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TL Audio PA2 mods and repairs for anyone interested Single-Channel Preamps
Old 1st April 2010
  #1
TL Audio PA2 mods and repairs for anyone interested

I recently had my TL Audio PA2 mic pre repaired and modded. This will be the first time I tried to attach any photos on gearslutz, so be patient with this post if not everything appears as I intended.

Anyway, a bit of history--I have read more than once on here where folks bashed the TL Audio gear, but only after I had purchased my PA2 used from eBay. I did not let the other folks opinions deter me however, and I decided I would get what I could out of the preamp.

The PA2 is a dual mic pre with IC op-amps feeding into a 12AX7. The 12AX7 receives high voltage. I have a decent amount of experience with preamp tubes from my guitar amps, so I decided to start mods on my own by upgrading the preamp with a pair of NOS Sylvanias I had. The result was wonderful. If you have one of these mic pres, do yourself a favor and swap out the stock tubes. They are rebranded Chinese 12AX7s and the difference was substantial. It really was the single most impressive change that was made to my preamp. The cost was about $50.00 for the pair of NOS Sylvanias--well worth the cost to bring this preamp up in quality but such a significant margin. I think there are some new tubes that would do well here--my faves being the JJ 12AX7 long plate, gold pin versions of the 12AX7 they make. If you are fortunate enough to get ahold of a pair of Svetlana 12AX7s, go for it, they are reported to be very, very nice. I can't get those.

Well all the tube switching was great, but I had a conversation with my very capable, creative and trustworthy repair guy regarding adding input transformers to a circuit. He said he could add an input transformer to any preamp. That got me thinking about making my one-trick pony TL Audio PA2 into something a bit more interesting. I knew from having the chassis open that it was an IC op-amp based input circuit and that something cool could happen if I got a pair of input transformers in there.

My repair guy, Tim, found a couple other problem areas with the PA2 circuit that anyone owning these needs to be aware of. I will point those out as I proceed with the project descriptions. I will now refer to the attached photos...

In photo 001 is the first area of concern--there are a couple of capacitors at the output of the preamp--they are in place to protect the circuit from receiving back-fed 48v from any other source. I bought my preamp used, so whoever owned it before me must have run phantom power into this thing by accident. The caps on my preamp were bulging, failing, but not yet leaking. These were replaced and upgraded to a pair of caps that will better handle filtering any 48v power erroneously fed into the preamp via the output connection. If you know for certain that phantom power never back-fed into your preamp, this will not be an issue for you.



Looking at photo 002, you will see the first mod--these are the IC op-amps that the signal first hits and then feed the signal to the tube. There is one for each channel here. Originally they were soldered to the board. Tim put sockets onto the board and got me a different set of ICs (the ones that are pictured are NOT the stock ICs). I A/B'ed the stock ICs with the ones that are loaded in my preamp now. I found the stock ICs were smeary in the high frequencies and the low mids and were not quite as precise or fast as the op-amps you see in the photo. You should also note that these ICs are 14 pin ICs as opposed to the eight pin op-amps found in a lot of other preamps these days. There certainly are other 14 pin op-amps available, and indeed, I have a pair of Burr Browns on the way to try out.



Moving on to photo 004--this is a detailed view of the front panel of the P2. What you see here is a switch that selects between a signal path that either goes to the op-amp on a single channel or else to the transformer I had Tim install. There is also a second gain control, which is used for the transformer side of the signal chain. Tim found a place in the schematic where he could bypass the op-amp and insert the input transformer. This was what I really was going for in the entire project. I found a used pair of Beyerdynamic input transformers on eBay for $30.00 plus shipping. Not bad. I think the transformers I have in there are a 10:1 ratio.



In photo 007 you can see the transformer peeking out at the bottom of the photo--it is hot glued directly to the chassis. A repair that you should note if you own one of these preamps is circled at the top of the photo: there are a pair of high voltage resistors that were failing on my preamp. They were charred and cracking. Fortunately they were not completely discolored and Tim was able to get the values off them--this was before I had secured the schematics from TL Audio. Tim thinks the original parts were a bit under-valued for their task and he upgraded them with a pair of resistors that will be able to handle the voltage, no problem. So again, if you own one of these, check into that pair of resistors.



