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Author Topic: preamp for LM3886?  (Read 35668 times)

sugrhigh

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Re: preamp for LM3886?
« Reply #15 on: February 20, 2008, 04:48:23 PM »
If I would use the circuits in that kind of parallel arrangement I would likely put some kind of a rotary switch to that setup and I would also try to switch at both output and input sides of the pre circuit just to eliminate noise (few hi-gain circuit in parallel may contribute a lot of hiss). If you do not "drop out" the "unused" circuits with some switching arrangement you need to consider the total impedance of all preamp final stages that are connected in parallel: Most of them have a resistor that ties the output to ground reference, when you parallel few of these the total resistance may end up being substantially low, there’s a good chance that some of the circuits cannot handle it and at at least it will effect the coupling/cut off frequency. The thing may need mixing resistors or buffers. I’m quite certain that you can’t just parallel the circuits and get away with it without any modifications.

Another issue is how wise this paralleling even is in practice. I mean, do you really need all those preamps or could you just work out an arrangement where you remove all the “unnecessary” parts and devise a couple of ways to make the circuits share the parts that are common to all. For example, quite a many of ROG circuits (and their variants) have almost identical attenuating volume and gain control arrangements, the final low-pass filter stage is similar in most circuits, most of them use FET gain stages where only differences are practically the values of gain and coupling capacitance/cut off frequency. These can easily be made switchable and controllable, which allows voicing one basic circuit with many alternative ways. Many ROG circuits also substitute a proper tonestack circuit with some simplified arrangement.

i tried to point out it was a very basic idea, so you would indeed need a way to mute the "off" preamps that i didn't deem necessary to outline my idea

but i do like your idea of using common parts

darwindeathcat

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Re: preamp for LM3886?
« Reply #16 on: February 25, 2008, 09:55:04 PM »
Okay, this is a fantastic thread! I'm learning a lot here....

    I didn't consider that paralleling the preamps would lower the overall output impedance of the preamp stage... That is a very important point. I assume that this effect only matters if each of the circuits are getting power. However, as you say, there could still be a "hiss" problem introduced by all the latent unused circuitry of any preamp channels left connected but unpowered (as I suggested to do in my previous post).
   I've been examining the ROG circuits, and I noticed exactly what Teemuk was saying: that there are many redundant elements between the designs... In fact, I've got a prototype circuit I've built that uses a 4PDT switch to flip from a "noisy cricket" to a "bassman ruby". These circuits only differ in a few places, and I use the switch to reroute these parts from the components of one circuit to the other.The same MPF102 buffer transistor and LM386 are used for either mode... As of now, I've got the noisy cricket part working, but I have some dead short in the bassman ruby portion, so it doesn't work... In theory, the switching I devised should work though...
    So, here's my new set of questions for this idea:
1) If you used the rotary switch method, would you wire it to intercept the signal between the preamp stages and the poweramp stage?
2) I suppose if you only had two preamp stages, you could get away with only DPDT. I'm think specifically here of the fetzer valve for a "clean", and a proffessor tweed for a "dirty"... This may be the easiest first build for a multiple channel amp...
3)I think I would use a passive buffer at the input of the poweramp stage like the circuit posted over in the tda7293 thread. Would that be enough compensate for any changes to the output impedance of the preamp stages? I'm asking this because I still wonder if it's possible to have two signal sources being fed into the power amp at the same time with only a simple type of buffer. It may be that an active mixer stage would need to be built between the pre and power stages for this to work, and I don't know if i'm willing to attempt that...
4) Also, a nice option would be to also have a jack that connects directly to the poweramp  input for connecting external preamps. How would this factor into any impedance matching calculations (just add another value in parallel, right? should decrease the total output impedence of the preamp stage when the external is plugged in, and increase back to normal when unplugged).

