Fostex FF165WK Measurements

[Joseph Crowe]

In this post I test the Fostex FF165WK 6.5" full range driver.

I purchased these drivers ten years ago for a project which was featured on my blog here.

2013 Blog Post 1

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2013 Blog Post 3

2013 Blog Post 4

2013 Blog Post 5

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2013 Blog Post 9

Testing again after ten years.

I was curious to see how these drivers would measure with my ARTA measurement software considering the vast database I've developed in the past five years. Being able to compare the FF165WK data against, for example, the Markaudio CHR120 was something I was personally interested in, considering how much time and effort I spent ten years ago on this driver. I must admit also that I have a bit of nostalgia for this particular driver, among man other single full range drivers.

Contour Network

Over the years I've improved in my ability to develop excellent passive circuits to help achieve the desired frequency response from various drivers. Examples of this is here and here. So I wanted to see if I could use my latest skill to further refine this driver for even more sound quality. Now, I must confess, that this driver is not necessarily anything stand out in terms of sound quality. It is pretty average if I am honest. But I feel like many enthusiasts have similar drivers where they've been tolerating the faults while enjoying what they get right, which is coherency and soundstage depth. In most cases, these faults can large be mitigated through careful contouring of the frequency response with a passive network. (see more later in this blog)

New Tapered Transmission Line Enclosure

In this blog post I also introduce a new tapered transmission line floorstanding enclosure for the FF165WK as a plan set that can be purchased downloaded on my site. This is also an area where I've improved in my ability over the past ten years. I believe the new TL enclosure maximizes the potential bass performance from this driver. Please contact me if you would like a custom enclosure designed for a specific driver.

Super Tweeter

Over the past six months I've been spending a lot time on super tweeters. The FF165WK lends itself to the addition of a super tweeter by virtue of it's natural roll-off above 10kHz. The driver can be used alone, or with a high frequency driver using a range of crossover points ranging from 2kHz - 10kHz. In the context of the DIY enthusiast, the FF165WK can serve has a good test box to evaluate various high frequency solutions. Considering that the overall cost of the FF165WK driver is relatively affordable, the enthusiast just may have money left over for a super tweeter as well.

Appearance

For me I really like the appearance of the FF165WK with it's various shades of grey. It has a vintage look to it as well which some people really like.

General Features of the FF165WK

The particular model stands out from the rest in that it uses a aluminum dust cap to extend the high frequencies as apposed to a whizzer cone. It's part of a family of FF series driver models which range in size from 3", 4", 6.50", and 8.00".

Features

• Double layer cone
  • Pulp base layer with Kenaf top layer
    • Kenaf layer provides high speed sound propagation
• Aluminum dust cap
  • Proprietary center dust cap ridge to distribute modal breakup
• Low loss urethan foam surround
• Relatively high sensitivity of 92dB/1w for a 6.50" woofer

Measurements

I measured the response at 1m mounted in a 20cm wide transmission line floorstanding speaker. I gated the measurement above 300Hz and then conducted a near field ungated measurement below 300Hz and spliced the two responses.

The on-axis frequency response can be summarized as follows.

• Very high sensitivity through the midrange (95dB 1w)
• +5dB rising response from 300Hz - 8kHz
• -12dB/octave falling response above 8kHz
• Surround resonance at 4.5kHz
• Sharp +15dB resonance at 18.5kHz (outside audible band)

Comparison for Context

Below is the frequency response of the Markaudio CHR120 for comparison.

If I overlay the Fostex FF165WK (Blue) for comparison we can see that the Fostex has significantly higher sensitivity. This is very important if using a low power amplifier since the amplifier is going to produce much less distortion at the lower output voltage. In this particular case the Markaudio will draw two times the power from the amplifier for the same output SPL. When looking at specific low power amps such as the Nelsen Pass Amp Camp Mini, this can be a distortion difference of 39dB in noise floor when comparing distortion between 1v output from the amplifier versus 4.00V (2w).

