Hi all.
I have a modern solid state active studio monitor speaker that after 25y bit the dust in a glorious cloud of smoke (Behringer Truth 2031a) and was beyond me to repair, especially when it was cheap to replace.
The amp for this was based around 3 x 38w LM3886T. with a bi-amplified design for the woofer and "ferro fluid cooled tweeter".
I thought it might be a fun project and an excuse to read my Merlin Blencowe high fidelity tube amp design book to convert this in to a valve amplified speaker.
My first port of call was to check the impedance of the speakers assuming they would read 4ohm.
The woofer did but the tweeter did not register for resistance and measured a really small capacitance.
My immediate thought was that the tweeter had fried also, but my friend suggested it may be piezo based as they can read like this?
The cabinet is a sealed unit so I haven't inspected the back of the speakers yet.
Firstly, does this sound plausible, and if so how can I test the speaker? Would it be safe to connect it to a consumer audio receiver?
If so can this kind of speaker be driven by valves given that the anode load is normally a product of output traf and speaker impedance?
Lastly if this seems doable would anyone have any schematics or component suggestions I look at as a starting point?
Thanks
I have a modern solid state active studio monitor speaker that after 25y bit the dust in a glorious cloud of smoke (Behringer Truth 2031a) and was beyond me to repair, especially when it was cheap to replace.
The amp for this was based around 3 x 38w LM3886T. with a bi-amplified design for the woofer and "ferro fluid cooled tweeter".
I thought it might be a fun project and an excuse to read my Merlin Blencowe high fidelity tube amp design book to convert this in to a valve amplified speaker.
My first port of call was to check the impedance of the speakers assuming they would read 4ohm.
The woofer did but the tweeter did not register for resistance and measured a really small capacitance.
My immediate thought was that the tweeter had fried also, but my friend suggested it may be piezo based as they can read like this?
The cabinet is a sealed unit so I haven't inspected the back of the speakers yet.
Firstly, does this sound plausible, and if so how can I test the speaker? Would it be safe to connect it to a consumer audio receiver?
If so can this kind of speaker be driven by valves given that the anode load is normally a product of output traf and speaker impedance?
Lastly if this seems doable would anyone have any schematics or component suggestions I look at as a starting point?
Thanks
Yes, a piezo tweeter will have a very high resistance. I think it will also be variable with frequency. It's more of a capacitor than a resistor. To make them fit into a more normal crossover I've seen some folks add a resistor in parallel with the tweeter so a crossover can work properly.
https://audiokarma.org/forums/index.php?threads/using-piezo-tweeters-wisely-a-how-to.79659/
https://audiokarma.org/forums/index.php?threads/using-piezo-tweeters-wisely-a-how-to.79659/
Piezos that we used years ago were roughly equivalent to a 100nF capacitor (0.1uF). They are best current driven so usually they come with a series resistor about 100 Ohms. The only reason to load them with an ~8 Ohm resistor is for the crossover ahead of them, ie usually a bad idea. A 100nF cap (0.1uF) in series and a 470 Ohm resistor shunt gives you a 12dB ~3KHz high pass, appropriate for a piezo. Note that connecting them directly to an amp without a series resistor may make the amp unstable.
Adding series resistance to a piezo tweeter protects the amplifier from oscillating, but rolls off the high frequencies a little.
Adding a resistor in parallel with the tweeter makes it look like an ordinary tweeter, but use 22 ohm as 8 ohm is too low a value.
The above diagram shows a crossover for a piezo tweeter that will be a safe load for a valve amplifier.
The parallel resistor is always 22 ohm. The following capacitance values relate to different size piezo drivers:
Small round piezo: C = 1.5 uF; larger rectangular piezo: C = 2.2 uF; large compression drive piezo meant for mounting on a horn: C = 4.7 uF.
More info here, including how to attenuate the piezo and how to tame the top end: https://www.frugal-phile.com/piezo-XO.html
Adding a resistor in parallel with the tweeter makes it look like an ordinary tweeter, but use 22 ohm as 8 ohm is too low a value.
The above diagram shows a crossover for a piezo tweeter that will be a safe load for a valve amplifier.
The parallel resistor is always 22 ohm. The following capacitance values relate to different size piezo drivers:
Small round piezo: C = 1.5 uF; larger rectangular piezo: C = 2.2 uF; large compression drive piezo meant for mounting on a horn: C = 4.7 uF.
More info here, including how to attenuate the piezo and how to tame the top end: https://www.frugal-phile.com/piezo-XO.html
Indeed Sonce! I did initially intend to check out the specs of the Behringer B2031A before posting, but got sidetracked!
The driver shown above is an "ultra-high resolution ferrofluid-cooled tweeter"!
