dbx 160vu clone

[damnyankee]

AC,

I have mine but I'm still going to build the VCA and RMS boards anyway; I've read there's a quality control issue between the cans used in the 160VU and the 117/119 (the analogy is the rebadged EV RE-10 mics and the genuine RE-15 mics).  If I've spent this much time on this project with the VCAs being such an easy build, I'm just going to go ahead and take the 1/2 hour and just build them.

DY

 

[abechap024]

Yes I agree,
and maybe we can find out some way to easily temperature match ??? (sounds like PITA!) all the transistors. That will def. be the tricky part. Arska pointed out we need to match "Ube and hfe and temperature drift as selection parameters."
HFe is as easy as plugging the DUT into a multimeter. The other will take some research. But all in all I'm glad we decided to keep the option for discrete VCA signal path....now I'm starting to think what if I made room for Discrete op amps in the signal path ;D ;D ;D

lol no.

AC

 

[damnyankee]

I have a feeling we're going to have to find someone with a curve tracer to match those transistors...

 

[abechap024]

http://www.techlib.com/electronics/curvetrace.html

???? maybe??
I'm not familiar with curve tracers , but they look really cool!


Cheers
AC

 

[arska]

Matching transistors for vca :  select hfe into 10% and Vbe into 1mV tolerance at low collector current (1mA max).  room temperature is fine......

Transistor tester:

http://www.sg-acoustics.ch/analogue_audio/index.html       

 

[damnyankee]

Thank you, arska.

I've never matched transistors before, so I'll read the tutorial.

DY

 

[matthias]

Matthais - your documents of the 200 VCA needs to be updated with a trace from drain for the input FET Q3 to the junction of R1 and C1.

thanks! I just corrected the pdf.

 

[damnyankee]

I read the tutorial (matching transistors are new to me).

I see a test circuit will need to be built to test Vbe.  And I just read on another board how critical Vbe transistor matching is with VCAs.  If they are not matched, "Splutter, distortion and cutting out on higher frequencies" will occur.

AC: did you say you were willing to do transistor matching and offer them on the pcbs?  (I assume for an additional fee).

Thank you in advance,

DY

 

[abechap024]

Right I'll probably be able to offer the matched transistors, we'll see how it goes. I'm doing a lot of reading on it
AC

 

[Rybow]

Yup. I'm still in for a prototype.

If I have read all the info properly, we will be able to:
-Build a stock 160VU with VCA's constructed directly on the main PCB
-Option will be available to use a modern power supply
-Use modern VCA's

If we want to use LED metering, will we still need to alter the meter circuit? $150 for a meter is a bit much for me.

I plan on building one channel stock and one channel "modernized" just to see what happens. Could be cool.

Looking forward to working on this project! Great work so far everyone.

 

[Biasrocks]

If I have read all the info properly, we will be able to:
-Build a stock 160VU with VCA's constructed directly on the main PCB
-Option will be available to use a modern power supply
-Use modern VCA's

Hopefully the VCA's and RMS modules will be separate plugin modules like the original so the PCB will accommodate the clone VCA/RMS modules and DBX originals which I happen to have sitting next to me.

Mark

 

[damnyankee]

[quote author=Biasrocks]
Hopefully the VCA's and RMS modules will be separate plugin modules like the original so the PCB will accommodate the clone VCA/RMS modules and DBX originals which I happen to have sitting next to me.

Mark
[/quote]
I think AC put both the 200 VCA and a THAT footprint (2180?) on the board so if the builder chooses, he could install both VCAs and toggle between them via the front panel.  I never thought of that and it was a brilliant idea.

I think right now AC is looking for people to confirm the schematic is correct so he can get the prototypes manufactured...

