PREDI’CT Build Thread (frozen)

[dirty1_1garry]

This thread dedicated to the PREDI’CT project - tube preamp for instrument signals.
This project originally was born from these thread:
http://groupdiy.com/index.php?topic=50529.0

Feel free to ask and comment!

22.02.16 update: !!! The project is frozen for an indefinite period until a there will be no budget for its renewal
Check Reply #76

So what’s that?
This is a pcb project that provides a simple way to convert an instrument signal to line level while adding warmth and character to the sound. It amplifies the signal with a tube-driven amplifier based on one 6DJ8 type tube directly into a transformer that converts it to a line level signal with a 600 Ohm load output.
With an input impedance of 1 MOhm, it provides about 18dB of gain, and a harmonically rich tone to the sound. It perfectly suits any kind of guitar pickups, adds richness to analog synthesizers, and enhances the character of any electrified instrument that is run through it. A great choice for bass guitar thanks to it's full low end!
This project allows for different variations based on your own taste.

Build aspects
Pcb contain signal chain, B+ power supply and voltage regulated heater supply.
For building ready for use standalone unit it’s also necessary external components:
output transformer, power transformer, jack connector, output XLR male, iec power plug, output pot, power  indicator lamp (optional). Project doesn’t contain rare components.

Output transformer options
there are few options like edcor GXSE5 or my custom C-core transformers specially made for this project.

Power transformer options
I test unit with 200V B+ voltage. But 200V not so common voltage for popular transformers.
I think that it’s possible to use power transformer with another value of secondary voltage, like 220V. It’s more common and simply to find. Will check this in future (unchecked yet).
At an early date I could offer proper custom power transformer with 110/220 input voltage.
Here is a places where it's possible to buy such trans:
http://www.toroidal-transformer.com/shop/high-end-audio/power-transformers/314/2n465-powertransformer-for-tube-amps---44va-.html#.VHgUQdymibB

Documentation and information
https://yadi.sk/d/HT3DvLO5dQzbB

Mouser carts
onboard:
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=e6494d3200
outboard:
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=6e1153d667



Case options
I think that it’s pretty comfortable to pack one channel into instrument case with appropriate sizes. Or it’s possible to pack it into 2U case. It’s possible to pack two unit’s into 2U case.
BUT! Very important! Power and output transformers should not be too close too each others! Otherwise it’s possible to get noise and hum.
At an early date I could offer examples of front panels for 2U cases. Also I will have 2U front panels for sale.
____________________________

UPDATES:
BOM edited - R7 = 39K, R8-R10 = 2K
no R11 on pcb - my mistake

27.03.2015 ALL TRANSFORMER KIT'S SOLDOUT. WAITING A NEW BATCH OF TRASFORMERS!

suitable 2U chip case:
http://www.parts-express.com/middle-atlantic-ec2-economy-rack-chassis-2u--262-442

Here is  an example of fully assembled pcb:

Regards,
Igor

 

[dirty1_1garry]

Here is an example of first V1.1 working prototype:

 

[dirty1_1garry]

Here is an example of  front panel design:

 

[collinTHEbrewer]

I love your cardboard prototyping skills. Kudos.

 

[ruffrecords]

Is this going to be available as a group buy or in the white market??

Cheers

Ian

 

[dirty1_1garry]

Is this going to be available as a group buy or in the white market??

Cheers

Ian

I think that I will open a thread on white marker in the near future. Matter of time.
I sold several kits as a private sale. About 12 kits may be.  PM me if you are in interest.

 

[theokbird]

Hey everyone,
first off: thanks, Gary and all the others who contributed, for all the hard work youve put into this project!


Now on to the problems 8)
I have some major buzz/hum issues, mains hum at 60hz and various other buzz throughout the audio spectrum.  i've come for help after i exhausted a number of potential fixes, and I wasnt sure what else to do! 

Here's what i've done so far:
-moving the power tx and output transformer away from each other, no difference in noise, even really way far away from each other, like further than any equipment i've seen.  I've even tried a transformer shield :http://www.don-audio.com/trafo-shield but this didnt make a bit of difference even when moving the TX far away.
-all my AC lines are tightly wrapped and fairly orderly
-i'm using high quality shielded cable for the in and out
-i know for 100% certainty every component and connection on the board is correct.
-unplugging the lamp makes no difference
-CJ mentioned the possibility of implementing a hum balance pot, however i believe Garry has implemented a fixed hum balance with R12/R13, both 120ohm resistors, one on each heater line with the other leg of each resistor going to ground.  I could be wrong about the purpose of these resistors, but looking at the PCB that's what i think they are.  From what i've read, this hum balance solution is just as good as the 'hum balance pot', but again i dont really know for certain!  Also i have absolutely no idea if this is really my problem

###this is my power tx:http://www.toroidal-transformer.com/shop/high-end-audio/power-transformers/314/2n465-powertransformer-for-tube-amps---44va-.html#.VHPhcpPF_Fc

###I am using the custom output tx from Gary, as well as his pcb.

here's a couple photos of the inside:

i have a multimeter and tone generator plugin in my DAW that i can use to test or take other measurments if need be, i just dont really know where to go from here, or if its even possible to squash the hum.  advance thanks for any and all help!

