I am doing a system that would have dipole bass up to say 250 Hz, ESL mids (89 db panels from E R Audio in Perth Australia) and G ribbon tweeters that I already have. Active bass, maybe passive mid & tweets.
ESLs seem to be hard to drive mostly in the bass and at 20 kHz(?).
I’ve considered:
- Rod Elliott’s MOSFET P101 or maybe P3A,
- a second hand ME (range from 60-150 watts),
- a 30 watt Aleph,
- a 25w Monarchy Audio SM-70 Pro (zero feedback in output stage), and a
- second hand 25 w Pass A-40 that’s just on the market locally
- something else?
For a mid-large room, just the mids - how many watts or amperes?
Is Class A better?
Is SE better?
What would be best?
Thanks
ESLs seem to be hard to drive mostly in the bass and at 20 kHz(?).
I’ve considered:
- Rod Elliott’s MOSFET P101 or maybe P3A,
- a second hand ME (range from 60-150 watts),
- a 30 watt Aleph,
- a 25w Monarchy Audio SM-70 Pro (zero feedback in output stage), and a
- second hand 25 w Pass A-40 that’s just on the market locally
- something else?
For a mid-large room, just the mids - how many watts or amperes?
Is Class A better?
Is SE better?
What would be best?
Thanks
If ampere specs are thin on the ground, are watts a useful indicator at all?
Anyone use an active ESL system?
My crude guess is if you take out the low-ohm treble, and the bass:
you would need less than a third the power – maybe only 25% of the requirement for a passive system??
Eg instead of 100 watts for a ESL passive system, for a ESL midrange - maybe only 25 watts is enough?
Anyone use an active ESL system?
My crude guess is if you take out the low-ohm treble, and the bass:
you would need less than a third the power – maybe only 25% of the requirement for a passive system??
Eg instead of 100 watts for a ESL passive system, for a ESL midrange - maybe only 25 watts is enough?
No inexpensive design will swing kilovolts. But if you're just running the ESL in the midrange, step-up transformer demands are greatly eased, which makes life easier for the amplifier. You can use less core and the ill effects of a high turns ratio are minimized.
If I were buying instead of building (perish the thought!), I might be inclined to look for a used Quad 405 in good condition to run the mids.
If I were buying instead of building (perish the thought!), I might be inclined to look for a used Quad 405 in good condition to run the mids.
Suitable amps
Though I have a set of John Lindsay-Hood amps built (boards) I have not got the supply done yet. From what I read this amp does a stellar job with stats. There are some here on this forum who run these so you can obtain direct comments. There is a specific version of this amp expressly designed for stats. You can find it on the class A web page at this url http://www.tcaas.btinternet.co.uk/index.htm.
There are also a number of direct drive design pages that you can look to for ideas if you check through the long list of ESL related sites that I have posted here for some reference. Hybrids really do seem to make the most sence though. Chip amps with a serious supply should be a very inexpensive way to go for very good quality sound and not a lot of cash out of pocket. Switching amps like the Carver might also do the job but I have no experience with how the Carver would deal with the reactive load but since it is a pro design it should be cool with the prospect of running stats. There are lots of Carver discussion/mod threads so you could look there. A good buddy runs a large 200 watts X 2 ARC amp on his 0ne plus 0nes and it sounds wonderful. Big old DH500 Haflers when modified have also sounded fantastic too and they are cheap. The Dyna 400 is a cool circuit that could stand a massage into the new century of better devices and they are cheap. Lots of options out there. Roger Sanders designed a SS (brute) amp expressly for ESL's also. The thread here on diyaudio on the JLH amp is huge and full of ideas. Carlos fm has also posted much on chip amp supply design and that would be a good reference.
Please keep us posted of you findings when you try different amps.
My guess is that from a dollar value and shear bulk point of view that multiple smaller amps in a hybrid system with good separate subs (and transformer stepup) is the going to prove to be the most cost effective way to go. Best regards Moray James.
