Comparing battery-powered phono pre-amps, from RS ($$) to ASR ($$$$) ... and beyond!

[laatsch55]

1/4 watt looks like..

 

[derek92994]

I am going to keep the original form factor/metal box, I like the size and the metal provides good shielding. Gold plated RCA jacks are also in the pipeline.

Spent some time listing and checking components. Plus checking resistor color codes against the schematic 3D posted, and learning which way they go.

Transistors
4x 2SC1740 -- replace with low noise version, possible replacement of something even better?

Caps upgrade low values to red wima caps e.g. 50v/1uf/0.015uf/47pf (c3,c4,c9,c12) the rest high audio grade electrolytic (c13/c10/c5/c6)
c11 16/470uf or 25/470uf (power cap)
c13/c10 16/10uf
c2/c1 50/1uf
c5/c6 25/47uf

C3/C4 0.015uf use red wima caps
C7/C8 0.0047uf (maybe leave these alone due to space restrictions, as C9/C12 upgraded will take up a fair bit of space)
C9/C12 47PF use red wima caps

Resistors upgrade to tight tolerance metal film
R1/R2 56K
R3/R4 1K
R5/R6 460 ohm
R7/R8 22OK
R9/R10 120K
R11/R12 460K
R13/R14 2.2K
R15/R16 16K
R17/R18 12K

Some progress there, comments welcome.

 

[derek92994]

Here's a likely replacement for the C1740 transistors, (BC237B) datasheet seems very close to the original.



BC237B PBFREE Central Semiconductor | Mouser Australia

Here is the datasheet for the original C1740 transistor:

 

[derek92994]

Hi 3D, parts are on order, using 470uf cap for storage. Its good to know that the 1000uf is still manageable. You will have a powerbank with that many caps.

All resistors and caps are being replaced with the tightest tolerance possible along with replacement transistors. Only exception is the 2x 50v/1uf capacitors which will be 5% and there was a pair of resistors that I could only get 2%, all the rest will be 1%. The remaining electrolytics will be 20% audio grade nichicons.

Its great how far you plan to take the experiments and testing. I heard a slight difference between the two audio files using akg reference headphones. Maybe even more difference will be observed with the higher quality parts. I will also test with 2 individual upgraded units, 1 channel per preamp.

 

[derek92994]

Game on

 

[laatsch55]

Get IT Darc!!!!!!!!!!!!!!!!!!!!!!!!!

 

[derek92994]

Got it, yes. Did about 10 hours work on it. Had to bypass a few tracks due to losing the solder pad. There were challenges due to capacitor size, managed to squeeze them in. The nichicon muse caps would not fit into the connector holes, the legs on them were too thick. I resorted to putting in separate legs and mounting the caps horizontally, not ideal and looks messy but it works, and thats all that matters.

Those BC237B replacement transistors have the wrong pinout, fuck! Had to put the C1740’s back in.

First test run the left channel was dead, tested Q1 which had an abnormally high value, took it out and tested out of circuit, no issues. Found C5 and C6 almost shorting, trimmed the legs to fix. Put Q1 back in, tested ok. tested for continuity and shorts all over the board. Now Im listening and it sounds sweet. Will record that same track used last time for comparison and share soon. No pics of soldering side of board, its a war zone
Rca input/output replaced with gold plated type.

Cap/Resistor make and type used:
Koa Speer Resistors mostly 1%
Wima MKP, FKP 2.5%
Cornell Dubilier CDE Grey Film (For the 50v/1uf 5% Cap, couldnt find Wimas in this value)
Elna Silmic 2’s (Electrolytic)
Nichicon UES and UKZ (Electrolytic)

Stocked up on 50v/1uf Nichicon Muse BP green caps, just for shits and giggles, these come in handy, forever running out of them.

By the way, used sil pads are handy to protect the board from the jaw clamps.

 

[J!m]

It’s a single sided board; I’ve been known to open the holes where needed. Never on a multi-layer board however.

Looks good from here!

 

[laatsch55]

A war zone eh?? Funny man Darc!! Good job getting everything on there!!

 

[derek92994]

It’s a single sided board; I’ve been known to open the holes where needed. Never on a multi-layer board however.

Looks good from here!

Can this be done without destroying the pads?

 

[laatsch55]

If you are very careful and hand drill from the pad side
..I've done it quite a bit Darc..

 

[George S.]

Yes

 

[laatsch55]

Thanks George! Exactly, I use a cutting torch tip cleaner, the drill bit type, comes with lots of bits. Just too damn handy..

 

[derek92994]

Need to find upgrades for those C1740's down the track, it is not finished until those are replaced. May have a go at making those holes bigger for the muse caps once those transistors are found.

 

[derek92994]

Audio file of Old Caps:
https://www.dropbox.com/s/yz0103gg9xsgy2i/Rat Shack Stock Caps.wav?dl=0
Audio file of Upgraded Caps 2:
https://www.dropbox.com/s/1bjs0ii67ckkxql/Rat Shack Resistor - Cap Upgrades 2.wav?dl=0

Not sure if I hear a huge difference, maybe changing the transistors will have more of an influence. Certainly can't go wrong tightening cap and resistor tolerances, good practice for the next run

 

[derek92994]

Could not get the Audio DiffMaker program to work on windows 10, it asks for soundcard to be selected, click ok then the app terminates. Might try it on the windows 7 laptop to see if that will help. Time for rest, back on late shift tomorrow. It has been a productive day.

 

[George S.]

Write your 2 transistor pin outs down on paper. Bend the leads on the new ones to match the old.
When using a pin vise to enlarge holes, always start on the copper side. Use very gentle pressure as a copper burr is usually created. Reverse direction a little, cock the bit so the sharp flute cuts the burr off, continue. Don't twist the pad off the board due to the burr! Small bits are available in equally small indexes.
Muse caps are usually physically larger than Fine Gold "FG" counterparts.
Have fun, I enjoy watching your experimentation!

 

[George S.]

Thanks George! Exactly, I use a cutting torch tip cleaner, the drill bit type, comes with lots of bits. Just too damn handy..

Lee, I'm going to look at those tip cleaners. Need one for my garage and torch anyway. Thanks.

 

[laatsch55]

Yeah George, they are just too damn handy. Bits are stored In the handle.