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Passive Attenuator: builder renditions

In answer to cfranz:
ashok and lorne, ... You will find, I think, that using a stereo, or 2 pole ganged pot and varying the shunt resister to match (or attempt to match) variations in the two 'sides' will make a more convenient attenuator.
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I may be one of the few who does not mind working with two independent pots — twirling two knobs. The Haller design is especially good at creating a precise balance by adjusting two separate pots. The Elliot design my be just a tad different due to the input impedance changes that occur. Still, the two pot balance directly through the volume pot has got me so convinced that I have considered modifying my active preamps to include them while jumping over the balance pots.
If it becomes that big an issue, what I've taken to is 'simply' building a 24 step attenuator and hand matching the resisters.
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Yep ... that same sort of thing was on my mind when I got into this stuff. And then I found out how cheap the parts were for the ones I built. I'd really be interested to know your assessments should you ever get a chance to compare other designs with your step device

Pain in the arse but a very satisfying result. I know of one guy who matched his resisters to .001%. He's nuts, but you have to admire the ingenuity...


See also, on the web somewhere a monograph by 'VoltSecond' on various attenuator designs. Just look for VoltSecond and attenuator, you should find it.



Good luck guys. I'll be following these threads with great fascination.....
 
In answer to doucanoe:

Hey lorne, I'm in the process of putting together a shopping list for a basic passive. Actually, its the one redboy built and not sure how similar it is to your versions.
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redboy and I have built the same Haller versions, but we have diverged in how we mounted the parts. So, if you look at mine, depending on which one it is, it may look different than redboy's, but the circuit is essentially the same.

Asok has drawn up another kind of circuit. It works great too. You can read an article about it on Rod Elliott's site, Elliott Sound Products (ESP).

http://sound.westhost.com/project01.htm

Rod suggests a 100K Ohm linear type B pot and a 15K Ohm R. Now, the R can be in ratio with the pot anywhere from 6:1 and 10:1, but the ideal log function comes in at 6.7:1. I recycled two 20K Ohm pots. I dug out four 6.8K Ohm R's I had in the bins and put them in parallel for 3.4 K Ohms per pot. This puts me a bit over the ideal mark, but I'm not anally retentive enough to fret about it.

The result — great. Good control and very good sound from the CD player. I'd like to make one using a 100K pot.

As for the Haller design that redboy has built for you: I used 10K and 20K pots in two distinct builds. I played with different R's but I started with the values suggested by Haller himself in his thread over at DIY Audio. I think that I usually came to something like a tenth to a fifth of the value of the pot. Gearhound did something else.
It's calling for a 100K ohm pot but you have noted using a 10K ohm pot. Would it be just the resistor values that would change?
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You can use different value pots, but your impedance matching may suit one specific value more than others. The value of the R's will correspondingly change. I think that I used in one case a 2K Ohm with the 10K pot, and in another 8K Ohms with a 20K pot. Don't hold me to these values. If I did keep notes, I'd have to dig for them. I know that I used various values. The Elliott device was easy. I just used the ratio factor.

I'm new to all this mumbo jumbo but am trying to follow along.
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Well, with any effort, you may very well overtake me, and I'll be asking you some questions in turn

It's great that redboy helped you out. But I gotta say that if you can cook breakfast and find your way to the county fair, you can build one of these things with four or five simple tools, a hand full of parts and a sandwich box. For some of us, it's understanding how the circuit works that takes a bit of effort. It's basic, but it means getting your head around how Ohms Law relates to basic principles of electrical theory. I can cook breakfast, but no one should rely on my physics:stupid:
 
Re: revelation

Good on ya von.ah!:thmbsp:
It took me 2.5 hours and it looks like it should be taken out of its misery.
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As long as the solder joints were not cold joints and you made a mechanical connection before applying heat and solder, then what it looks like is relatively unimportant isn't it? A person could make the most gorgeous looking piece but nevertheless use solder as glue and produce a heap of cold joints only to end up with a defective doorstop.

It really pleases me that you found excitement in your first project — especially one that is ... as you say ... a revelation. And by the way, I love those funky knobs that are relics from a bygone era.
 
So far.....that Haller inspired passive pre I built earlier (10K Alps Blue stereo pot/16.9K Dale RN-60 resistors).....has worked out GREAT with my 1988 B&K ST-140 solid state amplifier.

But.....I just picked up a pair of working Hammond PP 6gw8 tube mono block amps (about 10 watts).
I'm in the process of doing.....a COMPLETE rebuild (from soup-to-nuts) as we speak.
So......I just purchased a 100K Alps Blue stereo pot.....for another Haller inspired passive pre build to use with the Hammond amps.
We'll see how this one comes out...............................................................

