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A couple of things: 1st. Let's assume you're not listening at levels loud enough to clip. In that case, unless you want to listen louder, more power won't change the bass. 2nd. The impedance of your speaker may take a dive at low frequencies. Using the 4 Ohm output of your amp will help the amp keep the amplitude at the low impedance dips in line with the rest if the frequency spectrum. I've compared bass using my SET amps to bass using the Crown D-45. Within the amplitudes for which the SET is capable, I can't detect a difference. The Crown will certainly play louder, however. There is another possibility, and that is the frequency response of your SET amp drops off earlier than you would like. That's not a function of power, but of power supply capacitor size and transformer capability. Again, more power won't help that. Supply cap size (or parafeed cap size if you use that circuit) and bass-capability of the output transformer determine low frequency cutoff, not power. The supply caps (parafeed in my case) in my SET amps have a 3dB cutoff at 10Hz or lower. The iron is good down to 10Hz. My Chorus-II speakers are down 3dB at 40Hz (RF-7s around 32Hz). In this case, speakers determine bass cutoff, not the amp. Leo I just remembered, If you're not using a parafeed design then the size of the Cathode cap on the output tube is another and probably the biggest bass limiting factor. Unfortunately, although this one doesn't have to handle the high Voltage of the supply caps, it has to drive the low impedance of the tube Cathode. I've seen several amps in which the output Cathode caps were not really big enough and were limiting bass. Someone familiar with the circuit of your SET should know the bass cutoffs of the various components.

I added some additional damping to the RF-7s (1/2 lb each). There is a fair amount of damping already in the RF-7s (different from the Chorus-II in that the RF-7 uses ports and Chorus-II uses a passive radiator which may add its own damping). In any event, there was no significant change in the RF-7 sound.

Tube amps, SET or otherwise, are not generally the quietest amps available. Many ss amps are much quieter (they may not sound so great). My Crown D-45 creates no audible hum or hiss and sounds great. My tube amps create a small amount of hum (not audible at listening positions) and also sound great. I suspect you have a system issue and until you solve it anything will hum and hiss. SET certainly isn't going to solve a system hum problem. Another possibility is you have simply had a series of noisy amps and what you want is a quiet amp. Again, SET or any tube amp is probably not the quietest. Leo

DrWho, I completely agree. If the problem is the room, fixing it in the speaker is a band-aid and fixing the room would certainly be a better approach. Maybe someone with a more suitable listening room can give this a try and report on the results. I'm not convinced the Acousta-stuf is absorbing that much energy in the 40 - 60 Hz region where the radiator is intended to add radiating surface, However it seems to remove some box resonances that were coloring the sound and scrambling the image. You're probably pretty busy, but since you have a good ear, and understand the physics, possibly you could give it a try and let us know what you hear. The material is not objectionable to work with. Leo

Jeremy, The damping pretty much just cleaned up the frequencies covered by the 15" woofer: 40Hz - 600Hz. Highs are unchanged. Bass goes just as low, and with the same power as before, but notes and timbres are much more distinct, making them sound more powerful, or at least more musically meaningful. Low end of big grand pianos is more impressive and realistic. Bass imaging is much more in line with the excellent job done by the horns. Rock is more dynamic. I suspect a lot of cabinet echo and standing wave noise was being created and escaping via the woofer and passive radiator. The material damps those echoes. I had been aware of a rather muddy low end to the system, but, like I said, had always attributed it to the room. The room's still a problem, but no at all as bad as thought. DrWho, The nice things about this treatment is it's cheap, easy, fast, and easily reversed. As far as reducing efficiency of the passive, I'm not so sure. Doesn't the pasive simply add a half octave or so at the very low end, with anything higher being directly radiated by the woofer? In any event, I wouldn't say any appreciable amount of useful bass has been lost, just some mud and boxy coloration, making the bass more effective. I think it's worth a try. Leo

There was a post here a few weeks ago suggesting placing some Acousta-stuf inside a speaker cabinet (I think it was RF-7). My Chorus-IIs have always sounded a bit boomy in upper bass. I always figured it was the room. So, after reading the post I purchased some of the material, expanded it, pulled out the mid horns, and inserted a pound in each cabinet. Big difference. Bass is much more distinct. Worth a try. I'll try the RF-7s next since they were the subject of the post. I've never had a problem with the RF-7 bass, but who knows. Leo

Garyrc, I think the thinning out with crescendos it the old intermodulation distortion problem. With simpler material there isn't too much intermodulation distortion because there aren't that many frequencies (fundamentals and harmonics) that can intermodulate each other. However, when things get loud and crashy there's a lot going on .. more top end as instruments are pushed harder and upper harmonics are stronger and there are more of them. All of those frequencies intermodulate and the mids and highs get very loud and congested, filled with distortion products, swamping the bass, making things sound harsh and thin. Even with very loud passages, the highs still don't use much power so the low power characteristics of the amp still have substantial impact on how they sound. Low distortion at high power is good, but low distortion at low power is a must, and not always obtained. Leo

