DIY external preamp mod: Community project!

[poorlyconditioned]

good news on the mod front is I found a thread where a bunch of people who actually know what they are doing modded out the 8 chan version of this pre.  Seems like, if the basic pre design is the same,  they have already done a lot of the legwork for us!!!

http://www.prodigy-pro.com/forum/viewtopic.php?t=344&postdays=0&postorder=asc&highlight=pr8&start=0&sid=e19afe0246d51954c877e62f523fb090

I don't see the schematic they talk about.  Can you attach it here?

  Richard

[mmmatt]

http://www.andyharrison.net/diy/PR8_schema_correct.jpg

[poorlyconditioned]

good news on the mod front is I found a thread where a bunch of people who actually know what they are doing modded out the 8 chan version of this pre.  Seems like, if the basic pre design is the same,  they have already done a lot of the legwork for us!!!

http://www.prodigy-pro.com/forum/viewtopic.php?t=344&postdays=0&postorder=asc&highlight=pr8&start=0&sid=e19afe0246d51954c877e62f523fb090

I don't see the schematic they talk about.  Can you attach it here?

  Richard

Ah.  OK, I see it on the next page.

Comments:
- could probably improve circuit by putting constant current source on the Emitters, instead of a fixed
  resistor
- would a matched pair (LM394?) help
- I'm thinking that instead of trying to get *all* the gain in the transistors, which may be subject to distortion, ttry a fixed gain here (eg., 20dB) then do the rest with the Opamp.  Really, I trust an opamp, well designed and manufactured in a controlled environment, more than a DIY analog circuit!  So, use the transistors just to lower the noise floor a bit.

  Richard

[mmmatt]

I don't fully understand all the logic behind it, but they removed 3 caps and then replaced the op amps.  One other guy is removing the output pcb and is adding transformers for the output stage... don't think I'll go quite that far but the folks there seemed pretty pleased with just the basics.  they suggested adding sockets so that it was easier to swap opamps... I think I will definately do that and just get my hands on as many potentially good sounding opamps that I can.  it was suggested by a neewb at the end of the thread that one could conceivably use different flavor opamps at the input side than the output side.  Playing around with it will be a lot of fun!!! 
     I have to read through it all again to try and understand the cap cutting.  basically they are not needed I guess.  One sugested that they were put in the circuit to make a distiguishable difference between that pre and the higher level one.  removing thoes caps changes the freq responce from 20-20k to 20-90k.  Not a bad performance increas for just a snip!   Another newb popped in mid thread and asked for help because he was overloading the input side.  sounded like what we all know as brickwalling.  Another gentleman proposed a simple fix to knock about 5db off the input and another 10 (I think) from the output.  With the increased headroom of the new opamps this will allow for a gain stage that is more inline with our purposes, yet maintan a low-noise figure.  I'm waiting for a moderator to approve my login and then I will question them about it and see if I can't get them excited about helping with the 4-chan mod.  I'm really excited about this.  I will try to find time to read through that whole thread again and see what I can pickup on a reread.  Above is just waht I remember from the hour I spent today.
    One potential performance limit that was stated is that the pr8's (and assumably the pr4v) don't have an adjustable gain stage, but instead use resistance against a fixed gain.  I'm pretty sure I understood that right.  I think I saw someone quote Doug Oade as saying the same about the pcm660.  That was brought up in conjunction with the topic of the cleanest setting on an acm660 being full on.  If I'm understanding that right in this other forum, we will be wanting to ride levels as hot as possible.  I would think that if a better quality opamp is inserted the brickwalling will go away anyways... Man I wish I had a better understanding of all of this!!!!!
    A couple other things that are floating through my mind are from the standpoint of logistics... this unit has ins and outs on the front.  I will prefer outs on the rear, so I think I will probably want to run a pigtail out the rear with 4@ trs so I can more easily attach to my ultralight.  Having the jacks unused in the front will allow me to have a split from the rear out (for instance If I am running a stereo pair and someone wants to patch it) and/or create a low-z input for use as a DI.  I'm pretty sure that is a pretty simple circuit and with that, and a switch it would make great use of the front jacks when I want to run a guitar straight in.
    One other thing I am just remembering from the thread is that someone measured the oputput and it isn't truly a ballanced output.  techinically the voltage output of both sides should be symetrical.  I guess the + and - are actually 3db (or was in mv) 3% off from being properly ballanced.  Nobody wanted to tackle that issue and someone spoke up and said that wasn't that big of a deal... Here again, this is all WAY over my head.  I'm not sure of the down side of that (is that dc offset?), but "Matt Logic" (that same logic that gets me introuble and makes my gums flap) figures that I can run it unballanced and not worry about it since I will just be running 12" of cable to my ultralight!

