Tannoy SRM-12X Crossover Schematic Wanted

[walpurgis]

Anybody got a schematic for the Tannoy SRM-12X (or SGM-12X) crossover please. This would assist with a new project. Thanks.

[Qwin]

Hi Geoff,
have a look at this diagram on Hans site, I think it is the right model.

http://www.hilberink.nl/codehans/tannoy40.htm

[walpurgis]

Yes thanks Ken, that's interesting. I should be able to work from that. I'll compare the values to my Cheviot II crossovers, I suspect they may be similar.

Any further info most welcome.

[walpurgis]

Has anybody got any further useful info?

My intention is to use a pair of Lynx 12" drivers for Hi-Fi. The standard crossover is a fairly simple 2.5kHz item. This is a P.A. monitor, so the crossover point presumably kept highish to improve power handling. As this is basically the same driver as the SRM-12X, it appears that maybe a crossover point somewhere between 1.0 to 1.5 kHz would be better for Hi-Fi use, keeping the lower midrange cleaner.

I would like to achieve a crossover equivalent to the SRM-12X unit, but do away with the energy and roll-off sections and perhaps include a 3db or so L pad in the circuit to control the tweeter output.

[DSJR]

The HFC test of the SRM12X showed a rising response, the tweeter and upper mid well up on the mid-bass, giving an over-analytical and almost screaming balance. I'll try to scan the review and post later. In the meantime, it may be a good idea to balance the rebuild 'fixed level' crossover around the energy being set at -1 and the rolloff to -1 or -2, as I rember this giving the best result with the 12" driver at any rate in this size box...

[walpurgis]

Yes Dave, I was thinking a long those lines, although I may consider including -1db roll-off, I never actually need it with my current Tannoys which use the 3128 driver. The same unit, but with roll surround. An energy cut of -1 is occasionally called for, but not often. I'm assuming I can use a fixed inductor value to replace the tapped transformer.

[DSJR]

Well out of my comfort zone, but can't you use the existing transformer and just use the -1 tap?

[walpurgis]

No, it's a 2.5 kHz Lynx crossover. No transformer. That's why I want to build something that equates to the SRM-12X (or Cheviot II or Balmoral) unit. I have a big stock of good inductors and can also wind my own so no problems there.

[Qwin]

Hi Geoff,
Many of these Tannoy circuits are similar in layout. I modified my Cheviots to work with a fixed inductor instead of the Autoformer. (Big improvement)

Couple of things to bear in mind if you are going to use a fixed inductor and apologies if I’m teaching you to suck eggs here:

With reference to the diagram I linked to on Hans site, if you are going to reduce energy to -1.5 this is a good thing. The Autoformer performs 2 functions, it provides a variety of different voltages from the output taps and acts as an inductor of various values.

First - If you use a fixed inductor you should make it of the value for the Green -1.5 energy tap. This is the input and the output and is the only tap that has zero voltage change.
If any other tap is used the voltage is changed for the downstream components and there is no way a fixed inductor would do this, so the circuit characteristics will change.

Second – let’s say you have dropped in a fixed inductor of the value at the Green Tap.
When the energy level switch is in this position, the second pole of the switch, which is connected to the 50 ohm shunt resistor is connected to the Orange tap. This has a raised voltage over the Green input. Because you are not altering the voltage with your fixed inductor, you have to reduce this resistor value to maintain the same sonic effect. Too low a value and it will seem soft and shut in. Too high a value will be bright and edgy. On my Cheviots it went from 50 to 25 ohms. I did a direct comparison of the circuit with the autoformer in place, energy set to -1.5 verses the version with a fixed inductor and reduced shunt resistor. The general tonality was unchanged but the better quality inductor gave more detail.

So to sum up, use a fixed inductor of the value of the Green Tap and reduce the Shunt resistor to suit. All the other components work as normal if you do this, without any changes of values required.

Oh by the way, do not take anyone’s word about what the values of your Autoformer taps are. Measure them yourself, there is so much rubbish talked about this I gave up and bought an inductance meter.
Tannoy had a nasty habit of giving a generic part number to their bought in autoformers, but then added various thickness spacers between the “E” and the “I” pieces in the laminations. This has an overall effect on the taps output. So like I say, measure what you have between the Green and the black taps on your parts and get a fixed inductor of that value. Also if you are measuring the autoformer off the PCB, press down on the "I" lamination when you take a reading. If it is just sitting in position on the bench you will get a false reading, it is mounted under presure on the PCB (all found out the hard way).

Hope that makes sense and is of some use.

[walpurgis]

Thanks Ken, that sounds like useful info. I have an LCR meter and will investigate as you suggest. Fortunately my other speakers are a pair of Cheviot II and they have the 3128 driver, as opposed to the HPD 315 of the earlier Cheviot. I should be able to work from the crossovers in these and model something suitable. The 3128 is identical to the Lynx driver, other than the cone surround materials, so apart from the bottom half octave their behaviour is identical.