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Old 8th September 2012, 01:38 PM   #1
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Default power amp help please

Hi .
i have aquired these amplifier boards . they where removed from a mixing desk that had been partly destroyed by vandles. In the schematic at the top left there is a control circuit. i would like to remove or defeat this part of the circuit. Any help or advice would be very welcome .

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Old 8th September 2012, 02:27 PM   #2
AndrewT is offline AndrewT  Scotland
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just remove tr3.
insert a resistor to send current into the LTP.
Choose a value that sends the correct current into the LTP for the supply rail you have chosen..
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Old 8th September 2012, 03:09 PM   #3
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LTP, everybody knows what an LTP is except me and possibly madtecchy. Is the LTP the pair of input bipolar junction transistors like Tr4 and Tr5 in this schematic? They look like the input stage of an op amp. What is the second transistor for? Most of the BJT transistor schematics on solid state forum have the second one. None of my amps have them, the ST120 has a single input transistor and the Peavey's have op amps.
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Old 8th September 2012, 03:36 PM   #4
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Thankyou andrew.. @ indiana i know what a long tail pair is and looks like on a schematic . The question was about the control circuit that is in series with the positive supply of the ltp..If i am not mistaken the st 120 is a single ended amplifier . Now to find out the correct current for the ltp to run nicley..
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Old 8th September 2012, 06:36 PM   #5
RJM1 is online now RJM1  United States
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Just short TR3 C to E and it will be on all the time. The transistors are just switching the voltage on and off to R5,D2,D3,R6.
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Old 8th September 2012, 07:31 PM   #6
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Thanks RJM1. Also in the middle of the schematic there is somthing going on i have never seen before.
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Old 9th September 2012, 12:30 PM   #7
djk is offline djk
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Short-circuit protection, it's called fold-back current-limiting.
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Old 9th September 2012, 02:35 PM   #8
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Thankyou djk. im assuming this type of circuit is used primarily in pa equipment as a reliabilty measure.. I notice the current limit circuit connects to the feed back loop and the +and - signal rails . does this type of circuit have a negative effect on sound quality.
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Old 9th September 2012, 03:33 PM   #9
djk is offline djk
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"does this type of circuit have a negative effect on sound quality."

Not really, unless you activate it driving a loudspeaker. If properly designed, and the amplifier is used properly, you will not hear it.

Using the Leach amplifier as an example, if you drive a 2Ω load into hard clipping the limiter will engage. This will make a loud 'frapping' type of sound.
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Old 10th September 2012, 06:32 AM   #10
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Hi djk.
I have now managed to salvage the control /psu board for this amplifier and also the quite large toroid that powers it . everything looks to be fine on the boards . No bulging caps no burnt components or scorch marks. i have tested the output stage and all transistors tested ok on a DMM. I will fire it up later on a dim bulb tester . One thing i have noticed is the psu puts out approx 63 Vdc and the four smoothing caps are rated at 63Vdc. it has run like this for the last ten years so i guess it will be ok to leave them for testing . If the amp is ok i will replace them with 100 Vdc types when i do the re cap
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