The Mini-A

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#161
zycomatique,
The Zen amplifier uses a "normal" current source. A normal current source runs at the same current all the time, regardless.
The Aleph current source is modulated by the output signal. If the amplifier has no signal the Aleph current source runs at a steady current. If the amplifier is amplifying a signal, the resistor array at the output feeds a copy of the signal to the current source, causing it to swing with the signal.
The Mini-A, like the mainstream Alephs it is derived from, has a front end. The Zen does not. This isn't necessarily a big problem, just something to be aware of. The front end gives the Aleph amplifiers more gain and a way to use feedback. The advantage of the Zen is that it's simpler and cheaper to build. For a little more money you can have a baby Aleph.

Grey
 
#162
Mini-A Completed

Well, about a year after buying the boards from BrianGT it's done. It's amazing what you can accomplish while on disability.

Dead quiet, around 11 Watts a side.
10mV and 25mV DC offset.
66k uF per rail, 300VA 2x15v and 25VA 2x9v Antek Toroids.
Softstart board from Rob Cheng.
Thermistor controlled fan on the tunnel heatsink.

It's the first amp I've built up from scratch, so be gentle!

Now there's that A30 I need to finish...

Mini-A

Mini-A 2

Mini-A 3

Oh! And a pint to whoever can identify the chassis!
 
#163
I too just finished my mini-A after buying some almost complete boards from a member here.

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


I'm using a 225VA 12V transformer into the brianGT PSU board with 45kuF per rail. Output voltage under load is 15.7V. DC offset is <20mV and <30mV each side. I used a circuit breaker on the ground like you would in a valve amp. So circuit ground is connected to chassis earth via some back to back diodes with a power resistor and cap in parallel. No hum at all and the sound is very nice. I have 2 LEDs wired in just as an indicator for each rail.

Very good control of the bottom end without penalising the highs. I'm very happy! Oh yeah - I used 200mm wide by 150mm high by 25mm deep heatsink from fischer (farnell) and it gets hand hot - I'd say 45 degC in use. Something much bigger would be needed I think if I was to use the output boards.

Fran
 
#165
Grounding

First off Good for you Fran:
That work looks great. You seem to be on a bit of a role around here right now. Lots of good projects ont he go.

Could you draw your grounding scheme for me. I am having a bit of trouble getting a hum out of my miniA.

Thanks for the help
 
#166
Eh, well I'm a bit crap at drawing, but I can describe what I did:

An externally hosted image should be here but it was not working when we last tested it.

1.5mm copper wire used as a GND bus connecting both neg loudspeaker terminals - see the last pic.

PSU ground connected directly to this copper wire.

neg output from each aleph board goes to this bar too.

So at the bus bar there are 4 wires soldered on - the 2 outputs and the ground from the PS pcb. This is kinda like a star ground but my input grounds go to the inputs of the boards and I used insulated RCA jacks.

The 4th wire connects to the safety/chassis earth via 2 diodes wired 69 style paralleled with a 8R 7W resistor and also a 0.01uF cap.

So then from the pos rail at the end of the PS pcb I have 2 wires, one to each aleph board and exactly the same for the negative rail. The only ground connection at the aleph boards is the one from the output point that goes to the bus bar.

If you look at the last photo you can see how I did one channel - just ignore the black/red twisted pair - that just goes to the LEDs on the front panel.

I didn't use the balanced stuff at all, just jumpered the balanced neg to ground on the aleph board.

Fran
 
#169
Dernit!

Ok, first off I absolutely LOVE how this little amp sounds. Very tight, controlled bass and silky highs.

The problem I'm now having is that now I can hear something not right in the right channel- kind of a crackling during louder passages. It's not the amp as it follows the channel when I swap L&R. It's got to be something in either my cable box or my preamp (Adcom GTP-450).
I never heard it with the Hafler it replaced. Dernit!! 😀
 
#172
woodturner-fran said:
I too just finished my mini-A after buying some almost complete boards from a member here.

