Bill Fuss said:
Not exactly,
The A3 and A30 ran on +-25V. Remember, the A3 only had 4 Irfp240 per channel - 2 OP and 2 CCS.
I have built a couple 'mini' A on similar plus and minus "mid twenty volts" using the slightly fatter IRFP140 and IRF150 (2 total per channel).
As long as the heatsink is big enough you can get away with those voltages; and maybe use a tougher than Irfp240.
Member
Joined 2002
I think i'm running 19V rail's i'm not to sure tho, as i haven't used mine in a while.
I'm also running the SMD version.
I'm also running the SMD version.
Please don't flame me for this, but I need some help.
At long last I've got my Brian GT Mini A (30 variant) boards stuffed, my enclosures built, and I'm almost ready to throw everything together. I'm shooting for a bias of about 2.4 amps (my heatsinks are forced air cooled) so I omitted R13 and used 0R27, 3W metal films at R23-R26, R35-R40, R47 and R48, and installed a trim pot at R8. I've also permanently shorted the balanced (-) input and use only the Z1, Z2 and Z5 taps for diodes.
My chassis is quite cramped, though, and before I install the boards I might need to add some "test" leads that I can get to. I also want to be sure I understand the powering up and adjustment process correctly. I've read every post I can find on measuring bias and setting AC current gain, but I'd appreciate it if I could get the "Measuring Bias and Adjusting Current Gain for Dummies" version for this particular amp. Just to be safe.
1. At what taps do I measure bias, and how do I calculate it?
2. At what taps do I measure AC gain, how do I calculate it, and what formula do I use in adjusting it, given the bias I find?
Thanks.
George (Colescuttle)
Oh, I almost forgot, can I take the output boards V+ and V- directly from the power supply board's outputs? It would save some complexity, given my layout.
At long last I've got my Brian GT Mini A (30 variant) boards stuffed, my enclosures built, and I'm almost ready to throw everything together. I'm shooting for a bias of about 2.4 amps (my heatsinks are forced air cooled) so I omitted R13 and used 0R27, 3W metal films at R23-R26, R35-R40, R47 and R48, and installed a trim pot at R8. I've also permanently shorted the balanced (-) input and use only the Z1, Z2 and Z5 taps for diodes.
My chassis is quite cramped, though, and before I install the boards I might need to add some "test" leads that I can get to. I also want to be sure I understand the powering up and adjustment process correctly. I've read every post I can find on measuring bias and setting AC current gain, but I'd appreciate it if I could get the "Measuring Bias and Adjusting Current Gain for Dummies" version for this particular amp. Just to be safe.
1. At what taps do I measure bias, and how do I calculate it?
2. At what taps do I measure AC gain, how do I calculate it, and what formula do I use in adjusting it, given the bias I find?
Thanks.
George (Colescuttle)
Oh, I almost forgot, can I take the output boards V+ and V- directly from the power supply board's outputs? It would save some complexity, given my layout.
I'm ready to fire up my MiniA... yes a long time ago I've begun this project.
Now I'm a little bit confused before trying it in "real" conditions.
My PSU is tested Ok, fuses ready too, Amp PCB checked, MosFets are using Kerafol Keratherm Red Isolators.
How do I proceed, I've got some questions :
Do I need a load at the output ?
What are the usual measures to be done ?
Thanks for your advices and help 😉
Now I'm a little bit confused before trying it in "real" conditions.
My PSU is tested Ok, fuses ready too, Amp PCB checked, MosFets are using Kerafol Keratherm Red Isolators.
How do I proceed, I've got some questions :
Do I need a load at the output ?
What are the usual measures to be done ?
Thanks for your advices and help 😉
Member
Joined 2009
Paid Member
If possible, double check everything 😀
If you don't have a Variac (I don't) - what I did with my first amp was NOT to install fuses in the d.c. power rails, but use 10R resistors. If there's a major problem that results in excessive current draw these resistors will drop enough voltage across them to save the circuit from destruction. Oh, and they will start smoking pretty quickly so you know something is amiss. If it's OK at that point then measure voltages, especially those at the output. Keep a close eye for signs of temperature rises and any unusual smells (of things getting hot).
p.s. I also check with a multimeter the potential of the chasis to a reliable earth before I go near it - just in case for some reason it's at 115V AC !!
