If I may, I'd like to start a discussion on Rozenblit's tube amps. I'm leaving open ended to talk about any of his projects in Audio Reality. I've heard other express good thoughts about his amps and most people hold them in high regard. I'd like to hear what the diyaudio community thinks and possibly use this thread as a forum for creative thinking. Any ways to tweak Bruce's amps?
Better power supply, regulation, higher performance tubes, etc.
Eternalightwithin
P.S. As of right now, I'm modeling Bruce's 80W OTL amp in bSPICE. Grr. Took me a whole day and I'm having trouble finishing it.
I did learn how to import the SPICE data for a el509 tube, thanks guys. But... still can't seem to connect pins for grid and screen 
I think it's a bad thing when your simulation say 10 Resistors, and 5+ capacitors are floating
Better power supply, regulation, higher performance tubes, etc.
Eternalightwithin
P.S. As of right now, I'm modeling Bruce's 80W OTL amp in bSPICE. Grr. Took me a whole day and I'm having trouble finishing it.
I did learn how to import the SPICE data for a el509 tube, thanks guys. But... still can't seem to connect pins for grid and screen I think it's a bad thing when your simulation say 10 Resistors, and 5+ capacitors are floating
RE:Rozenblit amps.
Hi,
In the nineties mr.Rosenblit contributed to the US audio magazine Glass Audio.
One of the articles I'm particularly familiar with is an OTL amp he described and which I build with twice the output tubes and as monoblocks.
I have no idea wether the schematic is published in his Audio Reality book but i can say that it was well designed and is still in service to this day at a friend of mines' place.
While I looked at the design of the 80W amp, I noticed he ommitted the regulator for the input and driver stages.
It also uses the 12AU7A which I personally would replace in favour of the much better sounding 6SN7.
As the thread evolves I'll comment further if you like.
Cheers,
Hi,
In the nineties mr.Rosenblit contributed to the US audio magazine Glass Audio.
One of the articles I'm particularly familiar with is an OTL amp he described and which I build with twice the output tubes and as monoblocks.
I have no idea wether the schematic is published in his Audio Reality book but i can say that it was well designed and is still in service to this day at a friend of mines' place.
While I looked at the design of the 80W amp, I noticed he ommitted the regulator for the input and driver stages.
It also uses the 12AU7A which I personally would replace in favour of the much better sounding 6SN7.
As the thread evolves I'll comment further if you like.
Cheers,
Bruce Rozenblit amps
Yes, the 80W OTL schematic is published in Audio Reality, but I haven't attempted one yet. Looks to be a formidable project. In my case, with my bi-amped system, I would need FOUR monoblocks (or two stereo amps) to run my system. In that case, I would definitely be concerned about sourcing that many output tubes, and the cost.
The one thing of Bruce's that I have built (from scratch, using the schematic in Audio Reality) is the Grounded Grid preamp, and I can attest that this is one fine unit. If you want to see the results, check out my home page below:
http://mywebpages.comcast.net/cbrodersen/index.html
Yes, the 80W OTL schematic is published in Audio Reality, but I haven't attempted one yet. Looks to be a formidable project. In my case, with my bi-amped system, I would need FOUR monoblocks (or two stereo amps) to run my system. In that case, I would definitely be concerned about sourcing that many output tubes, and the cost.
The one thing of Bruce's that I have built (from scratch, using the schematic in Audio Reality) is the Grounded Grid preamp, and I can attest that this is one fine unit. If you want to see the results, check out my home page below:
http://mywebpages.comcast.net/cbrodersen/index.html
Wow, look at all the pretty colors
The sight of all those filaments glowing. Hehe. Wow that's 16 tubes for the output. I'll bet it's does a job on your wallet too.
$24 a pop. OTL must be something special to require that expense. I don't know how much transformers cost but they can't be more than $24 * 16 = errr David gets out his calculator... $384 Feel free to prove I'm clueless.
So I guess that would make around 160W ?
Yes, I noticed that he had jsut a basic PS. Now in his book he states that the amp really doesn't need regulation. But on his website (www.transcendentsound.com) he has an option for a low noise upgrade which in Bruce's words, "second choke/capacitor filter section, the signal to noise ratio can be improved to over 95 dBA"
Also, since the 90's Bruce has replaced the el509 with the 6C19PI tube. that's what the kit is for. Beats me as to why there is a difference. DOn't know much about the 6C19PI.
