I am thinking about doing a scratch-build PAS3 pre-amp for a SE EL84-based amp project I am working on.
Everyone pretty consistently recommends removing the tone controls, but one of the biggest needs for a pre-amp is that I *do* need tone controls. I am doing this whole thing inside an original Magnavox cabinet where the speaker mounting design is certainly less-than-perfect, so some ongoing tone adjustment will definitely be needed.
Further complicating this, it seems like the values of pots used for B/T on the PAS3 are not easily obtainable, so am I just asking for more trouble than it's worth? I do need a MM phono stage (as the original Mag had none), so I figured that by the time I build a power tranny and project chassis, I might as well add the line stage portion too. The PAS3 "project" is so common that it seems like a good choice, or are there good mods out there for a scratch build with tone controls that won't require me to screen my own boards?
This will be my first major DIY stereo project, so I suppose it doesn't have to be perfect in the first revision, but might as well try to do it as best I can. My analog electronics knowledge could be described as "just enough to be dangerous", so it's very likely I am either over-thinking or under-thinking this.
Any advice on this perhaps? Thanks!
Everyone pretty consistently recommends removing the tone controls, but one of the biggest needs for a pre-amp is that I *do* need tone controls. I am doing this whole thing inside an original Magnavox cabinet where the speaker mounting design is certainly less-than-perfect, so some ongoing tone adjustment will definitely be needed.
Further complicating this, it seems like the values of pots used for B/T on the PAS3 are not easily obtainable, so am I just asking for more trouble than it's worth? I do need a MM phono stage (as the original Mag had none), so I figured that by the time I build a power tranny and project chassis, I might as well add the line stage portion too. The PAS3 "project" is so common that it seems like a good choice, or are there good mods out there for a scratch build with tone controls that won't require me to screen my own boards?
This will be my first major DIY stereo project, so I suppose it doesn't have to be perfect in the first revision, but might as well try to do it as best I can. My analog electronics knowledge could be described as "just enough to be dangerous", so it's very likely I am either over-thinking or under-thinking this.
Any advice on this perhaps? Thanks!
Why on earth not buy an old PAS3x? You will then get the pots and a lot of other parts needed.
I don´t know if the guys telling you to remove the tone controls know what they are talking about. The tonecontrols are in no way in the circuit when centered. But this calls for the specialpots in the PAS3x. The older PAS3 and PAS3 aren´t built like that.
I don´t know if the guys telling you to remove the tone controls know what they are talking about. The tonecontrols are in no way in the circuit when centered. But this calls for the specialpots in the PAS3x. The older PAS3 and PAS3 aren´t built like that.
I'd thought about it, but the ones I've seen for sale seem to be pricey. I'm not so much looking for the fancy PAS 3X tone defeat controls, since I am planning to build in a bypass switch JIC they really do stink.
Given that, I'm having a tough time finding 750k and 400k stereo audio/log pots... Or did I read somewhere that they are actually linear? Forget finding them with 1" flatted shafts - gave up on that a while ago.
Anyone know of any sources for pots of those values, OR what mods would I need to do to use a more common 500k or 1m pot?
Thx guys!
Given that, I'm having a tough time finding 750k and 400k stereo audio/log pots... Or did I read somewhere that they are actually linear? Forget finding them with 1" flatted shafts - gave up on that a while ago.
Anyone know of any sources for pots of those values, OR what mods would I need to do to use a more common 500k or 1m pot?
Thx guys!
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Yeah, I tend to find that even more so on modern LPs that minor adjustment is necessary. At least a few new 180g pressings seem to just be really bright or too bassy, compared to originals which I can usually leave close to flat...
Option B is to build the PC5 with a cookbook baxandall type tone circuit, but I'm not all that great in my understanding of the nuances of input/output impedances so I'm afraid I'd just make it even worse.
I've got a PAS2 without the tone control switches, and I like it fine. 50 year old pots on tone, stereo cost cutters dynaco bankrupcy reject pot on volume(stuck). Putting in plastic film caps made the tone so trebly that I run with the treble pots down a bit. Paper caps were unbalanced, and of course it was the last one I changed that had the burned wax case( on the bottom). I suppose I should go back and put resistance in series with each cap to flatten it out. Did the electrolytics and 100k up carbon film resistors last year, too. Life marches on.
The reason the pots are so high is that the 12AX7 has only 250k output impedance. There are other pin compatible tubes that have more drive if you use 100k's. Don't know if the power transformer can support it. Mouser had a stereo 250k log volume pot from Japan I just bought, haven't put it in yet so no opinion of it yet.
The reason the pots are so high is that the 12AX7 has only 250k output impedance. There are other pin compatible tubes that have more drive if you use 100k's. Don't know if the power transformer can support it. Mouser had a stereo 250k log volume pot from Japan I just bought, haven't put it in yet so no opinion of it yet.
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To tone control or not to tone control? That question will eventually bring you responses with opinions that are sharply divided. It probably rates right up there with “what’s the best tube”, “triode vs. pentode” and “single ended vs. push-pull”. I have both a PAS 2 and A PAS 3X. I use tone controls because I find them necessary with varied music sources. But that doesn’t mean I like them! Both these preamps have a number of shortcomings compared to other designs, much of which is centered around the lack of cathode follower outputs. The tone controls exacerbate this problem, and I honestly don’t see the (X) feature as much of an improvement over the standard “pots”.
