Pre Amp?

a DIY, modified Mullard 5-20 monoblock design

Pre Amp?

Postby HOGHEAD » Tue May 31, 2005 7:51 am

Hi Shannon, I would like to know if you are going to release a pre amp in the near future. And the Eiclone is working and sounding great! Thanks, Jim
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Postby WA4SWJ » Wed Jun 01, 2005 5:03 am

Guys,

I agree too. I'd like one for my Stereo 70 as well as the ST-35 I've built.

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Postby TerrySmith » Wed Jun 01, 2005 7:54 pm

Well fella's,

I think it would be hard to beat a Dyna PAS series preamp. You can get the boards from Vintage Electron, and the transformer from Uncle Ned, the other nick-nack parts from Mouser, AES etc and have a brand new unit!

Just my $.02!
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Postby WA4SWJ » Fri Jun 03, 2005 7:42 pm

Hi All,

All of these are good suggestions but they don't have the little "diytube" logo on them like the 5 diytube amps I've built so far! ;-)

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Postby Thermion » Sat Jun 04, 2005 9:41 am

TerrySmith wrote:I think it would be hard to beat a Dyna PAS series preamp. You can get the boards from Vintage Electron, and the transformer from Uncle Ned, the other nick-nack parts from Mouser, AES etc and have a brand new unit!


This sounds like a very good idea. I have a PAS 3X and SCA 35 carcass that I plan to use for linestages. I will probably do this in the PAS 3.

My first diy tube project was a hybrid linestage from Welborne labs. I basically got two circuit boards, transformer, and little ziplocks with parts. Not for the beginner, but I did it anyway and I am still using this preamp. I stuffed it into an old PS audio chassis, so it looks fairly professional. But I digress....

In the SCA 35 chassis, I built an all octal (6SN7) linestage using a common cathode circuit with CCS on the cathode resistor. Shannon has been helping me troublshoot some issue with the CCS and ground loops. We are close, but still have some work to do. The point is that it is more difficult to do a linestage than one of the power amps. There are just more things to consider that can affect the sound. This is probably why Shannon hasn't introduced a product yet.

In the near term, I think the PAS 3 and Foreplay are good choices for DIYers. I don't think Welborne sells the hybrid linestage anymore. When I bought it, I decided between it and the Audio electronic supply kit.
http://www.audioelectronicsupply.com/cg ... m=AE-3_KIT
A bit pricey, but I have always regretted not having bought it. Back then, it used four 6SN7s, and had tube rectification.

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preamps

Postby Shannon Parks » Sun Jun 05, 2005 9:40 am

Thermion wrote:In the SCA 35 chassis, I built an all octal (6SN7) linestage using a common cathode circuit with CCS on the cathode resistor. Shannon has been helping me troublshoot some issue with the CCS and ground loops. We are close, but still have some work to do. The point is that it is more difficult to do a linestage than one of the power amps. There are just more things to consider that can affect the sound. This is probably why Shannon hasn't introduced a product yet.


Hi Guys,

Just got back from the great Northwest!

There are many reasons I haven't done a preamp, but Thermion probably hit the biggest one. His preamp has been a tough nut to crack. But we'll figure it out and be better for it. That's one of my goals this summer.

Thermion wrote:In the near term, I think the PAS 3 and Foreplay are good choices for DIYers.


I think a 'new' PAS3 is a great idea. Since you could get the original manual from Gary Kaufman, get yourself some spools of teflon wire and rebuild one. If it is half as fun as the ST70, it would be worth it and a learning experience. Can I mod any standard PAS3 to a PAS3X or does it require a special control knob? I guess you could always add filter bypass switches on the back.

I see Bottlehead has introduced the Foreplay III. While not as inexpensive as earlier Foreplays, this still seems like a value. For the budget minded, I think the previous Foreplay would be an easy one to 'clone'. Reverse engineering all the step-by-step sites dedicated to the Foreplay make this fun, too. It is also a good lesson on Paul Joppa's circuit grounding techniques. For example, his SNR for the Seduction is excellent.

