| Giaime |
Hello there! I'm an electric guitarist and I make up my mind to build myself a stereo Gainclone to power up a 4x12 cabinet, with the signal from my preamp. I'm not experienced in gaincloning, but I saw that with these power opamp you can get to 50W for channel, with minimal components. Would you more experienced suggest me what to look for? Keeping in mind that...
1) I don't need ultra hi-fi quality, it's for guitar, I just want it reliable.
2) 50W per channel or near, with the possibility to drive both 4ohm and 8ohm. This is important, because I haven't bought the cab, usually they're 4ohm for each channel, but I can change my mind and buy a 2x12 that has 8ohm per channel.
3) So I thought about the LM3886, but here in Italy I couldn't find them. Can someone suggest me a similar TDA sort of thing, because I can find them easier?
4) Can I use CPU ventilated heatsinks for the chips? I don't care for a few fan noise, because it's meant to be used on stage.
5) Since I don't need super hi-fi quality, what is the minimum rating for the transformer that I can use? I've thought to spend few money without a dual power supply....
Can someone help me? |
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| Giaime |
Bump.
I've seen that recently many guys are in the process of building a chip instrument amplifier. Maybe shall we create a permanent discussion? |
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| Giaime |
Thanks for the link! I think that the TDA2052 would do the trick, but the RMS power is a bit too low...
Suggestions are welcome! |
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| Nasse |
Well all I can tell I plugged my small modelling effects gadget to my lm3886 amp and it sounded very good
Maybe these chips are so clean, it is just better to leave the distortion stuff to be done in the preamp
I´m sure you can find 3886 in Italy, or buy it from some mail order component shop
www.generalguitargadgets.com
There is nice looking pcb pattern and power amp schem for 3886, I don´t have tested it but anyway.
Another edit: TDA7294 is about similar power specs as 3886 |
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| Giaime |
Yeah I will plug my PodXT in to it. I just need a clean power amp.
The pcb on that site seems good, I would use two of them.
But what circuit should I use for that 18v bipolar supply? Transformer ratings, ecc...
And the heatsinks?
Someone knows of some distributor in Italy where I can find the lm3886? |
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| jsteigs |
| i just finished a bridged lm3875 for my guitar. so, its 100W, single channel and beautiful sounding...i still have to make the preamp though. but why do you need stereo for guitar? all guitar amps and cabs are mono. also, a 4x12 cab is a bit of an overkill for a 50W chip amp. |
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| Giaime |
Thanks jsteigs! Post schematics! This forum is all about them....:D
a) I need stereo 'cause my preamp is stereo. If I would like mono I would keep my cold-souning Fender.
b) why a 4x12 is overkill for 50W? For example the Marshall 4x12 is a cab with 25W Celestions, so each channel is 50W maximum. I've seen 18W tube heads moving the air of 2 4x12 cabs...
Post Schematics!!!! |
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| jsteigs |
i just used the schematic from decibel dungeon, built it twice (each one about 50W), and put it in a bridge configuration resulting in an amp about 100W.
http://www.decdun.fsnet.co.uk/gainclone.html
what you could do instead of bridging then together is to have one amp powering one channel and another amp powering the other channel since you want stereo.
i agree that fenders dont give a very nice sound...i prefer marshall
usually you see 100W tube amps powering the 4x12 cabs and tube amps are generally louder per watt than chip amps. but u could still power a 4x12 with smaller amps. |
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| Giaime |
Thanks! I would build the 2X50W, but for personal knowledge, how do you bridge them? I'm not an expert...
I would try the generalguitargadgets PCB, but someone can suggest me a good power supply? And what about transformer ratings, ecc...? |
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| tja |
I bought four lm3886 of Carondimonio (he's a member here). Don't know if he has any left, but send him a mail.
Take |
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| jsteigs |
| quote: | Originally posted by Giaime
Thanks! I would build the 2X50W, but for personal knowledge, how do you bridge them? I'm not an expert... |
take the output of one amp and connect it to the speaker and to the inverting input of the 2nd amp. then connect the output of the second amp to the other side of the speaker. theoretically, there should 4 times the output power of a single chip but for the one i did, it is closer more than twice the power.
i built mine without a PCB because the circuit is so simple. and my transformer is 20-0-20V at 2 amps and its working fine. |
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| Giaime |
| So you think that for my 2x50W your 20-0-20 at 2 amps is fine? Can you post a power supply schematic? |
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| jsteigs |
| here is my power supply...its quite minimal but it works for me. |
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| Giaime |
Thanks! So what is the VA rating of the transformer for both channels? I opted for a shared power supply, with a single transformer with double secondaries. Any problems with this? I mean, everyone here is suggesting to have 2 psu... but like I said hi-fi quality is not my target...
