Those results are fine in practice. The voltages are not absolutes (the ACA is not a precision DC amplifier) and they also drift with temperature. The midpoint at approx half supply voltage allows the output to swing equally above and below that value meaning you get symmetrical clipping should you drive it to high levels.
The voltages across the low value resistors allow the quiescent current to be calculated. 0.312 volts across 0.47 ohm is 0.66 amp. You will find the same value calculates out for the 0.68 ohm. 0.452 volts is also 0.66 amp.
The actual vbe (base emitter volts of Q3 (how much voltage it needs to turn on) and the gate/source voltage of the power FET's also affect the result.
So all good there. I still think you need to swap out Q4, to at least prove it one way or the other.
The voltages across the low value resistors allow the quiescent current to be calculated. 0.312 volts across 0.47 ohm is 0.66 amp. You will find the same value calculates out for the 0.68 ohm. 0.452 volts is also 0.66 amp.
The actual vbe (base emitter volts of Q3 (how much voltage it needs to turn on) and the gate/source voltage of the power FET's also affect the result.
So all good there. I still think you need to swap out Q4, to at least prove it one way or the other.
Cam, I think I did something similar and found these ones with pre-soldered pigtails a viable alternative.
I don't see anything, nopajoe, what is this presoldered pigtail jack??
I managed to to identify this from Digikey: PART: 839-1292-ND - might even be the stock part? It's a 2.5X5.5mm jack that represents my best attempt to measure and spec from Digikey. Will get a 2.1X5.5mm version as well just in case I didn't get the plug spec right.
I intend to modify my soldering strategy significantly, but I'll have a backup extra in case.
I managed to to identify this from Digikey: PART: 839-1292-ND - might even be the stock part? It's a 2.5X5.5mm jack that represents my best attempt to measure and spec from Digikey. Will get a 2.1X5.5mm version as well just in case I didn't get the plug spec right.
I intend to modify my soldering strategy significantly, but I'll have a backup extra in case.
I found your Amazon link for the pigtail power jack, nopajoe, and then found this: https://www.amazon.com/Fancasee-Fem...-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1 a 4-pack with available Prime shipping. Would have saved me over $30 in FedEx overnight charge!
There you go! I think I bridged/melted something when I first tried soldering on the power & grounds to the jack so I found the pre-wired “pigtails” a nice shortcut. Seems to be working fine so far. I’m hoping to build another and run dual mono down the road so the extras should come in handy.
Now I’m just trying to block some time to do the Q4 swap Mooly’s suggested and figure out the static/channel imbalance.
Now I’m just trying to block some time to do the Q4 swap Mooly’s suggested and figure out the static/channel imbalance.
I wondered how it was all going
I'll just mention this for info but it is unproven although the simulation suggests it will work. If you haven't a FET to try for Q4 then a high gain small signal NPN transistor should work, something like a BC546 etc. Collector to +24 volts, emitter to R6/R9 and base to C3
I built two of these and they work great and guide is exceptionally through, but I was looking back at it for a quick reference on setting the voltage to 10 volts on the drain of Q1 and can't find it. My suggestion for here is to add a few words here about this final step to the build guide and a picture to make it clear which pin on Q1 is the drain.
I have a pair of "identical" v1.8 amp camp amp's with few mods mentioned on this forum.
With XLR connection (switch at the middle) speakers (88dB sensitivity) are totally quiet to my hearing. All ok from XLR.
When I connect RCA connector to amp, I get hum/buzz to a speaker (other end of RCA cable free).
Hum/buzz goes away if I short circuit open end of RCA cable. Connecting it to other electronics is not removing hum/buzz.
Both amps act the same. And so does Input A and Input B (when connected alone).
I usually use only XLR so this has not been a problem, but now I have B1 Buffer Preamp and it comes with RCA connections.
Hum/buzz is quite quiet, but it can be heard in quiet parts of music or when playing at very quiet levels.
Bridging resistors have been changed from original 39k to 68.1k ohms. Other mods are on boards / same for XLR and RCA, as I understand.
If I measure those resistors, those give only about 42k ohm (power off).
I have tested "switch up" and "switch down" possibilities. Same results. Even stereo (switch at middle; RCA) gives hum/buzz when RCAs' are connected to amp.
But if I connect both RCAs' (switch up), there seems not to be any hum/buzz (even from straight of my DACs RCA SE OUT).
