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Power (watts) and Valve Amp recommendations for 4-way studio monitors


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Hi Guys

 

I am looking at upgrading my amplifier in the next week or two. Have had a good run with a Classe Twenty-Five but am wanting to go back to tubes.

 

Previously I had a tube integrated but upgraded to a  Pre/Power combo as I was chasing more power. My main speakers are JBL 4315A and JBL 4315B, only difference being one set has full alnico drivers the other set ferrites. I do rotate between other studio monitors but these are the mainstays at the moment.

 

I was very impressed with the bass grip the Classe had and am chasing the same control from a tube power amp/mono's. Below is a picture of the impedance curve for my speakers. Are these a hard load to drive?

 

post-142892-0-65709800-1413702867_thumb.post-142892-0-77626300-1413703160_thumb.

 

How much power should I be chasing? I have been looking at Audio Research, VTL, Quicksilver amps.

 

Dr Hi Fi has a VTL ST-150 which is a bit below my budget but quite interested regardless. It is only 80 watts in Triode and I am concerned it might run out of steam with the 4 ways.

 

Any suggestions or insight is appreciated.

Edited by kelossus
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Are these a hard load to drive?

 

We see that there is impedance magnitude down to 2ohms.    This means that you need an amplifier which can supply a lot of current.

 

... but whether or not this is 'difficult' or not, depends on phase of the current and voltage draw in the amplifier... and for this we must see the phase of the impedance (not on your chart), to see if there are negative angles which correspond with the low impedance magnitude .... which would mean that your speaker is a difficult load to drive  (as well as needing a lot of current capability).

 

Circuits using tubes aren't ordinarily designed to drive down to 2ohm load ...... and they like to see a constant (resistive) impedance phase plot.

 

 

 

How much power should I be chasing?

 

This depends on one and only one thing.   How loud do you play them?

 

Really quick rough calc says that you could listen to them at 4m at 'reference level' (105dB) with less than 100w .....  but this is quite loud.   You may get away with < 25w .... but it's hard to say.  Power increases exponentially, so turning up the dial a little bit more, can make substantial demands.

 

Anything other than a test tone and SPL meter, using the volume setting you dialed in from "as loud as I would ever want" music .... is just guessing  ;)

 

 

concerned it might run out of steam with the 4 ways.

 

Not sure what you mean, but just in case....  4way (or 5way, etc)  speakers need less power not more than 2way (or 3way)....   more cone area needs less power to make the same noise

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concerned it might run out of steam with the 4 ways.

 

My previous tube amp was Triode/UL. On UL it was 50w which was not my preferred listening mode. When I use to turn the speakers up to a volume I found suitable it would start to clip.

 

The 250w Classe doesn't raise a sweat driving them. Was hoping a more modern Tube amp might be able to drive them. I thought VTL's were meant to be stable to 2 ohms?

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I thought VTL's were meant to be stable to 2 ohms?

 

Designed to drive 2ohms is like saying  "the forklift is designed to lift up to 300kg"

 

 

It doesn't tell you much about what happens when you're lifting 300kg though.    When you look at the phase of the impedance (shows how "constant it is in time") ..... it is like asking  "how violently can I shake around that 300kg weight when it's up on the forklift".

 

Do you need to drive the forklift very carefully with 300kg on it ? ........ or can you basically do donuts with it when carrying the load ?

 

 

The 250w Classe doesn't raise a sweat driving them

 

It is common 'mistake' to equate the extra power (watts) with the improvement you've noticed  ..... but this depends on actually how loud you are talking about.     Your speaker is about 32watts intput for 105dB output at a few meters .....  so if you are suffering from a lack of watts, then you must be listening quite loud.

 

Very much more likely.   Your old amplifier was running out of current (into the 2ohms), and/or unable to deal with the (not very flat) impedance phase that I would expect your speakers have.

 

 

 

The very short version of all this babble is that I expect your speaker needs an amplifier with low output impedance ... high current capacity ... and about 50w (SPL dependant).     This precludes most tube amplifiers.

 

Hope I'm helping   ;-)

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Thanks Dave, appreciate your insight.

