Perhaps it is. But if it is, then as I had said previously, I think it's a clumsy, ham-fisted solution.
It was that four deck switch that was John's excuse as to why the Blowtorch wasn't better optimized by placing the input switching and volume control back near the rear panel and moving the boards forward.
It's just sad how John goes on at length about how the best parts were used, Teflon circuit boards, hogging the chassis out of a solid block of aluminum, etc., but when you open it up, it looks like the cobbled job of a DIYer's first project.
se
why the 7 resistor part? thats the range I was mentioning for the 8 resistor part. why not NOMCT16031001AT1 ? thats the soic16, surely a more useful number? and its actually cheaper than the soic14 7 resistor part.
its got slightly lower tolerance than the 4 resistor thin film ORN I linked and of course left in the dust by the zfoil, but decent value really and may not make a difference in practice, certainly good value proposition if you are considering spending time matching a bunch of discrete SMDs.
GB? hmm 1k is a bit high for my needs if I was buying a few and i'm toying with the idea of a couple foil ones so I can get exactly the gain I need. but maybe, do let me know if you post a gb. the thing is since i'm over here, the saving buying say 10 of them in a GB would be pretty much wiped out by the postage vs just including them in a digikey order of my own.
the programmable parts from MAXIM like the MAX5426, or MAX5431 are interesting as well, for making a high performance balanced PGA. but they mention transistor counts of over 120, so I wonder if all of that can be just the switch logic and switches.
they all lend themselves to very nice neat multiloop PCBs
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I haven't been feeling very well over the last couple of centuries, but can you explain the benefits of such a 4-parallel part ?
If you order 25, it costs a dollar in SMD, $1.25 for 0.1% through-hole in 100/125 mW.
all are matched for not just global value, but also for TCR at a higher level again, they are cut from the same piece of material so the dynamic characteristics will match well, then they are stuck to the same substrate (often aluminum) and potted into a neat package.
its like buying a bunch of decent global 0.1% value match, then matching them for part to part for pairs or quads, then potting them after sticking them to the same heatsink. I tell you I would much rather spend a few dollars buying that than the time doing it after buying 100 pieces. basically if they drift or heat, they drift or heat together, very handy for an instrumentation front end, or maximizing CMRR
also you can buy then containing 2 or 4 1:1 1:2, 1:4 ... ratio pairs, that means they are also matched in pairs to each-other at 0.1 or 0.05% (better with foil) as well as being guaranteed to be within 0.1% of the marked value. when you buy a 0.1% resistor say on 1kΩ you can get anywhere between 0.2% spread, so 990Ω-1010Ω there is no relative matching like with these parts.
basically the very same reason you might buy a monolithic dual jfet, or stick a matched pair to a heatsink
its like buying a bunch of decent global 0.1% value match, then matching them for part to part for pairs or quads, then potting them after sticking them to the same heatsink. I tell you I would much rather spend a few dollars buying that than the time doing it after buying 100 pieces. basically if they drift or heat, they drift or heat together, very handy for an instrumentation front end, or maximizing CMRR
also you can buy then containing 2 or 4 1:1 1:2, 1:4 ... ratio pairs, that means they are also matched in pairs to each-other at 0.1 or 0.05% (better with foil) as well as being guaranteed to be within 0.1% of the marked value. when you buy a 0.1% resistor say on 1kΩ you can get anywhere between 0.2% spread, so 990Ω-1010Ω there is no relative matching like with these parts.
basically the very same reason you might buy a monolithic dual jfet, or stick a matched pair to a heatsink
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So then use two decks, four poles per deck. Don't see any reason for sending a wire to each deck.
EDIT: Actually, I think you would only need two poles per deck.
See above.
se
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The 7 resistor part cheaper, if you wait for the -5 version to get in stock. The 7 resistor part also works for the number of resistors I need, plus gives me an extra resistor to parallel with on of the others for a gain switch. I'm planning on giving them a go.
The MAX chips won't work for me, if I understand how to use them correctly, due to rail voltage limitation.
Tell that to the two deck input selector switch, Richard.
