Suggested Dynaco PAS mods

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Suggested Dynaco PAS mods

Postby jukingeo » Sun Feb 01, 2009 4:08 pm

Hello,

While cleaning out my electronics closet for a pending move, I located a couple of Dynaco preamps. Both are original in stock form. Since I have heard that many are modifying these preamps AND since I have TWO, I figured I would mod one of them.

I have looked up a few mods already, but don't know what would be the best ones to go with. One thing I did notice is that just about all of the mods bypass the tone controls. I DO NOT want to bypass the tone controls, nor the loudness. The whole fun of having a pre-amp is to have a central unit to adjust the sound with.

So I am looking into a mod that doesn't get rid of the tone controls.

One problem I DID notice with the PAS pre-amps is that the driving output impedance is pretty high. While this doesn't pose a problem to my planned tube amp designs...what if I want to use the preamp on a SS rig? Also driving with low impedance is usually better anyway.

I know a simple cathode follower would do the trick, but semi-recently I have been turned on to the SRPP pre-amp design. I have built the version in Bruce Rosenblit's book and I am pretty happy with that design. But that pre-amp is very much stripped down, it has no tone controls or anything. Just two inputs to select from and a loudness contour switch. I would like to take this a bit further with a PAS mod.

OH! I DO want to use the phono-preamp as well. So ideas for modifying this stage I am also interested in.

At any rate, I am all ears.

Thanx,

Geo
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Postby crispycircuit » Sun Feb 01, 2009 4:51 pm

I like the SDS power supply boards from Triode. They improve the sound. I usually keep the rectifier tube along with the board. I would do mods in stages with serious listening tests because they sound very good stock. A couple signal caps might do the trick. ... I agree tone controls are wonderful.... I would guess an outboard cathode follow is the answer for low impedance amps... or just change the resistors on the input of the amp...
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Postby joeriz » Sun Feb 01, 2009 7:22 pm

I use the SDS power-supply board in my PAS and it works well. But I will say that the filament voltages were a bit too high after installing it, however. They were up around 12.8v which some will claim is OK. However, I have come to believe that David Hafler meant to have the heater voltages on the 12AX7s around 11v. Moreover, the higher voltages seemed to add some midrange peakiness to the sound. I used a couple of diodes to bring the voltages back down to around 11.5v and all is well.

I agree with the need for tone controls sometimes (although the ones in the PAS are pretty weak, IMO...much too drastic in what they do to the signal) and the need to drive other-than-tube-amp impedances. To that end, here's a mod I'd really like to try. The tone controls look much more reasonable and it can apparently drive many SS amps.

http://www.normankoren.com/Audio/Spice_ ... l#ToneCtrl

Best of all, this guy (no affiliation) sells a board for this mod. Scroll about 3/4 of the way down the page...

http://home.comcast.net/~netminer/PAS-3.html

Let us know if you happen to try this.

Joe
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Postby CpuZapper » Mon Feb 02, 2009 5:20 pm

For me the biggest improvment was the removal of the tone controls.


Cut and Paste of a post by Joe Curcio at Dynaco Doctor.

RIAA FIX

The stock PAS phono stage compromises some performance in exchange for simplicity. Plus, its RIAA compensation departs from ideal by 4db. The output impedance is quite high (30K) so low impedance loads (less than 100K) will cause LF rolloff. While it is a major undertaking to lower the output impedance, it is relatively easy to address the RIAA compensation and the LF rolloff issues.
RIAA Fix:
1. Remove the 750pf caps (both channels) from the selector switch.
2. Remove the 27K resistors (both channels) from the selector switch.
3. Replace the RIAA 68pf cap on the PC-6 pc board with a 820pf (polysty or silver mica).
4. Parallel the 100K RIAA resistor with a 2M (metal film).
5. Replace the 2750pf RIAA cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7M RIAA resistor with a 2M (metal film).
7. FYI - DO NOT remove the 47K feedback resistors between the cathodes of each tube.

LF Rolloff:
1. Jumper the 0.1uF output cap connected from the plate of the second stage(s) with a solid wire (both chanels).
2. Change the value of the 0.1uF output coupling caps, connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) to 0.47uF / 400VDC.

After you complete these mods, you can use any of the 3 low level inputs (Phono, Tape, Special) for your turntable - all 3 will be RIAA compensated. RIAA error will have improved from +/- 2db to +/- 0.2db. Output impedance will not be changed so the original caution of low impedance loads (attached to the tape output connections) apply.