And to have about 50W per channel, what supply I have to look for? I mean what voltage of the transformer, it's ratings, capacitors... |
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| moving_electron |
| quote: | Originally posted by jsteigs
but why do you need stereo for guitar? all guitar amps and cabs are mono. also, a 4x12 cab is a bit of an overkill for a 50W chip amp. |
Some pedals can utilize stereo amps to good effect (no pun intended :) ). Check out the stereo output mods of Small Clone Chorus at Tonepad. Stereo effects are pretty rare though I image. Still if you are making one channel, two channels is not that much of an increased effort. |
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| jsteigs |
| quote: | Originally posted by Giaime
Thanks! So what is the VA rating of the transformer for both channels? I opted for a shared power supply, with a single transformer with double secondaries. Any problems with this? I mean, everyone here is suggesting to have 2 psu... but like I said hi-fi quality is not my target...
And to have about 50W per channel, what supply I have to look for? I mean what voltage of the transformer, it's ratings, capacitors... |
i have a single power supply powering both chips. its 20-0-20V at 100VA which most people here i think would say is a little low but i'm not having any problems. the caps are rated at 35V which IS too low but thats what i had at hand. they should be at least 63V |
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| Giaime |
| And what kind of heatsink I should use? what termal resistance? should I go with CPU heatsinks with the small fan (they're just an economy move)? Should I isolate the chip from the heatsink? |
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| jsteigs |
i'm not sure of the thermal resistance of my heatsinks, i got them in a grab bag...but i think that a CPU heat sink will work fine even without the fan unless it is one of those heatsinks that is only a fan. the fan may cause interference with the signal but i'm not sure.
yes u shoulld isolate the chip electrically from the heatsink since the back side of the chip is connected to the negative supply and the box you are mounting it in is grounded. |
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| Giaime |
| But with a 100VA toroid, small caps, shared psu, isolated heatsink (that means worse contact, and rise in temperature) what power I'm gonna get? and it is safe and reliable? You know, if it has a THD of 1% on the stage it won't matter, but if after 30 minutes you could fry an egg on it... |
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| kilowattski |
| Using a 100 VA transformer to produce 100 watts is not safe or wise. Neither is using underated electrolytic capacitors. Using underated components is a fire waiting to happen. |
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| jsteigs |
| quote: | Originally posted by kilowattski
Using a 100 VA transformer to produce 100 watts is not safe or wise. Neither is using underated electrolytic capacitors. Using underated components is a fire waiting to happen. |
which is why i went out today to buy larger caps...but a 100VA toroid is enough to get 100W though it may get a little warm. |
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| Giaime |
| Ok, I knew that was too little. Should I aim for 200VA? And what caps? How much power will this (and the www.generalguitargadgets.com board, lm3886) deliver? |
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| jsteigs |
you should get a transformer that has atleast 150VA...with the lm3886, you should get about 55W for each channel using the transformer they use in their project (18-0-18V). Using the lm3876, you should get about 45W per channel using their project's transformer.
i'm assuming your load is 4 ohms...is that correct? |
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| Upupa Epops |
| Before ten years I was made fourty 75 W RMS combos for guitar and bassguitar. There was 100 VA toroid, so by nominal output power was little bit overloaded. To this time I have any complaint. These combos was made by the same principle like Gallien - Krueeger and you can bring them on only by one finger ;) . Remember, that musicians ( on the beggining ) must take own apparatus by own arms :D. |
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| Giaime |
Yeah, but here we're talking about 120W peak power...
Yes, my load should be 4 ohm, but I want to build an amp that's capable of delivering some watts either in 8ohm. 2 and 16 ohms aren't used.
So, 200/220VA toroid at 20-0-20 (that's all I've found) - 25A bridge rectifier - 2200/4700 uF per side - generalguitargadget PCB - lm3886 - 55W per channel?
