Does a load box/headphone tap alter the impedance in any way?

[Mango Moon]

Hello Folks,

I know that the subject of amp/transformer output vs. net speaker impedance has been beat to death here and elsewhere, and I'd greatly appreciate your patience with me here on this entry-level (newb-type) question, but I've got a somewhat related concern that remains unanswered, so here we go . . .

I've got a low-powered amp (head) with a pair of parallel-wired 8-ohm Output jacks and no impedance selector switch that I normally use with a matched pair of 1x12 (8-ohm) cabs, one each plugged into each jack (i.e., a net 4-ohm load). Obviously, the amp was designed to handle such a 4-ohm load in this fashion or there wouldn't be two available jacks, and I've since confirmed this with the builder. Moreover, its sounds perfectly fine, so its all good!

However, I'd like to change things up somewhat by inserting a small (50w max) Load Box/Headphone Tap between the 8-ohm amp and the two 8-ohm cabs that would allow for silent (after-hours) use via its headphone jack. The problem is, unlike the amp, that Load Box/Headphone Tap has only one Output jack.

So, my question is this . . . will there be any risk in using a standard (12- or 14-ga.) 'Y' Cable made from high-quality 'speaker' wire to connect the two (8-ohm) cabs to the single (presumably 8-ohm?) Output on the Load Box/Headphone Tap?? In other words, does the Load Box (or its connection via 'Y' cable) pass the signal unchanged at 8-ohms or could it inherently alter the impedance in some way?

Many thanks!

[Chuck H]

I gotta be honest... I'm not getting the configuration your describing. If you run ANY cabinet off the jacks it's going to make noise and ruin any attempt at stealth. Other than that I would say that if the amp has two 8 ohm jacks it's designed to run one OR two 8 ohm cabinets. That said I would think that an 8 ohm load box plugged into one of the 8 ohm jacks would satisfy the load requirements. The fact that there is a headphone jack on the load box is of no importance to the amp since the impedance of the headphone circuit is probably high enough not to affect the primary load appreciably.

[Steve Conner]

I think he means the kind of attenuator that has a speaker in, a speaker out, and a headphone out. He's going to connect the amp to the attenuator, and the attenuator's speaker out to two paralleled 8 ohm cabs.

When the attenuator is set to zero attenuation, the amp should be connected straight through to the speakers, so it'll work exactly the same as before you had the attenuator.

As you increase the attenuation, the impedance that the amp sees will go away from the speakers' own and head towards the design impedance of the attenuator box. (They do have a design impedance: for instance the popular THD Hotplate comes in 4, 8 and 16 ohm versions.) So yes, the attenuator does alter the impedance in this respect. Your system is an 8 ohm amp feeding a 4 ohm load, so you have to decide what attenuator to buy, an 8 ohm one or a 4 ohm.

I would call the amp maker and ask him which impedance he thinks the amp would be happiest with for prolonged cranking: 4 or 8.

Final caveat: The amp might not have an impedance selector, but maybe there is a hidden one. Some Fenders had a switch contact on the extension speaker jack that selected a different tap on the OPT. If your amp is one of these, then you'll get a different result with just the main speaker jack in use, than if you used the main and the extension. Again one to ask the amp maker.

[Mango Moon]

I think he means the kind of attenuator that has a speaker in, a speaker out, and a headphone out. He's going to connect the amp to the attenuator, and the attenuator's speaker out to two paralleled 8 ohm cabs.

Yup, exactly! Except its not even an "attenuator" per se, its just a straight Loadbox/Headphone Tap with fixed (nominal) 8-ohm impedance. So the user can toggle between "Load" (no speaker output, only headphones) and "Speaker" (single 1/4", 8-ohm speaker out). Thus, the question remains . . . if I connect a standard 'Y' cable (made from suitable speaker wire) to that single (8-ohm) 1/4" Speaker Output jack on the Loadbox, and drive two (2) 8-ohm speakers with it in parallel (which is effectively presenting it with a 4-ohm load), could this damage the amp in any way?

I'll try to check with the amp builder and the manufacturer of the Loadbox/Headphone Tap directly (as suggested) to see if they can provide any additional insights on this as well, but this basic concern (as outlined) is essentially why I posted this thread to begin with.

