Hi
I have an old Dynaco PAS-3X I brought some years ago. I think it is time to fix this and get it running again.
When I look at it I do see some old modifications. I have no luck in finding any info about this old mods. Hope someone of you can tell me something about it.
The first mod is on the power supply. The old owner put in an extra electrolytic cap, and also an extra coil. Now the schematic looks like this:
Is it OK to have it in this way? Is it good, or should I do anything else? Maybe back to original?
Next old mod is on the PC5 - Line board. Here is a bunch of resistors and caps changed to new valeus. My plan is to change all of the components to better ones, but so far I didn´t notice any mods to the line board, just on the RIAA-board. Does anyone recognize this mod? Should I keep these values or change them back to original?
I DO want to keep the tone controls. Especially while it is a PAS-3X, so I have the possibility to center the tone controls and take them away from the signal path.
Here is a pic whats on the line board in my amp:
I also add a picture with the original values:
Anything else you may see that is wrong or also is an old mod?
My plan for this PAS is:
- Do the curcioaudio.com mod for the RIAA
- Change volyme-pot to an ALPS
- New electrolytic caps instead of the old ones
- Rewiring the input and maybe change the selector switch (it is in good condition)
- Take away Tapeswitch, Filterswitch and Loudnessswitch from the signal path.
Best regards.
Rampitsch
I have an old Dynaco PAS-3X I brought some years ago. I think it is time to fix this and get it running again.
When I look at it I do see some old modifications. I have no luck in finding any info about this old mods. Hope someone of you can tell me something about it.
The first mod is on the power supply. The old owner put in an extra electrolytic cap, and also an extra coil. Now the schematic looks like this:
An externally hosted image should be here but it was not working when we last tested it.
Is it OK to have it in this way? Is it good, or should I do anything else? Maybe back to original?
Next old mod is on the PC5 - Line board. Here is a bunch of resistors and caps changed to new valeus. My plan is to change all of the components to better ones, but so far I didn´t notice any mods to the line board, just on the RIAA-board. Does anyone recognize this mod? Should I keep these values or change them back to original?
I DO want to keep the tone controls. Especially while it is a PAS-3X, so I have the possibility to center the tone controls and take them away from the signal path.
Here is a pic whats on the line board in my amp:
An externally hosted image should be here but it was not working when we last tested it.
I also add a picture with the original values:
An externally hosted image should be here but it was not working when we last tested it.
Anything else you may see that is wrong or also is an old mod?
My plan for this PAS is:
- Do the curcioaudio.com mod for the RIAA
- Change volyme-pot to an ALPS
- New electrolytic caps instead of the old ones
- Rewiring the input and maybe change the selector switch (it is in good condition)
- Take away Tapeswitch, Filterswitch and Loudnessswitch from the signal path.
Best regards.
Rampitsch
Nice to see fellow swede here !
As for the powersupply changes made, i would replace with a board from triode:
SDS Labs Capacitor Board for Dynaco PAS, with parts & instructions
That would replce the selenium rectifiers and caps for the filament AND replace
the can-cap for B+ with fresh ones.
-As for the boards, yes i would replace the components to "factory values"
An alternative might be replacement boards, but i think it is overkill here.
-The curcioaudio.com mod for the RIAA is a waste of time and will degrade the
accuracy of RIAA See http://www.audioregenesis.com/documents/PAS_Phono.pdf
-replace volume with a 250k ALPS is beneficial
- let the selector be , it shorts unused inputs, if you replace with a "switch-only" some live but unselected inputs might bleed through. If you have a working one, leave it! ( maybe clean it and lubricate the shaft )
-loudess filter is removed from signal when open. No need to remove. tapeswitch
also.
Happy listening !
As for the powersupply changes made, i would replace with a board from triode:
SDS Labs Capacitor Board for Dynaco PAS, with parts & instructions
That would replce the selenium rectifiers and caps for the filament AND replace
the can-cap for B+ with fresh ones.
-As for the boards, yes i would replace the components to "factory values"
An alternative might be replacement boards, but i think it is overkill here.
-The curcioaudio.com mod for the RIAA is a waste of time and will degrade the
accuracy of RIAA See http://www.audioregenesis.com/documents/PAS_Phono.pdf
-replace volume with a 250k ALPS is beneficial
- let the selector be , it shorts unused inputs, if you replace with a "switch-only" some live but unselected inputs might bleed through. If you have a working one, leave it! ( maybe clean it and lubricate the shaft )
-loudess filter is removed from signal when open. No need to remove. tapeswitch
also.
