Help with Passive Headphone Box Stereo/Mono Switch

[MJ]

Hi!

I found this schematic for a passive headphone volume box on an excellent DIY site called "Syntherjack".

https://syntherjack.net/category/project/
I want to build it (actually a two potentiometer / two output version of it), but I want to somehow insert a single stereo/mono switch in it (just one switch to control the whole box), but am unsure what kind of switch and how to wire it?

Thanks!

 

[JohnRoberts]

are you trying to vary the line level feed going into the headphone amp, or the headphone level signal coming out?

JR

 

[ccaudle]

I found this schematic for a passive headphone volume box

That schematic would work for line level in/out with short output cables, but no way it would work anywhere close to reasonable quality for headphones.

I want to somehow insert a single stereo/mono switch

That sounds like amp abuse. A headphone amp is a (very) small power amplifier, and you can't just connect different power amplifier outputs together.
If you isolate the amplifier outputs with resistors you can combine signals, but now you have a poor quality amplifier with very high output impedance.

The standard way to do what you are asking is to make a combining circuit for line level input, then send the final signal to a headphone amplifier.

 

[MJ]

Hi thanks! The boxes are going to be fed by a couple of small 4 channel headphone amps. Not very powerful. They probably have enough power to feed two sets of headphones per output channel.

The headphone amps have mono switches, but I sometimes want to send a stereo signal, sometimes a mono, depending on the player, hence the need for a mono/stereo switch on each box.

 

[radardoug]

Just buy more amps like you already have.

 

[JohnRoberts]

the circuit you posted will not work for headphone level signals

JR

 

[MJ]

Thanks so much for all that helpful info!

 

[fazer]

the 100k pot should be 100 to 500 ohm IMO. Also the phones could have a series 100 ohm resistor feeding the pot. Then just cross couple the left and right after the switch to Left and Right. That’s the circuit used In headphone boxes used when I started in the 70’s.

We would put 25 ohm resistors from a 60 to 100 wat power amp. Then into 100 ohm resistor before the phones. And a SPST switch cross couple the. L to R. Feeds after the 100 ohm. 100k is to high for the volume pot when you consider that modern phones are 32 to 64 ohms. 100 ohm seems a better value to me.

 

[MJ]

Hey man that is it!

Thanks !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

 

[ccaudle]

100 ohm seems a better value to me.

Headphones are not a constant impedance, so that will change the frequency response pretty noticeably.

 

[fazer]

Bring it up to 500 or 1k. But a 100k is to high

 

[fazer]

Also making it mono I assumed was for a sectional performance like vocals strings horns. For mono (where you have one side on and one side off to hear the sections blend as a player. For critical listening I would listen on my favorite headphone amp.

The phones I used back in the day were akg headphones with a 400 ohm impedance and the sound was great with a 60 watt amp as I described but they were used in the studio for musicians cue. Not a very common system for today. I remember getting a hearback system for a studio with the 8 sends for mixing individual cues with 5 stations. The flexibility was great but the sound was dry and brittle from the amp in that system. I don’t know if it was the amp or the digital converter. The old system with the 60 watt amp and box as I described with the akg phones (400 ohm) was beautiful sounding. But as I say it was for cue mixes.

Akg 240 we’re the phones I believe

 

[Brian Roth]

I seem to recall using 500 Ohm pots with a series resistor (a few hundred Ohms??) in series between the wiper and the tip or ring of the 1/4" jack feeding the headphones. Early systems (1970's era) often used a Crown D-60 to drive the boxes but that wasn't enough VOLTAGE output to cause ear bleeding levels as often desired by the semi-deaf musicans. So, onward to D-150 or DC-300 amps. The larger amps weren't supplying "Lotsa Wattsa" to the distributed cue boxes, but a higher signal VOLTAGE.

I saw/built a number of variations of this scheme over the years. Early versions used a single stereo power amp with a switch on each box to select one of the amp channels that then fed a mono signal to both ears of the stereo headphones. So, the desk could establish two independent mono cue mixes.

A more elaborate version used two stereo power amps and passive switches in the cue box. Depending on requirements, a six position switch on each box could select:

Mono 1 (amp 1 left)
Mono 2 (amp 1 right)
Mono 3 (amp 2 left)
Mono 4 (amp 2 right)
Stereo 1 (amp 1 L and R)
Stereo 2 (amp 2 L and R)

Of course that required desk routing and often patchbay connections to establish the various possible combinations.

Bri

 

[Brian Roth]

There were several commercially available cue systems with individual "mixer pods" out in the studio. Furman was an early version but used 25 pair (!) telcom style cabling. The later HearBack used Cat5 cabling. One downside to all of these was that less than experienced musicians would end up hopelessly lost twisting the multiple knobs on the pods and the session engineer would have to go out into the studio and "fix the train wreck" mix on the pods.

One studio installed this Roland system, originally intended for live stage shows:

https://proav.roland.com/global/products/m-48/
It had the ability to allow the session engineer to access a given pod and "fix the mix" from the control room.

Bri

 

[Rob Storms]

Headphone outputs on receivers and the like are often just speaker outs with a (150 ohm?) resister in series. I remember (long ago) making a box that drove phones from the speaker out using a low value voltage divider ( 16ohms and 1 0hm?) that resulted in a much better damping factor. I think I remember that it sounded better.

 

[MJ]

That is interesting what you say about the 5-position switch. Someone actually gave me an old headphone-station box out of an old (70s?) recording studio that is that exact circuit. I opened it up and the switch inside is somethig I’ve never seen before. The box works beautifully too!