Impedance switch for SII?

[Deke609]

The Stereomour II ... retains the OT-2 output transformer, which has been quite successful. That means the impedance switch upgrade kit will also be available.

Based on what I read in other earlier threads, it sounds like this is the same impedance switch kit used with the Smack, S.E.X. and SI.  If so, are any of those kits still available - or even just the board and the manual?

I'm going to be experimenting with different impedances on my SII and a switch would be great for this. 

Many thanks,

Derek

 

[denti alligator]

Ditto, I would buy one.

 

[Natural Sound]

I asked about this a while back. I was told that the impedance switch kit was discontinued.

 

[denti alligator]

It was never available for Stereomour II, though.

 

[Paul Birkeland]

The answer to this is multi-faceted, and I'm sure I'll forget a few along the way and others can chime in. 

The Impedance Switch kit took a bunch of these little PC board mount slide switches, and they always seemed to either be out of stock or maybe there would be 3-4 available at a given vendor when we needed to order them.  When we released the kit originally, this part was well stocked at several different vendors by the hundreds.  There's nothing worse than having someone's Steromour packed and ready to ship but the switches for the Impedance Switch kit add on are back ordered for three weeks. 

The manual was another conundrum.  The Impedance Switch kit went with Stereomour I, Sex 2.1, and Smack (not as an option, but rather it came with the Smack).  Making a single manual for an upgrade that goes into multiple kits is pretty difficult, as is supporting this as the products develop.  As anyone who has installed the Impedance Switch kit in a SEX can tell you, the manual was already a bit jumbled just based on the variations of the SEX 2 that were floating around; adding a 3rd would have made the manual even more difficult to follow.  We would have also needed to add instructions for the Steremour II and the Seductor, so now we are up to three variations of the SEX amp (maybe 4 since you can put the iron upgrade in a SEX 2.0), two different Stereomours, and the Seductor.  This is not tenable.

Operator error also seemed to creep up a bit.  I know Doc B. will even admit that we (Doc B would tell you that it was probably me) would accidentally bump those switches underneath when moving gear around for a demo, and it's really embarrassing to have to debug why you have channel imbalance during a demo when an amp permanently wired at 8 ohms would never run into that problem.  If this caught us off guard periodically, how many headaches are we generating for everyone else with this kit?

The issue I would bring up is actual performance.  As I have mentioned on other threads, we actually eliminated instructions for anything other than 4 and 8 ohm operation in the SEX 3 manual to eliminate the repetitive threads of Grado/IEM users wiring their amps at 32 ohms and wondering why they hear hiss and why they can only turn up the volume knob two degrees before the amp is at full volume.  The available power on the 4 and 8 ohm taps is sufficient for 99.9% of the headphones out there and the noise floor is beautifully low in these configurations.

(I would mention that I haven't ever found a use for the 32 ohm setting)

 

[Jamier]

PB, why not include instructions, for example in the SII, for wiring the OTs with multiple Taps on the chassis like many commercial amps have? There might be room on the chassis for one common and three different impedance choices. The builder could choose to wire for 2,4,8 or 4,8,16 or whatever
the OTs can accomodate.

Jamie

 

[Paul Joppa]

The transformer is not designed with one continuous secondary winding; the impedances are made up with parallel or series combinations of sections. That's why it took three double-pole switches for each transformer.

 

[Paul Birkeland]

To add to PJ's comments about his secondary designs, if you have a tube amp with binding posts for 4, 8, and 16 ohm speakers, only half of the copper on the secondary is used when using a 4 ohm speaker.  This issue can be circumvented by having multiple windings and placing them in series/parallel arrangements to get what you need, or a couple of tapped windings.  The downside to this is cost and convenience.

 

[Deke609]

That's why it took three double-pole switches for each transformer.

Because I don't understand how the OT wiring works, the idea that an impedance switch for the SII requires only 3 On-On switches for each OT is completely baffling.

