HF time delay circuit?

[mt490]

I'm under the impression that it can't be done without difficulty using opamp circuitry?

I'm trying to shoehorn a LeCleac'h quasi-optimal crossover onto one of my existing XO boards, but since a delay for the HF driver(s) are needed, it doesn't look to be a very practical thing to do in analogue. Mechanically it's not straightforward either since I'm not using a horn tweeter.

Is there a trick to it? Or am I finally going to have to bite the bullet and change over to DSP?

[bwaslo]

You'll probably have to go for DSP. HF delays get tougher, because more cycles are needed to be kept somewhere 'in transit' for a given number of milliseconds delay than at low frequency. If all you have is lumped Rs and Cs and gain blocks (opamp circuits), where is the delayed multi-cycle waveform between the time it goes in and it comes out? It takes lots of poles and zeroes to get anything very useful.
With DSP (and enough memory) it's easy -- just stuff the bits into one end of a long enough shift register and pull them out the other end after a sufficient waiting time.

[DcibeL]

You can add delay passively with a ladder delay network. See this design for an example of the implementation.

Small "delay" can also be had by using asymmetrical crossover slopes.

[PLB]

mt490,

With a sixth order allpass, using 3 opamps, it is possible to get a good quality constant delay of upto 85uS (=30mm), but I don't know if this is adequate for your needs. If it is, let me know and I'll post the schematic.
If it isn't, bwaslo's recommendation is probably the best one. If this is the case, then the DCX2496 or the miniDSP would be a very convenient way to go.

Regards

Peter

[mt490]

Originally Posted by DcibeL

You can add delay passively with a ladder delay network. See this design for an example of the implementation.

Small "delay" can also be had by using asymmetrical crossover slopes.

Unfortunately this is not what I'm looking for as it has the same response as the line level active allpass filter. Whilst it's easy enough to delay the driver at the crossover frequency, the delay reduces as frequency goes up which means that the phase alignment is not presented as what the LeCleach crossover aims to achieve. Essentially I have to virtually move the tweeter away from me for all its frequencies, not just near the crossover.

Originally Posted by PLB

mt490,

With a sixth order allpass, using 3 opamps, it is possible to get a good quality constant delay of upto 85uS (=30mm), but I don't know if this is adequate for your needs. If it is, let me know and I'll post the schematic.
If it isn't, bwaslo's recommendation is probably the best one. If this is the case, then the DCX2496 or the miniDSP would be a very convenient way to go.

Regards

Peter

According to the calculations I need 109us. It would be close, I could make up that difference with physical alignment.

I'm guessing this is made up of three 2nd order allpass networks?

My two main concerns with making a daisy chain are additional noise and space requirements.

[mt490]

ED- going insane

[speaker dave]

Originally Posted by DcibeL

You can add delay passively with a ladder delay network. See this design for an example of the implementation.

Small "delay" can also be had by using asymmetrical crossover slopes.

Zaph achieves 90 degrees phase shift at crossover. 2 more orders of crossover slope would also achieve that, but I can believe that his solution was preferable in this case.

So, mt490, do you need actual broadband delay or just phase shift at the crossover point?

David S.

[mt490]

broadband - as I understand the tweeter has to appear as though it's 37mm further away than the woofer, meaning the delay needs to be constant for all frequencies up to 20kHz or greater.

for reference this is what I'm trying to implement

@PLB: After simulating some of the possibilities I'm not sure how you arrived at 85us with only a sixth-order allpass - certainly not all the way up to ~18kHz ?

The miniDSP is looking more and more enticing at the moment, certainly would be convenient having one around as an experimental platform even if I did decide to cut an opamp circuit for the final product.

[phase_accurate]

109 us should be feasible with an N-th order Bessel allpass.

I once ran a simu of a 100 us delay line with a bandwidth of 30 kHz. This was a 10th order Bessel Allpass. This 100 us could be easily expanded by 10 % at the expense of 10% bandwidth loss.

Regards

Charles

[PLB]

mt490,


"@PLB: After simulating some of the possibilities I'm not sure how you arrived at 85us with only a sixth-order allpass - certainly not all the way up to ~18kHz ?"


I think you may have made an error in your sims. I’ve attached group delay curves for the 6th order allpass I referred to in my last post. With the use of MC33079 opamps, you can obtain an “A” wtd S/N of 109dB wrt 2Vrms I/O.

Peter