dbx 160vu clone

[Biasrocks]

Unfortunately, the Antek will end up costing me $36 with shipping to Windsor.

A 25v'er, probably will work just fine. I'm going to verify that today when I hook my existing transformer up and take some measurements.

Mark

 

[abechap024]

I'm back to deciding what power transformer to use. I'm going to measure the voltage drop/gain with the one I'm using and decide whether I need a 24-0-24 or a 25-0-25.

Mark

I've looked all over in documents and while there is nowhere explicitly stating what kind of transformer they used. 2 x 24 - 25v should work fine. From all the pictures of the inside of dbx 160 all the transformers look surprisingly small.

Good luck!
AC

 

[Biasrocks]

They 160's are fused at 1/4amp so they draw very little current.

A .5 amp transformer should power 2 units comfortably.

Mark

 

[abechap024]

Important!!
As Mark pointed out the NTE129/128 ARE MIRRORED ON THE PCB (THE EMITTER AND COLLECTOR OR SWITCHED)
I READ THE DATA SHEET WRONG! SO ANYONE BUILDING THE PROTOTYPE YOU WILL HAVE TO BEND THE BASE PIN TO INSERT CORRECTLY!

GETTING THIS FIXED RIGHT NOW.
Sorry and Thanks!

AC

 

[Biasrocks]

As Mark pointed out the NTE129/128 ARE MIRRORED ON THE PCB (THE EMITTER AND COLLECTOR OR SWITCHED)
AC

Yep, a bit of a pain to fit those, it's a good thing I checked the schematic to verify what went where, otherwise pooffff!

You may want to double check the BC560, I had to do the bendy trick on that one as well.



I hooked up the transformer that I had laying around and measured a voltage gain of around 1.5volts to the main power caps. So it looks like a 24vct will do the trick.

Mark

 

[Biasrocks]

Alright, finally got around to hooking up a decent PT.

First off, it looks like a 24v PT is too much, I ended up getting around 28v's on my output transistors
instead of 24v's. Backing the variac down till I got 24v, I got around 20vac on the transformer secondaries.

On the sound front, no joy... I don't have a functioning unit. There is some sound coming through along
with a ton of noise. The controls seem to have some effect, the gain is working, threshold is working and
the ratio seems to change things. But all I'm getting is pffft and nothing resembling a good signal.

I've started to check voltages and it seems like all the opamps are getting proper voltages along with
the RMS / VCA modules.

I do have a question, I'm getting about 10v's from Q7 (BC263/560C transistor) to the "I" port of the 200 VCA.

On the schematic, it says to select Q7 to match the VCA. Any ideas what the procedure is for this and what the voltage is that I should be seeing?

Does anyone have a functioning unit that they could take some voltage measurements from?

Mark

 

[damnyankee]

Mark,

I have read a few years back on VCA repairs that if there's a pffffft sound (lots of noise), it's because transistors aren't Vbe & Hfe matched.  I don't know if that helps you or not...

DY

 

[Biasrocks]

Well since these VCA's came out of a working unit I doubt that's the problem.

I'm still curious about the selection process of Q7. I've got a bc177 in there right now, maybe I'll try to track down a few bc263's.

Did you get your's working?

I'd really like to have some sort of voltage reference for this guy.

Mark

 

[abechap024]

Hi test..I have a email from arno he wanted me to post - but it wont let me post?

But yet it lets me post this text....very strange...

When I try to paste in the regular txt from the email i get an error screen when I go to post it.....

Weird!

So I'll just give the main points till I figure out whats wrong...
He got his working with the THAT chip and threshold,ratio,gain work as they should...

Meter section is not working as of yet. He thinks it might be instability in the circuit, So we'll see.

Mark, Im looking to see if I can find any info what so ever on what and how to pic that transistor....

 

[damnyankee]

Well that's good you used a working VCA (as am I).  To me, that hissing/pffft may be one of your opamps not playing nicely.  I'm sure you checked the BiPolar and JFET...?

To be honest, I've been so busy homebrewing and finishing up my winter canning that I haven't had much time to finish this up.  I have the power supply put together and I have all the parts.  Throw in the kids activities (Scouts, Basketball, Soccer, etc), I just haven't had time to sit down and finish putting it together.

