Input requested

I think I can help you with the break-up problem. Before you change anything else, try putting the Goertz spkr cable aside and substituting -- not permanently; just for the purpose of this experiment -- some generic zip cord or $1/ft Monster or Radio Shack/Home Depot Monster clone. You may find that the problem is gone.

As VP of Threshold Corp during the "golden years" of the Forte' 1A and Model 4, I can tell you that THAT amp does not get along with highly capacitive cable.

If the experiment turns out as predicted -- please let the Forum know either way -- and if you do decide to switch speaker cables, there are many types that have more compatible specs: Cardas, MIT, Kimber, AudioQuest (I'm not familiar with the new models that need batteries), Monster, and TARA Labs, among them.

Best of luck!
Joe Abrams

Mprime: If you are running Goertz speaker cables without the Zobel networks, you ARE going to damage your amp ( if you already haven't ). This is especially true with Threshold / Forte' amps. Having said that, the Forte's with Goertz MI-2's are a fantastic combo IF using the Zobel's. I've run various Forte' amps into Goertz MI-2's or MI-3's into speakers that are 82 dB's and 2 ohms at high volumes for sustained periods of time with no problems. According to what Joe said above, i've put my own gear "at risk" doing this for several years, so i know that it can be done and done with NO fear whatsoever.

Other than that, I've run the flat Goertz speaker cables with every amplifier i've ever had and never had any problems. That is, so long as the Zobel networks were properly installed. Given that some amps have built in Zobel's or "high frequency protection", using external Zobel's may be redundant. If in doubt, ask the manufacturer of the amp. If the amp doesn't specifically mention having high frequency protection in it, USE THE ZOBEL's !!!

As some of you might have read, Kal Rubinson ran into high frequency oscillation with a recent Adcom amp that he reviewed. Kal uses Goertz MI-2's in his system. Had Kal used the Zobel's to begin with, he would have never had ANY problems. He stated that once he was able to figure out what was going on and installed the Zobel's, the problem was cured.

As most of you know, i have stressed time and time again that the Zobel's should NOT be considered "optional" for most installations. I have mentioned this to Goertz, but they seem to have their head buried in the sand on the subject.

As to Mprime's specific situation, my thoughts are that you are probably running out of peak power / dynamic headroom. If you like the sound that you're achieving but want more dynamic punch, pick up a Forte' 6/6A and bi-amp. This is basically the same amp as the 4 / 4A, but run in AB mode and rated at 200 / 350 wpc. Using this to drive the woofers with the 4/4A running the top end will give you the sweet airiness that you're used to with greater "dynamic wallop" and headroom for louder passages. Sean
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* Due to their design, Kimber "8" series ( TC / VS / PR ) cables are also high in nominal capacitance per foot. While not nearly as high in capacitance / low in nominal impedance as the Goertz, it is possible that they too may cause high frequency instability in certain designs. The use of a properly designed Zobel will not hurt the performance of a system and can actually increase stability in areas with high levels of RFI. In fact, there are companies marketing Zobel networks as "speaker filters" at very high prices ( for what you get ). Don't be afraid to use a Zobel and / or speaker cables that are designed to achieve maximum bandwidth with a low nominal impedance. Using anything else will compromise the results to one degree or another. Sean
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First, thanks for the replys Joe and Sean (especially since I could have been damaging my equipment!). It turns out I had the Zobel's factory installed, so if Sean's correct, then I should not have damaged my amp. Still, I took the Goertz out for the time being and returned to my AudioTruth Midnight speaker cables. I am too tired to listen critically, but I will say the sound is fuller though less controlled on the softer passages (when contrasted against the Goertz). Too late to play loud and assess the more dynamic passages.

I'll give them both a good listen tomorrow and get back on the results. It will be interesting if the issue is a power or cable one, rather than the limitations of the 805 - a myraid of concerns flow either way :-(

Best,

I saw your thread yesterday, but was a little reluctant to respond, with what I thought the problem might be.I then saw "Joeabrams" reply, and said Joe nailed it. When I checked again this morning, and saw "Sean's" reply, I said there is the frosting on the cake, and it looks like an easy, inexpensive fix...Your reply of already having the Zobels in place, then brought me back to my slightly out of the box thinking.

So here goes... I viewed your system, and I noticed a smallish room 10x14 with a wall mounted shelf.

Could it be, when you turn up the volumn, that enough energy from the room's primary resonate frequency point is exciting the wall, and shaking your wall mount shelf, which would shake the the preamp, and it's tubes. Heck, if you can yell into a tube directly, and have the sound of your voiced reproduced at the speakers, then why couldn't a vibrating wall induce some distortion into the tubed preamp, and have it appear as breakup at the speaker during loud passages.

Can you temporarily replace the tube preamp with a solid state unit, or move your tube pre from the wall mount temporarily. Another thought would be to try some sort of tube damping device.

A/gon member Jax2 (Marco) complained of a very similar problem awhile back, but I fail to reply then, again do to my reluctance, and lack of experience with tube equipment.

I told you it was slightly out of the BOX thinking.

Good Luck, Dave

Nelson Pass' designs are no more sensitive to high capacitance cables than other wide-bandwidth solid state amplifiers.
Painting with a broad brush, solid state amps whose high frequency response extends well beyond 20k can become unstable when used with a high capacitance speaker cable. My comment regarding the Forte' 4 was meant to get that point across to a user who might be having that exact problem.

Generally speaking: if you have a solid state amp that does not have a transformerized output, it's not a good idea to use high capacitance cable -- most times these cables can be recognized by their shape; either very flat or very skinny. Also, any cable whose marketing literature says that "inductance must be avoided" is highly suspect. Inductive cables are fine for solid state but not for tube amps. High capacitance cables are fine for tube amps, but not for solid state.