Hi, I'm a Brazilian newcomer to this forum, and I'm stumbled with
the following question: Along with its Specialist series of output
transformer for tube amps, Plitron also has another model with
cathode feedback - the VDV1070-UC - that is unity coupled, i.e.,
works with the McIntosh design. I have Menno van der Veen's
book, and it does not say much about this transformer except for frequency range measurements (which, by the way, are the most impressive I've ever seen). Does anybody know about an amp design using McIntosh's unity coupling and the VDV1070-UC?
Has anyone tried to do so? Or shoud I try to contact Menno himself (sorry for the dumb question)?
Thanks everybody...
the following question: Along with its Specialist series of output
transformer for tube amps, Plitron also has another model with
cathode feedback - the VDV1070-UC - that is unity coupled, i.e.,
works with the McIntosh design. I have Menno van der Veen's
book, and it does not say much about this transformer except for frequency range measurements (which, by the way, are the most impressive I've ever seen). Does anybody know about an amp design using McIntosh's unity coupling and the VDV1070-UC?
Has anyone tried to do so? Or shoud I try to contact Menno himself (sorry for the dumb question)?
Thanks everybody...
A unity-coupled output transformer allows you to make a superb output stage.
You now need a driver stage capable of supplying a very large voltage with low distortion. You have moved the problems from the output stage to the driver stage. This isn't a complete waste of effort, because the driver stage only needs to provide voltage, rather than power. Nevertheless, you either need a a driver HT of >700V or you do some rather dubious bootstrapping from the output transformer that seriously compromises stability.
Judging by your question, I suspect that you are not an experienced electronics engineer. Further, if this is your first valve project, I would suggest that a more conventional and slightly less ambitious project is more likely to reward you with good results. I have played with unity-coupled amplifiers, and was not convinced that they were worth the trouble. If I am wrong, and you are an experienced engineer, supported by lots of test equipment, give it a go, but expect HF stability or driver stage distortion to be the major problem.
You now need a driver stage capable of supplying a very large voltage with low distortion. You have moved the problems from the output stage to the driver stage. This isn't a complete waste of effort, because the driver stage only needs to provide voltage, rather than power. Nevertheless, you either need a a driver HT of >700V or you do some rather dubious bootstrapping from the output transformer that seriously compromises stability.
Judging by your question, I suspect that you are not an experienced electronics engineer. Further, if this is your first valve project, I would suggest that a more conventional and slightly less ambitious project is more likely to reward you with good results. I have played with unity-coupled amplifiers, and was not convinced that they were worth the trouble. If I am wrong, and you are an experienced engineer, supported by lots of test equipment, give it a go, but expect HF stability or driver stage distortion to be the major problem.
I'll third that- the Mac circuit uses a combination of negative and positive feedback. If you don't know exactly what you're doing, you'll come to grief.
If you're an advanced experimenter, I've had good luck with HV drive using cascoded diff amp, with a HV mosfet on top, tube on the bottom. I haven't used it with a Mac-type output stage, but it certainly can swing enough voltage if you've got a 1kV mosfet and an appropriate rail voltage.
A step up interstage transformer might be another interesting approach, though you'll probably have to restrict feedback to local loops.
If you're an advanced experimenter, I've had good luck with HV drive using cascoded diff amp, with a HV mosfet on top, tube on the bottom. I haven't used it with a Mac-type output stage, but it certainly can swing enough voltage if you've got a 1kV mosfet and an appropriate rail voltage.
A step up interstage transformer might be another interesting approach, though you'll probably have to restrict feedback to local loops.
Hi,
.....To do whot exactly?
And as Fela N'Kuti or whatever his name was said: ITT= International Thief Thief....
If you want to go down the socialist road come informed, otherwise show your best side.
In short, RS. Radio Spares is just another company trying to make a buck...Can they or do they get a veto?
Cheers,
..............?
.
.....To do whot exactly?
And as Fela N'Kuti or whatever his name was said: ITT= International Thief Thief....
