Flat Phono Preamp Based on John's P10 and 2SK389

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Gold
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Mon Aug 31, 2015 4:18 pm

Thanks John. Referencing Wayne's schematic on page 16, what would be the best way to implement the load in this balanced configuration? I'd think strapping the resistor across the differential input isn't the best approach.

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JR.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Mon Aug 31, 2015 5:00 pm

Gold wrote:Thanks John. Referencing Wayne's schematic on page 16, what would be the best way to implement the load in this balanced configuration? I'd think strapping the resistor across the differential input isn't the best approach.
OK hopefully Wayne can chime in if I guess wrong.

His schemo looks like 100k so is already a good match for your 1:32 transformer.

The 1:16 looks like it needs about a 36k at the optional J4. to bring it down to 25.6k

JR

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Tue Sep 01, 2015 9:39 am

+1 on JR's calcs.

With the values shown on page 16 it's 100K with J4 open and 47K J4 linked.
The 88K can be easily changed to provide whatever parallel termination is best.
The parallel resistor can also be installed on the Phoenix connector.

I'm wondering which will work out best:
The Denon seeing it's optimum termination reflected into the primary from the transformer secondary or the transformer seeing it's optimum secondary termination with the cart termination based on that.

With my AT-96 I ended up adding another 100 pF. That's 200pF preamp termination and about 90 pF leads.
I did that about the time I changed the front to the OPA2134 but it wasn't because of anything I heard.

If you look at some of the older front-ends like the UREI or Shure they have a fairly low passive pole at the input.

I noticed that the Lundahl spec sheet for that states that the primary requires one side to be grounded.
I wonder why?
It doesn't show to have an electrostatic shield tied internally.

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by Gold » Tue Sep 01, 2015 11:15 am

mediatechnology wrote:+1 on JR's calcs.

I noticed that the Lundahl spec sheet for that states that the primary requires one side to be grounded.
I wonder why?
It doesn't show to have an electrostatic shield tied internally.
I don't know. I didn't notice that requirement until after I had them. I didn't like it one bit. In practice it has been dead quiet. Other Lundahl MC transformers don't have that requirement. I was going to get a different model for the second set. After installing the first set without incident I decided to get the same ones. I liked them and it's one less variable.

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JR.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Tue Sep 01, 2015 4:00 pm

mediatechnology wrote:
I noticed that the Lundahl spec sheet for that states that the primary requires one side to be grounded.
I wonder why?
It doesn't show to have an electrostatic shield tied internally.
It may reflect how the winding is layered inside vs outside. Grounding the outside wrap will perhaps provide some shielding from external noise sources.. just a WAG :lol:

JR

{edit- probably not a great guess... the low turn primary is probably on the inside and secondary on the outside... [/edit]

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Thu Sep 03, 2015 2:39 pm

Would a hidden electrostatic shield have the requirement for that terminal being grounded?

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JR.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Thu Sep 03, 2015 3:06 pm

mediatechnology wrote:Would a hidden electrostatic shield have the requirement for that terminal being grounded?
Certainly connected to a low Z .

JR

klem
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by klem » Fri Sep 04, 2015 12:06 pm

mediatechnology wrote:+1 on JR's calcs.
I noticed that the Lundahl spec sheet for that states that the primary requires one side to be grounded.
I wonder why?
It doesn't show to have an electrostatic shield tied internally.
I think the data sheet is written as such because most situations, as I best understand it, require MC transformers be grounded at primary in order to provide a ground for the turntable cable shield. I think this way LL aren’t making any assumptions about what is/isn’t grounded on a turntable and tonearm.

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JR.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Fri Sep 04, 2015 12:13 pm

klem wrote:
mediatechnology wrote:+1 on JR's calcs.
I noticed that the Lundahl spec sheet for that states that the primary requires one side to be grounded.
I wonder why?
It doesn't show to have an electrostatic shield tied internally.
I think the data sheet is written as such because most situations, as I best understand it, require MC transformers be grounded at primary in order to provide a ground for the turntable cable shield. I think this way LL aren’t making any assumptions about what is/isn’t grounded on a turntable and tonearm.
MC carts are pretty low impedance so shielding of that primary side is probably not a huge concern, I'd be more concerned about loops and mutual ground connections in the vicinity.

JR

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Fri Sep 04, 2015 2:33 pm

klem -

What you said makes sense given the inherently unbalanced construction of coaxial RCA cables.
I suspect that with shielded twisted pair connections to the transformer input there would not be a primary grounding requirement.

It's the same problem attempting a true balanced connection with RCAs and moving magnet carts.
The balanced inputs work better with RCAs but only if the shield is grounded.
Try to float the shield and it hums.

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