Daven T attenuator/BA6A questions.

I have some questions about these Daven T attenuator things relating to a BA6A, if anyone could help but be gentle im fairly new & takes me a while to grasp these things thanks, done some searches & read whats been discussed before cant quite get my head round it yet.I built a BA6A from the manual schematic, just waiting for interstage tran to arrive, one of my probs is cant get 1 off these attenuators, so what can I put there instead ? I plan to use solid tube mic into my BA1A or V72 mic pre?s into the BA then into break out box into Delta 44 sound card to record onto PC phew, just to put you in the picture,I understand the BA puts out somewhere in the region of 10 watts,& I don?t want to smoke my soundcard, I take its this hi power is to drive the diode tube thingy, please put me right when talking crap, & then I spose the atten comes in to wash of a lot of power, hence pots cant be used coz of power & is it something to do with letting the O/P tran see the same 600R impedence? & also keeping it balanced ? is the daven on the BA kinda like to pots wired together ? & what would the value of each pot be 600R? if so, I was thinking maybe I could have 2 pole rotary switch with lots of wire wound resistors, or am I talking rubbish again thanks for your patience, sorry to ramble on.

Cheers Gary O.

On the Ba6a the Daven output attenuator apears to be of the Bridged T type. The main advantage of this is that you have the same input impedance & output impedance. It isn`t fully balanced. If you wanted it balanced it would have to be a H type attenuator.

You can still get these made since a company called Shallco bought out Daven. However the last time I got a quote it was $285 for one. You may be able to get one on ebay but they can still be £40

The large output is due to the fact that the kind of signal levels from this era were normally much bigger than those of today. Most older compressors like the sta level & the altec 436 need padding to interface with modern gear.

I read somewhere that you need to pad the output of the Ba6a by 40dB, though I can`t be sure this is true cos I`ve never played with a real Ba6a.

You could probably get away with padding the signal from the output transformer & then running a regular pot. The pot would be Ok since the signal is attenuated.

One other thing, on the original the dual 25k input attenuator is a dual Daven ladder attenuator (like a normal pot but switched resistors). I would think this could be useful because the tolerance between tracks on a normal pot is not that good. Many manufacturers quote 20% tolerance. With the dual ladder attenuator this is much more accurate because the resistors are matched. I would think that this might reduce the control signal breakthrough or thumping.

pad the output by 40dB

Click to expand...

40dB pad would be too much. In use, -14dB seems optimal when going into modern gear.
Also -14dB or less, at the I/P, depending on what is feeding it.

Also, you wouldn't need wire-wound resistors which are bulky, inductive and cost more.
You could easily make-up the i/p switch with a standard rotary (make before break contacts) and
1% resistors for the reasons Rob said.

:thumb:

I must admit I was a bit dubious about the 40dB pad, I guess it a great example of not taking everything one reads as gospel (which is why I didn`t.
Thanks for the advice on the 14dB pad, I`ll try that out when I get round to starting my one.
I think the stepped input control probably makes a more noticeable difference than the bridged T output attenuator. If you look at the diagram for the sta level they have an output attenuator that maybe adapatable for Ba6a use.

http://www.groupdiy.com/index.php?topic=2229

Percy audio (www.percyaudio.com) have the 45 position shallco 2 deck switches at $95 each.

FWIW, I'm pretty sure that the high level of the Ba6a had something to do with the fact that it was really intended as a broadcast limiter...i.e. for radio gear.

Cheers,

Kris

Hmmm cant afford a real Daven only had enough for the interstage & food for me & the cat {Marvin}, better put the violin down now, thanks for your thoughts so far more welcombe much to learn.For my input attenuator I have 2 pole 6 position rotary switch with 5 x 5K resistors on each pole {1 % tol I think} 5 10 15 20 25K is that how I should be wiring that up? it seems to work goes from no sound to lots, tho I have made slight cock up ive used 4K resistors so only adds up to 20K will that still be ok for now or will this upset the circuit from the start, next question do these attenuators add noise?. New york Dave pointed me to his nice pad box thingy thanks Dave perhaps I could make a 14db version and stick it in me BA? ouch! Or make the simpler gates sta level pad thingy,& do one or both
these output attenuators add noise too? & why was the original BA made not fully balanced?,would it be better fully balanced for nowdays use? If the BA was redesigned for nowdays kit I suppose it wouldn?t have such powerful amp but would it loose some of its famous big sound, So many questions thanks once again for your time & patience.

Cheers Gary O.

Gary your input attenuator switch would probably work slightly nicer if you calculated the values of resistor to give you a log scale. This is because its a gain control in an audio circuit & these are normally logarithmic.

What I`m intending to do for this is make a 12 way rotary switch with 2 wafers & make the attenuator from that.

It would also be possible to make a bridged T attenuator using the same switch kit for the output.

FWIW, I'm pretty sure that the high level of the Ba6a had something to do with the fact that it was really intended as a broadcast limiter...i.e. for radio gear.

Click to expand...

Yes, but as far as I`m aware much of this old gear ran at more meaty levels, not just radio gear. It is easier to do this with valve gear because you have much more headroom if you can swing say + & - 75v rather than the + & - 15v that many op amp circuits run on.

Also with Vari mu compressors many of them need fairly large control signal to pull the biasing on the vari mu valve down enough for it to really happen.
Take a look at the fairchild control amp. It doesn`t have anything to do with the audio path, & is purely there to provide a large control signal. With the BA6a & sta level this power stage it is in the audio path, but also provides a large control signal.

