choosing a rotary Attenuator for RCA pre

Hello Chaps, and Lass ( Kiira !!)

Just got me some UTC LS-10, and A-25 Iron, from an old friend,
and he gave me a circuit, which he reckons, comes from an RCA Mic Pre.

It's a simple circuit, based around 2 halves of a 12AX7.

only problem, there is no gain or level pot.
i asked him about this, and he told me, don't bother with a pot, better of going with a U Pad / attenuator.

I havent got a clue where to start. :? :?
how do i post a pic of the scematic, where do i start regarding the U pad ?

anyway,

any help is much appreciated !!

Steve :thumb:

H- L- and T-attenuators here:
http://www.gyraf.dk/schematics/bridge-attenuator.gif

Never heard of a U-pad.

½ 12AX7 will probably not give you a decent low output impedance.

For posting pics, use photobucket.com

Jakob E.

Here is some information on pads: http://www.uneeda-audio.com/pads/

Here's the schematic

http://tinypic.com/2p7vk

thanks for the help fellas

Just noticed, that this is not a 12AX7, but a 12AY7 tube. This makes more sense now..

(edited your link..)

I would suggest a balanced bridged-T (also known as a "balanced H" pad) on the output.
Here's a design chart:
http://electronicdave.myhosting.net/miscimages/bridgedh.gif
Your dB-per-step would be dictated by the number of positions you have on your switch. It's hard to find 3- or 4-pole switches with more than 6 positions. In the case of 6 positions, you could arrange it for 4dB/step, to give 0 to 20dB of attenuation. 4dB is a pretty coarse step, but you might find it adequate.

If you don't mind using two rotary switches, you could build it as a sort of ghetto version of the "decade attenuator" used in lab work. There would be two attenuators in series. Switch #1 would give 0-10dB in 2dB steps, and switch #2 would give 0dB to "off" in 10dB steps.

Sorry Jakob - my mistake

New york Dave, - I'm slowly getting my head around this attenuator thingy :shock: , so if i wanted 24 positions,without going too deep at the moment, i'd need a make before break , 3 pole , 24 way switch ?

also Dave, why put the pad at the output, before or after the transformer ?

sorry if it seems i'm being a little naive - thats because i am :?

would you be able to do a drawing ?

any help is appreciated,

Steve :thumb:

That is the mic pre from a RCA BC-2B console.
Do a search for BC-2B
Do a search on google "BC-2B mic pre" do not use quotes

There was a ton of info on the old forum.
Not too much here.
I have 8 channels waiting for a case.
It is a nice 40 dB gain block it does have limitations
having only 40 dB of gain. But it is one of the best
simple tube mic pre's.
Place a 250k audio taper por directly after the transformer.
Placing a pot any where else in the circuit is not good as it is inside the feedback loop.

Go here for a more clear schematic with a pot.
http://www.mastertraxstudio.com/tech_talk/RCA_BC2B/
I got my boards from kankaudio.com and he has a new kit similar
to this one but with a volume switched attenuator just like you want.
http://www.kandkaudio.com/proaudio.html

Place a 250k audio taper por directly after the transformer.

Click to expand...

After output transformer (600 ohm out imp.)you can place
600 ohm pad too. Properly loaded trafos sounds different
then unloaded. Try 1k;2k;5k audio pots.

BTW, according to your schematic,
http://tinypic.com/2p7vk
it is possible to change gain in amounts of some dB
while just changing R6 (180-750k, say) with switch.

Arrange the switch a way that highest value is allways in circuit,
you just put lower values in parallel, it eliminate clicks.
Maybe in parallel with low R6 values you will need some pF
(silver mica, say) caps to kill oscillations.

Also, if you have this qty of preamps, it is possible to do 8 to 2 console,
just add 2 preamps for summing and maybe aux.
Sweet thoughts about nice consoles....
))
('xcuse my bad english).
Igor.

'allo chaps,

thanks for the advice regarding this pre.

it looks like im going to go for a balanced bridged T attenuator, do i put this in place of the 250 k level pot ?
if i wanted input gain, as well as output level, how could i incorporate this into the circuit, or am i blaspheming

regarding the attenuator, i've had a look at daven attenuators on :evil: bay, is there any of these i can use ??

regarding other switched attenuators, any ideas. adrianh - those kits look sweeeeet, what make are the attenuators ?

any help is appreciated

Thanks

Steve :thumb:

I built a variation of this pre using UTC O-1 for input xformer.

I put a switchable 20 db pad in the input before the xformer for those high SPL's that tend to cause the circuit to distort terribly.

