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Author Topic: Replace those Daven attenuators  (Read 21261 times)
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k4kyv
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Don
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« on: April 06, 2007, 12:20:21 PM »

Is your audio processing gear connected to your transmitter using an old fashioned 500-ohm balanced line and a Daven attenuator?

Well, you need to throw out that obsolete junk.  Simply inserting a stepped attenuator or potentiometer in the signal path without an adequate buffer can lead to serious frequency response deviations due to the RC low-pass filter effect of the passive output impedance and any cable capacitance.

What you need is one of these
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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nq5t
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« Reply #1 on: April 06, 2007, 12:32:06 PM »

At a very affordable  $2900-5300 per unit, there's no reason why every well equipped station shouldn't have several of these .... especially since it has no "electronic glaze".

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The Slab Bacon
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« Reply #2 on: April 06, 2007, 01:08:11 PM »

Don,
      Where do you find this stuff!! that is one of the best yet! Who says P.T.Barnum is dead!! I guess the idea is practical, but return per dollar invested puts this one right up there with "audio grade" fuses and knobs!!

$2900 - $5300 for the variable transfoma vs $3-5 for a good pot there doesnt seem to be too much choice to me. It kinda makes you wonder if some of those people have any idea where "the point of diminishing return" really is.

The whole thing boils down to: How much is one willing to pay for bragging rights?? I wonder if it comes with audio grade knobs and case cover screws??

                                                   The Slab Bacon
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WA1GFZ
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« Reply #3 on: April 06, 2007, 01:12:39 PM »

Hey it's only money. I just spent $2K of my tax return to patch up our little mashed kitty...
but my daughter just walked by and gave me a big hug....
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W2XR
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« Reply #4 on: April 06, 2007, 01:48:54 PM »

Oh man, I use a number of those Daven 600/600 ohm T-type variable attenuators in the audio path of my shack to set the various gain structures. I guess I had better start saving for a bunch of these new latest & greatest Silver Rock attenuators. Should make a big improvement in the transmitted audio quality!

73,

Bruce
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« Reply #5 on: April 06, 2007, 01:55:27 PM »

You know,
              It would be kinda interesting to see the frequency response curve of them. I bet that the tapped inductance of those transfomas has a definate effect on it. And it might not be for the better  Grin
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k4kyv
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« Reply #6 on: April 06, 2007, 02:01:44 PM »

I wonder if it comes with audio grade knobs and case cover screws??

No, but you can order the knobs from the same company.

http://www.referenceaudiomods.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=NOB_C37_C

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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #7 on: April 06, 2007, 02:13:05 PM »

Couple of things.

First of all the article does discuss some of the freq response issues wrt various parameters...

Secondly, this guy is not the first to come up with the "tapped transformer" scheme.
Third, ur late to the party, this was a 2003 article, the idea was first presented as a high-end DIY idea probably 10 years earlier, and no doubt was used or thought of decades earlier.
Fourth, it is wound with silver wire, which isn't going to be a cheap, inexpensive product to manufacture in small quantities. Work out the math on it, add in the cost of doing business and marketing, see what price you want to sell them for... IF you can sell any.

Imho, I don't like the idea for hi-fi apps myself - having heard them myself.

Finally, fwiw, IF you are using Daven or similar attenuators (I do) and in case you have never opened up the case, you should know that Daven and others used whatever type of resistors they had on hand for any given value in the attenuator (except for the really early ones - they're all WW). That means WW, MF, and carbon comp are interspersed. Obviously for ham radio this doesn't matter at all, but in hi-fi situations, that's not so good. Ya know why? Cause yaz can hear differences in resistors! Yeah, one can.

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Todd, KA1KAQ
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« Reply #8 on: April 06, 2007, 02:16:14 PM »

It amazes me how any of this old equipment ever managed to work so well. How did they do it?  Roll Eyes

Even more amusing is the other side of the crowd, who has been scooping up this stuff, including Daven attenuators, and paying big bucks for it. They swear by it, the others swear at it?

Thanks anyhow, but I'll stick with my old 'tried and true' proven technology...
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« Reply #9 on: April 06, 2007, 02:31:14 PM »

I wonder if it comes with audio grade knobs and case cover screws??

No, but you can order the knobs from the same company.

http://www.referenceaudiomods.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=NOB_C37_C


Must be inflation, seems IIRC they usta only be around $300 Grin Grin

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k4kyv
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« Reply #10 on: April 06, 2007, 02:57:04 PM »

I have a whole box full of Daven attenuators on hand, picked up as single items at hamfests and classified ads, and salvaged from junked broadcast and professional grade audio equipment over the years.

Some of mine are low-Z variable T-pads, but the best ones are variable H-pads, designed to maintain balance in the line.  Others are high impedance potentiometers anywhere  from 100k to 1 megohm. I  have taken the covers off a few of mine.  The internal construction is of highest quality, and I have checked them with an ohmmeter and the resistances were always right on.