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


I'm using a 225VA 12V transformer into the brianGT PSU board with 45kuF per rail. Output voltage under load is 15.7V. DC offset is <20mV and <30mV each side. I used a circuit breaker on the ground like you would in a valve amp. So circuit ground is connected to chassis earth via some back to back diodes with a power resistor and cap in parallel. No hum at all and the sound is very nice. I have 2 LEDs wired in just as an indicator for each rail.

Very good control of the bottom end without penalising the highs. I'm very happy! Oh yeah - I used 200mm wide by 150mm high by 25mm deep heatsink from fischer (farnell) and it gets hand hot - I'd say 45 degC in use. Something much bigger would be needed I think if I was to use the output boards.

Fran
Click to expand...

Hey Fran, I wrote those numbers on the 9610's in your Mini A, 🙂

Looks good.

Anthony
 
#173
Cool - I'm not fully up to speed in all this design, but from what I've read the low DC offset is down to your matching - Thanks!

Lovely sounding amp this. Would beat off quite a few contenders i can tell you. Its at a friends house at the moment running his Quad 989s. He says it runs out of steam a little but only very loud - not bad for 10WPC!! Great build and now that the brianGT boards are back available its a great opportunity to build them.

Fran
 
#174
woodturner-fran said:
Cool - I'm not fully up to speed in all this design, but from what I've read the low DC offset is down to your matching - Thanks!

Lovely sounding amp this. Would beat off quite a few contenders i can tell you. Its at a friends house at the moment running his Quad 989s. He says it runs out of steam a little but only very loud - not bad for 10WPC!! Great build and now that the brianGT boards are back available its a great opportunity to build them.

Fran
Click to expand...

Is is true that with close matching of these FET's you can expect very low DC offset. All the sets I did, about 250 sets in all started from a batch of 1000 pcs. With such a large sampling I was able to provide FETs within a few mV of each other.

Glad to hear your project is working well for you, I built mine with the indentical transformers as you, with similar results.

Anthony
 
#175
Nelson once said "Check everything twice, once in the evening and once in the morning."

Well, I checked everything once and fired up my new Mini Aleph in the evening. Result: 12 Volts DC across both speaker terminals. The supply-voltage is +/- 16.3 Volts. No funny smell or noise, but I switched it off after several seconds.

My question for now is: Can I safely measure some more voltages with something being obviously wrong with the wiring/circuit or is it more likely that something will go up in smoke after a few minutes?

I have never encountered any problems of this kind before so any input is welcome. After the Zen Lite and the BOZ this is the third amp I "built" and the first one not working as expected.

All resistors > 0R47 are rated .6W, the 0R47 ones are rated 3W. The caps are 220µ50V. The IRF9610 are all matched. I did not measure/match the IRF644 so there is room for failure...
The circuit is p2p so desoldering the FETs would/will be a pain in you know where.
 
#177
MEGA-amp said:
it's possible one or more of your outputs is toast; not quite what you wanted to hear, but i'd check those first.
Click to expand...

True, not quite what I wanted to hear. See picture. I'd say they're quite hard to reach...

An externally hosted image should be here but it was not working when we last tested it.


I pulled some numbers from the circuit today, maybe someone can deduce something from them. They're the same for both channels.

0 Volts over R4
0 Volts over R3
3.2 Volts from collector to emitter of Q4
0.8 Volts over D1


I basically built Grey's original circuit, with the addition of a 1µ cap over D1, 221R resistors at all the gates and a 10p styroflex over R8. The circuit is attached.

Thanks to everyone helping and supporting, David
 

Attachments

#178
0,8V over D1? You sure you connected the zener correctly?

There should be some 9V over D1 if I am not mistaken.
It looks like you connected them backwards i.e. swapped cathode and anode.

Please double check. If 9V is not over D1 the zener is not zenering but just acting as a diode. So broken or connected wrong.

regards
 
#179
Looking at your photo I confirm the zener to be connected wrong.

Black band is connected to two resistors. Gate resistor and resistor to ground. Your R5 and R7.

The currentsource ( and the whole circuit for that matter) is not working because of this.

Check and triple check again
 
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