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DIY Switcher w/ Volume Control & Effects Loop

Blumlein 88

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An interesting approach, but I'm not quite ready to tackle PCB layout for A/D & D/A chips. If it can't match or beat the performance of the Topping D50s, I don't want to put it between my DAC and transducers. I admit though, I have long term fantasies of building my own AK4499EQ implementation, or whatever the "top of the line" is if I ever get to that point.

Alas, neither my own skills or home electronics laboratory are quite up to the task yet.
I believe what @restorer-john had in mind is using the output pins of this 10 bit chip to turn relays on and off. You have a relay on each output pin, feed a DC voltage thru a linear pot, giving fine control of the voltage fed to the ADC, and have the ADC sample that voltage. Turning the pot up and down switches the relays on and off giving you the effect of a plain volume control knob. If you have arranged the relays to control an R2R set of resistors you get a very nice volume control. But none of this is in the signal path. It only controls the relays. A very neat idea he had. Very neat.

https://www.ti.com/lit/ds/symlink/tlc1551.pdf
 

MRC01

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An interesting approach, but I'm not quite ready to tackle PCB layout for A/D & D/A chips. ... Alas, neither my own skills or home electronics laboratory are quite up to the task yet.
In that case, a passive attenuator is a great choice for first piece of audio gear to design and build. It was my first project because it requires only basic EE knowledge to design and the soldering/building is simple. You could get a Goldmark V47C and fill it with MF resistors. The hardest part might be finding the external case and knobs you want.

I believe what restorer-john had in mind is using the output pins of this 10 bit chip to turn relays on and off. You have a relay on each output pin, feed a DC voltage thru a linear pot, and have the ADC sample that voltage. Turning the pot up and down switches the relays on and off giving you the effect of a plain volume control knob. But none of this is in the signal path. It only controls the relays. A very neat idea he had. Very neat.
That's how Jan Meier makes the volume controls in his amps.
 

BDWoody

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I'd do it with an A/D converter with binary parallel outputs driving the relay resistor arrays via Tr buffers. One linear pot to control it all.

Something using the TLC-1550x if they are still around.

Simple, fast and could offer an instant mute/resume option too.

That's pretty slick...
 

restorer-john

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That's how Jan Meier makes the volume controls in his amps.

Don't know of Jan, what does he make?

When i think about it, the Schiit (pic above) unit may be using an onboard micro's A/D (they usually have several to play with) and sampling the DCV of the onboard alps pot, but depending on the outputs, they may have to use a serial to parallel conversion or a decoder to drive the 7 relays. The use of an standalone A/D means you dispense with the micro altogether (just a clock needed). And you may be able to shutdown the clock and hold the output with latches until it's touched again.

Many years ago, I messed around with National Semi LM3914/5 bar/dot lin/log display ICs to make a volume control using the parallel outputs to drive relay switched resistors with some logic to give a decent range. As that IC was hard to initialize without random outputs turning on, I gave the idea away.

An Arduino could be used to do it also. Trouble is the temptation to add more and more stuff just because you can, results in a whole ton of code and an onboard computer when all you wanted was accurate volume control and source control.

These ideas can snowball and before you know it, the original idea is lost. :)
 
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MRC01

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Don't know of Jan, what does he make? ...
He has a small (maybe even 1-person) audio company in Germany. Amir reviewed one of his headphone amps the Corda Jazz earlier this year, which led to a long discussion due to the unique design of its volume knob, which works similar to how you described above, changing resistors in the gain-feedback loop. The benefit was high SINAD at low volume levels, but the drawback was the type of resistors he chose limited the output voltage capability. In his higher priced amps he uses different resistors that don't have that voltage limitation.
 

restorer-john

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which works similar to how you described above, changing resistors in the gain-feedback loop.

Interesting. So changing the overall gain in the NFB loop? The bandwidth product would change unless it was kept to a relatively small range I would think. Also, there'd be a risk of open loop or oscillations perhaps especially if relay operated- NFB loops are very fast...

There's VCA options out there, but they put active components in the signal path that some people don't like.

So to prove there's nothing new in the world of HiFi, I did a bit of a search and found you can actually buy a volume controller (256 step) using the idea I posted above. Bah! :( It's been done already- and cheap too. It looks like 4 channels (or depending on how it's set up, it could do 2 x balanced using the second bank of relays)...

https://www.ebay.com.au/itm/4-chann...971460?hash=item3f76f90744:g:~rAAAOSwol5Y0zfb

1573870184703.png


I think I might buy one just for fun and have a poke about with it. Looks to be running an ADC and pot, just like what I envisioned with a partially populated secondary board for the parallel channels.

