dbx 160vu clone

[damnyankee]

More vca info, correct schematic for vca200

Hi arska,

I gotcha; I missed the 200VCA schematic; the file is mainly on the 202 & 2001 VCA.  ;D

DY

 

[damnyankee]

I guess my next question is:

There's rumour there are two different 200 VCAs...one that went for the 160VU and the others which didn't meet specs went to the 117/119.  The analogy as I understand is similar to the EV RE15/RE16 mics which when didn't meet QC standards were relabeled RE10/RE11 respectively.

Do you know if this is true???

DY

 

[arska]

More vca info, correct schematic for vca200

Hi arska,

I gotcha; I missed the 200VCA schematic; the file is mainly on the 202 & 2001 VCA.  ;D

DY

Daunload dbx202.pdf, on the first page is schematic of 200 and 202/203

 

[David Kulka]

I was skimming over this thread. Hope the project works out for you guys. I had a number of thoughts and later I'll try to post some of them. But for now...

So many questions about what circuits or modules to use for the VCA and RMS detector. Here's a thought. The dbx 303 cards (there were 4 or 5 versions) were used in various dbx noise reduction models, as well as the early Eventide harmonizers and Marshall Time Modulators. They include an RMS detector and a VCA, which are brought out to separate pins. Both sections were and are modify-able, and were configured in many different ways for dbx type 1, dbx type II, etc.

Probably with some R&D these sections could be modded to match the action of the VCA and RMS Detector in the 160. DBX NR used various pre-emphasis / de-emphasis curves where the 160 should of course be flat; this would not be hard to do.

Maybe I'll get motivated and R&D the changes for a 303 board. If so I would post the schematic and BOM gratis, then probably sell pre-modded 303 boards. These boards are not very hard to find but one snag is the different versions of 303 board -- there are many types out there and the mods would be a little different for each version.

The 200 and 208 are no doubt a major part of the 160 VU "sound". Whatever solution is used for the 200/208, you'll want to be very careful to stay as close as possible to the original operating parameters. One compromise with this solution is that the 200/208 were discreet and the 303's use opamps  (though better ones could be subbed).

This is a dbx 303 B version board:

 

[damnyankee]

Hi David,

Thank you for offering your expertise.  I think we're close to cloning the original.  All the parts for the main board, 200 VCA & 208/209 RMS are still readily available.  The only part unavailable of course is the VU meter (I contacted you about your re-pro'ed meters).  Oh, and the original 160VU box; I have no idea how to fabricate that so we're probably going with a standard 19" width rack..

As you are a dbx repair center, I have a question.  How much does the "under designed" power supply contribute to the unique 160VU sound?  A poster off our other board stated Les Tyler/dbx told him the power supply did play a role.  We've discussed this back and forth to keep the original power supply with the LM723 so in your experience, does the power supply contribute to that unique sound?  I think Abe is going to build the original power supply yet create jumpers so a modern power supply can be added if the cloner chooses to.

Your thoughts?

Thank you!

 

[abechap024]

I think It will be wise for someone to offer matched transistor pairs for the VCA. For someone to buy in bulk and then match the different resistors would probably a great investment for all - the person matching gets compensated for their time and the people buying don't have to spend a ton of money on transistors. I will volunteer myself and could just offer the option when sending for PCB

Lots of great info comming in.
thanks to all
AC

 

[damnyankee]

Hi AC,

I've been reading up on the importance of matching transistors (I'm an old tube analog guy; even my stereo is an old 1966 McIntosh 7591 preamp & amplifer...eek!). 

According to this article (http://sound.westhost.com/transistor-matching.htm):

"Matched transistors will rarely give you "better sound" (whatever that is supposed to be). In general, distortion will be almost completely unaffected, and there is no influence on frequency response or transient response. What you will get (for paralleled devices) is greater reliability, because the transistors will share the current more equally. "

It would be interesting to do an A/B .wav file of a dbx 160VU clone with match transistors and unmatched transistors...

