Hello all,
I'm an extreme nubie when it comes to circuitry, but I am trying to make some upgrades to my already extremely modified Dyna Pas 2. The modifications were done by someone else so I can’t take credit for that. However, I want to upgrade the RCA jacks and add an IEC power cord. I was wondering if any people that are familiar with this preamp could recommend some ways to reground/isolate the RCA jacks to provide better separation and imaging. Also, what should be connected to the ground prong of the IEC jack? I just don’t know enough to know what would give the best results. Any details would be greatly appreciated.
Jarrett
I'm an extreme nubie when it comes to circuitry, but I am trying to make some upgrades to my already extremely modified Dyna Pas 2. The modifications were done by someone else so I can’t take credit for that. However, I want to upgrade the RCA jacks and add an IEC power cord. I was wondering if any people that are familiar with this preamp could recommend some ways to reground/isolate the RCA jacks to provide better separation and imaging. Also, what should be connected to the ground prong of the IEC jack? I just don’t know enough to know what would give the best results. Any details would be greatly appreciated.
Jarrett
Dynaco PAS
Jarrett,
Take a look at http://www.curcioaudio.com/dynadr_3.htm. You'll find everything you need for the PAS. There's a great fourm too.
Jarrett,
Take a look at http://www.curcioaudio.com/dynadr_3.htm. You'll find everything you need for the PAS. There's a great fourm too.
Actually, your question is more mechanical. The power line ground ("safety ground") needs to be securely attached to the chassis. The RCA jacks on the back, as you recognize, need to be isolated from the chassis and attached to signal ground.
Unfortunately, the way Dynaco did the back panel, you're pretty much forced to strip and remove it, then drill or mill out the holes for new isolated RCA jacks. The original Dyna ones are simply awful and ought to be crushed and burned.
Unfortunately, the way Dynaco did the back panel, you're pretty much forced to strip and remove it, then drill or mill out the holes for new isolated RCA jacks. The original Dyna ones are simply awful and ought to be crushed and burned.
SY said:
An alternative is AVA's method. Buy a nibbling tool and cut away the portion of the back panel that holds those nasty OEM jacks. You close the rectangular opening created with a piece of FR4 substrate PCB material. The new jacks are mounted on the PCB material.
BTW, AVA does not isolate the jacks (except the phono I/P). They use the PCB laminate as a ground plane. I have experience with 2 AVA rebuilt Dyna chassis and find their method to work well.
Edit: fixed typos
About a year ago, I replaced the jacks in my PAS using the techniques that Eli described. Rather than use a nibbler to enlarge the opening, I simply settled for fewer inputs. I have no need for a "tape head" input as well as a "second phono" or "microphone" input, and I doubt that today, many others do either. If the low level phono compensation components were moved from the center section of the selector switch to the board, as they are in at least some of the PAS mods, these other low level options are no longer available anyway. If you can live without all these jacks, the task will be simplified, as you will have more space and fewer wires to deal with. Now is also a good time to rewire the input wiring, which is probably a rats nest, causing crosstalk and possible other problems. Also, plan on replacing the carbon 510K (or whatever value you're using) load resistors and 10ohm ground isolation resistors with new ones. These resistors are mounted on the rear jack panel, and aren't worth the effort to save and reuse them.
Thanks for the replies. I am not worried about preserving all of the inputs. I'm actually only going to save Aux 1&2 and Phono 1 so space isn't an issue. I guess what I'm still wondering is if there is a better place on the board to ground the inputs and outs (seperately) as not to have them all wired together at the jacks. Can the resistors be eliminated somehow? I'd like to clean it all up a bit.
You cannot delete the output loading resistors without dramatically altering the sound. In fact, these should be tailored to the input Z of your power amplifier.
