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ACA Mini kit


henry4841
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1 hour ago, henry4841 said:

Me too. You are the first one on this forum to actually have one. You are not going to find a class A SS amplifier this cheap that is this good. From the mind of the master of amplifier design. 

 

And it will perform better than much more expensive single ended tube amps and you'll get the same voicing as a tube amp. 1% THD at 5 watts.

 

Don't get me wrong you can make a 5 watt single ended tube amp with the similar distortion figures but the most common 5 watt tube amps I find have more than 1% THD.

 

I find it hilarious that most engineers will only use lateral Mosfets because of the negative temp coefficients for their designs yet Nelson Pass comes along and uses the cheap and much more common vertical Mosfets and most people love his amps. Again Nelson going against the grain and carving out his own niche.

 

 

 

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12 minutes ago, CWOReilly said:

Anyone know about making adjustments? The bias current value need to be hot or cold. At start up I was at .21 once hot .39. 

 

You are going to want to bias it down a nudge.

 

With the amp at ambient temperature set the VB to .15v

 

Let it sit for around an hour to warm up then come back and make sure it's around .30v to .35v

 

One could avoid this by using Lateral Mosfets that have a negative temp coefficient unlike the vertical types with positive temp coefficient. The hotter they get the more current they pull.

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41 minutes ago, captainbeefheart said:

 

You are going to want to bias it down a nudge.

 

With the amp at ambient temperature set the VB to .15v

 

Let it sit for around an hour to warm up then come back and make sure it's around .30v to .35v

 

One could avoid this by using Lateral Mosfets that have a negative temp coefficient unlike the vertical types with positive temp coefficient. The hotter they get the more current they pull.

So just sitting at idle it will reach .30 to  .35? The .39 I was getting was after a few songs. I’ll go back to it tomorrow. Thank you!

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31 minutes ago, CWOReilly said:

So just sitting at idle it will reach .30 to  .35? The .39 I was getting was after a few songs. I’ll go back to it tomorrow. Thank you!

 

Yes as the transistors warm up they'll  conduct more current.

 

From the man himself:

 

Initially we bias the amplifier low and let the bias drift up as the temperature of the transistors
rises. Most of the time the bias will drift up about 50% or so from cold to hot operation.
The two voltages on each channel are the bias current (measured as the DC voltage between
the two pads labeled VB) which is the voltage divided by 0.75 ohms, and the DC output
voltage, which is between the single pad labeled GND at the front of the board and the pads
labeled VO (one for each channel).
Initially the values will be at or close to 0 volts. While keeping an eye on both the VB and VO,
slowly alternately turn P1 and P2 clockwise. Eventually you will see some voltage appear
(don't worry if it's a minus voltage). Oh yeah, remember to turn on the switch.
We are looking to have a VO of 11.5 volts and a VB of about 0.15 volts. Keep alternatively
adjusting the pots clockwise until you get to these voltages. Sometimes you might have to
back off one one pot to get both these numbers aligned.
It's a little tricky, but it doesn't have to be very accurate at this point - we will fine tune that
after the amp has had a chance to warm up. When you get close to these voltages, just sit
back and watch the drift, no hurry, no drama.
Ultimately you will want to be at VO of 11.5 VDC and VB of 0.30 V to 0.35 V (0.4 Amps to
0.46 Amps of current). If you have a cool room or lots of ventilation you can venture into the
0.35 V territory. If not, there is little performance penalty at 0.30 V and that's how I run them
in my system. You will see some longer term temperature drift, so after you think it's stable on
those voltages, keep an eye on it for an hour or so anyway.
The VO is not very critical, and the range of 11.4V to 11.6 volts is plenty good enough

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31 minutes ago, captainbeefheart said:

 

Yes as the transistors warm up they'll  conduct more current.