Lol, or any tube for that matter.
In wrapping up this rambling post I must ask, is 80W really necessary? Maybe it is, for a tweeter no, but the fullrange speaker will have to reprouduce down to ~80 Hz. I guess that is where all the power gets sunk into.
I hear that most amps, particularly OTL like reallyresistive load speakers and not reactive. Someone want to comment on this as well?
Eternalightwith
I live for lively discussion
The sight of all those filaments glowing. Hehe. Wow that's 16 tubes for the output. I'll bet it's does a job on your wallet too.
$24 a pop. OTL must be something special to require that expense. I don't know how much transformers cost but they can't be more than $24 * 16 = errr David gets out his calculator...
$384 Feel free to prove I'm clueless. So I guess that would make around 160W ?
Yes, I noticed that he had jsut a basic PS. Now in his book he states that the amp really doesn't need regulation. But on his website (www.transcendentsound.com) he has an option for a low noise upgrade which in Bruce's words, "second choke/capacitor filter section, the signal to noise ratio can be improved to over 95 dBA"
Also, since the 90's Bruce has replaced the el509 with the 6C19PI tube. that's what the kit is for. Beats me as to why there is a difference. DOn't know much about the 6C19PI.
Lol, or any tube for that matter.
In wrapping up this rambling post I must ask, is 80W really necessary? Maybe it is, for a tweeter no, but the fullrange speaker will have to reprouduce down to ~80 Hz. I guess that is where all the power gets sunk into.
I hear that most amps, particularly OTL like reallyresistive load speakers and not reactive. Someone want to comment on this as well?
Eternalightwith
I live for lively discussion
Ground grid pre
cbrodersen,
Yes it does look nice, and simple too.
I'd like to make it into buffer preamp. That's the same as a line level, right? ie. no gain.
I don't know if it's possible to convert it though. I've just read through the book for the first time.
Currently reading Sloane and Self.
Eternalightwith
P.S. Nice webpage you have there.
Don't mind if I help myself to some of those interesting looking documents you have there.
cbrodersen,
Yes it does look nice, and simple too.
I'd like to make it into buffer preamp. That's the same as a line level, right? ie. no gain.
I don't know if it's possible to convert it though. I've just read through the book for the first time.
Currently reading Sloane and Self.
Eternalightwith
P.S. Nice webpage you have there.
Don't mind if I help myself to some of those interesting looking documents you have there.
THE PRICE FOR SONIC BLISS.
Hi,
Actually it's not all that bad, the grand total must have been something in the order of 2.500$ for both channels.
The most expensive part are the PTs which is a custom design at 800VA per channel.
Thing is, these amps beat all high-end stuff I heard so far provided ou use a reasonably resistive load.
ELSs are fine too but forget about low efficiency full-rang ribbons.
Nah, not for a 6AS7G/6080. I usually buy a 1.000 of them and sell whatever I don't need.
Nowadays these tubes can be had NOS in mint condition for about 6.00 to 7.00$/ea.
Both channels combined yes, your electricity consumption meter is going to give you different readings though.
Those are two totally different tubes, the EL509, once put in triode mode is a very powerful beast that can deliver lots of current...for a tube that is. i don't know why the idea was abandoned...someone commercial must have pushed the panic button.
The 6C19-Pi is kind of half a 6AS7G.
I never even saw one but but I have a myriad of ideas for where and how to put them to good use.
Look at it this way, you're going to need a bunch of tubes in // in order to lower the Zo of the amp.
You inevitably end up burning power.
The most economic way is to use a tube such as the very low Ri 6C33...I came across some other ex USSR references that may turn out to be excellent candidates for lower power reqs. but I'll have check that...
Good thinking, you don't need any gain from a pre if your main source is a CDP.
All you need is a buffered volumecontrol.
Jam started a thread on that a while ago in this section just to tease me (I think) and I'm waiting for the guy from the computershop to return from holidays- he got other excuses as well- so I can post the schematics.
It's not an "El Cheapo" project if built as intended but it's top of the pops.
Dual mono fully tube regged PSU, you name it.
Eh,eh...I already played pirate.
FYI, my own OTLs use 10 6080s per channel..hope you're impressed.
Cheers,
Hi,
Actually it's not all that bad, the grand total must have been something in the order of 2.500$ for both channels.