 

I do listen fairly loud as the dynamics of the big 4 ways are addictive.

 

When you say most Tube Amplifiers aren't suitable what are some of the exceptions without spending over 5k (if there are any)

 

Chris.

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Thanks Dave, appreciate your insight.

 

I do listen fairly loud as the dynamics of the big 4 ways are addictive.

 

When you say most Tube Amplifiers aren't suitable what are some of the exceptions without spending over 5k (if there are any)

 

Chris.

You need to seek out a big, push pull triode amp. Look for one with a low(ish) output impedance and at the very minimum, a 4 Ohm tap. Triode amps possess far superior low impedance load capacity. However, as Dave intimated, you might be better seeking out a decent SS amp. There are lots around, which may suit your needs.

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Thanks Dave, appreciate your insight.

 

I do listen fairly loud as the dynamics of the big 4 ways are addictive.

 

When you say most Tube Amplifiers aren't suitable what are some of the exceptions without spending over 5k (if there are any)

 

Chris.

Are you any good at DIY ? - This may suit - http://www.transcendentsound.com/Transcendent/Transcendent_Sound_BEAST_OTL_Tube_Amp.html

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When you say most Tube Amplifiers aren't suitable what are some of the exceptions without spending over 5k (if there are any)

There aren't many tube amps with 4ohm (or even better  3ohm) output taps AND the ability to produce 50W.

 

Having said that, 3-and-a-bit db down (20W) there's a whole pile of options - many of which handle transients better than conventional solid state.  But $5K RRP for three channels (ie. three monoblocks) is going to be challenging.  Second hand, perhaps easier.

 

I agree with Dave that your problem is likely to be the 2ohm dip.   I'd put a round of beers on the impedance having some seriously non-resistive points too. 

 

I'd see if you  can borrow/loan the usual suspects in high-current land: ME, Quad 404, (N)AKSA, even Krell  to name a few crowd favorites.

 

Having said all that, the VTL ST-150 datasheet claims to be able to handle 2 ohm loads.   In which case, 60W of PP triode will be more than enough.   Can you do a prepurchase trial? 

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There aren't many tube amps with 4ohm (or even better  3ohm) output taps AND the ability to produce 50W.

 

 

The 4315's are rated at 8 ohms. Should I be looking for an amplifier with a 4 ohm tap?

 

Having said all that, the VTL ST-150 datasheet claims to be able to handle 2 ohm loads.   In which case, 60W of PP triode will be more than enough.   Can you do a prepurchase trial?

 

 

I have spoke to James at Dr Hi-Fi and he is apprehensive to recommend the ST-150, he doesn't believe it will be able to drive the 12" drivers like I am use to.

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The 4315's are rated at 8 ohms. Should I be looking for an amplifier with a 4 ohm tap?

 

Heh.   Look at the chart.    You need a 2ohm tap   ;-)

 

More importantly....  if the impedance is not resistive  (which is the phase line which I was saying was missing from the impedance chart) .... then you need a "gutsier" amplifier.

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I have spoke to James at Dr Hi-Fi and he is apprehensive to recommend the ST-150, he doesn't believe it will be able to drive the 12" drivers like I am use to.

Then he hasn't looked at that impedence curve.     It's the mid range droop to 2 ohms that's confronting - below 100Hz is easy in comparison

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I've owned and used both speakers extensively.

IMHO, ME, Krell, Classe and similar do a great job on them. From memory, the best was an original Krell KSA 250, which had a great synergy with them. Something I've noticed with other JBL/Krell pairings.

I did bi-amp some 4350's for a while, with OTL'S on the more sensitive horns and tweeters, and Krell on the bass. Impressive, but hard to get it sounding coherent. But that was probably just me ;)

Valves will "work" but you won't hear what they're capable of without an awful lot of valves & current ability to boot.

Cheers

'Nutz

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Just bought a ME850 to give a go.

 

Probably shouldn't have bought it without listening but they have a great reputation and could probably get my money back if i decided to move it on. Now i have to wait for it to come.

 

Thanks guys.

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