ZAPsolute Mk4 - AMP Kit

From a business standpoint I know this: is too darn expensive, even at $995!
The people at diycable also know that very well.
This kind of prices are just not going to fly in the US, not when competing with Nelson Pass offering the Aleph amps for free (ok, almost, I wish, you get the idea!) that's why they try to pull this weird deal, order and wait...wait for what? a month? a year? Christmas?
If I was a european customer I would be pretty upset to have to pay the full price.
My guess is that you got plenty of time to make the 25 cut.
I have no prior knowledge of this make. I had a look at the schematic on their web site just now. It is a fairly basic, fully complementary bipolar amp that uses no overall feedback at all. It has extensive protection circuitry. Output dc offsets are minimized by a separate dc servo circuit. On the face of it it looks like a competent design and fun to build - the build quality looks good from the photos.

High-end audio? Hmm maybe. The schematic doesn't tell much - although there are some minor opportunities for improvements. It mostly depends upon transistor selection and this is not given in the schematic. Using no feedback at all is likely to be a problem sonically with a class AB design. I recommend you find some comparisons with other gear at an equivalent price or audition it yourself before purchase.
I cannot see what bias current they use from the info provided. I can see a spec of 50W into 8-ohms with 33V psu rails for the output transistors, implying an idle power dissipation of some 460W. Pretty hot!

I was not clear in my point above. The circuit is class A up to a certain output current and then transitions into B. Since most speakers are not resistors there will be times when the amp works in B. My real point is that the push-pull configuration is liable to particular distortions no matter what the bias current is set to and these are related to the matching of the transistors and the design of driver circuit. They actually brush on this in the "system construction" section where they talk about "metallic and strident noise" when paralleling devices and how feedback can cause "loss of detail". In fact this problem is also to do with push-pull stages where in class A you are, if you think about it, using a npn and pnp in parallel. They seem to have brushed this under the carpet by avoiding any feedback at all and chosen to tolerate inherent distortions in the circuit that feedback could have reduced. Another problem with this topology is that each stage tries to create an accurate version of the input voltage. But the inputs to the output devices need a voltage that is somewhat different from the input signal as it needs to comprise of both the input signal and the distortions of the output devices. Again, the output device distortions are not corrected.

Nevertheless if it sounds great for the money then who cares? :D I've designed no-feedback amps and they can sound extraordinarily lucid and relaxed and transparent. Is it good value for $US1000 diy? Could well be. I must admit I am sceptical about the Danish hifi reviewer claim that it sounds as good as the ML332 at US$8000. If this was generally accepted then I would expect more people would have heard of it and it would be a big name in the business.
Although there is no overall feedback the zap has a very large number of small gain stages and everything is held together by plenty of local feedback. There is an additional "hidden" stage in the form of the 2sa1633 rohm driver which is a composite pair with built in feedback resistor.
So this amp is FAR from being zero feedback. I looked for the rohm transistors a while ago and they appeared to be out of production (unless you want to buy some fakes from MCM or dalbani).
Looking at the size of the heatsinks, and considering the fact that the trannies appear effectively use only two out of the four heatsinks this amp cannot possibly be functioning with a bias current higher than 5-600mA ok maybe 700mA on a cold day in Norway. This really means a realistic 5-7 watts in class A on a real world speaker load.
I can't agree with traderbam. In ZAP you do NOT need loopback because output devices are linear and output impedance of transistors is low. I have seen many non-loopback design which has failed because they didn't have good driver stages, which has to be built to drive high currents without effect on voltage gain stage. ZAP solves it by 3x current gain stages between voltage gain stage and output stage. So you don't need to correct output for changes in speaker impedance, because ih has no effect on voltage gain. :cool:

Output impedance with one pair of output devices is about 0.1Ohm, which gives damping factor about 80 for 8Ohm load, so speaker impedance does not have effect on amp.
I have received a bit of email and posts on newsgroups concerning this amp. I must say, every person who has heard/owned one loves and recommends it.

I am yet to see something negative from someone with experience with this amp.

If you have experience with the ZapSolute MK4, I would love to hear what you have to say (good or bad)

to grataku

:D :D grataku:
Looking at the size of the heatsinks, and considering the fact that the trannies appear effectively use only two out of the four heatsinks this amp cannot possibly be functioning with a bias current higher than 5-600mA ok maybe 700mA on a cold day in Norway. This really means a realistic 5-7 watts in class A on a real world speaker load.

I have read the manual to ZAP about setting idle current. Current for 50W version is more than 3 Amps, which gives us class A on 8 Ohm load to 50W. And finally I don't think that heatsinks used in ZAP can't disipiate power of class A.
Yes I agree that an output resistance of 0.1-ohms is adequately small. By the way, I believe the proof is always in the listening and Gavin seems to be receiving lots of positive comments, so I have no trouble in believing the the amp sounds very good.

The thing about the output resistance, or impedance, is that it may measure 0.1ohms at dc. But how does this impedance change with output current, voltage, frequency and load impedance? This will largely determine the sound of the amp. How would you go about measuring these changes?
The Sanken transistors used are "extended beta" types which have a very linear Hfe curve over a wide current range, typically up to 7a. Also, they are very fast- Ft over 100 MHz.

As so measuring the output impedance over a wider range of conditions, to some degree this is straightforward, using another amplifier. The DUT amplifier is left with the input "grounded"; no signal. Connect the ouptut of a second amplifier through a load resistor (high power, 8-16 ohms, value not particularly critical; power handling is!). Run a sine wave test signal, or even a multi-tone or music test signal into the second amplifier; not necessarily at very high level, but at a sutiable level for 1-2A peak current through the resistor. Measure the voltage at the DUT amplifier output, with scope and wideband AC meter. Using a swept sine test, you can easily plot the amplifier output impedance versus frequency. For this type of NFB design with these transistors, it should be low over a wide range. Ceratainly, measuring up to 50 kHz or so should be no problem. This is a test I routinely do for measuring output impedance of an amplifier "wide range".

Best regards,