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The Peasant 31st March 2006 08:38 PM

Acoustat bass crossover?
I am looking for an easy way to filter out the frequencies below around 60 - 80 Hz from my Acoustat model IIs, in order for my subwoofer to produce these instead, lightening the load on the electrostatic panels. I realise that the best way of doing this is to filter the audio at the input to the power amp, however, my REL B3 subwoofer instructions state that for best results the sub should be connected directly to the speaker outputs of the amplifier, making this method impossible.

I have tried doing a simple passive 6dB/oct filter using a capacitor in series with the low frequency transformer, but due to the low impedance a very large and expensive capacitor would be required.

Is there some method of doing this that I may have overlooked? Perhaps something on the secondary side of the step-up transformers where the impedance is higher? Is there really that much difference running the subwoofer from line-level inputs as opposed to the speaker-level input? Has anyone ever tried something like this?

Thanks for any help or suggestions that you can provide!

Take care,

Bazukaz 31st March 2006 09:23 PM

So , what is wrong using an active filter for main amp and low-level inputs of subwoofer ?

The Peasant 31st March 2006 09:28 PM

REL recommends that for best sound the subwoofer should be connected to the speaker outputs, not low-level inputs, so that is not feasible. Connecting it this way ensures that the subwoofer sees the same signal as the main speakers, for best integration. I have not tried it using low-level inputs, I would have to obtain some long patch cords to do so.

Take care,

georgehifi 1st April 2006 11:56 AM

2 Attachment(s)
The Peasant, Doug, Acoustat' have two audio transformers, one for the bass mids and highs, and the other is just to augment the lower bass you could try to disconect the bass transformer that will give a much reduced lower bass, which could be ideal for what your trying to do.
Or if you change the bass transformer tap from red for the model 2, you could try orange, or even yellow these should give reduced bass output compared to the red.

Cheers George

The Peasant 1st April 2006 06:22 PM

Thanks for the suggestions, George. I have actually tried reducing the bass transformer output, but that also reduces the bass frequencies much higher than my intended crossover point. I don't want to lose any midbass as these speakers are already quite lean there, so unfortunately this will not work for me.

Any other ideas?

Take care,

georgehifi 1st April 2006 08:56 PM

Yeah, the 1ohm resistor R4, it's said that not only is it there to give the amplifier an easier load, but also acts as a subsonic fequency filter together with the input capacitance of the transformer, to stop panel flap at warp frequencies back in the days of records. So by increasing that resistors value you should be able to roll the transformer of earlier, maybe a cheap easier way out.
I've had acoustat 1+1, 2's and 2+2's and active xovers were always detrimental to the purity of the mids and highs, even a discrete transistor 24db one which was much better than comercial opamp ones were still taking away the magicical purity of no active xover in the signal path.
With my 1+1's I let them go down naturaly and the crossed over a pair of B-1814 Kefs (same as the B-139 except twice the size) in 12cu ft enclosures at 50hz 2nd order, that worked great. Seamless top to bottom flat to 20hz.

Cheers George

AW 25th April 2006 10:16 PM

I would agree with George and not bother rolling off the Acoustats at all. Anything you add to the signal path will worsen the sound from these great speakers - and about the worst thing would be capacitors to try & roll the bass off.

And the bass panels will not really be helped by rolling them off anyway. Quad 57's? - yes, but II's wouldn'd even notice.


moray james 27th April 2006 06:29 AM

don't take that 1 ohm resistor out!
that one ohm resistor stops the transformer from saturating at low frequencies so don't take it out OK!!! Best regards Moray James.

sreten 27th April 2006 12:08 PM


there is a way of doing it - but its not intuitive - and depends on the
REL having plenty of excess gain to spare on its speaker level inputs.

You apply a RCR filter to the input of the amplifier such that bass
rolls off say below ~ 75Hz but flattens out again at say ~ 7.5Hz.

Instead of connecting the REL directly to the amplifier outputs you
connect it via an RCR filter than has the inverse response of the
above filter. This will reduce strain on your speakers and reduce
voltage swing of the amplifier - giving a minor increase in dynamics.


bear 5th May 2006 06:45 PM

An appropriate cap at the input of ur power amp will do the trick.
It is nicer to provide a resistor at the driven end, and a resistor at the amp end, but there is already a resistor there now. Also, most power amps are AC coupled at the input, so you could also merely remove the input cap and replace it with an appropriate value. If you find an electrolytic cap there now, a film cap will do the job better and give you nicer sound.

Alternately, there is usually a cap to ground at the inverting side of the amplifiers differential input pair. The purpose of this cap is to force the feedback to maximum at some very low frequency - this pushes the gain of the amp down to unity at some very low freq, typically <1Hz. You can reduce this cap, push the frequency up into a range that is where you want the amp to roll off for the subwoofer implementation. Since this is usually an electrolytic cap, it may also improve the overall sound of the amp if replaced with a small value film cap for this roll-off trick.

You can change the input cap and the roll-off cap, then you'd get some sort of 12dB/oct (2nd order) filter effect.

You would have to check to see that the amp remained stable with the internal roll-off cap changed - it ought to remain stable.

If you have not replaced the electrolytic caps inside the interface box with film caps, do so.

Alternately you could put some nice film caps between the amp and the speaker.

I would not suggest removing the "bass" transformer. It is not really a "bass transformer"! The two transformers overlap quite a few octaves. You need both for flat response, iirc.

Also, there are two versions of the interface box - one has the "HF" xfmr coming in lower than the other by an octave or so...

_-_-bear :Pawprint:

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