Here's a 2 channel version. The ADC 0804 is on board. The beauty of that A/D is standalone operation and onboard clock generation. All in all, apart from the board layout around the resistor array, I reckon it's a robust solution. Buy a 4 channel unit and use the top and bottom sets of relays- one for each channel. That will help with channel separation which could be the downfall of the single board DPDT relay layout.

https://www.ebay.com.au/itm/2-chann...526910?hash=item3f6ff8f77e:g:eSgAAOSwHMJYNu3J

This listing allows you to really zoom in on the pics:

https://www.ebay.com.au/itm/HiFi-2-...255570?hash=item3f9a92dc52:g:hJoAAOSwOXBdkZkZ
 
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MRC01

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Interesting. So changing the overall gain in the NFB loop? The bandwidth product would change unless it was kept to a relatively small range I would think. Also, there'd be a risk of open loop or oscillations perhaps especially if relay operated- NFB loops are very fast...
Yes that's how I understand it. As you turn down the volume, NFB and bandwidth increase, noise & distortion decrease. You can find that thread here and read the details. Someone posted a schematic and Jan also commented on his design approach.
 

solderdude

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While R2R relay volume control can have excellent performance and fabulous tracking over the entire range this review shows that when the layout is not done perfectly you may end up with garbage from the circuitry around it messing up the performance.

I also wonder what happens after a while when pots become scratchy which control relays or feedback loops.
Certainly when there is DC on it.

About 25 years ago I used blue velvet pots (10k) in my pre-amp and in an passive attenuator.
Both are still in operation today.
Both post have excellent tracking and till today have never heard a single 'scratch' from them.

To me the simplest and easiest to use volume control option is to use those pots.
Have also used many other pots (the types used in most smaller sized gear these days) and a lot of them have balance issues and become scratchy and or loose the nice 'feel' as grease used seems to change over time.
 

restorer-john

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While R2R relay volume control can have excellent performance and fabulous tracking over the entire range this review shows that when the layout is not done perfectly you may end up with garbage from the circuitry around it messing up the performance.

Look at the thing. Let's not lump bad shielding and poor wiring dress, along with a transformer jammed next to completely unshielded cabling to RCAs and dubious earthing as representative of a relay driven R2R controller. That's not fair in my book.

But I too love Alps blues. They pretty much last forever and feel great. I've got them in 30+ year old gear and the plastics film ones are dead silent, track beautifully and just work.

And even better, the Alps Black Beauties found in the 1970s TOTL gear with their die-cast aluminium bodies and laser-trimmed printed resistor arrays- you can't beat them. They have guaranteed gang errors of <0.2dB on the best ones.

My favourite pots were from Noble, they made the Alps ones look cheap, but not many brands used them- Denon were one.

But people these days want no pots- goodness knows why, so let's give them the super accurate L/R tracking and no active circuitry- they'll be happy.

:)
 

solderdude

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Indeed ... its layout and shielding related for sure.

Do you happen to know if the current ones are still of the same quality ?
Haven't bought these pots in 25 years.. still have one in stock but 50k.

The thing with passive volpots that need power means there is some logic behind for control and would add remote in this case as well.
This means clocks etc. are needed and most would want a power supply near it (or use 5V DC wallwart).
Given the OP is a newbie in design I thought it would be best for his particular project (when he doesn't want remote control) to keep it Kiss.

One can use a nice shielded enclosure and don't need to worry about internal shielding. ground layout management would still be a thing though.
 
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Kal Rubinson

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Not what I would call a fun day with cables. :(
I know, I know. So, I do the VC in software in Jriver and I use two physical controls. The first one is at the audio rack with the keyboard/monitor:
71J-n40mwpL._AC_SL1500_.jpg

And the second one is wireless, sits on the coffee table and is within reach from the listening position:
dial200x200-500x500.png
 

MRC01

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While R2R relay volume control can have excellent performance and fabulous tracking over the entire range this review shows that when the layout is not done perfectly you may end up with garbage from the circuitry around it messing up the performance. ... Indeed ... its layout and shielding related for sure. ... One can use a nice shielded enclosure and don't need to worry about internal shielding. ground layout management would still be a thing though.
Yah, building a switchbox with ladder attenuators, all manual, no power, is much simpler and cleaner. It's more likely to work properly without any issues, especially for a first DIY project.
 
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restorer-john

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Do you happen to know if the current ones are still of the same quality ?

Not sure. They seem to be identical on the outside and it's been years since I've had the need to pull one apart to do a post-mortem. I'm lucky that they last so long as the majority of Blues used in commercial Japanese gear had the loudness tap (8 pins total) and they are not an off-the-shelf replacement.

The issue of course is there are more fake Alps pots out there, than real ones. There's mostly fakes in the teardowns we've seen on ASR in the cheap Chinese gear. The fakes are so perfect looking, you cannot tell and in the last 10 years the fakes have refined their duplication to the point there are no clues to separate real from copy.

As such, we can only buy from Mouser/Digikey/RS/Farnell etc with their audited supply chains (and higher prices).

Ask Tom (TCA)- he's been dealing with Alps direct, I believe, for a custom pot for his new HPA. I'd guess he'd be paying a premium for guaranteed gang error being minimised. Not sure if they custom manufacture or just test and pick the best for that application these days.
 
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