And why would you want to put the inverting and non-inverting leads for balanced on separate decks? Don't you want to keep the loop area of those two leads as small as possible to help keep noise pickup to a minimum?
se
OK yep that makes sense.
re the max stuff: i'm not certain, but I dont think you have to worry a great deal about the voltage, except for the supply voltage for the switch/logic circuitry. I think you need only worry about dissipation limitations/voltage drop across the resistors for the actual signal circuitry. the MAX5430/31 and MAX5420/21are +/-15v devices too, but there is no mention of limitations on the signal voltage, only supply voltage and dissipation limits are mentioned for the rest of the device.
what interests me maybe for another project is the 10 lead MAX5431and 8 lead sot23-8 MAX5421 versions. they have impedance matching pins for the non-inverting opamp input. the end to end impedance of the inverting/ladder/gain side is set in a ratio as to always equal the impedance seen by the inverting input, no-matter what gain setting; for matching input bias currents seen on inverting vs non-inverting inputs; sometimes thats important.
call me old fashioned, but my inner audiophile tells me the pure resistance only parts will sound best, logical or not and although they probably ARE thin film, the maxim makes no mention of the material. the simplified schematic of the parts does not explain where all those transistors are being used either.
Steve Eddy, why are you so concerned about the number of decks of the switches in John's preamp? Are you his bookkeeper?
Steve Eddy, why are you so concerned about the number of decks of the switches in John's preamp? Are you his bookkeeper?
It stemmed from a discussion about shaft extensions with regard to the passive preamp that John put up for discussion. That led to a discussion of optimizing the layout of the Blowtorch, vis-a-vis moving the controls to the rear of the unit and using shaft extensions out to the knobs on the front panel.
John said that couldn't be done because of the 4 deck switch that was used for switching polarity. And I questioned why a simple polarity switch required four decks, particularly when the input selector switch seemed to be doing just fine with only two decks.
Any other questions?
se
Not to make a big deal out of it, but I was originally referring to the multipole input selector switch that would have to be extended into 4 sections (along with the TKD input pots into 2 sections) IF and when a customer asked for balanced inputs as well as balanced outputs. The 4 gang polarity switch works very well for the job, even though we could have perhaps shortened it, IF NECESSARY for some reason.
Virtually all the area inside the CTC Blowtorch was designated for SOMETHING, and not to deliberately be left empty. In reality, we rarely used more that 1/2 to 2/3 of the space. This was due to customer interest, rather than anything else. For example, we left room for an active buffer, but we found no customer interest. We could even had mounted input or output transformers, if the customer wanted them. We had the space available.
Haven't any of you ever found that it sure would be nice to have more space available? I am working on a real problem like that just now, because we can't find any, and we have to go 3D with a daughter board to get what we want. What a hassle!
Virtually all the area inside the CTC Blowtorch was designated for SOMETHING, and not to deliberately be left empty. In reality, we rarely used more that 1/2 to 2/3 of the space. This was due to customer interest, rather than anything else. For example, we left room for an active buffer, but we found no customer interest. We could even had mounted input or output transformers, if the customer wanted them. We had the space available.
Haven't any of you ever found that it sure would be nice to have more space available? I am working on a real problem like that just now, because we can't find any, and we have to go 3D with a daughter board to get what we want. What a hassle!
Wide spacing between tabs on the switch or between layers/wafers reduces the c between wires and channels. Even on a single wafer, you would leave space between/gap for left-right gnd/signal.... wider seperation is better for minimizing cross-talk. It isnt so wide as to be an issue for noise pickup... other factors come into play to make that happen and there are other solutions besides close spacing for lowest loop area for rfi/emi pickup. All is a balancing act as to priorities etc. Each might choose a different priority if they have reason to believe something needs extra attension.
keeping cross-talk to a minimum or maximizing the channel to channel seperation is very important to spatial and imaging accuracy and detail recovery in reproduction.... thus it would be a priority.
Thx-RNMarsh
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Kind of expensive way to minimize the loops pickup. Have you considered depleted uranium for the case? It's MIL, has a heavy feel and would certainly be appreciated in a cost-no-objection device. Not to mention the media rap around.
Why? What on earth do you need 4 sections for? You could switch two channels of unbalanced or balanced inputs on a single 4 pole deck if you wanted to. But if you want more separation between left and right you could do it with two 2 pole decks.
Why do the inverting and non-inverting leads of a balanced input need to be separated by the spacing of a deck? I see absolutely no benefit to this except for Shallco. While I can perhaps see separating left and right channels by a deck spacing, separating the inverting and non-inverting leads of a balanced input makes no sense.
But a two section TDK attenuator would still be significantly shorter than a four deck Shallco and would allow the input switching and attenuation to be placed at the rear rather than up front.
Not spending more money on something that provides no benefit that can be seen seems like a pretty good reason to me.
se
I wasn't suggesting putting both channels on a single deck. I was suggesting one channel per deck, as is the case with the input switch.
And you still haven't explained how putting the inverting and non-inverting leads of a balanced input or output on separate decks is of any benefit.
se
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