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PostPosted: 27 Mar 2021, 09:30 
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Wow...looks like you're living the dream!
Thanks for your thoughtful reply about the speakers -- as a piano technician and electrical engineer, I am especially intrigued by planar drivers, but as a nascent audiophile I thought electrostatics was for headphones and Magnepan the only game in town for floorstanders.
Anyway, the immediate prospect is getting this PoddWatt built!
Thanks again,
Vin

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PostPosted: 06 Apr 2021, 17:07 
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Well, I had a pile of questions, and didn't want to be ... "that guy" ... so I spent hours today reading everything since 2012. And what a great experience. Most of my questions got answered, and it was truly a joy to have a window into so many DIY journeys, Bruce's generosity and the spirit of sharing among the users.

But some questions remain (and if I missed the answers, please give me points for trying).

I'm building Type E with both channels on a single chassis and just one PS. I would like to add a balance control. This is implemented in my Jolida 502 as a 100K linear pot connected across the two input channels after their volume pots, and with the center going to ground. Does that seem viable to you?

I feel really dumb about this one, but the line filter I bought includes a 3 amp fuse, but I see no way to disassemble the thing. Is it a circuit breaker?

In reading about designing for tubes, I learned that you need to set an operating point, and doing that requires knowing Vpp of the input signal. I see how to calculate that for downstream stages, but what about the the random device that drives the first stage? Seems sketchy since we plug devices and headphones into the same hole. (I'm having other problems analyzing the circuit, but I think those will resolve when I get past the rank beginning level...)

A builder wanted to add remote control and there was a good explanation about why you don't want to power the module from the filament side. But my question is, why not use the high voltage DC with yet another LR8 for powering low current accessories such as a bluetooth receiver module and remote control (if the mechanical type, I'm thinking the draw is perhaps significant when changing volume, but I would imagine we're listening less critically while changing volume.

The Fisher chassis I'm repurposing used brass ferrules/grommets/eyelets under the transformers to protect the wires from abrasion. Have we been doing this, or using rubber grommets, or are we finding that simply deburring the holes is fine?

Well, that's all for now...

Thanks in advance to anyone who can help!


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PostPosted: 07 Apr 2021, 12:45 
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Hi, it is possible to add a balance control. I don't really like it as there can be some unexpected issues. Bourns makes one, a type MN250K. One source is Antique Electronic Supply (tubesandmore.com). Other places carry them as well. Not costly. It can go in front of the 100K input pot. So far so good. It has a middle detent. In the middle range it does nothing. But as you mover further away from the middle it changes the input impedance to ground on the input channels. One goes up and one down and there is a corresponding change in the impedance between the source and the tube grid. This can upset the frequency response depending on the impedance of the source. A low impedance source could be seriously altered. As long as you are close to the middle it probably won't matter. I don't recommend them for that reason.

Your power filter....it is likely rated for 3 amps but if it is fused the fuse will be replaceable somehow. I have never seen one that wasn't. Many have a plastic cover on the power input side that needs to be pried out and the fuse (and often a spare) is hidden inside.

I am not sure what the second question is. All operating points in the amp are set via cathode sources. Resistors in the first stage create a voltage drop and a constant current IC in the output stage does the same. I do add a small amount of positive DC voltage to the output grids. This is unusual. It would increase the current flow through the tubes normally. However the CCS just adjusts the cathode voltage by a like amount and everything is cool. The voltage is added because the CCS becomes non-linear when it has less than 4 volts across it. So the solution is to make sure that it never gets that low.

I generally use large smooth holes for the wires when they are not going to be moved around. If they will move around then rubber grommets are needed. Additionally on transformers I clean off at least one of the feet so that it can make good electrical contact with the chassis. Depending on the style of transformer two or more may need to be scraped clean. The chassis also needs to be bare there. Two reasons for this. One, so if there is a failure in the transformer it is most likely to short to the chassis and blow the fuse. Second is to reduce electrical radiation from the transformer that might find its way into the signal portions of the amp.

