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Attenuator experiments
There were some attenuator discussions here some time ago.
I built a test attenuator using relays. I use 8 relays, so I get a range of 48dB. The attenuation from 0 to -31dB is in 1dB steps, and after that the steps are -33, -34, -36, -39, -42, -48dB and off - 39 steps from full on to off. I just used E12 resistors, so the actual steps are not that accurate... My conclusion so far is that 39 steps are enough, but I think 12 steps are too little and 24 steps is less than I would like also. But as the amp I use (2 6SN7 gain stages into a 2A3) is much too sensitive, so I find that I actually need more steps in the low end. I guess I should just add a 20dB attenuation on the input of the attenuator - or change the input stage on the amp to lower the gain... Or perhaps a 7-relay attenuator with a switchable 20- or 30dB pre-attenuator would be a good idea? I have read many places that make before break switches are essential, and that break before make switches cause clicks and other noise. The relay switch I use is both I guess, as the relays don't have the exact same switch time. But I get no noises at all - I can adjust the volume setting with no problems and without muting. Are the clicks only a problem on direct coupled SS amps? If anybody's interested the schematic can be seen he http://stiftsbogtrykkeriet.dk/~mcs/8bitatt.gif And the mono prototype can be seen here (behind the big, grey monster): http://stiftsbogtrykkeriet.dk/~mcs/00000005.jpg Best regards, Mikkel C. Simonsen |
#2
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Kirk Patton wrote:
Hi Mikkel, I really liked your elegant approach to a series (pot-emulating) design . . Thanks. I have built, tested, and revised a number of attenuator approaches in the past year or two, all have been part of an active line-stage. They have been located in between an active input buffer and an active output stage. Here are my thoughts . . . I built a test attenuator using relays. I use 8 relays, so I get a range of 48dB. The attenuation from 0 to -31dB is in 1dB steps, and after that the steps are -33, -34, -36, -39, -42, -48dB and off - 39 steps from full on to off. Of the basic question is whether or not the attenuator should support the same step throughout its entire range, I think that in practical purposes this can be determined by whether or not balance control (by the same attenuator) is desireable. I may add a balance control. Since I have two attenuators that can be controlled individually, adding a balance control would be easy. If this is the case, then the attenuator steps must of course be consistant, otherwise any introduced offset between the two attenuators for balance adjustment will not track correctly across the range. My plan is to aim for 1dB steps accross the useable area. If not, then I find that a taper that gives a finer adjustment in the "money range" than in the extremes is more user-friendly. I personally like a minimum of fuss when actually using the system, and having to switch volume control ranges really annoys me. Any range switching would be done automatically. Right now I just use a + and - button. The attenuator is of course linear, so the taper is done using a lookup table in software. That table could also contain extra bits for switching ranges. And using a special taper with 0.5dB steps in some areas and 2dB steps in others could also be done. I recently built a mechanical 47-position attenuator with the following curve: -1,-4,-7,-10,-12,-14,-16,-18,-20, -21 . . . (-1dB steps to -50),-52,-54,-56,-59,-62,-66,-70,-74. The associated active linestage has about 15dB fixed gain. I didn't include a 0dB setting or an "off" setting; the former is for impedance reasons and the latter is because when I don't want to hear it at all, I prefer to push "stop" or lift up the needle rather than turn the volume all the way down. With the design I've chosen the "off" step comes for free. That step can be used for muting (which I often need during the day when the phone rings) - the attenuator can very easily be returned to the previous position when the mute button is pressed again. I don't have to turn a knob after all ;-) But thanks for the values above. I have been wondering what attenuation levels are actually used. I find this gives me a single knob-twist that works well from low background "dinner-level" to rip-roaring loud, and gives me fine adjustment in the range where I do my most focused listening. I'm not using any balance control. My amplifer is moderate-to-lower input sensitivity (rated 2.5v for 500w output), 500W output? I get by just fine with 3W... and speakers are highish sensitivity (I think rated 92dB/2.83v/1M). I just used E12 resistors, so the actual steps are not that accurate... My conclusion so far is that 39 steps are enough, but I think 12 steps are too little and 24 steps is less than I would like also. But as the amp I use (2 6SN7 gain stages into a 2A3) is much too sensitive, so I find that I actually need more steps in the low end. I guess I should just add a 20dB attenuation on the input of the attenuator - or change the input stage on the amp to lower the gain... Or perhaps a 7-relay attenuator with a switchable 20- or 30dB pre-attenuator would be a good idea? I think that this is mainly an impedance/gain structure question. If the attenuator is in the traditional position in an active-linestage (at the preamp input) then using a fixed attenuator pad at the power amp input will probably help S/N at the cost of preamp headroom (which with a sensitive amp shouldn't be an issue). If, however, this is an all-passsive setup, then putting in a fixed series resistance before the attenuator and lowering the attenuator's overall impedance will make the range more usable, and lower the attenuator's output impedance (good). Right now my "linestage" is a small box with a rotary input selector, so it's quite passive... Given that you have competency in designing the control electronics and are thus freeing yourself of the limitations of a fixed number of switch positions, you could simply add more bits (relays) to your existing design and have as much range and resolution as you need, and simply set the practical operating range in the control system. I did some calculations. It seems I could quite a good range with 7 relays and a fixed 25dB attenuator switched by the 8th relay. 0 to -50dB in 1dB steps and 67dB max. attenuation should be possible. This would add only minimal cost (unless this is a production design . . .) The cost isn't that much of a problem. The relays I currently use cost about $1 each, and the control chips cost even less. But I guess keeping the number of relays as low as possible would be a good idea. Production? I don't think so, but you never know... :-) I have used Hamlin SIP reed relays (available from Digi-Key) for both attenuators and source-switching with excellent results -- they are really mechanically quiet as well. How much do they cost? Reed relays are normally expensive, and only available in single-pole versions. The relays I use now (made by Zettler) aren't quiet, but the noise isn't much of a problem (to mee anyway). Anyway, hope this gives a few ideas. It does, thanks for your reply. Best regards, Mikkel C. Simonsen |