[hebrer]

I have been using single-driver Fostex FE127eN speakers (91dB sensitivity in a Metronome enclosure) with quite satisfying (but not top-notch) results with a EL-84 push-pull tube amp.

Recently, I switched to a SET amplifier (Decware Zen design, DIY project) and the speakers are SLOOOW by comparison (at least that is the description that comes to my mind to describe how it feels).

I can think of two possible reasons:
1) Relatively low 91dB sensitivity of the speakers when coupled with 2x2W SET amp
2) Lower quality of SET amp output transformers (bought from ebay, made in Germany in 1960's)

Which do you think might be having a greater impact on the sound?

The ZEN amp has a reputation of being very FAST and I am unable to tell if it is better to replace the output transformers (to something like Transcendar SET transformers) or to get higher efficiency speakers (perhaps 10" Audio Nirvana)...

Thanks for your thoughts...
Peter

[avahifi]

In general, "slow" can commonly be a reaction to an underdamped low frequency response often caused by an inappropriate combination of amplifier and speaker.

It can also describe muted transient response broad band.

This is pretty easy to visualize with a square wave generator and scope.

Sometimes using the right test equipment can be the least expensive way to resolve listening issues.

[planet10]

Got pics of the diy ZEN?

dave

[doorman]

FE126's (eNA) in "Frugel Horn" cabs, driven by a factory Zen Select, were indeed lightning fast.
Hope you sort it out---
Don

[Mr Content]

Hi Peter, sounds if you have a problem somewhere there. Do you have any friends with higher sens. speakers that you could try your amp with? Might be the easy way to sort out if it is indeed your amp. The OPTs Steve put in his factory Zens are very cheap, maybe you could replace them with factory ones.

Mr C

[hebrer]

Here is a picture of the amplifier.
I may try to get hold of someone with high sensitivity speakers to try to locate the problem.



I have now inserted braces just behind the speaker drivers and added some more polyfill to the back walls of the speakers and the sound improved noticeably. Interesting...

[45triode]

hi peter,

i use a 2 watt 45 amp on 100db zu druids. this works fine, but does not go to ear deafening levels. 91db (although with a tube friendly impedance curve) might just be too low. i would rather suggest something around 25 watts. your el84 should sound better with the fostex, no?

[hebrer]

Hi 45triode,

Actually, the EL84 amp is more powerful and in some ways better. Still, I like the Zen a bit more because it makes the performers more "alive" and more "in the room". This could be the SET characteristic that many love so much (2nd harmonics etc.).

If I cannot make Zen work fine with 91dB Fostexes, I am going to replace speakers and not the amp. I just imagine with something like Zu Druids, the Zen must be truly great.

[DaveC113]

2W isn't enough for 91 dB speakers, IMO. I'd want ~100 dB eff. speakers.

[doorman]

Perhaps, but it still doesn't address the "slow" question.
Don

[DaveC113]

Perhaps, but it still doesn't address the "slow" question.
Don

I'm not so sure about that.... it could be the speakers.

[hebrer]

The more research I do on the web, the more I tend to think of changing the speakers: for a single-driver design driven by 2x2W SET amp, efficiency of 91 dB is probably too low (even if I hardly ever listen at high volumes). Besides, with a 4.5" driver, the bass is limited.

(quote from an interview with Bob Brines at http://handbuiltaudio.com/brines/index.html :
What do you optimize for?

- First and foremost, if a single driver speakers is to be taken seriously, it must have an in-room F3 (bass cutoff frequency) of no higher than 40Hz. This is required to cover the full range of the double bass, and one of the reasons I don't have any speakers in my catalog smaller that a 6" driver. Presuming the design passes this test, I optimize for the best overall balance.)

Maybe the driver magnet simply cannot move the cone fast enough when driven by a low-power SET amp.

I have finished the speakers only eight months ago and they were a clear step-up from commercial JM Lab Chorus 706. Discovering that maybe I should have made a better choice of Fostex does not make me all that happy.

[planet10]

FE127eN mets are going to be a challenge to drive with the amp you have... it is probably closer to 1.5 w and those OPTs, which i have had thru here, are more budget ones out of a German console. Hard to judge without some side by side, but i'd estimate they are half the mass of real ZEN Opts.

When we started on the path to working out an EL84 SE amp, we duplicated the Zen as close as possible (and started with more upscale OPTs). After 5 iterations we had a fairly standard RH-84 with 2x the power and better sound. A couple more iterations and we had monoblok variations on the RH84 with inspiration from both Eddie Vaughn and Doc Bottlehead. they are in a whole 'nother league from the Zen clones (and the reak Zens we also had). Those amps and a set of Fonken with the same driver really sing.

Your amp is no Zen, but you are getting at least a little bit of that midrange magic that makes an SE amp special. Before you do anything else you need to audition your amplifier on other speakers and see if it opens up, if it doesn't on any of them the amp is probably the limit. There is nothing lacking in the FE127eN Mets, you just have a case of bad synergy. I would also heartily recommend rewiring your amp into an RH84, it is much more capable of extracting the most out of a cheap set of output transformers.

dave

[planet10]

Maybe the driver magnet simply cannot move the cone fast enough when driven by a low-power SET amp.

More likely, with the OPTs you have in the amplifier, that the amplifier has an F3 of over 50 Hz, maybe way higher. The mono-bloks mentioned in my last post reach about 50, the consoles that the smaller EL84/EL95/ECL82 OPTs came out of were challenged to reach 100 Hz. They are also HF limited.

The other thing is that these typically have 4 or 4.5 ohm taps if they only have 1 set, and are typically 10k so that they could be used with both EL84 and EL95.

A tech could measure the power bandwidth of your amplifier. Or you can try a series of other speakers.

Another amplifier topology that works to get the most out of small OPTs is the STC from Japan. Jeff at Abraxas audio has done some good work with this.

dave

[hebrer]

Thanks Dave for this information.

As I mentioned in my original post and as you suggested also, the cheap OPTs may not be up to the task.
Size-wise, they are probably no smaller than the factory Decware Zen OPTs (the enclosure in the picture is 43 cm long), but it is very possible that they do not reach down to 50 Hz in the frequency range. Neither do Fostex FE127EN so actually both the OPTs and the drivers might be weak points in getting lower bass.

I do not have the option to go through five iterations of the amp to address this situation of poor synergy. The amp was built by a friend and using Transcendar OPTs seems better road for me than rewiring to RH84 (not that I woud be able to tell what it takes at this point). And, as I wrote above, additional damping and bracing the Metronomes did help noticeably so the synergy is becoming better.

Thanks again for sharing your experience with German OPTs. And as for Fostex FE127EN, in my Metronomes they sound very nice with 2x7W push-pull EL84 amp, even if this combination lacks SE "presence" magic.