-OK Bird

 

[theokbird]

So i just tried something and it made the hum basically vanish, which is that i disconnected the gain pot.  however now i dont have any control over the gain...what's up with that?  is there a way to have gain control without inducing hum? 

could this be a red herring?

 

[Winetree]

I haven't finished mine.
I'm still looking for a case and power transformer. What case are you using?
Photo of another builder's REDDI. Split P.S.B. and different O/P trans.
Could this picture this help with your hum problem? I'm probably going to have the same.
Any other PRED'CT builder comments?

 

[theokbird]

Thanks Winetree,

yeah i guess one option is to explore if this hum is inherent in the board design.  i dont really know enough about the reasons why hum is induced in valve amplifier circuits, but i'll go for anything at this point.

cases:
i originally went for the collective cases PSU case, which is very nice, but i found it to be a little tight for this job.  I also bought the Hammond 1402FV enclosure to hedge my bet, the downside being you have to drill the faceplates(or make your own in front panel design).  I designed my front panel in FPD and just printed a transparency of it, used removable spray adhesive to adhere it to the panel and started drilling.  I can post the fpd file, if you are interested or want to use it as a template. (it has provisions for a ground lift, combo jacks, thru, gain, a fender style jewel lamp, qualtek fused IEC with mains switch, and standard power switch)

However, speaking about this hum, i think if you can assemble it in a 1U case(which is the case DirtyGarry built and tested this board and OP tx in) you'd be better off, with the ability to separate the OP tx and power tx the farthest.  Also with the knowledge that he made it work in that type of enclosure.  That being said, my hum problem doesnt change when proximity changes between the two transformers...

what are you thinking about for power tx?

-okbird

 

[ruffrecords]

I just checked out the documentation but I could not find a schematic.

Cheers

Ian

 

[theokbird]

I just checked out the documentation but I could not find a schematic.

Cheers

Ian

Hey Ian, thanks for the gain control idea in the REDDI thread! i'm testing it out now, i'll let you know how it goes.

And yes, there hasnt been a schematic supplied for this particular PCB, which has made troubleshooting more tedious for me, particularly because i've already stuffed my boards and the solder mask is black which makes tracing the circuit on the pcb very tough.

DirtyGarry, any chance you can post that? I know when i asked for it before you pointed me to page 6 of the REDDI thread, but that doesnt provide the scheme for your PCB design, just a general concept of the circuit. 

thanks,

-okbird

 

[dirty1_1garry]

Hey everyone,
first off: thanks, Gary and all the others who contributed, for all the hard work youve put into this project!


Now on to the problems 8)
I have some major buzz/hum issues, mains hum at 60hz and various other buzz throughout the audio spectrum.  i've come for help after i exhausted a number of potential fixes, and I wasnt sure what else to do! 

Here's what i've done so far:
-moving the power tx and output transformer away from each other, no difference in noise, even really way far away from each other, like further than any equipment i've seen.  I've even tried a transformer shield :http://www.don-audio.com/trafo-shield but this didnt make a bit of difference even when moving the TX far away.
-all my AC lines are tightly wrapped and fairly orderly
-i'm using high quality shielded cable for the in and out
-i know for 100% certainty every component and connection on the board is correct.
-unplugging the lamp makes no difference
-CJ mentioned the possibility of implementing a hum balance pot, however i believe Garry has implemented a fixed hum balance with R12/R13, both 120ohm resistors, one on each heater line with the other leg of each resistor going to ground.  I could be wrong about the purpose of these resistors, but looking at the PCB that's what i think they are.  From what i've read, this hum balance solution is just as good as the 'hum balance pot', but again i dont really know for certain!  Also i have absolutely no idea if this is really my problem

###this is my power tx:http://www.toroidal-transformer.com/shop/high-end-audio/power-transformers/314/2n465-powertransformer-for-tube-amps---44va-.html#.VHPhcpPF_Fc

###I am using the custom output tx from Gary, as well as his pcb.

here's a couple photos of the inside:

i have a multimeter and tone generator plugin in my DAW that i can use to test or take other measurments if need be, i just dont really know where to go from here, or if its even possible to squash the hum.  advance thanks for any and all help!