My head is swirling over this.  Short of me fucking up the actual pcb with sloppy work, I think I will be able to really make this little sucker shine without dumping a bunch of money into it.  I'll be anxious to do some testing next week.  My next couple of weeks at the day job are going to be very busy, but I'm sure I will be working till my head nods playing with this little sucker.

Anybody can feel free to pull me back to planet earth here!
Matt

[mmmatt]

good news on the mod front is I found a thread where a bunch of people who actually know what they are doing modded out the 8 chan version of this pre.  Seems like, if the basic pre design is the same,  they have already done a lot of the legwork for us!!!

http://www.prodigy-pro.com/forum/viewtopic.php?t=344&postdays=0&postorder=asc&highlight=pr8&start=0&sid=e19afe0246d51954c877e62f523fb090

I don't see the schematic they talk about.  Can you attach it here?

  Richard

Ah.  OK, I see it on the next page.

Comments:
- could probably improve circuit by putting constant current source on the Emitters, instead of a fixed
  resistor
- would a matched pair (LM394?) help
- I'm thinking that instead of trying to get *all* the gain in the transistors, which may be subject to distortion, ttry a fixed gain here (eg., 20dB) then do the rest with the Opamp.  Really, I trust an opamp, well designed and manufactured in a controlled environment, more than a DIY analog circuit!  So, use the transistors just to lower the noise floor a bit.

  Richard

you lost me brotha

matt

edit  read that thread when you get the time Richard... I think some of that was discussed.  these guys are pretty hard core over there. 

matt

[Church-Audio]

From the schematic it looks like most of the gain is comming from the transistors, I would reverse that replace the 4555 with a OPA2227A opamp and change the feedback loop on that opamp with a 15k resistor and get rid of the shit coupling caps. Replace all resistors in the signal path and else where to 1% metal film if not so already makes a huge sonic difference. Make sure left and right caps are all the same.

good news on the mod front is I found a thread where a bunch of people who actually know what they are doing modded out the 8 chan version of this pre.  Seems like, if the basic pre design is the same,  they have already done a lot of the legwork for us!!!

http://www.prodigy-pro.com/forum/viewtopic.php?t=344&postdays=0&postorder=asc&highlight=pr8&start=0&sid=e19afe0246d51954c877e62f523fb090

I don't see the schematic they talk about.  Can you attach it here?

  Richard

Ah.  OK, I see it on the next page.

Comments:
- could probably improve circuit by putting constant current source on the Emitters, instead of a fixed
  resistor
- would a matched pair (LM394?) help
- I'm thinking that instead of trying to get *all* the gain in the transistors, which may be subject to distortion, ttry a fixed gain here (eg., 20dB) then do the rest with the Opamp.  Really, I trust an opamp, well designed and manufactured in a controlled environment, more than a DIY analog circuit!  So, use the transistors just to lower the noise floor a bit.

  Richard

you lost me brotha

matt

edit  read that thread when you get the time Richard... I think some of that was discussed.  these guys are pretty hard core over there. 

matt

[poorlyconditioned]

Hey Chris, thanks for helping in this thread!

