Problems on both J99s

[atiller]

Hi Tim,

I was hoping it wouldn't come to this, but alas. I just finished testing and installing the 6 modules I recently picked up from you. The only issues I'm having are with the J99s-- one being particularly worse than the other.

First of all, I am using the Forrsell JFET-993 opamps with 68pF compensation caps in place of the 680pF caps that come with the kit. I used this value on the recommendation of Mr. Forrsell. The squarewave response of the pres did not seem strange during testing. (1kHz @ 0dBV)

The worse of the two emits a sort of just-off-the-station radio buzzing that seems to change when I move the wiring harness. It is somehow related to the power on the adjacent troublesome J99 as well. The power in general seems to affect the amount of noise emitting.

This noise also seems related to both the gain and trim pot positions. It is at its worst around 11:00 on the trim pot.

I just also note that both the polarity and phantom switches seem to affect the buzz, but neither remedy it.

The worse of the two modules will not pass a clean signal: it either passes nothing or a massively distorted version of the input. I did not notice this distortion when I had the units on the scope however, which I find odd.

The slightly better one passes a signal, but the strange electrical buzzing comes in and out depending on switch positions and input level.

Both behave this way independent of input. That is, mic or no mic, they buzz.

I did have an issue with the build, which I discovered during testing. C24 (or the one adjacent to it, directly behind the two toggle switches) was in backwards. I did not notice this until I began to test the second of two preamps. So the preamp had received power over a significant period of time with the backwards C24. Everything tested fine in terms of the tests in the instructions however. I removed the cap and reinstalled it in the correct way.

Only one preamp of the two was powered with the backwards cap. I am assuming its the worse of the two that had the backwards cap.

I will record an audio clip of the two preamps in a moment and update the post.

Thanks for the help and the great products, Tim!

Adam

[atiller]

Ok, here are the audio clips:

http://atiller.com/01.mp3
http://atiller.com/02.mp3
http://atiller.com/03.mp3

01 and 02 are of the two J99s. The better one and the worse one respectively.

I start out with the trim pot at 100% and sweep the gain from 0-100% and back down again. I then sweep up to 100% and it remains there. Following this I toggle the polarity switch twice. Then I sweep the trim pot from 100% to 0% then back up to 100%.

These are done without a mic patched.

03.mp3 is an example of the sort of output coming from the worse J99. The better J99 outputs a similar sound, but just slightly improved.

New findings: the two J99s seem to be interactive. Even if I'm only patched into one of the two, changing settings on the UNPATCHED J99 results in changes in the other. This does not happen when I change anything on any other module in the rack. Just between the J99s.

Also, the problem seems to be significantly reduced if I power the J99s on their own. And by on their own I mean a) one at a time, and b) in addition to the two C84s which were always plugged in-- even during testing.

I hope this is enough info to track down the problem!

Thanks Tim!

Adam

[tpryan]

Are you testing with the front panel attached and the lock nuts installed on the J99 rotary switches?

Have you installed load isolators at L1 and L2?

What exactly is connected to the outputs, including cable length and connector types?

Do you have a scope?

[atiller]

Hi Tim,

Front panel is attached, yes. The nuts are installed on the rotary switches, minus the washers, as there never seems to be enough room on the shafts for both the washer and nut. None of the modules in my now-full rack have the washers. (The trim pots do, however).

No load isolators at L1/2-- just the zero-ohm resistors to jumper those points.

Right now the rack is patched to my TT (solder) patchbay via an 8-pair XLR snake of roughly 3.5' in length. I have tried the modules on different channels of the snake, which work fine with the other modules, and it makes no difference.

I have access to a variety of measurement gear of which a scope is included, yes.

I hope we can figure this out, Tim!

Cheers,
Adam

EDIT: So I just checked to see if the caps were blown by testing for a short across its leads-- all good there. I also decided to put one of the modules in PGND instead of CGND to see what would happen. With the polarity switch in the right position there didn't seem to be the hum, although output was distorted. With the gain at 100%, toggling the polarity switch essentially turned on the hum. A loud electrical buzzing type sound. Similar to what was in the audio files. Don't know if this is important at all, but hopefully it adds another piece to the puzzle.

Oh, I also retested the values outlined in the assembly instructions. All voltages seem good. Do you think this maybe has to do with the opamps? Should I try a 33pF for the compensation caps?

[tpryan]

What happens after the patchbay? Are you driving multiple loads? Are they all balanced? What happens if you connect directly to your headphone amp?

[atiller]

Hi again,

After the patchbay I go into my interface (RME Multiface). All of my gear is hooked up to the patchbay, but as we are testing this only a single channel of the preamp is going through the patchbay and into the interface. In terms of outputs, I have a pair of outputs going directly to a monitor controller (not through the bay).