DY

 

[abechap024]

Hi
I installed the footprint for the dbx 200 vca
and the footprint for the that 2180 vca
also the discrete parts inside the footprint of the 200 vca(if you want to build on right on the board)

I did not, however, implement a switch to be able to switch in between.
Doing so would've doubled a lot of the part count and to me serve little purpose. I don't think the sound is going to be THAT (pun) different. I put the VCA options in to reflect what parts were available and how much time people wanted to spend on the clone. Not so much for different "sounds". i plan on building my first set with the discrete VCA on the board. To me that most accurately captures the original second to using actual dbx modules, which i don't have an interest in looking for. I've already studied their insides till I went cross-eyed! The Discrete when properly adjusted should sound more interesting then a chip, I speculate.

I'll send an email out soon with more info to people interested in getting a prototype
thanks
AC

 

[arska]

Hi
I installed the footprint for the dbx 200 vca
and the footprint for the that 2180 vca
also the discrete parts inside the footprint of the 200 vca(if you want to build on right on the board)

I did not, however, implement a switch to be able to switch in between.
Doing so would've doubled a lot of the part count and to me serve little purpose. I don't think the sound is going to be THAT (pun) different. I put the VCA options in to reflect what parts were available and how much time people wanted to spend on the clone. Not so much for different "sounds". i plan on building my first set with the discrete VCA on the board. To me that most accurately captures the original second to using actual dbx modules, which i don't have an interest in looking for. I've already studied their insides till I went cross-eyed! The Discrete when properly adjusted should sound more interesting then a chip, I speculate.

I'll send an email out soon with more info to people interested in getting a prototype
thanks
AC

Design note: 2180 control voltage (IC pin2); buffer missing!!!! It needs to be low impedance drive, use opa (5534 etc.) to drive and match control voltage.
See THAT  corp design notes, please. It don´t work right , I have tested them all......... 2180/81 is not drop in replacement.

 

[damnyankee]

arska,

Will a OPA2134 work in lieu of the 5534?  Or is this not directly in the audio chain?

Thanks much!

DY

 

[abechap024]

Arska,
I'm not having the option of more than one VCA at work at a time. Put another way, people will only have ONE VCA on the board at a time. So pin 2 of the 2180 will be driven by one of the 4558 thru a 4.7k resistor and a 200ohm resistor. Am I missing something here or that should work well? Or do we need to change the 4.7k resistor to a lower ohm is using the 2180? Plus I'll also make a note of different resistor changes when using the 2180 - Like I know the 100k input resistor should be changed to 20k.
Thanks
AC

 

[abechap024]

arska,

Will a OPA2134 work in lieu of the 5534?  Or is this not directly in the audio chain?

I'm sure the opa2134 would work to replace the 4558 stock chip in te circuit. But to answer your question no a opa2134 won't replace a 5534 because the 5534 is a single chip!, but it is just buffering a DC voltage. I think why Arska put the 5534 there is because is probably the most convienient opamp most people have around. Pretty much any opamp will work there, with out too much of an effect on performance.

If we want to use LED metering, will we still need to alter the meter circuit? $150 for a meter is a bit much for me.

I agree! I left the meter section stock, so It should be able to hook up as is to an LED circuit. I honestly have not given it much thought, as I don't have much experience with LED circuits and the idea that this DBX uses a "center tapped VU" really threw me for a loop!! I don't know what that is and I don't know why DBX would use one. I figure one the circuit is in hand, we'll figure an easy way to fix it.

ac

 

[damnyankee]

Speaker of VU meters, I have a question (I'm learning)...

Can a traditional VU meter labeled -20 to 3+ be used instead of the dbx version labeled in decibels?

dbx 160VU: http://www.dancetech.com/aa_dt_new/hardware/images/dbx_160_main.jpg
Regular VU: http://www.sound-spot.co.uk/blog/wp-content/uploads/2009/08/VU_Meter.jpg

It appears the "0" marking is the same on both of them so as long as the voltage to drive the meters are the same, wouldn't both meters react the same?

Thank you in advance,

DY

 

[abechap024]

see thats what I figured? Any standard VU should ...but there has been talk that DBX had a special "center - tapped" monstrousity of a vu meter. I don't know if anyone can confirm this....

 

[damnyankee]

I'm going to the other board and see if Jim Williams or one of the others with this sort of expertise can help since David is on vacation.

DY