Pre-amp (Focusrite TT Plat) problem with hardware, HELP!

Posted: Mon Oct 13, 2008 8:37 pm

So, I'm guessing this is the right place to post this. It isn't a "Pro-audio" piece of equipment. If I'm wrong please move this topic.

I bought a Focusrite Twintrack Platinum off of craigslist a little while ago and I tested it out and everything seemed to be in working order. I have recently started to use it more because I am really digging the sound I'm getting out of it. Or so I thought.

It is having a problem I'm noticing now that I didn't notice at first. The level of a MIC plugged into the preamp is changing. What I mean is it is going up and down. It seems to be worse on channel 1. It also seems to be worse when I have the compression on. Now the funny thing is when I plug an instrument into either channel I no longer have this level variation.

Now, I'm hoping that someone can tell me what is going on with it and how to go about repairing it. I am very handy with a solder iron and if I have missed any troubleshooting steps let me know and I will get back ASAP.

I have contacted Focusrite's tech support and did not get a reply. Just as a side note, I will probably not buy anymore used gear as I really don't know how to test it and this was one of many purchases I have gotten the short end of the stick since joining this side of the audio world (the recording engineering side).

If anyone can help I will be greatly in debt to you.

BTW, in case you are wondering about my set up. I have a FP10 I'm running this into and recording with Cubase 4 LE. On a side note I have a DMPA that I don't have this problem with and I am using it the exact same set up on. I have tried to change all cables and have used/tested all Mics on different setups and all work great. Both external preamps are running into the line level inputs 4-8 with the gain staged all the way down (-20 on the FP10) so as not to use the preamps in my FP10.

Thanks again
--Eric

Posted: Mon Oct 20, 2008 10:38 am

So, could this possibly be a common symptom that could indicate an op-amp or a bad connector or something that I would not be able to figure out and I should just forever use this preamp as a DI?

Maybe I posted this in the wrong forum?

Posted: Mon Oct 20, 2008 12:00 pm

Eric- I don't have any help. Maybe if you put the name of the unit in the title, and gave a little more info on the symptoms it would help. (Maybe not. I have an Octopre, but there are not a lot of platinum series users posting here.) Let us know if you hear anything. And keep bugging Focusrite Tech support. I own a unit too.

Posted: Mon Oct 20, 2008 9:09 pm

It's probably a dry joint in the mic input section, or in the phantom power circuit if you are using a condenser mic. Do you get the effect with both dynamic and condenser mics?

I know you say it is worst on channel 1, but if it happens to some degree on all channels, then I would suspect the phantom power. Put a voltmeter between pins 2 and 1 on any XLR mic input. The meter should read somewhere between 44 and 52 Volts and be steady. If it's waving up and down, that's your problem.

It's unlikely to be the power supply to the amplifiers, as they are used for the instrument input as well as the mic inputs.

Posted: Tue Oct 21, 2008 12:48 am

I tried the VOM this morning and it seems to be holding steady at 46.2 after slowly climbing from 45.7 . I will try a dynamic MIC tonight. I am fairly sure I had already tried that but I will double check.

Posted: Sun Oct 26, 2008 8:34 am

So, it happened with a dynamic MIC (sm57). Everything was fine for like 5 min then all of a sudden I had an extremely significant loss in input/output gain. So much so that I could barely hear any output even with the input gain all the way up. It seemed to be both input and output at the same time. I could not peak the meter anymore and it was registering up to -6ish and the output was much lower then before when it was at -6 and it is extremely distorted. Could there be some overload I'm tripping or are these symptoms consistent with any other problems?

I am very willing to try any suggestions that anyone out there has to get this narrowed down.

Posted: Sun Oct 26, 2008 9:51 pm

Good - you seem to have eliminated the phantom power circuit as the culprit.

Was the effect exactly the same on all channels? Are you sure an instrument input is unaffected during this loss of signal on the mic inputs?

The extreme distortion clue would suggest a failing power rail, but that would affect all channels and also the instrument inputs.

Posted: Tue Oct 28, 2008 11:35 am

The effect is relatively similar on both channels but seemed worse on channel 1. I just checked the second channel once it happened on the first and it seemed unaffected for a bit then it to was effected but not for as long of a period. While the channel 1 was effected I switched it over to instrument and that has played fine. No cutting in or out that I could tell. The other thing I notice is that it seems to be distorting very early. After -10 it would almost seem. I don't know if that is normal for this preamp or not. Just thought I would add.

When its cutting in and out it kind of clicks (like a static sound or loose connection) and gets quieter (almost inaudible) then clicks (static) and goes back to regular volume. I can still see some movement in the level meter but hardly at all while its happening. I will also add that I was monitoring straight from the headphone out on the unit.

Last but not least it would seem that channel 1 is not popping back in once it has started happening and is still effected until I power cycle it. Channel 2 seems much more intermittent and unrelated but no where to the severity of channel 1 and not at the same time necessarily.

If I have left out anything let me know and I try to get back asap.

Posted: Tue Oct 28, 2008 10:57 pm

On balance, it sounds like a bad joint in the mic input of channel 1. However, that isn't going to affect other channels.

I would open it up and then with it switched on carefully go round with the freezer spray to see if you can find any sensitive areas.

Posted: Tue Oct 28, 2008 11:40 pm

I had a very bad experience with TTpro about 5 years ago. I bought a new unit and it was faulty. They replaced it with another unit and it was faulty too. I got it back to the shop and in the shop we have plugged in the third unit - same thing - crackles in the signal path. So I bought something else.

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