Bottlehead Forum
General Category => Technical topics => Topic started by: pro_crip on March 31, 2011, 03:12:45 AM
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I've heard mention again and again that replacing resistors in the power supply is a good thing. What exactly are the guidelines for doing this, and if it's too lengthy to respond here where would I read up on it. I don't suppose it's as easy as pulling out the resistor and dropping in a triad c-7x. Is there some sort of formula to follow, like replace x ohms with y mH? Thanks for listening.
Rich
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It was often done to the FP 2 (no specific kit mentioned here). In the case of the FP 2 you replaced a 1k ohm resistor in the power supply, or both, with Hammond 155J choke (15H, 1026 ohms, 30mA, 400V). The choke rejects noise that the resistor will simply pass. It also passively regulates current. The 1026 ohms (DC resistance) drops the same voltage as the 1000 ohm resistor.
I don't know the specs on the Triad C-7X. I have used the Hammond.
I'm not sure where to put the choke if you choose only one. Maybe PJ will weigh in.
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The C7-X has a resistance of 270 ohms, and 270 ohm resistors are used in several Bottlehead amps. The current rating of the C7-X is 90mA, more than enough for any of our current products. The Paramount power supply board has holes for a choke such as this one to replace the 270 ohm 5 watt resistor. The out-of-production Paramour II also has a 270 ohm resistor, and since it had the same chassis plate as the Paramount, there is a space behind the power transformer to mount a choke; this was part of the iron upgrade, re-purposing the original plate choke. The Stereomour in 2A3 form uses two 270 ohm resistors, one per channel. The Extended Foreplay III uses four 270 ohm resistors in the power supply (though the C7-X is larger than necessary in that application).
A caution is in order; if the choke is not positioned and oriented optimally, it can pick up 60Hz hum from the power transformer magnetic field, even while reducing 120Hz ripple in the high voltage. I have no data on how severe a problem this might be in practice, since I've never tried (at least, not on purpose!) using an inappropriate location. The Paramour II is the only case where a suitable location and orientation is built into the design.
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Would the correct orientation be at right angles with the power transformer? It looks like I could also use one in the eros as well. Thanks.
Rich
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Placement and orientation trade off. The easy way is to make the coil axis perpendicular and the location symmetric - look at the SEX amp for a fairly clear example. But other locations will still have an optimal orientation, though it may not be "square" on the chassis.
Yes, I forgot Eros.
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So if the Paramour II is the only one with a suitable location specifically designed into the amplifier, is it a good idea or not to replace the 270 ohm resistor on the Paramount power supply board with a choke? I.e. is it more likely to pick up 60hz hum in that application than the 120hz ripple it reduces? I know you said you haven't tried it so this is probably an unfair question, but did you not try it because you thought the negative effects would be likely or maybe the potential upside isn't worth the cost? Just wondering what your best guess would be knowing full well we can't know for sure without sticking one in there.
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Back when Paramount was being developed, I had in mind the cheapest possible 300B amp. Later, it looked like perhaps a very high end amp - like, a $5000 amp. Eventually of course we settled on the middle ground. The high-end version was to have had that choke, which would sit where the current output transformer sits - there was to have been a much larger output transformer mounted on top of the chassis. The PC board was designed around that time, and it retains the optional terminals for a choke. The current chassis plate however is not designed for that option.
So yes, I expect the change would be an improvement. Not a big one like better coupling and parafeed capacitors, but probably audible to most careful listeners.
The thing is, to make it practical requires some experimenting and forethought as to where to place the choke - especially, keeping in mind that it is at the full B+ voltage and the wires - even if they are insulated - should not be accessible. A simple slip of a screwdriver, or a cat's claw, could punch through the insulation, for example.
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Safety's the easy part. My listening room is a closed room and I'm the only one allowed in there. No pets, no quadruped infants, no one. It's my sanctum so to speak.
Rich
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I remember a post by Jim where he had a loaner set of Paramounts and the C7-X was mounted above the filiment choke on the back side of the transformer. Seems like the mounting holes would be the same or close to it. Would only need to drill one hole for the leads. Not sure if this would be the ideal location but it would be the easiest.
There were no comments as to any 60 Hz noise or not.
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Looking at the picture in the manual of the completed amp, I was thinking you could super glue it on top of of the filament choke. ;) But seriously, it seems way to crowed on the bottom of the chassis, mounting it on top seemed like another good option if it wouldn't be sitting too close to the power transformer itself.
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Being symmetrical and with a perpendicular axis, on top of the filament choke would be a good position for low hum pickup - though it might pick up some from the filament choke, which has the same axis. Only one way to tell!