Thanks for the responses so far... Like I said, this is a really great thread!
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darwindeathcat

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Re: preamp for LM3886?
« Reply #17 on: February 26, 2008, 03:16:57 PM »
Okay, some more searching through ROG schematics has yielded this: http://www.runoffgroove.com/doubled.html
The "double d" two channel distortion. I'm mainly looking at this schematic for the switching applications, and not for the particular distortion schematics used.
Basically it goes like this:

                                         |---distortion/preamp1---|
I ->Input Buffer-->Switch 1A-/                                   \-Switch 1B-->Output buffer-> O
                                         |---distortion/preamp2---|

Here, Switch 1 seems to be a DPDT, and the buffers are active JFET one transistor types. Each distortion/preamp channel has it's own volume and gain controls. It seems possible to stick a common tone section before the output buffer (or after? or replaced it with the tone section?). Following this would be the poweramp volume control, then the poweramp itself... Seems pretty straightforward... Any suggestions?
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syndromet

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Re: preamp for LM3886?
« Reply #18 on: February 27, 2008, 06:58:20 AM »
I haven't read through the hole thread, so forgive me if my answer is a bit off.... 8)

The idea you posted over here seems to be very good. if I was to build something simular 8 (wich I might..), I would drop the buffers, and use the preamps straight, just for simplicity. I think most of the ROG designs buffer the signal anyway.

As for wiring it up, I think the rotary switch option sounds good. You will however get a lot of pots, since every preamp will need it's own sets of controlls. that is great, if thats what you want. Otherways I waould listen to Teemuk, and make the "sound altering" components in one pre switchable.

iTzALLgoOD

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Re: preamp for LM3886?
« Reply #19 on: February 27, 2008, 01:34:12 PM »
It would be even better if the all of the channels were footswitchable.

darwindeathcat

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Re: preamp for LM3886?
« Reply #20 on: February 27, 2008, 01:59:39 PM »
II would drop the buffers, and use the preamps straight, just for simplicity. I think most of the ROG designs buffer the signal anyway.

    Actually, following teemuk's advice I was thinking that I would extract the buffer portion of the ROG designs, and put a single buffer in front of the switch (input buffer in my schematic). I think that most of the ROG designs use a very similar buffer. Either way, I think a simple fetzer valve can be used to replace the stock buffer in any of their designs (I know this is definately true for the ruby). I think your right about diching the output buffer. Then, I guess I'd have at least a gain pot per channel, and perhaps a couple of switches (for grit and whatnot, depending on the circuit I would choose). This way, you can have each channel adjusted the way you like it, and when you switch into it, you don't have to fiddle with any pots. Perhaps a volume pot per channel is not necessary if I have a master volume control right before the power amp? I'd stick a tone stack after the preamp stage, so it would be a common setting.
   The easiest switching would definately be a double gang rotary (or a dpdt if you only have 2 channels), and wire it to switch like in my schematic. Each preamp circuit would always be getting power, but would not be connected to a signal at it's input or output unless switched to. A footswitch would be very useful, but I have no idea how they work, so I couldn't even think about designing one!
   Right now I'm think of building a 4 channel setup with the channels being 1)Proffessor Tweed 2)bassman ruby 3)Odie 4)Clean ...

Maybe a lm3875 for the power amp?
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syndromet

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Re: preamp for LM3886?
« Reply #21 on: February 27, 2008, 04:10:11 PM »
II would drop the buffers, and use the preamps straight, just for simplicity. I think most of the ROG designs buffer the signal anyway.

    Actually, following teemuk's advice I was thinking that I would extract the buffer portion of the ROG designs, and put a single buffer in front of the switch (input buffer in my schematic). I think that most of the ROG designs use a very similar buffer. Either way, I think a simple fetzer valve can be used to replace the stock buffer in any of their designs (I know this is definately true for the ruby). I think your right about diching the output buffer.

This sounds like a good idea. Using the fetzer valve will also give you the possability to boost the signal to push the preamps a little harder. Maybe set the fetzer to unity gain, and have in switchable to +20dB or something. A lot of marshalls have this option, and it does change the sound quite a lot. You could ofcource use a pot, and label it "pregain gain" ;)
Quote

Then, I guess I'd have at least a gain pot per channel, and perhaps a couple of switches (for grit and whatnot, depending on the circuit I would choose). This way, you can have each channel adjusted the way you like it, and when you switch into it, you don't have to fiddle with any pots. Perhaps a volume pot per channel is not necessary if I have a master volume control right before the power amp? I'd stick a tone stack after the preamp stage, so it would be a common setting.

This is a hard one. What makes the JFET emulators sound somewhat simular to the emulated circuit has a lot to do with frequenzy shaping and tone controlls. Leaving the tonestack after the circuits, and finding the values wich sounds good with all preamps could be a PITA. Even if you find a tonestack wich does the job great, you will probably loose some of the caracteristics of the preamp ith just one tonestack.

