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resistive dummy load


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10/13/2005 1:23 AM
Andrei_T
resistive dummy load
Hi, I remember reading somewhere that itís better to use a resistive dummy load of letís say 16 Ohms with the amplifier set on 8 Ohms, but I donít understand why. Is this true? And can someone explain?  
 
Thank you,  
 
Andrei
 
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10/14/2005 11:50 PM
Jack Koochin

Andrei,  
I'm not sure what that reference was about... but if you're talking about doing power output tests into dummy loads, your amplifier won't develop full power out into 16 ohms when the OT is "expecting" 8 ohms.  
 
The reason to do this test is to see how the amp behaves under simulated actual load conditions, what the max power output is, and maybe freq. response or current draw. I'm not aware of any benefit of doing these tests at higher than actual load.  
HTH,  
Jack
 
10/16/2005 6:07 AM
Andrei_T

Thank you for your reply Jack, I built the resistive dummy load because I want to use it instead of the speaker cab, and then take the signal from the amp's LINE OUT and amplify it with a separate power amp. I want to have the amp's cranked tone at a lower volume. So the amp will run flat out most of the time.  
 
I read somewhere that because the impedance of the speaker is not say 16 Ohms throughout all it's freq range and it tends to rise on hi freqs and at the resonant freq, you need a dummy load higher than the amp requests to compensate those impedance nonlinearities. Otherwise you could fry the OT.  
 
Is it safe to use a resistive dummy load like this? and what would be the correct impedance?  
 
Andrei
 
10/16/2005 11:55 AM
Jack Koochin

Andrei,  
 
"I want to have the amp's cranked tone at a lower volume."  
 
I seriously doubt you'll achieve your purpose going from the line-out. This bypasses the whole output section. What you might get from the line out is a cranked preamp tone. Don't know what the amp is, either.  
 
If you want to use the line out, why not just yank out the output tubes, and run the amp as a cranked preamp? No speakers or dummy load required.  
 
If you had a resistive dummy load on the amp output, cranked it up, and fed a small sample of THAT output signal to the next amp, you would be closer to your aim. IIRC there are passive load devices out there (not sure if the Hotplate does this) that actually have an output you can do this with. Also, don't forget that the passive load won't interact the same way with the amp's output section that a speaker does, so it likely will sound different anyhow.  
 
Re: impedance... if you had a 16 ohm load on say, an 8 ohm output tap, the amp would draw less current, produce less power, and likely run a little cooler when cranked. I could see someone saying that you won't fry your OT this way. Also, the amp will likely have a different sound at 16 ohms. IMHO, I think the amp will be less likely to fry with a load one step larger than it is expecting to see (ie 16 ohm dummy, 8 ohm OT tap). Perhaps one of the techs with more experience on this could chime in here...  
 
I've heard stories of amps frying up using power soaks, but there is the theory there that the amp was not up to snuff in some way in the first place, or it wouldn't have fried (or would have fried anyways with a speaker load at that volume).  
 
Speaker impedance drops drastically at low frequencies, too, and the dummy load won't do this, so in that aspect, maybe they can be considered "easier" on the amp....? Depends here somewhat on the frequencies being amplified.  
 
Re: What is the correct impedance? What OT tap are you using? What does your schematic say is the correct speaker load for that amp?  
 
HTH,  
Jack
 
10/17/2005 2:13 PM
Andrei_T

Jack,  
 
The line-out in this amp (moderate-gain amp that I built, the schematic resembles a JCM800 but is voiced for more modern sounds) is taken from the 16 Ohm tap, so the power amp's sound is there.  
 
Andrei
 
10/18/2005 6:20 AM
Rick Adams
Andrei, I've been using a setup like this for years and it works fine. I've never had any problems at all with using an 8 ohm load (big resistor). What you will find is that the sound of the amp is not quite the same when you use only the resistive load compared to running through an actual speaker - that's why folks use things like the Weber MASS or a Hot Plate, etc. However, I find it works just fine for me with just the resistive load.  
 
 
 
The padded down speaker out doesn't sound very good if you run it directly to recording gear, however - you need some sort of cab emulator for that, or at the minimum you need to eq the signal pretty drastically. For running the signal to another power amp and then through speakers it works well.
 
10/19/2005 9:29 AM
Yan
There is good information on the subject on the Aiken Amps website, look in the techinfo section/ advanced, and dummy loads.  
 
http://www.aikenamps.com/  
 
Yan
 

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