The most expensive part are the PTs which is a custom design at 800VA per channel.
Thing is, these amps beat all high-end stuff I heard so far provided ou use a reasonably resistive load.
ELSs are fine too but forget about low efficiency full-rang ribbons.
Nah, not for a 6AS7G/6080. I usually buy a 1.000 of them and sell whatever I don't need.
Nowadays these tubes can be had NOS in mint condition for about 6.00 to 7.00$/ea.
Both channels combined yes, your electricity consumption meter is going to give you different readings though.
Those are two totally different tubes, the EL509, once put in triode mode is a very powerful beast that can deliver lots of current...for a tube that is. i don't know why the idea was abandoned...someone commercial must have pushed the panic button.
The 6C19-Pi is kind of half a 6AS7G.
I never even saw one but but I have a myriad of ideas for where and how to put them to good use.
Look at it this way, you're going to need a bunch of tubes in // in order to lower the Zo of the amp.
You inevitably end up burning power.
The most economic way is to use a tube such as the very low Ri 6C33...I came across some other ex USSR references that may turn out to be excellent candidates for lower power reqs. but I'll have check that...
Good thinking, you don't need any gain from a pre if your main source is a CDP.
All you need is a buffered volumecontrol.
Jam started a thread on that a while ago in this section just to tease me (I think) and I'm waiting for the guy from the computershop to return from holidays- he got other excuses as well- so I can post the schematics.
It's not an "El Cheapo" project if built as intended but it's top of the pops.
Dual mono fully tube regged PSU, you name it.
Eh,eh...I already played pirate.
FYI, my own OTLs use 10 6080s per channel..hope you're impressed.
Cheers,
Actually....
I was referring to Antique Electronic Supply 's EL509. they have them there as 24 buckaroos.
Damn, 1000's of them. I'm just a lowly grad student. Won't have any moeny if I'm buying 1000 tubes, even if only 7 bucks
So is it the fact that the tubes are hooked in parallel lower output resistance or just the fact that you have more gain to burn as feedback?
Isn't the Rozenblit OTL hooked in series??
"Both channels combined yes, your electricity consumption meter is going to give you different readings though."
LOL< must be those darn heater supplies.
The 6C19-Pi is kind of half a 6AS7G.
Is the 6AS7G anything like the 6KG6 ? Just wondering cause the only difference between the 6KG6 and the EL60 is that the Cg equals 37p for the former and 25p for the latter. Whatever that means.
I]David takes a wild guess...[/I] Tube Capacitance?[
WOuld be possible to build the 80W version with a lot of empty tube sockets and then just add tubes in parallel when the moeny comes in? I guess what I'm asking is would it turn into a flaming ball of death and destruction.
The most economic way is to use a tube such as the very low Ri 6C33
So what you're saying is that I can put a lot of these in // and get the same result? Low Ri and high current tubes, not too many that do both....
Eternalightwithin
I was referring to Antique Electronic Supply 's EL509. they have them there as 24 buckaroos.
Damn, 1000's of them. I'm just a lowly grad student. Won't have any moeny if I'm buying 1000 tubes, even if only 7 bucks
So is it the fact that the tubes are hooked in parallel lower output resistance or just the fact that you have more gain to burn as feedback?
Isn't the Rozenblit OTL hooked in series??
"Both channels combined yes, your electricity consumption meter is going to give you different readings though."
LOL< must be those darn heater supplies.
The 6C19-Pi is kind of half a 6AS7G.
Is the 6AS7G anything like the 6KG6 ? Just wondering cause the only difference between the 6KG6 and the EL60 is that the Cg equals 37p for the former and 25p for the latter. Whatever that means.
I]David takes a wild guess...[/I] Tube Capacitance?[
WOuld be possible to build the 80W version with a lot of empty tube sockets and then just add tubes in parallel when the moeny comes in? I guess what I'm asking is would it turn into a flaming ball of death and destruction.
The most economic way is to use a tube such as the very low Ri 6C33
So what you're saying is that I can put a lot of these in // and get the same result? Low Ri and high current tubes, not too many that do both....
Eternalightwithin
NASDAQ PRICES.
Hi,
Too bad George Fathauer isn't running that company anymore then...
Surely EL509s can be had for far less than 24$ a pop.
The gain is virtually none or less than unity, what you need to do is reduce Zo, i.e impedance as seen by the LS, therefore you need a lot of tubes since these are high impedance devices by nature.
