audio-talk

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Been thinking about building an amp for those days when I don't want to run up the leccy bills. The 3000B is a bit of a power hog. So here's my take on the great "El Spuddo".

Given my pechant for LED bias I will need to experiment with the operating points, however.

I have some nice Slagle iron at 5K, some of those snazzy IXCY constant current sources and it will fit in a case to match the pre-amp and I have a load of little red devil LEDs Evil or Very Mad

....here I go.

Comments and cast off 6C45 welcome Wink

-- Andrew

SteveTheShadow · Posted: Fri Nov 14, 2008 9:23 pm Post subject:

I've got a pair you can have for nowt Andrew.
Never even been heated up.

Steve

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Top man, Steve....a true scholar and an gentleman, to boot.

I will PM you my address..

many thanks,

-- Andrew

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Steve's pocket rockets arrived today, thanks Steve.

My sums, and Spice say I should be able to get close to 2.5 watts out of these babies with 1.5% THD, that's assuming I'm asking it the right questions, of course.

For a PSU I have a simple LC arrangement using a 10H choke and a 470uF cap, I am hoping the PSRR of the CCS and the step down will mean no hum at the speaker, we shall see. If it works it could prove a very simple circuit. Smile

-- Andrew

Dave the bass · Posted: Wed Nov 19, 2008 8:20 pm Post subject:

*Puts hand up*

"Question from the back of the classroom Sir?!"

'M1' in the Anode... is that an 'active load' or summat? And C1 also Sir, is that some sort of parafeeding arrangement?

Thank you Sir,

Master Bass (Class 3).

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Yes on on both counts, we shall have to get rid of the Red L plate, Mr Bass (where's the mods when you need them).....

Its an active load; a constant current source on-a-chip, can handle big voltages and quite high currents, certainly over 75mA, if memory serves.

The big cap? Yes parafeed. The active load means I have to parafeed, of course. The graphs I have of Mr Slagle's iron says its good for 300H without an air gap. The downside is the quality of the cap will make a difference, I suspect. I have some crappy Maplins for a first pass.

cheers,

-- Andrew

Dave the bass · Posted: Wed Nov 19, 2008 9:05 pm Post subject:

Thank you Sir, I understand now Sir.

*Tugs cap, does cheeky grin and runs away to write summat rude on the wall in the playground.*

DTB

SteveTheShadow · Posted: Wed Nov 19, 2008 10:19 pm Post subject:

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Hmm, interesting problem yesterday, I rushed the breadboard hoping to get something to play with whilst Colin and his wife were visiting.

Anyway, we checked the heaters OK, they were fine, AC heated so we didn't expect it not to work.

So then we started to wind up the voltage on the variac, the LEDs started to conduct, indicating acthode current, so it all looked good, bit more voltage, however, and the fuse blew. We replaced the fuse and tracked the problem down to the diodes going short.

THe PSU is full wave rectified, as if it were a GZ34 but with solid state diodes so it the anode to cathode to anode was short back to the TX windings. Variac was at about 50% I guess, I didn't look to hard as I was too busy throwing the off switch.

I need to investigate if something else was the cause, but the diodes were 600v @ 4Amps, could the reverse spikes have killed them?

Or, could the diodes have been killed by a short elsewhere before the slow blow fuse blew. I think this is more likely.

Any other things I could look for?

I might try a bench DC PSU I have one that will do 120v, which is good enough to test with and see if the audio circuit behaves itself.

And see if there's a short from the the HT rail to ground.

-- Andrew

Nick · Posted: Sun Nov 23, 2008 10:26 am Post subject:

Might be missing something. but if you have a LC supply generating 350v that would need something like a AC input of 388v, so if you have a 388-0-388 tx, with full wave rects as you have described, then isn't each diode across the full secondary of 776v or 1080v at startup. Or at least with the cathodes at 350, the anodes will go to -388 * 1.44, so thats 890v

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Nope, I think you're spot on Nick, same thought occurred to me. Choke input, mains TX is 350 ct. (that was a typo sorry)

So 350 * 1.4 * 2 is close enough to a thousand volts excluding and spikes etc so I probably zapped the diodes.

-- Andrew

Nick · Posted: Sun Nov 23, 2008 2:02 pm Post subject:

Seems likely if they are 600v diodes.

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

Hmm, thanks Nick, seems most likely place to start, time to double up on the diodes, or go hollow state....

-- Andrew

Andrew · Shed dweller Joined: 24 May 2007 Posts: 2228

I assume I'll need, at the very least, balancing R's say 1Meg across the diodes?

-- Andrew