Recommendations for tube preamp for lm3886 - diyAudio
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Old 6th June 2014, 01:47 AM   #1
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Default Recommendations for tube preamp for lm3886

I'm working on (re)building a bass cabinet and would like a nice tube preamp for it. My current setup is an Avel Lindburg 2x28.4VAC torroid, an XY power supply circuit and an XY LM3886 Power amp. Just running my bass through the power amp sounds ok but doesn't have any sort of push (not active pickups). I've tested it with a 9V battery powered LM386 "noisy cricket" circuit and it sounded better but since it's a battery powered preamp it had some bad hissing at higher gains.

I'd love to put a tube preamp on this to get a nice warm sound out of it but am not entirely opposed to other preamp options. Not looking for wall shaking wattage but a more sexy bass sound.

I'm newer to the DIY audio world and am still trying to read through the infinite documentation of audio circuits while trying to be a new dad as well... so any help is appreciated.
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Old 6th June 2014, 03:02 AM   #2
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Tube amps rule for guitar and bass. 3886 might be a little too wimpy, except for maybe a practice amp. Remember, 3886 has the SPIKE protection circuit.

I always wondered about putting a 12AX7 inside the feedback loop of a chip amp. It would most likely require a lot of sorting out.

A simple, single stage preamp would probably "sweeten" up the sound a bit (translation- introduce a simple distortion profile).

Nothing clips like a tube amp, that I've seen- nice, soft clipping. 3886 clips nasty. You won't like it.
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Old 6th June 2014, 12:20 PM   #3
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Thanks Eddie. This is more or less a practice amp. I gutted a burned out Peavey 40 amp and put the LM3886 amp in it (fits nicely!). It sounds good with the LM386 preamp but I wanted something a little stronger since I have higher voltage already available in the circuit (about +40V and -40V off the supply circuit) and didn't want to add another transformer. Trying to keep the weight down.

From what you're saying the characteristics of a tube + LM3886 wouldn't complement each other well. I've not had any clipping on the LM3886 circuits I've built so far and I've pushed some of them pretty hard. Only had one stop because it got to hot but it still works (had a heatsink that was too small...).

If there's a chip preamp or other preamp circuits that could help... Right now it's just too quiet to really do much good. If something besides an LM3886 would work better, I'm open for ideas. Like I said, I'm a new dad so I don't get much time to sit down to read through the endless volumes of all that is diy audio circuitry! :-D
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Old 6th June 2014, 03:15 PM   #4
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Look at the 3886 datasheet. It has waveforms illustrating the SPIKE in action.

I'm not saying it won't work. A tube buffer driving a chip amp will work fine. But if I was looking to build a cheap and dirty practice amp, I would be looking at gutting or modifying an old tube amp out of a console. They always show up in the alley or at resale shops. Even though the power will be way less, I bet you like the sound a whole lot better.

Have you ever seen an old Pignose practice amp? It runs off a 9 volt battery and employs a transformer coupled, germanium transistor amplifier. The reason I bring this up is because you could build a bigger version with silicon transistors.

For bass, you want clean power and a lot of it. Chip amps fall apart when they clip. A discrete circuit can be designed that is much better for that application. And transformer coupled amplifiers, with all their drawbacks, do clip cleaner as a rule.
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Old 6th June 2014, 03:29 PM   #5
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Also, the 3886 has been employed in some budget guitar amps. I recall reading a few posts where they kept blowing up.

Program material from a CD or MP3 (or even a turntable) is very much different from live input from an electric guitar or bass. The difference in dynamic range is enormous. Musical instrument amps have to run at full power indefinitely. Home sound system amplifiers run at much lower power, with occasional peaks to clipping. I've measured many consumer grade amplifiers. An amplifier rated "50 watts RMS" might indeed reproduce a 50 watt transient without clipping, but will typically produce only around 12 watts continuous without clipping. (I call the continuous watts "Marantz watts" because the old school receivers actually produced the power they claimed, continuously. ) It's just like a cheetah, which can sprint 70 miles an hour for about 60 seconds. It can't run 70 miles an hour from Chicago to Cincinnati.
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Old 6th June 2014, 03:30 PM   #6
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I'm only looking for a 1/2W or so preamp. So far using the LM386 I haven't had any clipping or sound distortion playing bass through it. The only issue is the LM386's low voltage tolerances and the hissing from the circuit I'm using being battery driven without earth grounds. I'd like a tube sound and know that the make and model can affect the sound and just wondered if there were certain tubes that had a good rep as being used for bass guitars. I hadn't thought of salvaging amps out of old equipment and may keep my eye out for any laying about.

