My New DIY Passive Preamplifier

RE: My New DIY Passive Preamplifier, posted on February 7, 2023 at 05:33:40
Steelhead Mike Audiophile Posts: 104 Location: New England Joined: September 2, 2000	Beautiful construction! How does it perform? M

RE: My New DIY Passive Preamplifier, posted on February 7, 2023 at 14:09:51

fastcat
Audiophile

Posts: 149
Location: Midwest
Joined: May 30, 2003

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.

RE: My New DIY Passive Preamplifier, posted on February 8, 2023 at 08:53:15
Mike B. Audiophile Posts: 26352 Location: OR Joined: September 27, 1999 Contributor Since: September 1, 1999	First class construction. I hope it sounds as good as it looks.

Where did you get that enclosure?, posted on February 9, 2023 at 07:15:41
1973shovel Audiophile Posts: 10117 Location: Greenville SC Joined: February 25, 2007	I'd like to upgrade my buffered passive to something like that. Beautiful work!

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 07:32:22
Tre' Industry Professional Posts: 17297 Location: So. Cal. Joined: February 9, 2002	"10K TKD pot with a resistor trick" What is the resistor trick? Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 09:57:35
Davey	You've got some exposed AC line voltage in there. Dave.

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 11:02:16
Tre' Industry Professional Posts: 17297 Location: So. Cal. Joined: February 9, 2002	Doesn't everybody? Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 11:20:58
Tre' Industry Professional Posts: 17297 Location: So. Cal. Joined: February 9, 2002	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"

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 13:11:48
Davey	No. Dave.

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 13:24:19
fastcat Audiophile Posts: 149 Location: Midwest Joined: May 30, 2003	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.

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 13:32:25

fastcat
Audiophile

Posts: 149
Location: Midwest
Joined: May 30, 2003

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.

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 15:01:53

Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002

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"

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 18:30:52
fastcat Audiophile Posts: 149 Location: Midwest Joined: May 30, 2003	Oops! I meant high pass filter at 94 Hz. The unit sounds quite good, and has good dynamics with my cables and equipment.

RE: My New DIY Passive Preamplifier, posted on February 9, 2023 at 19:25:05

Tre'
Industry Professional

Posts: 17297
Location: So. Cal.
Joined: February 9, 2002

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"

RE: Nice!, posted on February 11, 2023 at 05:52:51
B. Scarpia	Kinda makes my Pass B1 look like a 6th grade science project.

RE: My New DIY Passive Preamplifier, posted on February 12, 2023 at 22:07:04
opus64 Audiophile Posts: 23 Joined: September 7, 2009	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 ?

RE: My New DIY Passive Preamplifier, posted on February 19, 2023 at 09:06:41

fastcat
Audiophile

Posts: 149
Location: Midwest
Joined: May 30, 2003

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.

RE: My New DIY Passive Preamplifier, posted on February 19, 2023 at 20:02:38
opus64 Audiophile Posts: 23 Joined: September 7, 2009	Thank you.

RE: My New DIY Passive Preamplifier, posted on March 1, 2023 at 20:51:01
mingles Audiophile Posts: 158 Location: SF Bay area, Berkeley Joined: April 9, 2006	Very nice! Where did you get the chassis?

RE: My New DIY Passive Preamplifier, posted on March 3, 2023 at 17:40:10
fastcat Audiophile Posts: 149 Location: Midwest Joined: May 30, 2003	The chassis can be had at the diyaudiostore.com

RE: Nice!, posted on March 3, 2023 at 17:41:00
fastcat Audiophile Posts: 149 Location: Midwest Joined: May 30, 2003	How does your B1 sound?

RE: Nice!, posted on March 10, 2023 at 14:57:53
mark.korda@myfairpoint.net Audiophile Posts: 1253 Location: Maine Joined: August 16, 2011	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

RE: Nice!, posted on March 10, 2023 at 19:02:13
fastcat Audiophile Posts: 149 Location: Midwest Joined: May 30, 2003	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.

So, you have a muted output and a direct output?, posted on March 14, 2023 at 09:17:05
jedrider Audiophile Posts: 15166 Location: No. California Joined: December 26, 2003	Beautiful chassis layout.

RE: So, you have a muted output and a direct output?, posted on March 15, 2023 at 08:57:46
mark.korda@myfairpoint.net Audiophile Posts: 1253 Location: Maine Joined: August 16, 2011	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

RE: So, you have a muted output and a direct output?, posted on March 15, 2023 at 08:59:35
mark.korda@myfairpoint.net Audiophile Posts: 1253 Location: Maine Joined: August 16, 2011	Fastcat, the front

RE: So, you have a muted output and a direct output?, posted on March 15, 2023 at 13:22:18
fastcat Audiophile Posts: 149 Location: Midwest Joined: May 30, 2003	Mark: That has a very vintage appearance! How does your Dyna ST35 sound? I had one of those many years ago.

RE: So, you have a muted output and a direct output?, posted on March 18, 2023 at 20:46:06

mark.korda@myfairpoint.net
Audiophile

Posts: 1253
Location: Maine
Joined: August 16, 2011

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

RE: So, you have a muted output and a direct output?, posted on March 18, 2023 at 20:49:27
mark.korda@myfairpoint.net Audiophile Posts: 1253 Location: Maine Joined: August 16, 2011	Hi FastCat

Tweakers' Asylum

My New DIY Passive Preamplifier

99.70.239.115