Gary

I`m wondering if you saw the pics on this thread that CJ was kind enough to post. You can see the davens in some of them.

http://www.groupdiy.com/index.php?topic=2474&highlight=ba6a&sid=4029357fd328e7c122962cd290e52805

Yeah Rob I have seen those pics great stuff BIG! mothers those Davens, just read some old post where PRR was saying that they are basiclly just resistors & rotary switches, just very very good ones that would last a hendred years, on my BA i'll have to make do with the 6 position switch just need a little bit of control till I scrounge something better, & yep as you say log pots are used as level controls didnt think of that, so the total resistance still adds up tp 25K in this case, its the increments between that differ is it,its now I regret bunking maths, they said it would be useful too, thanks

Cheers Gary O.

There`s a site round somewhere which calculates the log resistor values. You just supply overall res & no. of steps & press go . It does the rest.

Maybe someone can chime in a tell us where it is. Might be on one of the META threads.

Cool i'll find that with google perhaps, who needs maths, thanks for that.

gary O.

Here, Gary, I drew this up for you:

22kB GIF

There are only six positions because six-position, three- or four-pole switches are not hard to find. If you could manage to score a three-pole switch with eight or more positions, you could add "0dB" and "off" positions, and/or use finer steps. But even as drawn, this should give you a reasonable range of control.

Oh what can I say thanks New York Dave :grin: ......what a nice man, straight down the resistor shop tomorow cheers!.

Gary O.

> I understand the BA puts out somewhere in the region of 10 watts,

Not that much. But as explained, it always runs at maximum level, to make control voltage. That can also be good when you have a 20 mile wire to the transmitter. If you need less, just waste some power in resistors.

> pots cant be used coz of power

Just use the right pot. A 1K 2W wire-wound pot will do.

> letting the O/P tran see the same 600R impedence?

1K is a near-enough match to 600Ω (these beasts are not hyper-fussy). The output impedance will be 0Ω to 300Ω, not at all a problem.

And you can fix a couple more problems. Put a 10dB L-pad after the pot, like 2K2 series 1K shunt. That knocks the signal down to sound-cardish levels, fixes the output impedance at 600Ω (irrelevant), and slightly slugs the linear taper so the center isn't -6dB but more like -8dB.

> something to do with & also keeping it balanced ?

The business about "balance" is totally pointless: if the output transformer is any good (it is), then a "floating" un-balanced pot or attenuator in the 600Ω line does not cause enough unbalance to care about.

> its a gain control in an audio circuit & these are normally logarithmic.

For a wide range of gain change, more than maybe 20dB, you really do need log-taper. Also if you demand constant dB/degree. But you don't change the output of a limiter very much. A few dB, not several dozen dB. And as a practical matter: 1K 2W linear is a $0.50 surplus item, a Daven or other log-taper pot/switch is a $90-$500 item. They sound the same (resistors is resistors) and I can tolerate a bent scale for the $89.50+ difference in cost. (If you actually build a switch: yeah, build it loggish.)

The input gain-set does need considerable range and logically can be a 25K or 50K dual audio pot, any watts.

[quote author="PRR"]The business about "balance" is totally pointless[/quote]

This is one of those rare instances where I disagree with you, PRR. :green:

I gave my (typically long-winded) explanation of why in an earlier thread...

http://www.groupdiy.com/index.php?topic=2192&start=0

But I grant you, in a typical home studio environment, with short cable runs, it may not matter as much as it does in a big broadcast plant.

Hey, if you don't care about the attenuator being unbalanced, here's another one for ya.

http://electronicdave.myhosting.net/miscimages/600bridgedt.gif

Thought id re awaken this old thread as Im still trying to find a pad for my BA6A still a few things I dont understand.....

I have made & have been using a cross between new york daves and PRRs sugestions with the 1K pot into my sound card...this has been working great as long as i I have the threashold on my limiter low so it compresses more keeping the output lower.....BUT if i want less compression the output goes up so I need to attenuate more at output..BUT when turn my pot down to get right level I start to loose bass ???

EKADEK has kindly posted these

http://ekadek.com/?page_id=34

This is AWAs version of the BA6A in the schematic you can see a 10db T pad followered by a bridged T...the RCA BA6A has 20 steps of 1 db so thats 20db right ( bang on ideas from PRr & dave as always thanks)

so now thas 10 db & 20db max ...total 30db...output tran to output socket. BUT as I understand it a real BA6 level would still be too high for todays sound cards (& is same for my DIY BA)

So I thought just make 1 pad that brings in right down to the level I want....but is that the way I should do it ??

doesnt seem to work unless Im missing something I tried a 20db T pad as expected level came down but way too hi still, so I went up I made a 30db T pad but like before I loose all bass, sounds terrible...what am i doing wrong ?

I wondered if was bad practise to soak up the power with one pad & like daves & PRRs & the AWA schematics suggestions I tried T pads 10, 14 20db followed by various balanced T pads but again when I get down to the sort of levels Im lookin for I loose the bass ?

Maybe Im doin something silly any ideas ?

Thanks for your patience.........................Gary O.

NewYorkDave said:

Here, Gary, I drew this up for you:

22kB GIF

There are only six positions because six-position, three- or four-pole switches are not hard to find. If you could manage to score a three-pole switch with eight or more positions, you could add "0dB" and "off" positions, and/or use finer steps. But even as drawn, this should give you a reasonable range of control.

Click to expand...

Sorry to dig up an old thread, but I'm having a very hard time finding a 3 pole 6 position switch for this attenuator.  Am I being massively stupid, but a 3 pole switch isn't the same as a 3 deck switch, is it?