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Re: Phono Transfer System

Posted: Sun Jan 29, 2017 5:59 pm
by Gold
I'm getting closer to getting the "commercial" build together. Here is an idea of what it will look like. This remote box was made by hand. I will have some custom remote enclosures made because it takes to long to make these by hand. These will be two Phono input systems. This one will have one remote for an MC cart and the other for an MM cart.




Re: Phono Transfer System

Posted: Sun Jan 29, 2017 6:18 pm
by mediatechnology
Very nice Paul! Someone's been busy this weekend.
The Hammond enclosures work out very well for the remote preamp.
The umbilical cable is nice too.

I just finished building a Dual Class A II today, have an MS to ship tomorrow, and then an MS board to build.

Re: Phono Transfer System

Posted: Sun Jan 29, 2017 6:54 pm
by Gold
It's been a little bit of work for the past few weekends. This one is prototype 1. It's going to Josh Bonati. He helped design the faceplate and said he would help me design an FPE made remote box. By designing a custom enclosure I'll be able to add more features. I also need to find a different kind of RCA jack. I think a pair mounted in plastic so I don't have to jump through hoops to galvanically isolate them from the chassis.

Flat Phono Preamp Variable Gain

Posted: Fri Mar 03, 2017 2:37 pm
by mediatechnology
John in an earlier post suggested a way to reduce the effect of cable capacitance when the gain pot is located off-board: viewtopic.php?f=7&t=753&start=53

I tested a driven shield approach and drew up the schematic.

Flat Phono Preamp (Instrumentation Preamp) with Variable Gain Using Driven Shields to Reduce the Effect of Cable Capacitance.

The summing nodes at the inverting inputs of IC1 are very, very critical.

Any minute stray capacitance imbalance from either inverting input to ground is disastrous to high frequency common mode rejection.
Capacitance appearing across Rg (which includes those referenced to ground) can also cause HF response peaking.
See: That Corporation's "Analog Secrets Your Mother Never Told You" pdf pages 16-18 ... ld_You.pdf

Avoid stray capacitance.

Flat Phono Preamp (Instrumentation Preamp). Avoid Stray Capacitance to Ground in the Gain Leads.

Because of cable capacitance a shielded cable with a grounded screen cannot be used for remote gain leads to a front panel pot.

When the cable shields are driven by the instrumentation amp outputs the effect of the unwanted capacitance is that it appears harmlessly in parallel with R8 and R9.

The effectiveness of the shields are the same as if they were grounded due to the low source impedance of the op amp outputs driving them.

Any small capacitance across Rg can also cause HF response peaking.
To avoid a shunt capacitance from appearing across Rg two separately shielded cables are required.
DO NOT use two conductor shielded cable and share a shield: Each shield needs to be driven by its respective output.

When constructing a unit with remote gain control its still important to keep the lead length as short as possible and the entire unit shielded in a metal enclosure.
Tests show that with careful lead dress using the approach shown in the schematic common mode rejection and low noise can be maintained.

My recommendation is that the gain pot be a stepped switch tapered reverse log.
The gain equation for the instrumentation amp is G = 1+[(R8+R9)/Rg]
Note that if THAT1240s are used after IC1 they provide an additional 6 dB gain.

Re: Phono Transfer System

Posted: Sun Mar 05, 2017 12:40 pm
by JR.


Good digital RIAA EQs ... accurate and Minimum Phase

Posted: Sun Apr 16, 2017 4:30 pm
by ricardo
Wayne, I think you posted somewhere a short list of the digital RIAA EQs which you'd found good ... accurate and Minimum Phase ... and the ones which weren't.

Could you please help a beach bum with the info ;)

I've searched the 2 huge phono preamp threads and this one without success

Re: Phono Transfer System

Posted: Sun Apr 16, 2017 6:53 pm
by mediatechnology
Were you looking for .vst?

These are based on the DH Biquad from Jim W: viewtopic.php?f=15&t=782

And these from our forum member Olaf Matthes: viewtopic.php?f=15&t=784

The built-in ones for Diamond Cut 8.5 are very good: ... age=page_2

On my last test I used Olaf's and I casually compared them to analog. They sounded pretty close.
Looks like a good test for AudioDiffmaker.

I've also used the DH Biquads but for some reason couldn't get them to run correctly in one installation of Audacity and haven't bothered to troubleshoot it.

The Case for Balanced Phono Cartridge Wiring

Posted: Thu Apr 27, 2017 3:46 pm
by mediatechnology
I'm about to do some Analog/DSP RIAA EQ comparisons and before I did wanted to grab a screenshot of the RIAA-EQ'd noise floor.

The first pic is the unweighted noise floor with both channel's inputs balanced.
The cartridge connections are shielded twisted pair.
The second pic is with the Left channel jumpered for unbalanced operation. No common mode rejection in the cartridge signal lines is realized in the Left channel.

Flat Moving Magnet Phono Preamp Noise Floor Both Channels Balanced.

Flat Moving Magnet Phono Preamp Noise Floor Left Channel (Blue) Is Unbalanced.

They may measure almost identically but they do sound different.

Update: December 18, 2018:
svart-hvitt: Almost identically means balanced measures a little better. Maybe the sonic differences are due to factors not covered by the measurements? Or maybe he hears voices in the air? ... post-49516
I'm talking about the signature of the noise floor sounding different. The noise level measurements, in dB, are nearly-identical, but the FFT, and the audible noise floor signature, are quite different in the inductive region of the cartridge above 1 kHz.

The original post at AudioScience read:
And, yet, we have phono stages costing thousands of dollars, with big toroidal power supplies that reek of engineering overkill for such small signals, and claims of new innovative topologies for a circuit that should have been solved 30+ years ago.
Can one of you EE or other electronics types explain why all this is needed?
It just seems like complete BS to me.
It is complete BS. The balanced, flat, preamp in this thread, when they're available, is a $39 kit. The lowest-noise performance is obtained using a 30 cent IC.

Re: Phono Transfer System

Posted: Sun Jul 16, 2017 4:57 pm
by quattro64
Hello, finally getting my build completed, Will be using as stand alone to feed a pcm4204evm adc. My phono leads are true balanced as shown in this thread though so I wonder, do I need to populate c4-c5 c-24 c-25? What value and voltage for those caps? Thanks!

Re: Phono Transfer System

Posted: Sun Jul 16, 2017 6:27 pm
by mediatechnology
Thank you for joining us quattro64!

I have never used the output coupling capacitors since the output offset is usually quite low with a balanced cart connection.

I had to look at the PCM4202 input stage to see if it has Vcm present on the inputs.
It does.
The PCM4202EVM uses an OPA1632 with Vcm supplied by the 4202.
Without input coupling capacitors installed on the EVM it does have DC just like my PCM4222 EVM. (C1-C4 on the 'EVM).
See: viewtopic.php?f=6&t=886

If the Flat preamp is going to be driving the PCM4202EVM directly you might want to install C4, C5, C24 and C25 with the + terminals pointing to the PCM4202EVM or install C1-C4 on the 'EVM.
Otherwise the THAT1240 outputs will have to sink about 2.5 mA DC.
Driving my PCM4222 EVM directly-coupled with most op amp outputs I don't see any increase in distortion but when you patch or switch the inputs it is pretty clicky since there's DC on the A/D inputs.