Every amp has some settings that make the amp sound like what it does. The tone stack of an overdriven Marshall is usualy set to boost the mids, while an AC30 is set to boost treble. The same tonestack settings would probably not sound great at both clean and OD, wich is why I hate amps with 2 or 3 channels and only one tonestack. If you tweak the sound to be great for cleans, it will usualy sound shitty on OD. Tweak it for OD and the cleans will suffer. If I were to build something like this i would leave the full tonestack on all the preamp cicuits. It would be a hell of a lot of pots, and make the offboard wiring a nightmare, but I would be able to get my favourite sounds by switching, and without having to tweak the knobs in the midle of a live set.

It would ofcource be possible to tweak the circuits to sound just the way you want it with thepost preamp tonestack set at the same possition, but that would recuire some serious breadboarding and comparing between the preamps.
Quote

   The easiest switching would definately be a double gang rotary (or a dpdt if you only have 2 channels), and wire it to switch like in my schematic. Each preamp circuit would always be getting power, but would not be connected to a signal at it's input or output unless switched to. A footswitch would be very useful, but I have no idea how they work, so I couldn't even think about designing one!

There is no doubt that a rotary sitch is the way to go. Futurlec carries a lot of different ones, so you will probably find something usefull there.

Footswitching could be done pretty easily with latching relays. I once planned to do what you described here with the rotary switch, and my plan was to have the last position on the switch be for footswitching, and sending the signal to relays.
So you would have one position for Clean, one for od, one for distortion and one for footswitch.
Quote
   Right now I'm think of building a 4 channel setup with the channels being 1)Proffessor Tweed 2)bassman ruby 3)Odie 4)Clean ...

Maybe a lm3875 for the power amp?
great choice of preamps. Personally i would dropp the BM Ruby for an eighteen, simply because I think the Proffessor tweed an the BM Ruby sounds a lot like each other. I also think the eighteen is the best pedal designed by rog ever...

I would personally go with the 3886 if I were to use one of the lm chips. I recomend you to look at the kits at www.41hz.com . I built an AMP5, and it sounds great with guitar. There is almost no need for a heatsink, and they recuire a lot less power per watt than the lm chipamps. This means that you will be able to build a lighter and more portable amp with a higher output.



Hope i didn's scare you completly of with my rambling. I'm really looking forward to see how this turns out.

darwindeathcat

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Re: preamp for LM3886?
« Reply #22 on: February 27, 2008, 05:00:37 PM »

Quote

Then, I guess I'd have at least a gain pot per channel, and perhaps a couple of switches (for grit and whatnot, depending on the circuit I would choose). This way, you can have each channel adjusted the way you like it, and when you switch into it, you don't have to fiddle with any pots. Perhaps a volume pot per channel is not necessary if I have a master volume control right before the power amp? I'd stick a tone stack after the preamp stage, so it would be a common setting.

This is a hard one. What makes the JFET emulators sound somewhat simular to the emulated circuit has a lot to do with frequenzy shaping and tone controlls. Leaving the tonestack after the circuits, and finding the values wich sounds good with all preamps could be a PITA. Even if you find a tonestack wich does the job great, you will probably loose some of the caracteristics of the preamp ith just one tonestack.

Quote

    I totally see your point here. I was thinking to avoid the "mess of offboard wiring" since this is where I usually screw up! But I did not look closely enough into the justification of why each circuit uses the tone controls it does have... I also agree that it would be best to have each channel totally set up the way you like it (gain, vol, AND tone), so that when you switch to it, you get just what you want with absolutely no need to tweak any settings!

great choice of preamps. Personally i would dropp the BM Ruby for an eighteen, simply because I think the Proffessor tweed an the BM Ruby sounds a lot like each other. I also think the eighteen is the best pedal designed by rog ever...

   I haven't built a proffessor tweed yet, so I'll take your word on it's similarity to the BM ruby (which I have built and really like). I took a listen to the Eighteen over at the ROG sound sample page and it does sound really good! It also seems like a pretty darn easy one to build (why I also like the odie and the prof tweed: great sound and simple builds). So I think that I'll have the fetzer valve up front (possibly switchible from unity to 20dB, like you say) then the switch to bufferless verisons of the Odie, the Eighteen, and the Tweed, and a clean channel (totally passive, except whatever effect of the fetzer input buffer).