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Nice! Just ordered two from Mouser. It will be a tight fit but worth a shot.
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I ordered my pair as well. We can meet back here and compare notes...
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I like to modify my equipment. So I ordered one to try in the Eros
Deb
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My Paramounts have top mounted opts so I will place the chokes in the original opts space. Some small spacers may be required to clear the opts wire and hardware.
Deb, where will you put the one in the Eros?
Anyone trying em in a FP3? I would think the adequate position here would be on the sides of the PT. In the same positions as the Sex amp parafeed chokes. Some spacers would be useful too to clear the wires coming to the front of the chassis.
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It looks like there is a space just to the left of the power transformer (looking from the top) it may just fit.
Deb
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Forgive me if this is a really silly question but I am curious. When talking about shielded wires the ones we use for building from bottlehead seem to have an inner foil. If I wrap the wires of the choke in aluminum tin foil and then put shrink unsulation over it to make sure it can't short anything out, am I providing extra protection from hum pick-up?
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Forgive me if this is a really silly question but I am curious. When talking about shielded wires the ones we use for building from bottlehead seem to have an inner foil. If I wrap the wires of the choke in aluminum tin foil and then put shrink unsulation over it to make sure it can't short anything out, am I providing extra protection from hum pick-up?
No. Aluminium provides virtually no protection against magnetic fields.
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Oh well, thanks for the response!
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I don't know. 12-24" maybe?
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I put a Triad C-7X choke into my Eros.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2FP1000344-2.jpg&hash=47ff975ce1ee5df69981c198117076d71db8b222)
Fairly easy to do, just had to drill two holes, remove the resistor and solder in the two leads. I also ran a ground wire from a solder lug I placed on one of the screws to the star ground lug on the power tranfo.
It did make a large difference in the sound. It sounds "brighter" and maybe more detailed now. I originally had a rear firing super tweeter on top of the speakers to add "a little twinkle to the high end". After the choke was installed I disconnected the super tweeters, it was just to bright. I may try and add a L-pad to the super tweeters to tone them down a bit, or play with the capacitor connected to them.
No problem with any 60Hz hum being picked up.
First impression is that I like the way it sounds with the choke.
Debra
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Cool! Thanks for documenting this; we all learn when one person experiments. The beauty of community....
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Debra,
Does the plate still fit within the base with the choke mounted there? It looks awfully close to the edge.
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Yes the plate still fits on the base. The perspective in the photo makes it look like it hangs over the edge.
I could have mounted it a little closer to the PCB. I had to sand down the inside of the wood case a couple of mm. Forgot to take into account the lip the top plate sits on.
Debra
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Thanks. I own a rotary rasp that makes quick work of nibbling out wood. I'll take it to Paul's house.
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I just used a Dremel tool. It took only a few seconds to sand the area down
Debra
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Hey Deb, do you have a scope? Your description of the sound makes me curious to see if maybe the choke is picking up some high frequency interference that makes the sound seem brighter. Might be interesting to look at the noise floor and see if there are any really high frequency spikes riding on it, like maybe up in the MHz range.
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Sorry, I don't have a scope. probably should look into getting one
Deb
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Actually I do kind of have a scope. It is the DPScope, a USB scope that goes out to about 1MHz. I have not used it in a few years and forgot about it.
I used it connected to one of the outputs with no input into the Eros
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2Feros4.gif&hash=1d022918a3695c5de5ef3a46985e76d7f96f8d75)
Since we were looking for possible high frequency spikes, I set the scope to do a FFT
40Hz/div Shows a 60Hz spike
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2Feros1.gif&hash=7d251a01c6e07d0e1904d0adc6446746a79baefa)
20KHz/div
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2Feros3.gif&hash=2c2c3376955f457beb6aadaeae54c6718cbd0335)
Don't know if this data is meaningful or not, or if I even made the measurements properly.
Probably should get a real scope with a higher bandwidth.
Debra
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Debra,
1M Hz bandwidth is enough to tell if you have any oscillations. And the software is more versatile than my Tek digital storage scope. Your FFT shows no high frequency noise. I am surprised that the FFT shows such high 60 Hz component, but that is the advantage of it versus the scope trace itself.
