Cello Amplifier troubleshooting

morrish · 27th January 2008, 08:28 AM

I have a Cello Duet-350 power amplifier. It is about 10 years old. It now generates its own gratuitous, random, static-sounding noise. Input level has no effect on the level of this noise. It is not objectionable with listening to Eric Clapton, but it is quite annoying when listening to Chopin Nocturnes. I have disassembled it as far as I could without resorting to a soldering iron; disconnected, cleaned and reconnected every removable contact I could find. I can find no parts inside that are obviously scorched. Does anyone, please, have any ideas about what I might try?

QSerraTico_Tico · 27th January 2008, 11:16 AM

Static noise in both channels?

lumanauw · 27th January 2008, 01:06 PM

Cello uses Jfet for input. Could it be the Jfet?
(Note : this is from Cello amp that has separated supply monoblocks (4pcs for stereo), don't know the type)

PigletsDad · 27th January 2008, 01:18 PM

Try some diagnostics to narrow down where it might be coming from.

First use scope to look at output - what does the unwanted signal look like - spikes? Bursts of oscillation? Bursts of white noise? Steps?

Now try looking at few points in the circuit to see if you can see the same thing.

Try the power supply rails, the input terminals (it may be being picked up in the cabling), the inputs to the JFET pair, the output of the VAS and anywhere else you can get an easy measurement.

This should at least give some evidence to build a theory from.

jacco vermeulen · 29th January 2008, 11:11 AM

David,

Encore-150

Other

lumanauw · 29th January 2008, 01:26 PM

Hi, Jacco,

Do you have the output stage schematic?

Nico Ras · 29th January 2008, 10:11 PM

Quote:

Originally posted by lumanauw
Cello uses Jfet for input. Could it be the Jfet?
(Note : this is from Cello amp that has separated supply monoblocks (4pcs for stereo), don't know the type)

Something pops to mind - is the capacitors maybe shot. Could there be arc through one one of the mica washers under the power trannies
. Finally is there some corroded joints even in the output fuses?

S.A.G. · 30th January 2008, 11:04 AM

Hi Lumanauw,

pag. 2 of the linked .pdf shows the output stage.

From the owner manual available at:

http://www.matthew-james.net/images/...ers-manual.pdf

it turns out that the Encore Amplifier is just a scaled down version of the Performance Amplifier (see pag. 4, "audio circuitry").

I might be wrong but the +/-Drive_2 signals on pag. 2 are not used in the Encore Amplifier. The only missing piece of information, beside the power supply section, is the EPB-1 board, a small board mounted across the output connector pins.

I'd be interested to hear some of the solid state gurus commenting the feedback scheme and the asymmetric input impedance of the balanced connection (330K on the + side, 5K on the -side according to the manual).

Regards

Giorgio

jacco vermeulen · 30th January 2008, 11:07 AM

Quote:

Originally posted by S.A.G.
the EPB-1 board

EPB= electronic protection board

S.A.G. · 30th January 2008, 12:38 PM

Thanks Jacco,

Do you have any schematic of it? I'd be interested to understand how it works.

Regards

Giorgio

27th January 2008, 08:28 AM	#1
morrish diyAudio Member Join Date: Jan 2008	Cello Amplifier troubleshooting I have a Cello Duet-350 power amplifier. It is about 10 years old. It now generates its own gratuitous, random, static-sounding noise. Input level has no effect on the level of this noise. It is not objectionable with listening to Eric Clapton, but it is quite annoying when listening to Chopin Nocturnes. I have disassembled it as far as I could without resorting to a soldering iron; disconnected, cleaned and reconnected every removable contact I could find. I can find no parts inside that are obviously scorched. Does anyone, please, have any ideas about what I might try?

29th January 2008, 11:11 AM	#5
jacco vermeulen diyAudio Member Join Date: Oct 2004 Location: At the sea front, just under Rotterdam	David, Encore-150 Other __________________ Looks like Sponge Bob has killed another thread.

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27th January 2008, 11:16 AM	#2
QSerraTico_Tico diyAudio Member Join Date: Oct 2004	Static noise in both channels?

27th January 2008, 01:18 PM	#4
PigletsDad diyAudio Member Join Date: Jul 2007 Location: South Worcestershire	Try some diagnostics to narrow down where it might be coming from. First use scope to look at output - what does the unwanted signal look like - spikes? Bursts of oscillation? Bursts of white noise? Steps? Now try looking at few points in the circuit to see if you can see the same thing. Try the power supply rails, the input terminals (it may be being picked up in the cabling), the inputs to the JFET pair, the output of the VAS and anywhere else you can get an easy measurement. This should at least give some evidence to build a theory from.

29th January 2008, 01:26 PM	#6
lumanauw diyAudio Moderator Emeritus Join Date: Oct 2002 Location: Bandung	Hi, Jacco, Do you have the output stage schematic?

30th January 2008, 11:04 AM	#8
S.A.G. diyAudio Member Join Date: Jun 2004 Location: Milano	Hi Lumanauw, pag. 2 of the linked .pdf shows the output stage. From the owner manual available at: http://www.matthew-james.net/images/...ers-manual.pdf it turns out that the Encore Amplifier is just a scaled down version of the Performance Amplifier (see pag. 4, "audio circuitry"). I might be wrong but the +/-Drive_2 signals on pag. 2 are not used in the Encore Amplifier. The only missing piece of information, beside the power supply section, is the EPB-1 board, a small board mounted across the output connector pins. I'd be interested to hear some of the solid state gurus commenting the feedback scheme and the asymmetric input impedance of the balanced connection (330K on the + side, 5K on the -side according to the manual). Regards Giorgio

30th January 2008, 12:38 PM	#10
S.A.G. diyAudio Member Join Date: Jun 2004 Location: Milano	Thanks Jacco, Do you have any schematic of it? I'd be interested to understand how it works. Regards Giorgio

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