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Here you will find useful information, tweaking tips and other useful information. Is there a topic you would like to see on this page? Let us know! Some of these are simply e-mails we have answered. Sometimes the answers are made separate from the question and others are integrated into the text in bold to add context to the answer.

Email response to customer regarding 6DJ8 type tubes.

Note: If you want to add to this send me an email.

As to excellent choices I recommend:

 Always make sure you can return noisy tubes, choose vendor wisely. The 6DJ8 family is more susceptible to noise than some other tube types.

Your Philips are often good, but so many variants like most brands. If they are quiet you can't go wrong with any of these.



“Lights Dimming” type of start up issue.

On 2/13/2017 Adam G. Wrote:

Note: Some new info on the DH-500/XL-600 thermal switches here.

Adam had concerns about data loss on computer on same AC circuit when he powers up his Hafler DH-500

> Disregard that question, the 5 ohm NTC resistor worked. It was only hitting the voltage regulator earlier cause it still had some charge in it.


> Thanks for the power supply!(Our PS-500)


I think your concern is fully understandable, but I've never seen an instance of problems. I have a bad-ass printer on my CAD system and I keep it at idle. When it starts a print cycle it causes my UPS to click in. Hundreds of such start-ups has never resulted in data loss.

First a UPS should help the computer though the reaction time would be paramount. They're not expensive.

Short of a time delay relay with large resistor bypass for X seconds you're going to get a drop of some significance. Some eBay vendors sell these though it's kind of left up to you to determine the best parallel resistor(s). A BIG wattage is recommended. I suspect the original thermistor, inrush current limiter some call it, was about taking some stress off the power switch. Original 500s tended to cause flame-outs there. I recommend going back to the original or like replacement.

Your 5R NTC resistor may have a very short life because the instantaneous wattage is harsh. I hope it is a physically huge part.

I have a computer in my sound system. Starting a DH-500 has never caused an issue for me but I do have a top-line Seasonic fan-less power supply which is awesome and not cheap. It's the job of a good power supply to instantly adjust to line conditions.


DH-500 Heat Sensors

Subject: DH 500


I know you prefer phone, but email is better for my schedule! I have an old Hafler DH 500 that was modified by Musical Concepts in probably ’96 with, as I recall, the LED driver input stages and maybe something else. Anyway, I still use it on my second system but it has developed the habit of going into thermal shutdown (I think) after it has been on a while. It shuts down, the front switch/light blinks and, in time, it will come back on for a while before doing it again. So I am considering whether it is time to give it a merciful sendoff or, if the probable issue is relatively minor, go through another mod. So since you guys did the mod years ago, I figured it best to contact you first!

I kinda hate to give up on the old DH500 – it was my first ‘high end’ amp!



Our response:

Hi Michael!

We have recently repaired a lot of these with aging thermostat heat switches. They shut down or have wacky fan speeds for the existing thermal conditions. Each DH-500 has two thermal shutdown thermostats and two that control the fan speed. We have seen so many of these malfunction that we recommend replacement as a matter of maintenance now. These switches are modestly priced. If we are doing another mod at the same time we will already have the amp opened up and so labor is less expensive than if we were to do it as just a repair without further mods. These great old amplifiers are worth saving every time especially given the sound of our latest upgrades.



Why not use low bias current?

Terry wrote:

I keep reading your website, and I never see one negative thing. I bought a mod from you for a Hafler Dh500 back in 2002. Isn’t a amp to amplify the signal and not change it? Why would I want to bias the amp deeper with higher ma setting? I was told from someone else to set the ma at 20ma-30ma, and that the setting does not make a difference. He said the most important part is to have the Dc output as low as possible? For neutral sound ,is it ok for me to have amp bias at 30ma, and does that setting have anything to do with power output?

Our Reply:

Higher current in the output devices moves them more into Class-A mode. Class-A is the most pure form of amplification. Unfortunately it is also the least energy efficient. A Class-A amp of any significant power will require a very robust power supply and an extreme heat removal strategy that takes the form of huge heatsinks, fan cooling, etc. Biasing the amp lower or higher has very little to do with the maximum power output.

