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In Reply to: RE: Measurements posted by Thermionic27609 on August 03, 2014 at 15:06:52
It's really a shame to put so much effort into an upgrade, only to see it fall down in some way that wasn't anticipated. Perhaps it's not too late to salvage it. There are dozens of very transparent preamp designs around, and some are extraordinarily simple to build. It might be worthwhile to consider how you could adapt one of those to the PAS. It might even be possible to reuse some of the high-end components you bought for the original circuit. I know your passive design is working well, but maybe it would prove advantageous at some point to have gain, or at least a low output Z that can drive various loads. If it were me, I'd put it aside and take some time to mull things over. I would decide what features and specifications are really useful, then look for a way to build that functionality into the original chassis using the existing power supply. Even just a rudimentary buffered linestage with an input selector and volume control would be more useful than letting it sit in the garage. :)
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You live and learn in audio as a hobby. If I wanted an active tube line stage at this point, I'm pretty sure I'd build some version of the Aikido circuit from scratch.
Maybe I'll run a thorough diagnostic of the PAS, but it's got to be something that affects both channels. The last time I checked the 12X4, it was strong.
I appreciate the advice on situations where a line stage is definitely needed. Very useful information. My passive preamp will probably end up installed inside my ST70.
My current push to get this sorted out started, btw, when I was testing a Pioneer SPEC-4 SS power amp I'd fixed for a customer in my main system. It had outstanding bass control when hooked to my KEF 104s, but when I hooked my PAS and ST70 back up, I realized that I was missing something essential to the music. Within its power limitations, my Dyna SCA-35 also has more articulate bass than the PAS/ST70.
OK, I ran some diagnostics on the PAS. Tubes all test > 100%. Voltages are where they should be. No high ESR in power supply caps.
Listening: the PAS sounds gorgeous, but that's really the problem. It's gloriously euphonic, but not honest. It adds just a bit of compression. It's like one of those TV shows like CSI Miami where we know that everything is run through filters to make everyone look good.
Switching back to the passive volume control, some recordings are less pleasant to listen to, but the instruments sound more like I know they do when heard live. Bass guitars and drums are much more defined.
I may be preaching to the choir, but you did not mention exactly what you did to your PAS3.
I've been repairing and modifying PAS3s for several decades. From what little I have been able to glean from your posts, it sounds like your line section is not working up to snuff.
I usually start with the filament supply, which should be rebuilt using at the very least a silicon rectifier instead of the stock selenium. I also replace the filter cap in the power supply; failure to do these steps will result in poor performance down the road. Its not a bad idea to replace the tube rectifier but if you do, do it with HEXFREDs; otherwise keep the tube rectifier.
If you are removing the tone controls, you don't also need the line stage circuit that is otherwise stock in the preamp. I usually use a 12AU7 instead of the 12AX7 and run it without feedback (there is a limit to the current you can draw from the power transformer)- this can provide similar gain but with a whole lot more bandwidth. I direct-couple the first half of the 12AU7 to the 2nd half, which I run as a cathode follower- if using a tube power amp the output coupling cap is then about 1 uf. This works a whole lot better than the original circuit- my circuit clips at about 15 volts output.
I have often built up the volume control out of a custom-built 23 position rotary switch. The existing volume control tends to have tracking problems. I usually retain the balance control, mono and separation switches. The volume control *can* be used as the grid resistor for the line stage, but if you do this make sure you install a grid stop resistor in the line stage or you can have problems.
If you do this right you will have a stable circuit that will play bass fine no worries- and be less colored than a passive control. There are a lot of things to mess up though, and if you don't know how to do this stuff there are a million and one ways to cause the circuit to fall short of the goal. Its important to keep this in mind; if you don't you wind up leaving performance on the table for no good reason.
To recap: I completely rebuilt my PAS-3X years ago from stem to stern with audiophile-approved parts, including the filament supply, volume pot (Alps Black Beauty), tube sockets, all resistors (Holcos before they stopped making them), and all film and electrolytic capacitors. It has increased B+ supply capacitance using Elna Cerafines following suggestions I got > 10 years ago from Gordon Rankin. Bass controls are out of the circuit, and treble controls are out of circuit at center. The filament supply did not escape notice.
I appreciate your recommendations, but it's quite a long way from "your line stage is not working up to snuff" to designing a new line stage. At the point where you're using half a 12AU7 gain stage direct-coupled to a 12AU7 cathode follower, isn't that a tacit admission that one might as well go ahead a build a different preamp? It's not really a PAS-3X anymore.