I was planning to post some audio samples, but I have decided against it, as there were no big audio surprises resulting from the mods. The IC op-amp side of the input signal path is precise, quick and has good detail across the audio spectrum. The transformer side has a warmer, fuller body to the sound. These characteristics are typical of an IC audio path or an input transformer audio path and those of you who know them will know what to expect. Those of you who don't know the difference between tranny sound versus IC sound, well, there are about a thousand descriptions and audio links posted already. One more thing regarding the transformer side--it has less available gain than the IC side. The transformer is adding color, but it really is only adjusting impedance and not providing any gain. Now MOST of the gain on this preamp comes from the tube, but in the stock set-up, the IC sort of assists by bringing the gain up a bit and then feeding it ti the tube. Therefore, with my transformer side of the signal chain, the IC is not employed and that small amount of gain the IC provides is not available.

I really am pleased with how everything turned out. The preamp now has a couple of different voices that can switch without any trouble. I can swap out the IC if I ever choose to, which is a very cool thing. $$$-wise it was well worth it as well--used the PA2 was about $300. I have seen them for slightly more and slightly less. $50.00 for the NOS tubes. $30.00 for the transformers. Less than $200.00 for the repairs and mods. Not bad--two channels, lots of color choices and it sounds great. The PA2 now compares very favorably to my Summit Audio 2BA-221, if that reference has any value to you.

Some points worth mentioning with the PA2--as you may or may not know, 12AX7 tubes have two gain stages/ two sides. On the mic pre with the P2, it only uses ONE side of the tube. The instrument input uses both sides of the tube. Another interesting thing with this preamp--the clip light varies in intensity, lighting as the tube voltage increases. When the LED is at full intensity, the pre is still something like 6dB below clipping. So if you want lots of tube color, ride the gain to get that clip light to glow. If you want less tube color, roll off the gain and don't light up the clip light.

So if you own one of these, you CAN get more life out of it and make it more useful. If you are interested in doing the same stuff I did, look around--they are available pretty often on eBay.

Also, if you do not have a trust-worthy repair person, I will gladly recommend my friend Tim to you at Full Custom Repair in Lemoyne, PA. He has the advantage over anyone else in the world right now in that he JUST completed this project a couple weeks ago. He knows the circuit. PM me and I'll give you his contact information--FWIW, his business is Full Custom Repair located in Lemoyne, Pennsylvania.

On a side note, the folks at TL Audio were very helpful and good to deal with. They very quickly e-mailed my the schematics for the preamp.

The caveat to this post is if a lot of you get interested in used TL Audio PA2s, I will not be able to get a second one as easily! Take care and I hope this effort will be useful to some of you.
Old 1st April 2010
  #2
Those OP11 quads are slow and dirty. Try a National LME49740NA or a Brown Dog dual SOIC to 14 pin dip adaptor for many good dual opamp choices.

Replace those dirty SSM2142 output driver chips with either BB DRV134's or that 1624's.

If the input mic preamp is the chip design using a SSM2017 or similar, you can swap to the SSM2019, BB INA217, that 1510 and the Audio Upgrades PIP hybrid modules at:
Audio Upgrades

Jim Williams
Audio Upgrades
Old 1st April 2010
  #3
Quote:
Originally Posted by Jim Williams View Post
Those OP11 quads are slow and dirty. Try a National LME49740NA or a Brown Dog dual SOIC to 14 pin dip adaptor for many good dual opamp choices.

Replace those dirty SSM2142 output driver chips with either BB DRV134's or that 1624's.

If the input mic preamp is the chip design using a SSM2017 or similar, you can swap to the SSM2019, BB INA217, that 1510 and the Audio Upgrades PIP hybrid modules at:
Audio Upgrades

Jim Williams
Audio Upgrades
Jim--thanks! I was wondering about more quad op amp options. I will look into picking up a pair of the National LME49740NA op amps.

I do not follow you on your input chip suggestions. I was under the impression that the quad op amps here were the input chips. But I am not kind of technician. There are a number of other op amps on the board, some of which are in sockets. Maybe some of those are the output drivers you mentioned?

