dBspl
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Posts posted by dBspl
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So I was down in Hope earlier this week and Jim Hunter was showing me a few things in the museum. We were down in the museum basement and I was just looking through some of the items in storage and noticed a piece of tube gear stuck in back of one the storage racks. We pulled it out for a better look, and it turns out that it is the PWK 6A5G amplifier based on the schematic shown above.
It looks to be in pretty good shape. It is missing the output transformer and the rectifier tube has separated from its base. As you can see, the chassis is designed to be mounted in an equipment rack (typical PWK). I'm guessing, but this is probably his laboratory measurement (and probably listening) amplifier from the late 1940's to early 1950"s.
dBspl
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I heard PWK remark several times that he couldn't hear above 5kHz. Of course, he was in his 80' and 90's at the time. He would say it was due to "shooting too many 45"s without hearing protection". But he was an experienced listener. I recall one time Hunter had purchased a vintage (& working) RCA theater speaker and amplifier for the museum. Before the speaker and electronics were moved to the museum he wanted to get an anechoic chamber measurement. We had positioned the speaker on the chamber door and decided to listen to it first. The loudspeaker when positioned on the door actually faces into the main part of the lab, so it actually makes it possible to perform a listening test from this perspective. Then you simply swing the chamber door to position the speaker inside the chamber to take your measurements. Anyway, PWK walks into the lab about this time, and as usual, takes an interest in what we are doing. He listens to the speaker along side us...and then walks over and grabs a piece of plotting paper (this was in the days when frequency response measurements were still made using X-Y plotters). He takes out a pencil and draws the frequency response of the loudspeaker we were listening to (and hadn't been measured yet). After a few more minutes of listening we placed that piece of plotting paper on the plotter and take the measurement. PWK's hand drawn curve was exactly right. I was a young guy then and I remember wondering how he had done that.
Although I probably haven't seen that curve in almost 30 years, I'm sure that piece of plotting paper is still in the archive somewhere.
dbspl
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4 hours ago, Chris A said:
I think this was the source of the problem--the idea that you can do something to the driver/horn assembly to change "its sensitivity". You can't change the horn/driver sensitivity unless you stick something into the mouth/throat of the horn or driver exit (thus lowering acoustics output in terms of SPL or power, and thus lowering "sensitivity"). It's set, whether collapsing polar or constant coverage horn displays frequency-varying "sensitivity' measurements.
If you're talking about measuring "sensitivity" at any point upstream than just across the driver's terminals (implying either a passive or active crossover...or any other internal gain or attenuation devices of any kind...are inserted into the circuit), then to make any sense of the word "sensitivity", you have to
1) tell the reader if you're doing that, and "sensitivity" no longer is being measured across the driver's terminals
2) tell the reader what kind of crossover (active or passive) is being used, and
3) tell the reader how much gain (positive or negative) is being used as a function of passband frequencies-for all frequencies being used.
Any any case, you're not actually changing the sensitivity of the horn/driver, you're changing the place and electrical/electronic circuit from which you're measuring "sensitivity".
Chris
I'm simply using a voltage reference to state sensitivity. Since an amplifier is a constant voltage source, you can "de-sensitize" the driver/horn combination however you wish. Your point is that the power response doesn't change, which is true, but I also mentioned that in the second paragraph of my original post.
Kerry
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It seems like we're making this out to be more complicated than it is. For a given compression driver, the on-axis sensitivity of a constant directivity horn will be less at higher frequencies than a similar horn with a collapsing coverage pattern. This difference in sensitivity is inherent in the driver/horn design. If the target magnitude response is the same in both cases, then that difference has to be made up somewhere - regardless how you go about it. As a result, you should always be able to measure that difference at the driver terminals.
The constant directivity horn will always require more power at high frequencies to achieve the same on-axis SPL.
Kerry
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On February 5, 2017 at 10:18 AM, jazzmessengers said:
This is something I've read from a few different posters on another forum, but with no measurements to back it up. What they're saying is a compression driver that needs to be EQ'd quite a bit, for instance the EQ required for a controlled directivity horn in the treble will diminish the sensitivity. How accurate is this statement, and how much sensitivity loss is there?
My guess at what they're getting at is that for a given compression driver, a constant directivity horn will typically be less sensitive (on-axis) at high frequencies than a horn with a collapsing coverage pattern. The difference in sensitivity will follow the difference in Directivity Index between the two horns. Since equalization is typically referenced to the least sensitive part of the horns useable passband - it follows that the constant directivity horn will be equalized to an overall lower sensitivity.
If the equalization is done properly - the efficiency of the system in both cases should be retained. And if the horns are of similar size, the power response in both systems should be very similar, too. My point here is that you're simply sacrificing sensitivity for a wider coverage pattern at high frequencies. However, nothing else has really changed.
