Hi to all,
I've decided to build a A75 Class A amplifier
from the article of Norm and Nelson.
And I need some advises:
1)- Replacing the finals by IRF 630 and IRF 9630
The IRF 230 and IRF 9230 reach 10 cad$ each...
That's too much for 48 devices + many others for matching.
2)- Doubling the power supply's with 2*500VA
3)- More final pairs perhaps 16 (12 origin)
4)- Are there PC Boards already available ?
5)- To be able to drive my Martin Logan
Aerius i .
I will be gracefull of any comment or advise
Regards.
I've decided to build a A75 Class A amplifier
from the article of Norm and Nelson.
And I need some advises:
1)- Replacing the finals by IRF 630 and IRF 9630
The IRF 230 and IRF 9230 reach 10 cad$ each...
That's too much for 48 devices + many others for matching.
2)- Doubling the power supply's with 2*500VA
3)- More final pairs perhaps 16 (12 origin)
4)- Are there PC Boards already available ?
5)- To be able to drive my Martin Logan
Aerius i .
I will be gracefull of any comment or advise
Regards.
for output FETs try IRFP240/9240 - under $2 US each. I bought 50 of each (fairchilds from mouser). I got four sets of 12 matched within .040 Volts VGS. The whole lot came in within .1 V. You shouldn't need too many extras to get matched sets.
Boards are available from audioexpress, although there are some errors and omissions on them. Search for A75 articles you should find most or contact me.
Not sure why you would want to add more output transistors - there is plenty of current capability already.
Dual mono/ monoblocks would be fine - I am using a single 1000VA transformer.
My pair of A-75s are waiting delivery of my cases, but the front end works well and sounds nice as a line stage. I just built an X-BOSOZ, but haven't had an opportunity to listen to it yet.
Others prefer the sound of the Alephs over the A-75. Haven't heard either yet, so I cannot comment except to note that you'll have a little less heat to get rid of with the A75 for the same outut power.
Boards are available from audioexpress, although there are some errors and omissions on them. Search for A75 articles you should find most or contact me.
Not sure why you would want to add more output transistors - there is plenty of current capability already.
Dual mono/ monoblocks would be fine - I am using a single 1000VA transformer.
My pair of A-75s are waiting delivery of my cases, but the front end works well and sounds nice as a line stage. I just built an X-BOSOZ, but haven't had an opportunity to listen to it yet.
Others prefer the sound of the Alephs over the A-75. Haven't heard either yet, so I cannot comment except to note that you'll have a little less heat to get rid of with the A75 for the same outut power.
Alain Dupont said:
I have assembled and tested complete front end boards, matched sets of output devices in TO3 cases (230 &9130), transformers with aditional windings for front end PS, PS for front end circuit based on Mark Levinson 27.5 amps, heatsinks, rectifiers and capacitors, plus any additional parts you might need
Here's the link to parts I'm selling: http://www.diyaudio.com/forums/showthread.php?s=&postid=244081#post244081
Deals are done
I have 2*750Va transfos +
30 Nichicon 5,600mF/63V capacitors.
on their way
And: 26 IRF 230 & 26 IRF 9230+
a serie of pcb's & pcb for power supply
on their way too.
Future...
Heatsinks waiting for price from acellthermal
Negociating for a stainless steel enclosure
A happy DIY'er.
Thanks to Nelson for the virus!
I have 2*750Va transfos +
30 Nichicon 5,600mF/63V capacitors.
on their way
And: 26 IRF 230 & 26 IRF 9230+
a serie of pcb's & pcb for power supply
on their way too.
Future...
Heatsinks waiting for price from acellthermal
Negociating for a stainless steel enclosure
A happy DIY'er.
Thanks to Nelson for the virus!
I will remind people that the A75 as published had about
twice as many outputs as it really needed. You can easily
cut back if your budget is low.
thanks, Nelson, for confirmingwhat I was just figuring out for myself. Now I have enough leftovers to build a min aleph and or mini XA.