DY

 

[Biasrocks]

I need to do a signal trace and figure out where things are going wrong, really haven't had the time.

Those voltage refs would really come in handy, so that I can rule that out.

Mark

 

[Arno]

I think it's got something to do with the "non-us" characters problem.
Here's what I wanted to post with "us" characters only


Mine is alive as well.

Passing audio, compresses and everything looks ok from here.
Measures high thd+n (several %) but no trimming done yet... Looks like 3rd order.
I'll take some more measurements as soon as I get the trimmers set-up...
-Btw, I've built it with the 2180 vca. I thought they were internally trimmed and that they need no sym adjustment on pin 4 ?

I can't get the meter to move though.
Regardless of meter offset trim or meter calibration, it's always sticking on 14,5V or so...
I don't really understand the meter section. My guts say it's just a bistable circuit but they're probably wrong.


Couple of (minor) pcb notes:

The nte129 and nte128 have their pinouts upside down so you need to bend the base backwards to get them in properly.
Some of the 22uF caps have 0,1" spacing (which is great) some have 0,2".
The 1nF caps are 0,3". Would be nice to have 0,2" as well (for wima's).
The connectors/pcb-pins for I-O and switches/pots are somewhat confusing. Some have pin1 on the left side, some on the right side.
Many of the ceramic pF caps suggested in the mouser bom don't really match the pin spacings. But it's always hard to get them all right anyway.

It IS possible to fit heatsinks on the nte129's in the psu but you'd need some of these, and something to isolate to avoid shorts.
http://be.mouser.com:80/ProductDetail/Aavid-Thermalloy/578305B00000G/?qs=2v7q0MSBcBMVIGpeoAPVAQ%3d%3d
mouser - 532-578305B00

Without heatsinks, they're cooking!

I'm using a dual 24V transfo. I'm reading +/- 34V on the collectors of the output pair (24V*SQRT2).
Not sure if the transistors can handle that but they seem to work fine and they're pretty cool (temperature-wise).


Overall very good job!

thanks !!



Edit: I think it was the greek "mu" that was causing the error

 

[Biasrocks]

Okay, well I'm getting signal up until after the .33uf capacitor, where I'm getting a -15vdc offset between the 100K resistor and the capacitor. The offset disappears on the other side of the 100K resistor; going into the VCA IN.

Can anyone confirm if this is this the expected behavior?

Mark

 

[Biasrocks]

I'm using a dual 24V transfo. I'm reading +/- 34V on the collectors of the output pair (24V*SQRT2).
Not sure if the transistors can handle that but they seem to work fine and they're pretty cool (temperature-wise).

I measured about 20VAC on the secondary in order to get +/-24 volts on the output transistors.

I have a 24V tranformer, but used a Variac to bring the voltage down to 20V.

Mark

 

[abechap024]

Okay, well I'm getting signal up until after the .33uf capacitor, where I'm getting a -15vdc offset between the 100K resistor and the capacitor. The offset disappears on the other side of the 100K resistor; going into the VCA IN.

Can anyone confirm if this is this the expected behavior?

Mark

Wow that is really strange...I don't know where that offset would be coming from. But probably a good place to look for problems. I'm looking over the schematic....I would guess its coming from the VCA somewhere?? not that your vca is defected but maybe needs to be optimized with the circuit..or something. Wish I could help more...

 

[Rob Flinn]

The That's Vca's are current driven devices which means you can't measure voltages in some places around them.  So this may not be a problem.

 

[Biasrocks]

I thought that might be it Rob, after the 100K resistor the voltage drops down to about .07vdc. It's only between the .33uf cap and 100K resistor that there's a -15vdc offset. I am only getting 10vdc to the +V side of the VCA which may be a problem, but I'm not sure until we get some voltage readings from a working circuit.

I may have to build this with a THAT VCA to get it working and then try the DBX 200 VCA again onces it working.

Mark

 

[arska]

That 2180 vca: high thd and noise

Possible HF oscillation in the side chain, read That design note 106.

 

[abechap024]

*edit*

 

[damnyankee]

Next week, I'm going to resume building past the power supply section.

Has anyone have a fully functional clone up and running yet?  As I move forward, I'll be asking some clarification questions...

Thanks in advance,

DY