PAS 2 - New volume control issues

bberkom

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I have been gradually restoring and tweaking a PAS 2 over the last several weeks. Replaced the can cap, swapped the rectifier for some diodes, and recapped the stock circuit. The first modification I made was removing the tone controls from the circuit, following the Audio Regenesis instructions, including the 1 uf caps to block DC from the preamp outputs. This mod didn't seem to affect the sound too much, but I didn't ever use the tone controls anyways.

The biggest issue I had with this preamp was the volume imbalance due to mistracking in the volume pot. I ordered this ALPS 250k pot (no affil) and wired it up. I had to cut about 1/4" off of the shaft with my angle-grinder so that the knob fit correctly. It doesn't have a loudness tap, so I just disconnected all of the components for the loudness.

I had read that this was a suitable replacement part, but it sounds terrible at anything but low level listening. The highs get really harsh and the bass muddy, and even at normal listening levels it hurts my ears. I have read elsewhere that removing the loudness tap can change the load presented by the original volume pot + loudness, but I am not sure how to correct for that. I read as much as I could find about this subject, but there is a lot of conflicting information out there about replacing these volume controls. The volume on this new pot also increases really quickly as well. It's a smoother volume control than the original, I just know something is off.

I have also seen many suggest that you can use a 100k pot that comes with loudness tap to replace the original 250k pot, but that it messes with the phono section because of the change in load presented. I really only use the phono section, and I don't want to make all of the modifications laid out by some to "fix" the phono section's RIAA errors or whatever. I think it sounded fine with the old volume pot, but I want to get rid of the imbalance! Thanks for any help.
 
Did you encounter the same bad sonics prior to cutting the shaft? These controls don't take kindly to vibration from cutting wheels and the like, so it could have severe internal damage.
 
Did you encounter the same bad sonics prior to cutting the shaft? These controls don't take kindly to vibration from cutting wheels and the like, so it could have severe internal damage.
That is something I didn't consider. With the soft aluminum of the shaft, I probably didn't need to go whole-hog and use my grinder. It got pretty hot... It sounds bad on both channels, but I suppose it could have affected both of them. I get equal and balanced sound out of both channels, so it's working, it just sounds harsh and has less bass. I suppose if cutting the shaft did hurt it, then I can measure it and see if I still get 250k ohms. I am guessing any damage would show up as resistance being too high or too low. I would hate to order another of these expensive pots and have the same issue.

Would removing the loudness switch and the caps/resistors associated with it have an affect on the sound and the taper of the volume? I've read that you can just discard those components but that you may need to compensate for a different resistance?
 
I suspect that you're just experiencing the normal flat frequency response of this preamp for the first time, because the original loudness function persisted well into mid-volume settings. It's entirely possible that 'normal' response in this case is a lot more distorted than it should be, due to some unrelated problem(s). Disconnecting the loudness circuit does affect control law, because originally there was a shunt resistor from the loudness tap to ground even with loudness defeated. You might try reconnecting the old pot with its loudness taps left open, just to confirm that nothing else has changed.
 
I suspect that you're just experiencing the normal flat frequency response of this preamp for the first time, because the original loudness function persisted well into middle volume settings. It's entirely possible that 'normal' response in this case is a lot more distorted than it should be, due to some unrelated problem(s). Disconnecting the loudness circuit does affect control law, because originally there was a shunt resistor from the loudness tap to ground even with loudness defeated. You might try reconnecting the old pot with its loudness taps left open, just to confirm that nothing else has changed.
I have kept giving this preamp more time for me to get used to since I figured it may just be how the preamp sounds without loudness and with a new control. It sounded really good before replacing the pot, so I am in the process of reinstalling the original volume pot now and will see how its sounds.

The ALPS pot measured about 230k ohms on both channels instead of 250k when removed. Would that much of a discrepancy point to some damage to the pot from vibrations or overheating? It's the exact same on both channels.
 
After Hakko soldering equipment, ALPS pots are probably one of the most often counterfeited components. And even reputable suppliers get bit sometimes:(
 
Well I installed the original pot, and it sounds much better now. It's a very noticeable difference. Deep bass has returned and the highs aren't hurting my ears. Listening to it now, I think the ALPS pot was causing some distortion. I guess cutting the post could have damaged the pot, and I think I'd suspect my foolishness before partsexpress, but you never know. I am still wary of buying another. It may just be how it sounds.

With the loudness control removed from the circuit with the original pot, the volume taper is now about the same as the new ALPS pot, which means wayyy faster than the original. I reach my listening level by around 9:00 on the control as opposed to closer to 11 or 11:30 before. The problem with this is that the pot is very imbalanced that low on the control. I have to put the balance to almost 3:00 to bring up the right channel's level. The pot becomes more balanced once turned up more, but now at that point it's extremely loud! I suppose I will just reconnect the loudness switch and see how that sounds, but I may end up purchasing another pot and try again.
 
We all make mistakes like this, its part of learning. I've never needed to cut the shaft of a pot, but its something I could see myself doing, then I can also see myself mad because I broke my pot :)

Listen first, grind later... Thats the lesson for today :)
 
I redid the loudness circuit today, and now the volume control is behaving normally. It's still imbalanced, the worst being the first quarter or so of the pot rotation. But installing the loudness circuit puts my normal listening levels past the worst of the imbalance, and I can use the balance pot to correct a little. I'll stick with this setup for a bit until I decide to try another new pot. For now it sounds great with the original.
 
I have gotten so fed up with the poor tracking and high prices of the supposedly ''high quality'' pots that I've gone to stepped attenuators for most everything.

We're long past the days of the 3'' diameter Switchcraft or CRL 23-position rotary switches that needed a huge amount of space and required so much torque to turn up that you needed a wrench instead of a knob!

