Something stirs in the Undergrowth

[IslandPink]

Scott-
I'm guessing that if you tried to move to a Fonken-style alignment for the 285GMF you'd either lose bass or have to use a very different ( bigger? ) cabinet to achieve it. It would just be too lucky to find you could narrow the vents and convert it into a 'Fonken' !

Sounded pretty tasty last night without the vent damping - much much better .
I also put the Horn onto the right channel and let it snuggle up the Onken , which actually sounded remarkably good , without any ( yet ) attention to crossovers, level matching or even playing a mono record

Tonight I shall try (i) a mono LP and (ii) setting up the autoformer to take the horn down about 8dB .

More soon , much fun to be had .

Mark

[Scottmoose]

deleted

[IslandPink]

Thanks Scott -sounds like I've got a pretty good compromise here.
I need the bass gain because I had to move from OB in my plan to a vented box to get extension down to 40-45Hz . I couldn't do that on OB without the series crossover , which didn't seem possible with the new horn-mid plans, and I want to stick to the 285GMF for now because the cone is so light and it gives me more chance of matching the dynamics of the upper bass to the horn ... which will not be entirely possible of course ! ....

Phew that was a long sentence .

I'll be able to check those resonances if Nick comes down with the microphone etc some time soon . These are of course trade-offs I will have to live with . I definitely prefer the bass tone without the damping anyway .

Mark

[Scottmoose]

deleted

[planet10]

Scott-
I'm guessing that if you tried to move to a Fonken-style alignment for the 285GMF you'd either lose bass or have to use a very different ( bigger? ) cabinet to achieve it. It would just be too lucky to find you could narrow the vents and convert it into a 'Fonken'

100 litres vents as attached... F10 ~38Hz

dave

[IslandPink]

Thanks Dave.
Interesting result ..
It's a significantly smaller box than the one I have, but I think the overall combination is impractical . The vent length of 56cm and the area of those 8 vents would probably be impossible to achieve in a 100-litre box .
Some clever individual may prove me wrong ...

I'm assuming F10 is freq at 10dB down ?
If so where is F3 ?

Cheers
Mark

[IslandPink]

So HELLO Mr. Three-Way !!
You sexy beast !

( Actually this is a "Fool's Three-Way" since the ribbon is just for show at the moment ! )

[andrew Ivimey]

& they fit into the back of a Mini!? Cor...

[IslandPink]

That may be a type of Mini I haven't seen before, Andrew .

Meanwhile, a side pic shows the rough & ready set-up at the moment . Some sort of padded 'rails' still need to be fabricated, to support the driver body .

Currently I'm running 1st-order crossovers on the Onken and horn and liking it rather too much . Hmmm .... maybe it will stay that way ! The 1st-order on the Onken is external by series choke ( 3mH ) at the moment. This took away some tone ( 'feron' core Mundorf ) . The 1st-order high-pass on the horn is by reduced coupling-cap size in the power amp .
Maybe I will rig-up an internal 1st-order low-pass in the left channel of the amp . I have had trouble finding any schemes on this , thought there would be more than one way of doing it, but looks like series resistor and cap to ground is the normal way. Will have to double-check if this is going to be OK for a pentode driver .

I did a rough test on horn position by playing 600, 700, 800, 900Hz tones , with the two drivers out of phase, and looking for a loudness null around 700-800Hz . I found a broad ( incomplete ) null with the horn driver quite a long way back , the rim of the horn only an inch or two proud of the Onken front-panel. This surprised me technically... but it's handy aesthetically & practically . I will re-test when I do more experiments on this . Maybe the autoformer on the horn changes the phase too ?

[Mike H]

& they fit into the back of a Mini!? Cor...

A stretched limo version



 

[IslandPink]

OK, as Harvey Rosenberg ( Gizmo ) said, "It's better to do it in the front than in the back "
Having tried out the 3mH chokes on the Onken cab to give a 1st-order low-pass, it didn't sound that great and took away some dynamics.
So having worked out how to do this filter internally on the amp ( remember, I'm driving bass speaker from left channel and horn speaker from right channel ), I did the 1st-order low-pass in the left channel of the amp.
I got a 0.0022u and a 0.0152u ( teflon ) together and took them from the output of the pentode ( C3m ) driver to ground , in front of the coupling cap .
I think ... this gives me an approx 820Hz low-pass when combined with the well-defined anode load of the pentode which is effectively 11.2K when combined with the internal impedance of the pentode. ( I made the crossover a little higher than the 750 high-pass on the horn because the horn roll-off itself provides a steeper cut-off than 1st-order on the horn , and I felt it best to push the bass crossover up a bit to compensate )

Playing this, this is the best yet, with good tonal and dynamic integration between the two speaker units . Very musical . I'm still not sure I can get away with 1st-order high-pass on the horn in the long run , but it's pretty good right now.

Doing the filter internally does not get round the phase shift effects ( which are relatively benign for 1st order ) but it does ensure the amp drives the speaker without any rise in driving impedance towards and above the crossover, which occurs with all passive crossover arrangements between amp and speaker . I've now proven to myself that this is better . Very educational !

The horn is currently reduced by 10dB by the autoformer to match the bass.... it might even be better at -12dB , which I can try .

More soon ...

[IslandPink]

Seems like Bruce Edgar prefers 1st-order slopes :
http://www.positive-feedback.com/Issue4 ... erview.htm
If it's good enough for Bruce, it's good enough for me .
(and for Steve S too ! )

I have tried an extra 1st-order high pass around 230Hz by means of a smaller ( 1.8u ) cathode-by-pass cap on the 300B . I thought it might clean up the horn sound by knocking down more of the low-frequency energy ; but it didn't do much from what i could tell . It also didn't affect the integration from bass to mid very much either, which was good ... but... I may go back to the normal 100u Black Gate on there in a couple of days .

[IslandPink]

I took out the extra high-pass filter at 230Hz . Sounds slightly better without it .
One reason is that a 100u /160 Black Gate NH sounds better in the mids and treble than a 1.8u Mundorf zinc/polyprop .
Shame Black Gates have gone - a major loss to audio .

[IslandPink]

Hmmm...
After my experiences so far with crossover components and autoformers, I'm wondering if I need to get a pair of these chaps :
http://www.bluearan.co.uk/index.php?id=BACDE35
and go tri-amp using a little 71A SE amp or maybe a 6E5P as per Romy to power the tweeters . That way I can hear the 288's properly, and there'll be no doubt that a single 2A3 will power the midrange .

First I need Nick to come over and really sort-out where the top-end of the 288's rolls off . I'm sure it's not 7kHz .

ILPS

[Mike H]

1st order slope is quite shallow actually. Maybe it needs more?

Something else just occurred to me ~ maybe both the drivers are contributing too much at the Xover point; you could A. wire one of them in opposite phase B. move the bass lower down (or the other up) to increase the gap.

"Mind the gap"