Yep, I have the bug bad. Starting to understand the devilish smiley
twice as many outputs as it really needed. You can easily
cut back if your budget is low.
thanks, Nelson, for confirmingwhat I was just figuring out for myself. Now I have enough leftovers to build a min aleph and or mini XA.
Yep, I have the bug bad. Starting to understand the devilish smiley
New A75
I have verified the paired devices I bought
IRFD 110 2.98 to 2.99v
IRFD 9110 a problem with 1 pair to be replaced (others paired at 0.01v)
IRF 520 all 3.14v
IRFD 9510 3 pairs 3.75 ; 3.79 & 4.05v
IRF 9231 paired at 0.1v max
I had a problem with the IRF 232
it took me all night and this morning a buzz in my stereo gave me the idea, except 1 they were oscillating, I had to modify the schema to avoid this.
Ereka, it worked. paired at 0.1 max
Grey could you help me with the modifications on the boards.
Regards.
I have verified the paired devices I bought
IRFD 110 2.98 to 2.99v
IRFD 9110 a problem with 1 pair to be replaced (others paired at 0.01v)
IRF 520 all 3.14v
IRFD 9510 3 pairs 3.75 ; 3.79 & 4.05v
IRF 9231 paired at 0.1v max
I had a problem with the IRF 232
it took me all night and this morning a buzz in my stereo gave me the idea, except 1 they were oscillating, I had to modify the schema to avoid this.
Ereka, it worked. paired at 0.1 max
Grey could you help me with the modifications on the boards.
Regards.
I just received the 2 PC boards for A75 from AudioXpress, great job + documentation included.
I also have the aluminium plates for the upper, bottom, back{4mm} and 8mm face plate from MetalSupermarket {MSC}
Does some one has advise for the face plate "brushing"... tools and "motus operandi"
Thanks.
PS: I am so excited. It reminds me my first HeatKit's ; many years ago...
{Receiver, Multimeter, stroboscope, etc...}
I will take the first pictures soon...
I also have the aluminium plates for the upper, bottom, back{4mm} and 8mm face plate from MetalSupermarket {MSC}
Does some one has advise for the face plate "brushing"... tools and "motus operandi"
Thanks.
PS: I am so excited. It reminds me my first HeatKit's ; many years ago...
{Receiver, Multimeter, stroboscope, etc...}
I will take the first pictures soon...
This is for Nelson
Hi Nelson,
You mentioned that the number of power devices could be reduced . My question however is about temperature .
What is the maximum acceptable heat sink temperature for the A-75 class A amp ? Since device lifetime decreases with increasing temperature what level is considered to be within "safe " limits.
I've seen some at about 60 deg C. Can that be exceeded or is it already too high ?
Do you find bipolar class A stage's long term dc stability at such high temperatures a problem ?
Thanks,
Ashok.
Hi Nelson,
You mentioned that the number of power devices could be reduced . My question however is about temperature .
What is the maximum acceptable heat sink temperature for the A-75 class A amp ? Since device lifetime decreases with increasing temperature what level is considered to be within "safe " limits.
I've seen some at about 60 deg C. Can that be exceeded or is it already too high ?
Do you find bipolar class A stage's long term dc stability at such high temperatures a problem ?
Thanks,
Ashok.
Hi Ashok,
I am not Nelson, but I think that I can help somewhat.
The critical temperature is really the die temperature. So you can run higher heat sink temperatures with more devices to keep the individual die tempeatures down. NP has mentioned keeping the die temps below 100 C for reliability.
Using off the top of my head numbers, IRFP240 die-case is about .8K/W, I measured about 1.2 K/W across silpads, so the die will be about 2K/W above the heatsink temp. so if you want to keep your die temps below 100 on a 60 degree sink, you need to dissipate less than 20 watts per device.
If you're looking at a stock A-75, then 8 output devices (four pairs) ought to do it thermally on a 60 degree sink with a small margin. I run 6 pairs of IRFP240/9240 in mine (per NP's comments) and have had no problems since I sorted out my construction mistakes. But I reduced the bias because I couldn't stand the heat.
