Drip V72 finished!

[JoleFIN]

When driving the unit loud (everything full CW and giving a little louder signal), the unit gives an annoying crackling noise. It's on the edge when the tube distortion is coming in nicely if you want it for effect, but you don't dare to drive it that much since the loud crackling distortion comes in soon next. I'm not familiar with tube gear, so I was wondering what's causing this? Is it some kind of short-breaking inside the tube itself due to higher voltages when driving it loud? I'm using the Tungsol EF806G tubes, so maybe I should test some other tubes too to see if this occurs. Would be nice to use it for tube distortion as an effect too since there's the direct input in front

And, does anyone have a suggestion for the +48V phantom circuit? I was thinking of this one http://sound.westhost.com/p30a_f2.gif

 

[peters]

I heard similar crackling sounds on my Drip V72 project.
I ended modifying the thing quite a bit, and it sounds a lot better and no crackling.
Can't say what it was specifically but the specified transformers are kind of dicey, IMHO.

You can change the stock Lundahl input from 1:20 to 1:10 by doing this mod on the PCB which will open up the frequency response a bit further, and lower sensitivity somewhat:
http://home.comcast.net/~ps8899/v72/PCB_input_1_10_mod.jpg

The specified Lundahl on the output side is actually an input transformer mounted backwards so is also subject to overloading.

I ultimately used a Jensen JT-115K-E for input and Sowter 1120 output, which was a significant improvement.

--Peter

 

[JoleFIN]

So it's not about the tubes, but transformers it seems... the 1:10 input mod would halve the gain on input? Thinking of all this, it would all point to the output transformer being too weak to handle the juices. The transformers would be a bit bitchy to desolder without wrecking them and I like the sound of the unit, though I don't have much to compare with (prefer it over the Focusrite ISA220 we have), so think I'll keep the transformers but will possibly try the 1:10 modding. It could also be made switchable between 1:20 and 1:10 with a 3 pole switch?

EDIT: And thanks for the clear modding picture

 

[peters]

I put up some response curves that I got early in the game (stock Lundahls), after the 1:10 Lundahl mod, and with the Jensen/Sowter replacements:

http://home.comcast.net/~ps8899/v72/fr_1_20_lundahl.jpg
http://home.comcast.net/~ps8899/v72/fr_1_10_lundahl.jpg
http://home.comcast.net/~ps8899/v72/freq_resp_24bit_192k.jpg

Yellow is the V72, green is a loopback cable on the Lynx box.

The input transformer and its loading resistor (R11) play a big part in the overall FR. When in 1:10 mode the R11 value should be around 150K. If using the input pot, it acts as the load resistor and you should generally leave R11 out altogether.

The output transformer change did not impact FR but I think the headroom went up.

--Peter

 

[JoleFIN]

Sounds reasonable, will do the 1:10 mod on the input to get a better response .. though it sounds nice already

Thanks Peter for everything, again!

 

[Clakejr]

Where did you get the case made from?

 

[JoleFIN]

Where did you get the case made from?

Sorry for the late reply, haven't been much into DIY in past decade but built a DA87 mic half a year back The case was from Tat Purusha I think, nowadays it's diy-racked.com if I'm not mistaking? But I don't see the V72 in their inventory.

 

[JoleFIN]

Regarding the V72 build; I've had the similar hum problem when connecting it in series with a Pultec EQ and 1176 sitting in the same rack. I read somewhere on this forum about a V72+175 hum problem, I guess it's something to do with the groundings doing a ground loop via the audio connections and the rack.

I'm looking into adding a 48V phantom to it to get it running with the DA87 mic, and also looking for the air lift mod that was mentioned somewhere on the original PDF plans but can't find anything about it online. Still got the Lundahl trafo on it too, haven't suffered the crackling effect when keeping it toned down a bit. Been into building a new work room for past two years and still haven't got anything decent for a rack here so it's mainly just ITB mixing for now

 

[JoleFIN]

Decided to finally revisit this build and added the 48V phantom power. I went with the Link Audio Design kit but was unsure of the needed coupling caps etc. Used 10uF bipolar film caps together with 10ohm 0.75W resistors that I had at hand, and these were followed by 12V zener diodes back-to-back against ground for added protection. I did blow a tube or two (replaced both) while testing but not sure if it was because of hitting them with a dropping tool at some point.

Printed some casings for the power switch and power connector to replace the old duct tape coverings, and also cases for the phantom PSU and coupling caps board. Even printed a part for holding the 48V signal red bulb above the meter which makes the unit look menacing The meter light changes accordingly between phantom ON/OFF and added the ”Air lift” mod switch on the front panel too.

The phantom PSU has 0.75mm steel sheet shielding in it which is grounded and the pieces were glued to the case and connected to each others with wire strips which were glued with conducting silver glue.

I’m still contemplating on changing the transformers from Lundahls to Sowters, but am relieved the project is done for now

 

[oldskool1]

Hey JoleFIN, I like the silver alpha dial on your K2000r. Back in the day a friend dropped by and asked how I got the finger indent on my alpha dial to be silver, and it was just from using the thing! I used the heck out of that synth/sampler for a decade, and my finger wore the paint off. Congrats on the LA2A too.

 

[JoleFIN]

Hey JoleFIN, I like the silver alpha dial on your K2000r. Back in the day a friend dropped by and asked how I got the finger indent on my alpha dial to be silver, and it was just from using the thing! I used the heck out of that synth/sampler for a decade, and my finger wore the paint off. Congrats on the LA2A too.

Oh yes I've been queried about the silver dial earlier too Bought the unit in mint condition in 1998 and after a few years of daily use the paint was chipping off from the dial, so I sanded it bare and it looks great -- it has the look it's in use, and k2k users will know

Ahh there's 1176 and Pultec, sadly I still haven't built an LA2A yet despite thinking of it for some 14-15 years... the software caught up back then and I went mixing mostly ITB, but the gear acquiration syndrome never really goes away.