Good Luck with the updates.
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Postby mesherm » Mon Feb 02, 2009 8:18 pm

Do you have PAS-2, PAS-3 or PAS-3x units?
Look at the backs of the tone pots. If all of them have 2 notches about 100 degrees apart or so then you might have a PAS-3x. The 3x had special tone pots that when set to center position passed the signal straight through making it unnecessary to remove them.
I have two PAS-3xs. One was non-op and I gave it a complete rebuild including a new selector switch and one was a factory unit that someone had started to replace the line amp PCB. The factory unit got a new line amp PCB with modern components and I also installed an SDS power supply PCB. Even with the SDS tweak the factory unit has some hum whereas the rebuild unit is quiet.
My suggestions:
1. Replace the stock selector switch and rewire all inputs with shielded and twisted cable.
2. Replace the PCBs with new ones using modern quality components.
3. Make the Curcio mods to the RIAA board and hard wire one set of RCAs in the back just for the RIAA input.
4. Replace the can capacitor and the voltage doubler components.
5. Clean all switchs and pots with DeOxit.
6. Disable all the 2 wire line plugs.
7. Use NOS 5751 tubes in the line amp and Sovtek 12AX7LPS tubes in the RIAA section.
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Postby jukingeo » Mon Feb 02, 2009 9:32 pm

joeriz wrote:
http://www.normankoren.com/Audio/Spice_ ... l#ToneCtrl

Best of all, this guy (no affiliation) sells a board for this mod. Scroll about 3/4 of the way down the page...

http://home.comcast.net/~netminer/PAS-3.html

Let us know if you happen to try this.

Joe



Hello all,

Ok, I took in what most of you written and I have been to the norman koren site before. I do like the idea that there is an option for modified tone controls, but the whole point to redoing the tone controls would be to get something that can easily be replaced down the road. At first glance this looked to be a cool circuit, but then I noticed that the bass control needs to have a second gang on it. I don't think I would have an easy time finding a dual gang 500k control. A dual gang 50k, 100k, 200k, yes...but a 500k??

Yes, I also saw the guy is selling boards but $109 for just the pcb's? That is a bit expensive.

I will say though that this circuit IS a step in the right direction, but I was wondering if there was something 'simpler'.

I did notice the circuit on the end with 'no feedback' and that is something that appeals to me. But how (if possible) would one add tone controls to a circuit like that.

CpuZapper wrote:For me the biggest improvment was the removal of the tone controls.


As I mentioned above, I don't really want to consider that an option. I WILL consider switchable filtered stages. As Joe pointed out to me with the site he mentioned, I DID see a selector switch tone control variant and that seems interesting, but it also seemed like that is the most complicated circuit.

I am very much a firm believer in "KISS" (Keep It Simple Stupid). But I already built a simple pre-amp and I would like to have one with tone controls.


RIAA FIX

The stock PAS phono stage compromises some performance in exchange for simplicity. Plus, its RIAA compensation departs from ideal by 4db. The output impedance is quite high (30K) so low impedance loads (less than 100K) will cause LF rolloff. While it is a major undertaking to lower the output impedance, it is relatively easy to address the RIAA compensation and the LF rolloff issues.
RIAA Fix:
1. Remove the 750pf caps (both channels) from the selector switch.
2. Remove the 27K resistors (both channels) from the selector switch.
3. Replace the RIAA 68pf cap on the PC-6 pc board with a 820pf (polysty or silver mica).
4. Parallel the 100K RIAA resistor with a 2M (metal film).
5. Replace the 2750pf RIAA cap with a parallel combination of 2200pf and 470pf.
6. Replace the 4.7M RIAA resistor with a 2M (metal film).
7. FYI - DO NOT remove the 47K feedback resistors between the cathodes of each tube.

LF Rolloff:
1. Jumper the 0.1uF output cap connected from the plate of the second stage(s) with a solid wire (both chanels).
2. Change the value of the 0.1uF output coupling caps, connected between teminals 1 & 3 (left channel) and 7 & 9 (right channel) to 0.47uF / 400VDC.

After you complete these mods, you can use any of the 3 low level inputs (Phono, Tape, Special) for your turntable - all 3 will be RIAA compensated. RIAA error will have improved from +/- 2db to +/- 0.2db. Output impedance will not be changed so the original caution of low impedance loads (attached to the tape output connections) apply.