If you want to go down the socialist road come informed, otherwise show your best side.
In short, RS. Radio Spares is just another company trying to make a buck...Can they or do they get a veto?
Cheers,
..............?Dear folks,
Thanks for the quick replies. I'll put my doubt into details following
each reply:
Yeah, I've already checked in some places that the driver stage
is the real trick in McIntosh designs. The MC schematics I've given
a bird's eye view on are very tricky...
You're right about that - I AM a newcomer, and with my
scarce budget coming from my PhD scholarship it's difficult
to acquire testing equip such as oscilloscope, signal generator,
spectrum analyser and so on... I'm having to rely on a fellow engineer (who, by the way, is not exactly enthusiastic about valves) to do the testing. I'm just evaluating if it's worth the effort
to invest $300 in a VDV1070-UC transformer and stick to a
McIntosh-like design, or to be down-to-earth and use a,
say, van der Veen design such as one of those in his book.
And, yes, it IS my first project.
Do you mean Frank McIntosh's original articles? I'd like to have
precise references, if possible.
Actually, this leads me to another aspect of my dilemma exposed above: I'd like to make experiments with three types of global
feedback: voltage, current and power. I don't know if it's possible
to do the last two if I use a McIntosh-like design, again because of the complexity of the schematics I've seen so far - will I have other instability problems besides those coming from bootstrapping (i.e., more LC loops, etc)? I was planning to do such experiments only if I'd chosen a barebone VDV design,
specially regarding power feedback loops such as the one
used in Stephen Fay White's POWRTRON design, which are
considerably more complex than bare voltage or current feedback.
Well, I've stalled long enough... Thanks all!
Thanks for the quick replies. I'll put my doubt into details following
each reply:
EC8010 said:
Yeah, I've already checked in some places that the driver stage
is the real trick in McIntosh designs. The MC schematics I've given
a bird's eye view on are very tricky...
You're right about that - I AM a newcomer, and with my
scarce budget coming from my PhD scholarship it's difficult
to acquire testing equip such as oscilloscope, signal generator,
spectrum analyser and so on... I'm having to rely on a fellow engineer (who, by the way, is not exactly enthusiastic about valves) to do the testing. I'm just evaluating if it's worth the effort
to invest $300 in a VDV1070-UC transformer and stick to a
McIntosh-like design, or to be down-to-earth and use a,
say, van der Veen design such as one of those in his book.
And, yes, it IS my first project.
Bandersnatch said:
Do you mean Frank McIntosh's original articles? I'd like to have
precise references, if possible.
SY said:
Actually, this leads me to another aspect of my dilemma exposed above: I'd like to make experiments with three types of global
feedback: voltage, current and power. I don't know if it's possible
to do the last two if I use a McIntosh-like design, again because of the complexity of the schematics I've seen so far - will I have other instability problems besides those coming from bootstrapping (i.e., more LC loops, etc)? I was planning to do such experiments only if I'd chosen a barebone VDV design,
specially regarding power feedback loops such as the one
used in Stephen Fay White's POWRTRON design, which are
considerably more complex than bare voltage or current feedback.
Well, I've stalled long enough... Thanks all!
Being unfamiliar with the Powrtron circuit, I waded through the rambling and turgid documents at:
http://www.cosmos2000.org/audio/powrtron1.htm
Fortunately, the original article ("Audio Anthology" Vol 3 pp13) was slightly clearer and comes clean about the feedback, confirming that it uses a combination of negative current and voltage feedback to achieve constant power.
Unfortunately, modern loudspeaker designers assume a constant voltage source, so supplying a loudspeaker passive crossover and drivers from a constant power source is likely to disturb the operation of the crossover and the bass alignment of the woofer. However, the Powrtron amplifier was intended for use with an active crossover, and would have provided HF equalisation to its associated drivers due to maintaining constant power across the rising impedance due to voice coil inductance. This might be useful, but that equalisation could just as easily be implemented by adding a step network at the input of the amplifier.
