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#1
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RIAA EQ... after the transformer
O Rats, Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? Regards, Barry Bialos |
#2
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RIAA EQ... after the transformer
Id ita wrote: O Rats, Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If it's a perfect transformer it doesn't. Why on earth would you choose to use one ? Graham |
#3
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RIAA EQ... after the transformer
Id ita wrote: O Rats, Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? Regards, Barry Bialos The transformer has primary inductance which shunts both the load and Ra of the tube in parallel. When the reactance of the Lp = ( RL//Ra )ohms, the LF gain will be down 3dB, and so Lp needs to be a high figure to avoid bass attenuations. The shunt C and leakage L of the transformer will also affect the outcome at HF, and unless the transformer is termiated properly, a horribly peaked HF response between 5k and 50k will result. Get rid of the tranny in this location. The use of a CCS anode dc load or high value choke feed to the triode driver tube would be better with a large value cap drive to the RIAA network. Patrick Turner. |
#4
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RIAA EQ... after the transformer
Barry Bialos wrote
Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? A google for "transformer equivalent circuit" gets lots of hits. Here is one: http://www.midcom-inc.com/Tech/tn17.asp Note only the central portion of the circuit shown is relevant...see bottom of diagram. The resistances may not be very significant in your case. That leaves the primary inductance, and winding capacitance in parallel, and leakage inductance in series as shown. cheers, Ian |
#5
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RIAA EQ... after the transformer
"Id ita" wrote in message
O Rats, Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? Your system design appears to be abysmal from a dynamic range viewpoint. Part of that is simply because it is passive eq, which sucks from a dynamic range perspective, and putting a questionably linear part like a transformer before the eq just makes things worse. Looks like you are headed towards coming up with the zero-feedback SET of RIAA phono front ends! |
#6
Posted to rec.audio.tubes
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RIAA EQ... after the transformer
"Id ita" Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? ** Yep - the case of a 1:1 tranny is very easy. Just sum the primary and secondary resistances and add that to the source resistance at mid band frequencies. For a given tranny - that is the best you can do. ......... Phil |
#7
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RIAA EQ... after the transformer
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#8
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RIAA EQ... after the transformer
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#9
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RIAA EQ... after the transformer
Id ita wrote: Date: Thu, May 10, 2007, 7:31am (PDT+7) From: (Eeyore) Id ita wrote: [...] How does the inclusion of a transformer ultimately affect the source resistance? If it's a perfect transformer it doesn't. Why on earth would you choose to use one ? It's not a perfect transformer, of course. Otherwise it would raise no concerns for you or I. You can get transformers from the likes of Lundahl that are perfect enough to be of no concern. It's being considered as the sonic alternative to a coupling cap, with measurable accuracy the lesser priority. Well..... a transformer will certainly have some 'sonic colouration' and a cap won't. Depends what you want really ! Graham |
#11
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RIAA EQ... after the transformer
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#12
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RIAA EQ... after the transformer
In article ,
"Arny Krueger" wrote: "Id ita" wrote in message O Rats, Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? Your system design appears to be abysmal from a dynamic range viewpoint. Part of that is simply because it is passive eq, which sucks from a dynamic range perspective, and putting a questionably linear part like a transformer before the eq just makes things worse. Looks like you are headed towards coming up with the zero-feedback SET of RIAA phono front ends! Oh c'mon Arny it isn't as bad as all that. Back in the days before stereo discs passive equalizers were frequently used in broadcasting applications. A passive LCR equalizer was often connected between a low impedance phonograph pickup and a microphone input of the broadcast console to play transcriptions and phonograph records, it worked just fine. Japanese audiophiles seem to love these passive LCR networks, and there is, or at least recently was, at least one Japanese company still making them. Today the Japanese audiophiles seem to use them following a stage of amplification, probably because phonograph pickups of the correct impedance aren't commonly available today, and audiophile preamps typically don't have low impedance microphone inputs. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#13