Good Luck with the updates.


I am taking notes here for the RIAA section and pretty much here I WILL do what it takes to get it right. I would like to use this pre-amp to 'warm up' the sound of records, not only for listening to, but also for CD creation via my computer. So I would like this stage to sound as good as possible.

mesherm wrote:Do you have PAS-2, PAS-3 or PAS-3x units?


They are PAS-2 units, but one might be a PAS-3. I am sure they are not the 3X variants with the special tone controls. I have heard of these and that the center detent position effectively bypasses the tone tone controls.

I DO think it is a cool feature, but I am sure that my pre-amps do not have it.


My suggestions:
1. Replace the stock selector switch and rewire all inputs with shielded and twisted cable.
2. Replace the PCBs with new ones using modern quality components.
3. Make the Curcio mods to the RIAA board and hard wire one set of RCAs in the back just for the RIAA input.
4. Replace the can capacitor and the voltage doubler components.
5. Clean all switchs and pots with DeOxit.
6. Disable all the 2 wire line plugs.
7. Use NOS 5751 tubes in the line amp and Sovtek 12AX7LPS tubes in the RIAA section.



I will say that overall I was going to replace the PCB's and update the power supply. But I am wondering if they might make 'hand wiring' PCB's. I am thinking more along the lines of going with a different circuit.

Believe it or not I was even thinking about going with something octal based, such as a 6SN7 tube. I have LOADS of these tubes.

I don't mind using an SRPP circuit for the line stage. In fact I do prefer it over the cathode follower. It is just that I do not know how to add tone controls to an SRPP.

As with my interest in SRPP, I do prefer a no feedback line stage design as well.

As I said above, I don't mind pulling the stock PCB's out and putting new ones in that can be hand wired from scratch. I would even use the Rosenblit SRPP pre-amp design again, BUT I just would need to know how to add a tone control to it.

This was my biggest complaint about my original Bruce Rosenblit pre-amp design. It had NO tone control. What I did do is buy myself a special volume control that had a 40% loudness tap and I just copied a popular loudness control contour into the design. So I had a bit of fun with that. The cramped quarters of my tiny 1 rack space chassis also forbade me to to install a standard selector, and relays would have been complex and I didn't have room for them. So I ended up only having the capability for 2 inputs. Not much for a pre-amp. But then again, I originally built that pre-amp because I wanted a good tube pre-amp to warm up my tracks when I had my recording studio. For that purpose I really didn't want the tone controls on the pre-amp. The loudness contour was effectively out of the circuit when turned off. So it was great for recording purposes.

But now that I want to build my own amplifier and settled on a design, I would like a nice pre-amp to go with it and since I have two PAS's already. May as well modify one.

But again, I am looking for a simple circuit, preferably NFB. Would be nice if I could use octals (6SL7, 6SN7). But some kind of tone control is mandatory. It could be something switched though...I am open to that idea.

Thanx for the input.

Geo
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Postby EWBrown » Tue Feb 03, 2009 6:32 am

Then there is what I call the "e-bay mod", Yellow_Light_Colorz_PDT_08 in which the stock original TElefunken 12AX7s are removed, tested and listed on e-bay, then they are replaced with the aforementioned 5751s or Sovtek 12AX7LPS or other ECC83 tube of choice.


Several times I have seen listed, by the same seller:

Item1: Dynaco PAS3 preamp with brand new Sovtek 12AX7s.

Item 2: Four tested Telefunken 12AX7s - and these usually went for big $$$ :o

I comb the yard sales and flea markets for vintage tube gear bargains, and have made some good "scores" over the years.

It's likegoing out hunting or fishing, most times you come back home "empty", but those rare occasions that pay off make it all worth while.

/ed B
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Postby jukingeo » Tue Feb 03, 2009 7:00 am

EWBrown wrote:Then there is what I call the "e-bay mod",

Several times I have seen listed, by the same seller:

Item1: Dynaco PAS3 preamp with brand new Sovtek 12AX7s.

Item 2: Four tested Telefunken 12AX7s - and these usually went for big $$$ :o

I comb the yard sales and flea markets for vintage tube gear bargains, and have made some good "scores" over the years.

