Moskido - Hybrid Aikido/MOSFET amplifier

[beau2317]

I don't know the gauge of the wire, but it's quite thick.

The gate resistor doesn't have to be heatsinked...just physically close, as you want that wire to be as short as possible or else it might oscillate.

There are no ground wires, no.

The big ground current is the speaker. Other than that, the bias board and the aikido board are star grounded. Hope that makes sense..

email me directly when you are building and need any tips or help. bob at beausoft dot com

[JC951t]

Hi Beau,
Manage to purchase 2 large heatsink measuring
7.5in W X 10in H X 2.5in T. To me it looks really huge.
I think this should be sufficient. Regarding the Caddock
MP930 resistors, does it dissipate alot of heat ? I'm thinking
of mounting it together with the Mosfet onto each heat sink.
As the Buz are T0-3 types which would neccesitate the use
of a L shape type mounting bracket, would it be better to
mount the bracket close to the bottom of the heat sink ?

Thanks again for the advice

[beau2317]

The caddocks don't dissipate a huge amount (say 0.25W) so there is no need to heatsink them. But you can if it's convenient.

It doesn't really matter where you mount the L shaped brackets, but most important is that they have good thermal contact.

Good luck with the build!

[JC951t]

Hi Beau,
Thanks again for the advice. I intend to build a mono
in all in one case. Just curious, do you think the close proximity
of the power trans will afffect the sound ? I'm using EI trans.
Lastly. how do I properly bias the amp ?

Thank you

[beau2317]

No the trafo will not be a problem I don't think.

How to set bias.

Make the circuit using a socket for the opamp. Start with the opamp out.

1) Turn Bias Adjust trimmer to minimum value.
2) Turn Zero Adjust to center
3) Power up
4) Adjust the Zero pot until output voltage is zero.
5) Turn up the bias to what you like. I use 2.4 amps total (1.2 per device).
6) Adjust zero again
7) Power off; plug in opamp; power on again.
8) Output should still be zero. Hopefully

One thing to bear in mind with my design is that during power-on the output voltage can swing to one or other rail. My speakers don't mind this (magnepans) but I am not sure if this might fry some normal speakers. So I suggest that you have a timer that connects the speakers after a few seconds with a relay.

While testing, don't connect up the speakers until after you power up, and measure the DC to be zero or thereabouts.

[JC951t]

Hi Beau,

Thank again for the advice. Re Caddock MP930. If I were
to use wire wounds or carbon resistors. What wattage
would suffice ?

Thank you again Beau

[beau2317]

I would not use carbon here, but wirewound would be OK. Use 3W or more.

Cheers

[JC951t]

Thank you again for the advice. I was looking at some
relays. Could you recommend a delay circuit.

Many many thanks for your guidance

[beau2317]

What you need is a fairly heavy duty, say 10A, normally open (no) relay. This means that when no power, the contacts are open. Some relays also have no/nc contacts.

Then get a 555 timer IC and look up the basic circuit for one shot timer, set up a 5 sec delay and build the circuit to drive the relay. Idea is that after 5 secs the relay is turned on and connects the speakers. When power goes off, relay opens ang disconnects the speakers.

[Stef]

For a simple relay I use the following:

http://www.tkk.fi/Misc/Electronics/c.../ampdelay.html

Its simple and you can adjust the time delay with a larger cap 2 (I use about 1000uF for a 30 second delay on start up - it disconnects after about .5secs on switch off).

Also I use the rectified dc from one channel so it works out pretty good. So long as you have about 24v then you are good to go.

Stef