If one were to use a 300VA Traf with dual 18vac secondaries(vs 15vac), and using a CRC supply (30mF/0.1ohms/30mF) what would be the power output into 8 or 4 ohms?
Any idea what the approximate supply ripple would be?
THX,
-Chas
There is an incomplete question.
What is the bias point? How many devices? These affect the end result.
Once you have decided these. You can use the attached spreadsheet to get the answer. Just watch out the SOA of the devices.
What is the bias point? How many devices? These affect the end result.
Once you have decided these. You can use the attached spreadsheet to get the answer. Just watch out the SOA of the devices.
Cool spreadsheet! Is it for Aleph in particular, or all class-A amps in general?
Thanks, Fred.
This is my first Class-A amp, so I'm getting my feet wet, so to speak.
Happy New Year!
-Chas
P.S. Anyone have info on the amount of ripple for 30mF/0.1ohm/30mF CRC with 18VAC traf?
Thanks, Fred.
This is my first Class-A amp, so I'm getting my feet wet, so to speak.
Happy New Year!
-Chas
P.S. Anyone have info on the amount of ripple for 30mF/0.1ohm/30mF CRC with 18VAC traf?
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download Psud11 and insert your data.
Allow about 1000ms after start up for the PSU to settle, then state for a time for the analysis period.
Allow about 1000ms after start up for the PSU to settle, then state for a time for the analysis period.
Hi Chas,
Happy New Year to you, too.
This spreadsheet is specific to the Aleph.
A sturdy 300VA 18VAC x 2 transformer is good for the Aleph 3 and Aleph 30 which are 30W into 8R and 40W into 4R.
Happy New Year to you, too.
This spreadsheet is specific to the Aleph.
A sturdy 300VA 18VAC x 2 transformer is good for the Aleph 3 and Aleph 30 which are 30W into 8R and 40W into 4R.
Recently I build a Mini Aleph (details at http://www.diyaudio.com/forums/pass-labs/207642-mini-aleph-build.html) and I'm now in the testing and tuning phase of the project.
For the initial build I used the values specified in the BrianGT schematic, these resulted in a bias current of ~0.9A which is a bit low for the Mini Aleph. I substituted a 200k ohm potentiometer for R13 (R19 on the Pass schematic) and was able to adjust the bias up to 1.17A, but cannot get it above that point. My target value is 1.2-1.3A based on heat sink capacity.
After analyzing the schematic I'm starting to suspect that the values of R14 and R16 (R17/R18 on the Pass schematic) are incorrect. The spreadsheet specified 1.5k ohm resistors for both the Aleph30 and the Mini Aleph, but there are a handful of posts that say that these should be 750 ohm for the Mini Aleph.
http://www.diyaudio.com/forums/pass-labs/2001-mini-42.html#post2427233
http://www.diyaudio.com/forums/pass-labs/2001-mini-24.html#post1575907
I could not find any definitive resolution on this, so I'm hoping the community can help out. All part numbers below are based on the "miniasch.pdf" schematic from 1/20/2009 which is linked at other places in this thread.
Based on my understanding of the circuit, Q7 is used as a current source to bias Q6 at ~1A. Q7 is controlled by Q5 which uses the voltage across the sense resistor R28 to set the bias current. The collector of Q7 is connected to the + rail through R14 and R16, and R13 is used to adjust the voltage at the base of Q7 and therefore the bias current.
In the original Pass Aleph 30 schematic it shows the following target voltages:
+26v positive rail
+15v at the bottom of R14 (11v drop across R14)
+4v at the bottom of R16 (11v drop across R16)
+0.8v at the base of Q5.
Based on the assumption that the Aleph30 and the Mini Aleph bias Q5 at the same current then is it possible that the voltage drop across the 1.5k ohm R14/16 resistors is too high to allow enough adjustment to drive Q7 above ~1.17A, assuming ~15v rails? If this is the case then it is strong argument for changing R14/R16 to 750ohm.
I'm itching to finish this build and any input on the above is greatly appreciated. Thank you.
- Matt
For the initial build I used the values specified in the BrianGT schematic, these resulted in a bias current of ~0.9A which is a bit low for the Mini Aleph. I substituted a 200k ohm potentiometer for R13 (R19 on the Pass schematic) and was able to adjust the bias up to 1.17A, but cannot get it above that point. My target value is 1.2-1.3A based on heat sink capacity.
After analyzing the schematic I'm starting to suspect that the values of R14 and R16 (R17/R18 on the Pass schematic) are incorrect. The spreadsheet specified 1.5k ohm resistors for both the Aleph30 and the Mini Aleph, but there are a handful of posts that say that these should be 750 ohm for the Mini Aleph.
http://www.diyaudio.com/forums/pass-labs/2001-mini-42.html#post2427233
http://www.diyaudio.com/forums/pass-labs/2001-mini-24.html#post1575907
I could not find any definitive resolution on this, so I'm hoping the community can help out. All part numbers below are based on the "miniasch.pdf" schematic from 1/20/2009 which is linked at other places in this thread.
Based on my understanding of the circuit, Q7 is used as a current source to bias Q6 at ~1A. Q7 is controlled by Q5 which uses the voltage across the sense resistor R28 to set the bias current. The collector of Q7 is connected to the + rail through R14 and R16, and R13 is used to adjust the voltage at the base of Q7 and therefore the bias current.
