it's the old ohms query

[LeoBurke]

Ok, so I have in my possession this little SS amp head, but I'm a bit scared of hurting it/my gear/myself because I'm a bit stumped on what I can and can't connect.

It has a stereo effects loop (yay) and stereo outs. The outs are 4 ohm at 75W each.

What are my options and restrictions? (Can I mismatch?, if so, how?)
What is my best bet? (Do i need to be aware of matching wattage values?)

I get the impression (from some reading) that if i run the outs into 8ohm speakers then the output wattage will infact be LESS than 75W. Is this so? How do I work out what the actual output will be given a discrepency of head/cab?

Are there obvious cab options?

Following a look through previous threads some extra info: i'm pushing the preamp with an overdrive/distortion/compressor, and using the stereo effects loop. I'll be using the stereo outs into a stereo cab (which i need to buy!!). I'm going to be making it loud.

Thanks. L

[Tom Phillips]

Leo,
Regarding your power questions.

In general, with SS power amps, you can mismatch the output load but there are some rstrictions
.
Since your amp is rated at 75 Watts for a 4 Ohm load, you will get the most power with a matched 4 ohm load. How close you get to 75 watts will depend on the honesty of the amp manufacturer and the aggressiveness of their marketing department.

You will get less power output if you use an 8 or 16 ohm ohm load. This is just the nature of the output circuit design of the SS amp. I'd guess that power output would drop to around 45 Watts with an 8 Ohm load.


If you mismatch the other way by hooking up a less than 4 Ohm load you MAY get the same or less or a little more power. However, this will produce more heat stress inside the amp and is NOT recommended for a musical instrument SS amp driven hard.

Regards,
Tom

[pdf64]

The rule with ss amps is not to go below the rated load impedance, else devices can overheat and go into meltdown quickly.
The rule with speakers is not to overpower them. Power amps are rated at their max clean power, but if overdriven can put out 50% more.

I get the impression (from some reading) that if i run the outs into 8ohm speakers then the output wattage will infact be LESS than 75W. Is this so? How do I work out what the actual output will be given a discrepency of head/cab?

An easy way to get your head around this is to think of your amp as a voltage amplifier. 75 watt into 4 ohms means it can put out 17.3V before clipping (V=PxR ^1/2).
17.3V into 8 ohms = 37.5 watts (P= V^2/R).
In practice though it will put out a little more than this because the lower currents will create lower volt drops so it can put more volts out = more power, but you get the idea?
So assume about 50 watts into 8 ohms.
Marshall do stereo 4 x 12 cabs ie two 2 x 12 cabs, in 1 cab. Or get a couple of 1 x 12 extension cabs with suitable drivers in. Peter.

[Enzo]

You will get approximately half the power with 8 ohm speakers as with 4 ohmers. but so what? half power is only 3db less loud - not a lot

And the two power amps are separate, so one has no idea what the other is doing, so if you feel like it one can have a 4 ohm speaker and one an 8 ohm. Not what I'd do, but certainly it will do no harm.

You can rub it to a stereo cab or two separate cabs if you like. Thew amp has no idea where its loads are located.

[Steve Conner]

Hoots mon! I've got a Hiwatt reissue 2x12" cab for sale in Glasgow if you're interested. I can wire it stereo for you. It will be loud.

steve

[teemuk]

Sorry for being (slightly) off topic, but I think it’s a damn shame they don’t fit these solid-state amps with those impedance matching autotransformers you sometimes see in some PA gear – and what were used at least in some Peavey, Marshall and Univox amps. I think it was a good idea. Someone sells such a device as an external box, though – unfortunately, I can’t recall the name of that product.

[Enzo]

PV sells their Automatch - or at least used to - that fits your description.

Yep, still there, 400 watt impedance matcher autotransformer with isolated secondaries for 70 and 100v lines. $94.

[Steve Conner]

There are several reasons why they don't fit those autotransformers any more.

1: Cost and bulk. Getting rid of the output transformer was one reason why the industry ditched tubes for three-legged fuses in the first place.

2: DC offset. A transformer has a much lower DCR than a voice coil, and a closed core that's vulnerable to saturation. If you connect it to a SS amp that has even a moderate DC offset, a large DC current will flow, the core will saturate, and bam! A new set of three-legged fuses please.

This is a consideration when retrofitting a transformer to a SS amp that didn't originally have one: I assume that any amp that had one from the factory will have this issue suitably taken care of. But I've seen app notes that recommended adding two huge back-to-back electrolytics as a DC block capacitor.

3: Distortion. It screws up the THD and damping factor. Good for musical instrument use, but looks bad on a PA amp's spec sheet.

4: Alternatives. You can add an impedance selector to a SS amp by changing taps on the PT: no output transformer is needed. Transistors have a broader "powerband", as it were, than tubes: if you feed them a lower voltage rail with a higher current capacity, they'll happily get down and grunt a lower impedance load around.

The reason you don't see this used more commonly is that Peavey have the patent on it. They don't use it any more, but they're still sitting on it.

5: OK, I lied. PA amps for driving 100 volt lines still (mostly) have the transformer. My first home-made guitar amp was a Philips solid-state PA amp that I got from a flea market, I ripped the 100 volt line OT out and installed a tube preamp in the space. My ears still hurt just thinking about it.

[teemuk]

The reason you don't see this used more commonly is that Peavey have the patent on it. They don't use it any more, but they're still sitting on it.

The selectable secondary voltage… Well, Peavey can’t possibly have a valid patent on that since I’ve seen that invention at least in some NAD HiFi amps from the early 80s, and if I remember right, it might have been used in some H||H amps too… Like that autotransformer impedance matching circuit, this as well is what you call “prior art”. But my memory is a bit blurry and I don’t bother to hunt down the schematic references.

Anyway, it’s a nice idea as well but naturally one has to rate parts for the highest voltage and design the circuit in mind that voltages in various points are subject to fairly broad change. Whereas with the autotransformer in the output the basic amp circuit pretty much remains the same while the transformer just “pumps up” (or down) the output signal voltage to establish the impedance match. It requires no changes to circuit and of course, you can buy such thing as an external device and fit it to any amp.

As for issues with DC, I think any decent SS amp should have a protection circuit against DC offset since DC is not very healthy for speakers either and fault modes where the speaker output latches to a high DC potential are quite common. This is again one of those things where guitar amps tend to drag few decades behind in comparison to current development of things. Maybe manufacturers think that one relay and few discrete components costs too much. Personally, I think they’re lot cheaper than a speaker of good quality.