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Old 12th February 2003, 08:59 PM   #1
amt is offline amt  United States
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Question dpdt attenuator

Hi I was wondering if this is a viable method of audio quality attentuation? It is used in other electronic applications and if it would sound good, might be an easier way to create a 25 step attenuator. It would use only 10 resistors for stereo and 5 high quality toggles. Anyone ever tried this?

amt
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Old 12th February 2003, 11:17 PM   #2
Bobken is offline Bobken  United Kingdom
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Hi,

The answer to your question here is no, in my case, and I wouldn't try it since there are just too many switch contacts and/or resistors in the signal path for it to sound very good.


It would also be a nuisance in use with so many switch toggles to operate if you wish to change the levels, I would have thought, compared with the usual rotary pot.

However, if you are not concerned about these drawbacks, no doubt it could be made to work.

What is not clear from the diagram is where any ground is connected, and from what you have drawn it seems that you are (may be?) under the impression that merely inserting some resistance in series with the signal will act like an attenuator.

If this is the case, unfortunately you haven't properly grasped the principle of operation of one of these attenuator devices, which needs to act like a potential divider with the input signal being split between the output and ground.

Series resistance, alone, will have some affect on levels, but will not reduce the power sufficiently to act like a conventional attenuator and reduce the output to zero, or anywhere near this.

For example, I have a DIY guitar amp which uses 3.3 MegOhms resistance permanently in series with the signal, and this is part of the ordinary circuit associated with the reverberation option.

I hope that this helps, and am sorry that it is such an apparently negative response.

Regards,
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Old 13th February 2003, 07:30 AM   #3
amt is offline amt  United States
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Bob, I have a reasonable understanding of attenuation and meant the diagram to only represent the resistance portion of the circuit. The ground would be just were it would for a standard rotary stepped attenuator, shunted or not.

I agree that it might be cumbersome to use but after talking with others that use stepped attenuators, most generally use only a few of the steps in their normal music listening. Once one figures out the right positions, it would be quick to switch.

My real concern is whether the contacts in the switches would be noisier than the contact and multible solder joints in a stepped rotary. Is a premium switch always going to be noiser than a soldered joint? If they are, the ideas dead, but if not, maybe Ill give it a try.

Art
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Old 13th February 2003, 10:43 AM   #4
Bobken is offline Bobken  United Kingdom
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Quote:
Originally posted by amt
Bob, I have a reasonable understanding of attenuation and meant the diagram to only represent the resistance portion of the circuit. The ground would be just were it would for a standard rotary stepped attenuator, shunted or not.

I agree that it might be cumbersome to use but after talking with others that use stepped attenuators, most generally use only a few of the steps in their normal music listening. Once one figures out the right positions, it would be quick to switch.

My real concern is whether the contacts in the switches would be noisier than the contact and multible solder joints in a stepped rotary. Is a premium switch always going to be noiser than a soldered joint? If they are, the ideas dead, but if not, maybe Ill give it a try.

Art
Hi,

I'm sorry and I didn't intend to put you down here, but as shown, it did make me wonder if you understood the matter well.

I have had a lot of experience of pots and switched attenuators over a great many years, see some of my posts on this subject in other threads recently, but I really wouldn't bother to progres this as a practical idea, if it was me.

Several switches and resistors in series do not 'sound' very good for many reasons, and it is better to minimise the number of soldered joints, too.

I have said before that anything (almost!) can be made to work, but this is just not a very good way to do it, in my experience.

When I tried a similar arrangement many years ago, when trying to increase the volume in consecutive steps, remembering the (relatively) complicated sequence of switch patterns was a pain in the a*s*compared with the simple and intuitive rotary control.

If I recall correctly (I have tried several dozen switched variations in my time, so there is lot to remember!) because one needed to reverse one switch but at the same time reverse maybe one or two the other way, in order to achieve the next step up or down, whilst this switching was taking place the volume level was jumping up and down, at alarming levels.