Do you really want something too hot to touch in your living room?
Hope this helps
I am not Nelson, but I think that I can help somewhat.
The critical temperature is really the die temperature. So you can run higher heat sink temperatures with more devices to keep the individual die tempeatures down. NP has mentioned keeping the die temps below 100 C for reliability.
Using off the top of my head numbers, IRFP240 die-case is about .8K/W, I measured about 1.2 K/W across silpads, so the die will be about 2K/W above the heatsink temp. so if you want to keep your die temps below 100 on a 60 degree sink, you need to dissipate less than 20 watts per device.
If you're looking at a stock A-75, then 8 output devices (four pairs) ought to do it thermally on a 60 degree sink with a small margin. I run 6 pairs of IRFP240/9240 in mine (per NP's comments) and have had no problems since I sorted out my construction mistakes. But I reduced the bias because I couldn't stand the heat.
Do you really want something too hot to touch in your living room?
Hope this helps
Thanks Bob.
You made the temperature requirements very clear . However I was aware of the basic requirement but wanted to know if the A-75 was designed to operate at some specific ( ht. sink) temperature - maybe lower than 60 deg C.
Do you find any degradation in quality by running the bias low and playing it loud when it might go into AB mode ?
I recently heard a Sugden ClassA amp . The case was quite warm
but not 60 deg C. Maybe close to 50 deg C I guess. Speakers were also supposedly good - Spendor's. I was disappointed with the sound. Can't guess what could have been wrong.
I made my first class A amp some 25 years ago. It was just 10 watts per channel. It ran too hot for comfort . It was thermally stable but I always worried about the bias running away and killing the amp and the drivers.
Cheers,
Ashok.
You made the temperature requirements very clear . However I was aware of the basic requirement but wanted to know if the A-75 was designed to operate at some specific ( ht. sink) temperature - maybe lower than 60 deg C.
Do you find any degradation in quality by running the bias low and playing it loud when it might go into AB mode ?
I recently heard a Sugden ClassA amp . The case was quite warm
but not 60 deg C. Maybe close to 50 deg C I guess. Speakers were also supposedly good - Spendor's. I was disappointed with the sound. Can't guess what could have been wrong.
I made my first class A amp some 25 years ago. It was just 10 watts per channel. It ran too hot for comfort . It was thermally stable but I always worried about the bias running away and killing the amp and the drivers.
Cheers,
Ashok.
Also, to those of you diving into this project, be aware that the original article was published with a couple of mistakes that were addressed in later issues of Audio Amateur. The addition of 150uf caps on the incoming rails of the driver card seemed to clean up a small but annoying oscillation which hampered startup. This amp has excellent bottom end and sounds really good with a pair of TL's. Mine, which can be seen at the Passlabs diy site ran at around 111 deg. F per device. The sinks ran a tad cooler than that. I was fortunate to have worked someplace with a lazer temperature gun and was able to shoot each output device individually. Good Luck.
A two level Class-A bias set by a front panel switch has some appeal. It is not uncommon to design an amp with a lot of headroom and then use just a small portion of this power for normal listening. More complicated sliding bias circuits seem unnecessary for typical home use.
Has anyone executed a dual bias Class-A and worked through the design issues?
Has anyone executed a dual bias Class-A and worked through the design issues?
NP said here that it should be fine as long as you keep > 100 mA per device. Thermal stability is an issue, so I moved the Vgs multipler to the main heatsink. It is a little overcompensated, but better than under.
With my current speakers I cannot hear a difference, my bias point (.6A total) gives me a little over 10 watts in class A. I doubt that I exceed that very often, but I may put a scope on it to check.
I may feel differently once I get my next speakers finished - what I have now isn't as revealing as the Focal 6W4254/ TC-120TD5 set I built for my fiance. This time its the same woofer and a Fountek JP3 for a dome/ribbon shootout.
