dspNexus 2x8 and phono use

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dspNexus 2x8 and phono use
« on: 23 Oct 2023, 01:05 pm »
The dspNexus 2x8 has an RIAA phono capability with balanced input from the phono cartridge.   There is a 40dB gain setting in the ADC system to work with the 47K Ohm load on the inputs.

The phono cable has to be wired to an XLR with the coils connected to the +/- input terminals and chassis ground to the ground pin. 

mix4fix

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Re: dspNexus 2x8 and phono use
« Reply #1 on: 23 Oct 2023, 07:51 pm »
Great. Bring a turntable.

 :popcorn:

VinceT

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Re: dspNexus 2x8 and phono use
« Reply #2 on: 23 Oct 2023, 07:58 pm »
I have the GR LS9s and know your active crossovers are well received with bi-amping these speakers and you have also done this with tremendous success with this model

As good as the DACs are with these units, I would have to think the analog signal is somehow affected. I too am interested in hearing the results.

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Re: dspNexus 2x8 and phono use
« Reply #3 on: 23 Oct 2023, 08:08 pm »
Al Clark at Danville Signal has designed multiple phono stages and one is patented. 

The ADC in the system is an AKM AK5578 that has minimum phase filters, just like the way the cartridge works. 

I have used the ADC's in this unit to record mics for use with my guitar rig and done recordings of a Steinway D with the earlier AK5397 ADC.  Sounds excellent for mics.

One customer will be using the new phono stage with his system in his new listening room that he is setting up.   He had earlier used the dspMusik for RIAA EQ with an external gain stage for his vinyl system with excellent results in his original room.

I have the LS-9 dspNexus 2x8 crossover ready for a customer that has the dspMusik 2x8 now and getting the dspNexus 2x8 as an upgrade to the system.   He does not have vinyl, but has 1/4in tape into the ADC and digital replay.     

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Re: dspNexus 2x8 and phono use
« Reply #4 on: 24 Oct 2023, 01:25 am »
Great. Bring a turntable.

 :popcorn:

I will see if the LINN Sondek LP12 my friend's dad gave me is up to the task, if I can rewire the phono cable for XLR use.

mix4fix

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Re: dspNexus 2x8 and phono use
« Reply #5 on: 24 Oct 2023, 11:58 pm »
I will see if the LINN Sondek LP12 my friend's dad gave me is up to the task, if I can rewire the phono cable for XLR use.

What about a simple adapter/box? I am never a big fan of cutting up something that's not broken.

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Re: dspNexus 2x8 and phono use
« Reply #6 on: 25 Oct 2023, 01:43 am »
Will have to see what is possible.

danvillesignal

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Re: dspNexus 2x8 and phono use
« Reply #7 on: 31 Oct 2023, 07:07 pm »
I thought I would add a few comments about the phono support in the dspNexus.

The differential input impedance of the ADC is 47k. It also has a group of capacitors for cartridge loading of moving magnetic cartridges.

Most tonearms/turntables isolate the coil from ground even when using an RCA phono. I would wire hot to center and cold to the ground return of the connector.
Connect the turntable ground to the XLR ground.

If you have a moving magnet cartridge you should have plenty of gain available and the 47k load is correct. You can also use high output moving coils. For example a Denon 103 should work but you will probably want to load into a lower input impedance. You can solder a small resistor in your XLR connector. I would use 400 to 500 ohms for a Denon. I have tried a Grado Signature which is low output when compared to most moving magnetic cartridges.

If you have a low output moving coil you will want a head amp or transformer.

A Rich mentioned, I do have a history designing phono preamplifiers. This was many years ago in my youth and I don't spend much time listening to vinyl these days.

The RIAA curve can be matched quite well in a digital filter when the sample rate is high, in our case 192k. The error calculates to 0.02dB which is better than anything you will build with actual analog components. I used Scott Wurser's calculations. You can change them to other EQs in Audio Weaver if you are so inclined.

At some point (NOT SOON), I will probably explore pop removal. The advantage of a digital crossover is that we can delay the signal for editing before its too late. Its also helpful to remove pops before the EQ is applied.

Of course, you can also use the ADC input without EQ and use your own analog phono preamp.

Al Clark



 

mix4fix

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Re: dspNexus 2x8 and phono use
« Reply #8 on: 1 Nov 2023, 01:31 am »
Would it not make sense to put RCA jacks to accommodate the majority of turntables?

danvillesignal

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Re: dspNexus 2x8 and phono use
« Reply #9 on: 1 Nov 2023, 03:43 am »
It may have been expedient to use RCA Phono connectors if our only interest was interfacing to a turntable. You would still want a ground return to the turntable.  The dspNexus inputs are used for other analog sources and we have no interest in dumbing down our product in this regard. If this statement offends anyone, so be it.

Analog phono preamps generally do not have a DC ground connection at the input stage. They usually included an RF connection (small capacitor) from the shield of the phono RCA to the board ground. The turntable chassis was then connected via a ground wire to the ground of the preamplifier. So in essence, you already have been making a three wire connection.

For the record, I think that unbalanced audio connections are a bad idea. I believed this 45 years ago when I first started designing high end consumer audio products. This has been accepted in professional audio for a very long time. Fortunately, we are seeing more and more consumer products moving in this direction.  Even if we accepted single ended circuits as good, the RCA phono is on the short list of dumbest connector ever invented. 

One of the advantages of a balanced front end is that it is easy to accommodate a single ended (unbalanced) circuit. This means that with appropriate cabling, you can connect whatever you like.

Al Clark


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Re: dspNexus 2x8 and phono use
« Reply #10 on: 1 Nov 2023, 05:57 pm »
To convert the turntable tonearm wiring to balanced XLR connection built an interface box.

Will check it out soon and then get it packed up for CAF2023.








mix4fix

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Re: dspNexus 2x8 and phono use
« Reply #11 on: 1 Nov 2023, 07:14 pm »
Is there a ground lug on the box?

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Re: dspNexus 2x8 and phono use
« Reply #12 on: 1 Nov 2023, 08:15 pm »
There will be. 

Just have to put the binding post on the box.

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Re: dspNexus 2x8 and phono use
« Reply #13 on: 1 Nov 2023, 08:37 pm »
Have the 5-way binding post for the turntable ground installed in the box.

Now to work on what is the best grounding scheme.



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Re: dspNexus 2x8 and phono use
« Reply #14 on: 5 Nov 2023, 04:35 pm »
The Phono Interface box is connected to the Oracle Delphi MKI modified turntable to the dspNexus 2x8 and working. 

The box has 4 grounding modes, so will also work with the LINN Sondek LP12 going to CAF2023.

Last checks are happening today before boxing up the CAF system for the demo room.


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Re: dspNexus 2x8 and phono use
« Reply #15 on: 6 Nov 2023, 05:25 pm »
The Linn Sondek LP12 turntable is packed up and ready to go to CAF2023 with the new interface box.

Packing is almost done.  Get the truck tomorrow and pack it up to travel to the show. :)

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Re: dspNexus 2x8 and phono use
« Reply #16 on: 13 Nov 2023, 08:23 pm »
Back from CAF.

Had problems with the phono interface box as someone wired it incorrectly, me :roll:

No soldering iron at the show, so it is now home for rework and try again.