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#1
Posted to rec.audio.tubes
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Transformer attenuators
I have noticed that a number of transformer
manufacturers are offering an audio attenuator transformer with a 24-tap secondary, for use as a preferred option to a passive controller. Sowter says, on their website: quote Attenuator transformers used in conjunction with a multi position switch provide a high performance volume control function. Sowter proprietary winding technique ensures 100 kHz bandwidth at all switch positions. The transformer action of these attenuators ensures the loading circuit sees a low driving impedance regardless of the switch position. This ensures the maximum bandwidth potential of the whole circuit can be realised at all gain settings. As the attenuation is related to the number of turns at the winding tap, the attenuation is exactly matched between stereo channels. unquote. Stevens and Billington say, on their website: Quote: The primary application is in so-called passive preamplifiers, more precisely passive control units. As such it allows a number of functions, the most important being the ability to change the output level of the passive control in multiple steps to control the volume of a high fidelity system. Applied in a passive control unit the TX-102 by applying transformer technology offers a material improvement over all previous such schemes based around resistive attenuation. The so-called passive preamplifier appeared on the map of High Performance Audio with the emergence of the CD Player. Modern Sources, such as CD-Players, DVD Players, SACD Players and others generally offer output levels sufficient to drive power amplifiers to full power (usually 2V RMS or more for digital full scale) and also offer sufficient drive for external devices and cables. Many CD-Players and similar devices have output impedances lower than 1kOhm, some are materially lower. Whilst "passive preamplifiers" initially created notable interest as a sonically extremely pure method of controlling volume and selecting inputs, they have soon faded back into obscurity, not surprisingly as most of these devices suffered from substantial impedance mismatches with either sources or loads. If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers! Thus the resistive volume control employed in passive control units must navigate a course between the Scylla of excessively loading the source, leading to increased distortion and the Charybdis of excessively high output impedance. unquote. Iain asks. 1.Does anyone have experience of such transformers? 2. Are they actually better than a passive stepped attenuator controller? Regards to all Iain |
#2
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Transformer attenuators
On Jun 6, 3:55 pm, "Iain Churches" wrote:
I have noticed that a number of transformer manufacturers are offering an audio attenuator transformer with a 24-tap secondary, for use as a preferred option to a passive controller. Sowter says, on their website: quote Attenuator transformers used in conjunction with a multi position switch provide a high performance volume control function. Sowter proprietary winding technique ensures 100 kHz bandwidth at all switch positions. The transformer action of these attenuators ensures the loading circuit sees a low driving impedance regardless of the switch position. This ensures the maximum bandwidth potential of the whole circuit can be realised at all gain settings. As the attenuation is related to the number of turns at the winding tap, the attenuation is exactly matched between stereo channels. unquote. Stevens and Billington say, on their website: Quote: The primary application is in so-called passive preamplifiers, more precisely passive control units. As such it allows a number of functions, the most important being the ability to change the output level of the passive control in multiple steps to control the volume of a high fidelity system. Applied in a passive control unit the TX-102 by applying transformer technology offers a material improvement over all previous such schemes based around resistive attenuation. The so-called passive preamplifier appeared on the map of High Performance Audio with the emergence of the CD Player. Modern Sources, such as CD-Players, DVD Players, SACD Players and others generally offer output levels sufficient to drive power amplifiers to full power (usually 2V RMS or more for digital full scale) and also offer sufficient drive for external devices and cables. Many CD-Players and similar devices have output impedances lower than 1kOhm, some are materially lower. Whilst "passive preamplifiers" initially created notable interest as a sonically extremely pure method of controlling volume and selecting inputs, they have soon faded back into obscurity, not surprisingly as most of these devices suffered from substantial impedance mismatches with either sources or loads. If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers! Thus the resistive volume control employed in passive control units must navigate a course between the Scylla of excessively loading the source, leading to increased distortion and the Charybdis of excessively high output impedance. unquote. Iain asks. 1.Does anyone have experience of such transformers? 2. Are they actually better than a passive stepped attenuator controller? Regards to all Iain This thread... I am watching with great interest. Thank you, Iain. Peter Wieck Wyncote, PA |
#3
Posted to rec.audio.tubes
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Transformer attenuators
