Solid state power output into different loads

Cirrus

I know we've touched on it before, but just looking for some opinions here...

I've got a Crate Powerblock as a backup amp. It's 150 watt bridged into 8 ohms or seperate L/R outputs which are 75w each into 4 ohms.

I've got a 50 watt, 8 ohm cab.

What do you think the ballpark power output might be if I plug one side into the 8 ohm cab? Close enough to 50 watts that I can use it in a pinch?

ecc83

In theory as the load impedance on a solid state amplifier doubles the power halves. That would give you 37.5watts.

In practice the drop is not usually quite as much so you should get a good 40, maybe 45 watts. Into a guitar speaker of the usual ~100dB/W sensitivity that should be pretty loud.

Dave.

ICBM

It depends on the power stage and power supply design. The Powerblock is a Class D, SMPS amp so the power developed will be almost linear with load impedance - ie you will get almost exactly half into double the load. I would be surprised if it's more than 40W - that should be safe into a 50W speaker assuming the 50W is a real-world guitar-amp rating and you're not thrashing the amp.

(Which would probably be a bad idea, even into double the load impedance.)

paulmapp8306

I bow to the more knowledgeable members above, but will say Ive always used the rule of thumb (loose 1/3 power for every doubling of impedance).  At that 1/3 your looking at 50w max. 

As ISBM says though - Class D SMPS amps closer top to the Theoretical.  In either case you should be fine.

ecc83

Din't know it was class D. So yes, they are rather weaker sisters into higher impedances than AB tranny amps.

Dave.

Cirrus

Great, thanks for that! And, presumably it'd struggle to overpower my 30 watt 16 ohm cab as well? Though into a 16 ohm load does the possibility of over-stressing the power block increase?

ICBM

Cirrus said:

Great, thanks for that! And, presumably it'd struggle to overpower my 30 watt 16 ohm cab as well? Though into a 16 ohm load does the possibility of over-stressing the power block increase?

It should be OK - you'll get as little as 20W into 16 ohms. (Linearity increases even more when you're already further away from the minimum load.) The amp should be under less stress too - although again try not to push it into distortion in order to compensate for the low output, Powerblocks aren't very reliable at the best of times...

You may also find that you get audible switching noise coming through the speaker, the effectiveness of the filtering is dependent on load impedance.

Cirrus

ICBM said:

Powerblocks aren't very reliable at the best of times...

I was going to replace it with a .22 or .44 EH power amp+ a joyo ac tone, but the EH power amps seem even less reliable - I had one that arrived broken, my friend got one that arrived broken, the guy looking for one in the classifieds here this week got one that arrived broken, and I've read of plenty of others finding the same - it's either broken straight away or decays to its natural broken state in short order.

It seems like there's a gap in the market for a higher power, very small solid state amp that's vaguely dependable. 

ThePrettyDamned

Cirrus;92698" said:

ICBM said:Powerblocks aren't very reliable at the best of times...










I was going to replace it with a .22 or .44 EH power amp+ a joyo ac tone, but the EH power amps seem even less reliable - I had one that arrived broken, my friend got one that arrived broken, the guy looking for one in the classifieds here this week got one that arrived broken, and I've read of plenty of others finding the same - it's either broken straight away or decays to its natural broken state in short order.

It seems like there's a gap in the market for a higher power, very small solid state amp that's vaguely dependable. 

I've heard good things about the isp one