[marlbly]

Well, I just had my el cheapo tube pre amp bugger up and need to whack together a quick passive pre for volume control 

I only have limited stuff on hand : a 100k linear dual gang 4 leg pot and two 180k 1% resistors, some decent silver wire and connectors etc.

I'm thinking of using those bits to put together a passive pre, but I'm not 100% sure about which pins I should hook up and whether the 180k resistor will be ok...



I think 2 is the wiper.  Resistance between 1 +2 and 2 +3 are inverse. Between 1 + 3 always 100k. Between 2 and 4 goes from 50 to 0 at half way back to 50.

Could someone please let me know if my schematic is right? Also will the 180K resistors be ok? I'm going from a DAC to an old Toshiba Aurex power amp. The Tube pre really wasn't suitable for where it was as you couldn't really go past 10 o'clock and TBH a passive pre is probably more suitable here anyway.

[Ethan Winer]

I don't know about pots with four terminals, but the correct way to wire a passive attenuator is like this:

Code: [Select]

>------------|
             |
             \
             /
             \<--------------> out
             /
             |
             |
           ____
           ___
            _

Hopefully the intent comes through past the bad ASCII art.

--Ethan

[Davey]

Your schematic would be correct if your intention is a shunt-style attenuator.  (Although the resistor values are too high.)

Best to configure for a standard, series-style as Ethan drew.  Leave pin 4 unused.  Pin 2 is the output, pin 3 is the input, and pin 1 is ground.

Cheers,

Dave.

[cheap-Jack]

Hi.

(1) I only have limited stuff on hand : a 100k linear dual gang 4 leg pot and two 180k 1% resistors, some decent silver wire and connectors etc.

I'm thinking of using those bits to put together a passive pre, but I'm not 100% sure about which pins I should hook up and whether the 180k resistor will be ok...



(2) I think 2 is the wiper.  Resistance between 1 +2 and 2 +3 are inverse. Between 1 + 3 always 100k. Between 2 and 4 goes from 50 to 0

(3)  I'm going from a DAC to an old Toshiba Aurex power amp. The Tube pre really wasn't suitable for where it was as you couldn't really go past 10 o'clock and TBH a passive pre is probably more suitable here anyway.

It sounds you were a cheaper jack than me.

(1) The linear pot (type B) you want to use is NO good for volume controlling. A type "A" pot (its internal resistance changes logarithmically, in line with our hearing characteristics) should be used.
So get a new dual ganged standard 100K type A pot. For only 2 bucks, no big deal. An Alp Blue Velvet would be ideal as it will sound much much bettter though it willl cost you more.

(2) Any standard pot gets only 3 pins to hook up. Following the same configuration of yr link, pin 1 is correct, pin 2 is for O/P to yr power amp. Pin 3 is for the signal return "ground".

(3) YES, if the gain was way too much - "couldn't really pass 10 o'clock", a passive control pot will surely do a much much superior job. Better headroom & therefore better sounding!

FYI, ALL my linestages are passive. Of course, I built them properly electrically, using finest silver plated
oxygen-free hook up wires.

G0 for it.

c-J

[Mark Korda]

Marbly,I made this passive preamp about 10 years ago.Those are Alps 100k pots.I used silver wire and toggle switches, and gold jacks.It is upside down so I could take the bottom cover off to photograph.I hope this helps.Sounds fantastic,I hope this helps you......Mark Korda

[Mark Korda]

Marbly,this is what it looks like.I got that wood from the hobby shop and had a trophy store make me a ingraved name....Mark Korda

It's a massive passive preamp so I rowed my dory up on it at high tide and waited for low tide to show the scale! I used knobs from a Dyna PAS-2.....Mark Korda,Portland,Me.

[Mark Korda]

Marbly,just in case I took a picture of the back.I used a Hammond aluminum inclosure and with many mistakes on the first boxes using drills,I bought a punch.A little costly,but it makes perfect holes just where you want them....Mark K

[Davey]

(2) Any standard pot gets only 3 pins to hook up. Following the same configuration of yr link, pin 1 is correct, pin 2 is for O/P to yr power amp. Pin 3 is for the signal return "ground".

No, I'm afraid not.

In his photo, if the control is turned fully CCW (minimum volume) pin 1 and pin 2 are now common.  Pin 2, being the output, should be at ground with this rotation position.
So, Pin 1 ground, Pin 2 output, Pin 3 input.

Cheers,

Dave.

[cheap-Jack]

Hi.

No, I'm afraid not.