"Peter Wieck" wrote in message oups.com... On Jun 6, 3:55 pm, "Iain Churches" wrote: I have noticed that a number of transformer manufacturers are offering an audio attenuator transformer with a 24-tap secondary, for use as a preferred option to a passive controller. Sowter says, on their website: quote Attenuator transformers used in conjunction with a multi position switch provide a high performance volume control function. Sowter proprietary winding technique ensures 100 kHz bandwidth at all switch positions. The transformer action of these attenuators ensures the loading circuit sees a low driving impedance regardless of the switch position. This ensures the maximum bandwidth potential of the whole circuit can be realised at all gain settings. As the attenuation is related to the number of turns at the winding tap, the attenuation is exactly matched between stereo channels. unquote. Stevens and Billington say, on their website: Quote: The primary application is in so-called passive preamplifiers, more precisely passive control units. As such it allows a number of functions, the most important being the ability to change the output level of the passive control in multiple steps to control the volume of a high fidelity system. Applied in a passive control unit the TX-102 by applying transformer technology offers a material improvement over all previous such schemes based around resistive attenuation. The so-called passive preamplifier appeared on the map of High Performance Audio with the emergence of the CD Player. Modern Sources, such as CD-Players, DVD Players, SACD Players and others generally offer output levels sufficient to drive power amplifiers to full power (usually 2V RMS or more for digital full scale) and also offer sufficient drive for external devices and cables. Many CD-Players and similar devices have output impedances lower than 1kOhm, some are materially lower. Whilst "passive preamplifiers" initially created notable interest as a sonically extremely pure method of controlling volume and selecting inputs, they have soon faded back into obscurity, not surprisingly as most of these devices suffered from substantial impedance mismatches with either sources or loads. If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers! Thus the resistive volume control employed in passive control units must navigate a course between the Scylla of excessively loading the source, leading to increased distortion and the Charybdis of excessively high output impedance. unquote. Iain asks. 1.Does anyone have experience of such transformers? 2. Are they actually better than a passive stepped attenuator controller? Regards to all Iain This thread... I am watching with great interest. Thank you, Iain. Hi Peter. I forgot to mention that I managed to obtain a pair of these transformers for evaluation purposes. So far I have only mounted them on a bulkhead plate which I intend to fit into my standard preamp chassis for AB testing. I am interested to know what the technical advantages of such a topology might be. Regards to all Iain |
#4
Posted to rec.audio.tubes
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Transformer attenuators
"Iain Churches" wrote in message
ti.fi If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. 5 K ohm load is severe??? If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! (a) This is a very atypical cable. (b) Iain, you shouldn't get your panties in a bunch about it. If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers! Using a potentiometer that is over 10 K as a passive controller is unwise. Thus the resistive volume control employed in passive control units must navigate a course between the Scylla of excessively loading the source, leading to increased distortion and the Charybdis of excessively high output impedance. I bet using that reference from greek mythology made you feel really intellectual, Iain. Iain asks. 1.Does anyone have experience of such transformers? No, because potentiometers work so well in real life. 2. Are they actually better than a passive stepped attenuator controller? They are clearly better for the retailer that sells them. Seems like they are right down your alley, Iain. To make your transformer attenuator complete, be sure to add the $500 wooden knob. ;-) |
#5
Posted to rec.audio.tubes
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Transformer attenuators
"Arny Krueger" wrote in message ... "Iain Churches" wrote in message ti.fi If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. 5 K ohm load is severe??? Hi Arny. That's a cut and paste from the maker's website, not my personal statement. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! (a) This is a very atypical cable. (b) Iain, you shouldn't get your panties in a bunch about it. See above. I have no opinion for or against at this juncture. I can however hear shortcomings in the passive controller topology, which is usually a 1m screened pair to a box with a stepped attenuator and a 1m screened pair leading out to the power amplifier. In addition to the changes in volume there are changes in timbre at different settings of the attenuator. If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers! Using a potentiometer that is over 10 K as a passive controller is unwise. Most people seem to pick 100k. Thus the resistive volume control employed in passive control units must navigate a course between the Scylla of excessively loading the source, leading to increased distortion and the Charybdis of excessively high output impedance. I bet using that reference from greek mythology made you feel really intellectual, Iain. Actually once again that was the manufacturers text not mine, Arny. I marked it clearly with Quote and Unquote. And please note, upper case, (capital) "G" for Greek. But yes. I did have the benefit of a classical education: Greek and Latin, plus French, German, and Middle English, I have since added Swedish, Finnish, and some Norwegian and Danish to that list. Something which I have never had cause to regret. A classical education is a god insurance against one ever becoming a compute repair man in Michigan:-) I delete the rest of your post as unbefitting of a Born Again Christian:-)) Cordially Iain |
#6
Posted to rec.audio.tubes
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Transformer attenuators
Iain asks.
