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Old 7th March 2001, 05:08 PM   #11
GRollins is offline GRollins  United States
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Join Date: Feb 2001
Location: Columbia, SC
doktor, Mark, everybody else...
Got sleep?
(Mark, do they have an ad campaign down there for milk with various celebrities with milk on their upper lips? I believe the ads are sponsored by some national milk industry board. This may not translate well to other countries.)
At any rate, I'm firing on more cylinders today. Back with an easier & cheaper solution.
Duh! (Accompanied by the sound of me slapping my forehead again.)
Put the volume pot into the amp itself.
-no pesky capacitances or inductances to worry about
-fewer boxes hanging around; why go to the trouble to have a box with nothing in it but a pot?
-cheaper! subtract 1 pair nice cables, 2 pair nice jacks (in and out), 1 enclosure, and of course the misc. rubber feet and hookup wire that always creep in the back door
The only disadvantage that I can come up with is that it's a little 'old fashioned,' as it is not current practice to put level controls on power amps. So what! It's your amp, your project...do what you want with it. Incorporate a switch and some extra input jacks if you want so you can hang a tape deck off the side.

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Old 7th March 2001, 09:33 PM   #12
GRollins is offline GRollins  United States
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3.33 dB sounds a little wide to me. Let everybody else weigh in with their opinions, then do something scientific...like flip a coin.
Two observations:
-I'd still check (using a cheap pot) to make sure you are happy with the volume attainable doing the direct drive thing. Some of these switches and the attendant resistors can get damned expensive. It'd be a pity to put out a huge amount of money, only to find that you're not satisfied.
-Keep in mind that there's no law that says that the steps on the switch *have* to be the same increment. You could have, say, six dB steps starting from the low end up to somewhere just south of a 'normal' listening volume, then drop back to 1 dB steps for fine control in that general region. This will save a lot of positions on the switch. If you haven't already checked, you'll probably find that 24 to 36 position switches are the most common, but that doesn't leave much elbow room for fine control if you try to divvy it up equally. Yeah, I know...this complicates the math, but you'll only have to do it once.

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Old 7th March 2001, 10:51 PM   #13
mefinnis is offline mefinnis
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Location: Adelaide, Australia
I hinted earlier that we have made a number of stepped volume controls and I would have to say these are my preferred option. You have much greater flexibility.

Cost *IS* an issue and the comment about checking first seems wise, just ignore the "white noise" in the background ;-)

For example, my preferred switch would be a Blore-Edwards 29 position gold contact job, which goes for the small sum of $AUS400 !!!! **If anyone has a spare, I'll pay the freight ;-)

I would propably settle for the ELMA 12 position, low-profile PCB mounting unit, its only $AUS100.

You get the idea ..... good switches cost real money :-(

Anyhow, doktor, Grey is absolutely on-track when he suggests making the steps different. This is what we have always done. Think about the way you listen ..... for me, I want fine control at low volumes (background use, multiple people in the room etc.), then as I start to get louder, less control and bigger steps is acceptable.

Suggestion: Aim for at least 6 x 2dB steps at the bottom end - 3.3dB is actually slightly greater than "doubling" power, ie. 2W, 4W, 8W, 16W etc.

So, if we realise that good quality, large No. contact switches cost lots, the design of the volume control in the Aleph P starts to make more sense: 8 DPDT good quality relays (not cheap), 8 switching transistors, 2 x 8 metal film resistors (peanuts) .... and 8bit AD-converter (not much), and a "cheap and nasty" pot (peanuts).

The end result, if you do it right is a 255 position rotary switch controlling balanced metal film resistors ...... starting to make more sense gents !

NP does his at quite high voltage for reasons that escape me. You can get 5-12V relays and the ADC0800 will run off 5V, so my plan was to have one of those plug-in DC supplies (like for your mobile phone) @ 12V which would be totally external and regulate it accordingly.

Comments welcomed! mark

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Old 8th March 2001, 02:07 AM   #14
GRollins is offline GRollins  United States
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Join Date: Feb 2001
Location: Columbia, SC
Let me know how your stepper circuit pans out. I considered something like that for my crossover (Now 4-way...can you imagine what 4 good multi-position switches with decent resistors [let's say Vishay, just for fun] would cost? Yikes!!!).

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Old 12th March 2001, 06:50 PM   #15
doktor is offline doktor
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Join Date: Dec 2000
Location: Saskatoon, Sk, Canada
Default cool

Mark just curious if you plan on making it remote controlled. Just got a chance to look at the service manual and it looks like a nice project. Good luck and thanks for your help
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