What do people want in a preamp? Some want a phono stage and some don't. I would want one in mine, so it would have the functionality of the Foreplay and Seduction. If a diytube kit, folks could 'leave out' the phono side if they so chose. I think I would also want to incorporate a stepped attenuator on the PCB itself - though I'm not sure how to switch it. Yellow_Light_Colorz_PDT_01

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Postby EWBrown » Mon Jun 06, 2005 5:06 am

I recently completed one of teh last of the Bottlehead FPII linestages, the design is elegant in its simplicity, and it works and sounds amazingly well. The circuit is very basic, a direct-coupled 12AU7 VA stage feeding a cathode follower, minimal parts count, and lots of attention paid to single point grounding. I have the C4S and "sweetest whispers" step attenuators upgrades, but haven't yet installed them. These probably will go into a future design of mine, which will combine the design of the "seduction" phono stage and what I'd call an "FPII.99" linestage - almost but not quite an FPIII. This will combine elements of both the FPII and FPIII, and use a nice 100K stereo step "ladder" attenuator I got off E-bay from a seller in Hong Kong - which is essentially a low cost copy of the GoldPoint attenuator. Perhaps it can be fit into a PAS preamp chassis...

I've used the replacement boards from "vintageelectron" on e-bay to rebuild a couple of PAS-3s, these have theidentical layout and circuit design as the originals, just on fiberglass PCBs and using better quality components.

This doesn't address the PAS series' main fault which is the very sloppy, long and complex audio signal path through all those controls and "filters" on the front panel, all done with plain unshielded wire, which is run back and forth in a complex maze of wiring. I'd ditch all but the volume and balance controls, and re-wire the input jacks with shielded coaxial, to start. New power supply electrolytics, and replacing the filament voltage doubler selenium rectifiers and two electrolytics is a very good idea, use modern caps and uF4007 diodes here, or better yet, iN5822s. Uncle Ned's replacement PA211 power tranny is fairly inexpensive, and there is also a 120/240VAC primary version for those who need the 240VAC power capability.

/ed B in NH
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Postby Hollow Fate » Sun Jun 19, 2005 7:55 pm

I’ve followed this thread for a while and would like to add my audio observations and suggestions. In the past, before I even owned a single vacuum tube (a year and a half ago) I was looking for a design to build and as the Thermionic Gods would have it, stumbled onto the Diytube site. I committed to Shannon’s ST-35 amp and built a very nice tube rectified amp with an octal front-end modification (6SL7 ~ 6SN7) that I like very much. The problem was what to use as a tube pre-amp? Since the Dynaco PAS3 and clones have been mentioned here as well as the Bottlehead gear; I’m mentioning my pre-amp because I’m thrilled with it. I’d actually been using it for many months pushing through a NAD amp before my tube power amp was finished. Searching and reading users opinions led me to purchase an Ultra 4A SE pre-amp from Mapletree Audio http://mapletreeaudio.com/ .

I couldn’t be happier. Dead quiet, lots of gain, looks terrific in a retro kind of way, and it has brilliantly clear deep sound that on good recordings almost sounds like live in the studio. It’s line-stage component is switchable between pairs of 6SN7’s and pairs of 12SN7’s. The beauty of the 12-volt switchability is that I am able to use a pair of JAN RCA 12SX7 GT tubes that clearly outclass any 6SN7 I have tried to date. The RCA 12SX7’s rarely leave the amp. Tube rolling involves only another 12SX7 pair (JAN Rogers). My stash of 6SN7’s and 12SN7’s never get used anymore. (Hello, Ebay) The phono stage is a pair if 12SC7 metal tubes. (The 12SC7 is like a 6SL7 or the miniature 12AX7 but with cathodes common). I did some experimenting with tube socket adapters and ultimately changed the phone stage over to a pair of Russian 6N2P-EV’s (kind of like a 12AX7) and I connected cathodes together (my version of a common cathode). I love the sound of vinyl on this pre with the 6N2P’s (more high end) but I’ve heard that others prefer the stock 12SC7’s (more low end). Either way, CD’s through this pre-amp really shine (which of course avoid the high gain phono section provided by my 6N2P’s or the stock 12SC7’s).

