Phono Transfer System Construction Information

Construction information for DIY projects, including the MS Mid Side Matrix, Elliptic Equalizer, Mastering Console, Phono Transfer System, Insert Switcher and the Dual Class-A Amplifier. You can post your baby pictures here.
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Gold
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Re: Phono Transfer System

Post by Gold » Fri Aug 12, 2016 3:43 pm

JR. wrote: If the gain trim being talked about is the 50K pot in the NF leg from op amp (IC5?) output to negative input,
I was talking about IC 1 on the preamp board. Bringing that out to pots caused me all sorts of problems.

3dB attenuation with makeup gain (+/- 1.5dB) before IC 5 is no problem. That's how Wayne designed it. Bringing the 50k trimmer point to the faceplate worked but I ditched it. I think Wayne got it right by keeping it as another calibration point. I'm building them as two phono input systems. The extra calibration point helps

Adjusting fine gain trim on the preamp board is the best way to calibrate it. That way the first active stage noise floor should be similar for both channels and the down stream circuitry won't add to any noise imbalance.

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JR.
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Re: Phono Transfer System

Post by JR. » Fri Aug 12, 2016 7:41 pm

I found several IC1s in this thread.
If you are talking about this IC1 Image

any stray capacitance there will have undesirable side effects. I dealt with the need to remote a mic preamp gain pot in a similar circuit and buffered the shields of two shielded cables with a CM signal I extracted.

If you are talking about some other circuit I give up...

JR

Gold
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Re: Phono Transfer System

Post by Gold » Sat Aug 13, 2016 6:07 am

Yep, that's the one.

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JR.
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Re: Phono Transfer System

Post by JR. » Sat Aug 13, 2016 11:28 am

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

Gold
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Re: Phono Transfer System

Post by Gold » Sat Aug 13, 2016 11:51 am

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?

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JR.
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Re: Phono Transfer System

Post by JR. » Sat Aug 13, 2016 1:11 pm

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

Gold
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Re: Phono Transfer System

Post by Gold » Sat Aug 13, 2016 1:15 pm

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.

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squarewaveind
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Re: Phono Transfer System

Post by squarewaveind » Tue Aug 16, 2016 12:13 pm

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.

Gold
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Re: Phono Transfer System

Post by Gold » Tue Aug 16, 2016 1:23 pm

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]

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mediatechnology
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Re: Phono Transfer System

Post by mediatechnology » Thu Aug 18, 2016 9:06 am

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.
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