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Old 22nd April 2011, 08:33 PM   #1
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Default PAS3 - are the tone controls really *that* bad?

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!
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Old 23rd April 2011, 07:38 AM   #2
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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.
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Old 23rd April 2011, 06:02 PM   #3
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at least with my old PAS-2 and -3, the removal of the tone controls improved the sound. Less muddy. I'm not a big fan of tone controls, but they are useful with some recordings!
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Old 23rd April 2011, 06:07 PM   #4
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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!

Last edited by Hippostar; 23rd April 2011 at 06:22 PM.
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Old 23rd April 2011, 06:16 PM   #5
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Quote:
Originally Posted by kstagger View Post
at least with my old PAS-2 and -3, the removal of the tone controls improved the sound. Less muddy. I'm not a big fan of tone controls, but they are useful with some recordings!
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.
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Old 23rd April 2011, 06:27 PM   #6
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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.
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Last edited by indianajo; 23rd April 2011 at 06:32 PM.
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Old 23rd April 2011, 06:29 PM   #7
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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.
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Old 23rd April 2011, 08:42 PM   #8
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Quote:
Originally Posted by ArtG View Post
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.
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.

Quote:
Originally Posted by indianajo
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.
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?
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Old 23rd April 2011, 08:49 PM   #9
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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.
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Old 23rd April 2011, 10:39 PM   #10
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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.
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