Though I have a set of John Lindsay-Hood amps built (boards) I have not got the supply done yet. From what I read this amp does a stellar job with stats. There are some here on this forum who run these so you can obtain direct comments. There is a specific version of this amp expressly designed for stats. You can find it on the class A web page at this url http://www.tcaas.btinternet.co.uk/index.htm.
There are also a number of direct drive design pages that you can look to for ideas if you check through the long list of ESL related sites that I have posted here for some reference. Hybrids really do seem to make the most sence though. Chip amps with a serious supply should be a very inexpensive way to go for very good quality sound and not a lot of cash out of pocket. Switching amps like the Carver might also do the job but I have no experience with how the Carver would deal with the reactive load but since it is a pro design it should be cool with the prospect of running stats. There are lots of Carver discussion/mod threads so you could look there. A good buddy runs a large 200 watts X 2 ARC amp on his 0ne plus 0nes and it sounds wonderful. Big old DH500 Haflers when modified have also sounded fantastic too and they are cheap. The Dyna 400 is a cool circuit that could stand a massage into the new century of better devices and they are cheap. Lots of options out there. Roger Sanders designed a SS (brute) amp expressly for ESL's also. The thread here on diyaudio on the JLH amp is huge and full of ideas. Carlos fm has also posted much on chip amp supply design and that would be a good reference.
Please keep us posted of you findings when you try different amps.
My guess is that from a dollar value and shear bulk point of view that multiple smaller amps in a hybrid system with good separate subs (and transformer stepup) is the going to prove to be the most cost effective way to go. Best regards Moray James.
SY
Thanks for the suggestion - the Quad 405 appeals on cost and built for ESLs.
Though if you look here: www.quadesl.org/Amplifiers/amplifiers.html it apparently doesn’t sound too good. But can be modded . .
* Have you tried either the original or Mark II with ESLs?
Moray
I believe JLHs are good. Though I have so many projects to do, a built amp appeals.
The link http://www.tcaas.btinternet.co.uk/index.htm for the class A isn’t working for me.
I wonder how a chip amp would go but while they’re bang for buck, I’d be sceptical
Class D Hypex UcDs are tested down to 1 ohm, and bass is good but I believe the mids not so.
Thanks for the suggestion - the Quad 405 appeals on cost and built for ESLs.
Though if you look here: www.quadesl.org/Amplifiers/amplifiers.html it apparently doesn’t sound too good. But can be modded . .
* Have you tried either the original or Mark II with ESLs?
Moray
I believe JLHs are good. Though I have so many projects to do, a built amp appeals.
The link http://www.tcaas.btinternet.co.uk/index.htm for the class A isn’t working for me.
I wonder how a chip amp would go but while they’re bang for buck, I’d be sceptical
Class D Hypex UcDs are tested down to 1 ohm, and bass is good but I believe the mids not so.
405 on non Quad stats
Hey Rick: I have run the 405 stock on both Acoustat 0ne plus 0ne's as well as on Two plus Two's. It is a good sounding amp. I have also run a modified 405 on both theese speakers and it sounds better again. Any 405 that you buy today is going to be old enough to require new electrolytics (if you don't want it to blow up) so you might just as well upgrade the whole thing. There are some new upgrades being done these days and I am sure that you would love the amp. It's a fun amp to truck around with to your buddies and blow away all kinds of big looking brute amps with your little grey/green shoe box! Let us know what you try out and how they sound. Regards Moray James.
Hey Rick: I have run the 405 stock on both Acoustat 0ne plus 0ne's as well as on Two plus Two's. It is a good sounding amp. I have also run a modified 405 on both theese speakers and it sounds better again. Any 405 that you buy today is going to be old enough to require new electrolytics (if you don't want it to blow up) so you might just as well upgrade the whole thing. There are some new upgrades being done these days and I am sure that you would love the amp. It's a fun amp to truck around with to your buddies and blow away all kinds of big looking brute amps with your little grey/green shoe box! Let us know what you try out and how they sound. Regards Moray James.
