Balanced MM phono stage : When Aleph Ono and Xbosoz meet...

[CheffDeGaar]

Warning : It's only simulations !!!

Hi there,

Just for fun, what about some genetic engineering to crossbreed Aleph Ono and Xbosoz ?
I've been contemplating the idea of a fully balanced MM phono stage since the hills were young, but I never had (and still don't have ) the skills to do it.
But never mind, I made an attempt…

I wanted to have it fully balanced from beginning to end. I browsed some web to have an idea of what was awaiting me. Not so simple, uh ? But this fabulous forum gave me some ideas... I found an interesting thread here, about balancing phono cartrigdes, the Xbosoz thread provided me with more dBs, and the Aleph Ono service manual gave me food for thought (thanks Nelson ).

Now to the result...

The input stage is the now classical JFET differential pair, with cascoding BJTs. As I didn't want to have a RIAA network inside a feedback loop, since I need 4 of them in a stereo version, I prefered to use the network as a drain load for the diff pair. Idea's not mine, I know... Useful info on this can be found here .
This configuration dramatically drops down the gain of the input stage, so I added a XBosoz after the diff pair, to reach around 45dB@1kHz from input to output.

Well, in theory, it should work... The RIAA curve is fitted within +/-0.5 dB above 100Hz, and the interstage coupling caps C1 & C2 filter the LF spectrum below 100Hz, to reach -3dB @ 10 Hz compared to the RIAA curve.

And now I'm coming to you before going to the breadboard, to seek your advice and to listen to your flames .

The main point I'm not sure of (added to know whether it will work or not ) is agout the noise level of this beast : is it better to use //ed JFETs at the input, replace the MosFets with NJFet and BJT for the xbosoz folded cascode, etc... ?

And yes, I will have to add some caps for decoupling the biasing stages... Not wanted to overload the schematic

Any comment (improving only ) welcome !

P.S. : A way to calculate the RIAA network can be found here. R1 on the schematic (Figure 1) is the drain resistor (4KOhms) in my case.

And the messy drawing :

[stefanobilliani]

Hi Cheff ,

beautiful subject, I'm glad that someone give it a try

The way is obtained the RIAA curve is absolutely interesting .
I myself made some experiments some times ago.

I feel to add that without input transformers the thing is unacceptable , lots of noise picks up .So the diff pair at the input must be a unbal/bal converter . To do this one input has to be grounded .

Please someone correct me if I'm wrong

[jam]

Cheff,

Looks good. You might want to double up the input fet's and buffer the outputs.
Looks similar to the approach Allen Wright uses for RIAA eq.

Regards,
Jam

[CheffDeGaar]

Thanks gentlemen, glad to have raised some interest

stefanobilliani
First, it also works with unbalanced cartriges. As you say, just ground one input of the diff pair. But I'm not sure to understand what you mean by your "noise pickup" issue. Most of cartridges are in fact balanced, and the ground connection that ties the two grounds of the cartridge's coils is done in the cartridge shell (at least on mine ). See here a nice post by GRollins about that. In my mind, balancing all the stuff from input to output had the advantage of noise cancellation... Am I wrong ?

jam,
Yes I saw your posts and links on the thread I mentionned above. And I sure was inspired by Allen Wright's approach for RIAA eq. Re. // Jfets at input, I surely will do something like that, but I'm not sure that the noise will dramatically decrease : I would have to decrease the drain current to 2.5mA for each Fet, and according to the 2SK289 datasheet, the noise of each Fet with increase a little if the drain current is reduced... But I still think the overall noise should decrease.
Also about noise, I'm not sure MosFets are the best performers here for noise, and I was considering using another 2SK389 JFet instead of the IRF610, and making the folded cascode with a BJT. Simulations I have made showed it could be possible, but I have no clue about the noise performances comparison of these two topologies... Any hint ?

Buffering outputs is also tempting, but I was a little bit reluctant to add devices in the signal path... Are these buffers really necessary ?

[CheffDeGaar]

Despite the huge interest for this kind of design , I'm still on the run .

I modified the schematic to include Fet buffers at the output, used a Jfet/BJT folded cascode instead of a full-mosfet one, removed the coupling caps, doubled the input fets, and modified the topology a bit .

I previouly mentionned that I had a deviation from the RIAA curve of more than 1.5dB for low frequencies, and thought the coupling caps where guilty. So I tried to remove the caps, but the deviation was still here. It finally appeared that this behavior was related to the two feedback loops (Rfb1 & Rfb2 on the first schematic). But I don't know why.... Can someone explain to donkey-me what happened here... ? Now the theoretical RIAA deviation is below 0.04dB

So I modified the topology to include the now famous X feedback, and we have here the new schematic. It's DC coupled, and output offset is nulled using VR1 & VR2. Gain at 1kHz can be selected "as usual "by adjusting the source-to-source resistor. Here a jumper can help to switch between 40dB and 60dB.

Before I draw the actual schematic of the beast I want build, may I be bold enough to ask you if you see any flaws in this design ?

[CheffDeGaar]

Some curves :
Top plot (red) is the output transfer function for a 40dB gain @ 1kHz

Bottom plot (purple) is the deviation from the theoretical RIAA curve (40.000m means 0.04dB )

[jam]

Cheff,

That looks good. How about mosfets for the buffers? Probably not necessary but you could bias them at higher currents.

So are you going to build one?

Regards,
Jam

[CheffDeGaar]

Thanks Jam.

Yes I'm going to try to build one . Just to hear the tremendous background noise it will make .

Since it looks good to you (and others ?) , I will draw an actual schematic with actual values for the RIAA network, proper decoupling caps, input capacitance switches, and other goodies...

Just a few things I'm not sure of : I love LEDs for CCS - they glow in the dark -, and in the ones I usually build, the LED is thermally coupled to the BJT(s), decoupling caps for each led are a 100uF 'lytic bypassed by a 100nF film cap. Instead of a resistor, The led is fed by a source degenerated Jfet (J108 or others) to provide it with constant current. But I'm wondering here if it is the best choice for low noise levels... Any hints ?

And the other thing is that I wonder if sources resistors for the //ed input Jfets are necessary in the "real life" ? Do I have to add some ?

Mosfet output buffer is looking nice - I love the sound of heavily biased Mosfets -, so I will try to plan a PCB allowing to test the two types of buffers. Thanks for the idea

[peranders]

CheffDeGaar, nice work (in theory). I see that you like something glowing, me too, but LED's aren't so predictable in voltage. Can go from 1.4-1.8 Volts, for red ones. I would go for one powerful voltage reference for all cuurent sources A LM431 maybe, with separate LP-filters for each transistor. 47-100 k + 100nF-10 µF. With this filter, it's much easier to filter out noise.

I have certain doubts about balanced pick signal going into a amp which aren't very good in rejecting common mode noise. This is only a hunch.

Have you checked http://www.klaus-boening.de/html/laboheme.html/]Klaus Böning's[/URL] balanced vinyl amp?

[Nelson Pass]

The problem you are likely to encounter is that both imputs of the cartridge will be able to pick up all manner of stray noise from the environment. If the common mode rejection figure of your input is high enough, it won't matter, but if it's not, the result is likely to be noisier than an unbalanced connection.