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Old 3rd April 2016, 10:20 PM   #1
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Default PHOBOS, a balanced MC phono stage

Intro: This is an extension of this thread, whose author suggested I start, as subject matter has moved from discussion stage to design stage.

MC Phono Amp first stage

Here is the latest schematic and board layout. The board is 4.7in X 3.9in designed to slide into a standard Hammond enclosure. It employs the ubiquitous 3.5mm trs jack for both input and output. If needed, cables that go from these to XLR are readily available at pro audio sites, not to mention ebay. I have installed 3.5mm jacks at the turntable, connected directly to the wires from the tonearm. Still some tweaking to do, should have one operating in a couple of weeks.
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File Type: jpg bitmap2 (2).jpg (249.8 KB, 625 views)
File Type: jpg Phobos+ (3).jpg (112.2 KB, 633 views)
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Last edited by polyphaze; 3rd April 2016 at 10:26 PM.
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Old 3rd April 2016, 11:38 PM   #2
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Quote:
Originally Posted by polyphaze View Post
Intro: This is an extension of this thread, whose author suggested I start, as subject matter has moved from discussion stage to design stage.

MC Phono Amp first stage

Here is the latest schematic and board layout. The board is 4.7in X 3.9in designed to slide into a standard Hammond enclosure. It employs the ubiquitous 3.5mm trs jack for both input and output. If needed, cables that go from these to XLR are readily available at pro audio sites, not to mention ebay. I have installed 3.5mm jacks at the turntable, connected directly to the wires from the tonearm. Still some tweaking to do, should have one operating in a couple of weeks.
Thanks for starting this thread Polyphaze. I have wanted to build a balanced phono stage for some years now, but I have never been able to work out a design that satisfies me. The Phobos looks just perfect for my needs and I will be building this up ASAP.

I will be running off +/-24 volt rails ( 4 x 12V SLA batteries) so circuit values may be different to yours. Are you able to update your circuit with all values so I can work through the values I will need.

Regards
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Old 4th April 2016, 12:24 AM   #3
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I would like to ask about resistors R31 and R32 (labeled in the attached schematic).

The voltage across each of these resistors is VBE, about 0.7 volts.

If their resistance is 1000 ohms, the current in each of these resistors is about 0.7 milliamperes.

So the drain current of the Pchannel JFETs is about 0.7 milliamperes.

The total current per side is 7.5 milliamps (15mA / 2). The PJFET gets 0.7 mA and the NPN gets 6.8 milliamps.

That is what I would to ask about. The current in the NPN is about 10 times larger than the the current in the PJFET. Why? Why this particular 10-to-1 ratio? Why not 2-to-1, or 1-to-1, or any other ratio?

Thanks for your time!
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Old 4th April 2016, 02:13 AM   #4
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I dont know. I'm a bit new to jfets. It just looks good in a sim. What do you suggest?
Hopefully Hazard500 will chime in.
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Old 4th April 2016, 02:26 AM   #5
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Quote:
Originally Posted by hazard500 View Post
Thanks for starting this thread Polyphaze. I have wanted to build a balanced phono stage for some years now, but I have never been able to work out a design that satisfies me. The Phobos looks just perfect for my needs and I will be building this up ASAP.

I will be running off +/-24 volt rails ( 4 x 12V SLA batteries) so circuit values may be different to yours. Are you able to update your circuit with all values so I can work through the values I will need.

Regards
It seems that no changes whatsoever are required to run with 24V rails. However, in the output stage, one could increase the CCS current, and decrease the R of the output load resistors, thereby lowering the output Z, a good thing.
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Last edited by polyphaze; 4th April 2016 at 02:29 AM. Reason: typo
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Old 4th April 2016, 05:08 AM   #6
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Quote:
Originally Posted by Mark Johnson View Post
I would like to ask about resistors R31 and R32 (labeled in the attached schematic).

The voltage across each of these resistors is VBE, about 0.7 volts.

If their resistance is 1000 ohms, the current in each of these resistors is about 0.7 milliamperes.

So the drain current of the Pchannel JFETs is about 0.7 milliamperes.

The total current per side is 7.5 milliamps (15mA / 2). The PJFET gets 0.7 mA and the NPN gets 6.8 milliamps.