 

From the man himself:

 

Initially we bias the amplifier low and let the bias drift up as the temperature of the transistors
rises. Most of the time the bias will drift up about 50% or so from cold to hot operation.
The two voltages on each channel are the bias current (measured as the DC voltage between
the two pads labeled VB) which is the voltage divided by 0.75 ohms, and the DC output
voltage, which is between the single pad labeled GND at the front of the board and the pads
labeled VO (one for each channel).
Initially the values will be at or close to 0 volts. While keeping an eye on both the VB and VO,
slowly alternately turn P1 and P2 clockwise. Eventually you will see some voltage appear
(don't worry if it's a minus voltage). Oh yeah, remember to turn on the switch.
We are looking to have a VO of 11.5 volts and a VB of about 0.15 volts. Keep alternatively
adjusting the pots clockwise until you get to these voltages. Sometimes you might have to
back off one one pot to get both these numbers aligned.
It's a little tricky, but it doesn't have to be very accurate at this point - we will fine tune that
after the amp has had a chance to warm up. When you get close to these voltages, just sit
back and watch the drift, no hurry, no drama.
Ultimately you will want to be at VO of 11.5 VDC and VB of 0.30 V to 0.35 V (0.4 Amps to
0.46 Amps of current). If you have a cool room or lots of ventilation you can venture into the
0.35 V territory. If not, there is little performance penalty at 0.30 V and that's how I run them
in my system. You will see some longer term temperature drift, so after you think it's stable on
those voltages, keep an eye on it for an hour or so anyway.
The VO is not very critical, and the range of 11.4V to 11.6 volts is plenty good enough

Ok. I’ll start over and see where it goes. Seems I’m looking for a max of .35 after warmed up. Thank you 

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4 minutes ago, CWOReilly said:

Ok. I’ll start over and see where it goes. Seems I’m looking for a max of .35 after warmed up. Thank you 

 

Here are the main takeaway point which hopefully may clarify and make it easy for you.

 

Basically you want to roughly set bias at ambient temperature.

 

Don't get hung up too much on making the initial settings of .15v perfect because transistors have a large tolerance and will drift after being warmed up.

 

Music or not while letting warm up may not make a huge difference but I'd just do it without any music because Class A devices will be at their highest dissipation at idle.

 

After it has warmed up take the readings again and if they are between .30v and .35v you don't need to do anything all is well. If they are above or below that region then adjust.  If one is .35v and the other is .30v you can leave them or if you want optimal matching between channels then adjust them closer to each other. Go very slow moving the trimpot in small increments and wait after making an adjustment. The transistor will need time to settle before making another adjustment. 

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29 minutes ago, captainbeefheart said:

 

Here are the main takeaway point which hopefully may clarify and make it easy for you.

 

Basically you want to roughly set bias at ambient temperature.

 

Don't get hung up too much on making the initial settings of .15v perfect because transistors have a large tolerance and will drift after being warmed up.

 

Music or not while letting warm up may not make a huge difference but I'd just do it without any music because Class A devices will be at their highest dissipation at idle.

 

After it has warmed up take the readings again and if they are between .30v and .35v you don't need to do anything all is well. If they are above or below that region then adjust.  If one is .35v and the other is .30v you can leave them or if you want optimal matching between channels then adjust them closer to each other. Go very slow moving the trimpot in small increments and wait after making an adjustment. The transistor will need time to settle before making another adjustment. 

That makes more sense than the “manual”. More patience. Thank you!

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19 hours ago, CWOReilly said:

So far it’s enjoyable. I’ve connected it to my LSI splits and it’s plenty. Despite low power seems to still have headroom. I’m listening to songs I listened to days ago running a big Carver and the biggest difference is it’s only 5 watts. 
 

Anyone know about making adjustments? The bias current value need to be hot or cold. At start up I was at .21 once hot .39. 

 

Which big Carver do you have and what speakers do you use typically?  What type of case are you going to put it into, if any?

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1 hour ago, avguytx said:

 

Which big Carver do you have and what speakers do you use typically?  What type of case are you going to put it into, if any?

Well it’s more a middle Carver. It’s a tfm-35. I was using it with Polk SDA2 then moved those out when I got the LSI Splits. Was waaaay to much for the LSIs in my space. It is going in for a recap very soon then probably gonna run my Polk SDA SRS. 
 

