Which amp for Monitor audio s6

[Rob_S]

Hello,

I have been reading through post for the last few weeks and have decided to build some diy power amps for my 5.1 system

The amps that I'm currently considering are P3a, P101, SKA (would consider non SS amps also).
Any advice on what could work well with the following setup would be great (the system is used for both Music and movies)

Speakers
Monitor Audio S6
Specification
Frequency Response +/- 3dB 38Hz - 30KHz
Sensitivity (1W@1M) 91dB
Nominal Impedance(Ohms) 6
Power Handling RMS(W) 120
3 way speaker

Here is the link to the spec (but there is not really any more info there, some pics though)
http://www.monitoraudio.co.uk/produc...duct=44&area=2

Plus silver 6 center and dipola rears

Rotel AV Pre Amp (currently using an integrated Rotel AV amp)

Arcam DV 88 as the source



There is the posibility of Bi-amping the main speakers but would need to take a look at the crossovers.

Thanks
Rob

[Rob_S]

Would be looking to have a 2 box solution, one for Center and Rears (with something like a gainclone) and the other for the main pair.

[AndrewT]

Hi,
the SKA gb150 running on ~+-50V (35Vac) would be ideal to match up the power into a 6ohm speaker (or is it a 4 to 8ohm speaker?).

I have seen various reports on the p3 and 101 but quality of the bootstrap and mirror bothers me. Some eg. AKSA, can make the bootstrap work very well but the research into optimising this for quality may be beyond many designers/builders (certainly beyond me).

As always I recommend VERY short cables and at least bi-wiring if the speakers have dual terminals. My Tannoys were single terminal but were easy to modify the crossover PCB to convert to bi-wiring and that was the biggest improvement I ever made to them.
Short speaker cables implies long line level interconnects.

[Rob_S]

Andrew thanks for you comments

The Speakers are measuring 4 ohm for single amp or 7 ohm for bi-Amp (getting another multimeter as i'm a little supprised by the 7 ohm result). I'm thinking of starting with just a single amp per speaker as a first project and see how we go from there. I guess if I go to bi-amp then it would be best to use 2 amp of the same design and move the crossover to before the amps?

As far as amp placment if possible go for an amp placed behind the speaker keeping the speaker cable very short (mono-block style)?

Would people recommend using the power supply module from SKA or build something myself?

I'll spend some time reading the SKA threads and see what info I can find.

Thanks
Rob

[AndrewT]

Hi,
the SKA PSU makes it easy to assemble but PSU building is very easy whether PCB or hard wired.

Bi-amping with the crossover BEFORE the amps is actually active speakers but our US friends mistakenly have called this bi-amping.
I sometimes refer to the real bi-amping as passive bi-amping and this passive version retains the original passive crossover between the amps and the drivers. Two terminal speakers need to be modified.
Four terminal speakers work straight out the box. Just connect an amp across each pair of terminals. But make sure EACH amp has a DC blocking cap on it's input.

Don't know why you measure 7ohm and 4ohm across the speaker unless you are measuring across different terminals.

If you are measuring bass @ 4r and treble @ 7r then you have a typical modern 4 to 8ohm speaker and sometimes referred to as 6ohm. This is deceptive, since the bass/mid is the main loading and the amp sees this for much of the frequency range and thinks it is driving a 4ohm load for 7octaves and sees 8ohm for just 3octaves where it cannot pass much current anyway. The treble amp gets an easy life, low current=low stress, short term transients=low stress
You could build an amp specially for this duty, but keep adequate voltage drive for those very peaky transient signals.

[Rob_S]

I have double checked the speakers and they are reading 4ohm or 7ohm for HF and 7ohm for LF, so not sure what us going on there. Getting a new multimeter so will see.

With the SKA is any of the following required / advisable?
Clipping Indicator
Speaker / Amp protection
Soft start

AndrewT if possible could you share your PSU design you have used in your SKA and the best sources of components in the UK (have seen you case suggestions in another thread, looks good)?

Thanks again for you help so far (and I'm in the process of purchasing 2 boards to start with and see how I go from there).

[AndrewT]

Hi,
if you decide to buy gb150 and or SKApsu you will get a whole host of build instructions and schematic with recommendations for ancilliaries.

I used 300VA 35-0, 0-35Vac with bridge rectifier and +-30mF smoothing. True monoblock!

I also tried RC snubbers across the 4diodes but removed them when I discovered they exaggerated a problem.

I tried a dual rectifier but could neither measure nor hear a difference to the single rectifier version. Extra cost and no reduction in hum seems poor value from dual rectifiers.

I now run both channels off a 450VA 4 secondary transformer (33-30-0, 33-30-0, 33-30-0 and 33-30-0) using the 33Vac to get +-48.5Vdc and keep the IRF9140 safely below the 100Vdc peak rating. Power about 150W into 6r using a 0.4C/W sink for the pair set at Iq=150mA (75mA/FET pair), making them suitable for 4 to 8ohm as well as 6ohm speakers, The recommended voltage is +-55V for 8ohm and +-45V for 4ohm speakers.

For full support, Greg B is available by Email and on DIYhifi.org

[mikeks]

Originally posted by AndrewT
Greg B is available by Email and on DIYhifi.org

Incidentally, did you know that Greg's amp is actually a Harman/Kardon design?

Verily there is nothing new under the Sun!

[AndrewT]

Hi,
you mean Harmon have stolen it already and calling it their own!

[mikeks]

No, H/k's design predates Greg's by at least a decade.