15" Tannoy Monitor Red crossover.

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dave slagle
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15" Tannoy Monitor Red crossover.

Post by dave slagle »

Have a friend that wants an all out crossover for his monitor reds. Rather than try to document the various things by email I'm putting it here. This is not intended to be a "design it once and use the most expensive parts and praise glory" type situation but a progress where we explore a few different possibilities.

Lets start with the stock crossover.
Image

This is quite different than the Gold crossovers which use a tapped choke which provides HF gain adjustment and a 2nd order filter on the high=pass. Below is the comparison between the stock red and gold crossover with adjustment of the autoformer on the golds.
Image

Aside from the differing slopes, the Gold crossover also has a notch filter at 3K to tame the "shout". The Red schematic list this as a modification and it is controlled by the value of {R3000} on the schematic. If the value of R3000 is high (1e9 or even 100K) then it effectively takes the filter out of the loop. Typically that resistor is a pot that you can adjust to suit your tastes and here is the result of changing the value. (20r 30r 50r 100r 100k)
Image

ultimately I think we want a fixed resistor in that position but for the first go around I am suggesting a rotary switch with assorted values. I still don't like the switch contacts but I think discrete steps will give a better feel for what is going on.

The other position that I am going to suggest playing with the value is for the resistor I labeled R700. This is fixed at 30 ohms for the stock crossover but adjusting that can manipulate the frequency in the 700hz range. Below I used 20, 30 and 50 ohms with the center line being the stock 30 ohm value.
Image

By the switching of those two resistors quire a range of tonal adjustment can be had and the below plots show the range of adjustment possible. I only selected two values for each to clearly illustrate the range. Image

This post is just meant to give a basic overview and my proposed switched resistors. I'm thinking a 5-6 position switch for each will do it. As for the resistors, chances are they will be copper wound.

dave
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dave slagle
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Biased Caps

Post by dave slagle »

One of the things we want to try is to bias the caps. One of our DC area friend had had great luck with biasing dielectrics so it seems like a cap would be the perfect place to try it. The problem becomes instead of 1 cap per position you now need 2 at double the value which will replace a single $140 2 mic cap with two $200 4 mic caps which almost triples the cap cost.

The 18uf cap for the Low-pass is already going tobe an expensive beast and the thought of a pair of 36u caps makes my head spin.

In looking at the crossover, there are already 2 caps in series so a possible trick might be to insert an additional cap in-between the 30 ohm resistor and the output. The picture below shows the three options. I used a 1000u cap which is over the top and all three lines overlap so a frequency plot would just cause excessive scrolling.
Image

The obvious question becomes, How small can we make that cap? This is where spice is a wonderful tool. I simply stepped the value of that cap in the following values 1, 40u, 20u, 10u. Note that spice sees a value of 1 for a cap as 1 farad which means for all practical purposes this can be considered a piece of wire and makes the green line represent the stock crossover. We can now see how the cap value changes the roll-off behavior.
Image

It is interesting to note that the cap value has an effect in two places, ~3K and below say 1K. We already have control over the 3K dip so lets look a little closer.
Image

In the 3K range the smaller cap causes a slight boost which is an alleged problem spot anyways. Since we already plan on giving the option of the 3K notch it is simple to adjust R3000 to match the ideal. and it turns out a value of 500R does the trick.
Image

This leaves us with a drooping response at LF which actually may not be such a bad thing but in the interest of completeness It appears that a corner frequency shift will correct the lowest part of the slope and then a tweak of R700 should bring us close. In actuality I increased the 4uf cap to 6uf and had to tweak both R700 and R3000 to come up with this.

Image

Some simple math gives us a better picture of how far we are off from the ideal and i'm not going to lose any sleep over it.
Image

I think the next step is to think of a natural progression is to leave the land of spice and listen to the various possibilities in practice. I'll lay out the plan in another post.


dave
Last edited by dave slagle on Wed Feb 24, 2010 11:07 pm, edited 1 time in total.
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My Proposed Plan.

Post by dave slagle »

I'll suggest we build the proposed crossover stock with the addition of the 3K notch filter. I'd like to see a 6 positon rotary switch added to the R3000 position to allow for ease of level tweaking. I'd also Like to do the same for the 30 ohm (R700) resistor.

This needs to be listened to and the knobs played with to get a feel for how they effect the sound. Once we are comfortable with the results we can abandon the "ideal" and use the "tweaked sound as the new reference.

I'd like to insert a 10u cap and try the "Hibred" biasing. Since the knobs are already adjustable we will be able to again play with the tone and correct things.

This 10u cap can be a cheap unit and if it shows promise, then we look onward. If the premature roll-off on the low end of the high-pass, then an additional 2uf cap can be added in parallel with the 4uf and the resistors tweaked to get the desired results.

If the 3K notch filter proves to be needed and substantial, then we can consider a better cap for the 5.6u value and possibly trying bias there.

I know this will be a bit of a process but I think it is the proper way to deal with the situation. All of this is just for the highpass. I'll touch on the bass in another post.

dave
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Post by dave slagle »

Got things finished up and ready for the cabinet maker to make pretty today. Going down to Ijaz's tomorrow to listen.

This can be set as the stock 15" red crossover and then the 3K notch filter can be slowly added in with a rotary switch. A second switch adjusts the highpass at the crossover point and a third switch is a bass attenuator to allow for an overall shift up of the highs.

The really cool thing is the sims and the measured responses are really close to each other. I don't have time overlap them but the images of both are below for the 700hz and 3K adjust. It isn't easy to output the series of plots in spice but they also closely match the simmed results.

Apologies for the scale difference in the frequency domain.

dave
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tubino
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Post by tubino »

What did you decide?

I have a pair of Tannoy GRF-Rs with Golds that I am going to live with and listen to daily for a while. I'm trying to figure out passive vs. active, or do a modest rebuild of the stock crossover.
dave slagle
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Post by dave slagle »

Hey Rick,

I ended up with the stock Tannoy crossover for the Reds and two switches.

One added in the notch filter and the other attenuated the bass in 1dB steps.

dave
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