Jim Shearer's Fostex FF85K and Eminence Alpha 15A Open Baffle Speaker - diyAudio
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Old 15th November 2009, 12:48 AM   #1
MJK is offline MJK  United States
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Default Jim Shearer's Fostex FF85K and Eminence Alpha 15A Open Baffle Speaker

I visited Jim Shearer this afternoon to hear his latest speaker build. You can see a picture of the speaker in another thread on DIYaudio.

New MJK Baffle Article

He has designed an OB system combining the Fostex FF85K full range driver and the Eminence Alpha 15A woofer with a passive 2nd order crossover. The speaker was being driven by a 70's era Yamaha SS receiver and a CD changer. The speaker design was based on my two way passive OB article but with Jim's ideas for making the baffle much more attractive hence producing a higher WAF. This was the first time I had a chance to actually hear this type of OB design and I was not disappointed in the least.

The performance was really amazing, in particular the depth and volume level of the bass output. The bass had to extend below 40 Hz and was solid without any bloat or bloom, everything was very tight and accurate. There were discrete and distinct bass notes. I had heard Jim's FF85K drivers in his Sprite enclosure earlier this year and was immediately impressed with the quality of the mid's and treble response. The only thing lacking in the Sprites was the bass. Now paired with the Alpha 15A for bass support, the FF85K could do what it does best. The combination worked really well together and I could have easily listened to the OB speakers for hours, a very enjoyable presentation. But we had a few other things to experiment with ......

This past summer I finally broke down and decided to try a tube amp. I won an EL84 integrated amp on e-Bay that produced a fairly decent 15 watts. That is the extent of my knowledge on the tube amp. When I connected it to several of my own speaker designs I was initially very disappointed. While the mids and treble sounded really nice the bass produced by the speakers disappeared. It was almost like a 200 Hz high pass filter had been placed between the amp and the speakers. It seemed to be a common problem with all of my speakers. Jim had been to my house earlier in the week and we had compared the tube amp and my SS amp and he heard the same issues with the significant loss of bass output. The SS amp was the clear winner.

I brought the tube amp, CD player, and my homemade CAT5 speaker cables to Jim's house so we could replicate my set-up but with his speakers. The first speaker we tried the tube amp on were Jim's Fostex F120A Metronomes. Jim and I both have pairs of F120A drivers, in different speaker designs, and we have both been disappointed with their performance. At my house we tried the tube amp on my F120A OB and Goldwood H frames and felt the F120A's performance improved but the bass rolled off significantly. So we started with his SS amp, listened to the Metronomes for a while, and then switched to the tube amp and CAT5 cables. When we connected Jim's Metronomes to the tube amp we were blown away. The improvement across the audio spectrum was dramatic, better and deeper bass with smoother extended highs. This was by far the best sound I have heard coming from a F120A driver and it was really really good. It was also the first time that I had heard the tube amp truely sound great. To try and understand if the tube amp or the CAT5 cables were the reason things improved so much, we switched to a thinner zip cord style speaker cable. Some of the improvement disappeared, so it became clear that both the tube amp and the CAT5 cables contibuted to the improvement in the sound of the F120A drivers. This combination of amp, cables, and speakers was an eye and ear opener. All the problems we had heard in the F120A driver were gone. The F120A sounded like a completely different speaker and more consitent with its stellar reputation.

So far so good, we now moved back to the FF85K/Alpha 15A open baffle speakers. Using the tube amp and CAT5 cables connected to the OB speakers, the bass totally disappeared. Again, it sounded like a filter had been placed between the amp and the speakers rolling off the bass below 200 Hz. How can an amp sound so good with one speaker and so inadequate with another? I am beginning to wonder how the passive crossover is interacting with the tube amp. My H frame speakers and Jim's OB speakers both have 2nd order low pass filters designed to work between 100 and 200 Hz. This means that there is a very large inductor, 12 or 13 mH, in the signal path. Could this be the cause? Higher power SS amps have no problems driving this load and producing excellent bass performance. The tube amp just did not work well at all.

I would be interested in hearing other people's experiences with two way dipole speakers using a full range driver and crossed very low to a helper woofer and in particular how they worked with SS and/or tube amps. Do people hear what I have been describing? Are OB speakers designed along the guidelines I have layed out really only going to work well with SS amps? Is it just the tube amp I bought or a generic issue with tube amps? Any feedback would be appreciated.
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Old 15th November 2009, 03:46 AM   #2
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Quote:
Originally Posted by MJK View Post

When we connected Jim's Metronomes to the tube amp we were blown away. The improvement across the audio spectrum was dramatic, better and deeper bass with smoother extended highs. This was by far the best sound I have heard coming from a F120A driver and it was really really good. It was also the first time that I had heard the tube amp truely sound great... All the problems we had heard in the F120A driver were gone. The F120A sounded like a completely different speaker and more consitent with its stellar reputation.


dave
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Old 15th November 2009, 03:49 AM   #3
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Quote:
Originally Posted by MJK View Post

He has designed an OB system combining the Fostex FF85K full range driver and the Eminence Alpha 15A woofer with a passive 2nd order crossover...