A college buddy of mine had a pair of Quad 57s. He saved up and bought a brand new Audio Research D75A to drive them. The tube amp could crank out more power than the original Quads could bear, but he was careful and the combination sounded wonderful. Then, along came the news of the “Current Dumping” Quad 405. Being a brand loyalist, my friend jumped at the opportunity and bought the new Quad amp unheard. To pay for it he sold me the almost new ARC amp at a discount. After a few days, he realized that he'd made a mistake. Although the 405 was not terrible sounding, it lacked the magic of a tube amp. I happily used that ARC amp for many, many years thereafter; later driving Quad ESL-63s during the late eighties and nineties, until I sold the ARC for more than my friend paid for it originally.
J. Gordon Holt said of the Quad 405 in Stereophile in 1976: “It dulls the extreme highs—the ones that give air to the sound of strings and crispness to the sound of hard percussion….Bass was somewhat sparse and a bit shy of impact, suggesting the possibility of some low-end (subsonic) filtering. Inner detail was fairly well-reproduced, depth was not very well-rendered, and the overall sound had a vaguely astringent quality to it—the antithesis of richness or warmth.”
My point? Good ESLs deserve good tube amps. It’s a match made in heaven, IMHO.
J. Gordon Holt said of the Quad 405 in Stereophile in 1976: “It dulls the extreme highs—the ones that give air to the sound of strings and crispness to the sound of hard percussion….Bass was somewhat sparse and a bit shy of impact, suggesting the possibility of some low-end (subsonic) filtering. Inner detail was fairly well-reproduced, depth was not very well-rendered, and the overall sound had a vaguely astringent quality to it—the antithesis of richness or warmth.”
My point? Good ESLs deserve good tube amps. It’s a match made in heaven, IMHO.
Hi Brian
Interesting story.
The mods that Steve McCormack made to the Quad 405 www.quadesl.org/Amplifiers/amplifiers.html six pages down under “MAC MOD 405” - supposedly “The Mac Mod 405 surpasses the original 405 in every respect : It was a great sounding amp which could compete with the most advanced high end amps in these days. Harry Pearson of the Absolute Sound wrote an enthusiastic review about it. I still regard it a pity that Quad-UK did not overtake the Mac Mod as standard, because the improved 405 is an exceptional amplifier.”
Tubes - an AR D75A would be hard to find, particularly outside the US.
But I could well imagine an appropriate tube amp would excel with ESLs.
So the question is what DIY tube amp qualities or design –
? SE (other than for a Quad 57, probably not enough watts)
? watts, ? amperes what to look for?
According to the quadesl.org article a surprising number of OTLs are listed. Also a
Dynaco ST-70 Lots of upgrade options for this
Velleman K4040! 4 * EL34 100 w stereo amp
Velleman K8010! KT88 65 w monoblocks
Anyone think they would be a good choice? Or . .
Interesting story.
The mods that Steve McCormack made to the Quad 405 www.quadesl.org/Amplifiers/amplifiers.html six pages down under “MAC MOD 405” - supposedly “The Mac Mod 405 surpasses the original 405 in every respect : It was a great sounding amp which could compete with the most advanced high end amps in these days. Harry Pearson of the Absolute Sound wrote an enthusiastic review about it. I still regard it a pity that Quad-UK did not overtake the Mac Mod as standard, because the improved 405 is an exceptional amplifier.”
Tubes - an AR D75A would be hard to find, particularly outside the US.
But I could well imagine an appropriate tube amp would excel with ESLs.
So the question is what DIY tube amp qualities or design –
? SE (other than for a Quad 57, probably not enough watts)
? watts, ? amperes what to look for?
According to the quadesl.org article a surprising number of OTLs are listed. Also a
Dynaco ST-70 Lots of upgrade options for this
Velleman K4040! 4 * EL34 100 w stereo amp
Velleman K8010! KT88 65 w monoblocks
Anyone think they would be a good choice? Or . .
ultralinear for ESLs?
