I've mostly been a reader only on the diyAudio forums up to this point, but I'm starting to diving into some more audio projects now and need to get more actively involved in discussion and questions. One of the projects on my to-do list relates to a pair speakers and amplifier that I found at a local estate sale. The speakers are "The Great Horned Heil" from the Horn Shoppe. This is a rear-loaded horn speaker with a ESS Heil Air Motion Transformer (AMT) on top. There is an external set of crossovers which were presumably designed to work with this pairing. The speakers also came with a First Watt F1 amplifier and "The Truth" Preamp. (I've found mixed reviews about The Truth preamp, so I'm going to ignore that component for now.)
Everything seems to work and I'm liking the sound I'm getting from the setup so far. (Although, I'm detecting a small amount of distortion from of the Heil AMT drivers on some recordings... The diaphragm may need to be replaced) I'm still experimenting with speaker placement and which room to put them in.
I understand that the First Watt F1 amplifier is an effective choice for efficient full-range speakers due to its transconductance (current drive) design. While this encourages better bass out of the speakers, I'm still interested in incorporating a powered-subwoofer to see if I can improve the overall performance.
Normally I would start by connecting the amplifier speaker outputs to the "speaker" inputs of the powered subwoofer. I'm worried about doing this with the First Watt F1 amplifier, however. Will the fact that it is a current source design create problems if I attempt to connect a sub in this way? I had found a short statement in a old forum post (via Google search) in which someone suggested that you should never connect this type of amp to a powered subwoofer. There was not enough detail in the post to fully understand the reasons. Could anyone advise me to what my options might be for incorporating a powered subwoofer for this system? Will I be limited to tapping off a line level signal back up the signal chain to a pre-amplifier?
Thanks!
Everything seems to work and I'm liking the sound I'm getting from the setup so far. (Although, I'm detecting a small amount of distortion from of the Heil AMT drivers on some recordings... The diaphragm may need to be replaced) I'm still experimenting with speaker placement and which room to put them in.
I understand that the First Watt F1 amplifier is an effective choice for efficient full-range speakers due to its transconductance (current drive) design. While this encourages better bass out of the speakers, I'm still interested in incorporating a powered-subwoofer to see if I can improve the overall performance.
Normally I would start by connecting the amplifier speaker outputs to the "speaker" inputs of the powered subwoofer. I'm worried about doing this with the First Watt F1 amplifier, however. Will the fact that it is a current source design create problems if I attempt to connect a sub in this way? I had found a short statement in a old forum post (via Google search) in which someone suggested that you should never connect this type of amp to a powered subwoofer. There was not enough detail in the post to fully understand the reasons. Could anyone advise me to what my options might be for incorporating a powered subwoofer for this system? Will I be limited to tapping off a line level signal back up the signal chain to a pre-amplifier?
Thanks!
Gosh, great question. If the subwoofer's amp is in parallel with the speakers, you will have a very light load (the sub amp) that is presumably constant as a function of frequency, e.g. resistive, in parallel with a reactive heavy load that varies as a function of frequency. I'm not sure that would mess too much with the F1's attempt to deliver a constant current (by varying the voltage as a function of load impedance) to the reactive heavy load of the speakers, but it would mean the voltage swing seen by the subwoofer amp is being modulated by something other than the music. Hm, maybe that's your answer. It would be distorted as a function of the speaker reactance curve.
nothing wrong wil happen if you just try it
but!
considering that F1 is bridged amp, with both outputs live and on significant DC potential, best way of connecting it to powered sub HiLevel inputs is - take hot to sub from one F1 output (doesn't matter which if you can flip phase on sub) through capacitor in series; say that 10uF/50V will do the job, any decent solid cap
and take GND to sub input anywhere from F1 GND
that way you'll take just one phase from F1, but no worries - nothing will buzz, you'll get modulation and all you need is just to hear are you satisfied with outcome
but!
considering that F1 is bridged amp, with both outputs live and on significant DC potential, best way of connecting it to powered sub HiLevel inputs is - take hot to sub from one F1 output (doesn't matter which if you can flip phase on sub) through capacitor in series; say that 10uF/50V will do the job, any decent solid cap
and take GND to sub input anywhere from F1 GND
that way you'll take just one phase from F1, but no worries - nothing will buzz, you'll get modulation and all you need is just to hear are you satisfied with outcome
Yes... I think I follow what you are saying here. I'm not sure, but this may point to the necessity of switching from using parallel filter network (for voltage amplifiers) to serial filter networks for current amplifiers.
@Zen Mod - When you say that the F1 is a bridged amp, are you referring to differential drive topology on its output channels? I revisited the F1 schematic just now to remind myself what it looks like. This reminded me that output of this amp operates with a non-trivial DC offset. You said the same above. In fact -- now that I think about the forum post which I found in the Google search mentioned in the my initial post, I believe it was in fact the DC offset that can cause issues if connected directly to a powered sub. (I think the author of that post said that making a connection like this toasted a resistor on the input path of the sub when he tried it.)
In the process of thinking more about your suggestion with the series capacitor, I also stumbled across a white paper that Nelson Pass wrote about Current Source Crossover Filters. If I'm interpreting the application of the series capacitor which you suggested above correctly, is it accurate to compare this to the single pole 2-way filter which Nelson illustrates in this paper? Or, am I on the wrong track here? (I'm also trying to sort out if I'll actually need to modify the crossover which came with these speaker to attempt what you are describing.)
just re-read what I wrote and you'll get it
hot to sub from one F1 out, through cap
GND from F1 to sub GND
it will work
hot to sub from one F1 out, through cap
GND from F1 to sub GND
it will work
I'm not sure the impedance of the speaker load and crossover network would end up flat enough either way to minimize the impact to the voltage portion of the amp output, but maybe it wouldn't matter much to just the bass end of the spectrum, which is what you care about here. I suppose it depends if there's a big impedance rise at the low end from the tweeter/midrange. The parallel shunt resistors in series crossovers are there to flatten impedance rises. Like ZM is saying, it may be worth a try.
Okay -- I did overcomplicate your suggestion, ZM. 🙂 You are just saying to wire up an AC-coupled signal from only one side of the differential pair on one channel of the F1. Return path to the F1 ground. This will be an easier experiment to hack together.
Thanks @ranshdow, for your input also. I was wondering what role the shunt resistors were playing in the series crossover.
Thanks @ranshdow, for your input also. I was wondering what role the shunt resistors were playing in the series crossover.