Happy listening !
If it was me I would leave the power supply per original, and would use an MU Follower configuration on the line stage, this would give you lower noise, better distortion figures , and better coupling with the input stage of ANY amplifier, even 10k!! And wouldn't over budget the 15mA trafo!! You would have a dang good preamp! I can post schematic for line stage if you need me to.
BTW, you would need heater elevation for the Mu-follower, but that could be incorporated into the new line stage board.
Lots of Mods for a Pas 3 available. Too many to correctly/quickly choose imo.
From experience Dynaco Pots were/are Really Cheap things ( Junk even imo)
Alps clone is a definite improvement.
Does that Dyna pot have a loudness tap? Likely.. making sourcing an Alps with the 4th pin a bit harder but not too difficult. Ebay is your friend for that.
From experience Dynaco Pots were/are Really Cheap things ( Junk even imo)
Alps clone is a definite improvement.
Does that Dyna pot have a loudness tap? Likely.. making sourcing an Alps with the 4th pin a bit harder but not too difficult. Ebay is your friend for that.
Do you have the original schematic of PAS 3X, esp RIAA section, kind of hard to see which component is which. I have downloaded the 3X manual already, hope to see where the mod is taken place, Maybe there are different versions.
Thanks for all your answer!
Interesting document you gave about the RIAA-mod, petertub!
I will try to print the original schematic for the Line board and write in the values I have in my unit.
I will do this with the RIAA-board as well, while it is a mod made on that board too.
Hope to fix this tomorrow.
//Rampitsch
Interesting document you gave about the RIAA-mod, petertub!
I will try to print the original schematic for the Line board and write in the values I have in my unit.
I will do this with the RIAA-board as well, while it is a mod made on that board too.
Hope to fix this tomorrow.
//Rampitsch
It appears from your photo that your PAS has the original selenium rectifier for the heaters. I would recommend that you replace it with silicon diodes as the very first thing that you do. At the same time, it would be worthwhile to replace the heater filter capacitors with new ones, of the same or greater values. Suggested values are shown in Norman Koran's schematic which is linked below. Additionally, attached is a simple inexpensive way of regulating the heater voltage using common regulators—LM7812 and LM7824.
There are dozens of PAS mods that have been published over the years. Some are very worthwhile, others are less so.
I like the work that Norman Koren published:
Spice and the art of preamplifier design, Part 1
There are dozens of PAS mods that have been published over the years. Some are very worthwhile, others are less so.
I like the work that Norman Koren published:
Spice and the art of preamplifier design, Part 1
Attachments
I hade some hour left this evening, so I printed a copy of the Dynaco PAS-3X manual, and wrote down the values that my PAS-3X is equipped with.
Here is the PC5 Line board (just one channel shown, but it is the same on both):
What can you guys say about this mod: keep it, change back or do something else?
And here is also the PC6 RIAA board (one channel shown here too):
Regarding to the RIAA, is it anyone that tried the Curcioaudio-mod?
Any comments on this mod?
I am sorry for the misleading photo. The photos in my first post is not from my unit, just a photo I found on the internet to find the original values (and I just found out that some of the values on the photo are not equal to the ones in the manual).
In my unit the old selenium rectifier is gone and replaced with silicon diodes.
Here is the PC5 Line board (just one channel shown, but it is the same on both):
An externally hosted image should be here but it was not working when we last tested it.
What can you guys say about this mod: keep it, change back or do something else?
And here is also the PC6 RIAA board (one channel shown here too):
An externally hosted image should be here but it was not working when we last tested it.
Regarding to the RIAA, is it anyone that tried the Curcioaudio-mod?
Any comments on this mod?
I am sorry for the misleading photo. The photos in my first post is not from my unit, just a photo I found on the internet to find the original values (and I just found out that some of the values on the photo are not equal to the ones in the manual).
In my unit the old selenium rectifier is gone and replaced with silicon diodes.
I put the values into spice and ran a AC Sweep, the curve looks way off, the yellow line is output from the inverse RIAA. the Red line is output without.
Attachments
Whether you implement is ideas or not, Norman Koren's article on the PAS pre is worth reading: Spice and the art of preamplifier design, Part 2
jack
jack
Here's the Schematic with the new values, you will have to leave in the cathode feedback, in order to have the needed gain at lower frequencies as a matter of fact it needs to be increased by lowering it to 22k instead of 47k. the response is almost ruler flat! BTW made a few adjustment in components to both biasing resistors, and the NFB.
