Building a stereo/mono switch box

[scotth]

As I eagerly wait for my Crack to arrive, I am working on making a mono switch for use with my turntable, as my integrated does not have one.

However, I see a lot of conflicting information on the Web and want to make sure I do it the right way. On some forums, there seem to be hundreds of people happy with the "double y cable method" of which I was planning on creating a switch box equivalent of.   As I started looking around I saw a lot of people suggesting that resistors be added to the summing switch and that just shorting the signals together is the "wrong" method. Although, some people seem to imply that whether or not it is "wrong" depends on what location in the signal stream the summing is done.

I have not seen a comprehensive explanation as to what the "right" method is for my situation, so I thought I'd reach out to the experts here in this great community once and for all.

As I only have an integrated in terms of amplification, the switch would go between my turntable and phono input. What method is the best or "right" option for my summing circuit?  Happy to provide any additional information that may be needed.

[denti alligator]

Note that you really only need a mono switch for LPs made before 1968 or so. Mono records made after that are cut with a stereo lathe.

[fullheadofnothing]

I added a SPST switch to my ForeplayIII that bridges the hot pins coming off the selector switch. When the switch is in the on position, the two channels are connected and therefore summed. When it is off, the right input goes to the right circuit and left input goes to the left circuit. Cheap, easy, and effective. No reason you couldn't do this with 4 RCA jacks and a switch. Since you said you want this for "your turntable," I should point out here that whatever switching method you use should be after your phono stage, not directly off the tonearm wiring.

One big advantage of a mono switch is that it minimizes any out of phase information between the two channels. This ends up meaning that a great deal of surface noise is reduced, because it will not be identical from side-to-side. This is true regardless of what cutting head was used. I also use my mono switch frequently on reprocessed for stereo records as well as overly panned records. It's a delightful tool that I wholeheartedly recommend for many situations, both in and out of the vinyl/shellac realms.

I also know people (not know of, actually know) people that are actively using mono cutting heads in 2016. The previous comment might be valid for most major label output, but it is not a wholly accurate statement.

[scotth]

It has been explained to me that summing the channels can greatly reduce surface noise lebel, even if cut with a stereo lathe.

[scotth]

I should point out here that whatever switching method you use should be after your phono stage, not directly off the tonearm wiring.

Could you explain why this is true so that I may understand it better? Is there any way to do this with my integrated; short of opening it up?

[fullheadofnothing]

You should not parallel the output of your cartridge because it will improperly load your cartridge. This will cause far more damage to the sound than the benefit of monophying your signal. It is worth noting that without a mono switch, the records will play just fine. The image will be centered between the channels just as if you had the switch in place.

The hows of this setup will vary greatly depending on the equipment used. What is the integrated? What is the turntable? What is the cartridge? How does any/all of this connect to the Crack?

[pboser]

 There is a lot of talk about mono phonowhich frankly is over my head.   I have created line level mono-izers for use with mono amplifiers.   I don't know if there is any risk of damage By just connecting the positives together, but I have read that a 10 K resistor on each leg before combining protects the source componeny.  I also know that Edcor makes transformers to accomplish this.  They make them both for speaker level and line level. Here is a page about their line level offering: http://www.edcorusa.com/wsmseriescouplers

 If you were only going to use this with your integrated amp, you could wire whatever method you use into your tape monitor loop.

 Some ideas – hope this helps.
Pete

[scotth]

You should not parallel the output of your cartridge because it will improperly load your cartridge. This will cause far more damage to the sound than the benefit of monophying your signal.

If this is true, why do so many people seem happy with the double Y method into the phono stage? Including Mr. SHtv himself? I'm not trying to be argumentative, just genuinely trying to understand.

The hows of this setup will vary greatly depending on the equipment used.

Turntable: Kenwood KD-550
Cartridge: AT15Sa; 2.7mV output MM
Integrated: Marantz PM5004; Phono input impedance 47k

I was planning on hooking the Crack up to the Tape Output.