Paul, Thanks for the very interesting comparison and writeup. The ss vs SET comparison has a lot of familiar themes. I suspect the SET simply has less objectionable and possibly simply less distortion at low power (I'm talking 10s of milliWatts at best here). Although there may be better high frequency extension with the ss amp, the ss distortion is accentuating those regions also by adding extra frequencies (harmonic cross products due to the non-linear amplification). With the classical music that's a disaster. I couldn't stand classical music on any system until I learned to use amps with low distortion at low power. The more transparent sound stage, again, is a result of better low power characteristics of the SET amp. Spacial cues, mostly room echoes, are way down in power compared with the direct sound. A lot of ss amps are distorting so badly at those powers ambient sounds are simply not recognizable. Low frequencies: hard to say. My parafeed SETs do as well with low bass as my Crown D-45. The Crown can simply go louder. I've done a lot of work on the SETs to make that the case (mostly using a very large, non-electrolytic Solen part as the parafeed cap). The Sowter iron is big enough to handle lows to about 10Hz, but small enough to have excellent highs because it is parafeed and handles no bias DC. I believe this is really a question of low distortion at low power, not SS vs SET. I have a real tough time distinguishing between my SETs and the Crown D-45 (except of course that the Crown can get louder). The Crown D-45 is made for horn-loaded speakers. It has very low distortion at low power. As an engineer, I trust it more than I do the SETs, but then, since I can barely tell the difference I enjoy both equally. The Crown cost 1/5 what I paid for the SETs. In the end, I think good PWM, like the Hypex designs, may have a substantial edge on both linear ss and tube amplification. Leo

carjulreyes You don't want any amps operating without a load. Can the switch(es) be wired so they load the non-speaker amp with an 8 or 10 Ohm 5W resistor? Also, are you going to switch the preamp from amp to amp? If you're driving both all the time, then you must load both and the larger amp may need a higher Wattage resistor. A note on tube amps, esp. no-feedback types (like the Paramour): I suggest wiring for 4 Ohm out on the amp .. better control for the impedance swings of the Chorus speakers. Leo

Rick, I sent email, but no reply .. I'd like to buy back the P6D at $220. Send me email or a private message through this system. Or, respond to my email. Leo

I think what one should strive for is sufficient gain to achieve full power out with the volume control at 3/4 to max setting. More than that simply adds to distortion and noise. The Wright mentioned above is a good example of an amp that won't achieve full output using a 1V line source and passive control .. so a preamp is required. In the case os the Moondogs, there is more than sufficient gain already in the amp and a passive control is a great way to go. The "noise" I am refering to is not simply a noise floor of humm and hiss, but also noise that accompanies the signal itself: distortion and noise modulating the signal masking lower level details There are also the issues of impedance matching, but they should be considered in the overall design of the system, not addressed by adding extra gain. Leo

Bruce, Quick thought about the hiss. The Moondogs alone have more than enough Voltage gain to develop their whopping 3.5W output from a 1V line source. If you're using a preamp all that preamp gain is simply adding noise and has to be padded down to a fraction of a Volt before being fed to the Moondogs anyway. Try a passive attenuator instead of the active preamp. or- Bypass the first stage of 6SN7 in the Moondogs .. easy mod for anyone who does that sort of thing. I just looked at the schematic and one disconnects C7 from V2-1 and connects the input to V2-1/R5 instead of V1-1/R1. That will reduce the Voltage gain by a factor of about 17. I've done it but needed an active preamp to make it really work. If it works you can pull V1 from the circuit altogether. Of course if you like it the way it is you can just listen to it. Leo

Bruce, Congratulations on the Moondogs. That amp chain (2 6SN7 stages and a 2A3) doesn't have to hiss. i've never experienced the problem. What are the 6SN7 tubes? And, as pointed out, the grounding may be involved (hf oscillation in a Gnd loop could sound like hiss). Anyway, when you're done, hiss shouldn't be an issue. Enjoy the Moondogs. Leo

Max, I hadn't realized you are working on a Hypex design. I have a pair of UCD180s waiting for me to find time to build them into an stereo pair. By specifications and reviews they are one of the best amps you can get (5 orders of magnitude of output power below 0.05% distortion and vanishingly low output impedance). I'm looking forward to your reviews. Good luck with them. Leo

Hi Dean, speakerfritz, If a Voltage divider is used, then one doesn't need, or really want a series resistor. The Voltage divider reduces the signal size, but keeps driving impedance to the phones low. By the way, Sennheiser phones have high impedance and are not particularly voltage sensitive, For some lower impedance headphones one may want something like a 2.2Ohm / 8.2 Ohm resister pair. Leo