That is what I would to ask about. The current in the NPN is about 10 times larger than the the current in the PJFET. Why? Why this particular 10-to-1 ratio? Why not 2-to-1, or 1-to-1, or any other ratio?

Thanks for your time!
Hi Mark, my simulator (SIMETRIX) doesn't have models for the exact components we use, but using generic devices I have tried out a few scenarios. What I observe is that, for a fixed current, the more current that gores through the BJT the higher the gain. Now we need enough current to turn on the fet, and keep it turned on for the full voltage swing of the first stage. The 2SJ74 fets I have measure Vgs of 300 to 400mV for drain current of 1mA, and this seems a reasonable figure. This implies that the drain resistors should be about 700ohm (but it would also be worthwhile measuring Vbe of the BJT at collector current of 7mA to check this figure. In fact I think you need to measure Vbe and match the BJTs for this to ensure that jfets get equal current for equal drain resistors.)

So, there is no magic ratio as far as I can tell, but I intend to start with fet current of 1mA and see how we go.
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Old 4th April 2016, 05:09 AM   #7
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Quote:
Originally Posted by polyphaze View Post
It seems that no changes whatsoever are required to run with 24V rails. However, in the output stage, one could increase the CCS current, and decrease the R of the output load resistors, thereby lowering the output Z, a good thing.
OK that seems OK - but you haven't included the values for the components in the eq network. What would these be?

Thanks, Hazard
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Old 5th April 2016, 12:34 AM   #8
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Here is an updated schematic, optimized for 24V rails and adding some component values.
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File Type: jpg Phobos24 (3).jpg (129.2 KB, 518 views)
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Old 8th April 2016, 06:38 AM   #9
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Quote:
Originally Posted by polyphaze View Post
Here is an updated schematic, optimized for 24V rails and adding some component values.
Thanks Poly, I've been playing around with this in SIMETRIX. I found some spice models for MAT02, and 2SJ74, so hopefully the simulation is pretty close to real life - its certainly very different to the outputs when I used generic devices. I have included both the circuit diagram and response curve. For inpuy of 0.5mV, output is around 2.5V @ 1kHz, so gain is just about right for ZYX and Denon, with a little overhead if real life circuit doesn't meet simulated results.

Note I have used +24/-12V rails. This helps to keep load resistors on the output stage, which define output impedance, as low as possible. Current in 2SJ74 is now 2mA, I need to check matching on my devices (which was done at 1mA). I will build this and test when I receive MAT02, due in today's mail.
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File Type: pdf phobos.pdf (12.0 KB, 71 views)
File Type: pdf response.pdf (11.3 KB, 49 views)

Last edited by hazard500; 8th April 2016 at 06:41 AM.
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Old 8th April 2016, 11:10 PM   #10
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Quote:
Originally Posted by hazard500 View Post
Thanks Poly, I've been playing around with this in SIMETRIX. I found some spice models for MAT02, and 2SJ74, so hopefully the simulation is pretty close to real life - its certainly very different to the outputs when I used generic devices. I have included both the circuit diagram and response curve. For inpuy of 0.5mV, output is around 2.5V @ 1kHz, so gain is just about right for ZYX and Denon, with a little overhead if real life circuit doesn't meet simulated results.

Note I have used +24/-12V rails. This helps to keep load resistors on the output stage, which define output impedance, as low as possible. Current in 2SJ74 is now 2mA, I need to check matching on my devices (which was done at 1mA). I will build this and test when I receive MAT02, due in today's mail.
OK, good progress. That response curve is very cool.

I was puzzled at first by your move to a -12V rail, but realized you must be looking for gain, as 74db is a lot more than I have ever needed. However, increasing gain is much better done by decreasing the emitter (source) resistors. So lets go back to -24V rail, decrease R11 R12 to 6r, decrease output resistors to 1k, and up the source current to 30ma. The circuit should now have the same gain profile, but with lower output Z and headroom galore. In fact, with an overload input of 5mv, output will be a very clean 25V with room to spare!

BTW, I'm leaning toward all BJTs in my implementation. No particular technical reason, I'm just a bit out of my comfort zone working with jfets.
You should probably try them in your sim, if just for a reference.
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