QED headphone control wiring - any guesses

[starbuck]

I have a relatively simple system upstairs of a Rotel CD player, passive pre amp built around a Dale/Vishay stepped attenuator, Quad 306 power amp and a pair of Spendor SA1 speakers.

I wanted to add the facility to listen via headphones and to that end, purchased a cheap QED headphone control box, expecting it to be one their little boxes with 2 pairs of speaker cable connections to connect from the power amplifier and another set of cable connections to go on to the speakers. What I have received (through no fault of the seller, who was very helpful and responsive) doesn't have the connections expected. I opened the box to find a screw terminal block with only 6 connections and without any wiring instructions, I am a bit stumped as to how it should be wired up. The blue cables exiting the box are what it came fitted with and if I was hazarding a guess, I would say that these were the ones to connect from the amp although I have no idea which would be left + and - and which would be right + and -.

I've taken photos of the box and the internals as best I'm able, I wonder if anyone can hazard a guess as to which connections should go where? Once identified, I will label it up and fit different coloured cables to identify things more easily. If it turns out I am unable to use it then no real loss as it didn't cost a lot. Anyway, some pictures:

20230111_144610 by Chris To Go, on Flickr

20230111_144636 by Chris To Go, on Flickr

20230111_144652 by Chris To Go, on Flickr

20230111_144707 by Chris To Go, on Flickr

20230111_144747 by Chris To Go, on Flickr

20230111_144753 by Chris To Go, on Flickr

20230111_144753 by Chris To Go, on Flickr

 

[Craig B]

I wired up a MkII version for a customer once. This had a clearly labelled 8 terminal strip out back which made things simple, as there need be 4 connections in and 4 connections out for L and R stereo from amp to L and R loudspeakers.

By comparison, with 6 terminal junctions, this original version would require one to common one pole (+ve or -ve) of each amp channel with the same pole of each loudspeaker at the terminal strip, with the opposite poles to the loudspeakers being made or broken by the switch. As 'pink' and 'green' connect directly to the switch, with the switch making or breaking contact between these and 'red' and 'brown' respectively, I would expect these to be -ve, as 'blue' and 'black' appear to be connected to the input side of the pots, therefore, Rch 'speaker +ve need be ganged with amp Rch +ve at 'blue', and Lch 'speaker +ve need be ganged with amp Lch +ve at 'black', with Rch 'speaker -ve to 'green' and Lch 'speaker -ve to 'pink'.*

The way that the external blue leads are connected at present indicates that these are from amplifier to headphone control box. You may want to mark these as follows.

Blue lead that connects to blue internal wire is Amp Rch +ve
Blue lead that connects to brown internal wires is Amp Rch -ve
Blue lead that connects to black internal wire is Amp Lch +ve
Blue lead that connects to red internal wires is Amp Lch -ve​

* Lch vs. Rch identification based upon the rear most pot of ganged stereo pots generally being assigned as Rch. YMMV.

 

[starbuck]

That's awesome, thank you very much for the explanation. I will have a look at it over the coming weekend, I'll do my initial testing on some sacrificial gear before using it with the upstairs system, in case I do anything wrong.

 

[Craig B]

Agreed, always best to take a safest approach to testing.

On that note, recommend simply connecting to the sacrificial amp according to the pattern above, and leaving the loudspeakers out until such time as you have chance to test via headphones only. Should headphone channels be the correct way round and in phase (at least wrt L vs R), with the volume pot working as it should, then you will know that all of the external blue leads are connected correctly. This would also make the proposed loudspeaker connection pattern a foregone conclusion (at least for moi), as there is simply no other way to include the 'speakers without them too going through the volume pot and/or not being switchable.

 

[starbuck]

Thanks again, will do exactly that with just some cheap headphones connected initially. Am I hoping for too much for it to sound okay, as and when I have it all connected up correctly?

 

[Craig B]

Am I hoping for too much for it to sound okay, as and when I have it all connected up correctly?

I wouldn't think so. After all, you will be driving your 'phones from a 306 through a volume pot, and your loudspeakers won't see a thing as QED always used a decent quality switch with silver plated contacts (plus the 'speakers don't go through the volume pot).