I have looked at your PIP modules and thought if I had a dual op amp circuit I'd go for it.

Hmmm.... maybe I'll have to pull this bad boy apart again and look into replacing some more chips.

Were your suggestions based upon what you see in my photos, on general experience, or have you worked on the PA2 preamps before? I certainly appreciate your expertise and suggestions.

We spoke on the phone once--I was trying to decide if I should hold onto a Valley People channel strip I picked up. It needed some work--the compressor was not functioning correctly. Upon your advice I took advantage of the sellers return policy and moved on from that one. I never thanked you--so thanks!
Old 1st April 2010
  #4
Quote:
Originally Posted by Jim Williams View Post
Those OP11 quads are slow and dirty. Try a National LME49740NA or a Brown Dog dual SOIC to 14 pin dip adaptor for many good dual opamp choices.

Replace those dirty SSM2142 output driver chips with either BB DRV134's or that 1624's.

If the input mic preamp is the chip design using a SSM2017 or similar, you can swap to the SSM2019, BB INA217, that 1510 and the Audio Upgrades PIP hybrid modules at:
Audio Upgrades

Jim Williams
Audio Upgrades
Jim--is there a difference between National LME49740NA, LME49740NABD and the LME49740NA/NOPB? I was looking at the spec sheets on them and from what I saw they were all the same???
Old 1st April 2010
  #5
Awesome!
I just got my hands on a model that doesn't say PA2 but looks similar. I also just bought JJ Gold pins for it. I'm glad they worked out for you.
Thanks for your post!
Old 1st April 2010
  #6
Quote:
Originally Posted by Damusica View Post
Awesome!
I just got my hands on a model that doesn't say PA2 but looks similar. I also just bought JJ Gold pins for it. I'm glad they worked out for you.
Thanks for your post!
Reply back here after you get the tubes installed. I would like to read how you make out with the change to the signal.
Old 2nd April 2010
  #7
Quote:
Originally Posted by Puffer Fish View Post
Jim--is there a difference between National LME49740NA, LME49740NABD and the LME49740NA/NOPB? I was looking at the spec sheets on them and from what I saw they were all the same???
I use the NA. Those others might be tape and reel or something. You can get them from:
DigiKey Corp. | Electronic Components Distributor | United States Home Page
or from
www.national.com
direct. If you give them a biz email they will send 5 samples for free. Otherwise you just pay a small fee that's less than the cost of buying and shipping them.

Jim Williams
Audio Upgrades
Old 6th April 2010
  #8
Nice

Quote:
Originally Posted by Puffer Fish View Post
Reply back here after you get the tubes installed. I would like to read how you make out with the change to the signal.
So I open this thing up and I find two different tubes. CH1 was a Edicron ECC83. CH2 was a Brimar 12AX7, logo looks like mid 1950's-60's according to the oracle google. I swapped in the JJ Gold pins and immediately heard a difference. I used memory and a couple of tracks to compare. I could still hear the unit taming transients but a nice clarity and harmonic detail was more apparent. The overdrive is more gradual and blooms nicely all the way up.
To me it was worth the upgrade.
thumbsup
Old 6th April 2010
  #9
How interesting. So you picked your PA2 up used as well--you must have--because the unit shipped with the Edicron tubes (rebranded Chinese tubes) in both sockets. It is surprising to me that someone would have swapped out only one of the tubes. Obviously if you ever want to use the pre for stereo source material, you need to have matching electronics--like a pair of tubes, not two different kinds.

I am glad your efforts are yielding favorable results. Your description reads similar to my experience with swapping out the tubes for something better.

Make sure you check out those caps at the output connectors. Like I wrote earlier, if someone ran phantom power on the back-end, those caps are likely to be in bad shape. Oh yes, and look at those high-voltage resistors and make certain they are not charred or cracked! And thanks for reporting back!
Old 7th April 2010
  #10
Lives for gear
 

Puffer, can you shoot me over those schematics? I have one of these as well (got it in a trade about 12 years ago) and would like to know whats in there should I need to work on it.