I haven't seen the Pignose amps but am vaguely familiar with transistor driven amps. The board I took out of the Peavey 40 had 2 opamps (i'm assuming for the tone stages to not drop signal) and transistors to drive the speaker at ~40W. The speaker is rated for 80-120 from the documentation I found so I'm not worried about the LM3886 over-driving that. The sound just needs a little "push" if I'm going to use it with a passive bass.

Thanks for all the info. I'm still new to the diy amp world and appreciate other more experienced opinions :-)
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Old 6th June 2014, 03:35 PM   #7
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Haha, yeah a buddy of mine has a Marantz system... GORGEOUS sound! The reason I'm using the LM3886 is a buddy of mine builds guitar amp cabinets and used to build his own amp circuits. Wound up finding a cheaper replacement that could be mounted easier and went with that. I got a box of LM3886 and LM4870 chips along with a few LM1875 (20W) chips and circuits and what-not.

Currently I'm using two of the LM3886 XY amp sets off of a PC ATX power supply to run some old Magnavox upright speakers with 15" woofers. Sounds great streaming audio from the PC or an MP3 player (especially considering I hacked a Dell PC case to make it ;-) ) The thing I like about the LM3886 chips is there's no hissing noise when you power the amp up. I forget that it's on most times and come back later and realize the LED is still lit.
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Old 6th June 2014, 03:50 PM   #8
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The main point of my ramblings is that for nice clipping behavior, it's hard to beat a well sorted pentode ultralinear output stage. Transistor amplifiers generate a horrid array of complex distortion when clipped. We all know what it sounds like. Much has been written about it (Douglas Self sums it up in his "blameless" amplifier tutorial).

There was a fad in the early 80s called "soft clipping." You could push a button and a circuit would clip the signal before the output stage, which kept it on the linear part of its curve. Of course these designs employed various diode clamping circuits which introduced distortion of their own. Walt Jung reviews them in his "IC Op Amp Cookbook." They were an attempt to make transistor amps clip more like the ultralinear amplifiers that were popular in the 50s. It never caught on with consumers.
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Old 6th June 2014, 03:56 PM   #9
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Quote:
I'm only looking for a 1/2W or so preamp. So far using the LM386 I haven't had any clipping or sound distortion playing bass through it. The only issue is the LM386's low voltage tolerances and the hissing from the circuit I'm using being battery driven without earth grounds. I'd like a tube sound and know that the make and model can affect the sound and just wondered if there were certain tubes that had a good rep as being used for bass guitars
You don't need a half watt preamp. 386 is optimized for battery operation and is a poor choice for a preamp and is not a hi fi power amp at all.

Here's my idea for a "tube sound" amplifier. Overdrive a 12AX7. Make the bias adjustable so you can adjust it for symmetrical clipping. Employ level controls at the input and output of the tube stage so you can fine tune the effect. You can go from clean to gritty gradually by just fiddling with the controls.
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Old 6th June 2014, 04:09 PM   #10
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Quote:
Originally Posted by m16gunslinger77 View Post
Currently I'm using two of the LM3886 XY amp sets off of a PC ATX power supply to run some old Magnavox upright speakers with 15" woofers. Sounds great streaming audio from the PC or an MP3 player (especially considering I hacked a Dell PC case to make it ;-) ) The thing I like about the LM3886 chips is there's no hissing noise when you power the amp up. I forget that it's on most times and come back later and realize the LED is still lit.
The 3886 is optimized for program source sound reproduction and as you know excels at it. The point that you need to understand is that program source material is nothing at all like the signal from an electronic instrument like a guitar. As a musician, you would appreciate this if you have ever been in a studio. The dynamic range of live music can easily exceed 60 dB. Program source material rarely exceeds 20 dB (FM program source material is 10 dB or less) and even those "super hi-fi" CDs that you pay 50% more for only have a dynamic range of 25 dB or so.

I bet that you could make that 3886 emit magic smoke if you hooked up a synthesizer to it and generated some full power 20 kHz harmonics .
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