Supply rails vs output

smithy666 · 15th July 2008, 01:47 AM

I have a friend's Denon AV receiver which I am testing at the moment. It is rated at 65W/channel "DIN" power rating 1kHz into 4R. I have the schematic and the supply rails for the amp section are shown as +/- 46V (no load, no signal) and these are the value I get with no load/no signal
. When I run a 1kHz sine wave through for testing, with an 8R load attached, it starts to clip at 23V peak-peak, or around 12V peak, way less than the 46V rail. The rails at clipping are still +/-45V. From my calcs this would imply an RMS power rating of only 9W into 8R. This amp has blown a couple of woofers.
Any suggestions as to why it would be clipping at such a low voltage and how does the power I am measuring at clipping equate to the rated 65W?
Thanks

unclejed613 · 15th July 2008, 02:06 AM

i work on Denon AVRs all the time. try measuring ALL of the channels. you will find that some of the channels clip at 23Vp-p and some at 90V p-p. i haven't had the time to find out why. i work in a production environment, and don't get a chance to dig into some of the "whys" of various designs unless the amp seems to be misbehaving in that particular area. i will check further. it may have something to do with the DSP "preclipping" the signal. center channels are usually the ones to clip at full output, with the rest "scaled down" according to their relative levels. this way, all channels clip at the same time, rather than having the center clip first, and then the others. at least that's what it seems like when i check surround amps.

smithy666 · 15th July 2008, 02:16 AM

Thanks for the response. This is a simple 2 channel receiver and both channels seem to be the same.

unclejed613 · 15th July 2008, 03:46 AM

what model is it? i can look up the service manual for you and find out why

smithy666 · 15th July 2008, 04:49 AM

Thanks. It is a DRA-455 which I think is basically the same as a DRA-385RD

AndrewT · 15th July 2008, 08:34 AM

Check the voltage on the supply rails going to the voltage amp stages.
The RC decoupling feeding these may have gone faulty.
R gone high resistance or C gone open circuit.

unclejed613 · 15th July 2008, 02:00 PM

check the voltage output from the +/- 15V regulators and check R105 which is the dropping resistor for both diff amp's negative rail. one other question..... did you check the rail voltages with the same oscope you measured the output waveform, or did you measure the rails with a meter? the reason i ask is that one thing that happens often, is that the oscope input channel variable gain pot (usually the middle knob in the input attenuator) gets nudged out of it's "CAL" click stop, and so all voltage measurements are incorrect.

smithy666 · 16th July 2008, 01:54 AM

I checked the rails with a DMM. My scope is a Bitscope PC oscilloscope, so it's all software based for the controls.

Thanks both for the tips - I'll check all the suggestions and report back

15th July 2008, 01:47 AM	#1
smithy666 diyAudio Member Join Date: Mar 2001 Location: Sydney	Supply rails vs output I have a friend's Denon AV receiver which I am testing at the moment. It is rated at 65W/channel "DIN" power rating 1kHz into 4R. I have the schematic and the supply rails for the amp section are shown as +/- 46V (no load, no signal) and these are the value I get with no load/no signal . When I run a 1kHz sine wave through for testing, with an 8R load attached, it starts to clip at 23V peak-peak, or around 12V peak, way less than the 46V rail. The rails at clipping are still +/-45V. From my calcs this would imply an RMS power rating of only 9W into 8R. This amp has blown a couple of woofers. Any suggestions as to why it would be clipping at such a low voltage and how does the power I am measuring at clipping equate to the rated 65W? Thanks

15th July 2008, 02:06 AM	#2
unclejed613 diyAudio Member Join Date: Dec 2006	i work on Denon AVRs all the time. try measuring ALL of the channels. you will find that some of the channels clip at 23Vp-p and some at 90V p-p. i haven't had the time to find out why. i work in a production environment, and don't get a chance to dig into some of the "whys" of various designs unless the amp seems to be misbehaving in that particular area. i will check further. it may have something to do with the DSP "preclipping" the signal. center channels are usually the ones to clip at full output, with the rest "scaled down" according to their relative levels. this way, all channels clip at the same time, rather than having the center clip first, and then the others. at least that's what it seems like when i check surround amps. __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/

15th July 2008, 03:46 AM	#4
unclejed613 diyAudio Member Join Date: Dec 2006	what model is it? i can look up the service manual for you and find out why __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/

15th July 2008, 08:34 AM	#6
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Check the voltage on the supply rails going to the voltage amp stages. The RC decoupling feeding these may have gone faulty. R gone high resistance or C gone open circuit. __________________ regards Andrew T.

15th July 2008, 02:00 PM	#7
unclejed613 diyAudio Member Join Date: Dec 2006	check the voltage output from the +/- 15V regulators and check R105 which is the dropping resistor for both diff amp's negative rail. one other question..... did you check the rail voltages with the same oscope you measured the output waveform, or did you measure the rails with a meter? the reason i ask is that one thing that happens often, is that the oscope input channel variable gain pot (usually the middle knob in the input attenuator) gets nudged out of it's "CAL" click stop, and so all voltage measurements are incorrect. __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: http://www1284177414881.v-dc.net/


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15th July 2008, 02:16 AM	#3
smithy666 diyAudio Member Join Date: Mar 2001 Location: Sydney	Thanks for the response. This is a simple 2 channel receiver and both channels seem to be the same.

15th July 2008, 04:49 AM	#5
smithy666 diyAudio Member Join Date: Mar 2001 Location: Sydney	Thanks. It is a DRA-455 which I think is basically the same as a DRA-385RD

16th July 2008, 01:54 AM	#8
smithy666 diyAudio Member Join Date: Mar 2001 Location: Sydney	I checked the rails with a DMM. My scope is a Bitscope PC oscilloscope, so it's all software based for the controls. Thanks both for the tips - I'll check all the suggestions and report back

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