IMHO, Norman Koren developed a very good solution to the problem, in his final circuits before he left audio for other interests, over ten years ago. His last line stage design for the PAS features a switched tone compensation circuit, mostly after the cathode follower that is truly “out of the circuit”, in the center position. It also uses a 6SN7 as a cathode follower, rather than a 12AX7, which may be a better choice. Scratch building this line stage is probably not a project for the novice, however. If you Google “Norman Koren Audio” you will find this information available, on the ‘net. The article available is an extension of the material he presented in “Glass Audio” magazine during the 1990s. If you can obtain copies of these, you will find that he offered a number of tone control configurations in line-stage circuit he developed.
It you do decide to build from scratch, etched circuit boards are not a necessity. Point to point is still an option.
IMHO, Norman Koren developed a very good solution to the problem, in his final circuits before he left audio for other interests, over ten years ago. His last line stage design for the PAS features a switched tone compensation circuit, mostly after the cathode follower that is truly “out of the circuit”, in the center position. It also uses a 6SN7 as a cathode follower, rather than a 12AX7, which may be a better choice. Scratch building this line stage is probably not a project for the novice, however. If you Google “Norman Koren Audio” you will find this information available, on the ‘net. The article available is an extension of the material he presented in “Glass Audio” magazine during the 1990s. If you can obtain copies of these, you will find that he offered a number of tone control configurations in line-stage circuit he developed.
It you do decide to build from scratch, etched circuit boards are not a necessity. Point to point is still an option.
I've seen them and was drooling at the option of building it! I looked at the blank boards for his designs that are sold on Classic Valve, But alas, they only sell the "purist" version, not the one with tone controls, which unfortunately is just one of the necessities of this project. A point-to-point build would be fun, but probably beyond what I can put into this one.
indianajo said:
I've read several modders have suggested replacing the volume pot with 100k anyway, which I was planning to do. This is where my ignorance of impedance shines, unfortunately. My big puzzle is if I do a "stock" build of the PAS3, and needed to sub different values for the bass/treble pots, is that something I could do easily or with small changes?
I should also mention, I'm using this preamp design to drive a re-built Magnavox 8601 EL84-based SE amp, with speakers that aren't quite exactly Cornwalls, so I'm sort of looking for that slightly imperfect sound -- warm, musical and full of tube! This replaces my main system which is a NAD C340, so I want something different than the flawless, sterile SS sound.
Warm is not a PAS2/3/3x. They use 1V pp of a 400 V range, so it is extremely linear. My op amp disco mixer with no tone controls, and the PAS2 with tone control down to remove brightness, sound exactly the same, except the disco mixer hums a little bit. You want warm, get a graphic equalizer and fool with the sound. equalizers with bad sliders are under $100, add some pots you are back to new. Some equalizers are quieter than others, 4558 op amps are noisier (hiss) than RC 4560 or MC33078 . My disco mixer, I replaced the 4558's with 33078 and reworked the power supply to get the 60 hz out of the steel op amp/slider box.
I removed the tone controls from my PAS3, as I imagine others have, and should have a full set if you can't find them elsewhere.
I don't think this is what YOU may want to do, but I plan to do it.
Referencing the article, "The Last PAS" by David Vorhis (AA 4/82), he removes the tone controls from the circuit as well as the tape output, and converts the circuit on the final tube section on the high level board to a CF configuration. He claims it can drive loads down to 10 ohms.
He makes some minor mods to the stock PCB, but changes most of the passive components. I decided to buy a 3rd party glass replacement PCB instead.
I'm going to take mine a step further and use a 12DW7 for the high gain PCB, and adjust the circuit values accordingly. That will give me the equal of a 12AU7 for the CF, which not only has a lower plate impedance, but will also drive at a higher current. (Some folks are not fond of using a 12AX7 for a CF, but in all fairness it has been done in a fair number of commercial circuits).
Vorhis did some excellent mods to the phono stage as well, and makes it truer to the RIAA curve. The high level board acts as a buffer to the phono stage, which is much needed.
If you are willing to do the high-level mod, the parts costs may set you back about $40 - $50, including the PCB. That's less than any outboard tube buffer, and you'll be simplifying the circuit and improving performance at the same time. PM me of you are interested in more project details. I'm putting mine in a nicer enclosure along with a regulated PS and a new Hammond tranny that is better rated. If it works as hoped, I'm going to have a Van Alstine type Super PAS for sale (the original PAS, not my project!)
I don't think this is what YOU may want to do, but I plan to do it.
Referencing the article, "The Last PAS" by David Vorhis (AA 4/82), he removes the tone controls from the circuit as well as the tape output, and converts the circuit on the final tube section on the high level board to a CF configuration. He claims it can drive loads down to 10 ohms.
He makes some minor mods to the stock PCB, but changes most of the passive components. I decided to buy a 3rd party glass replacement PCB instead.
I'm going to take mine a step further and use a 12DW7 for the high gain PCB, and adjust the circuit values accordingly. That will give me the equal of a 12AU7 for the CF, which not only has a lower plate impedance, but will also drive at a higher current. (Some folks are not fond of using a 12AX7 for a CF, but in all fairness it has been done in a fair number of commercial circuits).
Vorhis did some excellent mods to the phono stage as well, and makes it truer to the RIAA curve. The high level board acts as a buffer to the phono stage, which is much needed.
If you are willing to do the high-level mod, the parts costs may set you back about $40 - $50, including the PCB. That's less than any outboard tube buffer, and you'll be simplifying the circuit and improving performance at the same time. PM me of you are interested in more project details. I'm putting mine in a nicer enclosure along with a regulated PS and a new Hammond tranny that is better rated. If it works as hoped, I'm going to have a Van Alstine type Super PAS for sale (the original PAS, not my project!)
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