Re: Speaker level adjustment

I find that odd, as if it's wired correctly, an L-pad should give an adjustment range from 0- 100% of full power.

Another alternative would be to install a 100 volt speaker system with independent volume controls in each room, but that would mean buying a new amplifier,and some matching transformers and volume controls.

Well, AFAUK I have done it right, but I suppose that is a bit of a circular argument. ;-)

So, significant expense, compare with my proposal which you snipped. Just for the record, what would its advantages be?

Chris

Indeed. Obviously, you'd not deliberately or knowingly do it incorrectly. :-)

Completely independent control of levels on all speakers. Only one amplifier to go wrong, and 100 volt line amplifiers tend to be designed and built to last for ever even when abused. The downside is that the transformers can degrade the sound quality slightly, which may obviously be an issue.

I've used a 100 v line transformer I had lying around to adjust the level of a speaker fed from a normal low impedance amp. It worked rather better than a pad. It had five level taps on it - but it would be a bit of luck if it gave the amount of attenuation wanted. Pads always seem to upset the bass end - even on speakers which aren't exactly Hi-Fi.

There are only three terminals on a potentiometer.

The amplifier should go to the outer pair and see 8R load. The speaker should go to the centre tap and one end (the low end). If you choose the correct end then you will get a logarithmic adjustment and if you choose the wrong end you will get a sort of all or nothing behaviour.

Rotation 0 1 2 3 4 5 r (correct) 0 0.5 1 2 4 8 (correct) r'(wrong) 0 4 6 7 7.5 8

Swap the end you connect the speaker to and you might be surprised.

You still lose some bass damping this way but it may not bother you.

as can ANY form of passive attenuation to a loudspeaker designed to be driven from a

You can use a transformer to adjust power output via taps without upsetting the impedance - but resistive pads are not good for bass units.

As part of my essays into reproducing 'valve sound' for guitar amplifiers, artificially raising the output impedance of the power amplifier was routine.

The good old Vox A30 amplifier has an output impedance of some 80 ohms. Driving an 8ohm pair of loudspeakers, the effects are dramatic.

Massive bass resonance, about the bottom A on a guitar, plus overall treble boost as the rising impedance of the speaker voice coil (a lot of leakage inductance in a voice coil) is no longer a significant effect in reducing treble output. Boom an chink we used to call it.

Also all other resonances in the paper cones and cabinets are much less well damped, so the sound comes 'alive' with the sound of - well wood and paper!

The amplifier loudspeaker and cabinet are the 'sound box' of an electric guitar and give it an interest and colour it otherwise lacks. Hifi it aint. But it isn't intended to be, either.

I'm not familiar with the design of attenuator pads used in 100v and

I suppose the reduction of "Damping Factor"[1] is allowing the bass cone resonance to more prominently dominate the frequency response.

Since over 90% of the typical bass driver's "Voice Coil Impedance" was made up of its ohmic resistance, the actual "Damping Factor" was more like 1:1 using such an amplifier (the voice coil resistance being effectively in series with the amplifier's output impedance).

In practice, the damping effect was only of importance at the resonant frequency of the drive unit which is reflected as an increase of impedance which, for an 8 ohm driver, could climb to as high a value as 30 or more ohms.

In the case of a 32 ohm impedance at resonance, the actual damping factor applied by our "400:1 Damping Factor" amplifier would actually be a mere 4:1, just 1% of the claimed 'figure of merit' implied.

It's been a good 30 years or more since I last looked at such Hi Fi advertising 'blurb' so I don't know whether this misleading practice has stopped and the more useful "Output Impedance" figures are being quoted instead (e.g. "Output Z = 20 milli ohms", rather than "DF=400:1 for 8 ohm loads"). Quite frankly, the difference between 500 milli ohms and 20 milli ohms output impedance would be barely measurable let alone audible.

"The Natural Philosopher"

Lotsa valve guitar amps use PP output pentodes with little or no NFB, just like the AC30.

.... Phil

And QED used to make resistive speaker controls with wirewound pots. OK they could get a bit warm and crackly, but the seemed to work.

Brian

Actually, that isn't quite correct, see below, I had forgotten what I actually did.

Looking again at what I have done, if you remember my original problem:

I used the L-pad to reduce the volume in the dining room, which closer examination shows is actually working. However, to get the kitchen loud enough, the living room would be too loud, and I don't really want to mess with that output, since that is the best set of speakers.

Chris

Never had any problems with any of my designs attaching 100V transformers to em. LF response is pre tail;rerd to get rid of serious 'rumble' on most amps anyway and SOAR protection mens teh worst that can happen is serious distortion if you end up wit DC bias..

No not perfectly good. Acceptable. Barely.

Natch. How else?

Are they? Been too long..but your figures are as I recall from tests in terms of ferquency and gain..

Actually you would be surprised. They do NOT. Only the AC 30 (EL84) was in any way seriously high impedance. Most EL34 naps had some feedback - Marshalls come out around 8 ohms and the US amps (tetrodes) were much more 'hi fi' at less than an ohm.

I concluded that Fender, being an ex radio engineer, designed decent amplifiers. Whoever did the Vox was a post war penny pinching scoundrel, and Jim Marshall was a carpenter, who fiddled around till it was loud enough and bad enough to be acceptable to cloth eared blues guitarists.

IN short the British 'valve' sound was entirely due to a serendipitous coincidence of parsimony and incompetence.

Every British amp I dissected was essentially a Mullard standard applications note with feed back taken out to increase gain without more valves and components removed or downrated to make for a cheaper design. And a bit of clueless tampering to change the frequency response, and a total mistake in the Marshall preamp where it looks like he couldn't read the actual color code in the cathode resistors and put in 10k instead of 1k.,. thereby creating the Marshall sound entirely accidentally.

Then of course both Jim Marshall and the Vox boys were chasing decibels at low cost, so both employed ultra lightweight speaker cones that had vile coloration and zero stiffness. But louder. One imagines Celestion were glad to unload these at any price.

I do have a valve Marshall of course. Sounds the same as my designs, but better second hand value...

Every American amp was basically a faithful reproduction with decent sized components of an RCA Hi Fi. design. Plus occasionally a well designed extra pre-amp valve.

Which is why fender amps sound boring, and AC 30s and Marshalls do not.

British amplifiers and louspeakers are the sound of post war poverty.

Should be more than good enough for driving ceiling speakers. If you drive it off the existing speaker feed, via a resistive divider on each channel, it'll be more convenient too.

NT

Any suggestions for component values for the divider?

Chris

Thing with ceiling speakers is they're in a sort of infinite baffle so should have a good bottom end unlike a cheap small box design.

You'll need to play with the values, but could start with 10k in series then 1k in parallel

NT

not for loudspeakers. Try 220 ohms and 22 ohmns

Even driving a pair of headphones is only a couple of K

NT

Oh - missed that. Thats fine then