1.Does anyone have experience of such transformers? 2. Are they actually better than a passive stepped attenuator controller? No. I would be interested to hear from folk who have compared the two sensibly. We went through the impedance matching and possible bandwidth limiting from the combination of impedance and capacitance a while ago, in response to a question from yourself IIRC. It is not difficult, given short cables, to match a CD player with a valve amp via a potentiometer, because there can be a x100 difference between source impedance and power amp input impedance, thus allowing for 2 10-to-1 ratios. For example, a 10k pot should be just OK with 1k source and 100k load. In my view a resistor is preferable to an inductor if both work OK. So in most circumstances this is a question about whether a transformer is better than an active preamp such as your own. In which case the issue is distortion, presumably. There may be some folk who would prefer transformer coupling in any case, possibly because they object to using capacitors in the signal path, but it's a while since anyone owned up to this apparent fetish, and I forget how they argue their case. One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. cheers, Ian |
#7
Posted to rec.audio.tubes
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Transformer attenuators
"Iain Cherches Complete ASS "
Stevens and Billington say, on their website: If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! ** The authors have used very funny maths. With a 1500 ohm source and 1nF load: - 3dB is at 106 kHz - 1dB is at 53 kHz - 0.25dB is at 26 kHz With a 540 ohm source and 1 nF load: - 3dB is at 295 kHz - 1dB is at 147 kHz - 0.25dB is at 74 kHz Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low capacitance RCA leads have similar values. Takes a monster ** 50 foot long ** lead to create 1 nF. **Conclusion: The dB figures are false. The reasoning is totally false. What else would you expect from a marketing blurb ? ........ Phil |
#8
Posted to rec.audio.tubes
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Transformer attenuators
"Phil Allison" wrote in message ... "Iain Cherches Complete ASS " Stevens and Billington say, on their website: If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! ** The authors have used very funny maths. With a 1500 ohm source and 1nF load: - 3dB is at 106 kHz - 1dB is at 53 kHz - 0.25dB is at 26 kHz With a 540 ohm source and 1 nF load: - 3dB is at 295 kHz - 1dB is at 147 kHz - 0.25dB is at 74 kHz Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low capacitance RCA leads have similar values. Takes a monster ** 50 foot long ** lead to create 1 nF. **Conclusion: The dB figures are false. The reasoning is totally false. What else would you expect from a marketing blurb ? ....... Phil I believe RG6 can also be used. It has even better specs from what I understand. BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? |
#9
Posted to rec.audio.tubes
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Transformer attenuators
Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. Graham |
#10
Posted to rec.audio.tubes
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Transformer attenuators
Iain Churches wrote: "Arny Krueger" wrote Using a potentiometer that is over 10 K as a passive controller is unwise. Most people seem to pick 100k. For a passive control unit ? That would be exceptionally silly, but given the audiophoolery that routinely exists surprises me not one jot. At its -6dB position, a 100k volume control presnts an output resistance of 25k to the world. A relatively modest cable capacitance of ~ 300 pF will cause a -3dB low pass filter effect at ~ 20kHz. Now that'll be audible. Graham |
#11
Posted to rec.audio.tubes
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Transformer attenuators
Ian Iveson wrote: It is not difficult, given short cables, to match a CD player with a valve amp via a potentiometer, because there can be a x100 difference between source impedance and power amp input impedance, thus allowing for 2 10-to-1 ratios. For example, a 10k pot should be just OK with 1k source and 100k load. In my view a resistor is preferable to an inductor if both work OK. There is NO *MATCHING* involved whatever. It's purely a voltage transfer process. Voltage transfer works just fine with *ANY* practical ratio of impedances where the load impedance is typically ~ = 10 times the source impedance for minimal loading ( 1dB ). Matching refers a situation where 2 equal values of source and load resistance / impedance are used to maximise *power* transfer as opposed to voltage transfer. Maximum power transfer (impedance matching) is not a requirement for good performance in modern equipment and would indeed tend to degrade performance in modern equipment were it practiced. Please use the correct terms. Graham |
