BAN Build Thread

[Joe Malone]

Another thing: With the gain of the preamp to max and no mic attached (just a loose XLR cable), I'm getting some pretty good hum which is reading at -51 dB in my DAW. Is this normal?

2 MB WAV recording:
http://www.filedropper.com/nomiclidlooselyon

The chassis is currently still grounded via the lead I soldered to the bottom panel. I will switch it to one of the screws at the back later, though I don't expect that to change anything. All the ground screw terminals on the XLR board, preamp board, DC jack, and every raw chassis point I've tested test continuous. So it's clearly grounded.

I want to make sure everything is within spec. Noise floor is important to me and one of the reasons I build this.

if you get -51db with the mic pre at its full 66dB of gain would indicate a open noise floor of -117dB. The mic pre has a high input impedance so cannot be left open like you have tested.

Best test is nothing plugged into the mic pre with the pad switch on (as this will give a 120ohm input load) or no pad switch and load mic input with 51R between pin 2 and pin 3. And see what level you see on your daw meter then?

Is this in one of our BA2 or BA4 cases ? Is it using one of our 48v 520mA power supplies?

[mike]

It's a BAN4 project in the case you sent to me, with the 48V 0.52 amp 25W max output adapter you provided. So far I've only mounted one card.

With no input xlr cable, and the pad switched on, the noise floor idles around -50-60 dB, but it constantly fluctuates up and down by around 5 db in either direction. Periodically every 5 or 10 seconds it spikes up as high as -32 or -28 dB.

I'm still wondering: what is my target voltage at TP3? When I set the 10k trimmer half way it's reading ~40 V. It's reading around 34 V at the lowest with the trimmer turned all the way. What does this signify and shouldn't it be 24V (1/2 voltage)?

[mike]

I added 0.1 uF bypass capacitors from Pins 4 to 8 on both opamps with no improvement.

[mike]

Here are some pics of the board as it is. The only "mods" are now that I am using a SSM2212 mounted on a SOIC8 to DIP8 board via the underside (which seem to be passing sound and should be a straight substitution so I presume they're working fine). And I now added 0.1 uF bypass capacitors to the opamps which hasn't helped.

What can I do next? I've inspected and done continuity testing for close solder pads to make sure I don't have any tiny shorts and none have come up. I still can't explain why the lowest I can get is 34V at TP3

http://postimg.org/gallery/jqo9kao0/9a6e1a4b/

[mike]

Here is a screencap of what the waveform looks like when the noise bumps up to around -32 dB every so often.

[mike]

Further testing: I just completed a second channel. This one is measuring normal ~25 V at TP3. But it's still got the fluctuating noise floor.

I'm starting wonder if I have two problems.

1) First channel measuring abnormal ~38 V at TP3 when trimmer turned half way (for unknown reasons)

2) Faulty power supply

The power supply is the only thing in common between the two channels I've built, so that must be the source of the high/fluctuating noise floor. Any way to prove this? I don't have a spare 48V AC-DC adapter lying around to test with.

Both channels were built identically and sound normal besides the noise floor issue.

[Joe Malone]

HI
Do not add 0.1 across the opamp power pins as this will increase high freq SMPS noise by injecting SMPS noise into the half rail in the BAN.

Does the BAN case have 0ohms to 0vdc on all panels of the case?

Are you doing tests with BAN case fully screwed together with lid and everything and screwed into a rack?

Is your A/D D/A interface running at pro level +18dBu or more in and out?

What is the interface?

Let me know the voltages on all channels at every opamp pin for both opamps and SSM2212 pin so I can check this in the simulator how the SSM2122 performs?

Have you tried the standard BC transistors in one BAN to see if it works fine?

Also email me some large well lit clear in focus photos of the top and bottom of the BAN PCBs.

There maybe something wrong with the supply or it is has to small a load on it. These SMPS get quieter with more load. But 2 or more channels should be fine.

Please answer all questions and send me the photos so I can work out your problem.

[mike]

Do not add 0.1 across the opamp power pins as this will increase high freq SMPS noise by injecting SMPS noise into the half rail in the BAN.

Okay. Thanks. I've taken them off. It was a problem before I added those though anyway.

Does the BAN case have 0ohms to 0vdc on all panels of the case?

I'm reading 1 ohm from each panel to the ground tag of the AC jack.

Are you doing tests with BAN case fully screwed together with lid and everything and screwed into a rack?

No. I've been leaving it on the ground with the top open. I just put the top on though and there is no difference in any issues.

Is your A/D D/A interface running at pro level +18dBu or more in and out? What is the interface?

It's an Audiofire 12 and yes it's running at balanced line levels.

Let me know the voltages on all channels at every opamp pin for both opamps and SSM2212 pin so I can check this in the simulator how the SSM2122 performs?