I wanted to check my setup before answering. I soldered a 3.3 Ohm and 6.7 Ohm resistor (5W ea) in series, making a total of 10 Ohms. Using a dual banana plug as a single channel earphone plug, I soldered the resistor pair across the two banana terminals. I then tapped the earphone signal from across the 3.3 Ohm resistor. That signal drives one of the headphone channels. I wired a second dual banana jack identically or the other headphone signal. plug the dual banana jacks into the amp's speaker out, same polarity at both amp output terminals. If you keep the 3-circuit "stereo phone" jack on the headphones, you have to be sure the 3.3 Ohm resistor side of each dual banana plug goes to amp speaker out "Gnd." Wires from the "Gnd" side of the 3 Ohm resistors (one from each dual banana plug) can be connected together to create the headphone "common" or "sleeve" connection. If you mess this up you can easily blow one or both amp output circuits. What this does is load the amp with a friendly resistive 10 Ohm load and attenuates the signal going to the headphones. The amp will be operating at a little under a Watt where it has low dstortion and high signal to noise. With this setup and Sennheiser HD 600s I find I have to use a bit more volume out than I normally do directly driving my RF-7s or Chorus-IIs. I can plug the Sennheisers into any of my 3 amps using this arrangement (I have replaced the Sennheiser's "stereo phone plug" with the two dual banana jacks w/ resistors). If you do wire directly to the Sennheiser headphone wires, it's not trivial because Sennheiser uses very fine Litz wire. If one were do do this for a ss amp you might consider greater headphone attenuation like maybe a 1 ohm 5W and 9.1 Ohm 20W resistor, connecting the headphones across the 1 Ohm resistor. This would have the ss amp operating at something like 10W getting it out of the low level ss crud that is such a disaster with many ss amps. It actually makes a cheap ss amp sound quite nice. Leo

Cut-Throat, Do you use a decent rf filter on your power and possibly interconnects (rf beads on the interconnects)? Rf modulates everything, including the tape hiss that is on taped recordings (records or CDs or SACDs). RF modulated instruments sound harsh, with exaggerated attacks. Rf modulated hiss sounds like more hiss, sometimes a lot of it. I have 2 cd versions of Kind of Blue. Since I installed the line filters (and power supply cap upgrade in the Philps 963) hiss just isn't an issue. Leo

I have both speakers and find myself in agreement with the post above. I tend to prefer the RF-7s. I would recommend a very serious break in of the RF-7s. I finaly resorted (after owning the RF-7s for several years) to using a 20Hz - 20KHz sine sweep, loud enough so I wore earplugs when entering the room with the speakers, for 16 hours (8 hours straight - 2 days in a row). The sine sweep was made for the purpose, dropping in amplitude at higher frequencies so as not to burn out the tweeter. The Chorus-II can sound a bit more dynamic, but the RF-7s have a much cleaner high end, sound more natural, and go lower. I listen to classical music mostly and have concentrated on string quartets and chamber. However, since the break in I find I prefer large orchestral works on the RF-7s also (and that's with only 3.5Watts of SET). Now and then I put on the Crown (20 or 30 Watts) and can get higher output if I want (which I almost never do). RF-7s hold together in the most complex passages where the Chorus-II gets a bit confusing. I'm currently listening to the Living Stereo Ein Heldenleben SACD. An example of a difference at the low end: In Carry On by CSN&Y the bass is plucked in triplets. I can hear that on the RF-7s, but not the Chorus-II. Chorus-IIs, by the way, usually have the Crown as amp and RF-7s use the 4 Ohm output SET. You'll enjoy both. I certainly do. Leo

Max, Digital can be very good. It seems particularly susceptible to line and rf interference. I suspect your digital player has taken care of some of the worst noise issues. Recently I was having trouble with some glare throught the Philips 963 / SET / RF-7 system and was blaming the RF-7s or amp until I realized I'd left the video circuitry on (it can be turned off) on the Philips. Turned it off an the glare is gone. It doesn't take much to trash good digital sound. Leo

I never really understood the function of the risers on the Forte or Chorus. Locating the woofer closer to the floor is an advantage. I used to think the best listening alignment would be level with the mid or tweeters (argument for the riser) but I find the Chorus-II sound to be the best if ear level is a few degrees above the tweeter. My vote is risers are a marketing device. Leo

So what do you have that bends you mind, gives you the heeby jeebies or makes you say whoa, what was that!. thebes, Try reading "The Long Emergency" by James Howard Kunsler. That'll put you in a different world. Unfortunately, depending on what you get out of it there may be no way back out. It makes global warming look like a nice day at Disneyland. Leo Oh, going through all my records I stumbled across Little Feat "Day at the Dog Races" from "Time Loves a Hero." It helps me to temporarily forget about The Long Emergency until the next time I need gas, or electricity to run my stereo. Leo

Hello Craig, The loss of one's father is one of life's more fundamental rites of passage. Mortality, continuity, and responsibility take on an almost shocking focus and change in meaning. Hang in there, you and those you're responsible for will get through it, but now with someone who exists in a different way. Leo

In most cases they're my preferred speaker for music. Never tried movies. Leo

Larry, I find that a parafeed output extends both high and low frequency performance of a SET amp. Also, using a 4 Ohm output will provide better low frequency control. Compared at volumes compatible with my SET amp, my Crown D-45 produces identical bass. Leo

Good points .. thanks for posting them. Leo