Steve
 
So after a little bit of critical listening, I have noticed a few things:
One side gets louder before the other, though I'm not sure that there is consistently "more" voltage to that side as the pot is turned up.
The operative span of the pot is ~1/4 of a turn at most.

If I want more gradual usage of the pot, I should increase the resistance of R1, correct? If so, can I merely add a resistor from Input to Pin#1 and eliminate the signal wire, creating a series addition to the circuit?

Also, could I use differing R1 values to compensate for the difference in left vs. right signals?
 
When I did mine.....I started out with a 2K resistor.....and had NO play in the 10K pot whatsoever.....almost like an ON/OFF switch.
Then moved up to a 4K resister.....a LITTLE more pot usage.....up to a 7K resistor.....better, but still no cigar.
The bottom line was.....a lot of trial-and-error.....before I got to a pot value (16.9K) that sonically sounded good AND gave me usable pot range.

Steve
 
I do have all my components for the "build" in but have not found the time to get to ejfuds to prep. my chassis yet. I am really itch'in to put this thing together.

Maybe next week I can sneak over there if he has some time.


Hear that Gary, It's all your fault that I don't have this done yet :D

RC
 
One side gets louder before the other, though I'm not sure that there is consistently "more" voltage to that side as the pot is turned up.
The operative span of the pot is ~1/4 of a turn at most.
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If you have not got one, head out the door with $25 in hand and pick up a multi-meter. You can watch what your device is doing at the output. Cheaper pots are very liable to have dissimilar behavior as they are swept through their respective rotations. If you have a stack of them, you can meter each one and match them. And even then I suspect that some of mine have changed with use. Similar things apply to ganged pots. The solution for these issues are: (a) buy an excellent Alps pot; (b) buy and install TWO pots instead of the ganged type.

If I want more gradual usage of the pot, I should increase the resistance or R1, correct?
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Exactly what gearhound said. As Haller said himself, there is no free lunch. Your usable sweep may be in contention with the optimal sound, and that will vary from case to case depending on impedance matches. I've found this to be less of of an issue with the Elliott circuit, but then as asok has mentioned, your output impedance varies with the pot sweep.

No free lunch indeed, and versatility is not a strong point in favor of these devices. But, the intriguing thing is, when you dial all the factors in and get it right, it is simply brilliant performance for less than beer money.
If so, can I merely add a resistor from Input to Pin#1 and eliminate the signal wire, creating a series addition to the circuit.
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Yes.
Also, could I use differing R1 values to compensate for the difference in left vs. right signals?
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Well, I wouldn't. I think that this will just confuse the issue. Now, someone lash me if I am wrong here, but there is a point where impedance mismatching begins to have the effect of a filter — NOT BEING a filter — but having the effect of a filter. This may not occur with the compensatory variance that you are contemplating, but who knows what values and what matches you are going to get tied into. IMHO, it would be better to consider solution (a) or (b) as above.
 
gearhound wrote:
But.....I just picked up a pair of working Hammond PP 6gw8 tube mono block amps (about 10 watts). I'm in the process of doing.....a COMPLETE rebuild (from soup-to-nuts) as we speak. So......I just purchased a 100K Alps Blue stereo pot.....for another Haller inspired passive pre build to use with the Hammond amps. We'll see how this one comes out....
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I hear ya'. I'm doing a soup to nuts near stock rebuild of a an ST-70 (bigger PT and input filter). I want to build either a Haller or Elliott circuit with two 100K Alps pots. I've got 4 other projects on the go, so it may be later than sooner.
 
Hi Celt:

Very interesting. About how much does it cost to make a pot like the ones we see (in the link) — or did these come assembled. How many steps? Did you have lots of steps for low level?

TDS — the module. Can you explain, and/or give a reference where we can read up on it?

What did you sell it? Was/were there an issue/s
 
I sold the 50k ohm one right away. The 10k ohm was used between a CDP, Tuner and a Pass Labs Aleph 30 power amp for awhile. I assembled the Goldpoint pots, 64 steps as I recall, in 2db increments. I had several TDS modules, the one shown was the newest and the same used in Acoustic Research TDS boxes. The older ones were much larger with four trannies. The transformers added 3dB of signal boost (a good thing IMO) and also acted like a buffer. Music Direct had some of them on closeout several years ago. They do add some coloration, but many folks like the voodoo they do.
 