Until Thiele & Small came along in the mid-70s, the dynamics of loudspeaker bass alignment were very poorly understood, so a constant power source might have seemed appropriate. The effect of a constant power source on a modern loudspeaker would be to produce a change in frequency response proportional to the curve of loudspeaker impedance, exacerbating the effects of the LF resonance.
Simply because the loudspeaker is driven constant power does not mean that its acoustic power response has been corrected. The acoustic power response is the summation of responses at all angles, and is primarily affected by the relative size of the source to the wavelength being reproduced. Nevertheless, driving each loudspeaker driver from a dedicated amplifier is a good idea because it allows amplifiers to be designed and optimised for known loads that only have to be driven over a restricted bandwidth.
I see no particular reason why a McIntosh amplifier should be any more unstable using current or power feedback than when using voltage feedback. However, each time you change the feedback, you are going to have to go round the loop of making the amplifier stable again, which is trickier with a McIntosh.
Experiment with different forms of feedback, but be aware of the effect they will have. If you can borrow an oscilloscope and oscillator, you can measure the voltage response at the loudspeaker terminals with each change in feedback to assist in your deliberations. You will probably need to buy your fellow engineer lots of beer to pay for all the testing...
I don't want to be a wet blanket, but this will be quite a lot of work, probably for little gain. Complexity does not automatically equal quality.
http://www.cosmos2000.org/audio/powrtron1.htm
Fortunately, the original article ("Audio Anthology" Vol 3 pp13) was slightly clearer and comes clean about the feedback, confirming that it uses a combination of negative current and voltage feedback to achieve constant power.
Unfortunately, modern loudspeaker designers assume a constant voltage source, so supplying a loudspeaker passive crossover and drivers from a constant power source is likely to disturb the operation of the crossover and the bass alignment of the woofer. However, the Powrtron amplifier was intended for use with an active crossover, and would have provided HF equalisation to its associated drivers due to maintaining constant power across the rising impedance due to voice coil inductance. This might be useful, but that equalisation could just as easily be implemented by adding a step network at the input of the amplifier.
Until Thiele & Small came along in the mid-70s, the dynamics of loudspeaker bass alignment were very poorly understood, so a constant power source might have seemed appropriate. The effect of a constant power source on a modern loudspeaker would be to produce a change in frequency response proportional to the curve of loudspeaker impedance, exacerbating the effects of the LF resonance.
Simply because the loudspeaker is driven constant power does not mean that its acoustic power response has been corrected. The acoustic power response is the summation of responses at all angles, and is primarily affected by the relative size of the source to the wavelength being reproduced. Nevertheless, driving each loudspeaker driver from a dedicated amplifier is a good idea because it allows amplifiers to be designed and optimised for known loads that only have to be driven over a restricted bandwidth.
I see no particular reason why a McIntosh amplifier should be any more unstable using current or power feedback than when using voltage feedback. However, each time you change the feedback, you are going to have to go round the loop of making the amplifier stable again, which is trickier with a McIntosh.
Experiment with different forms of feedback, but be aware of the effect they will have. If you can borrow an oscilloscope and oscillator, you can measure the voltage response at the loudspeaker terminals with each change in feedback to assist in your deliberations. You will probably need to buy your fellow engineer lots of beer to pay for all the testing...
I don't want to be a wet blanket, but this will be quite a lot of work, probably for little gain. Complexity does not automatically equal quality.
What makes that experimentation even trickier is having to balance against the positive feedback to the driver tube.
I don't know if it's been incorporated into the Audio Anthology series yet, but I'd highly recommend an article by Norman Crowhurst, "Realistic Audio Engineering Philosophy," Audio, October 1959. He does a pretty thorough analysis of unity coupling and the driver stage tricks used by Mac.
I don't know if it's been incorporated into the Audio Anthology series yet, but I'd highly recommend an article by Norman Crowhurst, "Realistic Audio Engineering Philosophy," Audio, October 1959. He does a pretty thorough analysis of unity coupling and the driver stage tricks used by Mac.