In my attempt to figure out an impedance switching mechanism for the SII, I have treated the OT as a black box and assumed that for each of the 4 impedance wiring schemes (2, 4, 8, 16 Ohms) only those connections indicated in the manual may be made. So far, the only on-paper solutions I have come up with are: (A) one 2-polethrow, one 3-polethrow, and two On-Offs; or (b) a four position quad deck rotary switch that simply combines the previous solution into one switch.  Not very elegant.  And neither of these include a switch for balanced/unbalanced!

I won't post more details because I don't want to be responsible for someone toasting their amp, or worse.

I'm sure others who understand the OT wiring, and/or are better at combinatorial puzzles, can figure out a more elegant way of doing things.

I'm going to keep working at it. And then test the solution by having resistors with different values stand in for each of the 6 relevant OT terminals - the idea being that I work out what the net voltage drops for the various connection chains should be, and then use my meter to see if that's what I get in different switch positions.  Not yet sure whether this will actually work as a test of the solution.

cheers,

Derek

[edited to correct my mistaking "throws" for "poles"]

 

[Jamier]

Two, two pole switches will provide 4 possible combinations (2,4,8 and 16). Plus 1 switch for bal/ unbal. Right?( Both down, 1up 1down, 1 down 1 up, both up).I don't know the details of how the secondary windings were connected but I can imagine that the switches connected them in series or parallel to produce the desired impedance at a given switch position.If you knew the details of the OPT construction it would probably be easy to figure out the combinations, but I think that information is probably considered to be proprietary.

Jamie

 

[Deke609]

I think I figured out where I went wrong - I confused double pole with double throw and was trying to solve the problem with SPDT switches.  I'm going to rework the problem using DPDT switches - that should hopefully get me down to 2 switches.

 

[Paul Joppa]

It's not a secret, I think this has been posted before. Here's the circuit. They are DPDT switches, except the balanced/unbalanced which is SPDT.

 

[Deke609]

Awesome. Thanks PJ.

 

[Deke609]

Even just as a solution for impedance switching (leaving aside the bal/unbal part of the circuit), that is a very clever solution. It would have taken me a good long while to come up with it - and most likely I wouldn't have and instead settled for using more switches. Many thanks for sharing this, PJ.

cheers,

Derek

 

[Paul Joppa]

Haha! Yes, it took me a good long while to sort it out as well.

 

[Deke609]

Minimum current rating for the DPDTs?

 

[Paul Birkeland]

Ohm's Law can tell you this:

The amp makes 3.5W into 8 Ohms.  Solve for current, then add some safety margin.

(Repeat as necessary for other impedance taps, all of which will make 3.5W but into a different load)

 

[Deke609]

Hah!  Thanks PB. That was dumb!

 

[Deke609]

I'm going to build the impedance switches this wknd and have a few questions:

(1) If I were to leave out the bal/unbal part of the circuit and only build for impedance switching, I can leave out the resistors, right?  Their only purpose is to provide hot and cold references to ground in balanced mode?

(2) Is there any performance advantage (sound, health of the amp, etc) in implementing balanced at the OTs rather than at the speakers? If there is an advantage to balancing at the OTs, what value of resistor should be used? Anything in the same 50-100 Ohm range that applies when balancing at the speaker outs?

Many thanks,

Derek

 

[Paul Birkeland]

(1) If I were to leave out the bal/unbal part of the circuit and only build for impedance switching, I can leave out the resistors, right? 

In the schematic PJ posted, a resistive center tap is provided for two of the balanced impedance settings, while the other two use the junction of the two secondary windings as the center tap. 

(2) Is there any performance advantage (sound, health of the amp, etc) in implementing balanced at the OTs rather than at the speakers? If there is an advantage to balancing at the OTs, what value of resistor should be used? Anything in the same 50-100 Ohm range that applies when balancing at the speaker outs?

You're talking about putting resistors that handle very little current on one side or the other of a few inches of wire.