I've got a question about the input caps.  This also relates to some other gear, like the Edirol UA5 and R04.

Sometimes I see a single cap, like the PR8.  But I wonder if this is always correctly polarized.  Take a look at C6 or C7.  Ignoring phantom power, and letting the Base of Q1 and Q2 be approx. zero volts, it looks like a positive going input will correctly bias the cap, but a negative going signal will *reverse bias* it.  I thought electrolytics only "work" when there is a positive bias (like a DC blocking cap).  But I don't know what happens when they are reversed.  In fact, many of the caps (C9, C5) seem to have ambiguous polarity.

Similar wierdness happens in the Edirol UA5, but in this case, they take two caps and put them "back to back".  Again I wonder how this distorts because there will probably be one forward and one reverse biased cap.

I don't have a fear of electrolytics like some people do, but it seems like they are being *abused* often, and this might be why they distort.

Please enlighten me dude!

  Richard

From the schematic it looks like most of the gain is comming from the transistors, I would reverse that replace the 4555 with a OPA2227A opamp and change the feedback loop on that opamp with a 15k resistor and get rid of the shit coupling caps. Replace all resistors in the signal path and else where to 1% metal film if not so already makes a huge sonic difference. Make sure left and right caps are all the same.

[Church-Audio]

Well I do not know much about caps in that are used in this way, I always think that they use electrolytic caps to simply save money and they come up with these scheme to " make the cap work" A non polarized cap will always outperform a electrolytic the only problem is a 47uf poly cap is expensive and huge.

You have to also remember that these caps will only block dc not ac and will allow ac to go thru unaffected except for the uf of the cap so a 1uf cap will have a roll off at say 25hz-30hz  in a 10k circuit. So its purpose is only to protect the inputs this could be done with a single poly non polarized cap. But again it brings up the price.
I hope I am correct in my theory.

Hey Chris, thanks for helping in this thread!

I've got a question about the input caps.  This also relates to some other gear, like the Edirol UA5 and R04.

Sometimes I see a single cap, like the PR8.  But I wonder if this is always correctly polarized.  Take a look at C6 or C7.  Ignoring phantom power, and letting the Base of Q1 and Q2 be approx. zero volts, it looks like a positive going input will correctly bias the cap, but a negative going signal will *reverse bias* it.  I thought electrolytics only "work" when there is a positive bias (like a DC blocking cap).  But I don't know what happens when they are reversed.  In fact, many of the caps (C9, C5) seem to have ambiguous polarity.

Similar wierdness happens in the Edirol UA5, but in this case, they take two caps and put them "back to back".  Again I wonder how this distorts because there will probably be one forward and one reverse biased cap.

I don't have a fear of electrolytics like some people do, but it seems like they are being *abused* often, and this might be why they distort.

Please enlighten me dude!

  Richard

From the schematic it looks like most of the gain is comming from the transistors, I would reverse that replace the 4555 with a OPA2227A opamp and change the feedback loop on that opamp with a 15k resistor and get rid of the shit coupling caps. Replace all resistors in the signal path and else where to 1% metal film if not so already makes a huge sonic difference. Make sure left and right caps are all the same.

[mmmatt]

Well I do not know much about caps in that are used in this way, I always think that they use electrolytic caps to simply save money and they come up with these scheme to " make the cap work" A non polarized cap will always outperform a electrolytic the only problem is a 47uf poly cap is expensive and huge.

You have to also remember that these caps will only block dc not ac and will allow ac to go thru unaffected except for the uf of the cap so a 1uf cap will have a roll off at say 25hz-30hz  in a 10k circuit. So its purpose is only to protect the inputs this could be done with a single poly non polarized cap. But again it brings up the price.
I hope I am correct in my theory.

Hey Chris, thanks for helping in this thread!

I've got a question about the input caps.  This also relates to some other gear, like the Edirol UA5 and R04.