DY

 

[johnR]

According to this article (http://sound.westhost.com/transistor-matching.htm):

"Matched transistors will rarely give you "better sound" (whatever that is supposed to be). In general, distortion will be almost completely unaffected, and there is no influence on frequency response or transient response. What you will get (for paralleled devices) is greater reliability, because the transistors will share the current more equally. "

That might be true in power amplifiers, but VCAs are a special case. They need to be matched.

 

[abechap024]

Hi all
I have a question. I'm finalizing transistors and have found quite a few replacement choices all with different footprints of course, *sigh*.

So for the VCA we can have the NTE123/NTE159 they are in the big case and cost about 1.50 a piece. but they do offer matched pairs for not much more, I believe about 3.00$. We will need 2 matched pairs.
Or we could just used BC550/BC560 and they are only about 20cents a piece much more widespread.

Decided to use BC550/60  pair in VCA because they are cheaper and  easier to find matches



AC

 

[damnyankee]

AC,

I think I found us a source for those LM723CH 10 pin voltage regulators.  In stock.  Under $2 each.  I've sent an email to confirm TO configuration, manufacturer, and to see how many are in stock.  Keep yer fingers crossed.

As far as the BC263 cross-references to NTE159: Mouser sells matched NTE159's for $3.20 for the matched pair; they run $1.38 each unmatched so heck, isn't it worth 48 cents extra to buy two of them matched?

Finally, the BC109C cross-references to NTE123A: Mouser sells matched NTE123A AND NTE123AP for $3.20 for the matched pair; will this combination work???

Really, the only one I'm stuck on is Q3: KE3687 / U2749.  Any suggestions???

DY

 

[abechap024]

DY, at mouser they have the lm723 in dip-16 version. Much easier to come by, and its the pin out  I have on the PCB and its only cents compared to dollars. Circuit wise its the same.
Great! So Just finished looking over the RMS and VCA to double check schematics and everything is good.
I can almost smell those prototypes....

 

[damnyankee]

Excellent!!!

DY

 

[David Kulka]

Hi David,

Thank you for offering your expertise.  I think we're close to cloning the original.  All the parts for the main board, 200 VCA & 208/209 RMS are still readily available.  The only part unavailable of course is the VU meter (I contacted you about your re-pro'ed meters).  Oh, and the original 160VU box; I have no idea how to fabricate that so we're probably going with a standard 19" width rack..

As you are a dbx repair center, I have a question.  How much does the "under designed" power supply contribute to the unique 160VU sound?  A poster off our other board stated Les Tyler/dbx told him the power supply did play a role.  We've discussed this back and forth to keep the original power supply with the LM723 so in your experience, does the power supply contribute to that unique sound?  I think Abe is going to build the original power supply yet create jumpers so a modern power supply can be added if the cloner chooses to.

Your thoughts?

Thank you!

I wish I had a 160 here to confirm this, but I have not noticed any undue noise, softness, or lack of regulation on the 160 power supply rails so I don't see the PSU as a weak link in the unit or a factor in the sound. It certainly can't hurt to beef things up a bit but I don't think sonics will be affected either way. I really think it's all about the VCA and the sidechain circuit (which were both quite groundbreaking at the time) and the fact that it is a feed forward type compressor. (Was it the first one commercially produced?)

I agree that transistor matching is very critical in this circuit. I know that dbx used a lot of tricky parts selection and QC procedures to overcome the difficulties in using somewhat primitive semiconductors to execute their new and ambitious design concepts. One neat and somewhat challenging thing about these early dbx designs is that they are mostly direct coupled and based on current, not voltage. There's a layer of difficulty to understanding and troubleshooting these designs, because a lot of the circuit elements are hard to measure, and hard to visualize. I'm still amazed, impressed, and sometimes baffled by David Blackmer's brilliant ideas.

 

[abechap024]

Thank you David, your insight is much appreciated.
Regards
Abe

 

[shabtek]

@ David K
any hints/reverse engineering of those DBX cards would be appreciated.
I'm yet to decode the pin-out aside from the rails.
Looking forward to 160 action; keep up the good work boys.

 

[arska]

AC,

I think I found us a source for those LM723CH 10 pin voltage regulators.  In stock.  Under $2 each.  I've sent an email to confirm TO configuration, manufacturer, and to see how many are in stock.  Keep yer fingers crossed.