TH Velleman K4040 has been described by Thorsten as:
"circuit is simplicity itself, 12AU7 (use 12AY instead) preamp into the classic Dynaco ST-70 style circuit, using the 12AX7 - one voltage Amp into Concertina Phase-splitter into 4 * fixed bias ultralinear EL34s."
ultralinear - mmm
TH Velleman K4040 has been described by Thorsten as:
"circuit is simplicity itself, 12AU7 (use 12AY instead) preamp into the classic Dynaco ST-70 style circuit, using the 12AX7 - one voltage Amp into Concertina Phase-splitter into 4 * fixed bias ultralinear EL34s."
ultralinear - mmm
rick57 said:
No, I haven't. But since ESL57s are essentially midranges...
Brian- Best match I've ever heard with 57s is Fisher 50A with EL37 outputs. But the 405 did a very nice job; back in the day, all-Quad systems were pretty popular then with non-geek music lovers who didn't want to bother with the upkeep of ARCs. And for good reason. I've listened extensively to 5 or 6 such setups, and in each case, I was struck by how easy it was to stop worrying about the sound and just hear the music.
Long and rambling as usual...
Rick (or do your friends call you Rick57?),
No, I certainly wasn't suggesting trying to find an ARC D75A. I was just telling a story, something I do a lot more of as I get older. My point was that you may want to consider a tube amp of some kind rather than a 405 or variant. There are many better tube amps available today than the D75A, nice as it was in the seventies. The usual complaints about vintage tube gear’s soggy bass and rolled off highs are ameliorated in a mid-range only application. SY’s point about how nice an old Fisher amp sounded with the mid-rangy Quad 57s is spot on.
But to try to answer your original question: How many watts for the mid-range ESL? The correct answer is that it depends, but let me not cop out with that. With disclaimers, I’d say that for midrange only, in a TYPICAL room, with a TYPICAL ESL panel, with TYPICAL step-up transformer, driven to TYPICAL listening levels, 20 to 30 watts per channel (rated into 8 ohms) ought to work fine, about as you thought. You’ll want more power for the bass drivers, of course, but that wasn’t your question. If you choose tubes, EL34s in P-P UL is not a bad starting point. A Dyna Stereo 70 comes right to mind.
I would steer clear of direct-drive amps unless you’re totally nuts like me. Besides being dangerous, it’s also a difficult venture. Depending on your electronics experience and knowledge level, you really have three choices if you go tubular: build from scratch a published DIY tube design (and there are hundreds of designs posted on the web), buy and modify a vintage tube amp, or just buy a new tube amp from the likes of PrimaLuna or Velleman or few others. My suggestion? Option 2; it’s cheap, moderately easy and can be fun. And you can get great sound that way with a proven design. Find a decent old Dyna, Heathkit, Leak, Fisher or Scott amp (you Aussies may have other brands). Get it running, replace old electrolytics, enjoy it - and then from there you can make a career out of modifying and tweaking it per the thousands of tips found here and in other forums.
I don’t know much about the Australian ER Audio ESLs (I live in Melbourne too, but rather the Florida version). If the ERs act as a big shorted capacitor at high frequencies, then, with any amp, you may want to insert a small value power resistor in series between the amp and the ESL. This will help ensure stability by isolating the capacitance of the ESL a bit. The Quad ESL-63 uses a small series resistor to keep the input Z civil at 20KHz. It will roll-off the highs a bit but that shouldn’t be a problem in a mid-range only application. You could even make it part or your crossover design. Values? I don’t know. Start with 0.5 ohm or 1 ohm and work upwards as you listen. BTW, aren’t you tempted to run the ESLs all the way to the top to avoid the crossover, time alignment, and polar differences with a ribbon?
As an aside, much has been said of the apparent synergy of Quad amps and Quad speakers, as if they’d been voiced/tweaked together. As much as I admire the late and great Peter Walker, he would scoff at such suggestions. He wasn’t a tweaker. About the most he did for synergy was to ensure that his amps wouldn’t go unstable into an ESL load, and to ensure that his ESL impedances weren’t too horrendous. But many well designed amps, both SS and tube, are stable also. My point is that you shouldn’t feel duty bound to match ESLs with ESL maker’s amps. InnerSound might be an exception since they build amps that don’t mind the virtual short circuit that their ESLs present at high frequencies, a weak consumer product design IMO. Walker wouldn’t stand for that kind of thing.
