Active Speaker

[Crackle]

I have been looking at the tone control circuit from the Rotel amp.
It has some very specific component values.
Do you think the values are that critical to get a half decent smooth range of about +- 6 db on the bass and treble.
Should I make up the odd values with series resistors, and parallel caps, or just try the circuit with the nearest preferred value.
I can probably do most of the values like 1.8k, 2.2k, 22k, 1.5k, 8.2k from my spares.
   

Thanks
Mike

[Mark Hennessy]

Hi Mike,

That's a lovely bit of woodwork!

I'm guessing you prefer to use the Sinclair module because it will be quick and easy to implement. But if you're building tone controls as well, it's not much extra work to bung a power amp IC on there while you're at it. Looking at the schematics of that Sinclair, it's no wonder they were unreliable, given the complete lack of thermal feedback in the biasing circuit. And I know that you didn't mean it, but there's no way in this universe that they'd get down to 0.001% THD+N

The ICs I suggested earlier would be OTT for that drive unit. As you say, a couple of watts will do. There are loads of ICs that will do that. Years ago I used a lot of the once-ubiquitous TDA2002 - they will happily run from the single supply you have, and are easy to use. As well as the supply rail, the impedance of the speaker will determine the exact power output, naturally. Failing that (or equivalent), old PC speakers can often yield a suitable amplifier - ready built, of course. I recently took the time to desolder a TDA1517 from a 1990s soundcard before it went in the WEEE skip. With today's instant access to datasheets, old ICs can easily be given a second life.

In case you haven't already seen it, I built an "active" speaker a while back. If nothing else, you might find it an interesting read. The final instalment was added when I got a measurement setup - you can see how I tailored the frequency response to give a reasonable response from the small drive unit: https://www.markhennessy.co.uk/activespeaker/

Incidentally, I still haven't got around to properly measuring those 4" drive units that you kindly sent me a while back. A quick test looked very promising indeed. I plan to build another bench amp with several inputs and a built in mixer, based around one of those. I need to measure the T-S parameters to work out what kind of box they'd work best in first. No idea when I'll get the chance though...

Meanwhile, I started replying to some of the earlier posts before I starting putting the kids to bed. Now that we know about your specific project, a lot of it is redundant, so it has been edited down considerably.

---

The definition of an active speaker is simple: the crossover is implemented at line level, and there is a power amplifier for each of the frequency bands following on from this. A single power amp is a powered speaker. Yes, I (accidentally-on-purpose) made that mistake when I titled the article linked to above. I'll probably correct it in due course.

Meanwhile, there are many additional things you might choose to do while you're at it, including motional feedback, active servo technology (Yamaha), or a Linkwiz transform, but all of these are separate to the basic question of active vs powered vs passive. You could choose to do any of the above to either a powered or active design.

"Hybrids" exist - a good example is the Dynaudio Air 20, which is a three-way that uses a pair of power amps - one for the woofer, and the other for the mid and tweeter. The crossover between the latter is done using a simple series inductor for the midrange and a series capacitor for the tweeter. The low level crossover is actually implemented using DSP. They need occasional reboots.

Likewise, a "hybrid" powered speaker could use a passive crossover to split between LF and HF, but the BSC could be performed by a line-level bass boost circuit. That reduces the size of the inductor in series with the woofer by typically a factor of three, which equates to a significant cost saving.

The main benefit of MFB is to reduce harmonic distortion at low frequencies. If you use it to get bass extension, then you trade output level at LF. If you're smart, you do that dynamically (I've played with that sort of thing on an experimental sub years ago).

We're not terribly bothered by LF distortion, which is why MFB and similar schemes have failed to achieve significant market presence. Not helped by the fact that good drive units today have lower THD than the units available back in the '70s - motor design has come a long way... Far better to concentrate on mid-range distortion, where the ear is most sensitive.

[Mark Hennessy]

It has some very specific component values.
Do you think the values are that critical

No, the values won't be especially critical.