In the garden
Olá Pedrinho
Bem vindo ao forum da Diyaudio. Principalmente ao dos TUBES. Há poucos brasileiros aqui (acredito que nós dois).
Like you I´m just starting with tubes. Until now I am just building, mostly with schematics from Internet. Doing so to get a good amplifier to play while I am studying the theory.
I read menno´s book too. I even thought about building the Maurits, but Positron (member from this forum) recomended to find an other design. Search for maurits at this forum and you will find out why. Menno´s design with EL84´s runs the EL84´s at a higher than recomended dissipation. He says they can have it...
Have you thought about brazilian suppliers for output trafo´s? I know Lynx Audio http://www.audiolink.com.br/audiolink/transformadores.htm and Tesla transformadores (excellent service). Their number is 0xx21-2233-4975 or 0xx21-2233-4050. Ask for Manoel Pereira, or his son, Roberto. I haven´t used or heard trafos from neither company, but both seem to export to Europe (to the non-brazilians guys: a brazilian product exported to Europe is really fantastic). At the moment I´m using torroidals from www.toroid.com.br as OPT.
And the tubes? Where will you get them? The guys from Rei do Som in São Paulo are asking crazy prices, I wouldn´t buy from them. Eletrônica Rei das Válvulas (in RIO) sells 6A3 (don´t know the brand) for 55 reais/each. They have got 12BH7A (ultron SQ) for 15 reais/each, 6EM7 (westinghouse) for 9 reais/each. I bought PCL86´s at 8,60 each (RSD) to buid a DPP, using the schematic of an ECC83 and EL84. Or have you already bougt excellent audio tubes from a failed company? Maybe got them from the "lixão". I think this must be possible in Brazil.
Good luck with deciding and building your amplifier(s).
Erik
P.S. 1US$ ~ 2,90 reais.
Olá Pedrinho
Bem vindo ao forum da Diyaudio. Principalmente ao dos TUBES. Há poucos brasileiros aqui (acredito que nós dois).
Like you I´m just starting with tubes. Until now I am just building, mostly with schematics from Internet. Doing so to get a good amplifier to play while I am studying the theory.
I read menno´s book too. I even thought about building the Maurits, but Positron (member from this forum) recomended to find an other design. Search for maurits at this forum and you will find out why. Menno´s design with EL84´s runs the EL84´s at a higher than recomended dissipation. He says they can have it...
Have you thought about brazilian suppliers for output trafo´s? I know Lynx Audio http://www.audiolink.com.br/audiolink/transformadores.htm and Tesla transformadores (excellent service). Their number is 0xx21-2233-4975 or 0xx21-2233-4050. Ask for Manoel Pereira, or his son, Roberto. I haven´t used or heard trafos from neither company, but both seem to export to Europe (to the non-brazilians guys: a brazilian product exported to Europe is really fantastic). At the moment I´m using torroidals from www.toroid.com.br as OPT.
And the tubes? Where will you get them? The guys from Rei do Som in São Paulo are asking crazy prices, I wouldn´t buy from them. Eletrônica Rei das Válvulas (in RIO) sells 6A3 (don´t know the brand) for 55 reais/each. They have got 12BH7A (ultron SQ) for 15 reais/each, 6EM7 (westinghouse) for 9 reais/each. I bought PCL86´s at 8,60 each (RSD) to buid a DPP, using the schematic of an ECC83 and EL84. Or have you already bougt excellent audio tubes from a failed company? Maybe got them from the "lixão". I think this must be possible in Brazil.
Good luck with deciding and building your amplifier(s).
Erik
P.S. 1US$ ~ 2,90 reais.
Hi,
Welcome to this DiyAudio forum. More specifically to the tube forum. There are few Brasilian people around here. Basically just you and me...
Hey...One of you should speak Dutch too, right?
That was an easy translation...I'll check the technical content later on...
Bem vinido, buen venido, willkommen, bien venue, welcome, welkom...
Just kiddin...
Welcome to this DiyAudio forum. More specifically to the tube forum. There are few Brasilian people around here. Basically just you and me...