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RIAA EQ... after the transformer
In article ,
(Id ita) wrote: Date: Thu, May 10, 2007, 9:27am (PDT+7) From: (Patrick*Turner) Id ita wrote: [...] The transformer has primary inductance which shunts both the load and Ra of the tube in parallel. When the reactance of the Lp = ( RL//Ra )ohms, the LF gain will be down 3dB, and so Lp needs to be a high figure to avoid bass attenuations. The shunt C and leakage L of the transformer will also affect the outcome at HF, Hi Patrick. Excuse the botched formatting. You've described how a real world transformer performs as a bandpass filter. Every coupling transformer down the chain is going to nibble off some bandwidth, and subtract from the system's aggregate response. That's a given at any location, including where an xformer is loaded with a straight resistance. One of my concerns right here is that the reactive properties of the xformer will interact with the complex network that is loading it in a frequency-dependent way, and screw the playback curve. ...and unless the transformer is termiated properly, a horribly peaked HF response between 5k and 50k will result. Conversely, I'm concerned that the network will not or cannot correctly load the secondary in order to avoid peaking. The network values must be scaled to provide this, and at the same time present a practical load to the tube. I'd like the EQ series resistor to be 7 x Rp, but no higher than neccessary. It's a juggling act. How about including some sort of conjugate network so that the transformer sees a constant constant resistive load, or at least a nearly constant resistive load? Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#14
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RIAA EQ... after the transformer
John Byrns wrote: Japanese audiophiles seem to love these passive LCR networks For the response errors ? Graham |
#15
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RIAA EQ... after the transformer
"Eeyore" wrote in
message John Byrns wrote: Japanese audiophiles seem to love these passive LCR networks For the response errors ? For the *euphonic* nonlinear distortion and noise due to the bad set of choices, vis-a-vis dynamic range? We've been here before - with SETs. |
#16
Posted to rec.audio.tubes
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RIAA EQ... after the transformer
"John Byrns" wrote in message
In article , "Arny Krueger" wrote: Part of that is simply because it is passive eq, which sucks from a dynamic range perspective, and putting a questionably linear part like a transformer before the eq just makes things worse. Looks like you are headed towards coming up with the zero-feedback SET of RIAA phono front ends! Oh c'mon Arny it isn't as bad as all that. I think it would be amusing to do a proper DBT and see whether it sounds as bad as it looks on paper. Back in the days before stereo discs passive equalizers were frequently used in broadcasting applications. Please correct me if I'm wrong, but isn't there a possibility that expectations for dynamic range on AM radio were less then than now for a 21st centruy high quality audio system ? A passive LCR equalizer was often connected between a low impedance phonograph pickup and a microphone input of the broadcast console to play transcriptions and phonograph records, it worked just fine. Weren't a lot of the recordings being played 78s, and weren't a lot of the radio stations broadcasting in AM? I seem to recall that the modern stereo LP had a lot to do with the rise of FM. Japanese audiophiles seem to love these passive LCR networks, The whims and fancies of Japanese audiophiles are legendary, and not necessarily for sound quality. and there is, or at least recently was, at least one Japanese company still making them. Today the Japanese audiophiles seem to use them following a stage of amplification, probably because phonograph pickups of the correct impedance aren't commonly available today, and audiophile preamps typically don't have low impedance microphone inputs. Just because some people want audio in the worst way doesn't mean we should give it to them in the worst way. |
#17
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RIAA EQ... after the transformer
Id ita wrote: Date: Thu, May 10, 2007, 9:27am (PDT+7) From: (Patrick Turner) Id ita wrote: [...] The transformer has primary inductance which shunts both the load and Ra of the tube in parallel. When the reactance of the Lp = ( RL//Ra )ohms, the LF gain will be down 3dB, and so Lp needs to be a high figure to avoid bass attenuations. The shunt C and leakage L of the transformer will also affect the outcome at HF, Hi Patrick. Excuse the botched formatting. You've described how a real world transformer performs as a bandpass filter. Every coupling transformer down the chain is going to nibble off some bandwidth, and subtract from the system's aggregate response. That's a given at any location, including where an xformer is loaded with a straight resistance. One of my concerns right here is that the reactive properties of the xformer will interact with the complex network that is loading it in a frequency-dependent