Not all my gear is balanced, no. Everything is balanced on the patchbay (minus a couple of insert points) and unbalancing is done at the gear itself. All the grounds in the patchbay are bussed, and ground is had from the single circuit everything is on in the room.

I tried plugging the pres directly into a mackie mixer I have kicking around:

The preamp that I switched to PGND doesn't have the electrical buzz. However the output is much less than nominal, distorted, with significant noise floor, and lacking all low frequencies. Low frequencies return when I feed a hot enough signal (like a "loose connection" sort of effect). I should also note that switches (gain and toggles) create loud pops when engaged.

The preamp that remains on CGND continues to buzz like crazy.

I tried plugging the pres directly into my interface, bypassing the patchbay. The noise returns on the PGND module, and nothing changes on the CGND module.

I would like to say that I don't have grounding issues with any other gear in here, nor any other module in the rack, besides these two J99s. But obviously I will entertain the idea if it means having my SCA rack at 100%.

Hopefully we're narrowing it down here!

Thanks so much for the help!

Adam

[tpryan]

The squarewave response of the pres did not seem strange during testing. (1kHz @ 0dBV)

You observed the waveform on a scope?

[atiller]

I did, yeah. Using an AP rig. And I'm really, really surprised that I did not see the massive buzzing and noise.

The response was pretty much spot on aside from some 'overshooting' on the leading crest of the square wave, at lower gain settings. But I observed this with some of the other modules as well, so I assumed it fine.

In other words, there was nothing out of the ordinary on the scope. Which baffles me at this point.

[atiller]

Okay, so out of curiosity I decided to change the compensation caps from 68pF to 100pF. Not a big increase I know, but it made a difference nonetheless. Immediately the amount of electrical buzz is significantly reduced (even with the module set back to the original, very, VERY noisy CGND setting). The audio itself is still not right however. My findings:

-I had the module powered up, but not in the chassis. Touching the rotary switch made little shocks that while did not hurt caused audible noises on the output. They persisted and did not diminish, so I'm fairly certain it was not static. To reiterate, the module is now returned to its original CGND setting.

-Upon initial powering of the module, the frequency response (based on me speaking into a mic) is very bass heavy, but somewhat full bandwidth sounding. Within 5 seconds I can hear the low end slowly roll off and I'm left with a signal with essentially nothing below 500-700Hz if I had to guess.

Like I said before, even though the audio is not usable, at least the unit is not buzzing like crazy. The little shocks did kind of worry me though. But perhaps this is related to not being in the chassis.

I am going to try the 680pF caps to see what happens.

Adam

EDIT: Alright, so with the 680pF cap, things seem a little more stable. There's still a very small ground buzz, which is related to the rotary switch in the fashion mentioned above. This time however, I could get to 11 and 12 on the rotary switch while maintaining a degree of low end.

I have noticed that the sound remains somewhat stable until a loud enough transient, then the low end fades away and I think headroom disappears.

Could this be instability in the opamp? Are loud transients sending it into some sort of RF oscillation that I can't hear, and eating up all the headroom and low end (ie, voltage)?

I should also mention that, in general, the gain seems fairly low on the modules. For example, both my C84s on max will instantly send my open headphones and mic-in-hand into feedback (ouch). The J99s do no such thing. Max gain doesn't seem to feel even remotely close to feeding back my headphones.

[atiller]

So I mounted the module in the chassis with the 680pF compensation caps:

The weird shocks on the switch are now gone. And there doesn't seem to be any crazy electrical buzz at all. That is great.

However, the output is generally very low. Position 9 on my C84 will go to zero VU. Position 9 on the J99s will not even light up the meter hardly.

Positions 1-10 on the switch seem to be fine. Perhaps very bass heavy, and too quiet of course as mentioned above.

Positions 11-12 still exhibit this strange transient-activated low frequency roll off. As well is distortion. I toggled the phantom power, and then turned it back off and the output was tinny and distorted.

I have been checking a variety of things based on a knowledgeable friend's advice, but more suggestions would be much appreciated here Tim. I really want to get these guys working, and working at full potential.

Adam

[atiller]

Hi Tim,

The unit is on the bench awaiting your instructions.

Adam

[tpryan]

Sounds like you should send them in, clearly there are stability issues and possibly a problem with the servos. Send an e-mail to rma@seventhcircleaudio.com for return instructions.

[atiller]

Thanks Tim, will do.

Adam

[atiller]

Sorry, I meant to reply to this thread much earlier. My J99s have been going strong with the Forsell JFET-993 op amps for some time now.

The compensation caps need to be changed, first of all. In my unit, C6 = 27pF and C12 = 220pF. These values seem to work fine.

Secondly, the grounding also needed changing. Instead of the default CGND, use the jumper to change the ground to PGND (power ground).

Hope this saves someone a couple hundred bucks in bench time!

Cheers,
Adam