I was under the impression that the PAS power transformer didn't really have enough current capacity to run 12AU7s optimally. At least I think that that was one of the conclusions of Norman Koren's analysis of the amp.
Considering the reviews I've read of various versions of the Aikido preamp, I've been thinking that I might build one if I decided I wanted an active line stage. I even went so far as to buy an enclosure intended for a new preamp, but that was before the economic crash eliminated my job, sent me scrambling to keep my head above water, and forced me to turn my hobby into a livelihood. At this point, I'm a professional Hammond/Leslie and guitar amp tech. I have a shop, and I repair and rebuild amps every day. I've neglected my own HiFi system for a few years since I've been too busy fixing everyone else's stuff.
If I built a volume control into my ST70 so that there were no cables between the pot/attenuator and the input stage of the amp, would that make it any more acceptable? There is room. My ST70 has a custom base that's deeper to allow for PP power supply caps.
"I'm a professional Hammond/Leslie and guitar amp tech. I have a shop, and I repair and rebuild amps every day. I've neglected my own HiFi system for a few years..."
Man, isn't that always the way? I had my own company for about eight years, although that was in the era of the big SS receivers and amps. We did warranty service for companies like Yamaha, B&O, JVC, etc. etc. If I was still working on audio gear as my day job, my own system would look like something out of the Middle Ages. In fact, it's pretty much neglected even though I work in other fields now (RF/microwave). Hammond, you say? I still haven't quite finished the custom output/headphone circuit on my M103. Started it two years ago, so now it's at the point of chronic embarrassment...
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Buy Chinese. Bury freedom.
These days, I'm fortunate that most of the gear I work on is out of warranty. It seems to be a major pain to be an authorized service center in terms of what companies are willing to pay for warranty repairs. I know of several techs who have jettisoned Behringer because they end up losing money vi an endless flow of low-dollar work. A lot of companies are making it fairly clear through their policies that they have little interest in support for products, especially those out of warranty. Parts become NLA shortly after the unit goes out of production, etc...
There are exceptions. Parasound was very helpful in helping me fix an amp and preamp that got damaged by lightning.
I had to deal with Musical Fidelity once about my own DAC, and they were awful. The fact that the thing was assembled with special security fasteners should have been my first clue to the attitude I was about to encounter.
isn't that a tacit admission that one might as well go ahead a build a different preamp? It's not really a PAS-3X anymore.
I would have thought it was obvious that with all your work, that was already the case :) If you want the preamp to be a PAS3, expect it to have problems in the bass at the very least. IOW, saying that 'passive is better' is not the same as 'active is worse' if the PAS3 is left stock. You can make it a lot better!
If you are going to use a passive volume control, putting it in the amp is the thing to do. You may run into issues though- the line stage does 4 things:
input switching, volume control, add any needed gain, and control the interconnect cable.
Switching and gain may be your issues in this case.
At any rate, its a simple fact that the stock line section of the PAS-3 is problematic. I've been running 12AU7s in there for many years and they work fine, despite Norman Koren's remonstrations. To deal with the current limitations impsed by the power supply, I set the plate resistor of the first stage and the cathode resistor of the second stage both to 100K. This is not optimal for a 12AU7 but it allows for way more performance than you will be able to get out of the stock line section or variations thereof using a 12AX7! I've put the resulting preamp up against a lot of preamps that cost serious cash and its had no troubles keeping up.
The one missing value that you don't have to complete the line section as I have described is the cathode resistor of the first stage. That should be about 4.75K. This circuit is not hard to set up on the existing circuit board; try it.
Thanks, Ralph. I may try your suggestion.
My rebuild of the PAS-3X was based on what I knew and what I'd been advised to do a decade ago. It suited me for a while, but I think it's time for a change. My quick and dirty passive preamp box was mainly a cheap experiment to see how it would work. It may not ultimately be the ideal solution, but it does reveal the original PAS's colorations.
In many ways, I suppose we can credit Hafler & company for being smart about designing an inexpensive preamp whose failings are mainly euphonic.
-But even back then its failings were understood. Its always been fun to try and see what you can do with one; I draw the line where anything has to be outside the chassis to make it work. At that point you may as well get a chassis designed for what you want to do.
But I have had good success with them. There are a host of changes for the phono section too.
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