way, and screw the playback curve. ...and unless the transformer is termiated properly, a horribly peaked HF response between 5k and 50k will result. Conversely, I'm concerned that the network will not or cannot correctly load the secondary in order to avoid peaking. The network values must be scaled to provide this, and at the same time present a practical load to the tube. I'd like the EQ series resistor to be 7 x Rp, but no higher than neccessary. It's a juggling act. Thanks very much for your interest. Regards, Barry Bialos Lets assume the transformer primary inductance is high enough at the low signal level then the sec R termination for HF damping becomes R1 +R2 of a RIAA passive filter. The sum of these values just need to be kept low enough for damping at above 5kHz. Some trial and error using a reverse RIAA filter input signal should get you a flat response between 20Hz and 20kHz, -3dB points. However the tranny will have non linear inductance at low sig levels unless its designed for the application. Billington or someone would have something, but the tranny might not like the DC. What about a balanced input, with a 1 : 1 OPT transformer? The OPT has to have CT windings, to allow CT B+ connection and balanced DC. The sec can be grounded one end for a single ended output to power the RIAA filter. A 6DJ8 with 7mA per side and Ea = 80V would be fine, and give a gain of about 30. The Ra-a = 6k approx, and Lp should be more than 95H if you want -3dB at 10Hz. If the Lp was only 9.5H, response would be -3 at 100Hz, attrocious. You have to work all this basic stuff right out BEFORE you waste all your time trying to achieve what nodody has ever bothered to do. The only way to lower transformer distortions is to drive it with the low output resistance of a cathode follower or have a loop of NFB. Soon the parts count rises, and you'll wonder why you tried.... I prefer using a CCS active load for the triode, or a µ-follower. Then the standard cap coupled passive works fine. What you are doing would be only good for MM, since MC signal is so low that the recovered signal at the RIAA output will be low and compete with noise.... Transformers are used to get a phase inversion in a power amp, or get a balanced line input to convert to se, or se output to line out, or to get an impedance transformation downwards, as in the case of power amp OPT, or to get transformation upwards, as in the case of MC to tube input, which raises voltage 10 times and transforms typical MC Rout of 10 ohms to 1,000 ohms without increasing noise much. I cannot at all see any advantage that you gain sonically with a 1:1 tranny after a triode phono input stage. Patrick Turner. |
#18
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RIAA EQ... after the transformer
In article ,
Eeyore wrote: John Byrns wrote: Japanese audiophiles seem to love these passive LCR networks For the response errors ? Could be, you would have to ask them. On the other hand there don't have to be response errors anymore than with a modern RIAA equalizer. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#19
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RIAA EQ... after the transformer
In article ,
"Arny Krueger" wrote: The whims and fancies of Japanese audiophiles are legendary, and not necessarily for sound quality. Sound quality is a secondary issue in music listening, the important thing is whatever it is that floats your boat, tastes vary, viva la difference. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#20
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RIAA EQ... after the transformer
John Byrns wrote: Eeyore wrote: John Byrns wrote: Japanese audiophiles seem to love these passive LCR networks For the response errors ? Could be, you would have to ask them. On the other hand there don't have to be response errors anymore than with a modern RIAA equalizer. Show me an inductor with a tolerance of 1% ! Graham |
#21
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RIAA EQ... after the transformer
In article ,
Eeyore wrote: John Byrns wrote: Eeyore wrote: John Byrns wrote: Japanese audiophiles seem to love these passive LCR networks For the response errors ? Could be, you would have to ask them. On the other hand there don't have to be response errors anymore than with a modern RIAA equalizer. Show me an inductor with a tolerance of 1% ! I'm not sure such isn't available, but why is it necessary? Inductors were commonly used in equalizers in the days before opamps and such became the fashion. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#22
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RIAA EQ... after the transformer
On Thu, 10 May 2007 07:19:21 -0400, "Arny Krueger"
wrote: Your system design appears to be abysmal from a dynamic range viewpoint. Part of that is simply because it is passive eq, which sucks from a dynamic range perspective, and putting a questionably linear part like a transformer before the eq just makes things worse. Looks like you are headed towards coming up with the zero-feedback SET of RIAA phono front ends! My only personal comment is that you're being way, way, way too tolerant of poor design. But that's just me. Much thanks, as always, Chris Hornbeck |
#23
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RIAA EQ... after the transformer