I never paid much attention to the types of resistors involved, but nothing that affected the frequency range that I could measure using my audio signal generator and scope.  They always checked perfectly flat throughout the range of my signal generator. For my transmitter audio, I never worried much about frequency response as high as 20 kHz anyway.

One thing I have noticed is that the U.T.C. Linear Standard series audio transformers, long touted to be top-of-the-line, are nowhere near what they are cracked up to be. I have quite a collection of those, and have run frequency response checks on most of them.

UTC specifications in the catalogues lists these as +/- 1.5 dB or better from 20-20,000~, and many from 20-30K or 40K. Some of the older ones, manufactured before WW2 with the cast-iron cases, begin to roll off at about 5000~, or else they begin to have dips and peaks in the response.  The newer ones, with grey cases made of some special composition alloy, tend to show irregularities above about 11,000~.  I have seen very few of these transformers that registered flat all the way to 20K without some dips and peaks somewhere.

My HF-300 rig uses a LS-49 class-B audio driver transformer, that I purchased NOS.  The first time I tested it right out of the unopened box, I noted irregularities above 11K.  I connected the primary to a signal generator, and the split secondary winding, one end was connected to the horizontal deflection plate of a scope, and the other to the vertical, with the midtap to ground.  If the transformer is working properly, audio at each end of the split winding should be exactly 180º out of phase with the other.  The scope pattern should be a straight diagonal line regardless of audio frequency.  Mine showed a perfect diagonal line up to about 10K, but beyond that it began to separate into an ellipse at certain frequencies, and up between 11K and 20K there were some bizarre squiggles in the pattern.  This indicated phase error at the audio output from opposite ends of the winding.  This would produce distortion in a pushpull amplifier.

I didn't worry about it, since the transformer shows perfect response well past 7500~ which is the highest frequency I ever transmit over the air.  But I suspect  that if I tried to run hi-fi audio well above 10K, that phase error would generate some splatter beyond 10 kHz of my carrier.

My opinion is that these transformers, even the pre-WW2 ones came that way from the factory, and that UTC inflated the response ratings.  What could happen to a transformer, that does not have shorted turns, to cause the response to go ragged at the upper end after sitting for years on the shelf?

It's sort of a moot point, since most AM broadcast receivers of that era, especially pre-WW2 ones, very rarely had significant frequency response as high as 5000~.  I suspect that no-one ever noticed those transformer irregularities simply because the they couldn't hear  any audio that they produced in that frequency range.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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Todd, KA1KAQ
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« Reply #11 on: April 06, 2007, 03:56:41 PM »

you should know that Daven and others used whatever type of resistors they had on hand for any given value in the attenuator (except for the really early ones - they're all WW). That means WW, MF, and carbon comp are interspersed.


That's strange. I have a Collins 212A board from the late 40s/early 50s with ten Daven rotary attenuators. Of the ones I've looked inside of so far, all used identical types of resistors. But then, Daven wasn't a company known for shoddy construction practices from the little I know about them. Any can I've pulled open that wasn't repaired or otherwise molested by someone was the same: very uniform.

Quote
Obviously for ham radio this doesn't matter at all, but in hi-fi situations, that's not so good. Ya know why? Cause yaz can hear differences in resistors! Yeah, one can.

Really? With respect to the design and operation of an attenuator, and assuming known-good components - how does it sound? Beyond the 'weaker/stronger' reaction to more or less attenuation, I mean.

I do seem to remember 'JJ mentioning that Yaz has good ears, though...   
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« Reply #12 on: April 06, 2007, 04:01:19 PM »

Don,

I recall an old-time audio engineer telling me a number of years ago that UTC stood for "you take a chance"!!!

Your measured results regarding these UTC xfrmrs are not surprising.

Much better audio output and input xfmrs were made by Peerless and Triad, IMHO.

73,

Bruce
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« Reply #13 on: April 06, 2007, 11:24:50 PM »


I've taken apart a wide range of Davens, Shallcos, Tech Labs and similar.
Back in the day, they all came out of Jersey.

Most of the ones that I've had to take apart from the 60s and newer used a variety of different type and mfr of resistors. As I said, the earliest ones, with thick thick silver contacts and brown phenolic material seem to have scramble wound, free in the air or in the beeswax WW resistances.

Todd, for most situations, a resistor is a resistor. But as you actually get closer to a distortion free signal chain, especially in the speakers, one starts to hear rather small things make an audible difference in what you hear. It's not mass hypnosis or hallucinations either. Actually, it can get to be a bit annoying at times...

You can fairly easily hear a difference in resistors by taking something like an old McIntosh amp and changing out all the carbons (just about all the resistors) for MF and MO types, and listening. The change will probably be reasonably noticeable right away. Obviously, changing one resistor out of many is not likely to amount to anything in most places in most circuits. (it's absolutely trivial to hear differences in caps...)