Have you sat down and worked out the permutations of switch positions needed to acheive a gradual change like this, I wonder?

Also, the very best switches are rotary types because of their self cleaning action due to their wiping type of contacts, and toggle switches will perform less well because of this, especially after some time in use. If you haven't already seen it, have a look at my posts in the "Passive Preamp" thread for my opinions about switches and their sound quality.

The final difficulty, which I simply couldn't tolerate from the aggravation aspect, is caused by the fact that toggles are always (at least I have never seen any other type, and I cannopt imagine quite how they could be made) 'break before make' in operation.

This being the case you will have some brief time during changeover (each time any switch is operated) where there is no continuity, which will most certainly cause all sorts of pops and bangs to occur, and even worse, depending on your source equipment, could even cause some damage to it as, momentarily, it will be completely unloaded.

Some equipment will not tolerate this kind of abuse, because you are effectively unplugging and then plugging back again, a load whilst it is switched on, and that is never going to be a good idea with any electronic equipment.

Regards,

Edit: I overlooked this point, but I would say that a properly soldered joint will always sonically outperform *any* switch contact, including the very well made Shallcos (which I espouse in the other thread I mentioned above) even used with contact enhancers which I have commented on elsewhere.
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Old 13th February 2003, 12:29 PM   #5
Rarkov is offline Rarkov  England
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Hi amt,
There were some threads about something similar recently where nearly all of the problems pointed out by Bobken are discussed, albeit using relays rather than DPDT switches. It involved BrianGT and his flat mate...I'll try and dig out a link for you...

Edit:
Here is the link. It makes for quite an interesting read. Totally unreleated - do a search for forum member mhennesy and check out his website. This is one of the most beautiful solid state preamps I've ever seen!

Gaz
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Old 14th February 2003, 12:30 AM   #6
amt is offline amt  United States
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Default oh well

Bob/Gaz, Thanks for the help and explainations/links. It just seemed like an easy way to go if workable. I should have suspected that if nobody is using this method, it must not be viable. As shown once again (as an aged EE friend of mine used to say) "everythings already been discovered and rediscover and refinement are all that is left" I will go the rotary route.

amt
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Old 14th February 2003, 10:08 AM   #7
Bobken is offline Bobken  United Kingdom
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Default Re: oh well

Quote:
Originally posted by amt
Bob/Gaz, Thanks for the help and explainations/links. It just seemed like an easy way to go if workable. I should have suspected that if nobody is using this method, it must not be viable. As shown once again (as an aged EE friend of mine used to say) "everythings already been discovered and rediscover and refinement are all that is left" I will go the rotary route.

amt
Hi,

You are welcome.

Unfortunately, your EE friend is (mostly!) right but anyone claiming omniscience in these specific areas of audio should be viewed with some caution!

I certainly don't, and there is always something to be learned from life every day, but I do happen to have had quite a lot of experience with attenuators over the years, and, through listening tests on these devices, have discovered what generally contributes to good sound, and what doesn't!

I haven't looked through all of the link which Rarkov kindly posted, due to time constraints, but I can assure you that relays suffer from the same problems as toggle switches exibit, only, frequently, more so.

Again, because of the way their contacts come together, there is no self-cleaning action, and regrettably, their contact pressures are usually much lower than toggles, too.

Even with some very high quality noble metal, inert gas sealed, relays I have tried, which were quite good (sonically) to begin with, they soon deteriorated under use, and their performance dropped off as a result.

Also, note my comments about the differences in performance between the Shallco/Vishay switched attenuators and those which I have which are switched optically (with otherwise exactly the same resistive elements) as described in the passive preamp thread.

There must be some new innovations awaiting discovery, but because of all the difficulties associated with this particular choice of yours, unfortunately, I don't think this is one of them.

As I have said many times before, I never wish to discourage anyone in these matters, but I strongly believe that there are better avenues to explore as far as attenuators are concerned.

Regards,
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