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Here are some shots of my newly built passive preamplifier. It also has a unity gain active buffer to output to a sub woofer:
Follow Ups:
Beautiful chassis layout.
Hi Fastcat, here's what I came up with. I used silver wire,silver toggles, 100k Alps Black Beauty pots,gold jacks,Dyna PAS-2 knobs. A little walnut from the hobby shop and a Simplisitor nametag from the trophy store.I made this about 15 years ago and use it with a Dyna ST35....Mark Korda
Fastcat, the front
Mark:
That has a very vintage appearance! How does your Dyna ST35 sound? I
had one of those many years ago.
Hi FastCat
Hi FastCat, the Dyna St-35 is all I need.I have a small room. This amp is so good they brought it back, you can still buy the kit.I rebuilt mine from scratch and forgot to ground the new gold jacks, so it made a trip to Van AlStines to find that one out.
I saw an article in AudioXpress years ago about a simple transformer assisted passive amp.I buit one in a Dyna PAM mono preamp chassis, 1958 with Electraprint transformers,silver wire, and Goldpoint !0k stepped pots.The aim is to boost the signal with the transformers.I shellacked a walnut piece from the hobby shop for the faceplate.I took a couple pictures years ago and just put it back in with the ST-35
Very nice! Where did you get the chassis?
The chassis can be had at the diyaudiostore.com
I'm always pleased by these kinds of builds. Love the precision and the exceedingly tidy layout !
What is the purpose/construction of the block with the (look like?) foam channels ?
What are the dark & light grey materials lining the bottom & F/B ?
The CS designation on the rear switch ?
opus64:
The large black block has medium density felt pads that rub against the brass shafts for volume and input selection. They act as dampers to micro vibration that might arise in the shafts (and possibly getting into the pot or selector switch contact wipers. The dark and light grey materials are extensional damping treatments on the case to reduce any resonant vibrations. The rear switch labeled "CS" is to switch in or out a 3rd wire grounding of the case as electrical shielding.
Thank you.
Kinda makes my Pass B1 look like a 6th grade science project.
Novus Ordo Seclorum
How does your B1 sound?
Fastcat, If I need some brain or heart surgery I would request you instead of a doctor.Your wiring is impeccable! I need to connect all my grounds on the jacks of a Dyna PAS project. I was wondering what kind/mil. of wire did you use to connect the ground tab jacks? Once again....fantastic job...Mark Korda
Mark:
Thanks for the comments! This was a slow project that went on
for years, but finally done, and well worth it.
The ground wiring is 19 gauge silver plated long grain copper by
Wonder (Wonder wire). The signal wiring is dead soft silver (99.9999%).
All of the wiring is "bare" to avoid dielectric effects. And Wonder Solder Signature soldering.
You've got some exposed AC line voltage in there.
Dave.
Minor AC exposure inside unit is no problem. Unit is kept buttoned up (top on), and as an audiophile with an electronics degree, I am well aware
of what exposed AC can do.
Doesn't everybody?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No.
Dave.
I'd like to upgrade my buffered passive to something like that. Beautiful work!
First class construction. I hope it sounds as good as it looks.
Beautiful construction! How does it perform?
M
It actually sounds more transparent than a previous passive unit that I built in 1988. I moved up to a 10K TKD pot with a resistor trick, and an Elma selector switch. Wiring is dead soft bare silver with Wondersolder. Closer to the idea of a "straight wire with gain", but also closer to showing all of the "warts" that exist in many digital recordings and CDs. I have a 9.5 KOhm input impedance, and am feeding an amplifier with a 120KOhm input impedance.
What is the total shunt capacitance that you will be driving (cable capacitance + input capacitance of the amp)?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"10K TKD pot with a resistor trick"
What is the resistor trick?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
The resistor trick here is having 1/2W 100K Takman REY metal film resistors between the input and the wiper contact, and the wiper contact and ground. I believe this was first discovered by the late Bob Crump. The TKD pot becomes a parallel variable resistor with the fixed resistors. The actual input value is only lowered slightly. I have not measured the capacitance of the interconnects, but they are very low, and 0.5 meter or less in length. The input of my amplifier has a very low value capacitor in series, and with 120 K Ohm input impedance to form a 1st order high pass at 94 Hz.
The 10k pot will have an output impedance of 2500 ohms at -6db attenuation.
The 100k resistors will lower that a little.1st order low pass at 94kHz will shift phase all the way down to 9.4kHz
If you want 20kHz undisturbed then all your low pass filters need to be at 200kHz min.
I'm sure it sound just fine.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/09/23
Oops! I meant high pass filter at 94 Hz. The unit sounds quite good, and has good dynamics with my cables and equipment.
I'm sorry. I totally misread your post.
I understand now that you have a high pass filter installed, keeping the bass out of the main outputs, with a -3db point of 94 hz.
I was interested and worried about the low pass filter that is created by the output impedance of your pots and the total shunt capacitance (cable plus Miller capacitance of the amp being used) that that impedance is having to drive.
With short cables you probably don't have a problem.
Again, sorry for my confusion.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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