One-Bottle Preamp

[NewYorkDave]

Hey gang... I don't have time tonight to look back through the thread and answer all the questions that have been posted recently--and the other guys seem to be fielding them pretty well, anyway. But I wanted to pop in and mention that I used the "One Bottle" prototype to record a vocal last night and, after not having fired it up in a while, I was reminded of how much I like the sound of this preamp. (Of course, you must consider me very biased for obvious reasons!). Anyway, to those who are working through their own builds or are still in the throes of debugging, don't give up. Assuming you've used decent transformers, I think you'll be satisfied with the results when you're through.

 

[jrd]

hell yes!
i don't wanna let this thread die... i just finished my one bottle this weekend and it is great! i will post pics if i can get my hand on a friends digital camera. i think this should top the list of "the first lab project to try." who wants to f*(& with all the parts for a green or a gssl when you can do this... and actually have more understanding of what is actualy happening in there than you would from stuffing a pcb. my 2c. anyway, thanks for the circuit dave!!

 

[khstudio]

YES, Keep it alive.
I have all my parts in a pile ready to go but got sidetracked.

Love to see your pics :thumb:

Kevin

 

[Nadege]

we want pics, we want pics :grin:

 

[skipwave]

I have three more channels of One-Bottles waiting to be built. I might make a couple of them into Two-Bottles.

I used the two channels I have for makeup gain on my passive summing setup. Sounds super sweet. This is a great, useful design. So build it! :thumb:

 

[NewYorkDave]

Very cool. I'm glad to hear that people are getting good results from this little circuit. I'm planning to build more of them, myself.

 

[Freddy G]

hey jrd and skipwave,
which transformers did you guys use?
I'm still deciding on either lundahl or cinemag or edcor.
Freddy

 

[Nadege]

Hi,

wich power transformer will you recommended for two channel of one bottle pre amp?


I was looking for either an Hammond or a Welter, but they seems to be really big ones ...

Thanks for advice

 

[jrd]

transformers?... tried one from a scuttled biamp mixer, currently i've got one from edanderson that he sold on this board. just get whatever is cheapest is my motto for now. this week i'm going to order some edcor xsm for the output. currently unbalanced. and it makes a big difference what you patch into with the unbalanced out... so hopefully the output tx will make life easier in that regard.

if and when i post pics i want to talk power supplies again too.
skip-i'm using one of those 265/15 transformers you sold me. using the 15v winding to do regulated 12v heaters.. i just ripped off the circuit from the G9, 4 diodes, 5 caps and a fixed regulator and really easy. is there a way to build a 48v phantom circuit with this tx too... with either some kind of voltage tripling action on the 15v winding, with hopefully enough current to go around, or maybe something based off the other 110v primary that I am not using?.... i will have to look into this. perhaps i will find that just adding another tx may be easier.

 

[skipwave]

[quote author="jrd"]transformers?... [/quote]
For me its,
Input: Sescom MI-70
Output: Triad A69J

The next batch will use those Triad/Ampex inputs I got from FunkyDiplomat. I also have a couple of those inputs from edanderson, but they are supposed to go in another project. We'll see.

[quote author="jrd"]if and when i post pics i want to talk power supplies again too.
skip-i'm using one of those 265/15 transformers you sold me.[/quote]
:thumb:
Glad to hear it! I'm going run my next couple One-Bottles on one of those. I'm sure you can use a voltage tripler and an LM317 to pull phantom from the 15v windings. There should be plenty of juice if you are only heating 1 or 2 AV7s.

 

[jrd]

Hi gang. So I've been playing around more with my one bottle, and I have a question. For the power transformer I used one which I got from skipwave. The secondary is 265v. I have copied the schematic NYD posted,

http://groupdiy.twin-x.com/albums/userpics/10031/onebottlepsu.pdf

and by experimentation I have 15k at R1 instead of 4.7k. This gives me about 285v under load. Changing the value of this resistor does two things right? It has some effect on the current. And it has some effect on the frequency that is being filtered, in conjunction with C2. My question is... is this a good way to adapt this TX for use in a 300v supply? Dave says that one should *decrease* R1 and increase R2 for powering more bottles... so I assume that means one is making more current available. I assume that by jamming a 15k resistor in there I have decreased the available current? However, I was starting out with a more voltage also. Skip, have you tackled this yet? Any insights are appreciated. I am just a little unsure how to be applying the right math to the right values to get a useable solution. The thing seems to work fine though, which is good.

