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Converting car amplifier into home amp
Converting car amplifier into home amp
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Old 5th December 2017, 09:22 AM   #11
diy1995 is offline diy1995
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So this is what I measured:
55-0-55 from secondaries
-55 and +55 at diodes
-27 and +27 DC coming from diodes... is this normal?
-27 and +27 (27-0-27) at rails going to amp section
20171205_110722.jpg

Voltages was measured at idle.
If this is correct (55VAC to diode 27VDC from diode) that means that I need 55-0-55 300VA (as rsavas suggested) transformer ?
Thank you
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Old 5th December 2017, 02:28 PM   #12
indianajo is offline indianajo  United States
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The diagram post two showed +-40 v rails. So that's what I talked about.
The 55 VAC versus +-27 is a bit odd. I think they are splitting the AC into +- by deriving a center speaker ground somehow where you didn't measure. Either it is a center tap transformer and your missing the center tap point, or something. Center tap transformers often don't use bridge rectifiers, but single diodes.
So you want +-27 v rails at idle. Transformers are sold by the average voltage, not the peak voltage. (Average * 1.4)-1.4v = peak. The 1.4 v is the two diode drops of the bridge. So to produce +-26.5 you need a 20 v transformer with two windings, the two tied together make the center speaker ground. Toroids come that way. Expensive (now) E-frame transformers tend to have one winding and a center tap.
Confirm your 27 v reading by looking at the rating on the rail caps (the 4700 in diagram post 2. Should be 10-40% higher than that. BTW on an amp this old, the rail caps probably need replacing. I alway do after 20-30 years.
4 pairs output transistor for two channels is about 160 W so a 300 VA transformer would be adequate. Extra VA won't hurt the amp, too low a speaker resistance does. That's average or RMS power, but electronic or house music doesn't have much of a crest factor, so don't count on a big music power surge like I get on 1812 overture cannon shot.
Whether it can sustain 160 W/ch depends on how big the heat sinks are and how good the fan is.
If your rail caps are rated higher than 35 v maybe you can up the transformer to a two 24 v winding transformer which would be easier to find in stock.
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Old 5th December 2017, 06:07 PM   #13
jerluwoo is offline jerluwoo  United States
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Would need to use an oscilloscope to measure the AC because of the high switching frequency, use of a standard multimeter will give odd results.
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Old 5th December 2017, 06:18 PM   #14
thimios is offline thimios  Greece
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Yes,you can't measure at the output of a switching power supply without using a TRUE RMS voltmeter.
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Old 5th December 2017, 11:48 PM   #15
phase is offline phase  United States
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If you are patient you can find early 90s Kenwood KM-XXX, home 2-channel amps that make decent power and sound fine as well.
There was one on Seattle Craigslist for $40 when I looked last.
Might be a bit easier than the other whole car amp thing...
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Old 6th December 2017, 12:41 PM   #16
diy1995 is offline diy1995
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Hello, I`m thinking of building something with more power and selling this amp. I must do further research online and I will see.
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Old 6th December 2017, 03:07 PM   #17
indianajo is offline indianajo  United States
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Depends on your speaker. A 2 ohm speaker, 27 v rails is good, but with an 8 ohm speaker, four 2sd718 could be capable of 160 w/ch with 55 v rails. You could drive them with an op amp, see the op amp + pass transistor threads on chip amp.
I could laugh at phase; Slovenia is 4000 miles and a customs barrier from Seattle. I get junk receivers for $10 but they are chock full of electrolytic caps that need replacing, dozens of them, even down to 0.47 uf. Better a junk Peavey PV-4c for $40 + 30 freight; at least the heat sinks, case, fan are real. The driver boards could be scrapped & some IC design put in.
Look at the honeybadger boards under diyaudiostore up top. They are pretty powerful: 2sa1943/sc5200 outputs should be common in Europe. A 400 va transformer would be more suitable for that. Buy your transistors from the store with the boards or from an authorized distibutor like farnell or RS or that german one, reiser?: lots of counterfeits on the resale market.
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Last edited by indianajo; 6th December 2017 at 03:10 PM.
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Old 6th December 2017, 05:38 PM   #18
cbdb is offline cbdb  Canada
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If your using 8 ohm speakers 27 volt rails will give you maybe 50 watts. So using a 700 VA transformer seems like a huge waste.
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Old 6th December 2017, 10:43 PM   #19
phase is offline phase  United States
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Uh, I missed the Slovenia part..

Iím glad you like your Peavey amp!
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Old 6th December 2017, 11:45 PM   #20
indianajo is offline indianajo  United States
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Quote:
Originally Posted by phase View Post
I’m glad you like your Peavey amp!
The CS800s is no compromise; listening to it now @ 1.5 W/ch.
The PV-1.3k is a bit industrial at low volume, but really projects down the whole street when I play 1812 overture & Sousa marches on 4th of July. PV-xx don't hum, have great transformer screening. Great air flow, fan control (in the 1.3k), adequate heatsinks, case is roadworthy. The PV-xx compromises were in the circuits, designed for only high volume, and not adequately protected against shorted phone plugs to the speaker.
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Last edited by indianajo; 6th December 2017 at 11:50 PM.
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