Piezo Preamps and Transformers

I’ve been primarily using Triton Audio’s BigAmp Piezo preamps for piezo-based contact mic and hydrophone recording for years now. I previously used Hosa MIT-129 impedance matching transformers for the job, but I found that the BigAmp Piezos yielded me a better low frequency response thanks to the higher impedance. The Hosa MIT-129 hs a 50 kilaohm input impedance whereas the BigAmp Piezo has 7.5 Megaohm input impedance. Add to that the additional gain from the active preamp and you start to see why I generally prefer the BigAmp Piezo.

The selection of preamps and transformers that were tested.

In initial listening tests years ago I found that the BigAmp Piezo produced a wider frequency response generally, and it felt like it had a quicker transient response, when compared to the Hosa MIT-129. This sort of rounding off is as expected from the passive transformer, and is actually a pleasing, more analogue effect of the passive transformer device in some ways.

I also have a Barcus Berry 4000XL Piezo/Buffer/Preamp which has an input impedance of 2.2 Megaohms. This results in a fairly similar low frequency response to the BigAmp Piezo, but it has a much lower amount of gain compared to the BigAmp Piezo.

What about noise?

Noise can be a complicated issue, as you need to take into account how the impedances of the devices interact, and then there are differences at different frequencies. My comments below are based on my testing using a Sound Devices 788T with the inputs linked for precise gain matching.

I have found a little bit of variability in the self noise of the BigAmp Piezos – not heaps, but a little bit. In theory the Hosa MIT-129 has no self noise as such since it is a passive device, but when I compare a level matched recording under controlled conditions from a BigAmp Piezo and a Hosa MIT-129 I find that the BigAmp Piezo has a little less noise, but it is possible I am getting tricked in my level matching because of the tonal response differences.

I have found the Barcus Berry 4000XL to provide a similar amount of noise to the Hosa MIT-129 when I compare a level matched recording under controlled conditions. So overall the BigAmp Piezo sounds the cleanest for the equivalent amount of gain/signal, although again, it is possible I am getting tricked in my level matching because of the tonal response differences. The Hosa MIT-129 gives a thinner response overall with more high frequency hiss when relatively level matched.

What about matching?

As mentioned above, I have found a little bit of variability in the self noise of the BigAmp Piezos. Not enough to be a problem for most applications.

I have 6 x BigAmp Piezos, 4 of which are an older design and another 2 which I recently got that look a little bit different. The older design has a Triton Audio badge on it with a green dot below it, but no other text on the unit. The newer design has a different grey paint finish with no green dot, but it does have text on it (BigAmp Piezo :: made in Holland). In my testing I have found that the newer units have more low frequency response under around 100 Hz. The self noise profile of the newer ones seems to have a bit of a boost in the mids centred around 750-1,000 Hz. The specs sheets provided with both the newer and older units has exactly the same specifications on it, so I emailed Triton Audio to get confirmation of what I had found and they advised that there is “an extended low frequency response. Frequency response should be flat.”

The older BigAmp Piezo on the left, newer BigAmp Piezo in the middle, and a Hosa MIT-129 on the right.

In terms of frequency response, the 4 x older units I have are all quite similar to each other.

I have 6 x Hosa MIT-129s and I have found them all to have a very similar frequency response and amount of noise, so they match each other really well. As a simple passive device they shouldn’t have any self noise as such.

Conclusion

Using some form of preamp/buffer (active DI) or impedance matching transformer (passive DI) with piezo pickups is important if you want to capture a wider frequency response. The active Triton Audio BigAmp Piezos are an excellent low noise option with a great compact form factor for use in the field and are my preferred option. A cheaper option that doesn’t give quite the extended low frequency response is the passive Hosa MIT-129 impedance matching transformer. Being a passive option I would suggest it is probably a bit more rugged and less likely to have issues develop over time.

This entry was posted on Saturday, June 22nd, 2019 at 6:39 pm and is filed under Field Recording, Resources, Reviews - Gear. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

 

Leave a Reply