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Driving 5LEs w/ 4-8ohm OTC speaker amps a danger?

post #1 of 8
5/17/11 at 6:54am
Thread Starter 

 

Quote:

 

Originally Posted by Fang View Post

 

“For those who plans to drive HE-5LE by speaker amps: Please make sure your tube amp can drive a 38 ohm load. Some tube amp's output transformer only can drive 4 to 8 ohm. A solution is to wire two 8-ohm cement resistance in parallel. Solid state amp do not have such problem.” - Fang

 

Also: "If tube speaker amp's output impedance is only for 4 or 8 ohm, it will be risk to drive HE-5 which is about 25ohm."-Fang

 

Quote:

Originally Posted by immtbiker View Post

The theory is that a speaker amp that drives 4 or 8Ω driving 70Ω might damage the transformer on the output. I have asked

several engineers and have been told everything from "don't worry about it" to using the formula to add a resistor in parallel

which equals:

 

R total = R1 + R2

           --------------

           R1 X R2

 

which equates to approximately 8.8 ohms for the HE-6 and 11.2 ohms for the K1000's.

If you are using an OTL or SS amplifier, then there is no need for a parallel resistor.

 

I was thinking of using the fleawatt Miniwatt N3 as a speaker amp for HE-5LEs.  The above two quotes are from the HE-5LE thread. As Fang states, it might be a risk to drive the 5LEs out of a speaker OTC amp that drives 4, 6 or 8 ohms like the Miniwatt.  I'm not sure if that warning is applicable to the Miniwatt since it barely puts out any power - can anyone clarify?  

 

Also, the Miniwatt puts out 3.5W RMS @ 8 ohm.  I think that roughly translates to <1W for the 38 ohm 5LEs.  Is it a bad idea - too low powered?  Does it defeat the purpose of using a speaker amp in the first place?  I think the Lyr puts out something like 4.8W @ 38ohms.

 

Thanks all..

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post #2 of 8
5/17/11 at 7:50am

The risk with a transformer coupled amp, and an "inappropriately high" load (38ohms instead of 8) comes at you from a few angles. 

 

The first problem is that when transformers are underloaded (dont have a low enough impedance on the secondary) they tend to ring. Transients overshoot and blow the sneeze out of everything. If nothing else this sounds terible with a harsh edgy sound that nobody really cares for.

 

The second problem is actually more serious. If you have a pentode amp, or certain triode amps underloading the transformer causes the primary voltages to go SKY high. This is pretty bad and can lead to all sorts of interesting failures. 

 

A third danger comes with amps with feedback. The ringing and funky primary voltage swings can cause instability. Yea, its bad. 

 

On that note, all of this can be solved by paralleling an appropriate resistor like Immtbiker said. At worst it does nothing, at best it saves your amp.

 

Id personally use a 12ohm resistor for all the headphones - considering what passes as an 8ohm speaker it gets you close enough. If your amp has 16ohm outputs use them with a different resistor.

 

Regarding power: 

If you already have the amp, try them! Until you drive the amp into clipping (and you'l know instantly) there is no danger from using a low power amp. Speaker MFR's reccomendations that you use a 1200W amp stem from them (the speaker MFR) wanting to make sure you never go into clipping and blow up their tweeters. 


Edited by nikongod - 5/17/11 at 7:52am
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post #3 of 8
5/17/11 at 8:29am
Thread Starter 

Thanks.  Don't already have the amp and I wouldn't bother with paralleling a resistor, as I wouldn't know how.  Sounds like trouble.

 

 

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post #4 of 8
5/17/11 at 8:32am

You just wire the resistor across the outputs. 

 

If your having someone make you an adapter cable they can build the resistors in.

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post #5 of 8
5/18/11 at 12:56pm
Thread Starter 

Quote:

Originally Posted by nikongod View Post

You just wire the resistor across the outputs. 

 

If your having someone make you an adapter cable they can build the resistors in.


Ari, I really wouldn't even know how to do that, but since I don't even own the amp yet, it's easier to just not buy it.

 

So that aside, what about headroom?

 

I know the Schiit Lyr is capable of something like 50WPC peaks, but would that be enough for the 5LEs?  I'm sure RMS power of something like 4Ws+ at 38ohms is more than adequate but what about peaks?

 

There's a speaker integrated out there called the Alesis RA-150 which puts out 75Ws RMS at 4 ohms.  Head-fi'er Grokit uses this with a Grant Fidelity tube processor in his HE-6 rig and I'm guessing has all the headroom he could possibly need.

 

Would something like that be overkill or a reasonable option?  It costs just as much as the Lyr.  And tack this one on as well, is a tube processor a bad idea?  It does add one extra component to the chain.


Edited by sphinxvc - 5/18/11 at 1:01pm
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post #6 of 8
5/20/11 at 1:11pm

I'll chime in on this smile.gif

 

The consensus seems to be that resistors are warranted when driving headphones out of the speaker terminals of certain tube amps, but this is not a concern at all with solid state amps.

 

As far as the tube stage goes it certainly wouldn't be as desirable if for example you are using a source with a tube output stage like a Havana or Music Hall DAC. Also in addition to giving you that additional euphonic tube flavor I think the tube stage may help with impedance matching between components.

 

I really like the way I have the Alesis set up but I would ditch the Devilsound and the tube stage for the balanced Havana DAC in a heartbeat just to take full advantage of the RA150's dual mono design, and yes having one less component in the chain. But because the Devilsound is hardwired to it's own digital & analog interconnects, ATM it doesn't really seem like an extra component so much to me.


Edited by grokit - 5/20/11 at 1:14pm
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post #7 of 8
5/20/11 at 3:42pm
Thread Starter 

I'll be getting my XLR-speaker leads adapter early next week; waiting till then to see whether I'll want a tube buffer or not since the 5LEs are a touch warmer than the HE-6.  If only I could find a good, reputed OTL speaker amp that puts out 5Ws+ into 38ohms and costs about $400-500.

 

The Havana route is tempting.  I'd love to have a Havana + Stax Lambda rig sometime, I think it'll make a killer pairing.

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post #8 of 8
5/20/11 at 3:51pm

I used the Alesis without a tube stage for months. It makes a definite difference but the RA150 is quite good on its own as well.


Edited by grokit - 5/20/11 at 3:52pm
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