Sum to Mono w/ DPDT

[Dr. Toobz]

Here's an interesting question I asked about a year ago, but never got a response to:

"...how could I wire the other (DPDT) switch to take what's coming from the inputs and bridge it to mono? Some of my records are in mono, and some badly-remastered early 50's CD's I have took mono tapes and transferred them via stereo tape deck, resulting in the music being off-balance (or phasing between channels). It would be nice to flip a switch and get a perfectly-centered signal without "stereo" pops/crackles or unbalanced tape hiss."

I have two DPDT switches wired up in my newest Quickie. The first DPDT switch effectively lets me use the Quickie as a pass-through selector box when unpowered, allowing for the switching of my two sources. I had initially rigged up the second switch as a local feedback loop, but with the PJCSS, a different set of tubes, and the cathode bypass capacitors removed, distortion is already pretty darn low as it is (plus, if I don't want the added thickness of the preamp, I can simply throw the passthrough switch...). I think the mono switch would come in very handy, but I've seen about 20 ways of doing this online, and want to use the best method (one that creates the least chance of comb effects and other problems). To be clear, I would take the hot for each output jack off the center of the switch, and could use one half of the switch normally for stereo, and for the other half, I would somehow tie the channels together with resistors, which would sum the signal for the outputs. Or, I could use the switch as a DPST in between the channels, and short the two together somehow when it is flipped one way. Any ideas??

[Paul Joppa]

The reasonable way is a compromise. Connect each signal to a resistor, join the resistors, and take the mono from the joining point. Depending on the source, and the resistor values, this will reduce the stereo separation and the signal level. Larger resistances make for better separation but lower signal level. It may or may not be possible to find an acceptable compromise value.

The only ways I know around this limitation involve active circuitry or transformers or both.

[Dr. Toobz]

Sounds like I actually need a 3PDT switch to pull this off! The DPDT would give me one throw with stereo (one channel each side, center pins out to + of RCA jacks), and the other throw mono (channels joined by v-arrangment of resistors, then this junction would go to R and L sides of switch, as in above)...except that the resistors would still be shorting both channels together, even if the output jacks were not connected to them by the switch. For stereo, I would have to make sure the resistors on the other half of the switch are not connected together, in addition to disconnecting them from the output jacks as usual. I bet I could use the third bay of a 3PDT switch to do this, and just leave the corresponding 3rd pin on the mirror side blank, since throwing the switch to that side would disconnect the resistors anyway.

[Grainger49]

To answer the original question mono can be achieved with a DPDT switch.  Bring the right and left hots into the switch and on one position short right to left. 

The blend that Paul suggests is less brute force and allows the voltages generated to dissipate across a resistor rather than a dead short.

I once dropped a dime onto my ST-70 circuit board.  It shorted the drivers to mono.  The imaging sucked and it took me a while to find the dime (which I didn't know I had dropped).

[Dr. Toobz]

Ouch! At least all the dime did was short your channels to mono. I'm always living in fear of bits of wire or metal somehow going unnoticed after I work on something and then shorting HV paths like tube pins.

Re: mono: Honestly, if many of the jazz remasters I had didn't feature such poor tape transfers, this wouldn't be necessary. I have a number of mono recordings from the mid 1950's and earlier, like Sonny Rollin's Saxophone Colossus, that were clearly transferred on a stereo deck. Common sense would say to do a mono transfer and then duplicate it for the other CD channel, since Red Book doesn't have a spec for one channel. However, it seems like some engineers (including the venerated Rudy Van Gelder) have been using stereo decks, which results in an off-center transfer that wavers between channels at times. Truly annoying. The other no-no I run into is putting mono recordings onto stereo LP's or tapes, which yields stereo surface noise over a mono signal! I wish people would take better care of mono recordings, as they can be truly great if handled correctly. If anything, I think the introduction of stereo in the late 50's was initially a bit of a setback, as some recordings from the late 50's/early 60's have that tacky "ping-pong" type of arrangement where one instrument is in one channel, and the rest in another. This is course is totally unnatural from a live music perspective, leading to the odd situation where mono actually sounds more like a real performance than stereo.

[Grainger49]

How about this, put the mono switch after the output capacitors.  Tie the right and left into the middle terminals as I suggested with a dead short between one of the connected terminals, top or bottom.  The output capacitors isolate the signal well.

PJ may see a problem with this arrangement but I don't right now.

[Doc B.]

Ouch! At least all the dime did was short your channels to mono. I'm always living in fear of bits of wire or metal somehow going unnoticed after I work on something and then shorting HV paths like tube pins.

The trick is to pick up the upside down chassis, flip it over to right side up, and give it a shake. That will usually dislodge any offcuts.