Most amps are Class-AB, meaning that a few watts are reproduced in Class-A and as the amp is required to produce more power it "switches" to Class-B. This makes it possible to produce affordable amps that won't fry eggs. By reducing the idle current as suggested you dramatically reduce the power at which the amp switches to Class-B. Why is this a bad idea? When distortion is an appreciable percentage of the power being produced it is easier to hear. The switch to Class-B is accompanied by crossover notch distortion. If that notch is at a very low output power then the human ear, which is already sensitive to that form of distortion, is more offended by the distortion component. The tiny, delicate details that separate good reproduction from mediocre is harmed. There is, of course, a lot about this found by Googling "Class-A",  "crossover notch", etc.

I assure you I didn't make this stuff up. This distortion can be measured and heard given an adequate system. It has been accepted engineering practice since there have been amplifiers. If your adviser cannot hear the difference with low levels of bias then I accept his opinion, but it is based on one of three things, a.) a system that can't reveal the distortion, b.) a lack of hearing acuity, c.) he hears but he thinks the difference is inconsequential. I don't demean that either.

Having said all of that you can even run our amps with the bias turned off, ice cold. At that level crossover notch distortion will be more apparent, but not grievously annoying.

Wanna hear something funny? Several years ago we ran two of our Musical Design amps with zero bias at a CES show. Many knowledgeable people thought we were in the top three for sound at that show.

I think that tells us to just have fun with audio and worry less about the fussy details. If you like something go for it.

My advice is to listen to delicate low level sounds at different bias levels, you know like brushes on cymbals and drum heads, acoustic guitar, well recorded woodwinds, female voice and so on. Make up your own mind.




FAQ:  Your misbehaving Hafler or other MOSFET amplifier might have a front-end problem or perhaps bad MOSFETs(fets from now on). How can you tell 95% of the time with a minimum of hassle? Each fet has a resistor connected in series with the gate lead. If the gate has shorted then this resistor is not going to show a correct resistance value. For instance, in a Hafler we have both 220 Ohms and 470 Ohms( some have 470 Ohm for all) connected to these gates from a terminal strip attached to the heatsinks. Take a voltmeter and try to measure the resistance of each individual resistor while the resistors and fets are still in the circuit. Test each one for the actual value. If the value is correct then the attached fet is most likely fine. Let's say you measure a 470 Ohm resistor and it shows 110 Ohms. This particular fet is likely to be bad. If you find a "smoked" resistor that's usually a bad sign too. Then there's that 5% where you have a fet looking good that has not totally failed but has been damaged or might just be on the verge of a life span failure. You want to be sure - pull all the fets, marking from whence they came, then take them to a repair shop where they can test them for various failure modes. 




What do you set the bias at on the DH 220's.  I have 2 "factory" values of 250 and 275ma and will set it to the lower one without further direction? --------You can even set this at 400ma. Of course, you will pay a penalty in heat, electricity and very slightly reduced lifespan. You will gain a smoother sound with greater naturalness.  

They have the DH500's at 350ma but I'm going to use your value of 330ma - much safer. ---------I see your logic, but I doubt that you will find it safer. At some point, as you raise the current, your fan will kick into the second speed and this degrades the sound if you are anywhere near the fan. 

Thanks,  Eric



sirs, I've been reading your website, I had a Hafler DH500 amp.  A few questions, you mention sonicly superior SOLDER?  I always thought solder is just melted metal holding the components in place..., how can it have a sound quality?  --------We will soon publish more information on this subject in a new newsletter to be posted on our website. This will be a detailed description of solder properties as we believe they relate to sound quality. In the meantime, I think you will agree that most people believe that solder is either solid or liquid. Not so! Most solders actually solidify over a range of temperature.

Also you mention in replacing power supply caps, with superior sounding ones you supply, again, can you please explain how capacitors have their own sound qualities? --------------Do you believe that all flutes, violins and guitars sound the same? Then why would a musician pay hundreds of thousands of dollars for a Stradivarius violin? For that matter, are all cars the same, telephones, baseball bats, ice creams? If so, then you should buy the cheapest thing that will do a certain job. Pardon me for answering your question with questions, but understand that a capacitor is actually a complex device, appearing simple, with different resistance/impedance, inductance and dielectric noise problems. Things such as the methods of attaching the internal foils to the electrodes have an effect. The dielectrics and separators are issues that are the subject of ongoing, constant research. Unfortunately, not all of man's technical problems have been solved and will not be solved in our lifetime or that of our children.    

