Yet another PAS 3 question... PS mods....

[jeffdavison]

Looking to mod a power supply for a PAS 3...

In looking at the PS schems, I'd like to change the 10K ohm R1 resistor after the first capacitor after the 12X4 with a choke. Specifically a Dyna C354. Aim is to get more of the ripple out. Using a 300 - 0 - 300 PT, how would the the B+ change? I'm good at soldering but a bit weak in the math and theory dept. I have a couple of these chokes hanging around and thought I may be able to use them here.

C.Chong makes a replacement cap board that I'd like to use with the same values as the SDS board.

This power supply will be used in a dual mono Aikido line stage with 12sx7gt's that I'm building to match my ST70 monster. I'll be duplicating this setup for the opposite channel.
Thanks all!
Jeff

[mesherm]

If you are making a power supply for a preamp you won't need the current capacity of a C-354 which is 1.5H @ 200ma. I would consider a Hammond 156G or something similar. A 156G is 9H @ 40ma. 300 ohm DC resistance and the same price as a C-354.

[jeffdavison]

I have a couple C354's on had already on hand so the need to aquire them is a non issue, just need to know how the ones that I have will effect the PS.
The C354 will have way overkill the needed current capacity...but that shouldn't effect the way in which it should work?

Jeff

[mesherm]

For a power supply you usually want the inductance (henries) to be as large as possible. The higher the inductance the better the filtering.
The inductor acts like a high resistance to AC and a low resistance to DC which is an advantage when you need more current in a power amp. I doubt that a C-354 will be any great advantage over a resistor in a preamp power supply.

[jeffdavison]

I found this in one of the other DIY forums as a suggested PS for the Aikido:

http://www.diytube.com/phpBB2/album_pic.php?pic_id=24


Jeff

[mesherm]

Are you moding a PAS power supply or building a new preamp?
Either the Chong or SDS upgrade board will work fine on a PAS.
After that, the next best thing to do to lower the hum is to completely redo the input wiring. Eliminate the stock rotary switch. Part of its original function was to switch between RIAA to Tape EQ. Since Tape EQ is obsolete, you hard wire the EQ board to RIAA only and give it its own set of input RCAs on the back. Now you replace the stock rotary switch with a new 2P5T. The RCA inputs go to 4 of the positions and the EQ PCB output goes to the 5th.
Use shielded or at least twisted cable to wire up the new input selector switch and good wire dressing techniques. Your PAS hum will evaporate. I did this to one of mine and even at full clockwise there is no audible hum and just a smidgen of noise. Whereas on my factory wired PAS all I did was install the SDS board which lowered the hum a bit but it is still audible.

[jeffdavison]

Are you moding a PAS power supply or building a new preamp?

Yes.

I'm building a preamp using the Aikido line stage, but using a PAS upgrade power supply as it fills the requirements that the Aikido needs

Either the Chong or SDS upgrade board will work fine on a PAS. After that, the next best thing to do to lower the hum is to completely redo the input wiring. Eliminate the stock rotary switch. Part of its original function was to switch between RIAA to Tape EQ. Since Tape EQ is obsolete, you hard wire the EQ board to RIAA only and give it its own set of input RCAs on the back. Now you replace the stock rotary switch with a new 2P5T. The RCA inputs go to 4 of the positions and the EQ PCB output goes to the 5th.

Not using any PAS internals at all, will be building my own chassis, attenuatorsa and selectors.

Use shielded or at least twisted cable to wire up the new input selector switch and good wire dressing techniques. Your PAS hum will evaporate. I did this to one of mine and even at full clockwise there is no audible hum and just a smidgen of noise. Whereas on my factory wired PAS all I did was install the SDS board which lowered the hum a bit but it is still audible.

All those "hum-busting techniques are a given for this build, just trying to improve the power supply as much as I can. I like the Chong board because it is as almost plug and play as I can find. I'd like to tweak it to try and get as good as it can be given it's form factor. That's why I'm looking into using the choke in the circuit to the same effect as others who scratch built the schem I posted, which was well recieved by those who heard it.

Jeff

[Pyre]

Hey Jeff,

I too am building a Aikido line stage using a Pas power transformer. I am just in the end stages now before I put it in the fancy chassis. I built it all on a wood base this time first after having to pull a phono stage out of a chassis three times to fix a hum.

For a power supply I went with the Jung-Vorhis "last PAS" Regulated Suupply from tech-diy @ http://www.tech-diy.com/DIY_PWR_LastPass.htm

best 60 bucks I ever spent on a power supply. Its about as plug and play as you can get.

For the heaters I just went off the transformer into 2 diodes and 2 2000mfd caps. I am sure that I could come up with something more fancy for the heater supply but there is no hum what so ever so it does not seem necessary.

Oh and make sure you follow the plan and feed 1/4 the B+ voltage to the heaters. Made a huge difference for me.

What tubes and coupling caps are you planning on using? I currently am running 12au7 inputs and 12Bh7's on output with a 5.7Uf Solen on C1 and a .33uf Auricap on C2. The Solen sounds a bit muddy and the Auricap is a bit low on bottom end. Im Saving up for a set of 4uf Jensen's

[jeffdavison]

Here's the line-up for this line stage:

2 Octal mono Aikido boards, all tubes are 12sx7gt's
C1 caps are .47 Mudorf MCap Supremes Gold/Silver,
C2 caps are .1 VCaps. There will be a front panel switch for one or the other. Ithink the values should be fime as the are blocking DC and with the input impedance of the amp being driven (430K) that the low freq cut of should be well below 10hz.
C3 & C4 are .22 Multicap RTX's
Critical resistors are 1/2 watt Shinkoh Tantalum's, the rest are 2 watt Carbon film Stackpoles with copper leads.

the circut board will be capacitively grounded to the chassis groun.

A single Antek 150VA toroid with 300v - 0 - 300v, 12.6v and 6.3v outputs

The 6.3v will runheaters for two GEC CV4005 / 6X4 rectifiers. each will feed B+ to its own seperate SDS cap board, each slightly tweeked..first cap is 40uf then a 158q Hammond 5H choke then 3 100uf 'lytics. 12.6v out from the transformer will be diode rectified on the SDS cap board to feed the 12sx7gt's heaters (each diode - uf4007- will have a 680pf snubber.
Since the transformer has 12.6 and 6.3 filiments out, I can feed the rectifier and signal tube heaters their own source.

Each channel will have it's own stepped 100K 24 position atenuator with Caddock resistors between the inputs and the gain stage.

All this will fit in a 8"x14"x 3.5" Lansing chassis

not to forget the Corcom filtered power entry module and CL90 thermsistor.

JD