#12
Posted to rec.audio.tubes
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Transformer attenuators
"Eeyore" wrote in message ... Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. Graham Why are you always trying to pick fly **** out of pepper? Maybe you should frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like to see you post something in his language. Food for thought. west |
#13
Posted to rec.audio.tubes
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Transformer attenuators
west wrote: BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? They're not connected to a long length of cable are they ? Have you completely missed the point ? Graham |
#14
Posted to rec.audio.tubes
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Transformer attenuators
west wrote: "Eeyore" wrote Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. Why are you always trying to pick fly **** out of pepper? Maybe you should frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like to see you post something in his language. Food for thought. "Better" has no definable meaning. E.g. one often hears here that SET's are better than other amplifiers types yet they are demonstrably hugely technically inferior in almost every respect. If this is to determine what 'better' means then better = worse. How about asking a question that has a possible meaningful answer instead ? Or alternatively use the word 'nice' so there's no confusion with technical accuracy. The statement "SET's are nicer than other types of amplifier" would be a more honest expression of opinion than using the word 'better'. Graham |
#15
Posted to rec.audio.tubes
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Transformer attenuators
"Eeyore" wrote in message ... west wrote: BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? They're not connected to a long length of cable are they ? Have you completely missed the point ? Possibly. How much do you consider a cable which is long? I though Phil said that RG58 (or I mentioned RG6) has negligable capacitance in reasonable lengths. west Graham |
#16
Posted to rec.audio.tubes
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Transformer attenuators
"Eeyore" wrote in message ... west wrote: "Eeyore" wrote Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. Why are you always trying to pick fly **** out of pepper? Maybe you should frequent alt.lawyers. Let's stop this bs. I know what he means. I'd like to see you post something in his language. Food for thought. "Better" has no definable meaning. E.g. one often hears here that SET's are better than other amplifiers types yet they are demonstrably hugely technically inferior in almost every respect. If this is to determine what 'better' means then better = worse. How about asking a question that has a possible meaningful answer instead ? Or alternatively use the word 'nice' so there's no confusion with technical accuracy. The statement "SET's are nicer than other types of amplifier" would be a more honest expression of opinion than using the word 'better'. Graham Ok, I'll bite. However, it's better to evaluate the spirit of a post that the letter of one, especially when the poster is from a non-speaking country. west |
#17
Posted to rec.audio.tubes
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Transformer attenuators
"Eeyore" wrote in message ... Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. OK. Just for the sake of clarity, let's use the term "technically superior" I look forward to your reply. Regards Iain |
#18
Posted to rec.audio.tubes
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Transformer attenuators
west wrote: "Eeyore" wrote west wrote: BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? They're not connected to a long length of cable are they ? Have you completely missed the point ? Possibly. How much do you consider a cable which is long? A cable inside a piece of equipment like an amplifier is hardly likely to be longer thana foot or so. There'll never be enough capacitance there to bother a 100-250k pot. I though Phil said that RG58 (or I mentioned RG6) has negligable capacitance in reasonable lengths. Depends what you call 'negligible' and 'reasonable' ! How about some numbers ? That's all that matters. Load capacitance is what messes up passive attenuators using pots. Graham |
#19
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Transformer attenuators
"Eeyore" wrote in message ... Load capacitance is what messes up passive attenuators using pots. OK. Now we are getting to the crux of the matter. Can you give us a formula and some shirtcuff calculations, Graham? Best regards Iain |