For most applications,you really don't need anything better than one of these,they are readily available just about everywhere,and there are easy to use resistor-calculator programs for ladder attenuators on the google-webnet. You just enter the total resistance required,the number of switch steps,db per step,et voila!

This switch is about the same price (and footprint) as the pot you bought,and if you go with 1/4 watt 1% metal film resistors,you'll pay about 5 buck more for those:)
And the best part is that you can retain your loudness function if you wish.Just measure where the tap is on the original pot,and connect the network to the closest position on the switch.

connex_75572.jpg
 
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I have gotten so fed up with the poor tracking and high prices of the supposedly ''high quality'' pots that I've gone to stepped attenuators for most everything.

We're long past the days of the 3'' diameter Switchcraft or CRL 23-position rotary switches that needed a huge amount of space and required so much torque to turn up that you needed a wrench instead of a knob!

For most applications,you really don't need anything better than one of these,they are readily available just about everywhere,and there are easy to use resistor-calculator programs for ladder attenuators on the google-webnet. You just enter the total resistance required,the number of switch steps,db per step,et voila!

This switch is about the same price (and footprint) as the pot you bought,and if you go with 1/4 watt 1% metal film resistors,you'll pay about 5 buck more for those:)
And the best part is that you can retain you loudness function if you wish.Just measure where the tap is on the original pot,and connect the network to the closest position on the switch.

connex_75572.jpg
I'm with you on that, pots today suck.
 
I have gotten so fed up with the poor tracking and high prices of the supposedly ''high quality'' pots that I've gone to stepped attenuators for most everything.

For most applications,you really don't need anything better than one of these,they are readily available just about everywhere,and there are easy to use resistor-calculator programs for ladder attenuators on the google-webnet. You just enter the total resistance required,the number of switch steps,db per step,et voila!

This switch is about the same price (and footprint) as the pot you bought,and if you go with 1/4 watt 1% metal film resistors,you'll pay about 5 buck more for those:)
And the best part is that you can retain your loudness function if you wish.Just measure where the tap is on the original pot,and connect the network to the closest position on the switch.

connex_75572.jpg
I think that is a great suggestion. I looked into it a little and saw a few different kits for sale. There are some preassembled 250k attenuators that look almost exactly the same as the one you posted on ebay that ship from China for a decent price. They claim to use vishay/dale resistors and they look like it, but you never know. Do you have a recommendation for where to get a blank attenuator like the photo you posted?
 
I got mine from Partsconnexion. They're here in Canada,and they're one of the few distributors of Takman resistors,which are my first choice in either metal or carbon film.
I'm sure there are also lots of American-based or internet sources for that switch.In all liklihood,it's probably made in China;)

http://www.partsconnexion.com/controls_rotary_main.html
Thanks for the info. I forget about them since their shipping takes a little while from Canada. This is probably the way I will go once I get sick of messing with the original volume control again.

You mentioned creating loudness taps. I understand measuring the pot to find the resistance for the loudness taps, but would I just solder the components for the loudness taps to the same tabs on the switch as the resistors where I find the same reading? Sounds easy unless I am missing something.
 
I understand measuring the pot to find the resistance for the loudness taps, but would I just solder the components for the loudness taps to the same tabs on the switch as the resistors where I find the same reading? Sounds easy unless I am missing something.
Correct.
 
Thanks for the info. I forget about them since their shipping takes a little while from Canada. This is probably the way I will go once I get sick of messing with the original volume control again.

You mentioned creating loudness taps. I understand measuring the pot to find the resistance for the loudness taps, but would I just solder the components for the loudness taps to the same tabs on the switch as the resistors where I find the same reading? Sounds easy unless I am missing something.

You're not missing anything at all,you've got it. Just find the tab with the resistance closest to that you measured on the original pot. Not critical at all,close enough for horseshoes and hand grenades is just fine. I wish everything was that easy.

I remember reading a paper long ago that did an assessment of switched attenuators vs pots. They gathered several top quality pots,and also constructed a stepped attenuator using the crappiest old carbon composition resistors they could find. They fed a series of signals into the various controls and analyzed the output results with a spectrum analyzer.The pots lost that battle by a huge margin.The induced noise,especially when subjected to even low levels of vibration,was astonishing!
 
I have used several stepped attenuators in various projects and have been very pleased with
the tracking at low level volume as well as the performance at high level volume listening.

The last one I got was about one month ago, it was a 250k Dale 24 Step Attenuator Potentiometer
that I used in a DIY scratch build tube preamp, it did take about 3 or 4 weeks to arrive from China
and well worth the wait, it's awesome.

Also have used the stepped attenuators in a home built passive pre and it worked real nice in that too.

I got that last two stepped attenuators from e-bay seller >diyaudiokit
No problems with that seller at all and those Dale 24 Step Attenuators sound fantastic.


DIY Tube Preamp build
http://audiokarma.org/forums/index.php?threads/diy-tube-preamp-build.774580/

http://diyaudioprojects.com/Tubes/Universal-Tube-Preamplifier/
 
I have gotten so fed up with the poor tracking and high prices of the supposedly ''high quality'' pots that I've gone to stepped attenuators for most everything.

connex_75572.jpg

pro audio DIY guys mostly use stepped attenuators, its a cut above most pots. If matching is critical, you can guarantee it. If space permits, and you are going to stray from stock, its worth a look. There's plenty of cheap stuff to choose from, "Grayhill" is the goodstuff, I think there's a couple more noteworthy brands.
 
I suppose theres no reason you couldn't wire the loudness into one of those too is there? Just tap it at the appropriate place?
 
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