CHANNEL ONE (reading ~38 V at TP3 for unknown reasons)

IC1:
- Pin 8 - 48.0
- Pin 7 - 20.1
- Pin 6 - 29.5
- Pin 5 - 29.5
- Pin 4 - 0
- Pin 3 - 24.9
- Pin 2 - 24.9
- Pin 1 - 38.9

IC2:
- Pin 8 - 40.9
- Pin 7 - 23.9
- Pin 6 - 33.5
- Pin 5 - 33.5
- Pin 4 - 0
- Pin 3 - 33.5
- Pin 2 - 33.5
- Pin 1 - 21.9

SSM2212:
- Pin 1 - 33.5
- PIn 2 - 29.6
- Pin 3 - 28.9
- Pin 4 (NC) - 40.2
- Pin 5 (NC) - 38.8
- Pin 6 - 28.9
- Pin 7 - 29.3
- Pin 8 - 33.5

CHANNEL TWO (reading ~25 V at TP3)

IC1:
- Pin 8 - 48.3
- Pin 7 - 30.9
- Pin 6 - 29.4
- Pin 5 - 29.4
- Pin 4 - 0
- Pin 3 - 23.8
- Pin 2 - 23.8
- Pin 1 - 27.9

IC2:
- Pin 8 - 41.0
- Pin 7 - 22.6
- Pin 6 - 33.6
- Pin 5 - 33.6
- Pin 4 - 0
- Pin 3 - 33.6
- Pin 2 - 33.6
- Pin 1 - 22.9

SSM2212:
- Pin 1 - 33.6
- PIn 2 - 29.5
- Pin 3 - 28.8
- Pin 4 (NC) - 39.5
- Pin 5 (NC) - 39.6
- Pin 6 - 28.8
- Pin 7 - 29.5
- Pin 8 - 33.6

The biggest abnormality I see is pins 1/7 of IC1 for channel 1 are clearly not making sense compared to channel 2. Everything else looks reasonably balanced from channel to the other to my eyes.

Have you tried the standard BC transistors in one BAN to see if it works fine?

No since I thought the SSM2212 was a standard substitution but with possibly higher performance, and it was passing sound find I assumed it should work with that. I am planning to try that for my 3rd channel build which I will start next.

Also email me some large well lit clear in focus photos of the top and bottom of the BAN PCBs.

Done.

There maybe something wrong with the supply or it is has to small a load on it. These SMPS get quieter with more load. But 2 or more channels should be fine.

Just received a replacement one for $20 off mouser since I was placing an order for parts anyway and sadly it's not making any difference. The fluctuating noise floor persists, which tells me maybe there's something wrong with using the SSM2212 in this build?

Please answer all questions and send me the photos so I can work out your problem.

I hope so. I wish I had just paid you to build them for me. I've never had problems like this with a build before.

[Joe Malone]

Can you please check your IC pinout as pin 4 should be at 0v on both IC's and pin 8 should 48v one IC and about 40v on the next? Otherwise you have the ICs in backwards

[mike]

Can you please check your IC pinout as pin 4 should be at 0v on both IC's and pin 8 should 48v one IC and about 40v on the next? Otherwise you have the ICs in backwards

Sorry Joe, you're right. I rechecked and I have my ICs oriented correctly, but I reversed the pin numbers when I was writing down my voltages. I've corrected them.

Again, it appears like the TP3 problem with channel one is originating at IC1 pins 1/7 (output), as all voltages prior to that point appear roughly the same and +/- balanced between the two channels.

[mike]

To summarize, my problems appear solved after corresponding with Joe via e-mail, and the explanations are as follows:

1) Fluctuating noise floor - This has resolved to normal limits after adding a third channel to the build. It appears this was a shortcoming of the 48V adaptor that causes it to produce increased power noise at low current draw. I expect this will only improve further once I add the 4th channel.

2) Abnormal voltage at TP3 - Joe has clarified that TP3 voltage only indicates the balance between the two opamps which is not significant. I calibrated by unit with the following procedure instead:

(i) Measure voltage at the pin of 10K trimmer towards the front of the chassis, adjust 10K trimmer until this test point reads 24V.
(ii) Adjust 1K trimmer until voltage across gain pot is as close to 0.0 mV as possible.

Results
My specs running RMAA through the device with the pad engaged and the gain pot all the way down have been interesting. I currently have two channels with SSM2212 transistors installed and 1 with stock BC560's.

Many people have switched to the SSM2212 from other transistors for 990 type opamps saying these offer the best performance for the price. However, at least based on the three channels I have this does not appear to be the case.

SSM2212 Channel at Zero Gain:
Freq Response: +0.03 -0.05 db
Noise level -107.1 db
Dynamic range 107.0 db
THD 0.0015%
IMD+noise 0.0027%
IMD+noise swept 0.0033%

BC560 Channel at Zero Gain
Freq Response: +0.03 -0.05 db
Noise level -106.4 db
Dynamic range 106.4 dB
THD 0.0011%
IMD+noise 0.0018%
IMD+noise (swept) 0.0022%

Thus at zero gain, the BC560 channel is producing far less IMD, and less THD as well. This was consistent compared to the other SSM2212 channel (which had 0.0018% THD). The BC560 channel on the other hand has ~0.6 dB higher noise than the SSM2212 channels.

