Pro Audio Design Forum

OK, for that one it's pretty straightforward...

Two separate single conductor shielded cables. The shield of cable in series with R6 connects to pin 1, the shield of cable in series with R7 connects to pin 7. That way the shield capacitance looks like a small cap across the feedback resistors.

Easy peasy...

JR

JR. wrote:OK, for that one it's pretty straightforward...

Two separate single conductor shielded cables. The shield of cable in series with R6 connects to pin 1, the shield of cable in series with R7 connects to pin 7. That way the shield capacitance looks like a small cap across the feedback resistors.

Easy peasy...

I don't understand. The gain trimmer is arranged like a rheostat. It's not a grounded pot. Doesn't connecting the shield in series ground that side of the pot?

Gold wrote:

JR. wrote:OK, for that one it's pretty straightforward...

Two separate single conductor shielded cables. The shield of cable in series with R6 connects to pin 1, the shield of cable in series with R7 connects to pin 7. That way the shield capacitance looks like a small cap across the feedback resistors.

Easy peasy...

I don't understand. The gain trimmer is arranged like a rheostat. It's not a grounded pot. Doesn't connecting the shield in series ground that side of the pot?

Sorry I was imprecise... the shield of [the] cable [whose center conductor is] in series with R6 connects to pin 1. [The other end of the shield at R6 is left floating].

The whole purpose is to make the shield capacitance a harmless small C across each feed back R, not a harmful C from - input to ground.

JR

JR. wrote: [The other end of the shield at R6 is left floating].

The whole purpose is to make the shield capacitance a harmless small C across each feed back R, not a harmful C from - input to ground.

Ahh, that makes sense.

mediatechnology wrote: It's a good thing you did since it still is a balanced input and it realizes common mode rejection even if it is connected using an RCA.

The balance of co-axial cables isn't as good as true STP but apparently quite a few modern "co-axial" RCA cables are internally STP so the only unbalanced portion of the interconnect is the RCA where "-" ties to the outer conductor.

Hmm, what connection scheme are you talking about here? If the preamp input is an RCA, then the shield must be tied to circuit common on both sides, which means the "-" wire in the twisted pair is also tied to common, and the entire scheme is unbalanced. In this case the instrumentation amp will behave exactly the same as the equivalent single amp circuit. You could setup an impedance balanced interconnect, where the "-" of the twisted pair is connected to common at the cartridge end through a small resistance, but you would need a 3 conductor input at the preamp to keep "-" and shield/common separate.

squarewaveind wrote: If the preamp input is an RCA, then the shield must be tied to circuit common on both sides,

I think this is where the confusion lies. Both I and Wayne ignore the connector type and just treat is as the other end of the coil in the cart. A cartridge is a naturally balanced and floating device. It doesn't need a ground reference.

On an SP1200 the whole system floats until you connect the ground wire. On a lathe (not the VG66 drive electronics) the circuit ground is very polluted. I've found it dicey as a ground reference. That's why it's best to use the PTS in a balanced configuration.




which means the "-" wire in the twisted pair is also tied to common, and the entire scheme is unbalanced. In this case the instrumentation amp will behave exactly the same as the equivalent single amp circuit. You could setup an impedance balanced interconnect, where the "-" of the twisted pair is connected to common at the cartridge end through a small resistance, but you would need a 3 conductor input at the preamp to keep "-" and shield/common separate.[/quote]

Two separate single conductor shielded cables. The shield of cable in series with R6 connects to pin 1, the shield of cable in series with R7 connects to pin 7. That way the shield capacitance looks like a small cap across the feedback resistors.

Got it. Thanks.

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.

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.

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.