Kerry
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Seti,
You might get there before I do. I have another project that I committed too before I saw this one.
I'm not very familiar with tube circuits, but it hit me as odd the phase inverter circuit is also the front end of the amplifier.
DrWho,
I understand what you're saying. I suspect it would also be a challenge for me to get up to speed on the software, but I may take a look at it. I did find a couple of 6J5 and 6SJ7 tubes (metal envelope) in a box of old tubes I have. They tested good, too.
Thanks,
Kerry
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It probably wouldn't hurt to go through it and check for a cold solder joints, too.
Kerry
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I thought you guys might enjoy this...
I've probably passed by this picture in the engineering lab in Indy a 1,000 times. It finally dawned on me one day as I passed by it that it's an amplifier schematic drawn by PWK's own hand. Hunter was able to find the original drawing the picture was based, but unfortunately the drawing isn't complete. First, the power supply schematic is missing, and second, the amplifier schematic doesn't show an output transformer. There's also a puzzling comment about "crossover output (to speakers)", which doesn't make a lot of sense. Anyway, I was thinking it would be cool to build this amplifier given the connection to PWK.
The 6A5G output tubes are very similar to the 2A3, and the output is in the range of 10 - 15 watts push-pull. I'm guessing the schematic was drawn in the early 50's, but going all the way back to the early 40's seems plausible, too. It's little surprising the schematic isn't dated since PWK rarely wrote anything down without putting a date on it. Maybe the date is with the part of the drawing with the power supply schematic.
I'm planning to take a shot at designing a power supply, but I've never really done that before and I may need some help. The C+ & D+ voltage supply isn't specified, but based on the 6A5G data sheet, it looks like the C+ voltage should be around 325. The D+ supply looks like it needs 300 volts based on the 6J5 & 6SJ7 tubes. I'm not sure what the screen supply voltage on the 6SJ7 should be. It could be as much as 300 volts, but the little schematic shown on the lower left hand side of the drawing could be indicating the screen voltage should be 100 volts. I don't know.
From what I can tell, the output transformer primary needs to be 3000 ohms?
Kerry
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If the problem is common to both channels, then it's likely the power supply since that's the only part of the circuit both channels appear to share..
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I'm no expert, but I would start with the power supply to confirm all supply voltages are good. But it sounds like a leaky coupling capacitor in the output stage (charging & discharging).
I've owned a PV3 since the mid-80's. It's a great little pre-amp. I still use mine regularly.
Kerry
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Here's the demo pair of Forte III's setup in the Klipsch booth. We've already had great feedback and the show hasn't even officially opened yet!
I'll grab a better picture of the Forte III in California Black Walnut (also on display) and post it when I get a chance.
Kerry
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On December 28, 2016 at 11:30 PM, Chris A said:
The meaning of "collapsing polar midrange horn" is that below a certain frequency, the midrange horn loses its ability to control its acoustic output in one dimension or the other (i.e., vertical or horizontal). This means that its acoustic energy in that dimension (i.e., the vertical in the case of the K-400/401 midrange horn) goes all over the loudspeaker and the nearfield objects in that room. The fuzzy stuff across the top of the loudspeaker attenuates the acoustic energy with the highest SPL that's bathing the top of the loudspeaker from the midrange horn--the areas that are closest to the midrange horn mouth. That acoustic energy is being re-radiated by the top of the loudspeaker, thus unbalancing the sound of the loudspeaker to make it sound harsher and less coherent. Early reflections are not desirable since they unbalance the sound (which is part of a somewhat more complex subject of psychoacoustics). Absorbing the acoustic energy before it can be reflected/re-radiated audibly helps the overall sound and imaging.
While what you say is true, it's probably not quite as bad as you might think. At 400Hz the output is the sum of the midrange and woofer horn together. The apparent size of both apertures will help control vertical coverage. And because the summation continues (to a lessor degree) above and below the crossover point, the vertical coverage will be narrower than the if the midrange were operating by itself. The woofer horn should have some measurable effect on up to 800 Hz, or so.
Kerry
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17 hours ago, HDBRbuilder said:
If I remember correctly, in the demo room, that TASCAM replaced a Revox RTR...I also remember PWK having a Revox RTR at home in the 1970's.
Yes, the Tascam was the unit used for the slide/demo they had in the front office. PWK had a Revox B77 at his house, which is probably the one you saw. I believe the other unit was a Tandberg.
Kerry
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From at least the mid 80's to the end of his life PWK used a Crown D75 to power the left/right Khorns and a BGW Model 100 to power a Belle center channel. I want to say he used a McIntosh C28 pre-amp at some point, but I don't recall seeing it by the mid-late 90's.
I was probably the last person to go through the system and get it setup properly after he became too feeble to deal with it. I don't know what happened to that equipment after his death. He had a couple of really nice reel to reel machines, as well.