As for the ACA Mini case. I am undecided. I have a cigar box that’s almost perfect size. Just not sure I like the look. Leaning toward leaving it nude or maybe something with LEGOs. 

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  • 2 weeks later...
On 8/26/2022 at 10:00 AM, CWelsh said:

I'm anxious for an update! Now that you have a few hours on it, what do you think?

Missed your reply. At this point I’m enjoying it. I’m questioning my source. It’s bright, but there’s something else there. I’m using an old iPhone with pandora and some things are better than others obviously. Determining good from better is the trick. Gonna go for a CD player next. 

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On 8/3/2022 at 6:38 AM, henry4841 said:

Soldering is like welding, actually is welding, heat the parts enough to melt the solder and not the other way around. 

Soldering is not like welding, it's an entirely different joining process.  

 

Welding consumes both part surfaces being joined, solder is an alloy introduced at the joint that is melted and designed to wet the two parts being joined but does not, under any circumstances, consume or otherwise "melt" the parts being joined.  Welding temperatures are significantly higher than the two materials being joined, this promotes solubility and homogenization of the weld pool.  Welds also have mechanical requirements that are different than soldering.

 

Joining methods:

Soldering

Furnace brazing

Welding - many ways, resistive, friction stir, inertia, TIG, MIG, He-assist, plasma

Diffusion bonding

Explosive bonding

 

 

 

 

 

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11 minutes ago, John Warren said:

Soldering is not like welding, it's an entirely different joining process.  

 

Welding consumes both part surfaces being joined, solder is an alloy introduced at the joint that is melted and designed to wet the two parts being joined but does not, under any circumstances, consume or otherwise "melt" the parts being joined.  Welding temperatures are significantly higher than the two materials being joined, this promotes solubility and homogenization of the weld pool.  Welds also have mechanical requirements that are different than soldering.

 

Joining methods:

Soldering

Furnace brazing

Welding - many ways, resistive, friction stir, inertia, TIG, MIG, He-assist, plasma

Diffusion bonding

Explosive bonding

 

 

 

 

 

You are absolutely correct. Brazing is more like soldering. I was just trying to make the point that you want the parts you want to join together hot enough to melt the solder instead of the other way around. 

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3 hours ago, henry4841 said:

You are absolutely correct. Brazing is more like soldering. I was just trying to make the point that you want the parts you want to join together hot enough to melt the solder instead of the other way around. 

Brazing is actually a quite different as well.  In a proper braze joint, there's a metallurgical diffusion zone between the part surfaces and braze alloy where limited solubility exists.  Braze alloys are selected to provide diffusion at the interface.  Typical braze furnace cycle is to ramp up to braze temperature, hold a few minutes, then cool to an intermediate temperature and then long soak at the intermediate temperature to facilitate the diffusion process.  Braze joints can be very close to parent metal strength for some alloy systems like Titanium based for example where the braze alloys are "fortified" with Zirconium and Vanadium, elemental additions that provide potent strengthening effects   There's vacuum furnace brazing, dip braze, torch brazing, induction brazing.

 

 

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In the plant where I work (controls engineer) we braze propane cylinders together. We apply a braze paste of copper powder in a liquid carrier to the seam.

They are then put through a furnace with a reducing atmosphere at 2000F. Part of our quality control after brazing is to pressurize samples until failure. The steel fails before the brazed joint.

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15 hours ago, grendel23 said:

In the plant where I work (controls engineer) we braze propane cylinders together. We apply a braze paste of copper powder in a liquid carrier to the seam.

They are then put through a furnace with a reducing atmosphere at 2000F. Part of our quality control after brazing is to pressurize samples until failure. The steel fails before the brazed joint.

I had an old friend who was a welder who worked on an atomic plant in Alabama who said to get the job you had to pass a welding test much the same as you describe at your plant. That kind of welding is a 3 step process. To get the job, have to be certified as a welder first of course, is weld two pieces of plate steel together and then the welded plate is applied pressure to see at what pressure it fails. If I remember correct the weld is first x-rayed as well. If your work passes the test you get the job. 

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