Crossover question

[Joseph Crowe]

I was wondering how to wire two woofers in a MTM design, series or parallel? I wanted to achieve higher sensitivity by having the two woofers however if running parallel the impedance drops to 4 ohm. I have a vintage tube amp (Harman Kardon A-230) and my understanding was that tube amps don't like these difficult loads, but why? Increased distortion?

If I run the woofers in series it's a 16 ohm load, however when I model it in Passive Crossover Designer 7, the efficiency is only 89 dB.

Here are my options:
Series 89dB 16 ohm load
Parallet 95 db 4 ohm load

I'm not stuck on the MTM design, should I perhaps change my design to a 2.5-way?

The woofers are Fostex FF165WK crossed over at 2 kHz.

Thanks

 

[SY]

You have an interesting option with a tube amp- you can run the woofers in parallel and connect their crossover to the 4 ohm tap, while connecting the tweeter crossover to the 8 ohm tap. Or connect the woofers in series and drive their crossover from the 16 ohm tap.

 

[Joseph Crowe]

You have an interesting option with a tube amp- you can run the woofers in parallel and connect their crossover to the 4 ohm tap, while connecting the tweeter crossover to the 8 ohm tap. Or connect the woofers in series and drive their crossover from the 16 ohm tap.

My amp is unusual becuase it has a switch on the back...8 ohm or 16 ohm speakers with only one pair of connections.

 

[paulfx]

Those tube amps have a transformer to step down the voltage and step up the current to drive the speakers, and aren't nearly as stiff a voltage source as solid state amps.

http://www.diyaudio.com/forums/tubes-valves/165634-typical-output-impedance-tube-amps.html

But that classic HK has 8 and 16 ohm output taps, I think - you would get a bit more voltage and still get the 15 watts using the 16 ohm tap.

I'd say put an 8 ohm resistor in series with the tweeter, run the woofers in series and design the whole system for around 16 ohms.

 

[paulfx]

"Are there signifigant drawbacks to iron core as far as sound quality?"

Some time ago I laid out a lot of cash to replace the iron core coils in my Sapphires with Solen air coils. 7.0 mH if I remember. I would like to think they sound better...

 

[SY]

Some time ago I laid out a lot of cash to replace the iron core coils in my Sapphires with Solen air coils. 7.0 mH if I remember. I would like to think they sound better...

Besides the advantage of higher saturation current, you have to consider the tradeoff in DCR. If the DCR of the new coil is higher, it *will* change the frequency response.

 

[paulfx]

Yeah, I matched up the DC resistance, so they cost a fortune. Impressive hunks of copper...

 

[sreten]

Hi,

Design for 16 ohm as you have a 16 ohm tap.

You are confusing sensitivity and efficiency.
Say a speaker is 90dB/2.83V/1m.
Its efficiency if 8 ohm is 90dB/W, 4 ohms
it is 87dB/W and 16 ohms it is 93dB/W, as
2.83V is 1W 8R, 2W 4R and 0.5W 16R.

The efficiency of your drivers, does not change when
wiring a pair in series or parallel, its the same for both,
the nominal efficiency of a pair is +3dB over a single.

rgds, sreten.

 

[Joseph Crowe]

Hi,

Design for 16 ohm as you have a 16 ohm tap.

You are confusing sensitivity and efficiency.
Say a speaker is 90dB/2.83V/1m.
Its efficiency if 8 ohm is 90dB/W, 4 ohms
it is 87dB/W and 16 ohms it is 93dB/W, as
2.83V is 1W 8R, 2W 4R and 0.5W 16R.

The efficiency of your 8 ohm drivers, 92dB/W does not
change when wiring a pair in series or parallel, its the same.
The voltage sensitivity changes to 89dB for 16 ohm series.
The voltage sensitivity changes to 95dB for 4 ohm parallel.

rgds, sreten.

Thank you.

 

[Joseph Crowe]

Turns out 16 ohm is best. The tube amp need to be turned up more but goes just as loud, if not louder with less distortion.