Rick (or do your friends call you Rick57?),
No, I certainly wasn't suggesting trying to find an ARC D75A. I was just telling a story, something I do a lot more of as I get older. My point was that you may want to consider a tube amp of some kind rather than a 405 or variant. There are many better tube amps available today than the D75A, nice as it was in the seventies. The usual complaints about vintage tube gear’s soggy bass and rolled off highs are ameliorated in a mid-range only application. SY’s point about how nice an old Fisher amp sounded with the mid-rangy Quad 57s is spot on.
But to try to answer your original question: How many watts for the mid-range ESL? The correct answer is that it depends, but let me not cop out with that. With disclaimers, I’d say that for midrange only, in a TYPICAL room, with a TYPICAL ESL panel, with TYPICAL step-up transformer, driven to TYPICAL listening levels, 20 to 30 watts per channel (rated into 8 ohms) ought to work fine, about as you thought. You’ll want more power for the bass drivers, of course, but that wasn’t your question. If you choose tubes, EL34s in P-P UL is not a bad starting point. A Dyna Stereo 70 comes right to mind.
I would steer clear of direct-drive amps unless you’re totally nuts like me. Besides being dangerous, it’s also a difficult venture. Depending on your electronics experience and knowledge level, you really have three choices if you go tubular: build from scratch a published DIY tube design (and there are hundreds of designs posted on the web), buy and modify a vintage tube amp, or just buy a new tube amp from the likes of PrimaLuna or Velleman or few others. My suggestion? Option 2; it’s cheap, moderately easy and can be fun. And you can get great sound that way with a proven design. Find a decent old Dyna, Heathkit, Leak, Fisher or Scott amp (you Aussies may have other brands). Get it running, replace old electrolytics, enjoy it - and then from there you can make a career out of modifying and tweaking it per the thousands of tips found here and in other forums.
I don’t know much about the Australian ER Audio ESLs (I live in Melbourne too, but rather the Florida version). If the ERs act as a big shorted capacitor at high frequencies, then, with any amp, you may want to insert a small value power resistor in series between the amp and the ESL. This will help ensure stability by isolating the capacitance of the ESL a bit. The Quad ESL-63 uses a small series resistor to keep the input Z civil at 20KHz. It will roll-off the highs a bit but that shouldn’t be a problem in a mid-range only application. You could even make it part or your crossover design. Values? I don’t know. Start with 0.5 ohm or 1 ohm and work upwards as you listen. BTW, aren’t you tempted to run the ESLs all the way to the top to avoid the crossover, time alignment, and polar differences with a ribbon?
As an aside, much has been said of the apparent synergy of Quad amps and Quad speakers, as if they’d been voiced/tweaked together. As much as I admire the late and great Peter Walker, he would scoff at such suggestions. He wasn’t a tweaker. About the most he did for synergy was to ensure that his amps wouldn’t go unstable into an ESL load, and to ensure that his ESL impedances weren’t too horrendous. But many well designed amps, both SS and tube, are stable also. My point is that you shouldn’t feel duty bound to match ESLs with ESL maker’s amps. InnerSound might be an exception since they build amps that don’t mind the virtual short circuit that their ESLs present at high frequencies, a weak consumer product design IMO. Walker wouldn’t stand for that kind of thing.
Brian
Thanks again for your comprehensive answer.
I’m glad that someone with more knowledge and experience than me came up with a similar (appropriately qualified answer) re power needs!
> EL34s in P-P UL is not a bad starting point
I’m leaning towards P-P KT88s, to avoid or minimise the feedback in UL.
I'l leave direct-drive till I’m more advanced. I hadn’t thought of buy & modify a vintage tube amp before.
I’ll bear in mind your described use of resistors.
