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DIY Switcher w/ Volume Control & Effects Loop

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#1
Looking for a sanity check and some circuit design guidance here.

This one might be better suited to one of the more DIY-focused audio forums which I've long been an active lurker on, but lately I've seen most of the DIY engineers I've come to respect most posting here regularly. So, I'll save myself the account creation process for now.

Like many of you, I've consistently failed to keep my desktop audio setup simple and manageable. I really appreciate a well organized and tidy desk, and with two headphone amps, two computers, and a pair of speakers in scope shuffling equipment around and switching cables became frustrating quickly. Searching for a product that would let me use my equipment in the way I want on demand without doing this proved fruitless.

There's no shortage of switch boxes and passive pre-amplifiers on the market, but none of them quite fit my needs. So what am I trying to accomplish?

- Analog audio signal from a single source. A DAC fed by S/PDIF from my personal desktop and USB from my work laptop dock.
- Outputs to three different amplifiers. A Koss "energizer" for my usual headphones, a hybrid tube headphone amp for "fun with distortion", and a speaker amplifier.
- The ability to optionally pass the source signal through the tube amp (which has pre-outs), to muck up the nice clean signal with distortion on demand.
- A single, high quality, nice feeling volume control. The volume control on the DAC works well, but leaves much to be desired where the user experience is concerned.

To illustrate:
Component Map.jpg


There are plenty of products out there across a wide range of prices that will provide lots of inputs with a single output, a few with multiple outputs, and most of them have a volume knob in the form of a potentiometer or occasionally a stepped attenuator. As far as I could find however, nothing that would allow one to easily toggle sending the signal through an external device before routing it to whatever amplifier it's destined for.

I find this curious, as I remember in the not too distant past lots of home audio equipment came with "EQ output and input", which would allow you to send the signal through your choice of equalizer or other signal processing component before it made it to the amplifier section. Often times there was a "direct" button that would bypass this, or sometimes just a set of jumpers you'd place across the terminals if you weren't using one. This was pretty radical if you had something like one of those fancy Sony SEQ-555ES (R.I.P.) like I used to.

So, how would I accomplish this with commercially available products while keeping the form factor small? After some shopping around, the JDS Labs OL Switcher seemed like the best fit. Two inputs, two outputs, all switchable and with a potentiometer. Solving my problem seems to require two of them, but at $45 each that isn't too painful.

So what would that look like?
Switchers.jpg


Ok, well... That does appear to meet the requirements, if not in a particularly elegant way. Switcher 1 provides unified volume control and a toggle to route either through the tube amp or directly to switcher 2. Switcher 2 volume knob would stay at 100%, and you'd need to also toggle the input depending on how SW1 was set.

It works, but I'm not crazy about it for a few reasons. Now there are up to three potentiometers in the signal path incl. the one on the tube amp, two of them at all times. Assuming the OL Switcher uses the same pot as the JDS O2, it measures ok but not perfectly for channel matching. Best case scenario any imbalance cancels out, worst case it stacks. Any other objections I have are purely subjective. For one, setting the four toggle switches into the right configuration (well, three, one would be unused) wouldn't be the best UX. The JDS Switcher seems like a great product, but at 12 ounces I don't expect it to stay in place very well, and I need all the functionality to be one-hand friendly.

So, looks like my requirements are specific and odd enough that I'm in "custom solution land". Time to build my own! I haven't done any circuit design since I was about 12, with a radio shack learning lab and Forrest Mims "Getting Started in Electronics" books, but this has to be about as simple as it gets right?

As with any type of engineering, requirements first!

- Minimum one source input.
- Minimum one "EQ" input.
- Minimum three outputs.
- Output selector.
- Toggle for routing signal through one of the outputs, and passing the "EQ" input to another output.
- Volume control.
- Minimize sources of noise, distortion, crosstalk, and channel imbalance.
- Shouldn't explode, catch on fire, or risk damage to any connected component.

Let's try...
Circuit Diagram.jpg


Ok, this gives me three selectable outputs, with optional routing through the tube preamp for output 1. It only represents one channel, but a second channel is implied.

Probably painful to look at for a serious DIY'er or any professional. Simple software has gotten me this far, but I think it's time to move to some sort of SPICE editor (probably KiCad) to improve the quality and readability of my diagram which will be a learning experience for me. I think I'll also add an input selector and another input or two, just in case I want to add another source in the future. I'd also like to allow any of the outputs to be routed through the "EQ" path, rather than just one of them. Besides of course the output to the tube amp. Time for some refactoring!

In the meantime, I'd be extremely grateful for any feedback, ideas, or suggestions. I'll take anything into account and post my updated diagram here for "peer review" when done.
 
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saturnaal
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Thread Starter #2
Inspiration and resources:

Draw.io, a great multi-purpose tool. I frequently use it for flowcharts and network diagrams
https://www.draw.io/

Kicad, and excellent freeware circuit and PCB design editor
http://kicad-pcb.org/

Forrest Mims, my earliest source of electronics learning.
http://www.forrestmims.com/

This neat home-built passive preamp.
http://kotiweb.kotiportti.fi/audiovideo/DIY/preamp1/

Another DIY
http://grabel.tripod.com/etunen.htm

Great reference on attenuators posted by @Wombat
https://www.electronics-tutorials.ws/attenuators/attenuator.html

If anyone has any other references for building passive preamps, other threads they can link to, suggestions on commercial products I might have missed, or ideas, please post them!
 
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somebodyelse

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#3
Pot wiring is wrong - the wiper should go to the output, and the input to the end. Unless you're making a stepped attenuator you'll need to measure the pot for channel balance as they're only specified to 3dB matching, and even then only over a limited attenuation range. Probably buy several and select for the best. I may have misunderstood the 'effects loop' requirement - as drawn you can only use it with the headphone amp, not the speaker amp.
 