B1 is not totally quiet same way, there will come small hiss (when both RCA's are connected in the one amp, switch up). Yes, instructions say connect 1 RCA only to Input A, but I tested this also. May help you or not. Anyway, another RCA connected seems to make this more quiet.
This is hard to explain, hope you have simple answer
It would be great if I could measure what is wrong some how. Where I should start looking for?
With XLR connection (switch at the middle) speakers (88dB sensitivity) are totally quiet to my hearing. All ok from XLR.
When I connect RCA connector to amp, I get hum/buzz to a speaker (other end of RCA cable free).
Hum/buzz goes away if I short circuit open end of RCA cable. Connecting it to other electronics is not removing hum/buzz.
Both amps act the same. And so does Input A and Input B (when connected alone).
I usually use only XLR so this has not been a problem, but now I have B1 Buffer Preamp and it comes with RCA connections.
Hum/buzz is quite quiet, but it can be heard in quiet parts of music or when playing at very quiet levels.
Bridging resistors have been changed from original 39k to 68.1k ohms. Other mods are on boards / same for XLR and RCA, as I understand.
If I measure those resistors, those give only about 42k ohm (power off).
I have tested "switch up" and "switch down" possibilities. Same results. Even stereo (switch at middle; RCA) gives hum/buzz when RCAs' are connected to amp.
But if I connect both RCAs' (switch up), there seems not to be any hum/buzz (even from straight of my DACs RCA SE OUT).
B1 is not totally quiet same way, there will come small hiss (when both RCA's are connected in the one amp, switch up). Yes, instructions say connect 1 RCA only to Input A, but I tested this also. May help you or not. Anyway, another RCA connected seems to make this more quiet.
This is hard to explain, hope you have simple answer
It would be great if I could measure what is wrong some how. Where I should start looking for?
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Amp off: just SMPT connected to the wall and to amp (no audio)
RCA with one probe, 5 VAC
RCA with one probe and another probe "grounded to hand", 23 VAC
No DC to measure.
Amp on: just SMPT connected to wall and to amp (no audio):
RCA with one probe, 5-6 VAC (rising in time and heat?)
RCA with one probe and another probe "grounded to hand", 23 VAC
0.02-0.03 VAC from power cables (inside the amp)
0.2 VAC from RCAs (negative and positive to different probes)
24 VDC from power cables.
0 VDC from RCA connectors.
All seems to be 50hz (which is normal here), one probe to positive or to negative.
To make this even easier for you.. If I now connect XLR from DAC to amp (amp off and SMPT at the wall), I will get 0 VAC from RCA and power cables.
XLR seems to "eat" earlier measured VAC away. New way for it to escape?
Power on. Touching only one RCA outer ring there will be 0.2-0.3VAC, but when touching plus and minus VAC goes to 0 volts.
Also 0 VAC from power cables when XLR is connected to amp and DAC.
This might be the reason why I don't hear anything wrong when using XLR-connections. It seems that RCA is only problem.
Is my SMPT leaking AC to the amp, which is amplified (about tenfold) and that is the hum/noise I can hear from the speakers when using only RCA?
I can't find defects in amps and I can't hear any hum with XLR connections. Both amps act the same.
Noise comes when RCA positive pin touches its female connector (if XLR is not connected).
I have all hifi connected behind the same wall socket, so this is probably not a ground loop issue?
I tested also car use "ground loop isolator" ($15) and it did not help.
RCA with one probe, 5 VAC
RCA with one probe and another probe "grounded to hand", 23 VAC
No DC to measure.
Amp on: just SMPT connected to wall and to amp (no audio):
RCA with one probe, 5-6 VAC (rising in time and heat?)
RCA with one probe and another probe "grounded to hand", 23 VAC
0.02-0.03 VAC from power cables (inside the amp)
0.2 VAC from RCAs (negative and positive to different probes)
24 VDC from power cables.
0 VDC from RCA connectors.
All seems to be 50hz (which is normal here), one probe to positive or to negative.
To make this even easier for you.. If I now connect XLR from DAC to amp (amp off and SMPT at the wall), I will get 0 VAC from RCA and power cables.
XLR seems to "eat" earlier measured VAC away. New way for it to escape?
Power on. Touching only one RCA outer ring there will be 0.2-0.3VAC, but when touching plus and minus VAC goes to 0 volts.
Also 0 VAC from power cables when XLR is connected to amp and DAC.