#20
Posted to rec.audio.tubes
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote Iain Churches wrote: 2. Are they actually better than a passive stepped attenuator controller? Better ? Explain what better is first ! As I've discoverd, in the tube audio world, "better" has no correlation with measured technical accuracy / performance is is merely a subjective matter. I fail to see thefore how "better" can be anything other than a personal opinion. OK. Just for the sake of clarity, let's use the term "technically superior" I look forward to your reply. Well.... as discussed, they are virtually free of the capacitive loading problems that pots suffer from. In return you have to accept step volume changes and whatever colouration the transformer introduces. Plus a hefty hit in the wallet department. Personally I'd not use one. For a simple source selector and volume control I'd use a pot followed by a high-performance op-amp buffer of course. Tubist might consider replacing the op-amp with a cathode follower, which is about as blameless as tube audio gets. Graham |
#21
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Transformer attenuators
"Iain Churches" wrote in message ti.fi... "Eeyore" wrote in message ... Load capacitance is what messes up passive attenuators using pots. OK. Now we are getting to the crux of the matter. Can you give us a formula and some shirtcuff calculations, Graham? Best regards Iain Iain: I was just about to say the same thing. Graham: I defer the numbers to you. We are talking about mainly interconnects. Were you not the one who claimed that most interconnects were the same? Let's take a very common scenario ... CD direct to tube amp. First the numbers, then best way to hook up. west |
#22
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote Load capacitance is what messes up passive attenuators using pots. OK. Now we are getting to the crux of the matter. Can you give us a formula and some shirtcuff calculations, Graham? Assuming the source impedance is lowish compared to the pot value, the maximum output impedance / resistance of a potentiometer volume control is 1/4 the pots' value. For a 10k pot , that's 2.5 kohms. If we want to be confident that the pot's resistance and the load capacitance of the output cable's not introducing any colouration we need to choose a 'minus dB' @ say 20kHz that's small enough to be inaudible. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? Doing the sums elsewhere....... That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! That doesn't buy you much cable length. For very small values of the 'minus dB' - you can closely approximate other values arithmetically. So 76 pF would be approx -0.2dB @ 20kHz (worst case) and so on So, if using a pot, BUFFER IT ! That's my advice. Graham |
#23
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Transformer attenuators
"Eeyore" wrote in message ... Iain Churches wrote: OK. Just for the sake of clarity, let's use the term "technically superior" I look forward to your reply. Well.... as discussed, they are virtually free of the capacitive loading problems that pots suffer from. In return you have to accept step volume changes These transformers are made with 24 or 40 secondary taps, just as stepped attenuators are also made with 24 or 40 steps. and whatever colouration the transformer introduces. 0.05% at +20dBu 50Hz ! Plus a hefty hit in the wallet department. I got my pair as FOC for evaluation. Actually, even if you buy from the most expensive maker in the UK, a pair of transformers cost about the same as the TKD variable attenuator, which for many is a standard choice. For a simple source selector and volume control I'd use a pot followed by a high-performance op-amp buffer of course. Tubist might consider replacing the op-amp with a cathode follower, which is about as blameless as tube audio gets. Yep. That's the concept on which my own preamp is based. Iain |
#24
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Transformer attenuators
"west" wrote in message news:zIL9i.212$ng.61@trnddc07... "Eeyore" wrote in message ... west wrote: BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? They're not connected to a long length of cable are they ? Have you completely missed the point ? Possibly. How much do you consider a cable which is long? I though Phil said that RG58 (or I mentioned RG6) has negligable capacitance in reasonable lengths. I was looking at a typical commercially-built passive controller yesterday. It had cables of 1m50 on both input and output, and a 100k DACT stepped attenuator. Iain |
#25
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Transformer attenuators