To reduce it further overall NFB is used: a simplified calculation example for ten 6080s prior to NFB is applied: 280R (plate resistance of one triode) /2 for a // tube / 10 for 10 tubes = 14R add say 20dB of NFB and the net result is 0.14R.
In real life it's a bit more complicated but the idea is to give you some idea about why OTL and low output power don't really go hand in hand.
Nope, //.
Nope again.The 6ASG is a dual triode meant for regulator service whereas the EL509 was designed as a TV sweep tube (horizontal deflection).
Hmmm...Haven't really checked that but Cg just indicates the interelectrode capacitance between grid and anode...nothing of importance to us here.
Yes, this tube is no bed of roses either though...no free lunch.
I hope I haven't discouraged you too much, better not to spin any wheels I reckoned.
Hi,
Too bad George Fathauer isn't running that company anymore then...
Surely EL509s can be had for far less than 24$ a pop.
The gain is virtually none or less than unity, what you need to do is reduce Zo, i.e impedance as seen by the LS, therefore you need a lot of tubes since these are high impedance devices by nature.
To reduce it further overall NFB is used: a simplified calculation example for ten 6080s prior to NFB is applied: 280R (plate resistance of one triode) /2 for a // tube / 10 for 10 tubes = 14R add say 20dB of NFB and the net result is 0.14R.
In real life it's a bit more complicated but the idea is to give you some idea about why OTL and low output power don't really go hand in hand.
Nope, //.
Nope again.The 6ASG is a dual triode meant for regulator service whereas the EL509 was designed as a TV sweep tube (horizontal deflection).
Hmmm...Haven't really checked that but Cg just indicates the interelectrode capacitance between grid and anode...nothing of importance to us here.
Yes, this tube is no bed of roses either though...no free lunch.
I hope I haven't discouraged you too much, better not to spin any wheels I reckoned.
Are these subject headings useful?
I was talking about the 6KG6 and the EL509 before not the EL60. Sorry
When you say it's not a bed of roses, are you suggesting the EL509 is high current, high plate resistance or the 6080 tube?
In real life it's a bit more complicated but the idea is to give you some idea about why OTL and low output power don't really go hand in hand.
So what you're saying is that you can have low current, low resistance tubes, but that means you'll need more of them and so then your heater and other support electronics go through the roof. Not to mention your electricity bill
Eternalightwithin
I was talking about the 6KG6 and the EL509 before not the EL60. Sorry
When you say it's not a bed of roses, are you suggesting the EL509 is high current, high plate resistance or the 6080 tube?
In real life it's a bit more complicated but the idea is to give you some idea about why OTL and low output power don't really go hand in hand.
So what you're saying is that you can have low current, low resistance tubes, but that means you'll need more of them and so then your heater and other support electronics go through the roof. Not to mention your electricity bill
Eternalightwithin
Woops...
Hi,
No, something must have got stuck in the clipboard...I must have done something wrong...
I was refering to the 6C33 tube.
Once the EL509 is trioded it can put out more current than the 6080 but that wasn't my point.
The "no bed of roses" innuendo was addressed at the monstruous 6C33, have you ever seen one?
Well, allow me to rephrase that: what I'm saying is that you can have relatively high current , relatively low internal resistance tubes with high heater current demands and in order to drive an even lower impedance LS driver without an impedance adapting xformer you're bound to need a lot of // tubes and a fair amount of NFB.
Do you still care to hear the plus side?
Hi,
No, something must have got stuck in the clipboard...I must have done something wrong...
I was refering to the 6C33 tube.
Once the EL509 is trioded it can put out more current than the 6080 but that wasn't my point.
The "no bed of roses" innuendo was addressed at the monstruous 6C33, have you ever seen one?
Well, allow me to rephrase that: what I'm saying is that you can have relatively high current , relatively low internal resistance tubes with high heater current demands and in order to drive an even lower impedance LS driver without an impedance adapting xformer you're bound to need a lot of // tubes and a fair amount of NFB.
Do you still care to hear the plus side?
I'm running out of things to put up here
I'm all ears....
btw, I've seen a couple of biggies, don't know if it was a 6C33. Let's just safely assume I have not.
btw,why can't you use a 16 Ohm speaker in addition to all the other things. That way at those frequencies where the impedence drops a little, it will go down to like 11-12 Ohms rather than if you have a 8 Ohm LS going down to 4? Would this help the OTL a little bit?
