BA-3 As Preamp

[vdi_nenna]

A bit of advice - the pot will take the place of R2 on the PCB, input to the junction of R1/R2, output to the Jfet Gates and then connect the pot to ground. A little surgery or creative thinking will be needed for those connections. Nelson has suggested keeping R1, as it's there to stop parasitic oscillation.

Just this part. Its OK, but I don't get it. . I tried to visualize it, but without cutting traces or removing R1 and R2 and connecting them off PCB, I don't think it will work.

 

[6L6]

You are right. That's not gong to work as described. I think the easiest will be to place R1 on the pot or input jack and wire the pot into the R2 pads as best one can.

 

[6L6]

A little progress today. The most important thing was getting to my buddy's shop and using his mill to cut the hole for the IEC connector. The back panel needs only to be wired.

The transformer is a 120VA 20+20V Shielded Antek. Because of this I will use a LM317/337 regulator from TubeCad. Also the rotary AC switch and selector are from TubeCad. Very, very nice pieces. There are a number of good 'building blocks' available there. both low- and high-voltage.

The Attenuator is a series stepped Goldpoint with Dale RN60 resistors.

I have yet to make the final order to Mouser for components, that is why the preamp PCB is not currently installed.

 

[6L6]

A bit more wiring today.

 

[Audio Zen]

Looking good Jim, that chassis is from the store right? which one is it?

 

[Zen Mod]

I believe - nothing wrong with placing selector and attenuator all the way back , with extending shafts to front ?

 

[vdi_nenna]

6L6, you decide how you are going to connect the attenuator? My setup is a lot like yours.

 

[6L6]

Neville -- Galaxy 388 (3mm Front) - Compact with Quasi Heatsinks - Chassis

Vince - I'm going to try placing the R1 resistor in the output of the pot (attenuator). Probably right at the PCB. It's a gatestopper shared by both input Jfets, so hopefully should work. I will have photos when I wire it.

Choky - I don't have the necessary hardware for the shaft extension and internal switch/potentiometer mounts and such. I do like the idea, however.

 

[Hikari1]

Good place for extension shafts, volume knobs, etc:

Audio Catalog

That's what I am using. Shipping was pretty quick to the US (they are in HK) and the prices are great.

 

[Zen Mod]

shaft - plain M6 screw rods - any hardware store

mounting element for att and selector to bottom plate - piece of Al angle

wanked pots for shaft sleeves

plastic or rubber hose for joining extending rods to pot and selector

 

[Antoinel]

A circuit like BA3 FE maybe DC servo'ed per the attached concept [meaning doodle or untested] schematic. Downside is a change in the gain structure of the parent.

 

[6L6]

Selector all wired up.

Looking clean and organized - it's easy when the important part of the preamp isn't installed.

 

[PKI]

I like those coaxial cables, what are they? Don't be shy you always do a clean and organized job!

 

[Zen Mod]

have old CRT monitor or just (video) cable of ?

strip da cable

 

[PKI]

ZM, do you really need a coaxial, why not just those CAT5 twisted pair I told you about?

 

[Zen Mod]

sweet and short = 0.1mm - either foil or wire

when short , no need for twisting , so naked silvered copper is good enough

however - as I wrote earlier to you , if CAT is your poison , I have no objection ....... it's almost thin enough

 

[PKI]

That's a thinest I have! I was using that kimber tcss before which is much thicker.

 

[6L6]

The coax is stripped out of an S-video cable . It's thin, flexible and shielded .

 

[dodog]

The coax is stripped out of an S-video cable . It's thin, flexible and shielded .

Are you just using the core wire for one connection and the shield for the other? I have a few old CRT VGA cables available.... I should cut those apart and do something with them.

 

[6L6]

Yes. The center is the signal, the shield is ground. (Return)

The VGA cables are probably perfect!

I have been using cable scraps for hook-up wire for so long I can't remember the last time I actually purchased wire.