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Had to see what it looked like without the Choke
40Hz/div No Choke
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2Feros5.gif&hash=0e48d5d5af1407c5df67319e99fa7bfe76349545)
20KHz/div No Choke
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2Feros6.gif&hash=e254f7a932156dc567e9915ccca78bd15fa11b6a)
One more 20KHz with Choke ( Not sure if I captured the trace in the previous post correctly)
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi566.photobucket.com%2Falbums%2Fss102%2Frktdoc%2FEros%2Feros7.gif&hash=7b62614d63268763dff62611f9f97b8d4340ab91)
Debra
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I my limited experience with vacuum tube audio design I have come across things one wouldn't think audible, like digital filters that can radiate spikes in the 10MHz range. Under Buddha's tutelage I wrestled with filtering that kind of stuff out and found that the difference was audible.
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debk, I'm curious about the program you are using, and what is "acquiring" the signal for it to analyze.
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Check out
DPScope.com
Debra
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Thank you, I most certainly will.
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The PC board was designed around that time, and it retains the optional terminals for a choke.
Paul, the chokes are here and I'm curious if there are different terminals for the choke or simply removing the 270 Ohm resistor and wiring in the choke (Paramount use)? There will be a shorter wire run if I I use the present terminals when mounted above the filament choke. Lastly, will twisting the wires reduce potential hum pickup?
Thanks,
Aaron
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Paul, the chokes are here and I'm curious if there are different terminals for the choke or simply removing the 270 Ohm resistor and wiring in the choke (Paramount use)? There will be a shorter wire run if I I use the present terminals when mounted above the filament choke. Lastly, will twisting the wires reduce potential hum pickup?
Thanks,
Aaron
I'll attach a picture of the missing silk-screen layer. The two terminals at the far right, labeled "L," were for the choke. (This design is six years old...) Twisting is generally a good idea, and using the resistor terminals may in fact be better than using these terminals, which run through a large loop area on the board. In simplified theory the magnetic field should be pretty symmetric and orthogonal, hence not a problem either way, but the real world always holds unanticipated surprises for us!
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Thank you Paul. I'll post some pictures once finished. Another thing, I measured the chokes and one is outside 10% at 238 Ohms and the other is 251 Ohms. This is to be expected with the tolerances Triad specifies. Am I looking at any potential problems?
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Yep, I just measured both of mine and they both came back almost exactly 236 ohms. But they do state 20% tolerances, hopefully that isn't an issue for the Paramount either.
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I think the C-7X design by Triad goes way back. When we first bought them, they were actually labeled MagneTek (they had bought Triad from Litton Industries in the seventies) and made in Mexico. I dug one out of the stash and read 256 ohms. The current Triad web site has an interesting summary of ownership changes, and are currently owned by Axis of Taiwan, who were by that time (2004) actually manufacturing 95% of the product line. But the current production sold by Allied appears to be made by Hammond (look, feel, and boxes are distinctive) and has changed from the original layer-wound to modern nylon bobbins. No way to tell if anybody changed the internal design during any of those iterations. The C-7X is listed in the current Triad catalog, but I have no idea where it might be made, or if it is the same as what Allied sells.
Anyhow, since this is past the first cap, you can estimate the voltage loss fairly accurately. In a Paramount drawing 80mA, the 30-ohm difference will become 2.4 volts; less with other amps that draw less current. It's nothing in comparison to the effect of different power line voltages!
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My chokes are clearly labeled "Made in China"...
I'm not an electronics wiz unfortunately but it doesn't sound like 2.4 volts will make a huge difference in B+. My voltage is regulated at 120v. Had a thought; as it looks like the two sets of pads are in series. Could one use one set of pads for the choke and use the other set for a series resistor (30 Ohm, not sure on the wattage) to get a little closer to a total of 270 Ohms or is this overboard?
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They are in parallel, not series.
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We went ahead and installed the chokes. Will try to see if I can figure out how to downsize a picture and post it. We put shrink tubing around the wires and also put some thick wire mesh type material under the chokes to try to give some more shielding from the filament choke. On first blush it seems to us that what it did was make the highs a little clearer and sweeter if that makes any sense. But it did seem to make some difference so I am happy we did it. As far we can tell we didn't pick up any additional hum. Guess I will have to order one for the Eros!
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They are in parallel, not series.
Thanks Paul!
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I was just about to start my own thread related to using chokes in the power supply when I remembered this one.
I have a more basic question... what parameters are "better" when designing a power supply that uses a choke?
For example, the C-7X is rated 10H, 90mA, DCR 270 Ohms while the C-3X is rated 10H, 50mA, DCR 500 Ohms. I realize there will be a larger voltage drop using the C-3X, but is there any benefit to using the C-7X over the C-3X when my circuit draws only 20mA?
Also, the C-14X that is rated 6H, 200mA, DCR 150 Ohms. Assuming I don't need the current capacity (which I don't), is more Henries better?