Hey...One of you should speak Dutch too, right?
That was an easy translation...I'll check the technical content later on...
Bem vinido, buen venido, willkommen, bien venue, welcome, welkom...
Just kiddin...
Ik heb nederlands geleerd, aangemoedigd door mijn vader:
"Leer nederlands en je zou aangename momenten hebben wanneer je luistert naar de "plasante" uitspraak van de nederlandse taal door de vlaamse broeders".
I learned dutch because my father stated:
"Learn dutch and you will have good times listening the belgians speaking".
Geintje
Kidding too.
Erik
"Leer nederlands en je zou aangename momenten hebben wanneer je luistert naar de "plasante" uitspraak van de nederlandse taal door de vlaamse broeders".
I learned dutch because my father stated:
"Learn dutch and you will have good times listening the belgians speaking".
Geintje
Kidding too.
Erik
Hi,
That should be a good company to buy from in Brazil.
I recall meeting with the manager....damm I even recal what his office looked like.
King of tubes he probably wasn't, casecs regulations you know, but he seemed to know what he was doing...
Ah,...I had the idea it was the other way around....
Congratulations!
Cheers,
That should be a good company to buy from in Brazil.
I recall meeting with the manager....damm I even recal what his office looked like.
King of tubes he probably wasn't, casecs regulations you know, but he seemed to know what he was doing...
Ah,...I had the idea it was the other way around....
Congratulations!
Cheers,
Re: In the garden
It never occurred to me to build the Maurits - it seems to me
a very personal project, and according to Menno he used mainly
some spare parts he had in his workbench. It didn't particularly seduced me. I was rather thinking about Menno's barebone design (chapter 11 of Menno's book) with EL34's (30 to 100 W), or some improvement of it, such as the project of Rudy Godmaire (http://www3.sympatico.ca/r.godmaire/), because it gives some freedom of choice on the valves, and the type of feedback (if any) and phase splitting. Or this, or a McIntosh-like design, as I said in my former thread - that if I can manage to understand all aspects of such a design. I'll probably have to buy one or more volumes of the Audio Anthology, at least to be able to read Crowhurst's paper on McIntosh's unity coupled design ("Realistic Audio Engineering Philosophy"), since I couldn't find it on the Web for download.
I don't know about the other brands of toroidal transformers,
I decided to stick to Plitron because it's the only brand I have had some aural experience - an audiophile called Paulo Ramos, who lives in Santo André (Grande São Paulo) has built a VDV-like PP tube amp using Plitron and invited me and my father to have a testdrive. The result is simply exquisite.
The neighborhood of Rua Santa Ifigênia (center of São Paulo) has quite a few tube stores. My first surveys gave EL34 a price around 50 reais/each (Svetlana). Comparing with prices at, say,
http://www.thetubestore.com, ($27.95 for a matched pair, not including shipping costs), it's not that expensive. A JAN/Philips
ECC81/2/3 costs also around 50 reais/each ($12.95/each at
the site above). Scrap tubes are becoming scarce, and with brands I've never heard of - MESA Engineering, for instance.
I don't know about KT88/6550's. Real triodes are either unavailable (such as the 845) or outrageously expensive (300B) around Santa Ifigênia - certainly far beyond my present budget.
Thanks - the same to you... I also thank the warm welcome...
Pedro
P.S.: Regarding European ascendence, my grandfather was Danish (my middlename is Lauridsen), but I can't speak a single word of Danish...
ErikdeBest said:
It never occurred to me to build the Maurits - it seems to me
a very personal project, and according to Menno he used mainly
some spare parts he had in his workbench. It didn't particularly seduced me. I was rather thinking about Menno's barebone design (chapter 11 of Menno's book) with EL34's (30 to 100 W), or some improvement of it, such as the project of Rudy Godmaire (http://www3.sympatico.ca/r.godmaire/), because it gives some freedom of choice on the valves, and the type of feedback (if any) and phase splitting. Or this, or a McIntosh-like design, as I said in my former thread - that if I can manage to understand all aspects of such a design. I'll probably have to buy one or more volumes of the Audio Anthology, at least to be able to read Crowhurst's paper on McIntosh's unity coupled design ("Realistic Audio Engineering Philosophy"), since I couldn't find it on the Web for download.
