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Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

Re: Design matters greatly for passives....

Hi,

> this seems like such a slap-you-upside-your-head obvious way
> to attenuate volume, so why has it almost never been implemented
> before?

Who says it wasn't? The first such application I noticed in old WE Cinema Amps (in the 1930's), okay, the steps where rather larger than what we need now, but same principle. In later years Luxman made a commercial Transformer based Volume control. I kano input transformers with multi tapped secondaries as coarse volume controls from Pro-Audio gear.

It is however true that few commercial examples exist. Here is why:

An Alps "Blue" Pot will set you back $ 5 in quantity. Reviewers will comment how "good" your chocie of passive components is.

A pair of S&B Attenuator transformers will cost probably $ 300 the pair, a little less in quantity. The Switch to complete it will be at least $ 50. The time to wire it up.... Lets say such a Volume control will cost the maker probably around $ 500 to implement.

A good passive preamp with an Alps Blue or Bourns Pot sells at $ 500 - $ 1,000. Our Transformer based unit, given similar pricing and suitable quality of the other parts would have to sell at $ 50,000 - $ 100,000.... How many will you sell?

So you reduce your profit factor and sell the thing at (say) $ 3,000), barely enough to cover distributors margins, dealers margins and enough money you can buy food and shoes for your kids etc. How many are you likely to sell? Still very few if you ask me, as it is will be just "another passive preamp".

> I mean, sure it's expensive, but as you've noted there are some
> _very_ expensive resistive attenuators out there (including the
> Placette relay/resistor model for $1000).

Relais even in large numbers are a lot cheaper than a good switch and sound also much worse. Resistors once purchased in quantety come nicely down in price. Using a suitable binary encoding system you can get away with very few relais and resistors for a wide range of Volume settings. I know of VERY few manufatcurers that fit really high quality stepped attenuators to their gear, all but a few use $30 Elma Switches with $ 0.05 (in quantity) resistors.

> Therefore, I don't see cost as being the reason they haven't
> been used.

But it is the VERY reason. In addition, among Audiophools there is very unjustifiably a strong "anti transformer" sentiment. Otherwise such inherent misconceptions like the Output Transformer Less Valve Amp's would not even exist (Kudos to McIntosh who fit output transformers to their Solid State Power Amps!!!).

> So, there must be _some_ drawback, right?

Well, there is a certain limit to impedance levels, both Sowter and S&B Attenuators if left unterminated offer "only" around 30 - 40k Input Impedance. If the source impedance is high, the LF extension will be reduced and distortion will increase.

So a transformer volume control is not quite as universal as a resistive one, though IMHO not in the context of passive controls where a pot will have to be rather low resistance as well.

> Also a question: when using transformer-based attenuation, do
> you still see an advantage to buffering the signal,

I would see an advantage to buffering the signal BEFORE the transformer. Using a suitable valve buffer stage (or solid state) you could present the outside world with a very high impedance input, while having a very llow impedance output. Using a 5687 as Cathode Follower can give very low output impedances, of as little as 100 Ohm.... And one could have a switch that would allow "buffered" passive, or "passive" passive....

> or do the transformers pretty much solve all the current-drive
> issues of a purely passive controller?

Pretty much so, assuming the Source is up to it.

> If that assumption answers my own question, what kind of current
> would be needed from the source to drive a typical solid-state
> amp while using a purely passive xformer attenuator (20k ohms input
> impedence or more is typical, right?)?

Basically the same as needed to drive the Amp. A transformer is literally like a gearbox for AC electricity. If you don't build any passive gain into the circuit, then all that matters are insertion losses, which good transformers keep to a fraction of a db. So at the output of the transformer nearly the full power produced by the source is available. Does that make sense?

Ciao T


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