And the synergy current driving ability of Quad amps is decreasing, also after talking to a LT Quad user.
diyparadiso KT88 may be the best option at present
Cheers
Richard
Thanks again for your comprehensive answer.
I’m glad that someone with more knowledge and experience than me came up with a similar (appropriately qualified answer) re power needs!
> EL34s in P-P UL is not a bad starting point
I’m leaning towards P-P KT88s, to avoid or minimise the feedback in UL.
I'l leave direct-drive till I’m more advanced. I hadn’t thought of buy & modify a vintage tube amp before.
I’ll bear in mind your described use of resistors.
And the synergy current driving ability of Quad amps is decreasing, also after talking to a LT Quad user.
diyparadiso KT88 may be the best option at present
Cheers
Richard
KT-88s will do fine, but I wouldn't allow a stigma about feedback to color your view on the UL mode. After all, 100% screen feedback is called triode mode! UL is part-way between pentode and triode. The problem with pure pentode operation is that the output Z is so high that designers had to resort to heavy loop feedback to lower output Z and distortion. That is worse, IMO, than the local feedback from a UL screen tap. But you have another option with KT-88s. Run them as triodes. You‘ll get less power, but still enough for this application, and then you’ll get that nice triode sound and low output Z.
Driving ESL
Hi Brian
I've been reading your posting over and over because I found it very
interesting even though it speaks a lot about the Quads which I
don't own. I've heard them in the past especially the 57's and I
always found them absolutely breathtaking and as I never could
afford them (even when I was in Europe) about three years ago
I decided to built a pair of ESL from scratch.
Of course I didn't know didly about this art (today I consider this
an art) and I had to get the help of Rob Mckinly (from ER Audio)
who graciously helped me a lot. He also sold me the critical materials.
Today I'm in the process of building a second pair due to the
success of the first ones which I enjoy almost everyday.
And I have a question for which I ask your indulgence, you mentioned
a resistor in series with the...(what?), a resistor of 0.1 ohms to 1
ohms. Is this resistor really important to match the panels inductancewise
at high frequencies? Is it in series with the stators or the
membranes?
I have my panels segmented and wired in pararell of course
and besides the 20 Mohms resistors in series with the stators
I have no crossovers whatsoever, they run freely directly to
the valve amplifier that drives them. By the way the amp is A
modified Dynaco (sometimes) and another chinese valve job
with KT100 at the outputs which sounds really prime. Besides
these two valve amps sometimes I use a home brewed JLH
class A which drives the ESL with no effort or constraints.
I'm curious about that small value resistor you mentioned, I
would like to ask you to elaborate, please.
Cheers
John
Hi Brian
I've been reading your posting over and over because I found it very
interesting even though it speaks a lot about the Quads which I
don't own. I've heard them in the past especially the 57's and I
always found them absolutely breathtaking and as I never could
afford them (even when I was in Europe) about three years ago
I decided to built a pair of ESL from scratch.
Of course I didn't know didly about this art (today I consider this
an art) and I had to get the help of Rob Mckinly (from ER Audio)
who graciously helped me a lot. He also sold me the critical materials.
Today I'm in the process of building a second pair due to the
success of the first ones which I enjoy almost everyday.
And I have a question for which I ask your indulgence, you mentioned
a resistor in series with the...(what?), a resistor of 0.1 ohms to 1
ohms. Is this resistor really important to match the panels inductancewise
at high frequencies? Is it in series with the stators or the
membranes?
I have my panels segmented and wired in pararell of course
and besides the 20 Mohms resistors in series with the stators
I have no crossovers whatsoever, they run freely directly to
the valve amplifier that drives them. By the way the amp is A
modified Dynaco (sometimes) and another chinese valve job
with KT100 at the outputs which sounds really prime. Besides
these two valve amps sometimes I use a home brewed JLH
class A which drives the ESL with no effort or constraints.
I'm curious about that small value resistor you mentioned, I
would like to ask you to elaborate, please.