Re: mono: Honestly, if many of the jazz remasters I had didn't feature such poor tape transfers, this wouldn't be necessary. I have a number of mono recordings from the mid 1950's and earlier, like Sonny Rollin's Saxophone Colossus, that were clearly transferred on a stereo deck. Common sense would say to do a mono transfer and then duplicate it for the other CD channel, since Red Book doesn't have a spec for one channel. However, it seems like some engineers (including the venerated Rudy Van Gelder) have been using stereo decks, which results in an off-center transfer that wavers between channels at times. Truly annoying. The other no-no I run into is putting mono recordings onto stereo LP's or tapes, which yields stereo surface noise over a mono signal! I wish people would take better care of mono recordings, as they can be truly great if handled correctly. If anything, I think the introduction of stereo in the late 50's was initially a bit of a setback, as some recordings from the late 50's/early 60's have that tacky "ping-pong" type of arrangement where one instrument is in one channel, and the rest in another. This is course is totally unnatural from a live music perspective, leading to the odd situation where mono actually sounds more like a real performance than stereo.

I'd like to address this since I have heard the master tape of Saxophone Colossus and we have released it in tape format on our label. The master itself has a sort of "wide mono" aspect to it, that in our opinion is an important part of the feel and texture of that album. I my experience the image does not waver, but it does have a large scale to it, and you can actually see some slight out of phase stuff on an x-y display when you play it, for which I have no explanation other than to say other mono tapes do not exhibit this when we play them on the same machine. Paul Stubblebine may have more thoughts on how this wide mono effect occurred. I don't think that you would get that sense of scale in that recording if it was played back with a mono head. So I think your solution to do this folding down to mono at playback is the right choice for you.

We have also found that in regards to the early stereo era some of the legends about whether something was recorded in three channels and both stereo and mono masters were generated from that, or recorded on both a stereo deck and a mono deck simultaneously, etc. are just that, legends. It appears that once stereo decks became available, most often jazz was mixed on the fly to stereo and that was folded down to a mono transfer for the mono release.

[Wanderer]

While we are talking mono/stereo....

I have early Six-eye pressings of the Duke Ellington "Anatomy of a Murder" sound track in both mono and stereo. The mix sounds so very different going from mono to stereo that I could be convinced different mic setups were used. Could be I am hearing the differenace due to mix down but the mono pressing sounds much more "present" while the stereo sounds a bit distant and "echoey". This change in perspective makes me wonder.     


Anybody have any knowledge on if producers experimented with different mic set ups for mono during a stereo recording session?       

                        Kevin R-M 

[Doc B.]

I'd suggest folding the stereo version down to mono yourself and comparing the two.

[Wanderer]

I'd suggest folding the stereo version down to mono yourself and comparing the two.

Embarassing to admit I have not tried that. As Homer would say "Do'h!!"

                            Kevin R-M 

[Paul Joppa]

If you short the left and right "hot" signals, then the difference component sees a dead short. The preceding stage is probably not designed to drive a dead short, much less drive it with full fidelity.

[Dr. Toobz]

I'm probably going to use the 3PDT switch I have laying around, with the signal lines for each RCA output jack on the first two center pins, and then the right side of switch for stereo, left for mono. For stereo, I'll take the output off the capacitors to the first two pins on the right side of the center, which would be just like the normal amp when the switch is thrown that way. For the mono side, I'll take the outputs off the two capacitors into a 1k resistor per each channel. One resistor will go to the third/lowest center pin, the other to 3rd/lowest left pin. Throwing the switch to mono will therefore join these together. I can then take the junction off one of those pins to the first two left pins adjacent to the aforementioned RCA outs. Throwing the switch to stereo will therefore also disconnect the two resistors going across the channels, so when I don't want mono, there should be no damage to the stereo signal.

Now let's see what it actually ends up sounding like....

[Paul Joppa]

OK, I finally thought of a simpler method. Connect the outputs of the volume control with a SPST switch. As long as the gain is set below maximum, the top part of the potentiometer protects the source device from a short with difference signals, and when the switch is open there is no change from stock, and no loss of separation either.

I was concentrating so hard on the summing circuit, I didn't until now think of combining it with the gain control. Another Homer Simpson moment!

[Dr. Toobz]

That sounds more straightforward than my 3-pole switch idea! I'll give the SPST idea a try first.

Now, if I placed a resistor in series with each side of the SPST, wouldn't that protect my sources from a dead short if the pot was all the way open? I'd lose a bit of gain, but as long as I'm not going to run into trouble with an open gain control, there's no reason not to notch it up a bit more. (Actually, there's more than enough gain going into my SEX amp as it is...)

[Paul Joppa]

Draw the circuit and study it.

If you add the resistors as you say, then you must connect the grids to the downstream end of them to get mono. That is, they become large-value grid stoppers. Not likely to be a problem, but it does require more re-wiring.

If you put the resistors between the input RCAs and the gain control, you are equally well protected. Gain goes down a bit, but even 20K resistors will reduce the gain less than 2dB.

Obviously, millions of possibilities!