In the original Pass Aleph 30 schematic it shows the following target voltages:
+26v positive rail
+15v at the bottom of R14 (11v drop across R14)
+4v at the bottom of R16 (11v drop across R16)
+0.8v at the base of Q5.
Based on the assumption that the Aleph30 and the Mini Aleph bias Q5 at the same current then is it possible that the voltage drop across the 1.5k ohm R14/16 resistors is too high to allow enough adjustment to drive Q7 above ~1.17A, assuming ~15v rails? If this is the case then it is strong argument for changing R14/R16 to 750ohm.
I'm itching to finish this build and any input on the above is greatly appreciated. Thank you.
- Matt
decrease all big resistors for same amount and re-set bias an offset
shave them to next lower value or at least -30% , either replacing them or soldering some in parallel
shave them to next lower value or at least -30% , either replacing them or soldering some in parallel
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To clarify, are you suggesting that we drop the "big resistor" sense/source resistors from 0.47 ohm to somewhere around 0.33 ohm? That would definitely give a higher bias current for a given voltage across the sense resistor, but I still wonder if changing R14/R16 would have the same effect with less de-soldering and re-purchasing required.
Any input on if R14/R16 should be 1.5k ohm or 750ohm for the Mini Aleph?
- Matt
Zen Mod,
On that version of the schematic the resistors in question are R112 and R113 connected to the collector of Q5 and R114 which controls the bias. The values aren't 100% comparable because they are for the Aleph 30 and not the mini Aleph. To add to the confusion, the schematic in the Aleph 30 owner's manual uses a third component labeling scheme.
I've attached a copy of the schematic that corresponds to the chipamp.com parts list that many people use. That parts list is available at http://chipamp.com/diyaudio/mini-A.xls.
On both schematics the "big" 3w resistors are all 0.47ohm.
My question is whether or not R14 and R16 (R112/R113 on the schematic you posted) should actually be 750 ohm for the Mini Aleph.
- Matt
On that version of the schematic the resistors in question are R112 and R113 connected to the collector of Q5 and R114 which controls the bias. The values aren't 100% comparable because they are for the Aleph 30 and not the mini Aleph. To add to the confusion, the schematic in the Aleph 30 owner's manual uses a third component labeling scheme.
I've attached a copy of the schematic that corresponds to the chipamp.com parts list that many people use. That parts list is available at http://chipamp.com/diyaudio/mini-A.xls.
On both schematics the "big" 3w resistors are all 0.47ohm.
My question is whether or not R14 and R16 (R112/R113 on the schematic you posted) should actually be 750 ohm for the Mini Aleph.
- Matt
if you're using at least +/-15V for PSU , you can leave both R14 and R16 (in sch posted by you ) at 1K5 .
they're defining current through Q5 , without too much of consequence on Iq
for double Iq , comparing to present one , just halve values of R27 and R28 , and - to keep same Aleph CCS AC gain - values of R23,R24,R25,R26
fine re-set bias with R13
re-set offset with R8
they're defining current through Q5 , without too much of consequence on Iq
for double Iq , comparing to present one , just halve values of R27 and R28 , and - to keep same Aleph CCS AC gain - values of R23,R24,R25,R26
fine re-set bias with R13
re-set offset with R8
What if one's PSU voltage is an anemic +/-13.5V or so.......still leave them at 1K5?
Matt & Boywonder,
Grey's original mini-A schematics uses 750 ohm:
http://www.diyaudio.com/forums/pass-labs/2001-mini.html#post15054
Cheers,
Dennis
Grey's original mini-A schematics uses 750 ohm:
http://www.diyaudio.com/forums/pass-labs/2001-mini.html#post15054
Cheers,
Dennis
ZM, Thanks for the suggestions. My rails are right at +/- 15.5v under load, so I suspect I have maxed out the bias adjustment capacity of R13 at 1.17A. One option is definitely to lower the resistance of R27/28 and R23/24 to get a higher bias current.
Dennis, thank you for the post. That does seem to indicate there is a small "bug" in the chipamp.com build of materials.
- Matt
Someone please step in and help a guy out....
I have a set of Mini A boards built and tested. Both units have been benched and tested fine, good offset, etc. Well I have now cased these up and it is acting strange. The only thing I have done is added some bleeder resistors and added the parts on PSU pcb for the LED lights.
I powered up after putting it in case and used a variac, fuse, and light bulb tester. Ramped up to 115V AC nothing smoked! started checking the basics and noticed PSU is running perfect +/- 24V or so (no load from amp on rails)
The amp pcb is showing +42V on ch1 and -3.5V
amp#2 is showing -43V and +4.5
No wires are crossed, 12V across Zener5 (Brian GT pcb)
Nothing showing on source resistors. I am thinking about taking all power mosfets out and try it again. Is there anything I am missing?
I have a set of Mini A boards built and tested. Both units have been benched and tested fine, good offset, etc. Well I have now cased these up and it is acting strange. The only thing I have done is added some bleeder resistors and added the parts on PSU pcb for the LED lights.
I powered up after putting it in case and used a variac, fuse, and light bulb tester. Ramped up to 115V AC nothing smoked! started checking the basics and noticed PSU is running perfect +/- 24V or so (no load from amp on rails)
The amp pcb is showing +42V on ch1 and -3.5V
amp#2 is showing -43V and +4.5
No wires are crossed, 12V across Zener5 (Brian GT pcb)
Nothing showing on source resistors. I am thinking about taking all power mosfets out and try it again. Is there anything I am missing?