Shure portable preamp mod

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Ampampam

Active member
Joined
Feb 28, 2015
Messages
39
Hello guys!
I read this forum from years and the time is arrived when I will get involved with doing my first electric job. I want to ask your help about modding a Shure fp 42 four channel mic preamp. I own it from a while and find it to a really handful and lovely unit with great futures. The only problem is the relative loud hiss in it. You can read about this on forums (it is the same issue with the fp31) and usually the solution is to buy a Sound Devices unit for 10x more money. Unfortunatelly I don't have the money but luckily I have the passion to modify this lovely one.

Now. Here is the schematic in the manual:
https://www.google.co.uk/url?sa=t&source=web&rct=j&ei=hxmHVeutF4KrswHd7KqoDQ&url=http://www.shure.com/user-guides/us_pro_fp42_ug.pdf&ved=0CDgQFjAG&usg=AFQjCNEzfCeEtgqcTcbY3MorWn-qNIKz8A&sig2=nKTvZrn54Ms44EQ76p0qIw

I was thinking about to change the OP amps in it. The original one is the Raytheon RC4156DB. Here is the data:
https://www.google.co.uk/url?sa=t&source=web&rct=j&ei=ehqHVZC5LsKQsgH11oHQCQ&url=http://www.e2v.com/shared/content/resources/File/documents/QPSemi/Ray_4156.pdf&ved=0CB4QFjAA&usg=AFQjCNHd5a4ikRJV6PE6XLot5gs9Tg7cqA&sig2=i2vUYHrnFyL6_UAnNecq1g
As I read it before somewhere it can be replaced with the LME49740. Here is the datasheet:
https://www.google.co.uk/url?sa=t&source=web&rct=j&ei=SCCHVZa3JIKqswG7oYPIDQ&url=http://www.ti.com/lit/ds/symlink/lme49740.pdf&ved=0CCMQFjAB&usg=AFQjCNFxTg1lrkDVLC75PtGFgBsX4e-viw&sig2=Tygip2WFH9Lz-ke3ckF5sQ

I would like to ask your opinion about this idea. Should I have to modify the circuit to supply this amp or to match the signal levels? Any better solutions than mine? I am open for any modification ideas!

And please! I don't want to buy another unit. I want to improve this one and get involved in electronics.

Thank you in advance!
 
I've got the old M67, which is all discrete, no opamps, but I want to mod it anyways, or at least salvage all the great guts inside, transformers, etc...

If I were you I would install sockets in place of the opamps and mod just one channel at a time. That way you can compare it to the others/unmodded ones. If it squeals then you might have to add some caps... not really my area of expertise.

Watching this thread though :)
 
Ampampam said:
The only problem is the relative loud hiss in it.
Swapping OPAs is unlikely to reduce the hiss.

Super OPAs will also use more current and may be unstable in this circuit.

Have you compared the hiss with other preamps eg a Sound Devices?  You need to set the gain so it is the same on both with your mike.  Are you testing with the inputs unterminated?

How does the hiss vary with the CHANNEL LEVEL/CUE  and  MASTER controls?

BTW, Sound Devices was started by ex-SHURE people so they will know all about the faults and how to make a better one.  :D
 
Hymentoptera: The socket is a very good idea! I was thinking the same way. Basically I prefer the tube things but I have this one to mod now... :)

Rycardo: I believe that the OPA swap could make difference.  Probably the whole circuit should have been replaced with something better but not that is the plan. I read about that in a forum, the guys who worked at the shure has built the fp33 (upgraded version of this unit's small brother) after it is came to be successful about it's low noise and great futures and they left the company. They made the Sound Devices. That piece (fp33) is still double times more expensive than this first series.
I was looking for the fp33 schematic but couldn't find it.

About the gain settings: channel gain->4 master->3,5 and it brings a great signal on the output. Not peaking but not too hissy. I load it with a 100 ohm shotgun microphone.
 
So the work is in progress. I have some barriers what I didn't count with.

I ordered some texas instruments lme79470 and I have some sockets. I am willing to replace the u301-u302-u304 opa's in stages to see the difference.
The only problem is the technologie as it has been soldered on the board. Here are some pictures:

https://goo.gl/photos/5Wc4XU7UqFTL9Bks5

I tried to remove the first pins, but I saw that my solder pump doesn't work in this case. I don't have the required tools and as I saw a good desolderer is £500. I don't have that much money to repair this cheap unit.
Do you have any suggest how to remove it without burning the layers? (Don't forget it, I am not a very experienced solderer)

Ps.: I checked the unit without signal on the inputs. It has a recogniseable noise with the potmeters on zero, but as soon as you turn up the master trimmer the hiss is getting much louder. Around number 7 and after it sounnds terrible and even if you turn up the channel pot the noise is going to be the double. I am quite sure the problem is the quality of the opas.
 
