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Old 20th June 2009, 09:46 PM   #1
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Default Modifying 100 watt LXI amplifier circuit

Hello every one, I would consider this my second major solid state amplifier project, but with this one it will be like rebuilding an entirely new amplifier, so I am making an attempt to make the best one I can from what I've salvaged from this one and whatever else I can find in my junk piles.

The amplifier is made by Sears (go figures, considering there is no info what so ever about it). It produces 100 watts into 8 ohms and consider how cheaply made it is, it sounds really good and is a well designed amplifier. I even took the time out to draw up a schematic, which can be used for the convenience and personal use of others.

I have many ideas in mind of how to modify the circuit for improvements, but only understand the basics, so I am looking for ideas of what would make this circuit better, more current capable and efficient.

I have made a few posts on the Class "A" Hybrid Amplifier thread but have not had much help there since this is a class "AB" amplifier, that I indeed want to turn into a Hybrid of some sort, mainly to get rid of the obsolete chip fet at the input stage of the amplifier, so I am more than willing to be walked through the modifying process, as I now have all SUMMER!!!

Here is the original schematic that I drew up, if there are any reasons why what I drew up may not work please let me know! I will correct it as necessary.
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Old 20th June 2009, 10:01 PM   #2
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Now here is what I have come up with so far, removed the output short circuit protection (but will most likely make a seperate board for it as an addition). And also removed the output inductor (I hope this doesn't affect performance in any way, but this is how the ESP P3A is). Increased Emitter resistors from 0.27 ohms to 0.33 ohms (An attempt to lower distortion, but will reduce output power some what, not a problem for me). Lastly I thought that decreasing the 220pF caps on the drivers might make for some improvement (Also a point read on the P3A cercuit).

Other than that it looks the same, but with paralleled Less obsolete Toshiba's. And some components I don't really understand thier purpose, like the 3.3 ohm resistors on the bases of the outputs (changed them to 10 ohms like on the Leach Amp) or the 270 ohm resistor and 0.1uF capacitor between the driver transistors. And the two diodes that look like ordenary 4148's, if some one could identify them that would be nice (If you can understand the circuit in the first place. )

So here is the new schematic...
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Old 21st June 2009, 12:00 AM   #3
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The two diodes were part of the short circuit protection circuit.
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Old 21st June 2009, 12:19 AM   #4
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Soooo, exactly what would be the purpose of these diodes? Do I need them, is there anything gained from removing them?
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Old 21st June 2009, 12:26 AM   #5
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Quote:
Originally posted by CrazzyAbtTubes
Soooo, exactly what would be the purpose of these diodes? Do I need them, is there anything gained from removing them?

If your not using the short circuit protect then the diodes are not required.

The diodes stop the transistors reverse biasing.
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Old 21st June 2009, 01:13 AM   #6
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Thanks for the info, like I said I will probably lay the board out so that I can have the short circuit protection optional, just in case the amplifier is used on harsh loads some time in the future.

Now I just have to figure out the input stage, I can't find a data sheet for the UPA63H so I can find substitutes, unless like I said, the input stage could be some how subed with a tube but would require much chang in circuit design.
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Old 21st June 2009, 06:58 PM   #7
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Default i have been

priviliged to repair one of these amps ...made in korea for sears and actually left me totaly breathless with the performance

your approach though is wrong from two points
A) it seems to me that you dont have enough skills to do a thing like that
B) it also seems to me that you havent really evaluated what you allready have in your hands


you need first to decide what you like to do
A) built a new amp based on this schematic ( pcb layout and everything else )

B) repair and/or upgrade the existing one

if you go for section B then there is a few things that it can be done

basicly what i do is very wrong cause it seems that you dont know much about amps and this will bring endless emalis and a very long post that will only end up in smoke

what you describe as ''''''obsolete chip fet at the input stage of the amplifier''''''' is what make this amp unic and amazing performance without any improvemnts Imagine what is going to happen if you start tweaking things arround

i know some people that will kill somebody just to lay hands on chip fets like that


please dont missunderstand me i am not willing to make a personal attack to you ( i dont even know you ) but i think is better to listen from some one the "hard " way and save one amp from smoke

since i am familiar with the particular amp i will be very happy to help you taken as a fact that we speak the same "language"

happy regards
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Old 21st June 2009, 07:06 PM   #8
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Default now that i read more

carefully

...... god you cannot use advices from the P3a threads or esp pages !!!!! P3a isa bootstrap CFP ..... your is a ccs EFP ..... does all this sound chinese to you ?????

these amps are totaly diferent ...you cannot copy tips and fragments from one to each other ....

to nigelwright7557 :

you seriously need to clarify better which diodes you are talking about ....you are probably talking about the diodes that are part of the VI limiter but why i have a strange feeling that our friend is talking about the string of diodes that are located in the lower area and are part of a ccs ?????

here i will use one of the best Andrew T said

should i duck now or wait for the smoke first ????
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Old 21st June 2009, 07:46 PM   #9
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Default Re: i have been

Quote:
Originally posted by sakis
priviliged to repair one of these amps ...made in korea for sears and actually left me totaly breathless with the performance
I don't think my LXI is the same -- dual mono DC MOSFET -- but it is a surprising good sleeper amplifier. Pretty sure it was built by Hitachi. The accompanying Pre-amp is not bad either. One of the things that makes the amp/pre/tuner special is that my net cost was -$400

Like some of the Radio Shack stuff, some of these house brand items, built by solid OEMs, can be real sleepers. (Another Sears house brand, is their professional speaker, which provides 2 quite good Foster 28mm silk dome tweeters, and a 12" Foster midbass per speaker -- the same woofer & domw + a really nice Foster inverted 1" dome can be found in an AGS branded speaker).

I did think it was 100W but Daniel measured it at 150W and solid into 4 ohms. He srtipped out some wiring & switches, installed a gain pot, and increased the bias so that it stays in Class A till 10W. I use it as a woofer amp (mostly to drive push-push SDX7).

dave
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Old 21st June 2009, 08:41 PM   #10
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Default Re: now that i read more

Quote:
Originally posted by sakis
carefully
to nigelwright7557 :

you seriously need to clarify better which diodes you are talking about ....you are probably talking about the diodes that are part of the VI limiter but why i have a strange feeling that our friend is talking about the string of diodes that are located in the lower area and are part of a ccs ?????

here i will use one of the best Andrew T said

should i duck now or wait for the smoke first ????
Whoops ! apologies.......

I didnt see the diodes in the CCS, I just assumed he was talking about the diodes in the short circuit protect cct.
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