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Cheap Raal Direct Drive Amp

kevin gilmore

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On 8/8/2020 at 8:42 PM, kevin gilmore said:

wired in series for +/-24v and then connected to each channel

Kevin two UHP-350-24 connected in series is +/-48V.
Should we be using two UHP-350-12 in series for +/-24V?

I’ve ordered UHP-350-24....


Edited by johnwmclean
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On 8/6/2020 at 5:24 PM, themystical said:

If somebody is going to get these PCB's fabricated, I would be interested in a couple. Please PM me so that we can share costs?

I have ordered some of these PCB’s ( 2mm copper) so if somebody is interested please PM me. Probably makes more sense in the UK or EU because of shipping costs I suspect.🙂

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7 hours ago, n_maher said:

Sweet!  You up for giving it a shot driving speakers instead of ear speakers?

can try but it will be limited by voltage swing. 25W/8-ohm and 50W/4-ohm

as of right now here is where the difficult work actually starts, and the time consuming work. I'd be interested if anyone wanted to finish it up before putting in the $$/cost on their own unit. 

for future board revisions:

- make the jumper over the bass boost 3x 0.1" pins so a push-on jumper can be used + have a place to store it and/or add small signal DPDT 24v relay to switch it (add a 2nd value with moar boost?)

- don't block the heatsinked part mounting screws with the electrolytic caps

- holes for wires too small to accommodate 16awg, used 20awg instead

- caddock resistors need to be bent in an L, move them closer to edge of pcb if possible

- 2nd output hole for wiring another output

Edited by justin
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the jumper was supposed to be .1    newest software loves to keep switching to metric (fixed)

forgot about the size of the caps, can always put them on the bottom.

i had thought that 18awg was enough. size increased to 60th

not going to be able to move the caddock resistors much. only 25th. then runs into the output wire

Edited by kevin gilmore
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I received a suggestion that the grounds for the L/R output, which in this case is the 4-pin XLR but would also be the negative posts on the speaker terminals, be wired back to each L/R channel board rather to the star ground at the switching PSUs

open for debate on that

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16 hours ago, themystical said:

Seems to be working really well!! 
Any comments/initial reaction on SQ with and without base boost?

How warm is the heat sink getting?  Could it be much smaller?

keep in mind the thermal transfer for this package is not great, with a good insulator the IC will be 3C/W hotter than the heatsink. so you do need to keep the heatsink cool. temperature in this case with the RAAL is dependent on music and how loud you're listening. My guess is in most uses the heatsink will not exceed 40C. with no audio it will be room temp.

so if the heatsink reaches 40C w/ an average of 15W per IC then the IC temp will be around 85C

There is also the TO-3 package @ $150 ea. and Apex has parts of a similar price

Edited by justin
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  • 2 months later...

Here is a (my) version of Kevin’s opamp RAAL driver.

Board is kitchen made. Some parts are smd but schematic is same as original. Only one channel working and it’s powered by lab power supply max 3A. Occasional when playing, not very load , leds indicating max current hit are flickering. But I've two 340W Mean Well supplies at hand. Mouser don't sell them supplies to ordinary consumers in Europe so my wife bought them for me. Lucky me.

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On 11/8/2020 at 4:59 AM, manueljenkin said:

How to approach a design that can operate properly with such low impedance loads? Or in other words, what is it in the design of Opa 541 that makes it capable of driving the raal?

I have been listening more and more to Raal SR1a, and it really is a beautiful thing.  

I do have to wonder about how this little Jotunheim R's balanced, differential "Nexus" gain stage is claimed to deliver "up to 13 amps"  into Raal's low impedance, and how this cheap raal direct-drive amp will compare.  

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Aerial picture of opamp driver and psu.

I had some disturbing noise from the Mean Wells. After putting them their own enclosure the noise reduced considerable but there still is audible noise.

With AMB’s Sigma22 supply +/-29V the amplifier was noise free. I only tried sigma22 for a few minutes since it hasn’t the current capability as the Mean Well, but it sounded OK.

As for sound I find it enjoyable to listen to but I also think it has some sonically drawbacks (maybe) due to the switching mode power supplies.

Bottom line is that I agree with the designer

On 8/3/2020 at 5:15 PM, kevin gilmore said:

This is a bit cheaper and easy enough for just about anyone to do.


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