Sometimes I see a single cap, like the PR8.  But I wonder if this is always correctly polarized.  Take a look at C6 or C7.  Ignoring phantom power, and letting the Base of Q1 and Q2 be approx. zero volts, it looks like a positive going input will correctly bias the cap, but a negative going signal will *reverse bias* it.  I thought electrolytics only "work" when there is a positive bias (like a DC blocking cap).  But I don't know what happens when they are reversed.  In fact, many of the caps (C9, C5) seem to have ambiguous polarity.

Similar wierdness happens in the Edirol UA5, but in this case, they take two caps and put them "back to back".  Again I wonder how this distorts because there will probably be one forward and one reverse biased cap.

I don't have a fear of electrolytics like some people do, but it seems like they are being *abused* often, and this might be why they distort.

Please enlighten me dude!

  Richard

From the schematic it looks like most of the gain is comming from the transistors, I would reverse that replace the 4555 with a OPA2227A opamp and change the feedback loop on that opamp with a 15k resistor and get rid of the shit coupling caps. Replace all resistors in the signal path and else where to 1% metal film if not so already makes a huge sonic difference. Make sure left and right caps are all the same.

I think the guy who was replacing the output pcb with transformers was originally planning on switching the caps out to poly caps.  There are like 8 per chanel or something though so I don't see myself going through all of that.  I would like to know the purpose of the constant current on the emitters Richard mentioned though... I don't think that was discussed in the other forum.  I think I have heard that discussed before in another thead or article.  Seems to me it was an effort to get max performance from a 5v supply though... this, if I'm reading it right is +/-15v.  Can you elaborate on that thought Richard?  Hopefully somewhat non-techy.
     As for the gain stage Chris, I need to find again the part where they talked about that mod... it was shot down for some reason and suggested that the model up from the pr8 (the mkII) was configured that way and for $100 more it would be better just to start with that.  I think for our purposes even a fixed gain stage with a few steps of resistance will be fine.  If the pot in use on this sucker seems to cause a lot of noise or otherwise affects performance, I'm thinking  it could be discarded for a series of switched pads.  Thoes of us using this I think will be using it a frontend for another box that has variable gain.  Isn't this how Oade set his bricks up?  gain/attenuation in 10db steps?  It seems feasable that it could work this way, but if it isn't a performance issue there is no sence in messing with it IMO.  As long as the pre isn't clipping, then using attenuation from the next box in the chain is going to be a way of bringing it to a recordable level.  This all depends, assumably, on the quality of the other device in the chain.  I would think most any device could handle a +4dbu output though.  Since we aren't varying the input level at all, maybe it makes sence to not even try to attenuate the output.  Here again, this is" Matt Logic" so I'm not sure where this all fallis into the scope of reality.

also, thanks to both of you Richard and Chris for being a part of this discussion.  this is all obviously over my head.  I'm really trying to understand it though so just bear with me and feel free to please correct me when I'm wrong!

matt

edit... it was the concensus of the other thread to remove c13, c14 and c15 because the do nothing...not sure if you are refering to thoes caps Chris.

[mmmatt]

Ok  Good news and bad news... this isn't going to be as simple as replicating what they did at the prodigy forum with the pr8... I don't think.  the other pre had a seperate pcb for in and out... this is all on one.  I can see that there are 2 4558's on each chanel.  One is upright on it's side and one is flat.  Not that I know what the hell that means, but hopefully a picture will tell 1000 words!!



large size pic
http://img.photobucket.com/albums/v148/mmmatt/stocksmpropr4v001.jpg
http://img.photobucket.com/albums/v148/mmmatt/Stocksmpropr4v01.jpg

matt

[Chuck]

The 4558's can, and should be replaced with something better. Socketed 8 pin DIPs would be good for testing various op-amps. I agree with Chris and I think Richard in that using the op-amps for the gain and the transitors for buffering would be an advantage. I have built several DIY microphone pre-amps. None as complicated as this. The biggest problem I've had is with the phantom power circuit. I have recently found a surface mount device that may make building a small efficient DC/DC convertor possible. I really like the idea of a 4 in 2 out field mixer too, but I bet there would be a markert for a three in two out mixer. That's something that I could use. Maybe instead of full gain controls we could make "trim" controls with a steped gain control. Pots add noise and junk to the sound of microphone pre-amps.

