Long Preamp to Power Amp Cable Lengths

[Eli Duttman]

I've uploaded the 1st cut schematic for buffering the PAS. Assistance from CineMag is needed in properly sizing the DC blocking cap., that's in the O/P circuitry.

If a combined 12B4 line stage/buffer project is to be undertaken, the FET will be DC coupled to the triode. Fewer caps. in the signal path is "always" better.

[Loren42]

Originally Posted by Eli Duttman

I've uploaded the 1st cut schematic for buffering the PAS. Assistance from CineMag is needed in properly sizing the DC blocking cap., that's in the O/P circuitry.

If a combined 12B4 line stage/buffer project is to be undertaken, the FET will be DC coupled to the triode. Fewer caps. in the signal path is "always" better.

Thanks, Eli.

I ended up ordering two CineMag CMLI-15/15B transformers as per Dave's recommendation at CineMag.

This transformer is supposed to be located in the power amp.

CMLI-15/15B[PC] <Single Shield> - CMLI-15/15B2[PC] <Dual Shields>
Parameter
Conditions
Typ
Turns Ratio1 : 1.00
Input impedance, Zi
1 kHz, +4 dBu RL=15k Test Circuit 1
1 kHz, +4 dBu RL=10k Test Circuit 1
18.0 kS
13.0 kS
Voltage Gain
1 kHz, +4 dBu Test Circuit 1 Rs=600S RL=15 kS
-2.2 dB
Distortion (THD+N%)
1 kHz, +4 dBu Test Circuit 1 Rs=600 RL=15K
20 Hz, +4 dBu Test Circuit 1 Rs=600 RL=15K
0.0007%
0.02%
Max 20 Hz input level
1.0% THD; Rs=600S Rl=15KS Test Circuit 1
+20 dBu
Response, ref 1 kHz
20 Hz +4 dBu Rs=600S RL=15KS Test Circuit 1
20 kHz +4 dBu Rs=600S RL=15KS Test Circuit 1
-0.03 dB
-0.1 dB
Phase Shift at 20Hz
Phase Shift at 20 kHz
Referenced to source generator Test Circuit 1
+1E
-18E
CMRR Single Shield
CMRR Dual Shield
60 Hz Test Circuit 2 per IEEE Std 389-1996 ¶19 RL=15KS
1 kHz Test Circuit 2 per IEEE Std 389-1996 ¶19 RL=15KS
60 Hz Test Circuit 2 per IEEE Std 389-1996 ¶19 RL=15KS
1 kHz Test Circuit 2 per IEEE Std 389-1996 ¶19 RL=15KS
103 dB
76 dB
110 dB
87 dB
Operating Temp Range
Operation and storage
0E C Min 70E C Max

[Eli Duttman]

OK, then cross out the trafo in the buffer cuicuit I uploaded. The buffer's O/P cap. will be a 4.7 μF. metalized polypropylene (MPP) part, like a Solen. Bypass the MPP part with a film and foil part up to 0.47 μF. in value. The low cost bypass is a 716P "Orange Drop". The boutique bypass cap. is a MultiCap PPFX series part.

I strongly suspect you will be mounting the trafos you bought from CineMag in a separate enclosure that's positioned closely to the power amp.

If you already own LONG Blue Jeans cables, send them back to be remanufactured into a number of cable sets whose length is suited to use near the system control center. As I explained earlier in this thread, this situation requires that cable shielding NOT carry signal. As you are constructing a highly custom installation, make the interconnect cabling "captive", wherever possible. Fewer purely mechanical connections in the signal path is good. For instance, hard wire the cabling between the buffer and the wall plates to the buffer. Obviously, XLR connectors are necessary, at the wall. Strain relief of "captive" wiring is easy. The cable passes through holes in the sheet metal that contain rubber grommets. Inside the box, you tie a figure 8 knot, that holds a piece of wooden doweling, with the wire.

[Loren42]

Originally Posted by Eli Duttman

I strongly suspect you will be mounting the trafos you bought from CineMag in a separate enclosure that's positioned closely to the power amp.

Well, I plan to mount the two transformers inside the power amp close to the input PC board. I'll make a set of custom clamping brackets out of some 1/4" plate aluminum.

I have not bought cables. However, what do you recommend for the twinax line cable?

Normally, when I do a strain relief I use either a rubber grommet or I turn one from nylon on my lathe (it is good to have a private machine shop in your garage ) and use a nylon tie wrap on the inside to prevent the cable from pulling back out.

I'll post a wiring diagram and schematic once we get all the details ironed out so that you have a chance to do a final review of my intentions.

I appreciate the help you are providing as well as the education.

[Lavcat]

I see I am too late to be useful, but I have a Jensen CI-2RR on an input to my preamp for galvanic isolation and I cannot say enough good things about it.

A project I am currently working on is using Jensen JT-6110K-B's at the input to my power amp.

My advice for the cable for the long run between the preamp and transformer would be to use unshielded twisted pair rather than coax. I do not see the need to modify your preamp at all.

[Loren42]

Originally Posted by Lavcat

I see I am too late to be useful, but I have a Jensen CI-2RR on an input to my preamp for galvanic isolation and I cannot say enough good things about it.

A project I am currently working on is using Jensen JT-6110K-B's at the input to my power amp.

My advise for the cable for the long run between the preamp and transformer would be to use unshielded twisted pair rather than coax. I do not see the need to modify your preamp at all.

The PAS 3 preamp has no buffer stage. The output comes straight from the passive tone controls and Dynaco expects a terminating impedance of 100 KΩ or higher.

If the impedance is much lower it changes the tone circuit operation drastically.

[Lavcat]

Originally Posted by Loren42

The PAS 3 preamp has no buffer stage. The output comes straight from the passive tone controls and Dynaco expects a terminating impedance of 100 KΩ or higher.

If the impedance is much lower it changes the tone circuit operation drastically.

But you said your amp had an input impedence of 10K?

Had I seen the thread before you bought the transformers I might have suggested the JT-6110K-B step-down transformer to give you a higher input impedence, assuming you have enough voltage to use the JT-6110K-B and still drive your amp.

Still I would try the system with the transformers first before modifying the preamp. And I would not use shielded cable. Shielded cable grounded at the source can act as an antenna. I was getting pretty good shortwave reception that way here.

Forgive any ignorance on my part, my Dynaco is a PAT-4 and I realize I may have ventured into the wrong forum.

[Loren42]

Easy enough to try when the transformers arrive. I can raise the impedance of the power amp.

[Eli Duttman]

Guys pay attention, please! The sort of transformers under discussion need to be driven by low impedance sources. Look at the trafo data sheets, closely. Dyna PAS preamps are anything but low impedance sources. The approx. 470 KOhm I/P impedance of Dyna tubed power amps is not accidental. Sometimes, tweaking of the secondary side loading to stop ringing is necessary.

BTW, even if the PAS did not impedance mismatch with the ground loop blocking trafo, the capacitance of 20 ft. shielded cables will roll the HF info. off., very badly.

Build the buffer!

[Lavcat]

Don't use shielded cables then! (Though I would have chosen a higher impedence transformer, myself.)

But seriously, one other thing I thought of: if the preamp does not block DC you will want to do something to keep any possible DC from saturating the transformers.