I do wonder why you wish to only have +/- 6dB of range. I guess you've listened that speaker already and decided it sounds reasonable enough? I'd expect to need a bit of EQ to help it, but that's just based on what I've heard of similar units.

If you're thinking of using it for fault-finding and signal tracing (like I use mine for), you'd probably benefit from a more conventional control range. The 400Hz high-pass filter option is really handy. By default, my unit is connected to the Y-out on my 'scope, and I honestly can't tell you how useful that it.

[Crackle]

Hi Mark
Those 4 inch speakers gave a remarkable good account of themselves with the Z30 wound up to as full as possible before the distortion became too bad.
Thanks for the further information but I would like to complete the project with the Sinclair amp as a tribute to part of my past..

This was the sort of advert which got my imagination going when I was a young lad.
http://www.saturn-sound.com/images%20-%2...'%2069.jpg

"The definition of an active speaker is simple: the crossover is implemented at line level, and there is a power amplifier for each of the frequency bands following on from this. A single power amp is a powered speaker. Yes, I (accidentally-on-purpose) made that mistake when I titled the article linked to above. I'll probably correct it in due course."
That is what I call "bi amp" or tri amp.

I came across active crossover units when I was working in the theatre as a Chief LX. It would split the audio into separate bands to be amplified by different amps feeding their own speakers. But they were not known as "active Speakers" It was the xover that was the active part, not the speakers.
I go along with the What HiFi definition.

Mike

[Mark Hennessy]

"The definition of an active speaker is simple: the crossover is implemented at line level, and there is a power amplifier for each of the frequency bands following on from this. A single power amp is a powered speaker. Yes, I (accidentally-on-purpose) made that mistake when I titled the article linked to above. I'll probably correct it in due course."
That is what I call "bi amp" or tri amp.

No, "bi-amp" means something completely different.

Really, it should be called "buy amp". Rather like "buy-wiring"

When you bi-amp, you feed the same audio signal to two separate power amps. The crossover is a high-level crossover - the same high-level crossover that you had when you were driving the speaker conventionally. Assuming competently designed amplifiers are in use, it is purely a scheme to separate the gullible from their money as it brings about precisely none of the benefits of genuine active operation.

I came across active crossover units when I was working in the theatre as a Chief LX. It would split the audio into separate bands to be amplified by different amps feeding their own speakers. But they were not known as "active Speakers" It was the xover that was the active part, not the speakers.

This is the norm for PA and many other applications. Because you are filtering the audio signal at line level, this is very definitely an active crossover setup.

The physicality doesn't matter; the fact that the crossover and amps are not in the speaker cabinets does not change the fact that this was an active system. The fact the people you worked with did not call it an active speaker does not change the fact that is was indeed active.

An active speaker uses a line-level crossover with separate power amps for each frequency band. What you're building (and what I built) is a powered speaker.

BTW, Outside of the day job, I'm chief SX at our local theatre - one of the reasons why I'm here less and less these days.

I go along with the What  HiFi definition.

Excellent  

In fact, true active speakers (as opposed to the basic powered speakers you might have on your computer desk) have an amplifier for each individual speaker driver, and also active crossovers.

That whole page was nothing more than an advertorial for Dynaudio, BTW. At least they deigned to put "Sponsored" on the page. And the copy was almost certainly written by Dynaudio. As it happens, there's a similar article on their site: https://www.dynaudio.com/dynaudio-academ...-crossover

The frequency split is done before amplification with active crossovers. Either in the analogue or digital domain. After the split, the frequencies are sent to individual amplifiers running their own individual drive units.

You could also try Wikipedia: https://en.wikipedia.org/wiki/Audio_crossover#Active

Active speakers are extremely common in pro applications - we've got loads of them where I work. It's a shame active speakers haven't made much impact in the domestic market. Obviously, Dynaudio are having a bit of a push at the moment - good on them. However, the serious hi-fi types like to fret about amplifier and cable choice, and active speakers takes that away from them. But aside from them, most people do not want lots of 17" wide black boxes in their living rooms any more - but many of them would like something better than the typical iPod docks that pass for "hi-fi" these days. With their glossy white boxes and wireless operation, Dynaudio are clearly having a go at that part of the market. They're not the only ones...