Lastly, in your circuit boards description, you mention the superiority of yours versus Hafler's, eliminating "bounce effect".  i can't find that spec anywhere, can you explain.  --------Here I was mainly referring to the profile width of the foils we use on the circuit board. Circuit boards are as imperfect as many other of the materials that we work with. Our civilization is in it's technical infancy and we have to find ways to design around these burdens. Energy from the signal traveling on the board material is, to some degree, absorbed by the circuit board and over a period of time it is re-released back into the foil from which it came and the other foils in proximity. These effects are called dissipation factor and dielectric absorption. We use thin width, but thick foils to minimize the amount of this energy leaked into the board material.  

your add on are a bit pricey..a new Hafler model 9505 has a 7 year warranty, for the similar wattage, Hafler has an excellent reputation.  Why should I buy your kits over a new Hafler.  I'm trying to come to a conclusion in my search.  ---------If you believe that power and warranty are the only things to consider in your decision then you can easily find the warranty and power for even less money. The price of a used Hafler XL-280 or DH-500 would be under $400, maybe less. Adding our basic kit for $299 would provide you with an amp of considerable sonic prowess, well beyond that of any production Hafler. I should know, we were technical consultants on the original Hafler 9500 series. Many of them are outfitted with Musical Concepts capacitors.  If you seek further information and views on our mods, you can bypass magazines and such and go directly to the people who buy these kits and form their own opinions, some are found on our web site under "Customer Testimonials" and others at the web site www.audioasylum.com . Do a search for "Musical Concepts" on that site. 

Regards, Carl



Mr. Hillig: I own a Hafler XL600, 1990. Is there any reasonable method to eliminate the fan? If not, do you know a method to quiet it, yet sufficiently cool the amp? A high S / N ratio isn’t worth much if the noise floor is somewhat high from a cooling fan. If the amp is place in or near cabinets, the fan noise gets even worse. I’ve reduced it some by creating a muffled air way with scraps of carpet. But this fault alone makes me think about selling it.

 —––––––-Is it quiet when you first turn on the amp? It should be whisper quiet until the amp begins to warm up. Do you believe it to be operating at a normal speed? Are your speakers unusually inefficient? Do you play loud? At the lowest speed it should be very quiet even after a long "no sound" warm-up. There were several iterations of this product. Initial units were very unstable with reactive loads. Later "improvements" were aimed at taming the mercurial nature of the beast. Some versions would oscillate with little provocation. If you are playing your amp at low volumes or it is idling and you have a high fan speed + very warm air exhaust it might be oscillating or inappropriately adjusted. If you are not the original owner perhaps that person adjusted the idle current for more Class-A operation which results in a much hotter running design with high fan speed.


Why are your parts better?

I haven’t seen reviews on your mods for this unit in particular? What exactly is of marginal design quality in my original amp which would I get noticeable improvement from replacing? Hope you have a good suggestion.

Thanks in advance, KP

Reply: Every amp we have modified including ARC, Krell, Spectral, Levinson etc. has parts that we consider to be second-rate in certain sonic aspects. From a reliability and technical specification standpoint I suppose you might say that few of these parts are of poor merit, but that’s not what audio reproduction is about. Every part has it’s own sonic signature and we have spent years separating the poor and the outstanding. Our level of dedication to our art is uncompromising. If you would like to get the opinions of other customers, you can read a great deal about us at www.audioasylum.com and other forums by doing a search for "Musical Concepts".


Shorting Straps

I’ve another unrelated question: I’m selling an older Denon A/V receiver which used two "shorting pins" to bridge main out and front in preamp RCA jacks. I’ve lost the "pins" and ask if you have any suggestions on where I could get replacements. The local Radio Shack had NO idea what I was talking about...

Thanks, John

Our answer: "A very short loop of interconnect comes to mind. I imagine getting the parts from Denon with shipping is expensive, but worth a check. A lot of audio dealers may have these in their "parts bin". A bent piece of 10 GA bus wire in a “U” shape might do."

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