[fullheadofnothing]

According to the manual of the PM5004, hitting the T.Mode button on the remote will sum the Tuner input to mono. Perhaps it will for phono as well; it is worth trying as this is way easier than anything else.

It is unclear from the manual whether or not this unit has a monitor mode where it will send source signal to the tape out, but route sound output from the tape in. If it does, then that is the best place to utilize your summing device, as it would give easy switching into mono mode. If not, then really your only option is to do it out on the way to the Crack.

[scotth]

Is there a pro/con for using a transformer vs more conventional summing?

Unfortunately, my amp does not appear to have a tape monitor function. The T. Mode does not appear to have any effect either.

If I eventually get a phono pre, is there a way to hook that into my integrated and bypass the built in phono pre?

Are all the people who seem to be happy with the Y cables in between the turntable and phono input just oblivious to the loss in fidelity?

[pboser]

I forgot about this helpful page:
http://jelabs.blogspot.com/2016/04/stereo-mono-line-level-mixer.html?m=1

[fullheadofnothing]

Unfortunately, my amp does not appear to have a tape monitor function. The T. Mode does not appear to have any effect either.

I understand that it doesn't appear to have a tapeloop, but did you actually test it? Plug an interconnect into the in/out of one of the tape loops. Play a record on the phono input and hit the in button for the tape input and see what happens.

Hitting a mono button can sometimes a pretty subtle effect. When you tried T.Mode, were you playing a record that would be unmistakable (i.e. something heavily panned like some 60s records or a test record with channel identifications)?

If I eventually get a phono pre, is there a way to hook that into my integrated and bypass the built in phono pre?

Yes, just use any other input. Or plug it in directly to your Crack.

Are all the people who seem to be happy with the Y cables in between the turntable and phono input just oblivious to the loss in fidelity?

Probably. More likely they have never actually looked at the effects of their actions.

I measured frequency sweeps with the the channels summed going into a Reduction and coming out of a Reduction. Cartridge is a Grado wood body (I think it's a "Reference" model, but the point is it's a relatively standard cartridge).

Summing at the input of the phono stage does pad down the cartridge's output. The difference always at least 0.31dB, and at its highest is 1.1dB difference (that's a difference of about .6mV (which is more significant on a cartridge with 2.7mV than one with 5mV). Averaged out over the range, it's 0.72dB.

The overall trend seems to be that summing at the input of the phono stage leads to minor padding in low frequencies, steadily getting wider up until about 300Hz, where the difference is pretty consistent up through the rest of the audible range. The point here is that this will affect different frequencies different amounts, i.e. change the tonal balance.

The results measured here were less than I was expecting, but still enough to show that it is not a particularly good idea to sum at the output of the cartridge.

[scotth]

I understand that it doesn't appear to have a tapeloop, but did you actually test it?

Tried this today, no luck. I believe that whatever the source selector is set to is output through the tape out, so if I turn it to recorder 2, nothing comes through as nothing is being sent to the amp through the tape input.

When you tried T.Mode, were you playing a record that would be unmistakable (i.e. something heavily panned like some 60s records or a test record with channel identifications)?

I used a pretty well panned jazz album and listened through headphones. I also played with the balance control to make sure.

I may just go ahead and make the box since I have the materials. Would you recommend that I created a summing circuit with resistors instead if a simple shorting of the signal? If so, how do I determine which values to use?

Thank you very much for the detailed explanation. I guess I just need to start saving for an Eros so I can build a switch into it!

[denti alligator]

Bumping this up after many years because I now have an Eros and would like to build a mono-stereo switch. So: I‘ve got an enclosure (aluminum) and two sets of RCAs and I‘ve ordered a regular SPST toggle switch. How do I wire this? Simply connect both sets of RCAs to the two tabs of the switch? Connect the grounds? Is that all? What wire should I use? Thanks.

[Paul Birkeland]

If you have one of our amps or preamps, the way to do it is in the balance switch or in our attenuator upgrade.