If you don't have a Variac (I don't) - what I did with my first amp was NOT to install fuses in the d.c. power rails, but use 10R resistors. If there's a major problem that results in excessive current draw these resistors will drop enough voltage across them to save the circuit from destruction. Oh, and they will start smoking pretty quickly so you know something is amiss. If it's OK at that point then measure voltages, especially those at the output. Keep a close eye for signs of temperature rises and any unusual smells (of things getting hot).
p.s. I also check with a multimeter the potential of the chasis to a reliable earth before I go near it - just in case for some reason it's at 115V AC !!
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Member
Joined 2002
DO NOT put a load on the amplifier, just connect it as described, don't even connect a input signal till the amplifier is turned on and everything is ok. before connecting a speaker it is best to check the speaker terminals for dc, if there is dc then there is a problem, AT any level, there should be no more than 500mv on the speaker output.
J'
I've got 0.450 v at the 0.47R, less than 1A.
For DC offset I've tried with another DMM, the value is about 30mV decreasing.
Now I've to connect in real life and listen... keep you informed
For DC offset I've tried with another DMM, the value is about 30mV decreasing.
Now I've to connect in real life and listen... keep you informed
Member
Joined 2002
OK here's my mini Aleph:
Got my transformer today (Amplimo 2x15V / 160VA) and now have one channel wired up. Flipped the switch and then nothing happened, no smoke or noises. That's good! After that the left channel heatsink started warming up... Woohoo, it's working right at the first time!
Rail voltages are 18.4Volts. Voltage across the source resistor (0.47R) is 0.402 volts meaning a bias current of ~0.86 Amps. A bit too low, will it increase once it's properly warmed up? If not I'll have to solder a resistor across the 0.47R.
DC offset is ~70 mV, a bit high. Again will see what it does after an hour or so.
Tomorrow I'll wire up the right channel. I'll keep you posted.
Got my transformer today (Amplimo 2x15V / 160VA) and now have one channel wired up. Flipped the switch and then nothing happened, no smoke or noises. That's good! After that the left channel heatsink started warming up... Woohoo, it's working right at the first time!
Rail voltages are 18.4Volts. Voltage across the source resistor (0.47R) is 0.402 volts meaning a bias current of ~0.86 Amps. A bit too low, will it increase once it's properly warmed up? If not I'll have to solder a resistor across the 0.47R.
DC offset is ~70 mV, a bit high. Again will see what it does after an hour or so.
Tomorrow I'll wire up the right channel. I'll keep you posted.
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Well an hour has passed. DC offset is still ~70 mV. Any method of lowering that?
Voltage across the source resistor is 0.40 volts resulting in a bias current of only 0.85 amps. There's room for more bias current as the heatsink is only ~36°C (~97°F) at 20°C ambient temperature. How can I achieve a specific bias current predictably, or is it just a matter of trial and error?
Voltage across the source resistor is 0.40 volts resulting in a bias current of only 0.85 amps. There's room for more bias current as the heatsink is only ~36°C (~97°F) at 20°C ambient temperature. How can I achieve a specific bias current predictably, or is it just a matter of trial and error?
I'm using the black PCB's from BrianGT. I don't have that schematic at my disposal. There's Grey's schematic of course but parts numbering between the two isn't identical.
I came across this thread while looking for my next target, a small power amp that would be a step up from a chipamp. Would the mini-a fit the bill? If not, any other recommendations?
Now, about the mini-a, considering the small current through Q1, Q2, and Q3, would smaller (less capacitive) devices not make more sense here?
Thanks!
Now, about the mini-a, considering the small current through Q1, Q2, and Q3, would smaller (less capacitive) devices not make more sense here?
Thanks!
Member
Joined 2002
make mini Aleph J
+/-15V is absolute minimum
1A also absolute minimum for Iq
you can use IRF540 on output
chipamps are certainly better in some areas (power), but with enough efficient speakers , they simply can't match magic what Papamp have
After having successfully made a dual mono LM3886 chipamp I wondered what those Aleph PCB's BrianGT sells were intended for... Well I can say that making a mini-A is indeed a step up from a chipamp. It took me longer to read all the mini-A stuff and grasp parts of it. Having no experience in electrical engineering whatsoever I still don't understand a lot of stuff the better educated people here discuss when they talk shop. I'm still on steep learning curve here. Not really a surprise with the amount of knowledge present here. Still I'm eager to learn even while knowing that I will never reach their level. Respect to you guys. 
You know who you are.
How big a step up depends on your present knowlegde (or lack thereof) you bring to the table.
You know who you are. How big a step up depends on your present knowlegde (or lack thereof) you bring to the table.