This might be the reason why I don't hear anything wrong when using XLR-connections. It seems that RCA is only problem.
Is my SMPT leaking AC to the amp, which is amplified (about tenfold) and that is the hum/noise I can hear from the speakers when using only RCA?
I can't find defects in amps and I can't hear any hum with XLR connections. Both amps act the same.
Noise comes when RCA positive pin touches its female connector (if XLR is not connected).
I have all hifi connected behind the same wall socket, so this is probably not a ground loop issue?
I tested also car use "ground loop isolator" ($15) and it did not help.
Kind of hard to untangle it all... so first question, are you using the Meanwell PSU recommended for the ACA?
It is relatively common to be able to measure very high AC voltages from a floating SMPS to ground and this is usually caused by inductive leakage in the switching transformer and also from any deliberately 'designed in' coupling between the live and non isolated side and the secondary side. This can typically be a small very high voltage cap.
Although no mains hum should come down the DC output it is still possible to get a ground loop if the negative of the PSU output is connected to mains ground in the SMPS. I don't know if the Meanwell is a two or three core mains lead (so double insulated or mains grounded). A true ground loop would produce 50 (or 60) Hz.
The ACA should be silent on RCA inputs if just the PSU is attached and shorting plugs are fitted to the RCA inputs.
My SMPT's are not the recommended Meanwell PSUs.
I tested my amp with other "random SMPT" and got zero VAC coming out and now the amp works as expected.
Lesson to learn, NEVER trust chinesium (my PSU came with D-class amp, which tolerate more VAC by design, I guess).
I will get the preferred PSUs now (before I get linear power supplies. Recommendations on those?).
Anyway, another "electricity mystery" solved, thank you once again!
I tested my amp with other "random SMPT" and got zero VAC coming out and now the amp works as expected.
Lesson to learn, NEVER trust chinesium (my PSU came with D-class amp, which tolerate more VAC by design, I guess).
I will get the preferred PSUs now (before I get linear power supplies. Recommendations on those?).
Anyway, another "electricity mystery" solved, thank you once again!
I think the recommended Meanwell is the best choice for the ACA in terms in weight/cost/size and also performance.
I've no recommends on linear supplies beyond saying they would be big and heavy and they would (unless you went to great lengths) have some 100/120 Hz ripple component. A small SMPS beats 10's of thousands of uF capacitance in my book
I've no recommends on linear supplies beyond saying they would be big and heavy and they would (unless you went to great lengths) have some 100/120 Hz ripple component. A small SMPS beats 10's of thousands of uF capacitance in my book
Throwing up the Mooly Signal once again 🐮🔦…
I swapped the left and right Q4 and precisely nothing changed. I’m baffled.
Would it help if I make a recording of that YouTube test video and the audio? I have a decent mic that should be able to pick up the imbalance. It’s totally hollow from the higher midrange on up. I’m just not sure how to approach this.
As always, truly appreciate any help offered here.
I swapped the left and right Q4 and precisely nothing changed. I’m baffled.
Would it help if I make a recording of that YouTube test video and the audio? I have a decent mic that should be able to pick up the imbalance. It’s totally hollow from the higher midrange on up. I’m just not sure how to approach this.
As always, truly appreciate any help offered here.
You could try a recording I suppose. Looking back and I think we said all the voltages seemed OK (post #1260)
Are you sure it is nothing like the speaker output leads from board to socket being reversed on one channel (so speakers out of phase) although that wouldn't cause crackles and static noise.
Are you sure it is nothing like the speaker output leads from board to socket being reversed on one channel (so speakers out of phase) although that wouldn't cause crackles and static noise.
If it's at all helpful, here's my best attempt at a recording from this test signal.
Here's photos of the rear, left and right. LMK if you want to see the backsides, but I've checked all the soldering pretty much endlessly at this point. At the very least, I think my soldering is improving a bit.
Edit: I've also switched sources from Schiit Modi 3+ & Vali 2++ to Topping DX5 Lite. I swapped the RCA ins to check for any difference and now have pretty high confidence it's not a signal in issue.
Here's photos of the rear, left and right. LMK if you want to see the backsides, but I've checked all the soldering pretty much endlessly at this point. At the very least, I think my soldering is improving a bit
.Edit: I've also switched sources from Schiit Modi 3+ & Vali 2++ to Topping DX5 Lite. I swapped the RCA ins to check for any difference and now have pretty high confidence it's not a signal in issue.