You can try deleting the 10 ohm resistors, but, a friendly warning. Your posts state that you want to: 1/ add an AC ground, 2/ add a turntable ground, 3/ change the Dyna grounding scheme (which has it's drawbacks, but works pretty well). All this can lead to nasty "hum headaches", unless you know exactly what you are doing, to begin with. I think you should carefully consider your objectives before you start cutting!
You can try deleting the 10 ohm resistors, but, a friendly warning. Your posts state that you want to: 1/ add an AC ground, 2/ add a turntable ground, 3/ change the Dyna grounding scheme (which has it's drawbacks, but works pretty well). All this can lead to nasty "hum headaches", unless you know exactly what you are doing, to begin with. I think you should carefully consider your objectives before you start cutting!
Ok, so I don't know what i'm doing, but my objectives are simple. (a)Eliminate Crosstalk. Crosstalk is a major problem because one of my inputs is being used for my cable box. The problem is that it is always on. I like to watch the news and listen to music, and I don't want to have to unplug the TV audio when i listen to music. It seems that imaging and stereo seperations is being effected by the crosstalk. I am new to this Pas, and it might just be an inherent problem, but the Pas's reputation lead me to believe otherwise.
(b) Upgrade the RCA jacks, my interconnectcs are not allowed to touch such nasty jacks.
(c) Allow for the use of a higher quality power cord (for quality's sake, not grounding issues) Could this cause problems?
So, crosstalk is my biggest problem. I know that a better wiring job will solve some of those problems, but i was just petitioning for advice on better isolation. I guess the 10 ohm resistors are getting replaced. While im at it I think I'll replace the .1uf's with some Auri's. But as far as further isolation of the in's/out's, I guess since i don't know much, I'll just leave them grounded the way they are.
(b) Upgrade the RCA jacks, my interconnectcs are not allowed to touch such nasty jacks.
(c) Allow for the use of a higher quality power cord (for quality's sake, not grounding issues) Could this cause problems?
So, crosstalk is my biggest problem. I know that a better wiring job will solve some of those problems, but i was just petitioning for advice on better isolation. I guess the 10 ohm resistors are getting replaced. While im at it I think I'll replace the .1uf's with some Auri's. But as far as further isolation of the in's/out's, I guess since i don't know much, I'll just leave them grounded the way they are.
Let's take one item at a time.
First, if your power cord is in bad shape, by all means replace it, as soon as possible! A grounded three wire cord is an excellent idea, and for safety reasons it's the best way to go. Having said that, Dyna and other manufacturers built many thousands of pieces of audio equipment with the two wire AC feeds, with few problems. I've never heard of anyone being hurt because of this, but imagine it has happened. In a transformer supply the chassis should never be connected to either side of the AC line, and it would take a pretty oddball failure to cause it to happen, but again, I imagine it has occurred sometime, somewhere. I added an AC line fuse to my PAS, and I would strongly recommend that others do the same! I've never understood why Dynaco didn't include this very basic safety item!!
Second- I suspect that the audio output from your cable box is pretty hot, and this could be causing your problem. As there are no inputs on the back of a stock PAS marked "AUX", I'd check the wiring to be certain that you aren't feeding this into a spare low level input, by the way. I use a feed from my DISH network box connected to the PAS "Spare" input, and never noticed this situation, even before I rewired my inputs. As a temporary fix, just try turning off the cable box. A better fix may be rewiring your inputs, as suggested, perhaps using shielded cable as I did. This can get a little tricky in a number of ways. Finding good cable small enough is getting to be a problem, lately. Phono cartridge/ arm cable was readily available at one time, but no longer. I used RG-404, which is very small, and very expensive, and hard to work with, and worst of all, I now have reason to suspect that the center conductor is silver covered steel. Since steel conductors should be avoided at all costs, I'm planning to correct this soon, and I'm looking around for suitable cable.
If you do use cable, be sure to ground only the end of the shield at the back panel. Additionally you may want to consider the capacitance of the cable, particularly at the phono input, depending on your cartridge. Wiring with cable can also get a little hairy because of space limitations, and the soldering can be challenging if you don't have some experience!