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The chokes were installed today. It took about an hour or so and was very easy. Attached are a few pics of the amps afterward. There is zero 50 Hz hum (I'm on 50 Hz power if you're wondering) with 100 dB sensitive speakers when I stick my ear to the woofer.
There are changes to the sonics but it’s hard to put my finger on exactly what it is. It seems that the noise floor is a little lower and that there is more detail. On two of my favorite recordings with tracks I know well there are details that the Paramounts have never been able to reproduce before where other amps could (SS and PP). On one track there was a small change for the worse. I find that I want to turn the preamp up some and this may account for part of that. There does seem to be less sparkle for loss of a better way of putting it or lingering air around notes. In short a little less magic but this may change as the chokes settle. I’m going to leave them in for now and get used to the changes. Hopefully I’ll be better able to explain the changes with time.
The amps biased up the same and I didn’t notice a difference in B+ at A2.
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That is exactly how I installed mine, but I thought there was an immediate improvement in the high end, very small but noticeable. Well, I have made so many changes since we installed them a day or two ago no way to check now! Good luck, hope they improve. Any hum?
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A little 120 Hz hum with my ear to the woofer but no more than what I remember before. I haven't adjusted the hum pots for these tubes if that gives you an idea of how low it is.
(edited to save on posts)
Day two with the chokes. I'm listening at higher volumes and for longer periods without fatigue. There seems to be a little less room information if that makes sense in the music. The instruments are clear and focused but the room is less emphasized. Not sure if this is a decay issue but it’s noticeable in my system.
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Response to David Bilss' post on the previous page:
An educated guess here... If you are designing from scratch you can find a more inductive choke (higher Henries) at your rated current, 20mA, for a given price. Design around the needed current capability or higher and the needed DC resistance for similar voltage drop. More inductance rejects more noise.
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I have listened to my Eros with the choke now for about 10 hours. I like the way it sounds more and more. I still feel there is a definite improvement in the high end. Still have not reconnected the supertweeters, and don't feel like I need them as I did before the modification.
Debra
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I put my two in my Paramounts, wishing I had order three now! Guess I will have to pay to ship one more.
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As noted we put the chokes on top of the chassis for the Paramounts. The one picture I have seen on this thread for the Eros has them below. Is there anything to be gained theoretically by putting the choke on top of the chassis for the Eros so it is next to the power transformer like the Paramounts or is it better to go ahead and mount it below next to the power supply board? Happy to go ahead and try it but since I would have to drill an extra hole it would be nice to know which way has the most promise.
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The empirical way to figure this out is to do what PJ does - clip the leads of a 1mH RF choke to the input of a scope. You now have a neat little "60Hz magnetic field detector" that will couple with the power trans radiated field. Move it around the places where you want to put the new part and see what kind of field strength you pick up.
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Ok, that is a great idea. Next question (if there weren't another things would get boring) has anyone put a grid choke on the Paramounts? Just doing some reading and was curious as I know nothing about grid chokes, yet.
Never mind, found my answer:
http://db.audioasylum.com/mhtml/m.html?forum=bottlehead&n=123218&highlight=Grid+Chokes&search_url=%2Fcgi%2Fsearch.mpl%3F
Also from the old forum:
http://db.audioasylum.com/mhtml/m.html?forum=bottlehead&n=138238&highlight=Paramount+Grid+Choke&r=&search_url=%2Fcgi%2Fsearch.mpl%3Fsearchtext%3Ddussun%26topic%3D%26author%3D%26date1%3D%26date2%3D%26slowmessage%3D%26sort%3Ddate%26forum%3DALL
So as far as that is concerned, don't think I am even going to mess with grid chokes.
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Paully, no grid choke when direct coupled (per Paul).
Regards,
Aaron
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The empirical way to figure this out is to do what PJ does - clip the leads of a 1mH RF choke to the input of a scope. You now have a neat little "60Hz magnetic field detector" that will couple with the power trans radiated field. Move it around the places where you want to put the new part and see what kind of field strength you pick up.
This is worthy of a new topic folder, Tech Tips. I'm all for the empirical method.
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Paully, no grid choke when direct coupled (per Paul).
Cool beans, thanks!
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I finished installing MQ CP-08 chokes (cortesy of keto) in my six Paramounts. These are 12H / 246ohm / 75mA. The 75 mA are a bit on the low side but we all know Mike rates his iron conservatively.
The effect is subtle but noticeable. There's a little bit more of everything good in these already fantasic amps.
Can't wait to do the mod in my Eros, FP3 and Sex amps.
The Paramounts are the 2A3 version with lots of upgrades.