Thanks
Old 7th April 2010
  #11
gladly. Check your message box....
Old 7th April 2010
  #12
Lives for gear
 
beechstudio's Avatar
 

Thanks for doing this post Matt!! There's a lot of those PA2's floating around out there. I had mine up for sale, but now I'm having second thoughts. Do you have any audio clips you can post of the new modded unit?
Old 7th April 2010
  #13
Lives for gear
 

Quote:
Originally Posted by Jim Williams View Post
Replace those dirty SSM2142 output driver chips with either BB DRV134's or that 1624's.
Hi Jim,

I notice you specify BB rather than TI. Is there any difference between the original BB DRV134s and the newer TI branded chips?

Cheers,

Matt
Old 7th April 2010
  #14
Quote:
Originally Posted by beechstudio View Post
Thanks for doing this post Matt!! There's a lot of those PA2's floating around out there. I had mine up for sale, but now I'm having second thoughts. Do you have any audio clips you can post of the new modded unit?
I did record some A/B sort of stuff, but the more I listened back to the audio, the more I recognized IC op amp sound versus transformer on the front-end sound. As for what the PA2 sounded like BEFORE versus what it sounds like now... I do not have any examples.

Like I wrote earlier... the single biggest change was swapping out the crumby stock Edicron tubes with something better. I do not know why TL Audio put in rebranded Chinese tubes instead of something nicer--at the time the PA2 was under production, EI tubes were still being made and there was nothing wrong with them or they could have used Tesla tubes as another good new production tube. Of course the reason must have been cost, but I think they got a bad reputation for the preamp because of it. Anyway, the tube swap couldn't be easier, and if you go with a new production 12AX7, you could get a pair for less than $20.00. You could ask Damusica for some more info on the results of a new production tube in the PA2.

As for the IC op amp change, while you must do some work to the board to add a socket for the quad op amp, it opens up a lot of sonic possibilities that I think are worth it. You may have read already, Jim Williams suggested a different op amp from what I have in the box now and I have yet another IC on the way of a different make to drop in and try out.

....Hmmm... so as audio clips go, it appears I am not even finished with tinkering yet! I am sorry I do not have anything solid for you to listen to. My advice is keep your PA2, swap out the tubes if you have not done so already. After that, give it some time and decide if being able to change out the quad op amp has value to you. It did for me.

If you REALLY REALLY want to hear an audio clip(s), it would take me a week to get it all together. After I listened to the stuff and decided it did not have a lot of value to post on here, I did not bring the files out of the computer. They are still in the box, but it will take me some time to get to them. But I am reluctant to do so. I just do not think there is anything new to learn from the files. I am certainly not hiding anything--as evidenced by my 'tell all' post!
Old 9th April 2010
  #15
Lives for gear
 
beechstudio's Avatar
 

Yes...I actually swapped tubes out in the PA2 about 8 years ago. Sounded better, but I still feel it needed more work. The PA2 actually used to be my main pre for vocal/acoustic in the mid 90s when I was using Adats. It sounded really good paired with an AT4033.
Old 9th April 2010
  #16
okay then, go for the IC swap and have sockets added so you can drop in various quad op amps. And of course, you could always try another set of tubes...

I am sure you saw Jim Williams had some op amp suggestions that I am sure wold be great.
Old 26th January 2011
  #17
Lives for gear
 

Resurrecting this thread as I have PA2 and just need one question answered.
When you have removed the two ICs referred to above, are the others mentioned pin for pin replacements so I could just put two sockets in the board and drop the new ICs in, or is there some reworking of pin order involved?

Keen to try this out but still a little nervous.
I have no problem with the actual desoldering of the existing ICs, just dont want to turn my working PA2 into a pile of scrap or an expensive repair job for a pro shop!
Old 26th January 2011
  #18
I have NOT replaced the OP11 input ICs with the National LME49740NA, as per Jim William's suggestion. I have not gotten around to it. But it is my understanding that they are a pin-for-pin swap. I am sure that a peek at each of the IC datasheets would reveal answers to all your questions. I was certainly planning to eventually swap out the OP11 with the LME49740NA as a pin-for-pin swap.