Thanks Guys!

[Chuck H]

Yes, check with the amp builder as Steve suggests. How you plug in will depend on whether the there is any internal switching built into the jacks. But the way I see it, if there is internal switching then using the Y cable will set the amp up for a 4 ohm load. Since your using an 8 ohm box that wouldn't be ideal. Plugging from one 8 ohm output into the load box would be best in this case. If the amps outputs are parallel the Y cable will make no difference at all and so isn't needed. If you had a 4 ohm load box AND the amps output jacks are internally switched to set up the amp for a 4 ohm load when both jacks are in use THEN you would use the Y cable to set the amps impedance to 4 ohms for your 4 ohm load box. You do need to find out if the amps output jacks are parallel or switching and also what the ideal impedance for the amp actually is.

[Steve Conner]

I think he intends to use the Y cable to connect two speakers to one output jack, because the load box only has one, but the amp had two.

Not somehow connect both amp output jacks to the load box's single input. (Connecting two outputs to one input is almost always bad news.)

No, it shouldn't damage anything, the amp will always see either 4 or 8 ohms, and you already know it works with both of those. (but bearing in mind my "final caveat" above)

[Mango Moon]

Chuck/Steve,

I apologize for appearing to be so 'dense' on this subject and I have an e-mail into the manufacturer of the HA-50 Loadbox device itself (i.e., Weber VST) as we speak, but I'm afraid I remain a bit confused on this whole topic.

The amp I have has two 8-ohm outputs, and the builder tells me that they are "parallel" wired (no switching). He also tells me that the amp is designed to run perfectly fine into a 4-ohm load (using both of those outputs running to two 8-ohm cabs), and in practice, it sounds "awesome" (nice and fat!) when configured this way (i.e., when two 8-ohm cabs are used, one each connected to each of the two 8-ohm output jacks, presenting a 4-ohm load). Nice tone, nice low-end, just beautiful!

However, the problem (or concern) comes into play when I insert the single input, single output, fixed (8-ohm nominal) impedance Loadbox/Headphone Tap with two 8-ohm cabs running from it using a 'Y' cable. Since the Loadbox/Headphone Tap has only a single input and a single output, the system becomes configured as follows:

8-ohm Output #1 on Amp: Empty
8-ohm Output #2 on Amp . . . connected to . . . single (8-ohm) input on Loadbox/Headphone Tap . . . connected to . . . single leg on a 'Y' Cable . . . connected to PAIR of (8-ohm) speaker cabs via the double legs on 'Y' cable (4-ohm load on the Loadbox/Headphone Tap).

So, the amp to Loadbox connection should be just fine because the amp is seeing an 8-ohm load from the Loadbox (which both devices are designed to handle), but downstream, the fixed impedance (8-ohm) Loadbox will be seeing a 4-ohm load from the 'Y' cable and its two 8-ohm cabs, so that is the key question here. Will that 4-ohm load on the 8-ohm Loadbox cause any 'feedback' implications to the amp itself or could it cause potential damage to the Loadbox?

Again, your continued patience with me and my ignorance on this is duly appreciated.

[Chuck H]

What you propose is fine. The Y cable feeding the two cabs from the load box output should not affect the load box. I am assuming by it's nature that the speaker/load switch on your load box is a bypass switch (and perhaps marked that way) that routes the signal from the amp directly to the load box output jack. This will still present a 4 ohm load to the amp WITH the speakers in play. With the speakers NOT in play the amp will see the load box 8 ohm load. How the speakers are plugged into the load box has no affect on this.

One word of caution. Most Y cables are for small signal transfer and may not be capable of handling the kind of power your amp puts out. You do need to be sure that your Y cable is a speaker cable. I think they sell three female Y jacks at most major music stores.

[Mango Moon]

Excellent! I haven't heard back from the manufacturer on this yet, but what you've said here makes perfect sense and you sound like your quite confident on it, so that's what I was looking for the most. Its a great little amp and I simply didn't want to pose any risk of damaging it.

As for the quality and construction of the 'Y' cable, yes, I'll make certain that it can handle the task. It'll have a total of three jumbo male (not female) 1/4" plugs and be made from fully-coated, heavyweight (12-gauge) speaker wire.

Thanks again Chuck!