"Ian Iveson" wrote in message k... Iain asks. 1.Does anyone have experience of such transformers? 2. Are they actually better than a passive stepped attenuator controller? No. I would be interested to hear from folk who have compared the two sensibly. I am planning to do just that, when time permits. So in most circumstances this is a question about whether a transformer is better than an active preamp such as your own. In which case the issue is distortion, presumably. I am told that the tracking accuracy between channels is superior, althought with most stepped attenuators at betetr than 1% I wonder if the improvement is audible. I don't think it's a question of distortion either,and besides, the Sowter has a THD of 0.03% at +20dBu at 50Hz. There may be some folk who would prefer transformer coupling in any case, possibly because they object to using capacitors in the signal path, but it's a while since anyone owned up to this apparent fetish, and I forget how they argue their case. One thing I wonder about. It is common to use a capacitor in series at the input of a valve power amp, and this may be used to limit the LF response. If you use a transformer then arguably you don't need the series cap for its dc-blocking role. So it may seem that you can rely on the transformer also for the role of limiting LF. Would that be wise? I am concerned about the possibility of ensuing distortion. Most matchning and input transformers are not designed to have DC on them. Regards Iain |
#26
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Transformer attenuators
"Eeyore" wrote in message ... Iain Churches wrote: "Eeyore" wrote Load capacitance is what messes up passive attenuators using pots. OK. Now we are getting to the crux of the matter. Can you give us a formula and some shirtcuff calculations, Graham? Assuming the source impedance is lowish compared to the pot value, the maximum output impedance / resistance of a potentiometer volume control is 1/4 the pots' value. For a 10k pot , that's 2.5 kohms. If we want to be confident that the pot's resistance and the load capacitance of the output cable's not introducing any colouration we need to choose a 'minus dB' @ say 20kHz that's small enough to be inaudible. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? Doing the sums elsewhere....... That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! Typical cable seems to be 10pF/ft, so that represents a four-foot cable, which on most systems is probably typical. That doesn't buy you much cable length. For very small values of the 'minus dB' - you can closely approximate other values arithmetically. So 76 pF would be approx -0.2dB @ 20kHz (worst case) and so on So, if using a pot, BUFFER IT ! That's my advice. Or use a transformer? :-) Thanks Graham. I am looking forward to auditioning this pair of transformers. Iain |
#27
Posted to rec.audio.tubes
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Transformer attenuators
west wrote: "Iain Churches" wrote "Eeyore" wrote Load capacitance is what messes up passive attenuators using pots. OK. Now we are getting to the crux of the matter. Can you give us a formula and some shirtcuff calculations, Graham? Iain: I was just about to say the same thing. Graham: I defer the numbers to you. We are talking about mainly interconnects. Were you not the one who claimed that most interconnects were the same? Let's take a very common scenario ... CD direct to tube amp. First the numbers, then best way to hook up. With no level control ? The output Z of a *decent* CD player is so low that you don't really need to fret over the issue of capacitive loading. The only problem today would seem to be finding the damn output impedance data though ! This info on a Marantz suggest 200 ohms in this case. http://margo.student.utwente.nl/klaas/audio_cd.htm That would 'tolerate' 475 pF of cable capacitance for our -0.1dB @ 20kHz criterion. And this NAD has an output Z of 300 ohms ( only 317 pF of cable capacitance allowed for him ! ) http://207.228.230.231/info/NAD_C521i.pdf Using a figure of 30pF/foot for cable C, I was actually slightly surprised to see that this would be restricted to about 10 feet of cable for our suggsted -0.1dB point.. The tube amp has a high input Z so the voltage transfer will as close to 100% (0dB loss) as is practically possible. Here's what I'd use. I do use in fact. I've bought from this very supplier. It costs a very reasonable £5 ($10) http://cgi.ebay.co.uk/HQ-Twin-Phono-...QQcmdZViewItem And the 'way' to hook up. Errr, plug the connectors into the sockets. There's no magic here. Graham |
#28