There was a Marantz 8b in the museum, but I don't know its history. The model 7 pre-amp sat on a shelf in lab unit just a few years ago. I remember hooking it up once a long time ago just to see if it still worked - but it had issues.
Kerry
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9 hours ago, garyrc said:
The advertising spec sheet for the Klipschorn has it at 105 dB @ 1m @1watt. What would it measure at 2.83v, and would that be across the rated bandwidth, or what? I understand it is measured at 1/8 space. Thanks.
If you look at the current data sheet you will see that it is referenced to 2.83v and the sensitivity is still 105dB.
The test bandwidth is typically 100Hz - 10kHz using pink noise. The 100Hz cut-off is just a limitation of the test chamber.
Kerry
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2 hours ago, DrWho said:
Depends on the amplifier output impedance
Several of them flea powered amplifiers are matching output impedance to the source impedance (or at least they think they are).
This is of course obfuscating your point, which isn't intentional. Good amplifiers are constant voltage sources and that's the assumption throughout the entire audio path. For some reason some audiophile brands decide the constant voltage encoding no longer applies. Of course those are the same guys complaining about swamping resistors that flatten their frequency response.
You could always go to reactive components to flatten the impedance response - and then theoretically there's no real power loss. That might require unobtanium parts though. I've not bothered running the numbers because those passive parts are too non-linear for my taste. I like that clean clinical sound
I can just imagine how well it would go over if I bought a few 3.5 watt SET amplifiers to replace the audio amplifiers in our test racks. Nothing would ever test quite right, but on the other hand, the sine waves would sound more life-like.
Kerry
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37 minutes ago, DrWho said:
It's also worth noting that measuring SPL at 1m isn't a very good test because you're in the nearfield behavior of the speaker at that point - which will skew results even further. A 10W/10m test yields the same number as 1W/1m for a truly spherical source. This has become a somewhat newer "standard" for measuring sensitivity. It's probably better to say that's a more common trend. Very large line array PA systems would be another animal entirely and should probably be measured even further away 100W/100m? I've see a rule of thumb for 10x the acoustic size of the speakers (so if your speakers are 1m tall, then measure 10m away).
For the sake of clarity, Klipsch does measure the sensitivity at 3 meters in the chamber and then we convert it to the 1 meter equivalent (add 9.5 dB). The 3 meter measurement is a fairly standard distance for most loudspeaker measurements.
We've been using the 2.83V reference for quite a while. The 1 watt reference is really meaningless for the reason you point out. In addition, amplifiers are a constant voltage source...and not a constant power source. It's hard to screw up a voltage setting since it's as simple as setting a dial. Whereas the 1 watt reference is almost never really 1 watt, and very prone to setup errors.
Kerry
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On November 20, 2016 at 6:32 PM, wdecho said:
I am aware of the transmitting 845 tube but have never heard one used as an amplifier tube. I am talking about the 45 tube that was used abundantly for radio use years ago.
http://vinylsavor.blogspot.com/2011/09/tube-of-month-45.html
Tons of info using google. I do have a few other SE tube amps that sound real close one being Maynard's "Little Sweetie."
My 45 tube amp is built around this schematic with some changes I would call improvements.
http://www.electra-print.com/45silver.php
I've been looking at one of those electra-print 45 schematics to build. But it's the other 45 schematic in his list. I was attracted to the minimalist design and the fact there are no coupling capacitors. I'm in the process of slowly gathering parts. I haven't dropped the money on the OPT and PS Xfrmr yet. I'm still debating whether I really want to spend that kind of money on another amp. It gets expensive - quick.
Kerry
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1 hour ago, Deang said:
Jim had mentioned "real room vs. anechoic ..."
I brought up the same issue a decade ago regarding the RF-7, where Sound and Vision published 99dB/2.83v to the Klipsch 102dB figure. What we learned then was that Klipsch was factoring in room gain, which is typically 3-4 dB.
That is correct. All Klipsch loudspeakers include a factor of 4 dB for room gain. This would be equivalent to the SPL you would measure at 1 meter in an average listening room. The specification has been in use longer than I've been with the company (30 years), and I believe it originated with the implementation of the anechoic chamber (around 1980). Measurements before that were always made in listening rooms and I would imagine the engineers at that time needed a way to equate what they measured in the chamber with the real world. At that time a free space measurement was of little use, and of course, it isn't the way you normally listen to loudspeakers. You also have to keep in mind (early on, at least) Klipsch considered the room boundaries as an integral part of the equation by suggesting the loudspeaker should be positioned in the corner. The Klipschorn takes it a step further by actually using the corner of the listening room as an extension of the horn.
Kerry
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By the time I left engineering in 1994 the K-403 didn't exist yet. I believe it came much later. PWK was still experimenting with exponential horns over the period I was working in the lab.
