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Old 22nd September 2004, 06:45 PM   #1
mjarve is offline mjarve  United States
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Default Combining a Gainclone project...

....with a restoration project.

I have a 1974 Kenwood KR-3400 receiver with a bad power amplifier. Unfortunately, the power amp IC is next to impossible to find (KA-40W) and no one seems to know if it could x-ref with a newer one. It is a nice enough receiver, and I do not want to just throw it out.

So I was thinking (always dangerous)Ö Since the power supply is still good, and since the construction s modular, why not replace the original power amp with a nice little Gainclone. I would get experience building a nice little IC amp and at the same time revive a perfectly good receiver. The amplifier currently installed is rated at 16WPC, so I think a GC would be a good match in terms of power; Meaning of course that I should not have to putz with the power supply a great deal. It currently provides +/- 25 volts DC with a 19-0-19 Vac CT transformer. I also have three LM1876T ICís from a very cheap HT amp. I was thinking, since they are two channel ICís I would bridge one per channel, or would that work? I suppose I would need a beefier power supply (which is a possibility). Is there a good, simple, clean design for running a LM1876T in mono?

Thank you for the input!
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Old 22nd September 2004, 09:38 PM   #2
Nuuk is offline Nuuk  United Kingdom
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If it were me, I would use the transformer but add a new rectifier bridge and then use the 2 LM1876's to make a good sounding stereo amp.

I would also replace the preamp section with a buffer and stepped attenuator.
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Old 23rd September 2004, 12:39 AM   #3
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I recently put a stereo TDA7350 amp in a HarmonKardon 330b? reciever with a bad amp.I had fiddled with it so much trying to fix it,I finally gave up,added a 12V transformer inside along with the TDA7350 amp. I have each chip bridged to provide up to 22W per channel.
Besides the scratchy volume pot (Need to get somekinda pot cleaner.) it sounds pretty alright..
The best part,each power amp only has 4 parts (including the IC) per channel..Take -that- GainCard!
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Old 23rd September 2004, 01:00 AM   #4
Variac is offline Variac  United States
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Hmmm, there should be a big fad of replacing the amp portion of
high end 1970's receivers with gainclones. Then you have a great tuner with a great amp. I guess the problem is that the top of the line models with the good tuners have power supplies that put out too many volts for a GC. The receivers were around 110watts/ch
Any ideas? If the transformers had 4 input wires to allow either 120 or 220 volt use, they could be easily rigged to put out about half the volts.....
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Old 23rd September 2004, 02:02 AM   #5
ofb is offline ofb
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a fad of converting 70's receivers to gc and keep the tuner portion? interesting, variac.

just information to aid the discussion, i have two such units i'm salvaging for gc use. yamaha cr-240 and technics sa-303. these two are not particularly modular inside so it is unclear to me how one would keep the tuner section.

yamaha cr-240 transformer outputs 27-0-27 and 38-0-22.

technics sa-303 transfrormer outputs 31-0-31 plus has a second center tap that is offset 8v. 260va.

both have input only for 120v, no 240v.

other information i can give is the yamaha also yields a very good chassis, a nice heatsink, and a good existing arrangement for placing a gc inside. the technic's heatsink is a little less attractive, and the chassis is house-of-cards construction. it's very stout, but if you wish to replace a panel like the front or back with one of your own, it quickly seems better to use a different chassis. the integrity requires all the original interlocking pieces. the yamaha is more conventional.

(i realize the vdc is borderline for gc use. i only need moderate volume, so will use large heatsinks and see if i can get away with it. worth a try.)
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Old 23rd September 2004, 02:15 AM   #6
Variac is offline Variac  United States
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I'd be the last person who could tell you how to hook into the tuner section, but those old analog tuners are getting a reputation as being as good as tuners ever got!

The stand alone tuners from that era are rising in price, but I'll bet the high end receiver tuners were comparable.
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Old 23rd September 2004, 02:33 AM   #7
ofb is offline ofb
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oh, well shed no tears. these two were thrift store specials that no longer sound as good as my lesser technics sa-200 of the same vintage. its fm reception is very nice, but this seems due to a deliberate warmth to compliment the radio, not in any superior clarity.

perhaps the old ones also "sound better" because of the pleasant feel of the weighted tuner dial and the nicer display?

mjarve, i am sorry we are getting a little too far from your subject. were you able to find a schematic for your kr-3400? that might be helpful.
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Old 23rd September 2004, 12:41 PM   #8
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Quote:
Originally posted by DigitalJunkie

Besides the scratchy volume pot (Need to get somekinda pot cleaner.) it sounds pretty alright..
The best part,each power amp only has 4 parts (including the IC) per channel..Take -that- GainCard!
Add an IC stage, piggy-backed, to replace existing preamp or at least use/replace blocking cap to cut DC on pot. Change the pot if you can.


Quote:
Originally posted by Variac
I guess the problem is that the top of the line models with the good tuners have power supplies that put out too many volts for a GC. The receivers were around 110watts/ch
Any ideas? If the transformers had 4 input wires to allow either 120 or 220 volt use, they could be easily rigged to put out about half the volts.....
A better way out is to regulate it to get to +/-35v.


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