Off-Axis

Below is the off-axis frequency response overlay for 0, 15, 30 and 45 degrees off-axis. The driver's directivity starts to narrow at around 5kHz. This can be observed by the blue line (30 degree off-axis) at the 5kHz region where things start to fall off as you move up in the frequency spectrum.

Time Domain

Below is the burst decay out to 20kHz. The resonance at 4.5kHz is likely more audible due to the peak in the frequency response more than it's tail out to 12 periods. So if the peak is subjectively offensive then simply notching out this region is likely a good solution. The peak at 18.5kHz is entertaining to see.

Extending the burst decay out to 50kHz highlights this resonance even further. We see full ring out to 24 periods. That's a big resonance! Luckily this is outside the audible band.

The CSD is surprisingly clean with the exception of the 4.5kHz surround resonance.

Correction Network

Based on the above test data it appears the driver performs well with the exception of the following...

• Midrange peak at 1.5kHz
• Resonant peak at 5kHz

The below schematic is what I came up with the correct for the above issues. It is comprised of two series LCR contour circuits.

Below is an overlay between the raw response (red) and the affect of the filter (green).

I then re-measured the off-axis at 0,15, and 30 degrees off-axis.

Below is the time domain performance with the correction network in place.

Bass Cabinet No.2286 -- Tapered Quarter Wave Transmission Line (TQWL)

Below is the half space in-room response without the correction network. The speaker was placed about 30cm from the rear wall. No internal damping material was used in the enclosure.

 

[norman bates]

excellent work to an otherwise scary frequency response.

 

[mp9]

excellent work to an otherwise scary frequency response.

sure is, now to see Troy pull off the same magic act with the FE108SS-HP drivers on his shelf, that would be something👽

 

[Freedom666]

Putting it closer to back wall would fill the f response between bass and mids?

 

[freddi]

Very impressive work - - Here's a cabinet by GregB which would "work" with that driver. The prototype made of scrap was a bit less is depth and tuned to 60Hz with little damping material other than polyfil in the stub above the driver. In contrast,
Karlson's originals were two chamber bandpass style starting with "K15" from 1951 - it can exhibit less side-band distortion using two tone input than a reflex. I think K
15's front chamber tunes around 110Hz and system resonsnce ~48Hz.


I'd love to hear how you would approach a Karlson -typ0e cabinet -the the idea might not appeal to the scientific mind. (? - I like a lot of what they do vs compromised / small FLH. Are there front chamber shapes which work better than other? Can the curved reflector in Karlson's 3540544 patent do good and if so, how should it be implemented ?

For super tweeters I really liked the old Selenium phenolic diaphragm slot loaded tweeters and sad that category
has been pretty much replaced by 'bullet" style.

 

[Freedom666]

Magic of Karlsson is in my opinion proximity to two sides like floor and back wall giving +6db efficiency

 

[Joseph Crowe]

excellent work to an otherwise scary frequency response.

Thank you! I don't think the response is too bad all things considered.

Putting it closer to back wall would fill the f response between bass and mids?

Perhaps. I can try that. I've been listening to just a single cabinet for the past week and the bass is very nice. It has good detail and follows the mid-bass and mid-range quote well.

sure is, now to see Troy pull off the same magic act with the FE108SS-HP drivers on his shelf, that would be something👽

I've been seriously tempted to do that. Maybe a back loaded horn wide baffle.

For super tweeters I really liked the old Selenium phenolic diaphragm slot loaded tweeters and sad that category
has been pretty much replaced by 'bullet" style.

Thank you, let me look at this further. Interesting on the "Selenium phenolic diaphragm slot loaded tweeters", do you have a model number?

 

[pcgab]

The slot tweeter is the Selenium st320, I believe.

 

[freddi]

ST320 had an aluminum chassis- there was an excellent plastic chassis version - "ST324" which sold for around $25 a few years back

https://www.lautsprechershop.de/pdf/selenium/selenium_st324.pdf

 

[pcgab]

Oh, completely forgot about that one, thank you Freddi!

 

[DonM]

There is a full review of the project on YouTube (link is safe) with a preview of the speaker with and without the correction network. Check it out.