I'm all ears....
btw, I've seen a couple of biggies, don't know if it was a 6C33. Let's just safely assume I have not.
btw,why can't you use a 16 Ohm speaker in addition to all the other things. That way at those frequencies where the impedence drops a little, it will go down to like 11-12 Ohms rather than if you have a 8 Ohm LS going down to 4? Would this help the OTL a little bit?
Hi,
Was afraid you'd be...
Indeed it would, contrary to a solid state amp an OTL and indeed most tube amps out there will feel more at ease when driving higher impedance loads and tend to double their power rating as such.
So, an OTL amp rated at 20W into 8 Ohm will roughly spit out 40W into 16 Ohm etc. as opposed to a SS amp dumping 20W into an 8 Ohm load and roughly 40 Watt into a 4 Ohm load.
As for the advantages:
Very fast transient attack unimpeded by an OPT,expect a FR well over 100KHz.
Very transparent sound, what you feed it is what you hear.
The LS is taken into the feedback loop which tends to linearise speaker deficiencies to some extend, zobel corrected filters are recommended though.
When well designed only one capacitor per PP halve is necessary, with some trickery even this may be eliminated.
Little or no phase shift across the band, in most cases a bandwidth ranging from say 1Hz to 250Khz is no problem and is ruler flat when carefully designed.
PP design cancels even order harmonic distortion, so don't expect this kind of amp to whipe CD nasties under the rug.
In short it gives you an open window into the recording, not always a good thing to have but at least you'll know when the recording has gone through the mangle.
It's the kind of amp that will make you forget you have it in the first place and make you wonder what you can do to make the preamp better or any other source for that matter.
Cheers,
Was afraid you'd be...
Indeed it would, contrary to a solid state amp an OTL and indeed most tube amps out there will feel more at ease when driving higher impedance loads and tend to double their power rating as such.
So, an OTL amp rated at 20W into 8 Ohm will roughly spit out 40W into 16 Ohm etc. as opposed to a SS amp dumping 20W into an 8 Ohm load and roughly 40 Watt into a 4 Ohm load.
As for the advantages:
Very fast transient attack unimpeded by an OPT,expect a FR well over 100KHz.
Very transparent sound, what you feed it is what you hear.
The LS is taken into the feedback loop which tends to linearise speaker deficiencies to some extend, zobel corrected filters are recommended though.
When well designed only one capacitor per PP halve is necessary, with some trickery even this may be eliminated.
Little or no phase shift across the band, in most cases a bandwidth ranging from say 1Hz to 250Khz is no problem and is ruler flat when carefully designed.
PP design cancels even order harmonic distortion, so don't expect this kind of amp to whipe CD nasties under the rug.
In short it gives you an open window into the recording, not always a good thing to have but at least you'll know when the recording has gone through the mangle.
It's the kind of amp that will make you forget you have it in the first place and make you wonder what you can do to make the preamp better or any other source for that matter.
Cheers,
Terminology is your mother, they both humilate you
uhhh, ok. What exactly is a zobel corrected filter?
I think I heard that CD produce odd order harmonics? Is that what you mean by this amp not dealing with that?
Sounds like the kind of amp I want to build, but then I already knew that.
When well designed only one capacitor per PP halve is necessary, with some trickery even this may be eliminated.
We can talk about that later when I have all the pieces.
ok, well It's dreamland time here. Thanks for all your thought provoking responses. See ya tomorrow.
Eternalightwithin
uhhh, ok. What exactly is a zobel corrected filter?
I think I heard that CD produce odd order harmonics? Is that what you mean by this amp not dealing with that?
Sounds like the kind of amp I want to build, but then I already knew that.
When well designed only one capacitor per PP halve is necessary, with some trickery even this may be eliminated.
We can talk about that later when I have all the pieces.
ok, well It's dreamland time here. Thanks for all your thought provoking responses. See ya tomorrow.
Eternalightwithin
An alternative route
Eternalightwithin,
The high wattage (and cost) of Bruce's 80W OTL, as very well explained above, results from the requirement for low output impedance and was not the aim per se.
If cost is an issue and high power is not required, may I suggest the use of 6C33C-B. Thanks to its very low anode R of ~80 Ohm, just a pair is sufficient for the output stage. As a bonus, they come very cheap too at about $12.