The performance was really amazing, in particular the depth and volume level of the bass output.


Settled then, We are going active on ours.

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Old 15th November 2009, 04:14 AM   #4
TerryO is offline TerryO  United States
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Settled then, We are going active on ours.

dave
I wonder what the impedance curve for the woofer looks like. With SETs in particular, and perhaps to some extent PP amps, it can really cause problems...not to mention throwing an inductor into the overall mix.

Active solutions can just about completely eliminate this problem, especially with a SS bass amp.

Best Regards,
TerryO
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Old 15th November 2009, 05:24 AM   #5
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with a SS bass amp.
No shortage of those.

dave
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Old 15th November 2009, 08:50 AM   #6
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Quote:
Originally Posted by MJK View Post
I would be interested in hearing other people's experiences with two way dipole speakers using a full range driver and crossed very low to a helper woofer and in particular how they worked with SS and/or tube amps. Do people hear what I have been describing? Are OB speakers designed along the guidelines I have layed out really only going to work well with SS amps? Is it just the tube amp I bought or a generic issue with tube amps? Any feedback would be appreciated.
Hi Martin,

Valve amplifiers normally use considerably less overall negative feedback than SS amps and consequently have a very much lower damping factor and output impedance. As a consequence the passive crossover components have a much larger influence than with SS amps. This is most evident with single ended valve amplifiers which sometimes use no negative feedback at all.

Hope this was of some help,
Graham
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Old 15th November 2009, 08:57 AM   #7
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Quote:
Originally Posted by MJK View Post

I would be interested in hearing other people's experiences with two way dipole speakers using a full range driver and crossed very low to a helper woofer and in particular how they worked with SS and/or tube amps. Do people hear what I have been describing? Are OB speakers designed along the guidelines I have layed out really only going to work well with SS amps? Is it just the tube amp I bought or a generic issue with tube amps? Any feedback would be appreciated.
Oops, think I said that the valve amp has a lower output impedance - this should be higher... The damping factor is lower !
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Old 15th November 2009, 11:16 AM   #8
kmaier is offline kmaier  United States
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Martin, et al,

As has been pointed out already, tube amplifiers have traditionally low damping factors, generally below 20 and in the case of most SETs well below 10. As with your recently acquired P-P EL84 amp, depending on it's age, condition and design, it's damping factor could be even lower than original spec. In short, the output impedance is quite high and I suspect it has a negative feedback loop as well.

In your OB design (which I really like), you use a large woofer which is un-damped, i.e., no enclosure which also has a high moving mass. Couple this to the passive crossover between the F120A and the Eminence 15A and the back EMF generated from the 15A can easily be fed to the F120A effectively corrupting it's signal via intermod. (due to the high output impedance of the amplifier) and also affecting the feedback loop of the amplifier further exacerbating the problem.

The results obtained using just the tube amp directly to the F120A (which is the only way I've ever driven mine) seem to contradict all earlier experiences (which is good). The obvious next step would be to use the tube amplifier directly driving the F120A in your OB design and either a passive EQ driving the amplifier input or an electronic one... same for the bass driver, but a SS amp would be preferred to use the damping factor to advantage in controlling the woofer. If you do some additional experimentation around such a scenario I would be interested to hear your feedback on it.

Regards, KM
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Old 15th November 2009, 01:17 PM   #9
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One more piece of data to add to the puzzle: When I was over at Martin's place we also listened to his new Alpair 10's in ported enclosures--no XO, no BSC; straight from the amp to the driver. Using the SS amps, bass extension was good, sounding to me like to about 50 Hz. With the tubes, the bass vanished. This is why we were expecting the F120a's (which have no XO or BSC) to sound better at high freqs, just as Martin's did, but we were not expecting the bass to be full and rich.

(As to damping factor: do you really think that the high damping factor of the SS amp will survive the XO to reach the 15" driver? I have doubts that it could. And shouldn't a high damping factor lead to tighter, more controlled, less prominent bass anyway?)

So we have a more complex question than how can the tubes do so well on the F120a and so poorly on the two ways with XO. We have to understand how tubes can work well with the F120a and poorly with the Alpair 10. Was them amp somehow built specifically for use with F120a's?

I joked with Martin, (think voice of an ancient, alien android being from Star Treck, original series):
So long even I had forgotten! Tubes plus high capacitance cables plus ALNICO equals audio bliss!

Cheers, Jim
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Old 15th November 2009, 02:38 PM   #10
MJK is offline MJK  United States
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I think that the bottom line with this particular integrated tube amp is that it really does not work well with any of the speakers I have on hand scattered around my house. I have no reason to believe that the two designs I am probably going to build next will work any better. It is a fairly new design and construction. I have the option, from the manufacturer, of putting a variable negative feedback control into the amp at an additional charge or putting it back on e-Bay to recover some of the cost. The second option is probably the route I am going to take with this amp and I will stick with SS for the near future. Maybe I will try a different tube amp later.
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