The "cone-dome" diaphragm was basically the shape of an inverted loudspeaker cone. The problem was that the diaphragm side-walls were way too flimsy and there really wasn't much we could do to improve it.
PWK wanted a 2-way Klipschorn, but of course it had to be an improvement to the current 3-way design. I'm pretty sure PWK didn't believe there was a sufficient horn/compression driver combination available, at that time, that would fit his requirements. So he embarked on his own horn & compression driver designs.
Kerry
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As I recall PWK had at least three different projects that he spent a lot of time on. There was of course the extreme slope filters that he was preoccupied with most of time. There was the HF horn design for a two-way Klipschorn. And then there was the 3" compression driver design that was originally conceived by PWK as using the "Cone-dome " diaphragm geometry. It didn't work, but it transitioned to a more traditional design that later became known as the K-1132.
Kerry
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9 hours ago, Deang said:
Once again, I produced Al's white paper which neither you or Kerry want to address.
I'm not exactly sure what white paper you're talking about -- but I'm not inclined to read it because it's a solution I would never consider using. If I needed that kind of control over the loudspeaker, I would prefer to apply it to the front end of the amplifier where I have infinitely more control, and I don't have to worry about compromising the integrity of the passive crossover.
Kerry
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5 hours ago, Deang said:
Mike, I've built many of these type networks, and you're the first person to ever complain about this "problem" in 15 years. I have people running 45 and 2a3 SET with no reported issues.
I understand that there is always a cost involved, but in this particular case it's quite small. I paid John Warren to run a series of tests for me, including a test to determine the impact of the swamping resistor - it was minuscule - and he was unable to reproduce the results shown in the published plot submitted in "The Problem With Attenuators".
Al and me may have our issues, but I still like the earlier Universal and Super AA designs. They provide those who like to experiment with different horns and drivers a very simple and effective means of balancing the outputs of the drivers.
I agree with Honeybadger - this is largely hair splitting, and making something out of nothing.
I didn't see any "facts" presented, just subjective opinions. I posted Al's white paper on this topic - and no rebuttal was offered.
We can all enjoy this hobby in whatever way that makes each of us happy. And if this is a solution you're happy with, then so be it. But I'm not seeing how you take a normal 30 ohm load down to 8 ohms (from 500 Hz up) and claim it's insignificant, or minuscule. It looks like a significant percentage of the amplifier power above 500 Hz is simply being dumped in the resistor. It just hits me as a crude solution.
Kerry
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12 hours ago, mikebse2a3 said:
It's not decreasing the efficiency of the loudspeaker but it does add an additional load on the amplifier in the squawker frequency range. It's this additional load to low power amplifiers that the users of these amplifiers should be aware of and should take that information into consideration with their unique requirements/expectations from their systems.
Doug, I had the ALK networks over a year and tried several attenuation levels including one that closely matches the AK-3 level so no it wasn't a factor in my observation. This is simply a case of a limited power source (ie: Low Power Amplifier) being asked to drive an additional load (ie: swamping resistor) and the fact that there will be a reduction in available headroom in the squawker region with this method.
miketn
It does decrease the efficiency of the loudspeaker system (which includes the network). It doesn't have any bearing on the efficiency of any individual driver component (by itself).
15 hours ago, Doug said:This is super cool! Thanks for sharing!
In one of AL K's unnecessarily abrasive rants on his website, he points out that the total efficiency of a Klipsch heritage speaker is determined by the woofer, not the squawker since the mids are always attenuated in some way to make up for higher efficiency of the squawker vs. the woofer. Since there is no attenuation in his universal network between the input and the woofer, it seems to make sense (at least in my physics 101 level of electronics understanding) that the swamping resistor shouldn't decrease the overall efficiency of the loudspeaker.
The woofer doesn't determine the total efficiency of a loudspeaker - it essentially establishes a baseline for the sensitivity of the loudspeaker.
Loudspeaker sensitivity and efficiency are two different things.
Kerry
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PWK Amplifier Schematic
in Talkin' Tubes
Posted
I attached the only pictures I took of the underside. It's not very pretty. The #3 photo is the power supply and output tube side of the chassis. The #4 photo is the pre-amp and driver side of the amp, although this side also includes the (missing) output transformer. There is an discrepancy that I need to figure out. Jim confirmed the metal tubes are 6SJ7's as shown in the schematic (driver tubes), but the there is no 6J5. This is odd given the 6J5 in the schematic serves as the phase splitter. I'm guessing the front-end of the amplifier has a different topology than the schematic. It could be an earlier version?
I believe the rectifier is a standard 5U4G. The 6A5G is rated for 10 watts in class AB. A 10 or 15 watt output transformer with a 5000 ohm primary would probably be correct.
dBspl