Dbx Vca Construction

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Aha - now I get it, yeah, no, I meant I can redraw the DBX VCA PCB, haha, oh lord
I just read my first post and now I see what damnyankee meant - if you suppose it's
the whole device I mean I guess my post reads "Hmm I was thinking about building
one of your units but considering the VCA thing it probably sucks" or something like that  ;D

And then offering a "redraw" just adds oil to the fire, haw...
 
Livingnote,
If you really wanna redraw this sucker, let me know... I'll send you some 1200dpi scans (as seen on pg 1)


 
Lol! yea Sorry livingnote  ;D I need to read twice and type once  ::) and sometimes I think I need to read twice, think and repeat!
Anyway if you want to redesign the VCA pcb I would be interested to see the result, but think for me, at this point the PCB isn't the issue(hopefully)...figuring out how to trim/choose is resistors is my big question.
Thanks
Abe
 
Haha yeah - well you're not the only one, I guess I wasn't too explicit...so now I went looking
through the thread "this is what you get when you don't read every post" and then "hey but wait
a minute, what's that? No other PCB?"

Absolutely Davo, I'm in :)
 
Biasrocks said:
I think the biggest issue is going to be matching and thermally coupling the transistors.

I can make a coupler like the original one except out of copper or aluminium. The transistors would fit finger tight with a touch of heat transfer compound applied.
 
Thanks! If I am off to fight the dragon I am going to need an ultra big heat-sink!! ;)
Also, Silent:arts It might not be worth the time to make a heat sink like the one pictured. it might be worth noting that those bc109 transistors aren't being made anymore and the only replacements i could find are not in the same rounded can design, (the TO-19 I believe?) so I was just thinking of making the flat parts of the 2 pairs of replacement transistors (bc550c) face each other and then glue and wrap some sorts of metal ring around them all. But that was before people brought up the transistor array idea. Now I think a matched transistor array is the only way to go...

Plus if we get an easier PCB, If your going to go through the trouble of making a pcb livingnote, why don't you change the 4 thermally coupled and matched resistor to a IC chip transistor array. In retrospect its something I should've done on the 160...
 
Yep those are the ones. The That 340 4 transistor chip, should work in there dip14...http://www.thatcorp.com/datashts/300data.pdf
there's the data.
Though I haven't bread boarded and tested any of this.

AC
 
I've done this years ago, when I needed a VCA for my self designed compressor.
At the time I used selected BC547B/BC557B transistors. (Just because I had a lot of them...)
I matched them for Hfe and Vbe (identical values for PNP/NPN) and I got excellent results.
Low distortion, low control voltage feedthrough etc.
I glued the transistors to a copper strip, and encapsulated the whole assembly in expoxy resin.
It brings back a lot of memories, so I am follwing this thread with interest!
 
RuudNL said:
I've done this years ago, when I needed a VCA for my self designed compressor.
At the time I used selected BC547B/BC557B transistors. (Just because I had a lot of them...)
I matched them for Hfe and Vbe (identical values for PNP/NPN) and I got excellent results.
Low distortion, low control voltage feedthrough etc.
I glued the transistors to a copper strip, and encapsulated the whole assembly in expoxy resin.
It brings back a lot of memories, so I am follwing this thread with interest!


Great ;D good to hear that someone has successfully DIY a discrete VCA  ;D I knew it was possible but honestly your the first person to say they have successfully made one. Thanks for chiming in!
Abe
 
They look like this:
(Only the transistor array in a DIL-14 enclosure)

VCA.jpg
 
RuudNL said:
I've done this years ago, when I needed a VCA for my self designed compressor.
At the time I used selected BC547B/BC557B transistors. (Just because I had a lot of them...)
I matched them for Hfe and Vbe (identical values for PNP/NPN) and I got excellent results.
Low distortion, low control voltage feedthrough etc.
I glued the transistors to a copper strip, and encapsulated the whole assembly in expoxy resin.
It brings back a lot of memories, so I am follwing this thread with interest!

Very, very cool - and thank you for sharing!

The hfe matching seems straight forward but I am curious how to do the Vbe matching...?  Can it be done without building a testing circuit?

Thanks in advance,

DY
 
Some (even cheap!) DVM's have a diode tester that idicates the voltage across the conducting diode junction. You could use this to measure the B-E voltage.
Otherwise you could use a fixed voltage with a series resistor. (Say: 10 volts and 10 K.Ohms)
Then measure the voltage across the B-E junction. Take care of the temperature! Touching a transistor with your fingers will influence the B-E voltage.
For best results you should measure at different currents, so compare the voltages with a series resistor of 1 K.Ohms and 10 K.Ohms. In this case you measure the junction voltage at (almost) 10 mA and 1 mA.
If you get a good match at those two currents, you can assume that the match is close enough!
 
I don't want this to sound like I am actually encouraging this pursuit, but here is one way to match the devices for beta and Vbe at the same time.

It requires a bunch of precision resistors, + and - power supplies, and a pile of transistors.

First connect 10 or 20 transistors together at their emitters. So this voltage will be the same by definition. Connect each individual base to ground through a precision resistor (20-100k). Then connect each collector to the appropriate supply (negative for PNP, positive rail for NPN) through individual precision resistors (say 2k). Then connect the common emitter through a low value resistor to the other supply as appropriate (plus for PNP, minus for NPN). Size this common emitter resistor, and voltage it is connected to for the test current you want times the number of devices. So if you want 1 mA in each of 20 transistors you will want 20 mA total supplied at this common emitter node.

With all the emitters tied together, they will share the current between them based on the individual differences in Vbe. Further the resistor in series with the base will give you a direct reading of beta and further influence the current sharing. With a number of transistors connected, a quick reading of the collector voltages (at the 2k resistors) will indicate best Vbe and beta matching within the group. After identifying the best collector current matches, confirm that their base resistors read the same too. Weed out any where both collectors and base voltages don't match. You can confirm matching at several different currents by varying the total current sent to all transistors.

Depending on how many transistors you have and can test at once you can either pre screen all for closest rough matches first or just weed out and replace the worst ones from the 10 or 20 being tested concurrently as you converge on the best matches within your group. It might be nice to use one of those plastic plug in prototype strips to hold a bunch of devices at once without soldering to the leads which will heat up the part.

You might wait until after getting a promising group of matches before heat sinking them together for final best match, to save time. The bad outliers are not likely to get better after being heat sunk, but you want the best matches that stay matched when held to the same temperature.   

JR

PS: The VOM diode tester may be an easy way to prescreen devices but i wouldn't trust it for final matching.


 
All this points to one person going into mass production for everyone else.  Takers?   
 

Latest posts

Back
Top