Sparrow MTM-2 (SB13PFC25-04 with PRV D280Ti)

Sparrow MTM-2

It was in 2017 that I designed the Sparrow-MTM. In that version, my aim was to design a MTM that’s easy on the pocket. In this MTM-2, sound quality is what I’m after.

The first change I made is to replace the Pyle PDS221 with a PRV D280Ti. The D280Ti is not an expensive compression driver, only $35 but in terms of quality, it’s superior. 

Fig 1 – PRV D280Ti with PRV WG11-25 Waveguide

The first issue I dealt with is the impedance of the PRV D280Ti. Fig 1 is what it looks like when it’s loaded with the PRV WG11-25 waveguide. This is a disaster waiting to happen. I had no choice but to use a LCR network to tame the resonant peak. There’s no running away from this.

Fig 2 – PRV D280Ti with WG11-25 waveguide RAW and High Pass Response

Now that the nasty impedance peak is eliminated, the crossover of the D280Ti can be worked on. The Black plot in Fig 2 is the RAW response of the PRV D280Ti with the WG11-25 waveguide. The Red plot is with my High Pass filter.

Fig 3 – SB13PFC25-04 Low Pass with PRV D280Ti with WG11-25 High Pass

The Blue plot in Fig 3 is the Low Pass response of 2 units of SB13PFC25-04 wired in series. They are crossing at 3kHz with the D280Ti. 

Fig 4 – Sparrow MTM-2 Frequency Response

Fig 4 is the final frequency response of the Sparrow MTM-2. On the surface, it looks like any other plots. What it doesn’t show is the amount of components in the crossover to make it sound the way I want.

Fig 5 – Sparrow MTM-2 Null

Fig 5 is the Null response when I reversed the D280Ti wires. This is a beautiful, deep, symmetrical notch. It indicates excellent phase alignment of the D280Ti and the SB13PFC25-04.

Fig 6 – Sparrow MTM-2 Step Response

The Sparrow MTM-2 Step response shows the two SB13PFC25-04 are slightly ahead of the D280Ti. I’m surprised by the short delay because I’m not using first order networks.

Fig 7 – Sparrow MTM-2 Waterfall

Fig 8 – Sparrow MTM-2 Toneburst Energy Response

The Waterfall plot shows some artifacts at 15kHz and 18kHz. They are also recorded in the Toneburst as light blue slices from 10kHz~20kHz. 

Fig 9 – Sparrow MTM-2 Spectrogram

The Spectrogram in Fig 9 shows the cone breakup burst at 15kHz and 18kHz are not harmful as they dissipate by 1 msec. However, some cone ringing in the SB13PFC25-04 are recorded from 1.0kHz to 3kHz. I doubt they will smear the sound because they generally do not last longer than 6 msec.

A final note about the Sparrow MTM-2

I encountered a strange behavior when I tested out the Sparrow MTM-2. The vocals sounded compressed and lifeless, as though something is holding back the voices. I listened to it for a couple of days trying to figure out what the cause is. Then it struck me that it could be the bass bleeding into the midrange. I duly converted the bass reflex to a sealed box and the problem vanished instantly. The Sparrow MTM-2 now meets my expectations. The vocal clarity is superb and the bass is dynamic.  

For those that are contemplating which Sparrow MTM to build, I strongly recommend this second version. Though the overall cost is higher because of the D280Ti and a more complex crossover, the sound quality is worth it. 

Update – June 2, 2020

After running the Sparrow MTM-2 for about 10 days, I decided to listen to it again in a bass reflex. To my surprise, the strange, offending sound vanished. The crossover and the box are the same, no changes except for unsealing the port. 

After pondering over it, I concluded that the cause was the SB13PFC25-04 were not broken in. Unbelievable. Never expected that. Now, the Sparrow MTM-2 sounds perfectly normal in a bass reflex. 

Sparrow MTM-2 Impedance

Fig 10 – Sparrow MTM-2 Impedance

This is where wiring the 4Ω version of the SB13PFC25 in series helps in a MTM design. Had I used the 8Ω SB13PFC25, the impedance will drop to 3Ω when I parallel them. That will trigger some power amplifier’s protection circuitry. Not good for sound when that happens.

The box tuning is perfect for the SB13PFC25-04. Nice, symmetrical peaks. What is interesting is from 200Hz upwards, the impedance is fairly flat. Even the electrical phase is well behaved. This indicates the Sparrow MTM-2 more resistive than reactive. Power amplifiers love this.

Crossover is available on request. Free for DIY. Not for Commercial use.
Unless otherwise stated, all measurements were made with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.