In his photo, if the control is turned fully CCW (minimum volume) pin 1 and pin 2 are now common.  Pin 2, being the output, should be at ground with this rotation position.
So, Pin 1 ground, Pin 2 output, Pin 3 input.

Cheers,

Dave.

Yup, so reverse the pin connection on pin 1 & pin 3.

c-J

[cheap-Jack]

Hi.

Marbly,this is what it looks like.I got that wood from the hobby shop and had a trophy store make me a ingraved name....Mark Korda

It's a massive passive preamp so I rowed my dory up on it at high tide and waited for low tide to show the scale! I used knobs from a Dyna PAS-2.....Mark Korda,Portland,Me.

Being a sorta-kinda old radio collector, I like the old vintage look of yr passive linestage front panel.

How come the knobs of my 50-year-young PAS-2 look so much newer than yours?

One common overlooking by many DIYers is the proper running of the signal & signal return ("ground") wiring, active or passive regardless. The "ground" wiring of yr linestage needs re-run properly to get better sound, IMO.

For all the passive linestages I built, I pay tons of attention on signal & signal return ("ground") wiring.

c-J

[Mark Korda]

Hi Cheap jack,I don't remember ever cleaning the Dyna PAS-2 knobs.I know on the champagne knobs that Dyna used after the PAS-2 and FM-1 could show some grime if not cleaned up.I've never ever had any hum or noise from the way I grounded my passive preamp.All those jacks tabs,the little grounds on the sides of the jacks are all soldered together on the same wire.I don't know why,or forgot why to ground to the chassis.I have had all 4 high gain components,tuner,cd,dvd,ect,hooked up.When nothing is playing,both toggles are in the center position.I just flip one down or up to receive the signal.When I listen to my cd player,I just flip one toggle,where the cd is conectted down,and I'm getting the best sound I ever have had.I built a Dyna-ST-35 and forgot to ground the jacks,I put in new gold ones,not like Dyna did,and had their jacks all ready grounded to the chassis,and I got a terrific buzz,without drinking or smoking,to let me know about the ground not being grounded.I also picked up Pat Boones April Love with out any source hooked up yet.Frank  VanAlStine found my mistake.Cheap Jack,with the info I gave you,why isn't my pre amp giving me some clue about the ground instead of pristine music.Could some other component be grounded taking care of that?Some one gave me the answer on the grounding when I built it but it eludes me now.By the way,I told you I got a nice piece of wood for the face plate at the hobby shop,but if you have a Rockler wood product store close,they were not around here as yet,they have some real wild cool looking thin pieces of wood,not veneer,about 1/8 inch thick.Thanks for writing me.....Mark Korda

[marlbly]

Thanks Dave, that's exactly what I needed to know. I was planning on going the shunt style as I don't know how good the pot is but ill just have to wire it straight through instead.
It's only meant to be temporary until I can get over to the nearest electronics supply.

So if I was going to make a proper shunt style attenuator, what would I use? A 100k log pot and 10k resistors? I vaguely recall seeing a schematic somewhere with these values.

Also since I don't need multiple/switched inputs I'm thinking it probably is better to just wire directly to the pot eliminating the jacks. With grounding the plugs, on the input side ground should be connected to the plugs but not on the output plugs right?

[Quiet Earth]

One common overlooking by many DIYers is the proper running of the signal & signal return ("ground") wiring, active or passive regardless. The "ground" wiring of yr linestage needs re-run properly to get better sound, IMO.

For all the passive linestages I built, I pay tons of attention on signal & signal return ("ground") wiring.

Hi c-j,

Can you be a little more specific about grounding. I'm always looking for better ways to improve my passive volume control. Tips and tricks, etc.

Thanks man!,
Q.E.

[marlbly]

So I finished it off yesterday. Well it works! Unfortunately it came with some really bad news. When I went down to my garage to drill the holes through the case I discovered that my power tools had been stolen. I know who did it too, we rented a room out to someone recently and apart from turning out to be PITA it also looks like he helped himself to my tools when he left   
Really makes you wonder about some people and human race sometimes.
*sigh* anyway ...

Definitely better soundstage and channel separation than the tube pre, though that design did suffer from crosstalk. Definitely more listening fatigue though. I do miss that tube sound though I found it far more listenable before.

[cheap-Jack]

Hi.

Hi c-j,

Can you be a little more specific about grounding. I'm always looking for better ways to improve my passive volume control. Tips and tricks, etc.

Thanks man!,
Q.E.

A little so called 'design secret' to be released to those who do seriously love passive quality.
Please send me a PM so I can tell you discretely.

c-J

[Quiet Earth]

PM sent.