agedhorse

Yes, it is a myth when the speakers are designed with this in mind (and have the engineering/math to back it up). None of the efficiency equations contain any variables related to the size of the driver. Mixing drivers CAN be problematic, but it doesn't have to be problematic and in fact can provide the (knowledgeable, skilled) designer the ability to develop a range of voicings that is not possible (or practical) with a single driver.

In practice it MAY cause issues IF the cabinets were not designed with similar acoustic properties. This means sensitivity, power balance, phase response, complementary voicing, etc. The issues of acoustic summing occurs regardless of similarity of the speakers, it relates to identical speakers also, based on the number of point sources and the distance between the sources. Then there is the (usually) greater issue of boundary conditions, how the reflections within the room combine and the frequencies that each boundary act on (and at what level). The myth of only combining identical speakers without the considerations of all underlying factors that affect the summed response needs to be put to bed rather than be perpetuated.

If the Roland amp states that using the headphone out as a recording or aux output, that means that the designers determined that it was safe to do so. Different circuits can behave in unpredictable ways when used in ways the designer did not intend. This is why using a headphone output MAY not be safe unless stated so by the manufacturer.

Part absurd myth with a little bit of truth too. IF the speakers are designed to work together, that means sensitivity, power handling, phase response, complimentary voicing, then the combination may be better than 2 identical speakers. Of course, this can not be assumed, nor taken for granted. Where cabinets were not designed (in the engineering sense) to work together, it will be a total crapshoot.

Do not use a headphone out except for headphones. Some are not intended to be ground referenced and can either cause noise problems or in some instances damage. You can use a post-modeling line output into the effects return of the amp that is connected to the speaker. Check the owner’s manual to see if there may be other (safe) ways.

In general, unless there is a problem that can be traced to a bad tube, I recommend leaving well enough alone.

I see an awful lot of perfectly good tubes replaced, sometimes causing more problems than leaving well enough alone. For every player who is convinced a tube is "worn out", another player thinks that “bad tube” is the “best tube ever”.

At reasonable volume, a pair of Eon 515's should work pretty well (not ideal but certainly not garbage either). How exactly are you trying to us them, and how do you have things connected? I suspect that you have a connection/interface problem rather than the problem you think (unless you already damaged the speakers).

This is why I always recommend verifying that when buying any used cabinet you always check that the driver is original to the cabinet (the driver the manufacturer designed the cabinet around) and that the cabinet/crossover hasn't been screwed with. I see with enough regularity cases where the original driver was expensive and was either swapped out with a cheaper driver (and the original driver sold), or that the original driver was damaged and rather than spend the money they just slapped a cheap driver in the box. If the modification to your cabinet was done by somebody knowledgeable, you might be ok, but that will take a little research and effort to sort out.

I missed the date, hence their comments and conclusions being wrong for the times. 20 years ago, there were challenges, but electronic technology (especially power semiconductors) have come a long way since then.

All this talk of the "good 'ol days", how many of you were actually there in the good 'ol days, where hyperbole and bravado meant more that actual measured specs? First of all, when bravado claims about Trace amps being able to deliver XXX watts peak... for ALL amps regardless of brand, peak power equals exactly 2x the "RMS" power, so the AH-250 better be able to deliver 500 watts peak or it's not able to deliver 250 watts RMS. Next, since I worked for one of the companies that owned Trace before selling back to he original directors, I have more technical experience than most do about the product line. I looked up the technical docs I have for the AH-250 and the Vsat across the FETs is about 2 volts, add to this another 3V of sag at 4 ohms and you have a maximum voltage swing of 41 volts peak or 29V RMS. This equates to 210 watts RMS continuous, and with a 20msec burst rating of ~32V RMS or 256 watts RMS (burst). This is real world, and burst ratings were often used because they do represent how an amp feels. While Trace made an elegantly designed (IMO) product (especially for the time), there were stumbles in the execution throughout the company's life that resulted in reoccurring reliability issues that required a significant amount of correction. In fact, with NO real world losses (impossible to achieve) the maximum power of this power amp is a little more than 265 watts RMS. This is 530 watts peak, and not achievable with real world losses.

Except that the limitations on class D amplification are absurdly out of date (by maybe 20 or more years). In quality designs, these limitations are WAY to the right of the real world decimal point. In fact, it's possible to have HIGHER damping factors with class D not lower, the output reconstruction filters have almost no impact in the audio band due to the higher modern PWM frequency, and EMC/EMI ("radio noise") is very well under control as any commercial product is required by regulations/law to be compliant to strict standards.

Agreed, the 98dB sensitivity spec is suspect as both the No and TS sensitivity specs are much lower and agree with each other (at least for some form of broadband data set)

To add to Bill’s comment, generally as the power handling increases, sensitivity decreases because to increase low frequency power handling, trades offs are made that decrease the sensitivity. This can be offset but it adds a lot of cost and the driver is no longer “cheap”.

The preamps of the pedal and the amp are almost identical.

Also, verify that you are using genuine Neutrik parts. I have seen an awful lot of intermittent problems (exactly as you describe) from sub-standard Asian knock-off parts, and cables improperly assembled by tinning the wire before assembly. About 1/2 of the intermittent speaker issues have traced back to defective SpeakOn connectors (generally the cable mount type).

The critical information being left out of the statement above is that "underpowering" is not what causes the damage, but the clipping which causes damage primarily to the high frequency drivers. Otherwise, how would all of the overdriven tones work... they are various forms of clipping. This information is from a pro audio perspective, one that generally avoids high level clipping, and also generally has better (more effective) processing to prevent mechanical damage form too much power. Speakers have both thermal and mechanical failure modes. Clipping is generally a thermal failure mode in the high frequency driver, which is why players who use overdriven and distorted tones generally play cabinets without tweeters also. Not only do tweeters disproportionally reproduce the harmonics generated by clipping, but they don't last long doing so at higher power levels. In bass guitar, the most common form of speaker damage is mechanical, and this always occurs from too much power. This is either from an amp that is too large for the speakers, or a misunderstanding of how the amp and the speaker are rated (ie. RMS, program and peak descriptors). When a speaker is rated at say 1000 watts, without any descriptor, the FIRST question to ask is HOW that rating is made.

Explain to me how it's possible for an amp to be so popular and well talked about (bragged about even) in Europe, yet have the reliability and support issues that are being discussed here?

There has been quite a bit of player discussion on the combinations of cabinets over at TalkBass. This may be a more helpful resource.

Your impression is exactly correct, all models within the Subway line are designed with similar phase response, sensitivity and power bandwidth to work well together. The engineering and math have been done, listening tests agree with the predictions, and it's not influenced by the marketing department. Note that this is not true mixing the Subway cabinets with other manufacturer's cabinets, nor mixing cabinet models from Mesa outside of the Subway models (for example, PH and Scout cabinets may not mix well with Subway cabinets). The most common combination is the 115 with the 210, but the 115 with 112 is not far behind.

I just suggest that you have some real experience. I see a fair amount of unrepairable damage caused by attempted repairs.

I did suggest a factory authorized service center as they will have the necessary access to resources and parts. I would have expected them to be fully supported and serviceable for much less then the cost of buying a new one. It's not an inexpensive amp is it? (I don't have a good grasp of European/UK prices of course)

Why not have your amp repaired. I would expect that a qualified MarkBass authorized service center would be able to get this taken care of for you.

I have not seen anything that specifically states that the + and - signals are fully isolated from pin This is an essential requirement for use with ANY amp that contains bridged topology.

Yes. The direction is screen printed on the PCB. Use all safety precautions, the PCB does not need to be removed. It’s a double sided board with plated holes so be be gentle.