"Phil Allison" wrote in message ... "Id ita" Consider a standard passive EQ network, inserted after a transformer that is coupled to a triode plate. The transformer's pri/sec ratio is 1:1. For the sake of accuracy, we have calculated R'p as a source resistance and included it in the network. In practice, R'p and a few other stray factors are variable, but my question is academic: How does the inclusion of a transformer ultimately affect the source resistance? If the ratio is 1:1, the secondary Z is about equal to plate resistance, but how might transformer losses figure in? Is there a rule of thumb for adding them? ** Yep - the case of a 1:1 tranny is very easy. Just sum the primary and secondary resistances and add that to the source resistance at mid band frequencies. For a given tranny - that is the best you can do. It's ironic Phil. You get so many angry at you, yet you make a 1 sentence remark and end the discussion. Way to go. Shssh, don't tell anyone, but I am somewhat of a fan, even though I'm an autistic, criminal, f--k wit... west ........ Phil |
#24
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RIAA EQ... after the transformer
Show me an inductor with a tolerance of 1% ! If you really need to, you could use adjustable inductors. |
#25
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RIAA EQ... after the transformer
Date: Thu, May 10, 2007, 1:28pm (PDT+7) From: (John*Byrns) In article , (Id ita) wrote: Date: Thu, May 10, 2007, 9:27am (PDT+7) From: (Patrick*Turner) Id ita wrote: [...] ... I'm concerned that the network will not or cannot correctly load the secondary in order to avoid peaking. The network values must be scaled to provide this, and at the same time present a practical load to the tube. I'd like the EQ series resistor to be 7 x Rp, but no higher than neccessary. It's a juggling act. How about including some sort of conjugate network so that the transformer sees a constant constant resistive load, or at least a nearly constant resistive load? Regards, John Byrns -- A resistive divider across the secondary would simplify several factors- it would swamp out changes in Rp, for one. But the loss factor would be fatal to gain and noise. I'd like to drive the filter from a 600 ohm winding, for that matter, but it's unaffordable without adding an extra stage. Regards, Barry |
#26
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RIAA EQ... after the transformer
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#27
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RIAA EQ... after the transformer
John Byrns wrote: Eeyore wrote: John Byrns wrote: Eeyore wrote: John Byrns wrote: Japanese audiophiles seem to love these passive LCR networks For the response errors ? Could be, you would have to ask them. On the other hand there don't have to be response errors anymore than with a modern RIAA equalizer. Show me an inductor with a tolerance of 1% ! I'm not sure such isn't available, but why is it necessary? For an accurate RIAA curve. In comparison 1% resistors are commonplace and 1% or 2% caps aren't that tricky to find either. Graham |
#28
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RIAA EQ... after the transformer
robert casey wrote: Show me an inductor with a tolerance of 1% ! If you really need to, you could use adjustable inductors. Ok, how about tempco then ? Graham |
#29
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RIAA EQ... after the transformer
John Byrns wrote: Inductors were commonly used in equalizers in the days before opamps and such became the fashion. They still are used by some ppl. I've just been working on a 'boutique' eq section that uses LCR filters and op-amps. It's a replica of a Neve design from the late 70s. Graham |
#30
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RIAA EQ... after the transformer
In article ,
(Id ita) wrote: I submitted a design problem that was not intended to endorse any one particular approach. I have no manifesto, and would not have posed my questions had it been assumed the approach in mind was workable. So if you considered the approach to be unworkable, why did you pose your questions in the first place? -- Surf my web pages at, http://fmamradios.com/ |
#31
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RIAA EQ... after the transformer
"Id ita" wrote in message
Date: Fri, May 11, 2007, 3:37am (PDT+7) From: (Chris Hornbeck) On Thu, 10 May 2007 07:19:21 -0400, "Arny Krueger" wrote: Your system design appears to be abysmal from a dynamic range viewpoint. Part of that is simply because it is passive eq, which sucks from a dynamic range perspective, and putting a questionably linear part like a transformer before the eq just makes things worse. Looks like you are headed towards coming up with the zero-feedback SET of RIAA phono front ends! My only personal comment is that you're being way, way, way too tolerant of poor design. But that's just me. Much thanks, as always, Chris Hornbeck The only purpose of Arny's troll, it seemed to me, was to superimpose his belief system on his presupposition of my belief system. An irrelevant response deserved no reply. Yeah sure, I'm just commenting here in order to make trouble among the holy and sacred. I'm sure that's what you think. I submitted a design problem that was not intended to endorse any one particular approach. Good, then when you hear some negative comments, you would be happy to consider them carefully before you dismiss them. Trouble is, you didn't do that. In fact you immediately dismissed the negative comments. I have no manifesto, and would not have posed my questions had it been assumed the approach in mind was workable. It surprises the hell out of me, Chris, that you would ignore that distinction to recieve your portion of the prey. Id; your defensive, hostile, dismissive response makes everything you've just said, say shall we say, highly questionable. |