Don, I tend agree about the LS series. The older non LS series is not so good. The lams tend to be thick and not such good steel. Some of the LS series is quite good. I have an LS-10X here that measures nicely out past 40khz, how much past, I don't recall now but I think maybe out to 60-75kHz. I think they want to have the requisite load on the secondary when you go to measure the freq response... in case you didn't have the load on, they might measure better with it in place. Some of the LS series output iron, I have never liked the way it sounded, dunno why though. The ones I have measured in that spot don't measure badly. Always put them in the "?" category for audio outputs.

In general, it's possible to build (or buy) audio iron that is good out as far as 100kHz.! I have some here that do that trick in sizes up to ~250watt cores... but most commercial iron doesn't do that well. The ones that do tend to be pricey. Input iron like Jensen, Cinemag and Lundahl (and others) do that trick depending on turns ratios. There is output iron that is similarly good. Tango and Tamura are highly regarded Japanese brands. Some mfrs are using the Amorphous steel now for even better HF results. Permalloy (Nickel Alloy lams) have long been used for better HF response in the high-fi world - but they seem to not do as well on the bottom end.

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« Reply #14 on: April 06, 2007, 11:44:03 PM »

When testing audio transformers you have to run them at DC bias to get the real picture. The core offset has a big effect on the frequency response. I had to design an audio interface in a mil system a while ago and used a 5 Watt PICO part.
It was rated at a good frequency range for what I needed but the power level was under 1 watt. This exteneded the frequency range a lot well beyond spec.
It is very hard to limit leakage inductance in a transformer with a lot of turns and still have a good low end.
I bet many hams couldn't measure audio response back in the days before guys could afford good test equipment. gfz
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N3DRB The Derb
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« Reply #15 on: April 07, 2007, 06:31:12 AM »

Quote
Hey it's only money. I just spent $2K of my tax return to patch up our little mashed kitty...
but my daughter just walked by and gave me a big hug....

stuff is stuff, but saving loved ones is priceless.
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« Reply #16 on: April 07, 2007, 09:39:28 AM »

Don and all:

In the 1980's I built a stereo console with 10 Daven 100K stepped attenuators into an OP-Amp summer.

Flat to 40Kc with no reactions or loss of HF.

The Daven's were the highest construction quality.

And they sounded the same as carbon pots.

73,
Dan
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k4kyv
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« Reply #17 on: April 07, 2007, 10:41:43 AM »

When testing audio transformers you have to run them at DC bias to get the real picture.

Since those transformers are designed to operate without DC through the windings, and I use blocking caps and plate resistors in the actual circuit, I don't test them with DC, even though the specs may say up to 10 ma or so.  I load them down with a resistor the same value as the spec sheet gives for the secondary impedance.  I also add some resistance in the primary circuit because my signal generator is a fairly  low internal resistance signal source, usually by inserting a 10 dB or so T pad.

Those were some older LS-10X's that I described that began to peak and dip at about 5000~.

The newer ones with hypermalloy  cases (that's the alloy I mentioned in the previous post) seem to be closer to specs, but even those often go weird up past 11K.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #18 on: April 07, 2007, 11:55:33 AM »

Not too long ago I witnessed the use of carbon resisitor pots in an audio board used in a college FM radio station. The format was free speech and great commercial free music that was driving the commmercial stations in Washington DC up the wall with their rigid format.
Any way twisting those carbon pots 24/7 between turntables made for a scracthy sound after 6 mos.Then the stereo channels would be unbalanced as you tried to find the sweet spot of the worn out Allen Bradley carbon pot. I bet those Davens could be twisted day and night with just a spray of De-oxit once and a while. They are hard to break.
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Fred KC4MOP
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« Reply #19 on: April 07, 2007, 12:44:41 PM »

Don, I'm surprised at the results you got with the LS-10X. I am sure I did not see any anomaly in the response at 11kHz. Possibly there is another explanation? Btw, the hypermalloy refers only to the laminations, not the zinc alloy dress case that UTC used. In general "permalloy" refers to a steel transformer lamination that is high in nickel content.

The LS-10X that I have here is not that new, it's in the cast alloy case, but is painted factory black - not grey, so it is earlier than the grey ones...

Dan, I don't want to belabor the issue, but switched wiping contact attenuators do not "sound the same as carbon pots" any more than high performance racing tires are the same as Firestone's cheapest tires. On a car doing 45mph, probably no difference, but on a Nascar vehicle at 200mph, quite a lot of difference.

(the scratch carbon pots are due to some DC appearing on the pot, which in turn is due to either bad design or leaky caps... I'd have changed out the caps in that board)

Similarly, opamps do not sound the same, even types intended for "audio application", even though they spec at very low "distortion figures". Same idea applies, as for most things, they're all about equally good and the same, but for high performance situations differences begin to appear and become evident.

I can demonstrate differences in opamps without much difficulty, should anyone care to hear such a thing. You will not have to strain to hear it, or "make believe". (bring ur favorite single and dual opamps in DIP-8 package for comparison - doesn't matter which ones they are at all)   Grin

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