SSM2212 Channel at Max Gain
Freq response +0.05 -0.06 dB
Noise level -69.2 dB
Dynamic range 69.2 dB
THD 0.0097%
IMD+noise 0.097%
IMD+noise (swept) 0.101%

BC560 Channel at Max Gain
Freq response +0.04 -0.05 dB
Noise level -68.9 dB
THD 0.0095%
IMD+noise 0.099%
IMD+noise (swept) 0.103%

When testing the channels at full gain, the results were almost identical however with no clear differences.

Remaining Questions
Joe I'm wondering if you can comment on a few things:
- Are these results normal or less than optimal?
- Does it "make sense" that the SSM2212 would have higher distortion but slightly lower noise?
- Do you think an LM394 would be superior to both? On a previous post you suggested it can provide 1-3 dB noise floor benefit over the BC560.

Thanks,
Mike

[Joe Malone]

Remaining Questions
Joe I'm wondering if you can comment on a few things:
- Are these results normal or less than optimal?

Yes in this test setup.

- Does it "make sense" that the SSM2212 would have higher distortion but slightly lower noise?

Not a big enough number of test sample to be sure but the higher current drawn by these multi transistor matched pairs mean they have emitter share resistors to help the matching which can cause this. But THD for all is so low it matters little. The main feature is usually best noise floor which multi matched pairs can give 1 to 3db theoretical improvement which is shown in the results.

- Do you think an LM394 would be superior to both? On a previous post you suggested it can provide 1-3 dB noise floor benefit over the BC560.

While SSM2212 and LM394 give a slightly better noise improvement I have always found I like the sound of the BC560 pair better to my ears.

[mike]

Looks like my challenges with this build aren't quite done yet. I finished the fourth channel (using 2SA1804 transistors - accommodating for their different pin configuration appropriately).

Somewhere along the line testing this fourth channel, I started developing a bizarre fluctuation in the voltage supplying my PCB's. At first it would happen when I would just turn the unit on. The power light would pulse on-off... on-off... on-off... then stay steady on. After a few times of turning the unit on/off while trying to trim the fourth channel, the pulsation became unending.

Now when I turn it on, the green power LED just pulses on-off... on-off... over and over and over. When I measure the voltage at the jack or fuse, it is fluctuating up and down in concert with the LED lights. If I turn on the phantom, the phantom LEDs pulse up and down in sync with the power lights.

I have two PSU's and I tried them both with the same result. I tried detaching the new fourth PCB 48V (red) power cable but the voltage pulsations are continuing nonetheless.

I tried changing the fuse with no effect. I tried taking out the fuse altogether (to break the circuit) and measured the voltage at the adaptor jack. It measures 48V steady when there is no fuse in. As soon as I reinstall the fuse (reconnect the circuit) and turn it on, the voltage starts pulsing/spiking rhythmically again.

Any idea on earth what this is now?

[Joe Malone]

Looks like my challenges with this build aren't quite done yet. I finished the fourth channel (using 2SA1804 transistors - accommodating for their different pin configuration appropriately).

Somewhere along the line testing this fourth channel, I started developing a bizarre fluctuation in the voltage supplying my PCB's. At first it would happen when I would just turn the unit on. The power light would pulse on-off... on-off... on-off... then stay steady on. After a few times of turning the unit on/off while trying to trim the fourth channel, the pulsation became unending.

Now when I turn it on, the green power LED just pulses on-off... on-off... over and over and over. When I measure the voltage at the jack or fuse, it is fluctuating up and down in concert with the LED lights. If I turn on the phantom, the phantom LEDs pulse up and down in sync with the power lights.

I have two PSU's and I tried them both with the same result. I tried detaching the new fourth PCB 48V (red) power cable but the voltage pulsations are continuing nonetheless.

I tried changing the fuse with no effect. I tried taking out the fuse altogether (to break the circuit) and measured the voltage at the adaptor jack. It measures 48V steady when there is no fuse in. As soon as I reinstall the fuse (reconnect the circuit) and turn it on, the voltage starts pulsing/spiking rhythmically again.

Any idea on earth what this is now?

If you read the top of this BAN thread this is fully explained. But the simplest thing to do since you have already built is just remove the 470uF 50v next to the 10R resistor at the back of each PCB. This will let the power supply start easier and leave the preamps working fine.

[mike]

Yeah you're right Joe. My mistake. I had read a couple times we probably didn't need the soft start mod for the new PSU's so I didn't recognize the hickup protection for what it was.

To confirm, do you mean remove the cap I crossed off in red, or the one circled in blue? Would I need to put a jumper across where I remove it?

BAN800[1].jpg

Is there any comparative disadvantage to the expected specs by removing this capacitor vs. doing the soft start mod? I don't mind the mod. It looks like it's just:

- Add jumper wire connecting two pads
- Cut connection to one pad
- Remove jumper and install BD681
- Install 10K resistor

If that gives better performance, I'd rather do that. I've put so much into this already, and I'm due to order some parts from Mouser anyway. I can pick up 4 BD681's no problem.

Also, while I'm ordering, is there a Mouser part number for the blanking plugs you use for the DI jacks? I'd like to pick up two more.