#32
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RIAA EQ... after the transformer
Date: Fri, May 11, 2007, 7:22am (PDT+2) From: (John*Byrns) In article , (Id ita) wrote: **I submitted a design problem that was not intended to endorse any one particular approach. I have no manifesto, and would not have posed my questions had it been assumed the approach in mind was workable. So if you considered the approach to be unworkable, why did you pose your questions in the first place? -- Correction: I would have not posed my questions assuming the outcome of the experiment for better or worse. It was expected that some readers would dismiss this idea right off the bat for ideological reasons, or preferably some technical reason that would benefit me to know. That comes with the territory. I build phono amps continually, and pick a different poison every time. Hitting a design wall-- a useful truth-- has come to be painless. Regards, Barry |
#33
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RIAA EQ... after the transformer
In article ,
flipper wrote: On Fri, 11 May 2007 07:22:45 -0500, John Byrns wrote: In article , (Id ita) wrote: I submitted a design problem that was not intended to endorse any one particular approach. I have no manifesto, and would not have posed my questions had it been assumed the approach in mind was workable. So if you considered the approach to be unworkable, That isn't what he said. That was the way I parsed it, but then I was never very good in English class. He said "would not have posed my questions had it been assumed the approach in mind was workable." To me that sounds like another way of saying that he considered the idea unworkable. Haven't you ever had an 'idea' you weren't sure of, either way? Sure but in that case I would have said I didn't know if the idea was workable or not, and I was looking for some help in figuring out if it was workable. why did you pose your questions in the first place? He's probably asking himself the same thing by now. I thought it was an interesting question, and worthy of discussion until he said he didn't consider it workable. Unfortunately, workable or not, there are too many in this group, like Arny, who consider ideas like this to be unworthy on religious grounds, and put them down on that basis. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
#34
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#35
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RIAA EQ... after the transformer
Id ita wrote: Date: Fri, May 11, 2007, 7:22am (PDT+2) From: (John Byrns) In article , (Id ita) wrote: I submitted a design problem that was not intended to endorse any one particular approach. I have no manifesto, and would not have posed my questions had it been assumed the approach in mind was workable. So if you considered the approach to be unworkable, why did you pose your questions in the first place? -- Correction: I would have not posed my questions assuming the outcome of the experiment for better or worse. It was expected that some readers would dismiss this idea right off the bat for ideological reasons, or preferably some technical reason that would benefit me to know. That comes with the territory. I build phono amps continually, and pick a different poison every time. Hitting a design wall-- a useful truth-- has come to be painless. Regards, Barry I understand Barry. I never build an amp which is exactly like the last one either, and I don't like boring myself to death. But using a 1:1 tranny to couple an RIAA filter to a tube is a challenge alright because if the issue of the primary L mainly, and the loss of LF gain. Possibly you could exploit the leakage L of the tranny as the HF attenuation element. And possibly you could get best possible bass performance if you used a 12AX7 for gain then with a cathode follower output, say 12AU7, and then Rout will be less than 1,000 ohms to allow Lp to be 16H at 10Hz. I can't see what you'd gain in sound quality the tranny and would put the effort into balanced and/or using j-fets to reduce noise. Patrick Turner. |
#36
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RIAA EQ... after the transformer
Ok, how about tempco then ? It'll drift out more than 1%? Okay... |
#37
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RIAA EQ... after the transformer
"robert casey" Ok, how about tempco then ? It'll drift out more than 1%? Okay... ** No if made right. Ferrite inductors were widely used for 31 band graphic equalisers, master tuning oscillators in early electronic organs ( 12 of them, one for each semitone ) and precision notch filters with only a few Hz bandwidth in 10 kHz in AM tuners. In all cases, drift was well under 1% for the room temp range. ........ Phil |