I also plan to swap out the SSM2142 output IC with the suggested Burr Brown DRV134. That is an 8 pin IC. I can't recall if it is already socketed or not. I do know that some of the ICs were already socketed, just not on the input IC.

In case you don't know about the ssm2142 or the DRV134, they take an unbalanced signal and make it into a balanced signal--they are intended to take the place of an output transformer. They use +/- 4 ~ +/- 18 VDC for powering the IC and make for a much lower parts count than if you tried to do the same thing with a handful of active electronics. Hmmm, they are probably quite a bit cheaper than if you had a bunch of other ICs, resistors and so on. I have recently gotten quite interested in these little guys for some projects rather than trying to scrape up enough cash for all the output transformers I would need for the same task. Heck, even if you wanted to use an output transformer for 'the sound' I could still see using one of these ICs to send a signal to something like a meter from the same circuit. Well, maybe not, because the output would be different, oh whatever, they certainly have their uses.

So back to the original question--pull the ICs that are hard soldered to the board, add sockets and drop in a pair of LME49740NA ICs from National. Get yourself a pair of DRV134 for the output and drop them into the circuit. You should expect small improvements from these changes--do not expect miracles. The biggest perceived changes I had from all my tinkering came from the tube swap--and it was the easiest of all the changes to make. I expect that if TL Audio were making the PA-2 today, they would use these various different ICs discussed here--I am only guessing, of course, but I suspect these little guys were not available when the original design was made--probably 15 years ago. How time flies!
Old 27th January 2011
  #19
Lives for gear
 

Thanks for that!
Off to check out my parts suppliers site!
Old 28th January 2011
  #20
You are welcome. When you get your parts and change stuff out, report back with the results. I am sure other folks would like to read about it. I know I would. Good luck!
Old 11th February 2011
  #21
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jimjazzdad's Avatar
Hey Matt,

Thanks for the advice about what to look for on the PA-2. I also have the crumbly 2 watt resistors and the bulging caps on the output. I have ordered replacements (I'm upping the resistors to 3 watts). I also ordered new op amps - the Burr Brown INA217s, the DRV134s and the National LME49740NAs. Unlike your PA-2, mine has no sockets for the ICs, so I will install them at the same time - then I'll have some fun swapping things around and report back. I think the tube change I made with the 1959 RCA Blackplate 12AX7s is probably the biggest improvement, but the rest of the goodies are less than $50, so its worth a try...
Old 11th February 2011
  #22
My PA-2 did not actually have the sockets when I got it, they were added later. I had the same experience with improved sound quality mainly coming from the tube upgrade. Changing out the IC on the input did make a small difference. Of course there are better ones for me to try, so I expect additional small increments of improvement if/ when I ever get around to swapping out some of those other ICs.

Overall, these changes we have all discussed on here make the PA-2 a safer and better sounding preamp and thus more useful. The transformers I dropped in--they are nice for another flavor, but they only get used some of the time. The unit as a whole gets used frequently, so for the $$$ spent on the purchase and then the upgrades, I think it is a worthwhile effort. If you can pick one up in the $300 US range, then spend another $100 on upgrades/ parts, even $200, you are getting two good channels with real tubeyness for not so much money.
Old 11th February 2011
  #23
Lives for gear
 

I was in a local pro audio shop recently.. and they had one in the used rack - in pretty rough shape. I think they were asking 4 or 500 for it. I cracked up. I'd happily sell my MINT one for that much.
"-)
Old 11th February 2011
  #24
Quote:
Originally Posted by biggator6 View Post
I was in a local pro audio shop recently.. and they had one in the used rack - in pretty rough shape. I think they were asking 4 or 500 for it. I cracked up. I'd happily sell my MINT one for that much.
"-)
Yes, based upon what I have seen them go for on eBay, etc, if they were more than $400, I would keep on walking.
Old 15th February 2011
  #25
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jimjazzdad's Avatar
PA-2 Tune up results

Well I got my bag of bits from DigiKey today, so I plugged in the soldering iron...and put on my magnifier glasses. Now I remember why I used to prefer working on tube stuff that was point to point - I could SEE it and I could get my fingers in there. Must be getting old.

