F5 power amplifier - Page 1018

lhquam · 31st January 2011, 04:13 AM

I finally finished my F5 and decided that it was trustworthy enough to install in my stereo system (Squeezebox, McIntosh C504 preamp (circa approx 1986) and B&W CDM1 speakers).

I found that the offset voltage of each channel went to about 60 mv (from near zero) when connected to the preamp. Measuring the dc level of the preamps verified that the preamp channels have about 10 mv dc output. Apparently the 10uf non-polar DC blocking caps in the preamp are leaking a little.

My question is whether to simply adjust the F5 to accomodate that offset at the inputs, install blocking caps in the F5, or replace the blocking caps of the preamp.

jameshillj · 31st January 2011, 04:48 AM

1st things first - get your preamp sorted - [B].... replace the blocking caps of the preamp[/B] - good quality film ones will also give a big improvement in sound quality - film caps do usually take awhile to "settle down" to their final sound - also have a look at replacing all the other electros, as 25 years old.

Lerg · 31st January 2011, 04:51 AM

Don't break your F5 to compensate for your preamp. Fix the preamp.

EUVL · 31st January 2011, 05:20 AM

You can always give DC servo a consideration.

lhquam · 31st January 2011, 05:47 AM

Maybe it is time to consider building a replacement preamp.

h_a · 31st January 2011, 07:59 AM

60mV...that's about 1 milliWatt into your 4R speaker ;-)

Voice coil displacement must be in the micron-range.

I would say - enjoy your F5 and use the time to listen to music ;-)

AndrewT · 31st January 2011, 08:31 AM

Quote:

Originally Posted by h_a

60mV...that's about 1 milliWatt into your 4R speaker

1mW is ~-47dB ref maximum power of the F5 (50W into 4r0).
If one had noise at a level of -47dB one would be in tears after spending all that money.

Output offset is a "noise", if anyone will allow DC to be a noise. It certainly should not be there and certainly not considered acceptable.

If high efficiency speakers are being used with the F5 then it's even more important to minimise the output offset.

Marra · 31st January 2011, 09:10 AM

I connected up my F5 to the mains this morning without the bulb tester and after several seconds the fuse blew. The right channel was disconnected. The left channel did not blow the fuse but adjusting P1 does not do anything; P2 adjusts the voltage across R12 and alters the dc offset. Nothing went pop on the pcb's and there was no smoke. Have I cooked Q3 on the left channel? Any advice as to what to check would be much appreciated. TIA

AndrewT · 09:17 AM

Unplug.
Discharge all the capacitors with a power resistor.
The electrolytics will appear to recharge due to energy storage in the di-electric.
Discharge them at least twice and measure voltages before you start the next stage.
Measure the resistance of the Output mosFETs.
D to S and S to D.

jpedro · 31st January 2011, 09:25 AM

Hi CViller,
I made an order for a PSU for the F5, and also the payment procedure.
Best regards,
Pedro Simões

31st January 2011, 04:13 AM	#10171
lhquam diyAudio Member Join Date: Mar 2010 Location: Olalla, Oregon: Land of the 100 Valleys	To DC block or not, that is the question I finally finished my F5 and decided that it was trustworthy enough to install in my stereo system (Squeezebox, McIntosh C504 preamp (circa approx 1986) and B&W CDM1 speakers). I found that the offset voltage of each channel went to about 60 mv (from near zero) when connected to the preamp. Measuring the dc level of the preamps verified that the preamp channels have about 10 mv dc output. Apparently the 10uf non-polar DC blocking caps in the preamp are leaking a little. My question is whether to simply adjust the F5 to accomodate that offset at the inputs, install blocking caps in the F5, or replace the blocking caps of the preamp.

31st January 2011, 04:48 AM	#10172
jameshillj diyAudio Member Join Date: Feb 2005 Location: Melbourne, Aust	1st things first - get your preamp sorted - [B].... replace the blocking caps of the preamp[/B] - good quality film ones will also give a big improvement in sound quality - film caps do usually take awhile to "settle down" to their final sound - also have a look at replacing all the other electros, as 25 years old. __________________ ... jh

31st January 2011, 07:59 AM	#10176
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	60mV...that's about 1 milliWatt into your 4R speaker ;-) Voice coil displacement must be in the micron-range. I would say - enjoy your F5 and use the time to listen to music ;-) __________________ fresh matched IRFP240/IRFP9240 fets \|\| AlephJ/JX-kits F5 transistor kits \|\| Burning Amp BA-1/2/3 transistor kits

31st January 2011, 09:14 AM	#10179
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Unplug. Discharge all the capacitors with a power resistor. The electrolytics will appear to recharge due to energy storage in the di-electric. Discharge them at least twice and measure voltages before you start the next stage. Measure the resistance of the Output mosFETs. D to S and S to D. __________________ regards Andrew T. Last edited by AndrewT; 31st January 2011 at 09:17 AM.

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31st January 2011, 04:51 AM	#10173
Lerg diyAudio Member Join Date: Oct 2010	Don't break your F5 to compensate for your preamp. Fix the preamp.

31st January 2011, 05:20 AM	#10174
EUVL diyAudio Member Join Date: Oct 2003	You can always give DC servo a consideration.

31st January 2011, 05:47 AM	#10175
lhquam diyAudio Member Join Date: Mar 2010 Location: Olalla, Oregon: Land of the 100 Valleys	Maybe it is time to consider building a replacement preamp.

31st January 2011, 09:10 AM	#10178
Marra diyAudio Member Join Date: Nov 2009 Location: Near Lincoln	I connected up my F5 to the mains this morning without the bulb tester and after several seconds the fuse blew. The right channel was disconnected. The left channel did not blow the fuse but adjusting P1 does not do anything; P2 adjusts the voltage across R12 and alters the dc offset. Nothing went pop on the pcb's and there was no smoke. Have I cooked Q3 on the left channel? Any advice as to what to check would be much appreciated. TIA

31st January 2011, 09:25 AM	#10180
jpedro diyAudio Member Join Date: Jul 2010 Location: close to Lisbon	Hi CViller, I made an order for a PSU for the F5, and also the payment procedure. Best regards, Pedro Simões

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