My Ultra 4A is not available as a kit but two other offerings of Mapletree’s owner/designer Dr. Lloyd Peppard are. Prices for the two pre-amp kits offered are comparable but higher (+23 to +30% over the Bottlehead offering). The Magic 5 pre can also be fitted with the legendary 12SX7 GT tube I (rightfully) rave about. There’s a reason that the JAN RCA’s sell for upwards of $80 a pair on Ebay and it ain’t the pretty box. I always found that the Bottlehead amps always looked to me like wooden shoeboxes with tubes topside. But aesthetics is a tricky business. To each his own.

One last comment. I received an Ebay-sourced Dynaco PAS3 last week which I purchased for an economy-minded friend. All original, no upgrades, and of course it sounds far better than any transistor pre-amp I have ever owned. Against my Ultra 4A with the 12SX7 tube pair? Sorry, no contest. That said, for anyone contemplating a tube pre-amp now, so as to get away from a transistor pre, the PAS3 is worthy of consideration. Thirty-five-year-old switches and other components can be a bit dicky but you’ll probably re-sell it on Ebay after a couple of years of use for what you paid for it. In the mean time you can save up the pesos as you search out the tube pre-amp of your dreams.

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Look in yard sales and flea markets, and even the town dump

Postby EWBrown » Mon Jun 20, 2005 5:52 am

I have found Dyna PAS preamps (and FM tuners, Fisher amps, etc) at yard sales, flea markets and even at the local town dump, for very cheap and even free. There is so much of this stuff arounhd, just gathering dust in attics, cellars and barns, and sometimes it eventually surfaces to be discovered.

Last summer, I snagged a very clean PAS-3 complete with all four original Telefunken 12AX7s inside, for the princely sum of $10. The seller was, in his own words, "happpy to get rid of that old tube junk..."

How many times has the same seller on e-bay had for sale "Dynaco PAS-3 with brand new Russian Tubes" and his next item is "four Telefunken 12AX7s". Guess where they came from. Yellow_Light_Colorz_PDT_10

Got it home, the controls and pots weren't scratchy, everything worked and sounded just fine.

Summer's here, so the "hunting" should be good once again...

/ed B in NH
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Preamp for the Masses

Postby Shannon Parks » Wed Jul 13, 2005 6:29 am

Ok. This thread took me down a crazy path. I sketched up numerous designs. Ultimately, they end up with relay switching the signal paths and start getting too complex. How about a real simple preamp based on the RCA 12AX7 phono stage (uses a single tube per channel) and a single tube linestage ala the VTV design (voltage gain into a cathode follower)? It would be 9-pin only, but adventurous folks could flying wire their octals to it. The phono stage would be relegated to using the 12AX7, but the linestage portion would support any 9A base tube, along with three jumpers to support 5687s - which is what I would use. Attentuators and switching would be up to the DIYer. I think this design would be simple enough to churn out quickly and do some prototype work and testing. Comments? Anyone built the RCA phono?

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Postby WA4SWJ » Wed Jul 13, 2005 7:30 am

Shannon,

You've probably seen this but what about something like it? I think there is some excellent work done here and this approach would make a great preamp circuit. I keep thinking I'd like to get a Dyanaco PAS-3X and fix it up but I'd rather build something - especially one of yours with better PC boards. Plus, I think the PAS-3X is overpriced from what I can see on eBay:

http://www.normankoren.com/Audio/FeedbackFidelity.html

A loyal diytube-er.

Regards,
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Postby HOGHEAD » Thu Jul 14, 2005 8:30 am

Well Shannon on the pre amp as one would say "get er done"! I my self would be happy with a line amp as all I have is a tuner and cd player for inputs and my Eiclones. Thanks and look forward to any news on a project pre amp. Jim Yellow_Light_Colorz_PDT_04
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