Anyway, I replaced 4 bad 330u caps on the outputs, 2 charred power resistors, removed all the ICs, installed sockets and put in the new ICs - Burr Brown INA217s, DRV134s and the National LME49740NA quad op amps. I feel good about fixing up the bad caps and resistors but, as expected, the op amp swaps didn`t net huge improvements. The mic preamps are a bit quieter at high gain so there's a little more useable gain (still not really enough for ribbons tho). I haven't recorded anything complex yet, so its hard to tell if the quads and the drivers are faster and less smeary at this point. Also, I think everything needs to burn in and settle down for a bit.

All in all, I'm glad I found this pre - I think its good value (with tubes & repairs/upgrades I'm still <$400), it has a pleasant warmth without excessive colour and now, hopefully, it will last me many more years. Thanks to Matt and Jim Williams for the advice and support.
Old 15th February 2011
  #26
Quote:
Originally Posted by jimjazzdad View Post
I feel good about fixing up the bad caps and resistors but, as expected, the op amp swaps didn`t net huge improvements. The mic preamps are a bit quieter at high gain so there's a little more useable gain (still not really enough for ribbons tho). I haven't recorded anything complex yet, so its hard to tell if the quads and the drivers are faster and less smeary at this point. Also, I think everything needs to burn in and settle down for a bit.

All in all, I'm glad I found this pre - I think its good value (with tubes & repairs/upgrades I'm still <$400), it has a pleasant warmth without excessive colour and now, hopefully, it will last me many more years. Thanks to Matt and Jim Williams for the advice and support.
I agree, this preamp is not always the best choice for ribbon mics or dynamic mics in general for quiet sources.

I am glad to read that your efforts met with success! A big thumbsup on the resistor and cap replacements--that should indeed make this preamp a useful tool for a long time--along with the tube replacements. If you have any further experiences worth telling about regarding the IC swaps, I certainly would like to read about it. Thanks a lot for posting back with how you made out on this project.
Old 24th April 2012
  #27
Here for the gear
 

TL Audio PA-2 Dual Valve MIc Preamp DI

dear friends.
I have a preamp TL Audio PA-2, and has changed the gain controls for 10Kohms, not what the original value of the mic amp. Please sendme that value.
Thank you very much.
Old 24th April 2012
  #28
Quote:
Originally Posted by Luis Rodriguez View Post
dear friends.
I have a preamp TL Audio PA-2, and has changed the gain controls for 10Kohms, not what the original value of the mic amp. Please sendme that value.
Thank you very much.
I actually tried to send you a very comprehensive reply yesterday regarding the question you sent me. I was traveling for work and I think the email got lost in the realm of internet nothingness when I hit "SEND". Sorry about that.

Anyway, after reviewing the schematic for the PA-2, the original gain pot value is 5K.

If you read through this thread and some others on Gearslutz that I have posted, you'll surely have learned already that the four 330uf capacitors protecting the output drivers have too low a voltage tolerance to handle the +48v they might get hit with. look them over, if they are bulging or leaking, replace them with 50v or 100v 330uf capacitors. 470uf caps with the same tolerance will work as well.

There is a lot of other stuff I could say about the PA-2. I actually already did when I wrote you an email yesterday. Sorry about that. If you have more questions, I will try to assist you as I am able. Take care and good luck getting your PA-2 up to good working condition!
Old 8th May 2012
  #29
Here for the gear
 

"Anyway, after reviewing the schematic for the PA-2, the original gain pot value is 5K."

thank you very much for your kind help, ask now if it's a 5k linear pot, or antilog, because the gain versus degrees of rotation does not have a good relationship, the gain is increased when it reaches the maximum cursor position.

regards, Luis Rodriguez
Old 22nd January 2017
  #30
Here for the gear
 

The different tubes (mismatched) sounds to me like the previous owner was using both channels as a dual-stage mono pre-amp.
Meaning that the output of ch1 was being fed into the line-level input of ch2 for overdriving saturation.
This could be where the +48v came from that burnt (bulged) those "blocking" capacitors
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