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I just want to say thanks for this thread. I'm in the midst of building a passive and have been experimenting with different ways to wire the circuit and think I like this one best so far. http://sound.westhost.com/project01.htm

This was given by Lorne in one of his earlier posts. I'm having a little trouble with volume settings but am on the right track now. I've got some different values of parts ordered and will keep experimenting till it's best it can be.
Although it sounds awfully good now. Running into ss amp and passive really seemed to increase the frequency range and I'm hearing things I didn't before.
 
Time to resurrect this thread. I finally got around to taking some pictures of my passive channel-selection-plus-attenuation device.

site1099.jpg


Channel selector on the extreme right, volume control to its left.

site1100.jpg


Three inputs starting from left. Two outputs.

site1101.jpg


Internal view. Sorry for the slightly out-of-focus picture. You can see the +/- 15VDC power supply (Marchand) on the extreme right. To its left is the familiar Bugle phone preamp. Battery holders are visible, although preamp is powered by the DC power supply.

To the upper left of the Bugle is a small floating board, which is the opamp buffer, with a very low output impedance. Under the opamp board is one of the 1:1 MQ signal transformers. The second 1:1 transformer is just below that.

site1102.jpg


A better picture of internals. You can see the transformers and the opamp board better.

Here is a partial schematic:

site1054.JPG


The phono preamp and buffer are on the primary side of the 1:1 transformers. Notice the volume control is on the secondary.
 
ashok: From my point of view this unit a triumph — a rationalized, minimalist preamp. To complete the picture ... how does it sound (subjectively) ... to you of course? Assuming that it sounds really good, I expect that a lot of us would love to have a full schematic, or at least a parts list. But you are neither an industrialist nor a publisher (as far as I know). You must have a real life. So any scanned crude drawings, doodles or notes would be cool, assuming that they remain stuffed in a drawer or a file case.

One would think that the net is crammed up to the tits with detailed variations on this theme. But, unless I am completely incompetent with search engines, descriptive presentations that would encourage us more pedestrian hobbyists to build our own are few in number and generally offering less information than what it takes to build a project.

I for one am particularly interested in the transformers that turn the passive concept into a active one. This has to be the way to go if the enthusiast is interested in building a project that side-steps some of the impedance matching issues. I think Rod Elliott has outlined a project that uses op amps, and I was thinking of trying to build one of those in the future ..

Thanks for the post and pics.
 
Hi theophile

I found the same thread and tried to get stuck into it when it was only a few pages long — and now look ... over 60 pages! I got baffled with George's first post; I just couldn't understand the circuit and still can't. I'm not proud to admit it, but there it is.

George talks about experiments and then goes on to describe attempts by the Melos company to circumvent problems caused by switches and contacts. Fascinating, but I was quickly swamped. The principal of using an LED instead of a common resistor will have my further attention as soon I can find something I can get my synapses around. (May wait a long time:stupid:)

I'd like to see everything you have about the device you mention ... like what attenuator ... what is a Burson and so on? Photos, parts, schematics, drawings? I'm all eyes and ears.
 
Looks like a poormans attenuator. Passive attenuator are noisy and not exactly precise but does the simple job of padding down the signal. It is very useful in connecting directly across speaker out of a radio, tv, etc... and hooking it up to a microphone input for recording such as computer mic's input . Not exactly for matching but I used 100K and 10k to get 10% attenuation. It's a useful tool indeed if you need a quick solution :yes:
 
Hi Fisher-man
Ok, I'm of very limited experience with this kind of stuff, but why not just a pot?
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Actually, a log pot on each channel and nothing else might work OK — as in could, may ... Same for ganged pots, but many of them do not track very accurately. I like two single pots because you have an effective balance control and tandem tracking is not an issue. (If there one best/good reason for outboard attenuation this would be it IMHO. Balance controls on the preamp tone board are a real PITA — they just don't work very well.)

The original idea of the ESP Project 01 pot was to be able to use a cheap linear pot. On the whole, ordinary resistive carbon linear pots track better than log pots. Log pots produce a curve that is adjusted to the way that humans hear sound — as opposed to linear pots which are .. er ... linear. The problem comes in cheaper (affordable?) log pots because they are compromised by their construction which produces a compounded curve that is attempting to appear contiguous — but ain't. Project 01 gets around this by using a linear pot and some resistors — in effect "faking" a log curve. It works, but, as ashok said a few posts ago, passive attenuating devices are very system dependent. Not many of us are going to get the ideal passive device to be the optimum goody for more than one or two system rotations at a time. Hence we get all sorts of variations on the passive concept in order to try and overcome some of the compromises. Some people are satisfied with a simple pot between the source and the amp.
 
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