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Old 12th April 2012, 10:42 PM   #1
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Default Amplifier for Driving Altec 414z's

or some time i have been using some altec 414z's in an Onken arrangement driving them with a pair of Mullard 5/20 monoblocks. I have been wanting for some time to get the bass response tighter by changing to SS amps to improve the damping factor.

I have tried a number of different SS amps from Rotel monoblocks through to recently an audiolab 8000p. All of the SS amps don't drive the Altec's very well at all (flat lifeless response). For example with the audiolab i need to attenuate the other amps in the system by around 30db to get the levels comparable with the Mullard 5/20. ( Even my 7 watt 6B4G's drive them better)

( I have modified the cabinet volume to account for the lower resistance ( running active - so no crossover resistance)

I know that i will get some reduction as the drivers are 16 ohm, so feeding them from a 8 ohm SS output will lose 50% of the power, but i cannot understand why its this bad. Can anyone explain to me whats happening. My initial thoughts were that the Alnico magnets needed recharging - but then logically i should have a problem with the valve amps also.

If i wish to continue using these drivers with SS what type of amp do i need, is it a number of amps bridged to increase the voltage or what?
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Old 13th April 2012, 12:26 AM   #2
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Try a 3-5 ohm resistor in the speaker line, thus destroying the improved damping factor of the transistor amps.

Some of the Heathkit Altecs recommended this series resistor in the manual when using solid state. i forget which ones...it has been a while.

Give it a shot and see what happens.
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Old 14th April 2012, 07:35 AM   #3
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Thanks for the info.

I tried it just ended up with less gain. Maybe the heathkit advise was to keep the same damping factor with speakers when changing amps from Valve to SS
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Old 14th April 2012, 03:20 PM   #4
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Yes, the idea was to provide a higher ohm (looser) damping factor. I mentioned it to address the "dull and lifeless" complaint. 3-5 ohm resistor is a strong experiment which would really loosen things up. Heathkit may have recommended 2 ohms...don't really remember the exact number but it was enough to whack the DF.

I think the reason you are not getting more responses is that this doesn't sound right. It is baffling, so I can understand why you are confused!

Lots of people use SS amps on Altec woofs. I tried it and liked PP feedback amps more but the SS worked fine.

Input sensitivity is probably different with the Mullard but 30dB is a huge difference.

I believe that the Onken is underdamped a bit. Try using the 5-20 and blocking off one or more of the slots with foam rubber. I messed with this approach on a 416/large Onken and it did tighten things up.

Are you driving the SS amps with a source that can drive them properly? The input Z on some of the old Mullard implementations could be as high as 1 meg! I'm guessing the SS average is 10k.

What amp/transducer is on the top? I'd expect a sensitivity gulf between a horn and a 414...but not 30dB!
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Old 14th April 2012, 11:19 PM   #5
Pano is offline Pano  United States
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I agree with Joe that some series resistance tends to help when using speakers voiced for valve amps driven by S.S. It helps, but never sounds quite right with most amps. There is more going on than just damping factor. The way the transformer interacts with the drivers has to have something to do with it.

I don't "know", but I suspect that low distortion drivers like the Altecs tend to sound better with an output transformer and its gentle distortion and that modern, higher distortion drivers sound better with tight, dry Solid State amps. How the music was recorded, mixed and mastered make a difference, too.
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Old 15th April 2012, 12:14 AM   #6
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I'm running Iconic 165-8G (Altec 515-8G) in homebrew Onken (Jensen Ultraflex designed with help of the ubiquitous Onken spreadsheet by Marc Cyre Debien) cabinets and the qts of the woofer is absolutely critical in getting the box tuning correct. Changing the amplifier damping factor changes the qts significantly and you are probably now seriously over damped. I designed mine for a total source impedance of 3 ohms including the dcr of the choke in the X-O, and have found they work great with most of my tube amps, and not so well with any solid state amp I've tried.

You might be better off improving the performance of the 5-20 amplifier, improvements in power supply, and LF performance of the OPT might help. A fixed bias design based on the 5-20 will probably perform better in the bass based on my experience.

Not sure how you are compensating for the change in amplifier damping factor playing with the box, my recollection is that with a given driver and desired cutoff the lower the 're' the bigger the box becomes.
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Old 15th April 2012, 01:26 AM   #7
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Whoa...didn't even know about the ubiquitous Onken spreadsheet!

Things have sure come along in Onken land. See what happens when ya don't pay attention...

All I can say is "Don't mess with success!" Out of everything I tried when biamping my Onken/288/1505 system, I ended up with a Dyna ST-35 on the woofs. Can't explain that one.
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Old 29th April 2012, 08:04 PM   #8
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Sorry for not replying sooner. I have been driving the Onkens recently with a Behringer A500 amp, with a lot more success than any of the other SS amps i have tried. The 30 db difference i mentioned is still there, but thats probably to do with the sensitivity of the amps. I am driving the SS amp from a WOT preamp with 600 ohm output so should be fine for driving the 10K input of the SS amp.

The bass extension is alot better but i have a 70Hz peak (7 db) which is clouding the listenining experience. I am not sure if the "thick bass" sound is suited to my room.

Looking at alternative speaker designs currently. Has anyone experience of the blue thunder (VOTT style) speaker, mentioned in Sound Practices, as i fancy making a downscaled version for the $14'z.
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Old 30th April 2012, 11:03 AM   #9
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I heard the Blue Thunder and i will be with Christian who built it this week in Munich!

The BT was built for TAD 15". You will need to tune the port for 414s. Extra box volume is good for Altecs.

I say you might need to tune your Onkens. You have the box already, so why start over. You can tune the internal volume by adding phone books. Block off ports.

You can even block off all ports and run it sealed box. That should work real well.

I'd fix the box you have.
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Old 1st May 2012, 06:45 PM   #10
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I still cannot work out how to tune the onkens properly which is probably the basis of the problem. I have been tuning them like BR units, measuring impedance v frequency - minimizing the peaks. The problem is when you add the series resistance required to measure impedance, it changes the Qts, so you are not tuning it with the same resistance it will see when connected to the amp only. The only other way i know of tunning is by ear, which for LF will be very subjective.
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