[/quote]
I would personally go with the 3886 if I were to use one of the lm chips. I recomend you to look at the kits at www.41hz.com . I built an AMP5, and it sounds great with guitar. There is almost no need for a heatsink, and they recuire a lot less power per watt than the lm chipamps. This means that you will be able to build a lighter and more portable amp with a higher output.

[/quote]

    I was thinking of the lm3875 because of the wattage rating of the speakers I was planning to use are only 30w (and 8ohm)... I do not wish to over power them! I've looked at the "class T" kits over at 41hz, and I'm glad that you have given one a shot... It's good to hear that they make good amps for guitar... Another option that I might take is to use a tda7396. This is probable a chip that never gets used for instrument amps, but it does have some interesting possible advantages. It's a class AB, in BTL configuration, so it's capable of high output wattage even though it's a single supply chip (It's meant for car audio).  The datasheet indicates that I can get about 30w into 4ohm (my two 8ohm speakers in parallel) powering it at +15vdc. I have an old ATK computer power supply sitting around that I can use to get that voltage quite easily, nicely regulated and at any ampage I'd like. I can also plenty of other voltage levels out of it to power all the preamps! It would save me a crapload of time, and an even larger crapload of money, not having to build all the powersupplies and rectifiers I'd need if I were to use a traditional transformer.
  These chips are going for $2.29, and the circuit will only require something like 6caps and one resistor(I'll post a schematic if interested). Can't get too much simpler than that!
  Anyway, I think I might start to build this one over the weekend... At least I'll start puting the preamps together... Keep the advice coming though, it really helps!
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iTzALLgoOD

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Re: preamp for LM3886?
« Reply #23 on: March 06, 2008, 10:41:43 PM »
Has anyone tried the DRP-1 from the earlier post??  Is it a verified layout?

Quote
http://www.matsumin.net/diy/jisaku1/DRP-1/index.html

How about the original?  Do they sound good. 

rob_nl

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Re: preamp for LM3886?
« Reply #24 on: March 12, 2008, 06:05:07 AM »
I dunnow, using preamps that different together doesnt make sense to me when used in a combo. The loudspeaker does a lot for the tone as well. You could use an English sounding speaker with for instance the VOX pre-amp, but I would definatly want an american sounding speaker with the 18watt preamp. Just my 0.02$

joecool85

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Re: preamp for LM3886?
« Reply #25 on: March 12, 2008, 06:53:41 AM »
Has anyone tried the DRP-1 from the earlier post??  Is it a verified layout?

Quote
http://www.matsumin.net/diy/jisaku1/DRP-1/index.html

How about the original?  Do they sound good. 

I haven't tried building one, but I owned the original until I sold it to my brother, it rocks!  Made my LM3886 sound like a Marshal for sure.  The overdrive was my favorite part (although it had good clean), it wasn't a metal distortion or anything, but a good creamy overdrive.  The EQ was a bit touchy, but really flexable.
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iTzALLgoOD

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Re: preamp for LM3886?
« Reply #26 on: March 12, 2008, 10:20:30 AM »
I etched and populated a board from the layout but haven't put power to it yet.  I'll get to it sometime this week.  I have a some TDA2030's and TDA2050's I'm going to use for poweramps.  I'll report back when it's up and running.  Is there any harm in leaving off the speaker sim/line out board and just use the preamp out or is that part of the mojo?

rob_nl

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Re: preamp for LM3886?
« Reply #27 on: March 12, 2008, 11:11:18 AM »
Is there any harm in leaving off the speaker sim/line out board and just use the preamp out or is that part of the mojo?

It definatly shapes the tone, so if you leave the cab sim away prepare for a more sterile sound. Unless you run the amplified signal trough a real cab that is. Either way, it won't sound the same as with the cab sim because your cab will have it's own sound to it.

iTzALLgoOD

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Re: preamp for LM3886?
« Reply #28 on: March 12, 2008, 12:04:59 PM »
I plan on making a small combo style amp for practice.  I think that I'll try it both ways and see what I like.

Robin755

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Re: preamp for LM3886?
« Reply #29 on: April 18, 2008, 12:35:53 PM »
@syndromet and others: I'm planning to build a 4 channel amplifier with some of the projects with fet's of runnoffgroove for use as channelpreamps,
but do they give enough output voltage to the power-amplifier? Is it then better to run them on 18Volt for use as a preamp?
or should I just place a real preamp between these fet based circuits to boost the signal enough to run it to the power-amplifier?

 

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