Issue number two is heaters. My regulated heater set-up gives me 11.8vdc. Not quite 12.6vdc, but it works. I assume this is close enough. After all, it works.

 

[jrd]

Wait a sec... maybe I'm not as dumb as I think. That 15k resistor is dropping 107 volts across it. 107volts, divided by 15,000ohms, gives me a hair over 7ma. Which is totally fine. Right?

 

[jrd]

And 7ma times 107volts equals 0.75 watts.. so I'd better make sure that resistor is a 1 Watt resistor. Which it is. Right?

 

[jrd]

Right, the tube is drawing 7ma no matter what, and it's up to the supply to provide it, if it can. As long as that resistor can dissipate enough wattage, that transformer won't have any trouble coming up with 7ma. Have I moved the filtering cutoff frequency appreciably with the 15k resistor to affect anything?

 

[jrd]

I think I am ok? But maybe misunderstanding what is happening. I calclated
the cut-off frequency twice. Once with 15k and 22u, the other time with 4.7k and 22u. I used the equation 1/(2pi*CR). The answers were realllly tiny. Like 1.5x10^-6 and 4.8*10^-7. So I have two possible conclusions: 1. My modification to the supply to accomodate my transformer has had no appreciable effect on the supply's ability to do good filtering. In real life it seems to work and be quiet. or 2. I have misapplied this equation here and perhaps I do need to change the value of C2 or something?....

 

[skipwave]

I've never looked at the filtering frequency of the power supply RC poles. I don't hear any hum. :green:

I think that lowering the value of R1, was to keep the B+ as designed, halving it for twice the current draw, thus a consistent voltage drop. Raising C2 was just to make the increased current available, double the current so double the value seems to be the rationale.

Flashback to page 16 :shock: of this thread, where Dave taught me about power supplies. :wink:

 

[jrd]

right, it's all a lot clearer now. dave chose that value anticipating a draw of 7ma. i can kind of choose whatever resistor i need to get me to 300vdc for b+. if i decide to power another av7 with this psu, that'll be another 7ma it's drawing and i'll have to halve that 15k resistor to get back to 300v ish. and add a beefier cap to keep up. this is stuff that everybody knows but that i'm just figuring out right now by doing. so apologies. and thanks!

 

[jrd]

actually, one last thing on my mind. the power tx is center tapped. should i have that grounded.. right now it is just hanging loose. things seem ok, but just wondering what i "ought" to be doing.

 

[skipwave]

[quote author="jrd"] the power tx is center tapped. [/quote]

Do you mean the SHIELD? I had to ask about that, but its not a center-tap, just a protective measure. You can connect it to chassis.

A center-tapped transformer is more efficient, capable of slightly higher current draw. You can use only two diodes, instead the 4 in a full-wave bridge setup. Of course, center-tapped costs more, from what I've seen, and rightly so.

 

[pstamler]

[quote author="jrd"]I think I am ok? But maybe misunderstanding what is happening. I calclated
the cut-off frequency twice. Once with 15k and 22u, the other time with 4.7k and 22u. I used the equation 1/(2pi*CR). The answers were realllly tiny. Like 1.5x10^-6 and 4.8*10^-7. So I have two possible conclusions: 1. My modification to the supply to accomodate my transformer has had no appreciable effect on the supply's ability to do good filtering. In real life it seems to work and be quiet. or 2. I have misapplied this equation here and perhaps I do need to change the value of C2 or something?....[/quote]

You got those results because you computed:

1/(2 x pi x 15000 x 22) and 1/(2 x pi x 4700 x 22)

Your answers are correct...for 22 farad capacitors. But these are 22 microfarads, so really the computation should be:

1/(2 x pi x 15000 x 22 x 10^-6)

and ditto for 4700.

Peace,
Paul