It's likegoing out hunting or fishing, most times you come back home "empty", but those rare occasions that pay off make it all worth while.

/ed B


I guess in so many words that is your way of saying, "sell the PAS's" and scratch build once again?

I will say that my cabinet making skills are not the greatest and as it is, You can pretty much see the pattern. The Rosenblit preamp was built into an existing Paia Tube Head chassis, I am looking to build my amp within a new ST-70 chassis. (See the pattern?) That is another reason why I wanted to use the PAS pre-amp (or at least use part of it). Then everything would be in matched "Dynaco" chassis/cabinets. Granted, what is inside wouldn't be much of original Dynaco anymore.

Geo
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Postby EWBrown » Tue Feb 03, 2009 7:18 am

I wouldn't say that, go ahead and give the project a go.

If your PAS's still have the original Telefunken 12AX7s / ECC83s, by all means keep them, if they are still good.

The reason for the use of these seemingly "boutique" tubes is that back in the 1950s and 1960s, the US Dollar had a lot more strength than today, and it was less expensive for the manufacturers to use "cheap" imported tubes like Telefunken, Mullard, etc. They cost less than RCAs, GEs, etc, did back then.

In finding and rebuilding several PAS's I have only encountered one bad Telefunken, and it had a dead section.

IIRC, Roy Mottram had PC boards for upgrading the PAS series of preampes over at www.tubes4hifi.com I haven't checked there inthe past couple years,so I don't know what, if anything, is available today.

Vintage Electronics (an e-bay seller) also has PAS upgrade boards that fit right into and onto the existing chassis, no mechanical re-working needed.

My "chief beef" with the PAS (and also the SCA-35) is the long, convoluted and messy, unshielded, signal path through all of the controls and switches behind that front panel. The only shielding is the top cover, and it isn't all that effective...

The other upgrade is to get all of the ceramic disc caps and PECs out of the audio signal path and replace with film caps of the same values. That alone will be a major improvement. The PECs can be replaced with standard components (look under the "Those Pesky PECs" topic for an explanation).

My own choice would be to remove all of the tone controls, filters, blending/mix switches, etc and retain only a volume control and some form of balance control.

But that is personal preference, and not based on any sound technical "science". If the original volume and balance pots are still good (not scratchy or intermittent) then use them, otherwise they should be replaced. Contact cleaner is only a temporary band-aid and not a permanent cure.

Now, given all of that, my last two PAS rebuilds, I kept everything as "stock" as possible, and I usually save the total makeover jobs for the units which are in less than excellent condition. Then I don't feel bad about gutting and rebuilding them into becoming something completely different than they once were.

Of these two amps, one was a factory built unit I got at a sunday flea market for a princely $10, the other was a gift that my dad gave me, he built it back around 42 years ago, and I wanted to keep it as original as possible. That one had the hald-bad Teletube, so I swapped it out with another from my "stockpile". I kept the half dead one, because it is also half good... Yellow_Light_Colorz_PDT_08

/ed B
Last edited by EWBrown on Tue Feb 03, 2009 1:58 pm, edited 1 time in total.
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Postby CpuZapper » Tue Feb 03, 2009 11:15 am

By the way I don't think anyone said remove the lamp yet, as it taxes the power supply.

Jukingeo, what kind of sound are you looking for?

For me the stock sound of the PAS is bloated and flabby and just not as toe tapping lively for me. Replacing the power supply helps, new components help, removing the controls made me sit and listen to the music, I still use my pas like this. The Tele's are quite good tubes but for me the Tungsram's are killer in the PAS (Well balanced and detailed). My Tele's are sitting on the shelf.

If you are looking at replacing the boards why not consider a new project from scratch? Clean up the PAS or sell as is, I think more people like to buy it stock rather than modified. Use the funds to off set a new Pre-amp with more of what you want.

If you are going to record the music on PC can you not EQ it as well on the PC?

For me I wanted to play so I went ahead and modified a PAS 3x, but now I know that I would not do it again. Current project is a Aikido line stage, parts are still on order for my power supply. The boards from JB at tube cad are very well made and retail for $39.00, you can pick from octal, or 2 versions of 9 pin. I don't know if he has any info on adding tone controls on his site or not.

Have fun, I hope the info posted helps you make the right decision for you...
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Postby joeriz » Tue Feb 03, 2009 11:18 am

Geo,

You didn't really say what your budget was or how involved you wanted to get with the mod so I didn't take that into consideration.