In place of R4 (680k) I use a pot for output control. I've used values from 500k to 1 meg.

Works for me.

byron

well, I´ve always heard that this is bad practice to put a pot inside a feedback loop, like you did. But, I´ve also did it in my Pultec MB1 and it worked with no preblems... Anyway, I got it out and now I´m using anoher method of controlling gain there...

I would really like to know what are the bad consequences of using a pot to attenuate the signal before the second triode, inside the feedback loop...

OK, think about it this way: you have an amplifier enclosed in a feedback loop, but the open-loop gain of the amplifier is variable, so the feedback margin (open loop gain vs. closed-loop gain) is variable--and also, the phase-shift through the circuit is varying as the pot is adjusted. Think about the consequences.

[quote author="gyraf"]H- L- and T-attenuators here:
http://www.gyraf.dk/schematics/bridge-attenuator.gif
Jakob E.[/quote]
Is here some comparation of attenuators in the mean of
its NOISE FIGURE ?
I have measured output noise voltage of the same
preamp (to HI-FI set, not mic) and it have this noise
dependent to volume pot position. And it was worse at middle
of the pot position. Not at maximum volume what I expect.
If noise figure (or input noise voltage) is this voltage divided by gain,
noise figure of preamp is volume pot position dependent.

Are H or T attenuator better in that sense?
Why are designing inputs with best noise figures if simple pots
this destroy.

xvlk

Well since you are attenuating inside the feedback loop you are simply reducing the 'open loop' gain. The overall gain will remain essentially the same as long as the open loop gain is greater than the desired closed loop gain set by the feedback.

A measure of feedback is always healthy in my opinion as it reduces error due to component tolerances and drift in the operating parameters.

Hi xvlk,

An attenuator' noise will always be highest at the point where it's impedance presented to the next stage is highest.

So in a standard attenuator that will be in the middle position. For constant-Z-out-attenuators (bridged-T, bridged-H) it will be the same all over.

Jakob E.

more reading? :green:

http://beradio.com/notebook/passive_attenuators/
http://www.microwaves101.com/encyclopedia/attenuators.cfm

[quote author="gyraf"]Hi xvlk,

An attenuator' noise will always be highest at the point where it's impedance presented to the next stage is highest.
So in a standard attenuator that will be in the middle position. For constant-Z-out-attenuators (bridged-T, bridged-H) it will be the same all over.
Jakob E.[/quote]

Hi gyraf,

If I have understand, in constant Z attenuators his noise is the same
at all position. ? some problem.
In passive circuits with constant impedance his noise figure is
deppendent to power gain.
There is equation (only linear, ... van der Ziel ?)

To=G*Ti+(G-1)Tg
To is overall noise temperature, Tg is temperature of attenuator
and Ti is temperature of the input and G is power gain (in passive
circuits <1)

It is not probable, that noise temperature of the att. is the same that of the input. And your "the same noise overall" is kind of mistake.

In the passive attenuators with constant impedancy noise temperature
monotonically varries.
We can cold it for better noise.

in pots there if some strange.
Why potts are still used?
In some japan electronic pots is the same problem. I have TV
set and if I (at night) set the TV on low volume, I hear
big noise. If I (in the quiet passages) turn electronic vol up,
noise become inhearable.

Why some producer is not producing constant impedance electronic
pot. It can be benefit for him.

xvlk

[quote author="cuelist"]Well since you are attenuating inside the feedback loop you are simply reducing the 'open loop' gain. The overall gain will remain essentially the same as long as the open loop gain is greater than the desired closed loop gain set by the feedback.[/quote]

Sorry, but that's incorrect.

G = A / 1 + AB (in the case of a noninverting amplifier)
where G=closed-loop gain, A=open-loop gain and B=feedback as a fraction of output voltage. It's easy to see that, with the value of B fixed, changing the value of A will alter the value of G. This can bite you in the ass particularly with tubes, which have a relatively low open-loop gain to begin with.

The only way the value of G can remain "essentially the same" even when A is varied is if the value of A remains very large compared to G under all possible conditions. When you put a volume pot in between two stages that are enclosed within a feedback loop, you're varying A between zero and the full open-loop value, rendering the feedback useless at best.

Ignoring the possible instability at high frequencies caused by phase shift from inserting series resistance into the signal path, it has been demonstrated that small values of feedback can cause generation of annoying higher-order distortion products--so the sound of the amp, as well as its gain, will change significantly as the value of A changes with B remaining fixed.

I noted on Igor´s schem that C2 is .22uF. I´m using a big 22uF/400 electro cap there, like in the schem. I thought a big value was needed there... Do you think I can go lower and use nice film caps?