#38
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RIAA EQ... after the transformer
Phil Allison wrote: "robert casey" Ok, how about tempco then ? It'll drift out more than 1%? Okay... ** No if made right. Ferrite inductors were widely used for 31 band graphic equalisers, master tuning oscillators in early electronic organs ( 12 of them, one for each semitone ) and precision notch filters with only a few Hz bandwidth in 10 kHz in AM tuners. In all cases, drift was well under 1% for the room temp range. Is this ferrite the similar sort used in ferrite cores shaped like cotton reels for speaker crossover inductors? What is the distortion like in such cores? In my own experiments making bridged T LCR notch filters to filter out fundmental F from a sample signal, the ferrite core i first tried was the ferrite rod antenna stuff from DSE/Jaycar, and it sure made the Q higher but introduced some 3H. Even air cored windings reacted with a steel case to produce 3H, unless carefully placed. When I began to try to measure THD down to 0.002%, using L anywhere was a PITA, and opamps with R&C filter elements only were more reliable for low THD and noise. So anyone using L in an RIAA filter should worry about the distortion caused by the iron. Hum pick up is another worry in coils used for low level signals. The other phenomena is that when testing THD using an LCR notch filter, say with 40Vrms of amp signal. My notch filter uses air core L only. I can adjust the phase of the oscillator signal and the phase of the 1kHz notch filtered waves to within a few degrees, and thus get a very deep null of the fundemental right down to below the noise floor, maybe -100dB, 0.4mV, but then when the input signal being tested is reduced to 2Vrms, the oscillator and notch filter have to be re-adjusted finely with the extra fine set pots to get the deeper null. Seems to me the L or the C values change slightly with applied voltage to cause a change to the Fo, maybe by only a fraction of a Hertz, but its enough to prevent accurate THD readings with signal changes unless I constantly adjust for deep null, while monitoring with a CRO to make sure I have the null I want. Patrick Turner. ....... Phil |
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RIAA EQ... after the transformer
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#40
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RIAA EQ... after the transformer
Id ita wrote: Group: rec.audio.tubes Date: Sat, May 12, 2007, 12:45am (PDT+7) From: (Patrick Turner) Possibly you could exploit the leakage L of the tranny as the HF attenuation element. That would be some leak. It would be easier to exploit for the 3µS rolloff you mentioned in another thread. Wouldn't the turnover tend to wander with changes in the plate resistance? You'd lose the advantage of C2 attenuating some of the noise generated across R1+R2, plus everything else upstream. Quad exploit leakage inductance in their ESL57 step up tranny to give attenuation of HF above quite a low F. A 1:1 tranny wound on a C core with windings well apart on each leg of the core would givs a lot of leakage L, and the more turns put on for a high amount of Lp, the greater the LL, which is proportional to Lp. Frankly, I think it'd be difficult to get the LL to give an exact 75uS time constant, and R&C are so much damn easier and precise. And possibly you could get best possible bass performance if you used a 12AX7 for gain then with a cathode follower output, say 12AU7, and then Rout will be less than 1,000 ohms to allow Lp to be 16H at 10Hz. So far, would rather use a single stage with R'p ~3K-10K, which still allows the transformer to be a tenable beast. I did listen and learn from your suggestion to make the stage balanced in the primary to avoid inadequate L at low levels. Considering that also for the external noise rejection benefits of a balanced cartridge input, and maybe reduced sonic artifacts from the power supply/decoupling components. The power supply noise simply should never be a problem because of the excellent filtering and/or regulation given to working rails in phono stages. See http://www.turneraudio.com.au/preamp...hono-2005.html Balanced design doesn't reduce noise unless it is common mode, and one has to minimise it at the input first before anywhere else. Hum pick up by stray magnetic induction pick up is very easy wherever you have a transformer in an input stage, so the tranny and amp should be within a thick steel case and shielded. I have never built a pukka balanced phono stage which performed to justify the extra components and circuit complexity, and it seems pointless whe at the output someone connects an SE amplifier to make the speaker sound. The SE phono circuits i routinely make have less than 0.05% N&D maximum at any peak listening levels, and the thd in the power amp/speakers will always be a lot more. The input fet abolishes the noise problems one gets with tube phono inputs without a step up tranny and MC. Allen Wright has more to consider at http://www.vacuumstate.com Patrick Turner. Thank you again. Barry Bialos |
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