Regarding the 500k dual-ganged control, I believe Mouser or Digi-Key sells it (see the Norman Koren site). If they still have them you could always buy extras.

In any event, here's a mod that I think can be done for not much $ in parts and seems to be in keeping with your goals.

http://www.audiokarma.org/forums/showth ... dynaco+pas

Joe
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Postby jukingeo » Tue Feb 03, 2009 11:56 am

EWBrown wrote:I wouldn't say that, go ahead and give the project a go.

If your PAS's still have the original Telefunken 12AX7s / ECC83s, by all means keep them, if they are still good.


I don't think they do. BUT that doesn't mean I don't OWN some Tele 12AX7's :).


IIRC, Roy Mottram had PC boards for upgrading the PAS series of preampes over at www.tubes4hifi.com I haven't checked there inthe past couple years,so I don't know what, if anything, is available today.


I been to the site and it looks like the board(s) are still available. However, they only show the board in the PAT-4 pre-amp and they show pricing on the BOARD (meaning singular). There are two boards in the PAS.

Vintage Electronics (an e-bay seller) also has PAS upgrade boards that fit right into and onto the existing chassis, no mechanical re-working needed.


Yes, someone mentioned these earlier. But the boards are not a mod. If I am going to replace the boards, obviously I would like to address some of the problems with the PAS.

My "chief beef" with the PAS (and also the SCA-35) is the long, convoluted and messy, unshielded, signal path through all of the controls and switches behind that front panel. The only shielding is the top cover, and it isn't all that effective...


Understood. When I built my Rosenblit pre-amp, I used fairly heavly shielded input cables. I did run out of room very fast in that pre-amp. The big issue was the power supply. I didn't want to deal with a large HV umbilical so I went LV. I have a huge 12v transformer outside of the case, and then I have a much smaller 220v to 12v transformer connected in reverse for the B+. I used very large capacitors in the unit (Sprague Atoms for the PS and Solens for the cathode bypasses). I followed proper grounding and kept the inputs on one side and the power supply on the other. The pre-amp worked first shot.

Really overall, I don't mind doing this design over again as a mod for the PAS, BUT I would like to incorporate the tone controls. I wouldn't know how to do that with an SRPP circuit.

The other upgrade is to get all of the ceramic disc caps and PECs out of the audio signal path and replace with film caps of the same values. That alone will be a major improvement. The PECs can be replaced with standard components (look under the "Those Pesky PECs" topic for an explanation).


Blech! Yeah, I know what you mean here. In the very least, I have used Sprague orange drops for coupling. I mostly use these caps to re-cap guitar amps, but if I need something 'good' in a small space then I use these. If I have enough room then for coupling I would either use Solen Fast Caps (which are in my Rosenblit pre-amp too), or PIO caps. I am going to use PIO's in my next project.

My own choice would be to remove all of the tone controls, filters, blending/mix switches, etc and retain only a volume control and some form of balance control.


That is exactly what I did with the Rosenblit pre-amp. It was strictly bare bones. The only 'luxury' was the loudness contour, which I could switch completely out of the circuit.

I guess I COULD do an off beat design with a switched tone control. I would keep it more or less 'passive' but choose selections that are 'musical'. In this manner it would like dialing in selected filters and yes I would have an 'off' or 'bypass' option. The only downside to something like this though is that it will probably complicate the heck out of the wiring and is making me think that perhaps I would be better off with standard Bass/Treble controls in the first place.

But that is personal preference, and not based on any sound technical "science". If the original volume and balance pots are still good (not scratchy or intermittent) then use them, otherwise they should be replaced. Contact cleaner is only a temporary band-aid and not a permanent cure.


Oh, I normally try to use new controls as much as possible. So more then likely I would be replacing the controls anyway (if I can).

Now, given all of that, my last two PAS rebuilds, I kept everything as "stock" as possible, and I usually save the total makeover jobs for the units which are in less than excellent condition. Then I don't feel bad about gutting and rebuilding them into becoming something completely different than they once were.

/ed B


That is just it. One of these PAS had had a 'hack' attempt on it before (not by me) and it is clearly in worse shape than the other one. Opening the unit up the other day I did find out that the tone controls were bypassed and cut out of the circuit, (but they are still present).

