B1 preamp build thread

Jumper connections to permit 3-way selector switch

If you only want 2 sources, then use a DPDT switch and connect to the PCB. If you want to use a rotary for 3 or more sources, connect to one set of inputs on the board, jumper the PCB switch connections and switch everything with the rotary.


Hi 6L6:
Your first photo in the build guide shows these jumpers running from the PCB pads in the middle to pads labeled L1 and R1, respectively; later photos show the jumpers running from the middle pads to pads labeled L2 and R2. Does it matter at all which configuration one chooses? I'm going to use a 3-position rotary switch to select among three signal sources.

Thank you for the build guide and your help to the DIY community!

Larry Wright
 
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Oh, never mind...

Hi 6L6:

Well, that was a dumb question. My trusty old Beckman multi-meter shows with a couple of continuity checks that it doesn't matter, as long as I am sober enough to pick the right set of input pads to connect to the output of my selector switch. (Of course, that contingency introduces another whole set of un-good possibilities.) :D

Larry
 
Hi Zapped,
I read about your need to connect 3 inputs to the B1.
I built an AC/DC B1 during the lockdown.
It has 3 inputs selectable with a rotary switch like yours.
In addition it has 3 selectable outputs with a rotary switch.

I have seen some using parallel outputs.
I do not like this option because one amp loads the other
and when you switch on both amps the sound comes from both.
Why people do not have a rotary switch selectable output on their
preamps is because the unused amp will start howling when
an input RCA cord is connected to its input unterminated.
This is because the cable which is connected and left open
at the preamp end acts as an antenna due to the high gain
of the unused amplifie, there by making it howl or hum.

The trick I used was to terminate all 3 stereo outputs with
470K ohm resistorsto ground at the output jacks of my AC/DC B1.
I used 6 resistors 1/4 watt, 470K from all outputs to ground.
Thereby I have terminated the preamp end of my unused
outputs and the amplifiers which are connected and
kept switched on are silent till I connect my input to
them with the rotary.The 470k output impedance
load is nothing to a power amp which are all configered
low impedance.
Now I have the luxury of selecting(listening) to my Fet amp,
SE tube amp or PP tube amp at a flick of the rotary selector.
No need to wait for warm up.
Can compare and continue listening with the
best combination with the selected source.
Now I feel like top of the world.
Thanks again Pops Nelson and Salas for these wonderful stuff.
Left for you to try and enjoy!
 
Amps to compare

Hi Srian:

I like the idea of being able to compare combinations of amps and the B-1 ! It's rather like having a switching arrangement to compare components, with the amp being the only variable, and somewhat simplifying the the gain factor. Thanks for the idea!

I have an F5 (built 2018, works great), a ZenV4 (built 2004, ditto), and and an old Leach amp (working virtually nonstop since I built it in 1978). I also have an Audio Research ST-7-C3 ( a kit to modify a Dynaco ST70 which operated flawlessly for 10 years and then gave up the ghost on one channel in 1989, and continues to resist repair, despite a near complete rebuild in 2015), and a Pass-A75 (which I royally screwed up circa 2003). I refuse to give up on the ones that do not yet work!

Torn between the idea of getting the ST-70-C3 or the A-75 working, versus building an Aleph 3 or 5 (which I have all the parts and heatsinks for). Since I am 71 years old now, I have to think about what should be my main goals (not to mention staying ahead of COVID-19 !!).

Cheers,
Larry
 
Problem with "Volume" input level Potentiometer

I've built a dual mono (2x PCB's) for XLR version. I used two 12VDC switching power supplies in series (future plans for a Torroidal/Rectifier module). I cheaped out on everything except the Capacitors (5uF SoniCaps in parallel and Clarity 1.5uF Caps that were on sale). Resistors are all 1% 1/8W Surface Mount Vishay's. For switching, I had some old Bose Series 2 EQ switches that I salvaged form my collection of crap (where do you think the SoniCaps came from ?).

Anyway, it sounds great. Nothing added and nothing taken away...LITERALLY !

The 20kΩ pot I bought (cheap Bourns 6 gang from Mouser) either attenuates completely at the 20kΩ position or full "Volume" at 1.6Ω position. When I wired the onto the two PCB Pot locations, I was careful to make sure that I was polarity correct all the way from the XLR Inputs to the XLR Outputs.

I've tried with fixed resistors when I was testing the boards both individually (single ended) or as a dual mono system and had the same problem. All resistors are mounted in the correct locations with the correct values.

Has anyone else had issues controlling the input level via the Potentiometer or fixed resistors ? I have absolutely no noise/hiss/hum issues !

These are the Cheaper Chinese PCB's but electrically identical to the Schematic on the Pass DIY page.
 

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Just the pot and it's wired and GND'd correctly.

I treat each PCB as an individual channel where the "right" non inverting pin is actually + or "hot" and the "left" inverting pin is actually -. GND XLR pin tied to Chassis GND. No ground loops either. It's so weird. I'm expecting it to track both channels perfectly, but I should be able to have some control over the input level.
 
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!@$#!!&*!!

I'm just getting around to building up a B1 preamp board that I bought from the store some time ago and I can't find the four JFETs anywhere. I was getting a little casual with my projects and fear I might have tossed them by accident--either that, or I'm going to find them a year from now.

If I give up and buy another set from the store, what exactly is it that I need--a matched quad of LSK170?
 
So has anyone else had issues controlling the input level using the 20Kohm POT. If not, then what's the secret ?

I've built two B1s on boards purchased from Pass DIY -- one using dual GoldPoint attenuators and the other using a TKD potentiometer. Absolutely no problems from this superior, elegantly simple design.

I would suggest that if you are having problems, it is likely due to the Chinese clone boards you are using or the implementation of your balanced configuration -- not the design of the B1.
 
@rawjugsw I believe several folks have commented on this before. Please see previous posts. The B1 is a passive preamp with a buffer. Therefore, how loud the volume gets is a function of the output of what is feeding it. This is because the B1 doesn’t add any gain, itself. Too little gain from your amp and you’ll really need to turn up the volume. Too much gain and you’ll have too much volume. Depending on where you are in this spectrum of things and the pot you are using you may be able to add a few resistors at the pot to smooth things out. That said, the volume levels are ultimately controlled by the gain coming from your amp.
 
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I've built two B1s on boards purchased from Pass DIY -- one using dual GoldPoint attenuators and the other using a TKD potentiometer. Absolutely no problems from this superior, elegantly simple design.

I would suggest that if you are having problems, it is likely due to the Chinese clone boards you are using or the implementation of your balanced configuration -- not the design of the B1.

Did you have a preference on the Goldpoint vs TKD? I’m working on another B1 and have been considering the TKDs. Using a Khozmo attenuator right now, but plan to keep it with another pre.
 
Member
Joined 2019
Paid Member
I've built two B1s on boards purchased from Pass DIY -- one using dual GoldPoint attenuators and the other using a TKD potentiometer. Absolutely no problems from this superior, elegantly simple design.

I would suggest that if you are having problems, it is likely due to the Chinese clone boards you are using or the implementation of your balanced configuration -- not the design of the B1.

Did you have a preference on the Goldpoint vs TKD? I’m working on another B1 and have been considering the TKDs. Using a Khozmo attenuator right now, but plan to keep it with another pre.