DIY 5687 Preamp

[BillyF]

This is my first DIY preamp. Design taken from Master Yeo's Web Diyparadise.com.

5687 Black Beauty

The look, with those cabinet handles, pretty cool.
AC filament supply.
The sound, it's very dynamic, fast and transparent. Imaging is also great. Vocals ok but not that fantastic.

Problems,
1. slight hum
2. when no signal input, noise can be heard when volume pot reaches 9o'clock. Noise louder as I increase the volume pot.
Need some help to from all the Guru's here on the board.
Thanks for all your help.

[li_gangyi]

did you use AC or DC for your heaters?? Ican't see the heater circuit in the schematic...and is it floating (if you used DC...)

[Colt45]

what is the hum like?

50hz or 100hz? or some hashy noise..?

(singapore uses 50hz power, right?)

[BillyF]

I used AC filament supply.
It has a slight hum maybe 100Hz but that Hashy Noise is bad. It's loud when no signal input.

[li_gangyi]

yeah Singapore uses 50Hz mains...which was my problem last time...just couldn't get thos clocks running properly...anyways here's a link for the worlds power...hehe
http://www.user.fh-stralsund.de/~ema...n/0921271x.pdf
Hope it helps...

[li_gangyi]

Ok...try using a DC heater instead..or re-route ur heater paths...and cross the other wires at a 90 degree angle...twist them together tightly also...

[Original Burnedfingers]

I built the same circuit except I used 2 gas regulator tubes in the power supply. I used a VR150 and a VR90.

I had the same problem with hum. I went to a DC supply for the heaters.
I also used a 100ohm resistor after the input pot to the grid. This totally took care of all the hum problems.

Microphonic problems:

I noticed that Tung Sol tubes are great for microphonics. I finally switched to JAN phillips 5687's and it is dead quiet.


I would highly recommend using VR tubes in the power supply as it will result in a very noticeable difference.

Joe

[audiobot]

Hashy sound doesn't seem like a power supply problem to me. I would suspect a bad solder joint or HF oscillation. Did you use a grid stopper resistor? Try a 100 to 1000 ohm soldered right to the grid pins. One thing you can try, to see if it is a bad solder joint, is to poke, gently and very carefully, the various solder connections with a chopstick or other nonconducting tool while listening for changes in the noise. Sometimes that will show up a bad connection. Another thing is to slowly pass your hand closely over the circuit, ie on the bottom side with the cover off. The capacitance of your body will sometimes show up a spike in noise when you are near a bad connection. Do you have a scope? If so check for HF oscillation. If not, an old time method to check is to put a small neon bulb on the end of a stick and probe around the circuit with it. When it is near a source of HF oscillation it will light up.

Good luck
M

[li_gangyi]

if he has a scope...it will tell you if there is still ripple in the supply...

[li_gangyi]

be careful of the hand method..dun SHOCK yourself~!!!