The driver shown above is an "ultra-high resolution ferrofluid-cooled tweeter"!
Hi all..
Thanks for the response.. So yes.. fried tweeter.
Just for fun and being a bit skint at the moment rather than chucking it all in the bin.
I thought I'd have a go at getting this working as a passable passive speaker, then if this works look to drive it with a premade PCB or when finances allow try making a simple Monoblock valve amp.
I purchased this tweeter:
https://www.amazon.co.uk/dp/B000L9A4VK?ref=ppx_yo2ov_dt_b_fed_asin_title
and this crossover:
https://www.amazon.co.uk/dp/B0BCW7CMKP?ref=ppx_yo2ov_dt_b_fed_asin_title
Addition of some plywood, silicone, and matt black paint and its holding together well enough and producing sound when connected to a hifi.
I realise I've already messed up somewhat in that despite spending hours looking for a cheap 4ohm tweeter to match the woofer I've accidentally purchased an 8 ohm model.
What would the impact be here... half the efficiency/volume on the tweeter?
I still get a 4ohm reading on the input of the crossover.
Does anyone have any suggestions on how to proceed to try get this working as an active speaker?
Would the impedance mismatch suggest id be better off amplifying the speakers separately and applying high/low pass filters to the respective amps or will it be fine as is? Will I be straining the crossover somehow with the mismatch?
As a side note the original speaker was using a 32-0-32 toroidal transformer which is going spare..
I was considering something like this to drive it:
https://www.ebay.co.uk/itm/156718821632?
Thanks for the response.. So yes.. fried tweeter.
Just for fun and being a bit skint at the moment rather than chucking it all in the bin.
I thought I'd have a go at getting this working as a passable passive speaker, then if this works look to drive it with a premade PCB or when finances allow try making a simple Monoblock valve amp.
I purchased this tweeter:
https://www.amazon.co.uk/dp/B000L9A4VK?ref=ppx_yo2ov_dt_b_fed_asin_title
and this crossover:
https://www.amazon.co.uk/dp/B0BCW7CMKP?ref=ppx_yo2ov_dt_b_fed_asin_title
Addition of some plywood, silicone, and matt black paint and its holding together well enough and producing sound when connected to a hifi.
I realise I've already messed up somewhat in that despite spending hours looking for a cheap 4ohm tweeter to match the woofer I've accidentally purchased an 8 ohm model.
What would the impact be here... half the efficiency/volume on the tweeter?
I still get a 4ohm reading on the input of the crossover.
Does anyone have any suggestions on how to proceed to try get this working as an active speaker?
Would the impedance mismatch suggest id be better off amplifying the speakers separately and applying high/low pass filters to the respective amps or will it be fine as is? Will I be straining the crossover somehow with the mismatch?
As a side note the original speaker was using a 32-0-32 toroidal transformer which is going spare..
I was considering something like this to drive it:
https://www.ebay.co.uk/itm/156718821632?
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Your off the shelf crossover says it suits 4 to 8 ohm, so provided the drivers are blending together smoothly I wouldn't concern myself.
(The different impedances of the two drivers may mean that the woofer roll off frequency and tweeter roll in frequency are not exactly the same.)
The 4 ohm reading you get is the DC resistance of the woofer. The tweeter doesn't register as it has a capacitor in series that blocks DC measurement.
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The reason for Urumiko's thread was to ask if it was safe to drive a piezo tweeter with a valve amplifier.
(Note that his question no longer applies as he does not have a piezo tweeter.)
The thread could now usefully be transferred to a more appropriate section of the forum.
Thanks for the insight. By the sound of it this is where a DC resistance is a bit useless and an impedance curve measurement through and beyond the crossover value would be of interest but I still worry that the increased impedance above the crossover freq would result in a sharp dropoff in the amplitude at those frequencies?
You're not wrong. My query was posed from the desire to use this speaker as a platform to have a go at building my own valve amp to power it as a fun project. I've mainly just worked on guitar amps and not scratch built anything before. My initial concern was about what I thought wrongly was a piezo speaker. Now that's resolved the thread has gone off topic but I'm now just looking for practical suggestions from those more experienced on what would be a suitable starting point/ reference circuit. I'm still pondering the idea of going all out and trying to build a kt88 push pull monoblock, Vs throwing in a prefabricated solid state board (possibly with some tame power supply work), or if people have other suggestions for someone of my limited experience and budget.
It sounds like you haven't tried out the crossover yet. It's what actually happens in practice that counts.
The sensitivity of your tweeter is not specified in your link, but is likely to be higher than that of the mid/bass driver, meaning it would sound louder.
A tweeter may require to be attenuated by a resistor, or combination of resistors, to bring its level down to match that of the mid/bass driver.