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Class D Amp impedance mismatch


James Nada
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Apologies if simmilar has been posted before...

My knowledge of matching cabs to amps is thus-
Valve amps must always be matched to a cab with an impedance equal to or lower than the amps stated impedance.

Class A and B solid State amps must always be matched to a cab with an impedance equal to or higher than the amps stated impedance.

What about Class D amps? It seems logical to me that they would follow the same "rules" as the above mentioned class A and B solid state amps. Is this correct?

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Actually no...!

All amps must be matched with [color=#282828][font=helvetica, arial, sans-serif]a cab (or cabs) with an impedance equal to or higher than the amps stated impedance. If you use two (or more) cabs [/font][/color]
[color=#282828][font=helvetica, arial, sans-serif]the combined [/font][/color][color=#282828][font=helvetica, arial, sans-serif]impedance is lowered e.g. two 8 ohm cabs = 4 ohms [/font][/color]

[color=#282828][font=helvetica, arial, sans-serif]A Class D amp will work the same as any other amp. [/font][/color]

Edited by peteb
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[quote name='Cuzzie' timestamp='1503008232' post='3355095']
Does the combined ohms rating not depend on whether they are in series or parallel as in series doubles, parallel halves?
[/quote]

Yes it does, but in practice multiple cabs connected to an amp will almost certainly be connected in parallel, as this is the way amps and cabs with more than one speaker connection are generally wired.

A cab with multiple speakers may be wired in series or in parallel, but this will be reflected in the total impedance of the cab. So a 2 x 12 cab with two speakers of 8 ohms can be wired in parallel to give a total impedance of 4 ohms, or wired in series to give a total impedance of 16 ohms.

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[quote name='peteb' timestamp='1503005740' post='3355077']
Actually no...!

All amps must be matched with [color=#282828][font=helvetica, arial, sans-serif]a cab (or cabs) with an impedance equal to or higher than the amps stated impedance. If you use two (or more) cabs [/font][/color]
[color=#282828][font=helvetica, arial, sans-serif]the combined [/font][/color][color=#282828][font=helvetica, arial, sans-serif]impedance is lowered e.g. two 8 ohm cabs = 4 ohms [/font][/color]

[color=#282828][font=helvetica, arial, sans-serif]A Class D amp will work the same as any other amp. [/font][/color]
[/quote]

Fair point - I was just answering on the basis of one cab. Same principle once you have more than one cab in that the combined impedance mustn't be lower, I just didn't think to add that bit :)

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[quote name='peteb' timestamp='1503005740' post='3355077']
Actually no...!

All amps must be matched with [color=#282828][font=helvetica, arial, sans-serif]a cab (or cabs) with an impedance equal to or higher than the amps stated impedance.[/font][/color][/quote]

This isn't true.

Transformer coupled amps (which is all valve amps and a handful of solidstate amps - the only recent one I know of being the Hellborg power amp) need a load on the transformer which allows current to flow, which is why they can't be used without a speaker plugged in (infinite impedance load). The transformer transfers power most efficiently if the impedance of the speaker matches the rating on the output tap - however, as impedance is never constant it isn't that perfect.

For valve amps the rule of thumb to use is that total impedance of the speaker(s) must be within 2:1 either way. If you have a choice between high impedance speaker on a low impedance tap or vice versa, then go for whichever gets you closest to matching but lean towards the speaker being a lower impedance than the tap. So basically, it's kind of backwards for valve amps, compared to solidstate amps.

With solidstate amps, the amp puts out a certain maximum voltage, and the lower the impedance of the speaker(s) it's driving, the more current will flow for that voltage, hence more power (watts = volts x amps). The minimum impedance is the point at which the amp can't deliver any more current without the output devices overheating or breaking down. This holds true whatever the class of solidstate amp (apart from the very rare exceptions with output transformers).

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But with regard to all this, assuming that you're not driving your rig overly hard then does it actually "matter"?

I like lots of headroom and never get anywhere near using half the power I have on tap. I don't ignore impedances completely, but I'm pretty bloody relaxed about them.

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[quote name='Happy Jack' timestamp='1503053982' post='3355335']
But with regard to all this, assuming that you're not driving your rig overly hard then does it actually "matter"?
[/quote]It does. You may get away with too high an impedance load on a valve amp or too low an impedance load on an SS amp for brief periods but you still risk triggering the protection circuits of SS, and worse, damaging the output transformer and/or valves with a valve amp.
[quote]I...never get anywhere near using half the power I have on tap[/quote]How do you know?

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[quote name='Bill Fitzmaurice' timestamp='1503056064' post='3355347']
It does. You may get away with too high an impedance load on a valve amp or too low an impedance load on an SS amp for brief periods but you still risk triggering the protection circuits of SS, and worse, damaging the output transformer and/or valves with a valve amp.
How do you know?
[/quote]

I feel a QI moment coming on 😁

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Well, "know" is putting it strongly, but my reasoning is as follows.

My understanding is that, broadly, to double the volume you need to increase the power by a much greater factor, perhaps as much as 10x.

If I know that my rig is capable of a given (ridiculously loud) volume, but I am actually playing at a much lower volume, then I'm pretty confident that I'm using much less than half the power of my amp.

My method of testing this is to count the number of amps that I have managed to blow up so far. By a bizarre coincidence, it turns out to be exactly the same number as the number of bass strings I have snapped to date.

Zero.

:)

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[quote name='Happy Jack' timestamp='1503057152' post='3355357']If I know that my rig is capable of a given (ridiculously loud) volume, but I am actually playing at a much lower volume, then I'm pretty confident that I'm using much less than half the power of my amp.[/QUOTE]

This is pretty logical - but it fails to take into account how things can escalate as gigs progress. I don't know if you play with any other drummers but Paul is the kind of drummer who actually has proper volume control, so you may be right. Replace him with a less wise youngster and all bets are off! :P

Usually when bassists say they don't use more than half power it's because the gain and volume knobs aren't past half way - with most players and basses and amps that's enough gain to get full power quite often...

[quote name='Happy Jack' timestamp='1503057152' post='3355357']IMy method of testing this is to count the number of amps that I have managed to blow up so far. By a bizarre coincidence, it turns out to be exactly the same number as the number of bass strings I have snapped to date.

Zero.

:)
[/quote]

Amps rarely blow up from being driven too hard, they're pretty well designed like that nowadays! I've only ever snapped one bass string in my life and I beat basses to death - and that snapped one had been boiled quite a few times (in my student days with a bass that didn't do brightness very well...)

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[quote name='alexclaber' timestamp='1503058282' post='3355370'] I've only ever snapped one bass string in my life and I beat basses to death - and that snapped one had been boiled quite a few times (in my student days with a bass that didn't do brightness very well...) [/quote]
I hope you took the string off the bass first? :huh:

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[quote name='markstuk' timestamp='1503057017' post='3355356']
I feel a QI moment coming on
[/quote]You have to bait a hook before you can set it. B)

[quote]The issue is you cannot tell by just keeping the volume knob down.[/quote]+1. Besides, it's not power that trips protection circuits, it's current. You simply can't tell if you're pushing the current envelope until your amp shuts down. Having a big amp with lots of head room helps, but with today's typical puny power supplies you probably don't have as much headroom as the amp power spec would lead you to believe. Even if you really do have 10dB of headroom a hard transient can eat up that 10dB in a heartbeat.

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[quote name='alexclaber' timestamp='1503062843' post='3355438'] I was saved from that mistake because it wouldn't fit in the pan, even dipping the headstock and neck in and waiting for it to soften up like spaghetti... [/quote]

Ikea sell big pans (for lots of strings that is).

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