Cheers
John
John,
The working assumption with most ESLs is that they present a large capacitive load to the amp – even “backwards” through the step-up transformer. Depending on how low the impedance drops at the high end of the audio band (or just above the band), and the phase angle of the load (how reactive versus resistive it is), the driving amplifier can oscillate or ring on transients. Some amps are happier with this than others. It’s especially rough on feedback amps since this load can create additional phase shift in the feedback loop reducing phase margins. Another problem is that the step-up transformer’s leakage inductance will resonate with the ESL’s capacitance near the top of the band, which really drops the impedance seen by the amp. Above that resonance, the impedance climbs again and is inductive. Isolating the amp a little from the heavy capacitive load and/or the resonant dip, by using a small series resistor (between amp and speaker, not after the step-up transformer) might help. Cable resistance helps too, by the way. The exact value can only be determined by experimentation since there are so many variables, but if it’s made too big, there can be a roll-off in the top octave. Quad used a 1.65 ohm resistor in each transformer primary (two transformers were used with primaries in parallel and secondaries in series) in the ESL-63. This is equivalent to a 0.825 ohm series resistor. This would raise the minimum impedance a bit, but perhaps more importantly would keep the phase angle from being too severe. Sometimes all it takes is a little change to keep things happy. For a midrange application, one could make the resistor a couple of ohms without worrying about in-band roll-off. That should be more than enough to make any amp happy.
I wasn’t citing this as mandatory tweak, but rather as way to keep a quasi-stable amp from going nuts driving a large panel. It’s an easy experiment to try using ears (with music) and square waves (with a scope). If you have no problems, there is no need to bother with it.
The working assumption with most ESLs is that they present a large capacitive load to the amp – even “backwards” through the step-up transformer. Depending on how low the impedance drops at the high end of the audio band (or just above the band), and the phase angle of the load (how reactive versus resistive it is), the driving amplifier can oscillate or ring on transients. Some amps are happier with this than others. It’s especially rough on feedback amps since this load can create additional phase shift in the feedback loop reducing phase margins. Another problem is that the step-up transformer’s leakage inductance will resonate with the ESL’s capacitance near the top of the band, which really drops the impedance seen by the amp. Above that resonance, the impedance climbs again and is inductive. Isolating the amp a little from the heavy capacitive load and/or the resonant dip, by using a small series resistor (between amp and speaker, not after the step-up transformer) might help. Cable resistance helps too, by the way. The exact value can only be determined by experimentation since there are so many variables, but if it’s made too big, there can be a roll-off in the top octave. Quad used a 1.65 ohm resistor in each transformer primary (two transformers were used with primaries in parallel and secondaries in series) in the ESL-63. This is equivalent to a 0.825 ohm series resistor. This would raise the minimum impedance a bit, but perhaps more importantly would keep the phase angle from being too severe. Sometimes all it takes is a little change to keep things happy. For a midrange application, one could make the resistor a couple of ohms without worrying about in-band roll-off. That should be more than enough to make any amp happy.
I wasn’t citing this as mandatory tweak, but rather as way to keep a quasi-stable amp from going nuts driving a large panel. It’s an easy experiment to try using ears (with music) and square waves (with a scope). If you have no problems, there is no need to bother with it.
Brian
> I wouldn't allow a stigma about feedback to color your view on the UL mode. After all, 100% screen feedback is called triode mode! UL is part-way between pentode and triode.
Thanks ~ I’ve just read of others that concur
> But you have another option with KT-88s. Run them as triodes. You‘ll get less power, but still enough for this application, and then you’ll get that nice triode sound and low output Z.
The amp that I’m leaning to (like a few others) has a switch between the two – 25 watts triode or 40 watts UL
Cheers
> I wouldn't allow a stigma about feedback to color your view on the UL mode. After all, 100% screen feedback is called triode mode! UL is part-way between pentode and triode.
Thanks ~ I’ve just read of others that concur
> But you have another option with KT-88s. Run them as triodes. You‘ll get less power, but still enough for this application, and then you’ll get that nice triode sound and low output Z.
The amp that I’m leaning to (like a few others) has a switch between the two – 25 watts triode or 40 watts UL
Cheers
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