The boards are still the original though with some of the caps replaced...with those little yellow jobs. I don't like to use those small yellow caps. Only very rarely I used them in a jukebox amp rebuild when the working space is very tight and I can benefit more from using a small cap.

I guess from here, since it is looking like I probably will have to redo the power supply anyway, perhaps I would be looking into a different line stage circuit. It looks like I can 'rebuild' the RIAA circuit and don't have to deviate too much from the original design, but I think it would be nicer to go with something that doesn't have feedback for the line stage. An SRPP designed around 6SN7's (or 6SL7's) would be REALLY nice, but then I do need to follow some kind of circuit to at least get some kind of tone control.

Thanx,

Geo
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Postby jukingeo » Tue Feb 03, 2009 12:04 pm

CpuZapper wrote:By the way I don't think anyone said remove the lamp yet, as it taxes the power supply.

Jukingeo, what kind of sound are you looking for?


A very wide open soundstage that is warm and has good bottom. I mostly listen to modern music (usually from the 50's to early 90's).


If you are looking at replacing the boards why not consider a new project from scratch? Clean up the PAS or sell as is, I think more people like to buy it stock rather than modified. Use the funds to off set a new Pre-amp with more of what you want.


I could do that, but funds are pretty low and I can't really foot the expense for the cost of building (or rather having built) a chassis for the project. I am more or less after a nice finished cabinet here. I am very well aware of the associated problems with the PAS. Maybe I might sell it off. I don't know.

If you are going to record the music on PC can you not EQ it as well on the PC?


I mentioned that on the archiving end of things and for other uses of the music material. But in terms of the amplifier project, it mostly will be putting in a CD and listening too it from there, not the PC. However, I do have a turntable and some records as well. I would like to dust them off and see how the vinyl sounds with all tube sound.



Current project is a Aikido line stage, parts are still on order for my power supply. The boards from JB at tube cad are very well made and retail for $39.00, you can pick from octal, or 2 versions of 9 pin. I don't know if he has any info on adding tone controls on his site or not.

Have fun, I hope the info posted helps you make the right decision for you...


I heard of this pre-amp design before, but not heavily looked into it. But I know it was mentioned several times before in this forum. $49 is fine for a board. That is better than the $109 that I saw on the other site for the PAS replacement boards.


joeriz wrote:Geo,

You didn't really say what your budget was or how involved you wanted to get with the mod so I didn't take that into consideration.


That is true. I didn't mention it because no one (up to now) asked. Let's just say the budget is very very very very small. I am mostly digging into my excess supply of parts and working around that. I am really not in a position to buy anything 'out there' cost wise. BUT that is saying nothing without a number attached to it so, here goes:

New empty PCB or board: $20 to $50...yeah OK. $75 to $100 Forget it!
New project enclosure: Most likely over $100, forget it.
New power supply, up to $75...yeah OK.

In simpler terms, I would need to do this with my existing parts and less than $200 additional. (Lower the better).

The good thing is that I do have most of the parts already (in my days when I did make much more money).

In fact it was my preparations to move out of my house when I realized how much stuff I was sitting on. So being that I am strapped for cash, now is the time to actually starting 'using' that stuff and get to building something.

Regarding the 500k dual-ganged control, I believe Mouser or Digi-Key sells it (see the Norman Koren site). If they still have them you could always buy extras.


I order from Mouser...lemme see...Oh yeah...they do! Alpha controls makes them. Less than $3 too (Ahhh, gotta love Mouser).

In any event, here's a mod that I think can be done for not much $ in parts and seems to be in keeping with your goals.

http://www.audiokarma.org/forums/showth ... dynaco+pas

Joe


I can't access those pages. Is it posted somewhere else where I can see it? Or could you briefly describe what it is?

Thanx,

Geo
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Postby joeriz » Tue Feb 03, 2009 2:54 pm

It's an old article from the Sensible Sound that outlines various mods to the PAS. I can't recall if this is the whole shootin' match but...

1) The filter switch becomes a tone in/out switch.
2) The accuracy of the RIAA phono eq is increased.
3) I believe an improvement is made to the loudness function.
4) The mod enables it to drive a greater variety of power amps.

If you PM me your e-mail I can send you a copy.

Joe
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Postby CpuZapper » Tue Feb 03, 2009 8:04 pm

If you like the SRPP would it not be better to add tone controls to it?

With a quick google I found some SRPP preamps with tone controls.
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