Some might complain of one too many pots in the signal path on headphones, but your Dale/Vishay stepped attenuator might just as well not be there.

If you do find that you perceive some degradation via your Spendor speakers you could always fit the amp end of the QED wires with stackable bananas and run both the QED and loudspeakers directly from the amp. Not as convenient as a switch but absolutely no possibility of sound degradation. I've found these Deltron ones to come in very handy over the years. Rega also used these on their very cool headphone adapter from yesteryear.

deltron-553-series-4mm-inline-stackable-plug-datasheet.jpg​

 

[starbuck]

Thanks again for your reply, Craig. I look forward to adding the QED headphone option for early morning/later evening listening. Another option which I can consider for the future if I wanted to, is that I remembered that my Rotel CD player has both RCA phono and balanced XLR analogue outputs, so the addition of a dedicated headphone amp could be achieved by using both of those outputs to feed separate speaker/headphone set ups. I expect the QED will be good enough for my needs, though.

 

[Craig B]

RCD-1572?

Just FYI, @starbuck, manufacturer's, including Rotel, advise against connecting both unbalanced and balanced outputs at the same time. They typically don't state why, however, the reason is that this places the circuits of both connected units in parallel, but only on the one signal leg of each balanced channel. IOW, the in-phase leg of the balanced connection ends up loaded by the unbalanced connection while the anti-phase leg isn't, therefore the signals on the two legs of the balanced circuit will be unequal in amplitude, which will mean they are no longer functioning as a balanced differential pair of signals, potentially losing any inherent advantages of balanced operation.

Looking on the positive side (pun), no damage should result, only the balanced side may not perform to spec wrt extraneous interference rejection; not generally a problem with home hi-fi over short runs to begin with.

Another thing that comes to mind is that, should the time come that you wish to try a dedicated headphone amp upstream, there are some interesting combined DAC/headphone amps about which could plug straight into your Rotel's SP/DIF socket with no negative affect upon the analog side and vice versa. Essentially, you'd have two independent un-compromised systems sharing the same source.

 

[starbuck]

Its an RCD-991. Thanks for explanation/advice re. using both analogue outputs, I think it will be a non-issue given the QED will hopefully suffice for my purposes. I'll keep in mind the idea of a combined DAC/headphone amp for the future and will keep my fingers crossed I get the QED working over the weekend. Will report back when I have it connected, knowing it works along with your excellent instructions may be useful for anyone else on t'internet who may stumble upon the thread in the future.

 

[Craig B]

Once you get the QED going, don't despair should there be any volume pot noise. This thing may not have been used for some years and, although looking spotless inside, may need a bit of pot tracks/wipers exercise. A dozen or so full turns, end stop to end stop, will likely as not clear things up should it need it. I always prefer to do this before resorting to contact cleaner sprays. The switch is likely to be fine, as these were always decent quality on the early QED boxes.

BTW, I stumbled upon a Google pic of the hookup instructions for a same vintage QED Speakers Volume Control Type 4/5 (for use with remote speakers in another room, from long before these became common). Same 6-terminal block within, this time with the amplifier and 'speaker negatives ganged together on each channel.

 

[starbuck]

I think I found the same Type 4/5 volume control image you did whilst searching for the wiring instructions for the headphone control (there is an ebay listing for one here: https://www.ebay.co.uk/itm/14459065...d=link&campid=5338728743&toolid=20001&mkevt=1 ). I imagine a household of such devices would have seemed quite futuristic at the time they were first released.

I've now had an opportunity to wire up the headphone control to an old Cambridge Audio amp as per your excellent instructions and I have music (and no scratchy volume control, either). Next up is to try it with some old speakers attached and then to buy some more banana plugs for termination of the wires as the 306 doesn't accept bare cables. I tried it on some old earbuds initially and tentatively experimated with the volume control(s) to get sound and a useable range on the QED volume knob, and it sounded good, so risked a better pair of headphones and it sounds great. I think it will be more than adequate for my needs.

Thanks again for all your help, I owe you a few beers.