diytube.com

Put this together for less than 50 bucks excluding tubes. I guess I could have used a bigger box, but I wanted to see how small I could make it. Ran out of room for a volume control, but that's ok. I built this 'cos my Adcom GTP-600 didn't have a phono stage. I rectified the filament supply and increased capacitance in the PS. Over all, it sounds better than the SS phono stage on my old NAD 1130.

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SDS-PAGE wrote:Put this together for less than 50 bucks excluding tubes. I guess I could have used a bigger box, but I wanted to see how small I could make it. Ran out of room for a volume control, but that's ok. I built this 'cos my Adcom GTP-600 didn't have a phono stage. I rectified the filament supply and increased capacitance in the PS. Over all, it sounds better than the SS phono stage on my old NAD 1130.

Do you have a schematic of this preamp?

Yellow_Light_Colorz_PDT_02

Tight quarters too, eh? You must be very dexterous.

Great job.

Is there much noise/hum with no audio input (say, hooked up but no vinyl playing)? Congrats!

Would this be a suitable power transformer from AES?




TRANSFORMER, POWER, 125 V, 15 mA

P-T442

Primary: 117 volt, 50/60 Hz
Secondary: 125 V, 15 mA
Filament winding: 12.6 volt, 0.3 amp
Mounting centers: 2", U-bracket
Weight: 0.7 lbs

That power trannie would barely be sufficient, and would not permit DC for the filaments.

I modified a Bottlehead "foreplay" linestage long ago, it originally had the same or a very similar AES (or Allied) hammond-made power trannie, to feed two 12AU7s, and it was just barely sufficient. I replaced it with a 125-0-125 25 mA and 6.3V, 1.2A trannie so that I could DC power the filaments and eliminate the magnetically-induced AC hum. The trannie was from Bottlehead, and was about $40, and it had magnetic shielding, and was over-built fo the power requirement. BTW, this is the exact same Bottlehead power trannie which is used in their "seduction" phono stage".

The "HV" secondary winding could be 125VAC, 15 to 25 mA should be more than enough, but the filament winding should have at least 6.3VAC @ 1 amp, or 12.6VAC @ 500 mA, if you plan on powering the filaments from DC, as the process of rectifying the AC voltage will increase the AC current demand to about 1.5X the actual DC filament current consumption.

This looks like the well tested back- to-back filament trannie scheme, and the fact that they are (more or less) shielded will help reduce AC magnetic-field induced hum.

HTH

(Instant Update)

The PA-211 trannie from Triode (intended for the PAS2 / 3 preamps) migne be an excellent choice here. The 330-0-330 HV winding is a bit high for this application, but just add in some extra CRC in the filtering for the B+ line, and it should be a winner! The filament winding is 12.6 VAC at around 800 mA, they doubled itup to 24VDC, and ran the four 12AX7s' filaments in a series-parallel arrangement, which drew 300 mA (and the 12X4 rectifier and pilot light filament ran straight off the 12.6VAC).

/ed B in NH

Is there much noise/hum with no audio input (say, hooked up but no vinyl playing)? Congrats!

Actually, there is a hum when hooked up with no vinyl playing, which increases with the volume. Guess I failed to notice it through songs playing. Anybody knows how much it is expected to hum? There is a hum level control and on the manual it says the control is for getting the hum down to an acceptable level?

Would this be a suitable power transformer from AES?

Not sure about 0.3A for a continous operation. I think 0.5 A minimum ought do it. I used a pair of Radioshack's 117V to 12.6V 1.2 A trannies.

The circuit layout and a good single-point grounding scheme gets very "critical" in these higher-gain preamp circuits, as ground loops and magnetically induced AC hum can be a "tough nut" to crack.

When you're dealing with millivolts (and microvolts), the hum-burglar can be tough to defeat Yellow_Light_Colorz_PDT_02

Oh, yeah, you really do need the tube shields in this application, which may very difficult to implement inthat compact construction...

/ed B in NH

Ya, I might just have to get myself a bigger box. Thought I could get away with rectifying the filament PS, but who knows what's going on w/ AC lines, even though twisted, bridging all over the place. I also kinda think that it might be the ground loops. I gounded at two places: a star ground @ RCA inputs and filter cap section to the chassis. I will need to change the latter. I am using a 2-prong AC power cord. I like 12AX7s, but with amp factor at 100, anything and everything will get loud. BTW, my B+ is 117VDC. This ok? The schematic calls for 100 VDC.

If you could find a pair of the push-on type tube
shields, they might work. Solder ground leads right
to them, rather than rely on the spring clip at the
bottom. The other thought would be just to
move the power transformers to the outside of
the box ... same mounting holes, you'd just need
holes for the wires ... it might be good to keep the
115 VAC outside somehow too ...

The higher DC B+ voltage of 117VDC is OK, and it should make very little difference in performance. 12AX7s can go up to 300VDC B+ , so you're OK...

A four-diode bridge rectifier and higher filter capacitance in the power supply may also help, just increase the R27 or R28 value to compensate if the voltage gets too high (like above 130VDC)... Looks like the circuit draws only around 1.1 mA B+ current, from the given figures.

The "seduction" phono stage uses two 6DJ8s, with around 150-160VDC B+ , and a passive RIAA RC netweork between them.


/edB in NH

A four-diode bridge rectifier and higher filter capacitance in the power supply may also help, just increase the R27 or R28 value to compensate if the voltage gets too high (like above 130VDC)... Looks like the circuit draws only around 1.1 mA B+ current, from the given figures.

Ditto... this is how I set it up: four-diode bridge - 10uF/450VDC - 10k-47uF/450VDC-22k-47uF/450VDC. I also wrapped layers of aluminum foil around the tubes (I've watching MacGyver) and it didn't make any difference. I am yet to try placing the trannies outside. The hum seems to be mostly low to mid freq. It increases w/volume. This for sure is a AC hum, possibly caused by induced magnetic field due to the trannies?

One quick-n-dirty test to see if the hum is caused by the circuitry itself. or by outside influences (interconnect cables, AC power line, etc) is to turn off (or unplug). the AC power, and then the tubes should still "coast" along for a few seconds while the PSU caps slowly discharge and the filaments cool off.

You might want to turn down the volume, then remove the preamp's AC power, then turn the volume back up again, this will eliminate the inevitable loud "POP"...

If the hum suddenly stops, then it's caused inside the preamp, if it still continues to hum, and slowly dwindles down to silence, then it's external causes. Check the cables (and any ground wire) from the turntable to the preamp.


If you have a "third wire" power line ground (green or green & yellow wire) it can cause an AC ground loop, if the power amp also has a 3rd wire "safety" ground.


/ed B in NH

Has someone got a transformer to suggest that will work here?

Don