amateur crossover question

[ryanmerc]

Hi,

I have a vintage philips ag9018 se 800 ohm tube amp.

I am able to get a pair of 800 ohm 6 inch full range drivers and and a pair of 8 inch 800 ohm full range drivers. my plan is to take off the whizzer cone on the 8 inch and diy them into a 2way system crossed around 400hz.

I would like just a simple 1st order crossover.

My question is is there a circuit I could make so that my amp will see 800 ohms?

Thank you

 

[AllenB]

You can apply Zobel compensation to the speaker after you cross it. This will probably involve a series RLC 'notch' filter across the amp around the crossover, and maybe the same near the woofer resonance. Reaching exactly 800 ohms probably isn't as important as simply smoothing the impedance so that it won't translate into response anomalies.

 

[picowallspeaker]

I'd worry about the added Re provoked by a > 100 mH bobbin

 

[AllenB]

Would you lose sleep if it were 50 or 100 ohms? I have a 100mH choke with a resistance of 250mΩ 😀https://en.wikipedia.org/wiki/Omega

 

[picowallspeaker]

There must be a reason why 8 Ω speakers has been set as a standard !
And...which speaker ?
Certainly not an old paper cone with fabric suspension and 5-10 W power.
Those might be ok for acoustic guitar & voice ( reproduction ).
I'd lend towards an electronic crossover before the amplifier(s) in this case; at least use it once just to hear how that speaker behaves when band-limited.

 

[AllenB]

I don't know, if 800Ω was common I'd do an OTL. 8Ω is pretty convenient for SS.

An electronic crossover is going to want extra amps.. I suppose this isn't going to be all that difficult. It would be better if the OP could measure impedance but this could possibly be done without. Even if the crossover itself is adjusted to compensate for the output impedance of the amp then nothing else would be needed?

 

[phase_accurate]

I remember that an old Philips TV set of ours had an 800 Ohms speaker like that driven by a some single-ended type of OTL. There is a reason why the didn't use 8 Ohms with that tube output stage ! 😉

Regards

Charles

 

[picowallspeaker]

There is a reason why the didn't use 8 Ohms with that tube output stage ! 😉

..nor multi-way arrangements in any way ( woofer-tweeter-1st order, 2nd order... )

8 Ω seems fair for the values of a crossover: inductors may range from 0.1 to 10 ( or more ) mH; capacitors from 1 to 100 uF ( or more ); resistors from 1 to 10 Ω ( or more ).

 

[phase_accurate]

Capacitors for 800 Ohms crossovers will be nice and small. Inductors not equally so. But they won't have to carry the same amount of current as if used within a low-impedance situation.

Regards

Charles.

 

[ryanmerc]

Whoa! I think I'll have to read up and go over the crossover basics thread first and study-up. Thanks for all your replies.

 

[AllenB]

If this amp was used without an output transformer I would be tempted to use a shunt crossover to low pass the woofer, but this would require careful consideration, not for the faint hearted. If output coupling capacitors are used I'd also make sure they are healthy.

 

[sreten]

Hi,

A series first order @ 400Hz order is the way to go most likely.
Capacitor is easy, high voltage 0.5 uF. The inductor is not and likely
will need to specially wound, your looking at a low current 0.3H coil.
You need a relatively low power core and lots of fine windings with
decent insulation. DCR can be up to 100R without any real issues.

FWIW : Philips AG9018 - Manual - Stereo Integrated Amplifier - HiFi Engine

No way is that SE at 12W per channel, it does not mention the OTL
option, elsewhere does, but I cannot find any details on that option.

rgds, sreten.

 

[AllenB]

'D' leads from heater centre tap/chassis.