[Crackle]

I am currently making the connection panel for the input and controls out of aluminium, unfortunately it is 3mm aluminium but that is all I had in the way of anodised aluminium.
This is where  I have go to at the moment.
Mark can probably explain more about the treble speaker,  where it came from and what it is like. But coupled with the 10 inch Goodmans and a 1.8uf cap* it sounds quite good to me and that is with out the new back I have made fitted. The new back panel is also made out of solid oak, but it is a thin panel with t&g joints, it is from an old wardrobe. I have added 2 bracing ribs to the panel to prevent the t&G joints from splitting in future and also to brace it so it acts less like a drum.
I am waiting for some figure 8 chassis mains plugs to arrive so I can drill and cut the correct hole size for one in the aluminium.

So today I will try and put together the tone control and see how it works. If there is too much loss in the TC then I have the option of tweaking the gain of the amp a little by altering the feedback, or adding another preamp stage.
I have all the resistors, the caps were a little more tricky, I have a 1500pf mica but I may make one up with a ceramic 1n and 470p.
   

I have to say that all this is not being done to any scientific method so the result will be a compromise between a dead pile of scrap oak and a useful active speaker for an ipod or small radio.
It is also being made with scrap and spares that I mostly have to hand. The only things purchased so far specially for this is the mains socket.

* This was an ex ring capacitor from a BT socket, I had a few of these after we chucked a load of BT sockets and I cropped out the caps.
I dont know if Mark could comment on the value and its suitability, have I guessed on the value about right.
           

Mike

[Crackle]

hi Mark
This is one of the 2 speaker cabinets I made for those 4 inch speakers. It was made out of old wardrobe chipboard panels, glued together with epoxy. The port is a piece of a vacuum cleaner pipe.
I have 2 lumps of foam from seat cushions in there to help damp the internal resonances.
The dimensions are internal measurements and the port is 120mm long. I may have got the dimensions on line, I cant remember for sure now.
           

For what they are, very simple, I cant fault them, I wondered if there was a speaker design tool, how they would come out in theory.

On a hot day with them turned up, you get a nice breeze from the front and the port. Not that you would really want to do that as they are very loud and it would hurt my ears.

Mike

[Crackle]

Here is the prototype tone control wired just as in the diagram. It works satisfactory, the bass control is smooth and just enough cut and boost. The treble control is subtle, but does just make a noticeable difference, it is wired backwards at the moment for layout convenience.
   

I now need to decide how I an going to finish it, probably veroboard.
It is supplied from the ear phone socket of my workshop DAB radio, the tone circuit has still left plenty of drive for the Z30.
Normal listening levels are with the DAB on 7 or 8 (out of 30) and the Z30 goes into just noticeable distortion at about 13/30 on the DAB volume.
The power capability of the Z30 is at the moment limited by the 14v PSU. If the PSU was changed for one of 25 or 30 volts then it would make a noticeable difference to the volume and be pretty ear splitting in my small workshop room. I am not too sure if the speakers would handle the 15 theoretical watts at 25 volts. It is not recommended to take the Z30 to its full 30 volts max supply with a 3 ohm speaker.

What would be the best values and which components to change to give a little more treble cut and boost?

thanks
Mike

[Crackle]

Trial and error shows that an extra 560pF cap across the 1500pF capacitor improves the treble control range.
Is that the correct component to change?

Thanks
Mike

[Terry]

What would be the best values and which components to change to give a little more treble cut and boost?

Mike, from the references to available gain in that post, it sounds (pardon the pun!) as if you have enough gain available for the full ±15dB circuit you started with or, at least, somewhere in between.

Absolute component values aren't important so, as you have two sets of component values in front of you, you can experiment with whatever intermediate values you have available to get the range(s) of adjustment you want. As it's completely passive, there is no way you can do any damage!

EDIT

I got interrupted while I was composing this post, so missed the your last one, Mike.

I think this post really answers your question - experiment as much as you like until you get the result that you want!