As far as the BC263 cross-references to NTE159: Mouser sells matched NTE159's for $3.20 for the matched pair; they run $1.38 each unmatched so heck, isn't it worth 48 cents extra to buy two of them matched?

Finally, the BC109C cross-references to NTE123A: Mouser sells matched NTE123A AND NTE123AP for $3.20 for the matched pair; will this combination work???

Really, the only one I'm stuck on is Q3: KE3687 / U2749.  Any suggestions???

DY

KE3687= 2N3687    I am using J201 jfet

NTE123A IS NOT close enough  to BC109C,          use BC549C,BC550C. 2SA970GL  etc..


Why not use THAT corporations transistor arrays?????    THAT340 is possible solution.....2npn 2pnp in same die.....just an idea, not for clone projects.
...or MAT02/MAT03

 

[damnyankee]

[quote author=arska]
KE3687= 2N3687     I am using J201 jfet

NTE123A IS NOT close enough  to BC109C,           use BC549C,BC550C. 2SA970GL  etc..

Why not use THAT corporations transistor arrays?????    THAT340 is possible solution.....2npn 2pnp in same die.....just an idea, not for clone projects.
...or MAT02/MAT03

[/quote]
Hi arska,

Thank you for the heads up on the NTE123A.  I will be following your recommendations on the BCxxx transistors.

As for me, I'll be sourcing all the original components to create a faithful clone (with the exception of the VU meter of course).

As for the THAT chips; it's a great question.  My first question is: would these various THAT chips make the unit sound more like the newer generation 160_ (x, xt, a, s, sl) compressors?  I don't know...

DY

 

[David Kulka]

Here's the schematic of the 303-A Board. This is the early type with 4 vertical trimmers. It is not nearly as stable as the versions that came later. I only chose this schematic because it is closest to the 200/208 design, and is the basis for the RMS block diagram (to follow). The VCA section is on top and the RMS Detector section is on the bottom. The pinouts are the same for all versions of this board. QA1 is a CA3046 IC and QA2 is a CA3083. Both contain matched transistors.

(Come to think of it, maybe the CA3046 and CA3083 could be used in your clones. Those IC's are not too hard to find.)

NOTE:  I see that resolution is fairly bad on these posted images. I'm willing to upload them to my own site and post links but I'm going out of town soon. If someone else here can publicly host high resolution versions, let me know and I'll email them today.

 

[David Kulka]

Now, here's a block diagram of the RMS Detector section. I don't know who created this, but obviously a lot of thought went into it, and it contains a lot of very helpful information!

Remember that this block diagram and the schematic are for the noise reduction version of the RMS Detector, and so they contain a pre-emphasis / de-emphasis section that will not be found on the 208.

The Blackmer VCA and RMS circuits were both ground breaking designs, but it was the RMS Detector that rocked the audio design world. VCA's already existed, but the RMS Detector was a whole new animal that took an audio signal, discarded troublesome LF and HF information, and outputted a highly dependable but "low bandwidth" log DC voltage with ultra fast attack time and slow-ish release. It was "just right" for controlling a VCA for audio noise reduction applications and unlike Dolby, did not require calibration for correct decoding. (Dolby ultimately won the format war of course, but to me the dbx concept is much more interesting!)

 

[damnyankee]

David,

Thank you for sharing your expertise.  I love old dbx compression - there's nothing like it.  The more I learn about these old VCAs/RMSs, the greater appreciate I have for the complexity of the design considering they are built from only a handful of common components.  It's brilliant engineering, no question about it.

That 303 looks eerily similar to the one in my dbx 118; I'll have to check that when I get home later tonight.

I remember all to well the dbx vs Dolby NR war.  I'm looking through the eyes of a teenager, but I seem to recall once Dolby was able to get his Dolby B circuit integrated onto the home cassette tape deck market, everything snowballed for Dolby and dbx wound up being at the bottom of the hill.  However, it seemed Dolby kept putting out different versions of noise reduction (C, then S) and after that, I lost track.

Anyway, interesting stuff!!!  Thank you again for your contributions!

DY