IIRC, NP said the number of output devices in the A-75 design was at least partially driven by the heat sinks he had on hand. They were pre-drilled and to get the thermal performance needed, he ended up with spaces for 12 pairs of transistors.
He's pointed out several times that you can use 6 pairs safely. this should give a little more top end extension, due to lower total gate capacitance. BTW, the fairchild equivalents I used have lower capacitance than the IR originals, and other specs are comparable.
While MOSFET characteristics change with temperature, I suspect that change is minimal to inaudible. Total bias seems to be much more of a sound quality driver.
When I burned out my last matched pairs of irfd220/9220s (forgot a heat sink isolator on Vgs multiplier) I bent the pins of a set of irf610/9610s and used them in the input differential. Out of curiosity, I left the other channel stock and cannot hear the difference (may be speaker limited again). So you can substitute easily if you need to.
A friend purchased a set of Spendors to replace a dipole speaker he had for greater SAF. Initially he placed them roughly as the dipoles had been and he was disappointed. After his bride rearranged the furniture to suit her sensibilities, the speaker placement was different and surprisingly, their sound improved to the point that he dropped his quest to keep the dipoles. Maybe the ones you heard were not placed properly in the room. I haven't heard them yet.
With my current speakers I cannot hear a difference, my bias point (.6A total) gives me a little over 10 watts in class A. I doubt that I exceed that very often, but I may put a scope on it to check.
I may feel differently once I get my next speakers finished - what I have now isn't as revealing as the Focal 6W4254/ TC-120TD5 set I built for my fiance. This time its the same woofer and a Fountek JP3 for a dome/ribbon shootout.
IIRC, NP said the number of output devices in the A-75 design was at least partially driven by the heat sinks he had on hand. They were pre-drilled and to get the thermal performance needed, he ended up with spaces for 12 pairs of transistors.
He's pointed out several times that you can use 6 pairs safely. this should give a little more top end extension, due to lower total gate capacitance. BTW, the fairchild equivalents I used have lower capacitance than the IR originals, and other specs are comparable.
While MOSFET characteristics change with temperature, I suspect that change is minimal to inaudible. Total bias seems to be much more of a sound quality driver.
When I burned out my last matched pairs of irfd220/9220s (forgot a heat sink isolator on Vgs multiplier) I bent the pins of a set of irf610/9610s and used them in the input differential. Out of curiosity, I left the other channel stock and cannot hear the difference (may be speaker limited again). So you can substitute easily if you need to.
A friend purchased a set of Spendors to replace a dipole speaker he had for greater SAF. Initially he placed them roughly as the dipoles had been and he was disappointed. After his bride rearranged the furniture to suit her sensibilities, the speaker placement was different and surprisingly, their sound improved to the point that he dropped his quest to keep the dipoles. Maybe the ones you heard were not placed properly in the room. I haven't heard them yet.
initially I'd planned to go the dual level bias route, using a phototransistor to switch another resistor in parallel with the bias pot. The cases I bought through a group buy here didn't leave me enough room to do that or the soft start/ remote turn on I wanted.
PassFan, Good point about the errors - I found a couple of silkscreen errors on the AudioXpress boards and found decoupling caps on the board absolutely necessary.
As for the board errors, IIRC, two spots are labeled R24, one is really R81 so some trace following is in order. The PSU board fan controller is not quite right, You'll end up putting full unregulated voltage on your fan. There is an appropriate but unmarked hole that fixes it. Sadly, neither of these was mentioned in the errata package supplied with the board. They do tell you about the extra diode you need to protect your differential, though.
PassFan, Good point about the errors - I found a couple of silkscreen errors on the AudioXpress boards and found decoupling caps on the board absolutely necessary.
As for the board errors, IIRC, two spots are labeled R24, one is really R81 so some trace following is in order. The PSU board fan controller is not quite right, You'll end up putting full unregulated voltage on your fan. There is an appropriate but unmarked hole that fixes it. Sadly, neither of these was mentioned in the errata package supplied with the board. They do tell you about the extra diode you need to protect your differential, though.
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