PAS4 Preamp

I wasn't that impressed with the linestage. I had built your SP9 and found that unit at least as satisfying.

I also replaced the volume and balance controls with Alps Black Beauties - Volume control is stepped.

New project is to add a loudness circuit and possibly tone controls which can be bypassed completely if needed.

They still do pop up from time to time:
http://www.ebay.com/itm/142276806568?ul_noapp=true
No affiliation, just sharing the love.

Well, this sure is interesting. I see, from Bob's post number 31, that the four 1uf coupling caps have been increased in value to 3.3uf. That seems to me to be a huge increase, over 3 times in value, from the original schematic dated rev A, 1 July 93. Since I'm getting closer to up-grading the caps in my PAS-4 I'm wondering if I should stay with 1uf's or increase the value to 3.3uf's as shown in the newer schematic. There are also some changes to the power supply but those don't concern me at this point. Oh the joy...

I would focus more on the type of CAP rather than the size. Increasing the size is usually more important in a power amp rather than the pre-amp. I replaced all of the caps in my PAS4 with newer ESR's from Panasonic for the electrolytics. The wima caps were all replaced with a combination of caps (same theory that overpriced MultiCap uses). Remember that the usual purpose of changing the type of cap is to capture the nuances of that particular cap. In other words, if you simply piggyback (parallel) another cap on top of an exsting cap, the combination will not take on and exhibit the charateristics of the new bypass cap. By combining different types you can really tailor and fine tune the sound you are looking for. In my PAS4 I replaced all of the Wima's - red rectangular ones in the photo. I personally have always thought Wima's were overated. Good in their infant days - but now not so much.

Since Caps usually have a tolerance of 10 -20%, adding a bypass cap of 5% of the original value works fine. In my case, I replaced the Wima's with identical value (but higher voltage rating) Solen Fast Caps because they tend to really pass the original detail of the source. Then I bypassed them with Russian PIO caps which really smooth out sound and bring out the liquidity of the tube sound while the Solen's maitain the crisp detail. PIO's really make a great difference in tube power amps. The Russian PIO's appear in the various selling boards like ebay from time to time but are beginning to go up in price as they become more rare. When USSR fell, there were lots of them on the market as the military personnel deserted and grabbed whatever gear they could carry. The Russian PIO's are glass sealed and very high quality as the Russian military used mainly tubes in their radar installations and these caps were essential to their performance. I bought 12 boxes of different popular values. They came in boxes of 10 - all compartmentalized, leads perfectly formed and each individually wrapped within the box. Each box of 10 was sealed with the USSR military seal and labeling - tied with a red cord sealed with wax seal. They really loved their CAPS.

Sorry I've been gone so long from the Forum but life threw me some curves and I'm now back into the music. I just finished building a custom turntable and am getting ready to set up my system again. I've always been an AudioTechnica fan from my days of selling their products back in the 70's and I think I have a winner. Table consists of 2 plynths from a Marantz/Clearaudio 15s table seperated by a high density MDF matching plynth seperated by thin discs of sorbathane. Motor is mounted so it is not in contact with any of the 3 layers. The platter is mounted on the top plynth and the tonearm on the middle plynth. Cartridge is a new AudioTechnica Art9, using a modified AT650 Step Up Transformer. Tone arm is a like new Signet XK50 / AudioTechnica 1100 with interchangeable arm tubes. I just finished modifying one of the tubes from a 9" metal to 12" carbonfiber with Cardas wiring. Mass is the same as the stock metal arm with all the benefits of carbon fiber. Interconnects between table and SUT and SUT and PAS4 are AudioQuest Leopard Cables.

I'll try to posts pics soon. Vinyl and Tubes - doesn't get any better than that!

Enjoy the music!

Bob Latino wrote:Someone requested a PAS-4 schematic .... See below ..

NOTE - If the whole image does not show up on your screen then right click on the photo and then "Save image as" and you will save the full sized photo on your computer.

Bob,

reviving an old thread. do you still recommend upgrading the coupling caps to 3.3uf from the current 1uf caps that are in there?

unless you have an amplifier with very low input impedance, say under 10K, the output caps on the PAS4 at 1uF should be fine
for very low input impedance amps, you can increase the PAS4 output caps to 3.3uF to extend lowest bass

tubes4hifi wrote:unless you have an amplifier with very low input impedance, say under 10K, the output caps on the PAS4 at 1uF should be fine
for very low input impedance amps, you can increase the PAS4 output caps to 3.3uF to extend lowest bass

im running a VTA ST-70, so im assuming the 1uf cap should work ok then.