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote Iain Churches wrote: OK. Just for the sake of clarity, let's use the term "technically superior" I look forward to your reply. Well.... as discussed, they are virtually free of the capacitive loading problems that pots suffer from. In return you have to accept step volume changes These transformers are made with 24 or 40 secondary taps, just as stepped attenuators are also made with 24 or 40 steps. and whatever colouration the transformer introduces. 0.05% at +20dBu 50Hz ! Plus a hefty hit in the wallet department. I got my pair as FOC for evaluation. Actually, even if you buy from the most expensive maker in the UK, a pair of transformers cost about the same as the TKD variable attenuator, which for many is a standard choice. Whose standard ? There's *no way* a variable R costs as much as tapped transformers in the real world. For a simple source selector and volume control I'd use a pot followed by a high-performance op-amp buffer of course. Tubist might consider replacing the op-amp with a cathode follower, which is about as blameless as tube audio gets. Yep. That's the concept on which my own preamp is based. I was toying with the idea of a DC battery powered CF stage actually. Rechargeable via a wall wart. Graham |
#29
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Transformer attenuators
"Eeysore" For a 10k pot , that's 2.5 kohms. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! ** You have shifted a decimal point. A 2500 ohms source with a 38 pF load gives: - 3dB at 1.68 Mhz. - 1dB at 840 kHz - 0.25dB at 420 kHz - 0.1 dB at 230 kHz Using a 10 kohms pot allows up to a 800 pF output cable to be used, with only 0.25dB loss at 20kHz - worst case. ......... Phil |
#30
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Transformer attenuators
Iain Churches wrote: "Ian Iveson" wrote So in most circumstances this is a question about whether a transformer is better than an active preamp such as your own. In which case the issue is distortion, presumably. I am told that the tracking accuracy between channels is superior, althought with most stepped attenuators at betetr than 1% I wonder if the improvement is audible. 1% is near enough 0.1dB. That's close enough for me ! I don't think it's a question of distortion either,and besides, the Sowter has a THD of 0.03% at +20dBu at 50Hz. There's more to transformers than just a simple THD figure. Graham |
#31
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Transformer attenuators
"Iain Cherchus Asinine ****wit " Typical cable seems to be 10pF/ft, ** B O L L O C K S !!!!!!!!!!!!!!!!!!! .......... Phil |
#32
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote in message Doing the sums elsewhere....... That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! Typical cable seems to be 10pF/ft, so that represents a four-foot cable, which on most systems is probably typical. The cable I use isn't 10pF/foot ! More like 3 times that. And I use longer cables too. Graham |
#33
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Transformer attenuators
Iain Churches wrote: "Eeyore" wrote So, if using a pot, BUFFER IT ! That's my advice. Or use a transformer? :-) Thanks Graham. I am looking forward to auditioning this pair of transformers. Beware of transformers' sensitivity to *source* impedances. There's usually an optimal range of source Z. Graham |
#34
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Transformer attenuators
"Iain Churches" wrote in message
ti.fi "Arny Krueger" wrote in message ... "Iain Churches" wrote in message ti.fi If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. 5 K ohm load is severe??? Hi Arny. That's a cut and paste from the maker's website, not my personal statement. You seem to lack the knowlege and candor it takes to intelligently comment obvious on weirdness, Iain. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! (a) This is a very atypical cable. (b) Iain, you shouldn't get your panties in a bunch about it. See above. I have no opinion for or against at this juncture. You posted it without commenting on how strange it was. I can however hear shortcomings in the passive controller topology, which is usually a 1m screened pair to a box with a stepped attenuator and a 1m screened pair leading out to the power amplifier. In addition to the changes in volume there are changes in timbre at different settings of the attenuator. No doubt a badly designed passive controller. If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers! Using a potentiometer that is over 10 K as a passive controller is unwise. Most people seem to pick 100k. Nonsense. 100K is a good value to use in a preamp where the volume control is followed with some kind of active buffer stage. Counter-example: http://www.mhsoft.nl/MySystem/HiFi.asp "Some important points: "Use as short interconnect cables as possible "Use cables with low capacitance "Use low attenuator resistance values (recommended total resistance: 10kOhms) |
