Hi
I recently purchase a brand new RX120RH from a store that imported unsold units over europe and I'd like to know if I can use it without load (speaker)
I havent seen anything in the manual. Randall solid state amps, if I understand correctly, use some weird continuous power in the power section, not sure if it's related to Class A/B but some people on a forum reported they were not sure if it could be used without a load
Please post the forum where they posted about it. I would like to read what they posted.
Quote from: DrGonz78 on November 24, 2024, 11:58:14 AMPlease post the forum where they posted about it. I would like to read what they posted.
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/961808-randall-rh200sc-head-repairing-power-amp-board-gs-200s-b
https://www.ssguitar.com/index.php?topic=798.0
I could not find power section schems for this, but I found a photo of the internals. The power board shows a pair of big transistors and a pair of smaller ones all on a heatsink. A pair of ceramic resistors nearby is also visible. By these superficial features, it looks like a regular Class B or AB emitter follower output stage. The only way such a thing would be unstable without a speaker plugged in would be if it had no voltage feedback, only current. I think, nobody does that. It wouldn't sound good. And an oscillating solid state power amp with no load should not damage itself.
There is such a thing as solid state amps that have output transformers. It was more common in the late sixties maybe early '70s? Using one was one possible design which allowed non complementary transistors to be used: i.e both of them NPN. Another way is a non-complementary totem pole output stage.
We had a thread recently about a Radio Shack Realistic brand PA amplifier is not quite that old and has an output transformer, like a tube amp. That lets it have multiple outputs for different speaker impedances, which is nice.
I suspect that a solid state amp with an output transformer could possibly have issues with no load, due to the flyback voltages and whatnot, like a tube amp. I didn't see anything like an output transformer in the picture of the Randall's internals.
I really wouldn't worry about it. Maybe there's a user manual for it out there? If you find one it doesn't mention this issue I would definitely not worry about it.
Quote from: Kaz Kylheku on November 24, 2024, 05:12:28 PMI could not find power section schems for this, but I found a photo of the internals. The power board shows a pair of big transistors and a pair of smaller ones all on a heatsink. A pair of ceramic resistors nearby is also visible. By these superficial features, it looks like a regular Class B or AB emitter follower output stage. The only way such a thing would be unstable without a speaker plugged in would be if it had no voltage feedback, only current. I think, nobody does that. It wouldn't sound good. And an oscillating solid state power amp with no load should not damage itself.
There is such a thing as solid state amps that have output transformers. It was more common in the late sixties maybe early '70s? Using one was one possible design which allowed non complementary transistors to be used: i.e both of them NPN. Another way is a non-complementary totem pole output stage.
We had a thread recently about a Radio Shack Realistic brand PA amplifier is not quite that old and has an output transformer, like a tube amp. That lets it have multiple outputs for different speaker impedances, which is nice.
I suspect that a solid state amp with an output transformer could possibly have issues with no load, due to the flyback voltages and whatnot, like a tube amp. I didn't see anything like an output transformer in the picture of the Randall's internals.
I really wouldn't worry about it. Maybe there's a user manual for it out there? If you find one it doesn't mention this issue I would definitely not worry about it.
Appreciate it your help!
Finally setting up
On the back it says 220V/50hz and 250V/50hz marked, I assume they could have written 220~250V but for some reason it's double marked. Here where I live it is 220/60hz, I hope there wont be issues
It's heavy as a truck. 30kg (~66.5 pounds)
The size of the heatsink on the back, I assume it is to handle the class AB power section because there is no fan.
Mine RX120RH has no line out, there is an headphone out jack (I hope it doesn't have any sort of speaker emulation eq curve) and a send/return. Hopefully I can get line level out from this send
There are two external speaker jack, it simply says "TOTAL: MIN 4ohm". I can assume it can handle 16ohm and a 8ohm cabinet, but I'm planning to buy a 4x12 8ohm and I hope it will work with no issues. Or perhaps a 2x12 8ohm and later another 2x12 8ohm for a 'stereo' effect plugged in each side of the room, I assume it will work. The manual doesn't have much specification on the speaker configuration
Perhaps the design was meant to work with a 4x12 60w in one jack and the other output speaker with another 4x12 60w
https://imgur.com/a/PEXvKrZ
Not sure if here is the right place to ask, but if I buy 4 speakers 8ohm each, I should connect in parallel right? The total amount will be 8ohm still? and if each speaker is 25W (4x25) = 100w total, safe for this amp right?
NO
do not connect them in parallel because you will get 2 ohms and destroy the amp.
You want them connected in series/parallel to get 8 ohms.
2 - 8ohm speakers in series is 16 ohm (make up 2 of this)
2 - 16 ohm in parallel is 8 ohm
Look at example 4 on this page:
https://www.amplifiedparts.com/tech-articles/speaker-impedance-power-handling-and-wiring
Thanks for the help
Btw
Do you guys know any way to make the preamp circuit or the poweramp less noisy? When I engage the distortion channel it has too much noise, it cant be fixed with a noise gate
I tried already having a dedicated Neutral + Earth from the circuit panel
Also tried a 2 choke + 2X + 2Y cap circuit and it's not EMI related
Any idea how to make the circuit less noisy? thank you
Quote from: nkzmme on March 16, 2025, 11:42:04 AMThanks for the help
Btw
Do you guys know any way to make the preamp circuit or the poweramp less noisy? When I engage the distortion channel it has too much noise, it cant be fixed with a noise gate
I tried already having a dedicated Neutral + Earth from the circuit panel
Also tried a 2 choke + 2X + 2Y cap circuit and it's not EMI related
Any idea how to make the circuit less noisy? thank you
Put the gate in the fx loop if it has one.