Lots of electrolytic caps and resistors in the signal path are going to be heard in the end. Ideally using transformers for the gain stage would eliminate the DC blocking caps and provide relatively clean (but expensive) gain.

Anyway, Since I have found this thread, I'm in. I've wanted to see the taping community come up with a "project" pre-amp along the lines of what the headphone amp guys have been doing. Tested and approved circuit boards etc...

If I can help in any way, let me know.  I love DIY stuff.

Chuck

[poorlyconditioned]

Ok  Good news and bad news... this isn't going to be as simple as replicating what they did at the prodigy forum with the pr8... I don't think.  the other pre had a seperate pcb for in and out... this is all on one.  I can see that there are 2 4558's on each chanel.  One is upright on it's side and one is flat.  Not that I know what the hell that means, but hopefully a picture will tell 1000 words!!



large size pic
http://img.photobucket.com/albums/v148/mmmatt/stocksmpropr4v001.jpg
http://img.photobucket.com/albums/v148/mmmatt/Stocksmpropr4v01.jpg

matt

That picture is not bigger.  Can you post one with a closeup of the "vertical" 4558s.  Are they SIP (single inline pin) chips?

  Richard

[mmmatt]

for some reason photobucket keeps resizing the pic... It is less than a mb so it should be fine.  I'll shrunk a bit more so I can attach it.  I will try to get a closeup of the chip, but you may have to settle for a description.  I circled them both on onbe of the channels.  In the pic, the "vertical" one is lower down towrd the front of the unit.

matt

[mmmatt]

The 4558's can, and should be replaced with something better. Socketed 8 pin DIPs would be good for testing various op-amps. I agree with Chris and I think Richard in that using the op-amps for the gain and the transitors for buffering would be an advantage. I have built several DIY microphone pre-amps. None as complicated as this. The biggest problem I've had is with the phantom power circuit. I have recently found a surface mount device that may make building a small efficient DC/DC convertor possible. I really like the idea of a 4 in 2 out field mixer too, but I bet there would be a markert for a three in two out mixer. That's something that I could use. Maybe instead of full gain controls we could make "trim" controls with a steped gain control. Pots add noise and junk to the sound of microphone pre-amps.

Lots of electrolytic caps and resistors in the signal path are going to be heard in the end. Ideally using transformers for the gain stage would eliminate the DC blocking caps and provide relatively clean (but expensive) gain.

Anyway, Since I have found this thread, I'm in. I've wanted to see the taping community come up with a "project" pre-amp along the lines of what the headphone amp guys have been doing. Tested and approved circuit boards etc...

If I can help in any way, let me know.  I love DIY stuff.

Chuck

welcome to the fun Chuck and thanks for helping.

I put it back together and did a little recording with it.  it doesn't sound to bad.  As it is it seems a bit better than the MOTU pre's.  More dynamic anyways.  Not to bad I guess.  The dynamic range is closer to that of my ONYX pre's and I have always thought of them as being very dynamic.  Brittle mid/highs though to a point of being grating.  I just strummed my guitar a bit and listened.  It is very hard for me to tell much more with just a guitar.  Funny how a studio guy could probably hear major differences here but if you give him an ambient concert recording with it he would probably think it all sounds like crap!  I can hear differences far easier that way.  I assume it is because of what I'm used to.  I will redo the comp in the near future for all to hear my specfriggtacular acoustic prowess   The one I recorded tonight was quick and I need to do it again with more symetrical levels so that I can keep it a bit more acurate.

Matt

[poorlyconditioned]

So, what voltage does this thing take?  AC or DC?  I'm guessing it is AC, right?

Does it output P48, or a lower voltage?

  Richard