#35
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Transformer attenuators
"Iain Churches" wrote in message
ti.fi "west" wrote in message news:zIL9i.212$ng.61@trnddc07... "Eeyore" wrote in message ... west wrote: BTW Krueger states that any pot over 10K is "unwise." If that's the case, then why do so many commercial tube amps have a 100K-250K input pot? They're not connected to a long length of cable are they ? Have you completely missed the point ? Possibly. How much do you consider a cable which is long? I though Phil said that RG58 (or I mentioned RG6) has negligable capacitance in reasonable lengths. I was looking at a typical commercially-built passive controller yesterday. It had cables of 1m50 on both input and output, and a 100k DACT stepped attenuator. There's nothing typical at all about 100K. I *own* a couple of commercially-built passive controllers, and they use 10K pots. Balanced in and out. |
#36
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Transformer attenuators
Iain Cherches Complete ASS "
Stevens and Billington say, on their website: If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! ** The authors have used very funny maths. With a 1500 ohm source and 1nF load: - 3dB is at 106 kHz - 1dB is at 53 kHz - 0.25dB is at 26 kHz With a 540 ohm source and 1 nF load: - 3dB is at 295 kHz - 1dB is at 147 kHz - 0.25dB is at 74 kHz Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low capacitance RCA leads have similar values. Takes a monster ** 50 foot long ** lead to create 1 nF. **Conclusion: The dB figures are false. The reasoning is totally false. What else would you expect from a marketing blurb ? ........ Phil |
#37
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Transformer attenuators
"Eeysore" For a 10k pot , that's 2.5 kohms. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! ** You have shifted a decimal point. A 2500 ohms source with a 38 pF load gives: - 3dB at 1.68 Mhz. - 1dB at 840 kHz - 0.25dB at 420 kHz - 0.1 dB at 230 kHz Using a 10 kohms pot allows up to a 800 pF output cable to be used, with only 0.25dB loss at 20kHz - worst case. ......... Phil |
#38
Posted to rec.audio.tubes
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Transformer attenuators
"Eeysore ****ing Pommy **** " For a 10k pot , that's 2.5 kohms. I've known ppl detect as little as -0.3 dB @ 20kHz so let's say -0.1 dB shall we ? That -0.1dB @ 20kHz will be caused by a tiny *38pF* of load capacitance ! ** You have shifted a decimal point - ****HEAD ! A 2500 ohms source with a 38 pF load gives: - 3dB at 1.68 Mhz. - 1dB at 840 kHz - 0.25dB at 420 kHz - 0.1 dB at 230 kHz Using a 10 kohms pot allows up to a 800 pF output cable to be used, with only 0.25dB loss at 20kHz - worst case. ****HEAD !!!!!!!!!!!!!!!!!!!!! ........ Phil |
#39
Posted to rec.audio.tubes
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Transformer attenuators
Iain Cherches Complete ASS "
Stevens and Billington say, on their website: If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! ** The authors have used very funny maths. With a 1500 ohm source and 1nF load: - 3dB is at 106 kHz - 1dB is at 53 kHz - 0.25dB is at 26 kHz With a 540 ohm source and 1 nF load: - 3dB is at 295 kHz - 1dB is at 147 kHz - 0.25dB is at 74 kHz Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low capacitance RCA leads have similar values. Takes a monster ** 50 foot long ** lead to create 1 nF. **Conclusion: The dB figures are false. The reasoning is totally false. What else would you expect from a marketing blurb ? ........ Phil |
#40
Posted to rec.audio.tubes
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Transformer attenuators
Iain Cherches Complete ASS "
Stevens and Billington say, on their website: If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6db attenuation, while at -20db attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3db attenuation at 20kHz for a 20db attenuation setting, practically showing the absolute permissible limit for load capacitance. The worst-case attenuation at 20kHz almost reaches 1db!!! ** The authors have used very funny maths. With a 1500 ohm source and 1nF load: - 3dB is at 106 kHz - 1dB is at 53 kHz - 0.25dB is at 26 kHz With a 540 ohm source and 1 nF load: - 3dB is at 295 kHz - 1dB is at 147 kHz - 0.25dB is at 74 kHz Ordinary co-ax cable ( ie RG59) has only 22 pF per foot - good, low capacitance RCA leads have similar values. Takes a monster ** 50 foot long ** lead to create 1 nF. **Conclusion: The dB figures are false. The reasoning is totally false. What else would you expect from a marketing blurb ? ........ Phil |
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