Ampampam said:
I checked the unit without signal on the inputs. It has a recogniseable noise with the potmeters on zero, but as soon as you turn up the master trimmer the hiss is getting much louder. Around number 7 and after it sounnds terrible and even if you turn up the channel pot the noise is going to be the double. I am quite sure the problem is the quality of the opas.
Are you testing with the inputs unterminated?
 
What is the noise like if you put all the inputs into "cue" mode? ( pull all the input knobs out).  This will remove all the inputs from the mix bus, and ground the bus.
 
Yes, it is less noisy, and I heard that some people use the headphone's output to record,but I wouldn't do that. Otherwise the channel has the same problem as the main output without the addition of the master bus's noise. I would use that nice trafos if they are built in...

Meanwhile I found a great tutorial on the youtube about removing IC-s:

https://youtu.be/47xGYT-QGLI

I think I will excercise on some kind of electric waste, and after do it.

What is your opinion now Ricardo? Now I answered all of you questions detailed...  :) ...and Thank you very much for taking care about my case! You are very nice guys!
 
Do you have desoldering braid? I'm not a fan of the stuff much anymore now that I have a Hakko 808, but I still find uses for it from time to time.

It's nice to have a nice selection of sizes of braid, especially the really wide stuff for desoldering multiple pins at once. Just be careful how much heat you apply, and of course try not to pull up any traces! Chemtronics is a good brand I like.
 
When I work doing EFP / ENG  gigs out of a rental place , they eventually phased out the shures and replaced them with sound devices ,  Soooo much better !  some of the hiss  maybe the headphone amp ?  It was the worst combo to use those with the bright Sony headphones. I'm not sure I'd take a Shure mixer if someone gave it to me, are they cruising on a 70's design ?
 
They are more like the 80's design... Under £200  this is the only four channel unit with six built in trafos. It has a not so bad channel separation, and a very good limiter. You can't overload it. It is also very useful at loud vocals. I love the 80's transistor sound and don't mind if noone agree with me. I thought I could upgrade it with some nice opas, but I would have to buy a new soldering iron becouse my one is a 30w toy what is not able to heat up the pins to the proper temperature without harming the main board.

The sound devices units are a bit overpriced in my opinion. It is not a very popular product like an Iphone and they have to earn money, so they put a very high bill on them. I don't understand how the behringer doesn't started to produce ENG mixers though.  The market is very tight with crazy prices. There is a Hungarian analouge mixer brand with relative good prices but this is the 21st century. If I can buy a brand new Macbook air under £1000 I don't know why should I have to pay  £3500 for a portable digital recorder...

About the fp 42... Now I am thinking to cut off the pins and desolder it from one side and suck out the rest from the other side. I will think it over during my holiday and will let you know what's happened. Meanwhile I received the opas with the slots. Getting to be excited!! :)
 
Hello again!

I am ready to report the result:

LESS HISS!!! 8)

I swapped the u302,u303 and u304. Now the unit has less noise and more highs. It sounds more open than before.  Actually my AT875R has more hiss than the preamp. (And it is still not a rode...)
But!  Only on the first two channel. The reason is that, I bought an Lfn347n opa from the maplin (£1) only to compare with the TI lme79470(£3). The noise floor is much lower with the ST than with the Ti. Tomorow I will buy some more of that...

Thank you everyone for your care and instructions! I am very happy! ☺

Attila
 
hymentoptera said:
Do you have desoldering braid? I'm not a fan of the stuff much anymore now that I have a Hakko 808, but I still find uses for it from time time.

Thanks for that! The desoldering braid is very useful. My father told me to use flux as well... :)
 
Ampampam said:
Thanks for that! The desoldering braid is very useful. My father told me to use flux as well... :)

Flux can make all the difference when using that stuff. In fact all but the cheapest braid has a little flux already in it. I've even used a bit of finely stranded hook-up wire with flux to desolder with in a pinch (don't recommend it though) :)
 
Ampampam said:
I bought an Lfn347n opa from the maplin (£1) only to compare with the TI lme79470(£3). The noise floor is much lower with the ST than with the Ti. Tomorow I will buy some more of that...
What is the ST?
 

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