Lab Series L9

[Dave Pipkey]

I was just wondering why all the Lab Series amps are rated 8 ohms min Impedence . Most SS amps are very happy down to 4 ohms and some even down to 2. is it the power transistors they use circuit design or maybe both. thanks.

[Enzo]

Hi, welcome to the forum.

A solid state amp can be designed to do whatever you want it to do. When these were designed, they certainly could have easily designed it to run 2 ohms or 4 ohms if they had wanted to. The output transistors on that are RCA1B04 types. Four per side, eight total. Those are 200v, 7A, 150W parts. Probably wouldn;t blow up at 4 ohms, but it just isn;t rated for it.

Just an opinion here, but the reason an amp would go down to 2 ohms would be so it can drive a boatload of speakers. No one has a 2 ohm cab, so that implies at least a pair of 4 ohm cabs, or four 8 ohm cabs. But consider who the amp was designed for. Not, in my view, the screaming Eddie Van Halen or Pete Townsend types. No wall of speaker stacks. The Lab Series is the amp BB King plays. It is clean, strong, and steady. Not deafening.

[Dave Pipkey]

Hi, welcome to the forum.

A solid state amp can be designed to do whatever you want it to do. When these were designed, they certainly could have easily designed it to run 2 ohms or 4 ohms if they had wanted to. The output transistors on that are RCA1B04 types. Four per side, eight total. Those are 200v, 7A, 150W parts. Probably wouldn;t blow up at 4 ohms, but it just isn;t rated for it.

Just an opinion here, but the reason an amp would go down to 2 ohms would be so it can drive a boatload of speakers. No one has a 2 ohm cab, so that implies at least a pair of 4 ohm cabs, or four 8 ohm cabs. But consider who the amp was designed for. Not, in my view, the screaming Eddie Van Halen or Pete Townsend types. No wall of speaker stacks. The Lab Series is the amp BB King plays. It is clean, strong, and steady. Not deafening.

I wasn't going to run it at 2 ohms but I was hoping I could run an ext. speaker and all mine are 8 ohms so I'd be running it at 4 ohms. I'm a bass player I know the amp doesn't care what I play through it the speakers might. Haven't looked at the L4 schematic yet but do they use the same out put transistors I know there 200watt The preamp is a little diferent also but thats not to importent I think the only difference is the reverb and mabe a little different voicing in the tone stscks. Thanks

[jrfrond]

Actually Doug, Peavey has a 4 x 12 2ohm ported bass cabinet, and it's not an accident. The specs are on the nameplate on the rear. Four 8 ohm speakers wired in parallel. Exactly WHY they made it, I don't know, but I recently worked on one and it surprised the heck out of me.

Concerning solid-state amps and speaker loads: Speaker loading = current draw. The lower the speaker impedance, the more current you draw. The more current you draw, the hotter the amp operates, and if the semiconductors get hot enough, they'll short or burn open.

It is THEORETICALLY possible for an amp to offer unlimited current draw to the load, providing the components are kept cool enough. Of course, in practice, this won't fly, but cooling is of the utmost importance.

Speaker impedance is a nominal rating at a single frequency, and changes with frequency, because it's an inductive load. Impedance is lowest at lower frequencies, so if you are a bassist, or a sound engineer concerned about power amps on subs, you have more to think about, because bass frequencies hog power amp current. You probably COULD operate at 4 ohms, with only a slight decrease in reliability.

[Dave Pipkey]

difference in output wattage?

I was looking at the power amps for my L9 and the L4 they both use the same transistors and both circuits look the same to me but I might be missing something. so why the difference in wattage the L9 is 100 watts at 8 ohms the L4 is 200 watts at 8 ohms? what am I missing are they just driveing the output transistors harder or what.

[loudthud]

It would have to be the rail voltage. It takes about 1.4 times the voltage to double the power.

[Enzo]

John
You are right, there is an exception to most anything, just meant that looking down the list of available speakers, a 2 ohm cab would be an unusual item.

[J M Fahey]

Hi dave pipkey.

the L9 is 100 watts at 8 ohms the L4 is 200 watts at 8 ohms?

There are two versions
The board and the general schematic is the same, but the transistors and the supply voltages are not.
The 100/8 is +/-63V and uses certain Delco transistors; the 200/8 uses RCA 1B04 as mentioned and +/- 83V.
I think the 8 ohm choice came because the original amplifier was designed around the very high quality (and heavy) EVM15L or EVM12L , at the moment available in 8 ohms, and with some difficulty in 16 ohms.
It would also instantly differentiate the amp from all other SS contenders.
Most were 100/120W into 4 ohms, and high power Acoustics were 200W into 2 ohms.
That's what you could accomplish with then standard 2N3055/40411/MJ802/4502 workhorses and their typical +/-40V power rails.
100/8 was accomplished by some Yamahas and 200/8 by no one else.
To put things in perspective Crown DC300 were just born.
As a side and personal note, in the 70's my own guitar amps were 200/2 driving 2 2x12" cabinets and my 400W bass amps were really 2x200/2 driving 2 4x12" 2 ohm cabinets.
PA systems were poor and stage levels were hairy.