DIY phono preamp - cheap and simple!

[nobody special]

Mine is about that size, too. It's all through hole parts though. It is JAM PACKED. I have some resistors on the back of the board. Garbage, your's is definitely neater than mine. I wasn't so sure mine would even work. I'll try and post some pictures of it later.

[garbage]

Originally posted by nobody special
=I wasn't so sure mine would even work.

neither was i. i only did rudimentary voltage checks.

i will take up richard's suggestion to remove the jackets of the lytic caps next.

forgot to make a correction in my post...
the last pic is of the cart that we listened to last. it is a denon 103R elliptical, not shibata as indicated in post 187.

also the pics of the carts.
1st pix: bps garrott shibata in background, 103R garrott elliptical in black headshell, and the 103R stock in silver headshell

2nd pix: 103R garrott elliptical

cheers
garbage

[nobody special]

i only did rudimentary voltage checks.

Well, then you did more than I did! I just fired it up, and hoped it didn't fire up! Actually, one channel didn't work at first.. until I found I hadn't plugged in the RCA connector all the way. DOH!
I've done a lot of prototyping / soldering in the past 12 years or so at work, so I knew what to look for. I did have to touch up one area, but I caught it before I tried it out, so no problem.
I can't really get to the chip on my board with a probe to check it from the top. I am still amazed it worked, and was as quiet as it was. My grounding scheme must have worked well, because I have zero hum. There is a little noise only when the amplifier is cranked all the way up. At normal sane levels, it's dead quiet.
The noise I'm getting sounds like rectifier noise. I used a lot of capacitance in the raw supply. Do you think snubber caps across the diodes would help? What's a good value, .01uf?

[garbage]

hi steve

i'm not sure if snubber diodes helps. if i'm not mistaken, most people use 0.1uF. i tried that with my buf634/opa627 head amp, but found no audible difference. the rectifiers used for that application were 4 x mur860 (actually the same psu that i used during my testing of the vsps).

cheers
garbage

[rjm]

The diode noise does tend to get through and form the low level background hiss. A quick test is to try batteries and see how much lower the noise floor is.

If it bothers you, there are four options:
[list=1][*]substitute a different diode for rectification[*]experiment a little more with snubber circuits, including RC types[*]lower the amount of capacitance in the power supply[*]finally, use batteries as a permanent solution[/list=1]

-rjm

[nobody special]

I guess it (the switching noise) really doesn't bother me. It's at a very low level, and isn't audible at any sane listening level.
I set cartridge up a little better last night, and listened to a few more lp's. The grainy upper mids are subsiding. I broke out an unopened copy of Phil Keaggy's "Way back Home" LP. It's mostly acoustic guitar and vocals, with some strings. I have heard this on cd many times over, and I was hearing things in the LP I hadn't ever heard before! There is something going on with the inner dynamics that is really amazing. I've never heard it before on my system. Very cool.
On the other hand, there is still a flat spot in the mid/upper mid that is a little annoying. I'm wondering if it isn't my AT cartridge, which is known to sound a little on the warm side. Could it be the RIAA network?
Overall, this project has made me consider getting a really good TT. I've been listening to lp's since I was 8 years old (When I got my first real system- marantz turntable and receiver) but this is the first time I've heard the real potential of the LP.

[rjm]

Its worth giving audio circuits at least 100h of break in before making any real conclusions.

In your case, though, I really do think you need to urgently attend to point 3, above. I suggest removing, if only temporarily, the 6800uF caps and 22ohm resistors from the power supply and giving it a listen.

/rjm

[nobody special]

Could you explain to me how too much capacitance is a bad thing? And, why not the RC section? My thinking was, as long as my voltage coming in to the regulators is sufficiently high at the max current being drawn, how would a little resistance hurt? It definitely knocks out some of the noise; it suppresses it at least 10x by my simulations. As long as the input voltage is in the regulator's range, and there is sufficient filtering in close proximity to the opamp, why would it make any difference what is supplying it, as long as it's clean? I could see the caps near the opamp being critical in value.
I'll give it a try when I get the chance. I would really like to hear what you think about what I've said. I wasn't saying that you're wrong- only that I don't see what the problem is. It wouldn't be the first time I was wrong

[rjm]

I could make a pretty good case for 6800uF being too high. I dont have time to run through the numbers, but the as you add capacitance the diodes and transformer have to pass more current in a shorter space of time to maintain the same steady state output current. Both components become progressively less happy.

If you are into simulations, it is interesting to calculate the conduction angle as a function of filter capacitance and output current.

Of course if you remove the caps, you can't leave the resistors in there or you raise the power supply impedance to the value of the series resistance.

I could continue and try to explain the virtues of low impedance, low capacitance power supplies, but it would be quicker really if you just gave it a shot and decided if you liked it or not.

/R

[nobody special]

I understand. I 'll give it a shot. If anyone can point me toward a calculation for the conduction angle, I'd like to see for myself what is going on.