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| Letters -- July 2003 KR amplifier July 28, 2003 To Ross Mantle, I read your review of the new Blue Circle amp. I own the Kronzilla Stereo amp with two 1610s, and read with interest your comment about being ready to speed dial the fire station when you turn on the KR amp. The Kronzilla has a soft-start turn on, which seems to manage the process very well. I know we have different models with different tubes, but is there something I don't know that I should? What kind of problems have you had? David Zigas My KR has been a bit of a labor of love. Since I bought it three years ago, I have burned out three sets of 300 BXLS output tubes, completed numerous power-supply and other internal upgrades, and learned basic electronic repair. From what I understand, your Kronzilla is substantively different in design from the KR 32 bsi, in that it uses a solid-state gain stage, and requires higher power-supply voltages for the large output tubes and therefore a different power supply, so my troubles may not apply to you at all. I would estimate that my KR has broken down about 15 times, sometimes due to faulty repair work done by me, but mostly due to problems with the design of the power supply. I was warned about reliability problems before I bought it (expressly without a warranty by the way), but the price was so good and the sound was so fantastic that I bought it anyway. Here's my interpretation of the main problem. The power supply of my KR uses a "dual stacked" configuration. In rough figures, in order to generate 600V, two solid-state rectifiers take 150V AC from the transformer and rectify it to 300V pulsatile DC. The dual 300V potentials are fed to a capacitor network that stores, theoretically, 600V. After passing through a choke and various resistors, you end up with something like 550V at the plate of the 300BXLS output tubes. The problem is that stock capacitors come in at around 385V. To get a capacitor that can store 600V, you either need unusual and expensive custom caps, or you network the caps in series pairs, which is what KR did. Unfortunately, as with networked resistors, for the network to function properly, all the caps in it need to perform up to spec (or within 20% of spec), or some of the caps take more of the load than they should and then fail, causing very undesirable results -- burned-out ceramic resistors, et cetera. There are something like 20 caps in that power supply, and it gets as hot as Hades. What I think may have happened to me is that an initial faulty cap led to damage in other caps and continued breakdowns through a kind of domino effect. The problem can be solved simply by placing a 1M-ohm resistor across each cap, which allows excessive voltage buildup across any one cap to be distributed through the network. What I actually did was to rebuild the entire power supply using expensive Black Gate Rubicon capacitors, at a cost of about $2000 in parts. The amp sounded even better and has not broken down since. I have also played around with a number of other modifications, like placing 10-microfarad caps across all the rectifiers, and changing the wiring, that have also resulted in big improvements. Next, I'm going to replace the coupling caps in the gain stage. My KR remains among the best, if not the best, amplifier I have ever heard, and most of what I've learned about audio can be traced back to it, so I don't regret the purchase.....Ross Mantle Reimyo ALS-777 July 18, 2003 To Mike Silverton, I just received a Reimyo ALS-777 on a 30-day trial. I currently use a BPT 2.0 balanced-power unit for everything. (1) Do you still like this unit? (2) Where do you think it would make the most difference? CD, DVD, plasma TV, digital or analog? (3) How long does it take to really notice any improvements (break-in)? Oded Zyssman I continue to like the Harmonix Reimyo ALS-777 very much indeed. As I mentioned in my review, my initial interest was piqued by this Japanese unit's having incorporated in its design something I was already familiar with (and using): Bill Stierhout's Quantum QRT technology. As to where the Reimyo is best put to use, I've little to say or recommend. From the start, I've used it only with my Mark Levinson No.390S CD player. My power amps (again, as mentioned in the review) have their own dedicated 20-amp outlets. In order to draw on that kind of power reserve, I'd need a Reimyo on both outlets, and even at that, I'd have access to 15 amps per side -- the Reimyo's meter shows red between 15 and 20 amps -- not a good idea. I could barely afford the one. Two are out of the question. The Reimyo requires little to no burn-in. You ought to hear differences straightaway. While on the subject of differences, I'll soon be reviewing (I hope) another line conditioner that uses Stierhout's QRT technology, this one from Walker Audio. It lists for less than the Reimyo. We shall see....Mike Silverton Passives and more July 14, 2003 To Ross Mantle, In general, I agree with you about passive controllers. There certainly can be a loss of dynamics, but when listening to classical quartets, jazz combos, and solo acoustic instruments, I love the transparency and lack of electrical grunge of a passive. I have an Audio Synthesis passive. When I had Theta Digital adjust the output of my DAC to 4V, the extra gain made a great difference. I am looking at the passive from Sonic Euphoria, which uses autoformers in a process similar to AVTAC and is cheap. Let me know what you think. Victor Savage An autoformer is not a transformer. It's a coil that uses its inductance to down-convert voltage in exchange for up-converting current. Apply an AC signal to either end of a simple coil, take a tap from the center, and at the tap the voltage is halved and current is doubled. The voltage-for-current exchange in an autoformer is similar to what you get in a transformer-based attenuator, but with an important difference. In the simple coil, the source is directly connected to the amp through the wire in the coil, while in a transformer, the source is decoupled from the amp by the gap between the primary and secondary coils. This decoupling accomplishes two beneficial things. First, it does not allow DC through the gap and blocks ground-loop and other noise. Second, because the source signal is passing through a fixed segment of the primary coil, the source sees a constant, nearly infinite impedance -- i.e., little or no current from the source signal is being used up. When no current is drawn, the original voltage profile that encodes the music remains intact -- so your music survives to this point with no distortion. In an autoformer the source signal is directly connected to the amp, so the amp can draw current from it, which will drop signal voltage and distort the music. Of course, the amp can also draw current from the secondary in a transformer, but transformer designers have more flexibility to minimize this effect by matching the impedance of the secondary with the amp. In addition, I'm told there are also more practical problems in designing simple coils with linear bandwidth than there are in making a linear transformer. Evidently, magnetics are hellishly complicated. To sum up, an autoformer may outperform a resistive attenuator, but my sources tell me that a true transformer-attenuator will trounce an autoformer any day. I have heard an autoformer product from Antique Sound Labs that was even cheaper than the one you mention from Sonic Euphoria. It was certainly not in the same league as the AVTAC Pasiphae. Your best alternative to the top-end AVTAC stuff is Bent Audio's product, which is based on the same Stevens and Billington TX-102 transformers. I don't know whether it sounds as good. It lacks many of the features of the Pasiphae and is certainly not as attractive, but it is a way to get into true transformer attenuation on a more modest budget....Ross Mantle
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