-OK Bird

Hey dude, your build looks great!

R12/R13 resistor for output gain potentiometer only and they should be 300R (as mentioned in the BOM). It's design is from original unit, not my own invention

Looks like you did not connect ground > case wire as mentioned in connection schematic.

 

[theokbird]

Looks like you did not connect ground > case wire as mentioned in connection schematic.

I did, it's done from the back of the PCB to keep the wiring neat, you just cant see it from the pictures 
I've used this point to implement a ground lift switch, however i still cant tell whether or not this is truly the audio ground.  If we could get a schematic thats specific to your PCB design it'd make things a bit easier.

thanks,

-okbird

 

[Winetree]

Theokbird, I've got my 2 boards built but I've held off on my power transformers and build until the hum problem is solved
I'm taking some wild guesses at just looking at your pictures.
The unbalanced input jack looks like is crammed into the output transformer.
Even though the transfo is shielded should they be that close?
You said when the output pot was disconnected the hum went away.
The output pot traces on the P.S.B. run right next to and very close to the tube traces.
Could the traces be cut and the pot be connected closer to the output transformer?
Did you use a S.M.D. or a soldier bridge to connect the points under the tube ?
I wonder if Dirty's rack mount is hum free. I hope you can find a solution. I'd like to finish mine.

 

[dirty1_1garry]

Looks like you did not connect ground > case wire as mentioned in connection schematic.

I did, it's done from the back of the PCB to keep the wiring neat, you just cant see it from the pictures 
I've used this point to implement a ground lift switch, however i still cant tell whether or not this is truly the audio ground.  If we could get a schematic thats specific to your PCB design it'd make things a bit easier.

thanks,

-okbird

I don't think that it's a right place for case connection. Using it  could gives you a ground loop.
Just done everything like sad in manual and you would be -  8)

 

[dirty1_1garry]

Theokbird, I've got my 2 boards built but I've held off on my power transformers and build until the hum problem is solved
I'm taking some wild guesses at just looking at your pictures.
The unbalanced input jack looks like is crammed into the output transformer.
Even though the transfo is shielded should they be that close?
You said when the output pot was disconnected the hum went away.
The output pot traces on the P.S.B. run right next to and very close to the tube traces.
Could the traces be cut and the pot be connected closer to the output transformer?
Did you use a S.M.D. or a soldier bridge to connect the points under the tube ?
I wonder if Dirty's rack mount is hum free. I hope you can find a solution. I'd like to finish mine.

solution is ver easy - do how it's sad in manual

 

[dirty1_1garry]

One more happy builder! 
Build in 2U rack case from parts-express.com
No hum, no buzz.
Additional isolated thru output (rightmost)  with Jensen split transformer.

 

[theokbird]

Hey everyone,

i've come to confess...
after pulling my hair out i've figured out my hum problem and they had nothing to do with the board, wiring, or anything related to the Garash Labs equipment or my build.  After trying a few different sources with it i realized that

1) the single coil instrument i was using was introducing hum to the signal(even though i dont hear hum while using it through other amplifiers, it was clearly revealed in the DI)
2) a certain cable(a reliable monster cable) i was using in combination with this instrument seemed to create even more hum, especially as i moved the instrument around, which is classic single coil noise.
3)the gain pot solution doesnt seem to be a problem in this circuit


So after swapping cables, using different sources, i realized that this particular combo of cable and instrument produced the offending hum, and I also realized that the positions of the output tx and the power tx were good. 

I will say that there is still a very tiny amount of 60hz hum, but its only audible at levels that i would never use in the first place, and i can definitely live with that.  I should also add that using a power transformer shield does knock down this hum even further, and i'm sure that if you install this in a case that leaves plenty of room between the two transformers, then you will likely  be *nearly* completely hum free.

Thanks Garry!

here are some complete build photos:

 

[dirty1_1garry]

Hey everyone,
i've come to confess...
after pulling my hair out i've figured out my hum problem and they had nothing to do with the board, wiring, or anything related to the Garash Labs equipment or my build.  After trying a few different sources with it i realized that

What a story!)
Great that you get a good result. I'm happy at least you

Where do you get these nice cases? I will add them to the topic as available cases.

Your power and audio transformers really close to each other. I always got a hum with a such placement . Looks like you've done a nice job for finding a perfect place for them Cool!