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#5
Man I have the same problems (or similar enough, minus the EQ). I have need for switchable inputs (easy) and switchable outputs, some that are full line level (1 to headamp, 1 to tube amp with volume control), some that are volume controlled (go to amps/monitors). Right now I have this for switching coupled with short xlr leads and this for volume control. Ideally i'd love a slightly better system, in a single box, but i've been searching for years and haven't found it yet. I reckon a custom box with switchers and pot all in one might solve the issue.
 

Kal Rubinson

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#6
You guys have it easy. I am looking for the same but with 6-8 channels and remote control. :eek:
 

JeffS7444

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#7
Before getting too carried away with using the tube amp's line-out, I'd verify that this isn't itself a passive output, and the way to test is to see whether you get clean signal out of these jacks even when the amplifier is powered down. If yes, than forget about getting "tubey" sound in this manner.
 
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saturnaal
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Thread Starter #8
Pot wiring is wrong - the wiper should go to the output, and the input to the end. Unless you're making a stepped attenuator you'll need to measure the pot for channel balance as they're only specified to 3dB matching, and even then only over a limited attenuation range. Probably buy several and select for the best. I may have misunderstood the 'effects loop' requirement - as drawn you can only use it with the headphone amp, not the speaker amp.
Thanks for the heads up about the pot wiring. Regarding the pot, I'm leaning towards using a nice stepped attentuator right now. There are some nice options out there, and a few of them have the ability to add remote control if I decided I wanted to go that route in the future. I recognize that as drawn there the "effects loop" would only work with the headphone amp, I'll probably change that in V2 once I have time to get up and running with KiCad.

Hi
Maybe you should have a look at this.
Nice find. A little pricey, though. Thanks for posting it here!

Before getting too carried away with using the tube amp's line-out, I'd verify that this isn't itself a passive output, and the way to test is to see whether you get clean signal out of these jacks even when the amplifier is powered down. If yes, than forget about getting "tubey" sound in this manner.
Good suggestion, fortunately in this case I know it's an active output.
 
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saturnaal
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Ok, I've made some updates and moved to a more appropriate software tool. I think I got it right this time.

3 inputs, 3 outputs. Probably a stepped attentuator. Toggle sends full line level signal regardless of input or output through the "loop".

Circuit2.jpg


Any adjustments pending, if all checks out I'll start sourcing components. I'll probably skip doing a board layout and etching / sending off to have one printed and just point-to-point wire this. Any suggestions on components?
 

Wombat

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#11
Ok, I've made some updates and moved to a more appropriate software tool. I think I got it right this time.

3 inputs, 3 outputs. Probably a stepped attentuator. Toggle sends full line level signal regardless of input or output through the "loop".

View attachment 38662

Any adjustments pending, if all checks out I'll start sourcing components. I'll probably skip doing a board layout and etching / sending off to have one printed and just point-to-point wire this. Any suggestions on components?
Technicalities:

FWIW: Inputs are usually on the left in simple schematics.

It is a switcher, not a preamplifier(it doesn't amplify).

Volume control normally has the wiper connected to the output.

The simple volume control has wide impedance variation re in-out matching.


Nerd.png
 
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saturnaal
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Thread Starter #12
FWIW: Inputs are usually on the left in simple schematics.
Makes sense, the Western world sure does love its left->right flows.

It is a switcher, not a preamplifier(it doesn't amplify).
DIY Switcher w/ Volume Control does seem a more appropriate title.

Volume control normally has the wiper connected to the output.
Good catch, someone had mentioned this on the previous iteration and I overlooked it on v2. I'll correct this.

The simple volume control has wide impedance variation re in-out matching.
Could you elaborate a little on this? My intention here is to use a quality stepped attenuator. Any recommendations on what specs to target, considering 2v line level signals are the intended input? Any suggestions on ways to improve the design?

I want to make the switcher itself have as little effect on the sound as possible, besides volume control and the loop of course.
 

Blumlein 88

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#13
snip...
Could you elaborate a little on this? My intention here is to use a quality stepped attenuator. Any recommendations on what specs to target, considering 2v line level signals are the intended input? Any suggestions on ways to improve the design?

I want to make the switcher itself have as little effect on the sound as possible, besides volume control and the loop of course.
In regards to impedance variation with simple volume control (usually thinking of a pot). You typically see a fixed input impedance. 10 kohms if a 10 k pot. Output impedance is anything from zero to 2.5 kohms. 2.5 kohms would occur at -6 db where you have tapped halfway between 10 kohms on the pot. This could mean variable FR with volume position.
 

solderdude

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#15
This circuit won't work properly under certain circumstances.

Look what happens when the attenuator is all the way to 0.
As it is switched now you actually SHORT input 1 as wel as loop input 1

It could be solved by adding a 10k resistor between wiper and signal wiring.
 

Wombat

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#16
This circuit won't work.

Look what happens when the attenuator is all the way to 0.
As it is switched now you actually SHORT input 1 as wel as loop input 1

It could be solved by adding a 10k resistor between wiper and signal wiring.
…. or using the wiper connection for the output which is more usual.

A battery operated buffer stage would make life easier.
 

solderdude

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#17
In this case he wants to be able to route all his inputs via the outputs of his other amps to increase gain or 'insert' tube-ness to his SS amps so he has to attenuate output signals as to not overdive the inputs.
 

Wombat

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#18
A simple fixed attenuating impedance-matching network for each path would be how I would do it, disposing of the problematic volume pot(this is how I match pro to consumer gear using matched tolerance resistors per channel). Volume control is then via the SS amp, which I am assuming is an integrated one.

Passive attenuators Note specific tutorials at end of main article.
 
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Blumlein 88

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