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Resistors to improve amp/speaker synergy


tube fanatic

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It is no secret that tube amp/speaker synergy is very important.  This is especially true with single ended pentode amps which do not employ negative feedback and are sensitive to speaker impedance variations across the audio band.  A trick which I, and others, in the design world have always used is a resistor across the amp output terminals, or speaker terminals, to keep the impedance the amp “sees” more constant (i.e.“flattening the curve”, an expression with which all are now familiar).  It can make a huge difference and allow amp/speaker combinations which may not sound great to suddenly become glorious.  
 

So, it is gratifying to see a speaker manufacturer actually promoting such a tool:

 

https://www.zuaudio.com/diy/loading25r

 

The resistors they furnish can be purchased for a few dollars less but will not be matched.   As someone who designs single ended amps which are used almost exclusively with Klipsch speakers, I can tell you that using the resistors can be a worthwhile tool.

 

Maynard
 

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4 hours ago, tube fanatic said:

As someone who designs single ended amps which are used almost exclusively with Klipsch speakers, I can tell you that using the resistors can be a worthwhile tool.


As long as the power wasted from the amplifier into the “snubber” resistor doesn’t limit your amps ability to supply the power required for the desired SPL Level from your speakers.

 

ie: Its not a free lunch 🙂

 

miketn


 

 

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Unless I am wrong when you put resistors in parallel, the speaker is considered a resistor, the resistance the amplifier sees will be lower then the speakers nominal resistance. Am I wrong Mike? 

 

I first heard of putting a resistor in parallel with the speaker was from Nelson Pass on one of his amplifiers that ran best with a 8 ohm speaker and a customer had 16 ohm speakers and the amplifier did not perform as it was intended. After placing an 16ohm resistor in parallel with the speaker everything worked as intended and the customer was happy. Unless I am mistaken it was a low power Firstwatt amplifier. 

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Agree Mike!  But that is the nice aspect of using Klipsch speakers most of which are so efficient (as you know!) that the power loss is not generally an issue.

 

The resistor does not turn the speaker crossover characteristics into a purely resistive situation but will lower the effective impedance seen by the amp.  The graph on the Zu link above demonstrates this.  Even if the speaker impedance drops to 4 ohms, the resulting slight decrease should not be a problem.  I usually use 27 ohms which covers most situations nicely.  It is the very large impedance increases which the resistor really helps to contain.

 

Maynard

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This remedy addresses issues with pentode amplifier characteristics (high output impedance and high plate resistance) as opposed to deficiencies in crossover characteristics.  I don’t know many audiophiles who would be qualified to reconfigure a crossover which, of course, would change what the manufacturer had in mind.  Installing a resistor to make the amp “happier” is something anyone can do.

 

Maynard

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Some additional information can be gleaned from this old thread on the forum:

 

https://community.klipsch.com/index.php?/topic/34263-how-does-impedance-matter/

 

When one considers that the output impedance of some SEPs without feedback can hit 30 ohms or so, holding the speaker impedance to a narrower range is certainly valuable.  
 

My work with SEP spud amps (no fb) has been enlightening in this area and all the future builds will include an internal, switchable, snubber resistor.  Such amps can really sound nice!
 

The SET crowd snickers at us guys who still use pentodes...😄

 

Maynard

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its super timely that this topic came up. I recently build a spud amp (6cl6 based) and it sounds great on my cornscalas running alk crossovers. I recently tried the same amp on my forte ii and it was a different story. I know that the forte ii are supposed to be a pretty rough impediance load and the alk crossovers are supposed to create a nice easy speaker to drive. 

 

The amp is borderline unlistenable on the forte ii but 90% as good as anything ive ever heard on my cornscallas. Does this seem like the issue that Im running into?

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Many Klipsch 8Ω-labeled systems use 4Ω woofers to boost bass output measurements by +3dB.

OK on horn-loaded systems where the acoustical loading increases the AC impedance at the amplifier.

Questionable game of specsmanship when the marketing guys, not the engineers, write the spec sheets.

Early KG4's were labeled 4Ω, quickly changed to 6Ω. Two 8Ω woofers in parallel are 4Ω, not 6Ω or 8Ω, kids!

My Quartets, Forte II's, and Chorus II's - all sold as 8Ω speakers - have 4Ω woofers to generate that big BASS!

Transistor amps deliver twice the current at 4Ω, tube amps limp along with transformer limitations & distortion.

Measure DCR at the speaker terminals to see what you have for a load - the AC impedance won't be much higher.

 

 

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7 hours ago, HiFi Heaven said:

Many Klipsch 8Ω-labeled systems use 4Ω woofers to boost bass output measurements by +3dB.

But in LaScala or KHorn, horn loading increases impedance. So even if nominal free air or infinitive baffle impedance is 4 Ohm when placed inside of a horn enclosure, impedance raises, it can be even doubled.

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8 hours ago, brl0301 said:

its super timely that this topic came up. I recently build a spud amp (6cl6 based) and it sounds great on my cornscalas running alk crossovers. I recently tried the same amp on my forte ii and it was a different story. I know that the forte ii are supposed to be a pretty rough impediance load and the alk crossovers are supposed to create a nice easy speaker to drive. 

 

The amp is borderline unlistenable on the forte ii but 90% as good as anything ive ever heard on my cornscallas. Does this seem like the issue that Im running into?


It may be.  Can you get the impedance plot for the speakers with the ALKs?  With the factory crossovers there is a huge impedance rise in the midrange which the resistors would bring way down.  
 

In what respect are they unlistenable?  
 

 

Maynard

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13 hours ago, brl0301 said:

I know that the forte ii are supposed to be a pretty rough impediance load and the alk crossovers are supposed to create a nice easy speaker to drive. 


This a common misunderstanding about the ALK Crossover with the swamping resistor. It is actually a “more demanding load” on the amplifier to maintain a more constant impedance load for the amplifier.

 

When making the speaker load impedance more constant when using amplifiers with relatively “higher internal output impedance” like many single ended tube amplifiers have will result in the loudspeaker frequency response error due to amplifier/loudspeaker impedance interaction to be minimized.

 

miketn

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I was hoping that Al furnished the Forte plot with the crossovers.  You may want to ask him for it.  It is strange that you are hearing slight distortion at low volume levels.  Is your amp the Spudkit?  If so, ask Jerry if he has encountered this with other Forte users.  What other amps have you used with the Fortes successfully?

 

Maynard

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3 hours ago, tube fanatic said:

I was hoping that Al furnished the Forte plot with the crossovers.  You may want to ask him for it.  It is strange that you are hearing slight distortion at low volume levels.  Is your amp the Spudkit?  If so, ask Jerry if he has encountered this with other Forte users.  What other amps have you used with the Fortes successfully?

 

Maynard

Ya its the spud kit amp. The alk crossovers are on a set of cornscalas and the forte ii are running stock crossovers. Ive run the fortes with my "dynaco" st-120, a class d audio and a tubelab tseii. Fortes sound good with all other amps but really struggle with the spud

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Im in southern california. 

 

I just hooked up the amp again to get a fresh idea of the sound and maybe distorted wasnt the right word. Def very bass light. Mids and top sound ok at best but bottom end is mostly missing. If i run the volume up, it just sounds shrill. 

 

Its weird because this amp sounds amazing on my other speakers but just doesnt seem to get along with the fortes. Maybe ill drag the fortes into the main listening room and see if the combo sounds any better in that room. 

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