I don't know about the other brands of toroidal transformers,
I decided to stick to Plitron because it's the only brand I have had some aural experience - an audiophile called Paulo Ramos, who lives in Santo André (Grande São Paulo) has built a VDV-like PP tube amp using Plitron and invited me and my father to have a testdrive. The result is simply exquisite.
The neighborhood of Rua Santa Ifigênia (center of São Paulo) has quite a few tube stores. My first surveys gave EL34 a price around 50 reais/each (Svetlana). Comparing with prices at, say,
http://www.thetubestore.com, ($27.95 for a matched pair, not including shipping costs), it's not that expensive. A JAN/Philips
ECC81/2/3 costs also around 50 reais/each ($12.95/each at
the site above). Scrap tubes are becoming scarce, and with brands I've never heard of - MESA Engineering, for instance.
I don't know about KT88/6550's. Real triodes are either unavailable (such as the 845) or outrageously expensive (300B) around Santa Ifigênia - certainly far beyond my present budget.
Thanks - the same to you... I also thank the warm welcome...
Pedro
P.S.: Regarding European ascendence, my grandfather was Danish (my middlename is Lauridsen), but I can't speak a single word of Danish...
Tubes
Hi Pedrinho
R$ 50,00 for an EL34 is a good price. Rei do Som is asking R$ 190,00 at their site.
Rei das Válvulas has ECC83 for R$ 30,00 / each.
If I remember well (one of) the phase splitters used in Menno´s amplifiers is based on a ECC82. I remember Frank de Grove stating that 9 in the 10 designs using an ECC82 in the circuit sound better when you use a 12BH7A (R$ 15,00 at R. das V.)instead of the ECC82. He can surely tell you more.
In case you find a batch of 6AS7 or 6080 for a good price, give me a hint. No, it´s not for the Maurits, but for an OTL I´m building.
Thanks.
Erik
Hi Pedrinho
R$ 50,00 for an EL34 is a good price. Rei do Som is asking R$ 190,00 at their site.
Rei das Válvulas has ECC83 for R$ 30,00 / each.
If I remember well (one of) the phase splitters used in Menno´s amplifiers is based on a ECC82. I remember Frank de Grove stating that 9 in the 10 designs using an ECC82 in the circuit sound better when you use a 12BH7A (R$ 15,00 at R. das V.)instead of the ECC82. He can surely tell you more.
In case you find a batch of 6AS7 or 6080 for a good price, give me a hint. No, it´s not for the Maurits, but for an OTL I´m building.
Thanks.
Erik
Re: Tubes
Hi Erik,
Nice price for the ECC83... What surprises me is the difference
of price between this valve and the EL34 in your case.
Regarding the ECC82, some higher power designs use the 12AX7WA instead for the phase splitting, specially when using negative feedback. I've seen the 12BH7 being used before only as a voltage amplifier stage. Anyway, this is always an optimization I can make later - I just have to see whether I'll have to change some component values in order to achieve the correct operating point in the course of substituting the valve.
As for the 6AS7, I'll probably visit the tube stores in the next days - I'll see what I can do.
Regards,
Pedro
Hi Erik,
ErikdeBest said:
Nice price for the ECC83... What surprises me is the difference
of price between this valve and the EL34 in your case.
Regarding the ECC82, some higher power designs use the 12AX7WA instead for the phase splitting, specially when using negative feedback. I've seen the 12BH7 being used before only as a voltage amplifier stage. Anyway, this is always an optimization I can make later - I just have to see whether I'll have to change some component values in order to achieve the correct operating point in the course of substituting the valve.
As for the 6AS7, I'll probably visit the tube stores in the next days - I'll see what I can do.
Regards,
Pedro
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