Racking a Tascam DBX Card

Hello,
I have a spare dbx card from my Tascam 85-16B tape machine that I would like  to rack with an external PSU and use the encode mode as a fixed ratio compressor.
I have a PSU that supplies +15V , -15V and 0V.
So I need to connect the input signle to connection point 1 on the dbx pcb, the +15V to connecttion point 2, -15V to connecttion point 4, output signal to connecttion point 5, 0V to connecttion point 6.
Now what should I do with connecttion points 3, 7, 10, 11 and 12?

Also when I'm connecting the +15V, -15V and 0V to the card, the card is operating in decode mode, how should I connect connection point 12 so it will operate in encode mode?

Here's a link to the dbx card schematic:
https://drive.google.com/file/d/1WN3JaRWrSEwRB0TRbPSbPQfSCVzeHiZo/view?usp=sharing

As I recall dbx NR is not only a fixed 2:1 compressor but has roughly 12 dB of HF pre/de-emphasis... i.e. HF boost during encode.

To defeat the HF pre-emphasis you probably need to remove a few parts (capacitors) in audio path and the side chain that will cause similar HF boost.

JR

PS: The HF emphasis boost from Dolby B cassette NR was more gentle and many tolerated the extra compressions and sizzle, the dbx HF boost is less gentle IMO. 

smilan said:

Here's a link to the dbx card schematic:
https://www.keepandshare.com/doc14/22791/dbx-pdf-2-1-meg?da=y

Click to expand...

I can't see it. It asks for login.

moamps said:

I can't see it. It asks for login.

Click to expand...

I'm sorry. this link should work
https://drive.google.com/file/d/1WN3JaRWrSEwRB0TRbPSbPQfSCVzeHiZo/view?usp=sharing

smilan said:

I have a spare dbx card from my Tascam 85-16B tape machine that I would like  to rack with an external PSU and use the encode mode as a fixed ratio compressor.

So I need to connect the input signle to connection point 1 on the dbx pcb, the +15V to connecttion point 2, -15V to connecttion point 4, output signal to connecttion point 5, 0V to connecttion point 6.

Click to expand...

IMO, for output you should use enc send pin.

Also when I'm connecting the +15V, -15V and 0V to the card, the card is operating in decode mode, how should I connect connection point 12 so it will operate in encode mode?

Click to expand...

Mode can be flipped by connecting pin 12 to gnd.

Thanks,
I've connected the card as flowing:
1---------- Input
2---------- +15V
3----------X
4---------- -15V
5----------X
6---------- Gnd
7---------- Out
8---------- X
9---------- X
10-------- X
11-------- X
12-------- Gnd

This is how it should be connected?
It works but it's very noisy.
As you can hear in this video, when the dbx card is in bypass mode it's quite but when it's in encode mode it's very noisy.
https://youtu.be/UBbnbIEdn2k

The dbx encoder compresses higher levels, jacks the low levels way up, and pre-emphasizes HF.

The loud buzz in your recording  tells me that there's some probably some lesser amount of buzz at the input of the card, which the encode process is exaggerating, or there's some kind of grounding problem at the interconnect, or the power supply for the dbx card isn't clean.  Regardless, I don't think there's any way to make a useful audio processor out of a dbx encoder card. Music in, garbage out. Sorry. 

smilan said:

Thanks,
I've connected the card as flowing:
1---------- Input
2---------- +15V
3----------X
4---------- -15V
5----------X
6---------- Gnd
7---------- Out
8---------- X
9---------- X
10-------- X
11-------- X
12-------- Gnd

This is how it should be connected?
It works but it's very noisy.
As you can hear in this video, when the dbx card is in bypass mode it's quite but when it's in encode mode it's very noisy.
https://youtu.be/UBbnbIEdn2k

Click to expand...

As I tried to explain before this is the encode half of a (tape) noise reduction system. The way it works is a full range 2:1 compressor that boosts up the low level signals and reduces high level signals. A symmetrical playback decoder expands that compressed signal back out 1:2 restoring the original dynamics while at the same time expanding down path noise (like from a tape recorder). In addition to this 2:1 compression there is 12 dB of HF boost, with a symmetrical 12 dB of HF cut in the decoder.

You could defeat the HF pre-emphasis by removing a few caps, but would still be left with all that compression.

I probably won't try to explain this to you again, but who knows?

JR 

I'm understanding what yo're saying.
I would like to use this 2:1 compressor between my mixer aux send to the input of my Roland re-201 echo machine to reduce loud signals that causing  too much saturation.
When I connecting the re-201 to my Tascam 85-16b (in input mode with the dbx cards are in encode mode) it works very good for this purpose.
This is why I'm trying to connect this dbx card to external PSU (it doesn't make sense to turn on the tape machine each time I would like to use the echo).

smilan said:

I'm understanding what yo're saying.
I would like to use this 2:1 compressor between my mixer aux send to the input of my Roland re-201 echo machine to reduce loud signals that causing  too much saturation.
When I connecting the re-201 to my Tascam 85-16b (in input mode with the dbx cards are in encode mode) it works very good for this purpose.
This is why I'm trying to connect this dbx card to external PSU (it doesn't make sense to turn on the tape machine each time I would like to use the echo).

Click to expand...

It will reduce loud sounds that are above it's internal 0dB (unity gain) threshold  (while boosting quiet sounds that are below threshold).

You can play around with hitting it harder (louder) to realize gain reduction, then scaling the output level as needed.

You will still have the HF pre-emphasis.

JR