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.
Unless otherwise stated, all measurements were made with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.
May 18, 2020Projects