3/ I agree with your point (Sy's too) about the jacks, although I don't think you will damage your ends if you use them for now.
4/ Upgrading the caps sounds good but, it's usually best to tackle one thing at a time.
Do you have any information about the mods that were done by prior owner? Were they "homebrew" or done commercially?
First, if your power cord is in bad shape, by all means replace it, as soon as possible! A grounded three wire cord is an excellent idea, and for safety reasons it's the best way to go. Having said that, Dyna and other manufacturers built many thousands of pieces of audio equipment with the two wire AC feeds, with few problems. I've never heard of anyone being hurt because of this, but imagine it has happened. In a transformer supply the chassis should never be connected to either side of the AC line, and it would take a pretty oddball failure to cause it to happen, but again, I imagine it has occurred sometime, somewhere. I added an AC line fuse to my PAS, and I would strongly recommend that others do the same! I've never understood why Dynaco didn't include this very basic safety item!!
Second- I suspect that the audio output from your cable box is pretty hot, and this could be causing your problem. As there are no inputs on the back of a stock PAS marked "AUX", I'd check the wiring to be certain that you aren't feeding this into a spare low level input, by the way. I use a feed from my DISH network box connected to the PAS "Spare" input, and never noticed this situation, even before I rewired my inputs. As a temporary fix, just try turning off the cable box. A better fix may be rewiring your inputs, as suggested, perhaps using shielded cable as I did. This can get a little tricky in a number of ways. Finding good cable small enough is getting to be a problem, lately. Phono cartridge/ arm cable was readily available at one time, but no longer. I used RG-404, which is very small, and very expensive, and hard to work with, and worst of all, I now have reason to suspect that the center conductor is silver covered steel. Since steel conductors should be avoided at all costs, I'm planning to correct this soon, and I'm looking around for suitable cable.
If you do use cable, be sure to ground only the end of the shield at the back panel. Additionally you may want to consider the capacitance of the cable, particularly at the phono input, depending on your cartridge. Wiring with cable can also get a little hairy because of space limitations, and the soldering can be challenging if you don't have some experience!
3/ I agree with your point (Sy's too) about the jacks, although I don't think you will damage your ends if you use them for now.
4/ Upgrading the caps sounds good but, it's usually best to tackle one thing at a time.
Do you have any information about the mods that were done by prior owner? Were they "homebrew" or done commercially?
The mods were done by Audio Dimensions in Cali, I'm guessing about 25 years ago. Modifications include an additional 3 position impedance selector switch for matching the main outputs to your amp. 3 phono inputs, 2 aux inputs, a tuner input, and a switchable tape monitor are provided. The tone controls and scratch filter have been removed. Precision resistors as well as several high quality film and poly caps have been installed in critical spots to improve signal quality. Chokes have also been added to improve B+ power supply decoupling. Extra filtering has been added to the filament circuit to lower hum & noise. Diodes replace the original 12AX4 rectifier, reducing strain on the power transformer and improving reliability. An internal fuse has been added.
I purchased this Pas with these modifications done; however, it had some problems. There were trace fractures on the board, some of which had been "fixed", others were not. The .1 uf capacitors are in very bad shape (they look like a can with botulism). The RCA jacks are very corroded, and some of the porcelain is cracked/chipped. The insulation on the power cord is also in bad shape.
I purchased this Pas with these modifications done; however, it had some problems. There were trace fractures on the board, some of which had been "fixed", others were not. The .1 uf capacitors are in very bad shape (they look like a can with botulism). The RCA jacks are very corroded, and some of the porcelain is cracked/chipped. The insulation on the power cord is also in bad shape.
Audio Dimensions made very nice improvements to the PAS, probably better than most, considering the time period. It really sounds as if this unit would be well worth bringing back to top condition. It's possible that broken traces on the boards could contribute to some of the problems that you've described, so I'd look repairing these, perhaps even before you rewire the inputs.
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