A nice design for a start can be found here http://www.audiodesignguide.com/my/OTL_version_82_ocl.gif
I have build it several months ago and I am delighted with the results. It gives about 22W/8Ohm which should be sufficient in your case.
As every other design, it can be improved in certain areas but even as it is, it plays beautiful.
Cheers
Eternalightwithin,
The high wattage (and cost) of Bruce's 80W OTL, as very well explained above, results from the requirement for low output impedance and was not the aim per se.
If cost is an issue and high power is not required, may I suggest the use of 6C33C-B. Thanks to its very low anode R of ~80 Ohm, just a pair is sufficient for the output stage. As a bonus, they come very cheap too at about $12.
A nice design for a start can be found here http://www.audiodesignguide.com/my/OTL_version_82_ocl.gif
I have build it several months ago and I am delighted with the results. It gives about 22W/8Ohm which should be sufficient in your case.
As every other design, it can be improved in certain areas but even as it is, it plays beautiful.
I second that, in fact it was the first thing that came to mind when I connected the beast for the first time in my system
Cheers
OTL.
Hi,
A Zobel is a special circuit designed to compensate for the rise in impedance that occurs at or near the resonant frequency of a speaker.
Often it comprises a cap and resistor in series wired directly across the drive units of a speaker.
In older OTL designs developped for 16 ohm speakers a Zobel network was built into the amp itself across its speaker terminals.
What was meant is that it won't be forgiving of the source material, be that CD or anything else.
Cheers,
Hi,
A Zobel is a special circuit designed to compensate for the rise in impedance that occurs at or near the resonant frequency of a speaker.
Often it comprises a cap and resistor in series wired directly across the drive units of a speaker.
In older OTL designs developped for 16 ohm speakers a Zobel network was built into the amp itself across its speaker terminals.
What was meant is that it won't be forgiving of the source material, be that CD or anything else.
Cheers,
Is it possible
Can you have empty sockets and just add pair of output tubes to increase current capacity, or would that change the load that mains transformer sees and throw off the whole circuit?
Any ideas on how the power supply could be better? I noticed there was no filtering on this circuit.
Also, I've learned from the forum that increasing the VA of your Mains transformer will benefit the voltage rails stay within specs, improve sounds, etc. Would this be something to consider for this project?
How much would a good 1000 VA transformer cost with all the extras?
Eternalightwithin
Can you have empty sockets and just add pair of output tubes to increase current capacity, or would that change the load that mains transformer sees and throw off the whole circuit?
Any ideas on how the power supply could be better? I noticed there was no filtering on this circuit.
Also, I've learned from the forum that increasing the VA of your Mains transformer will benefit the voltage rails stay within specs, improve sounds, etc. Would this be something to consider for this project?
How much would a good 1000 VA transformer cost with all the extras?
Eternalightwithin
Re: OTL.
This is a common misconception. A Zobel is actually for the other end where there is an impedance rise due to the voice coil inductance.
A parallel RLC resonance trap is used to tame the resonant peak (to some extent this can also be done with a TL tuned towards the aperiodic).
dave
fdegrove said:
This is a common misconception. A Zobel is actually for the other end where there is an impedance rise due to the voice coil inductance.
A parallel RLC resonance trap is used to tame the resonant peak (to some extent this can also be done with a TL tuned towards the aperiodic).
dave
ZOBEL.
Hi Dave,
You're absolutely right.
And you know the most stupid part about it is that I actually checked this on the net and the same wrong ideas keep cropping up.
In the case of the OTL amps and naturally the same goes for MOS-FET amps using the same kind of topology is that the amp sees a more homogenous impedance when a speaker is hooked up.
Nowadays decent speakers have these corrections included in their x-over so this network can often be omitted.
Anyway, thanks for setting me straight and I do hope I have it right this time...
Cheers,
Hi Dave,
You're absolutely right.
And you know the most stupid part about it is that I actually checked this on the net and the same wrong ideas keep cropping up.
In the case of the OTL amps and naturally the same goes for MOS-FET amps using the same kind of topology is that the amp sees a more homogenous impedance when a speaker is hooked up.
Nowadays decent speakers have these corrections included in their x-over so this network can often be omitted.
Anyway, thanks for setting me straight and I do hope I have it right this time...
Cheers,
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