing with some old dual JFETS back in the day but as i recall they weren't very well matched or very low noise.

JR
 
sodderboy said:
Why not try to get the preamp less than 2 meters from the cart, and make the distance with line level?
Mike

For me that would mean either doing the limbo sideways when I want to empty the chip jar or drilling holes I don't want, to mount the preamp on the lathe. It's always something. There are also grounding considerations between the preamp, the lathe amplifier rack and the lathe its self. I have gotten my setup pretty quiet but I've had to jump through hoops to do it. Other cutters tell me that they have trouble getting their setups quiet. Many give up and do critical listening on a dedicated turntable.

If the only penalty to using a balanced configuration is 3dB more noise from the input devices I'll take it. It will still be way below the noise floor of a record.
 
JohnRoberts said:
I guess "old" soldiers is a relative term..
2sk389's are a couple years discontinued..
http://www.micross.com/pdf/LSM_LSK389_SOIC.pdf

..I remember this causing quite a stirr in diy-hi fi communities

PITA with fets is they have to be -somewhere- DC-decoupled, even the matched-pair ones.

My POW:
Trimming=introduces more noise -somewhere- so WTF
DC-servo=YOWZA and overkill unless this is an "expensive" design (a BJT pair needs less attention and cheaper)

for a 47k input, perhaps a BJT-pair = LESS PITA?

Another angle:
03046.png


Perhaps for a preamp that will invariably "work" in less-than-optimal environment, a simple instrumentation circuit could be adapted for RIAA - a RIAA network should be somehow inserted in place of the Rgain on the picture, and HPF (two caps) inserted at the outputs of the first two opamps - before the debalancer.

In the end, should be cheaper, easier on your mind etc...



Yet another thought - perhaps one could use INA217 or THAT 1510 chips as a front end using larger "input" resistors - smaller, faster, badder ....
... and somehow flex the RIAA network in place of the Rgain ... and DC servo the thing ... and ... whatever.
 
If he has a good phono pre that he wants to keep, He could make a simple balanced instrumentation amp out of some of the new generation low noise JFET input opamps..  Run this balanced instrumentation amp at modest gain (say less than 20 dB) and then single end and pad it down for presentation to the real phono preamp input.

I just offered my old design since it already offers the floating input capability, and works well (IMO). But back when I designed this JFET input opamps weren't  as low noise as they are now.

JR


 
tv said:
Yet another thought - perhaps one could use INA217 or THAT 1510 chips as a front end using larger "input" resistors - smaller, faster, badder ....
... and somehow flex the RIAA network in place of the Rgain ... and DC servo the thing ... and ... whatever.

I thought of using a THAT 1510 but the data sheet says it's "optimized for low source impedance". I thought that meant it would be noisy with an MM. I don't know enough to look at the data sheets and know if it would be okay in this application.

Since this is for a calibrated system the pre amp would need to have coarse and fine gain adjustment. I would need to calibrate 0VU to 7 cm/sec lateral.
 
Gotta love the internet... all kinds of competing advice.

From Wayne's post, the typical MM cart impedance is  (500mH+1K)||150pF, So 1K resistive with a dash of inductance for even higher impedance at HF...

So IMO not a good match for a mic preamp optimized for 150-200 ohm source impedance, but apparently opinions vary...

JR

 
Hey, this is the 21st century!! If we can't lose the wax, at least we should have "phono cartridges" which take a bit of DC and output a digital stream in S/PDIF or USB flavors.

> a long line between the cartridge and the preamp

Phono interface grew-up on the assumption that preamp would be 3'-4' away, going to a hi-gain tube. While we can lose the tube's grid capacitance, 10 feet of cable shifts the ~~15KHz resonance down, smooths the cymbals (and groove shatter), which you don't want in a process monitoring chain. Some carts tolerate more, and I like Grado, but I would not chain myself to his products.

And as JR sez, you must re-wire from 4 conductors (one signal cold also cart shield) to a 5-conductor (two twist-pairs and a shield). And lose the RCA connectors, of course.

> If he has a good phono pre that he wants to keep, he could...

OTOH, he could cobble "four" preamp channels into stereo differential.

I'm not a passive-EQ guy _but_ this problem brought to mind the Hagerman Bugle. The half-kit is very affordable, even if you buy a pair. The PCB is easy to cobble. Little cobbling is needed. Omit the phono-in jacks and wire balanced input connectors. Omit a bunch of parts. Take one 13K0 resistor, check for match, and tack-solder it cross-"channel".

> need to calibrate

Overall gain can be trimmed hugely with one resistor, no interaction with EQ. (NFB-EQ is not so convenient.)
 

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That would work too... while I am not a huge fan of asymmetrical differential inputs, it would work just fine.

The whole passive eq thing is based on the premise that non-inverting one stage phono preamps diverge from the true 75 uSec RIAA pole up where only bats can hear, but lathes also don't keep up that high so it's mostly much ado about nothing.

Another old theme is that splitting up the gain between two stages makes the 60 dB gain at 20Hz easier for a lowly opamp to deliver but that RIAA gain drops from 60 dB to only 20 dB by 20 kHz, so even modest opamps can usually keep up since they have beaucoup gain at 20 Hz.

I am not up to date on opamp numbers, if rolling with opamps I would suggest using some of the modern low noise JFET input opamps, but opinions vary about even that.

JR

PS: I need a calendar check... this is 2011 and were still debating phono preamps? The end must be near.  8)

 
 
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