Attachments
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I’m currently using the phono board values that are recommended by Curico, and find that it does improve it, but I still feel that the performance of this stage is “lacking”.
I’ll replace it completely someday, but it’s not a priority, as find myself using the phono stage less and less.
Keep in mind that the signal from the phono stage still has to pass through the line stage, and the original line stage has a myriad of issues. One of these issues is the huge subsonic rise in response below 5Hz. Another is that loading (the type of power amplifier used) greatly affects the performance of this stage. Dynaco was aware of the loading issue, and addresses it in the manual.
The minor changes shown on your diagram do not solve these problems. But, if you decide to use it as is, I‘d check the capacitors between the bass controls and the outputs. Dynaco used an electrolytic, which should be replaced with a poly type of the same value.
I’ll replace it completely someday, but it’s not a priority, as find myself using the phono stage less and less.
Keep in mind that the signal from the phono stage still has to pass through the line stage, and the original line stage has a myriad of issues. One of these issues is the huge subsonic rise in response below 5Hz. Another is that loading (the type of power amplifier used) greatly affects the performance of this stage. Dynaco was aware of the loading issue, and addresses it in the manual.
The minor changes shown on your diagram do not solve these problems. But, if you decide to use it as is, I‘d check the capacitors between the bass controls and the outputs. Dynaco used an electrolytic, which should be replaced with a poly type of the same value.
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Denny9167: In the first one in Spice, do you noticed that the feedback resistor on 47k is gone and not replaced by anything (it´s wide open between the tubes)? I can´t se that part in the first printscreen from you.
I do not know what this preamp sound like with this values, since it has to much hum to be accurate. My plan were to fix the hum and at the same time do a Little uppgrade.
I have read all the info in the article by Norman L. Koren, and it do sounds good, but as the first step, I don´t want do rebuild the PAS that much. I think if it´s time to mod the PAS that hard, to make it sound good, then it´s better to build from the scratch.
What I now looking for is to investigate what mod my PAS-3X has, and what I should do with it, without rebuild it totally. Maybe go back to original values on the boards? Or make som minor changes?
I´m up for ideas 🙂
Regards
Rampitsch
I do not know what this preamp sound like with this values, since it has to much hum to be accurate. My plan were to fix the hum and at the same time do a Little uppgrade.
I have read all the info in the article by Norman L. Koren, and it do sounds good, but as the first step, I don´t want do rebuild the PAS that much. I think if it´s time to mod the PAS that hard, to make it sound good, then it´s better to build from the scratch.
What I now looking for is to investigate what mod my PAS-3X has, and what I should do with it, without rebuild it totally. Maybe go back to original values on the boards? Or make som minor changes?
I´m up for ideas 🙂
Regards
Rampitsch
Yes your correct, the drawing you provided shows the feedback resistor removed, was taking that into consideration. The second reply I posted shows how I have built it, and have done extensive listening to it as well, it has a superb sound from top to bottom. BTW in my builds, I've increased the B+ to 250V.
The values used by Curcio are correct, but you have to also change the positive feedback resistor, other wise the bass will be very weak, probably around 24K will work, if you use Dynaco's stated load resistors, and B+.
Curcio PAS boards
I installed the power supply and line stage in a PAS some years ago. It is a fully regulated modern design that drove solid state amps and tube amps very very well. I thought it was a terrific upgrade. Not cheap, but terrific.
I installed the power supply and line stage in a PAS some years ago. It is a fully regulated modern design that drove solid state amps and tube amps very very well. I thought it was a terrific upgrade. Not cheap, but terrific.
As starter i would get things back at "original values". Fixing the hum is of course
importent, a replacement powersupply(1) _should_ take care of that.
As regards for reverting components, one could also obtain complete
replacement boards ( i do have a pair on the shelf)
As for components, note that not all components are located on the riaa-board, some
are mounted on the selector.
(1) reffering to boards like : SDS Labs Capacitor Board for Dynaco PAS, with parts & instructions
et.al., several look-alikes are available, the beauty of these is that all power
supply caps will be replaced with modern ones.
importent, a replacement powersupply(1) _should_ take care of that.
As regards for reverting components, one could also obtain complete
replacement boards ( i do have a pair on the shelf)
As for components, note that not all components are located on the riaa-board, some
are mounted on the selector.
(1) reffering to boards like : SDS Labs Capacitor Board for Dynaco PAS, with parts & instructions
et.al., several look-alikes are available, the beauty of these is that all power
supply caps will be replaced with modern ones.