Good listening
Bruce

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PostPosted: 10 Apr 2021, 03:01 
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Thanks, Bruce!

Thanks for your thoughts and sources on the balance control. The only time I use it is when I feel the soundstage needs a tiny bit of tweaking, or to isolate a single speaker when setting up or troubleshooting. It had not at all occurred to me that changing what looks to the amp like input impedance might also change the voice!

My second question had been about the voltage swings we put on the grid in the first stage; how do we know what DC voltage to place on the grid so the positive and negative swings on the source signal stay in the linear region on the load line. But after writing you, I figured out the right phrase to google and got my answer: line level voltage for consumer gear, is +/- 0.447 v And maybe that was an unfair question for this thread since it was more general about amp design than the PodWatt :|

Turns out I ordered a line filter rated at 3 amps, not including a 3 amp fuse. Since there's already a hole for a fuse in my chassis, I'll use it. But maybe that's a bit of learning that some future builder & reader of this thread could learn from.

Anyway, thanks again!

Vin

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PostPosted: 10 Apr 2021, 14:28 
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Hi, the "standard" line level signal is not really a standard. Typical values given ramge from about 1/2 volt to 1 volt. I design for 1/2 volt on most things and in case of the biggest power amps 2 volts. That difference means I don't need an additional gain stage in the amp. Any line stage preamp worth taking home will deliver well in excess of that. Most (mine included) can deliver more that 20 times that much. Depending on the actual tubes used (some have a bit more gain than others) the Poddwatt is designed for 200 mv input for max output. This is partly because iPods put out that much ...thus the name as well. A Poddwatt uses about 1 amp so your line filter is fine.

Good listening
Bruce

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PostPosted: 19 Apr 2021, 09:42 
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Hi, I would like to switch power to the amp on through a relay. What would be the easiest way to get 5 or 12V dc ? I was thinking of rectifying the 12V secondary of the Edcor to power a relay but I don't know the math to get to this exact voltage, or would it be better to use some sort of step down converter connected to the mains ?


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PostPosted: 19 Apr 2021, 13:52 
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Hi, Either way would work. A simple bridge rectifier off the 12 VAC feeding a filter cap of anywhere from 100uf and up would give you a bit over 12VDC. So a value of resistor could be calculated based on the current needs of the relay to bring it down a bit At the same time the AC 120/240VAC to 12 VDC modules are really inexpensive and would do the job. I believe the last bunch I got were about $5 each and good for 1.25 amps at 12 VDC. I would expect one to drive a relay just fine.

Good listening
Bruce

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PostPosted: 21 Apr 2021, 17:04 
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Hi! I note that the UF4005 rectifier diode's current rating is 1 amp. But somewhere along the line I got the idea that the recommended fuse is 3 amp (presumably slow-blow). This seems inconsistent. Am I falling into that AC v DC math trap? Or have I somehow gotten the wrong fuse value into my head?

Thanks in advance
Vin

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PostPosted: 21 Apr 2021, 20:26 
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Hi. the sizes of the two components are not related. The fuse (I use standard blow ones in this size amp) is for the whole amplifer which includes the heaters as well as the high voltage. It needs to be a bit larger than the expected maximum power usage to allow for the surge when turned on. The rectifiers just change the high voltage AC from the transformer into DC. One amp is a pretty much a standard rating for solid state high voltage rectifiers. The actual current requirement for the tubes is about 1/6 of an ampere.

Good listening
Bruce

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PostPosted: 21 Apr 2021, 20:36 
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Hi, I'll try this again as I just got an SQL error message on this response.

Anyhow. The sizes of the fuse vs rectifiers are not related. The fuse (I use standard blow in this size amp) handles the total power of the amp including the heaters. It is sized to handle the initial turn on surge that is much larger than the actual running demand. The rectifiers are a standard size and most high voltage solid state rectifiers are rated for the same current. Different voltages yes, but one amp is pretty much standard size. The actual B+ current used is about 1/6 of an amp,

Good listening
Bruce

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