Open baffle response with tube amps

[otto88]

Using Unibox to model a vented box with a solid state amp, then bumping up Rs (under External components) to estimate the effect of tube amp, usually shows
- sensitivity (max SPL) decreases,
- a lower-mid bass hump increases (warmth/ possible tubbiness is added)

Unibox doesn’t model Open baffles - when a tube amp is used to drive OBs, are the same two effects experienced?

[otto88]

I know in an OB, driver Qts > 0.5 gives better more extended bass.
But if it makes a difference, in my case for > 80 Hz, I’m going to use Lambda mid bass units with a Qts about 0.35

[john k...]

Originally Posted by otto88

Using Unibox to model a vented box with a solid state amp, then bumping up Rs (under External components) to estimate the effect of tube amp, usually shows
- sensitivity (max SPL) decreases,
- a lower-mid bass hump increases (warmth/ possible tubbiness is added)

Unibox doesn’t model Open baffles - when a tube amp is used to drive OBs, are the same two effects experienced?

First, max SPL is dependent only on driver area and excursion. Max SPL at any give frequency will not change. Sensitivity of the driver will not change either. What will change is the the voltgae applied across the driver due to the voltage drop across Rs. Thus, if referenced to the amplifier output (which is what Unibox does) the sensitivity of the driver/Rs system will change.

That said, when used in OB format similar effects would be observed.

[StigErik]

Originally Posted by otto88

- a lower-mid bass hump increases (warmth/ possible tubbiness is added)

That is due to higher total Q and slightly less level loss around the driver impedance peak at Fs. The same thing will happen with OB.

[otto88]

Thanks John and StigErik, that answers the initial question posted.

Does either A B C Dipole, or MJK's OB worksheet, model the size of the lower-mid bass hump/ driver impedance peak at Fs?

And - haven’t read dipole theory for a few years now: need to refresh!
– isn’t SPL at any given frequency dependent on eg baffle width and the 6 dB/ rise which iirc begins at Fequal?

[StigErik]

Yes, with OB max SPL at a given frequency is also dependant on the effective baffle size.

[otto88]

So it seems that at present we can not model the size and frequency of this lower-mid bass hump . . ie model a crossover driven by a tube amp

[Rudolf]

Originally Posted by otto88

So it seems that at present we can not model the size and frequency of this lower-mid bass hump . . ie model a crossover driven by a tube amp

I believe that MJKs MathCad worksheets cater for an in-line-resistance.
I know that Boxsim does it.

Rudolf

[john k...]

To model the hump correctly the full driver impedance in ncessary. The effect of Rs on Qts can be directly calculated and the modified Q input to ABC DIpole, giving an indication of the Q boost. But it will not consider the additional attenuation that occures in the mid band.

[CLS]

Yes, the key point is the combination of output impedance of the amp and the Re of the driver, thus the Qes is altered, then of course the Qts.

The wording "tube amps" in the topic is somewhat misleading, sand amps can have high output impedance, too.

Here are my previous posts about this: Low Q Woofer in OB with High Zo Amp

Have fun