But what termal resistance of the (isolated) heatsink I'm looking for? |
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| jsteigs |
a 20-0-20V transformer will give you about 28V DC which make the lm3886 68W into 4ohms or 38W into 8ohms. so, that PSU should be fine with the caps and 25A bridge rectifier. for my PSU, i only use 220uF caps...its quite a bit cheaper and works well enough for my purposes.
as for thermal resistances, i'm not sure...i've never built one but you can find the equations for thermal resistances on National's datasheet.
http://cache.national.com/ds/LM/LM3886.pdf |
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| jsteigs |
| i just looked at the datasheet and if i'm reading it correctly, you need a 2 C/W heatsink |
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| Giaime |
| Thanks! But I'm having trouble finding the right toroid... Here near my town all I can find is smalleer than 150VA... |
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| jsteigs |
you could either order off the internet or you could get two 100VA toriods and connect them in parallel
a cheap place is www.parstexpress.com but they have high shipping costs espescially outside the U.S. |
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| zei |
I have been following this tread and i,m curious if it is a success? :)
I,m in the process of building a power amp for my Pod Preamp and the PCB at guitargadgets looks nice.
I,m most curious if it sounds any good?
Please keep posting!:D
Best regards |
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| zei |
having a funktion so that you can choose between a 2X50W stereo amp and a bridged 100W mono amp?
would,nt that be a nice thing? just a small switch in the front or something..
i know i would like that. anyone here good and kind to make a small scematic on how to make that work? Proberly very simple. :)
regards
Z |
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| hitsware |
>having a funktion so that you can choose between
>a 2X50W stereo amp and a bridged 100W mono amp?
Make 1 channel inverting, and the other non-inverting.
For bridged use a ' Y ' cord into the inputs.
For stereo connect one of the speakers out of phase. |
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| Giaime |
Hello zei! I want to do precisely what you want to do. I also have a Pod (the xt version :D ) and would like a pwr amp to drive a 2X12 cab or 4X12... but I am doing nothing at the moment because I haven't got the money yet, and I've got to decide what can I do.
Here in Italy things are difficult to do: you can't buy a chassis, for instance, you've got to recicle something ( :xeye: ) or build something from scratch. The only useful thing I have seen is on this site www.audiokit.it, they list some heatsinked chassis at not too high prices.
Transformers here are hard do find in the shops: my favourite one (30 minutes by car from my home) doesn't carry anything over 10VA, and you could order models up to 150VA, not more.
Also I haven't found a place where I can get the LM3886, I've seen a website that lists them for 20 euro (quite expensive for DIY, I need two... but what happens if I burn one :hot: )
I think I won't do anything yet... My hope is to find an old hi-fi integrated amp or poweramp to cannibalize... or if it's powerful enought I will just use it...
Sorry but I can't help you. I suggest you to ask at www.generalguitargadgets.com for some info on that, they might have some feedback from someone that has built one... |
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| Giaime |
"Make 1 channel inverting, and the other non-inverting.
For bridged use a ' Y ' cord into the inputs.
For stereo connect one of the speakers out of phase."
Good that! But in the "bridged" mode, how will I connect the speakers? I don't understand real well.. if the outputs are out of phase won't they cancel? or all the current would flow from a chip to another, avoiding the speaker? Sorry I am not an expert!
We will keep it as an option, but we would try to make the two channels with the same PCB on www.generalguitargadgets.com |
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| matjans |
| if i were you i'd go with one of briangt's boards, the board offered on guitargadget look a little sloppy. he offers component kits too. |
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| Giaime |
Sorry but I'm against selling things between DIYers. I want a free board layout and wanna make my own. Someone has a link to a better done PCB?
(and why is that board bad? tell me I am not an expert) |
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| hitsware |
>Good that! But in the "bridged" mode,
>how will I connect the speakers?
The speakers connect between the 'hot' sides
of the 2 channels. The grounds are not used
(bridged mode only).................. |
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| jsteigs |
| i finally got a camera and took a pic of my amp. i still havent got a chassis for it yet though |
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| jsteigs |
| its kinda hard to see but for the bridged config, the output of one amp is connected to the input of the other amp and the speaker. the output of the second amp is connected to the other side of the speaker. both of the amplifiers are inverting. since the amps are out of phase, while one amp is pushing current through the speaker the other amp is pulling current through it which is why bridge mode is a.k.a. push-pull. |
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| Giaime |
| Wow! that's a very DIY picture (in the bad sense of the term :D ) |
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| jsteigs |
| yea i know...i used my cell phone cuz i cant find my camera... |
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