Sony amp feedback

jean-paul · 07:48 PM

Hi, this amp has a sensitivity of 150 mV for full power output which is way too much for nowadays sources. With the volume potentiometer at 8 o clock it is darn loud already. I wish to temper the sensitivity to around 300 mV or even 1 V.

My guess is that lowering feedback resistor R361 to 33k is enough. But what to do with R382 and C357 ?

Frank Berry · 7th October 2011, 07:35 PM

Why not just put an attenuator on the input of the amp?
Changing the feedback will have multiple effects. It may make the amplifier oscillate.

DF96 · 7th October 2011, 07:37 PM

You could try increasing C357 a little too, but HF stability is hard to predict as it depends to some extent on parasitic components which don't appear in the circuit diagram. You could find you have made a power oscillator! An attenuator at the input would be simpler.

jean-paul · 08:20 PM

It has a volume potentiometer but if I barely touch it it is very loud. Gain is very high which made me think to change the feedback circuit. I also thought of possible oscillation but my knowledge in this area is lacking to say the least. It occurred to me that the resistors are very high in value too for a power amp feedback circuit. Long ago I was taught that the impedances at both inputs of a differential pair should be equal but I forgot the reason

That would imply that 51 kOhm would be OK. What to do with R382 then ? I should have made more I amps I see now.

Putting an attenuator before the volume potentiometer to have the signal later on amplified a lot again seems counterproductive IMO but I guess it is the safest approach.

tornislv · 7th October 2011, 08:12 PM

With rail voltage around 40V it seems to be non-ES series SONY amp. It may seem a little strange you have problems with over-sensitivity. I have a bunch of SONY in my possession currently (630 esD, 650esD, F470, F519R), and even with SACD player with 2Vpp output the volume is rather "normal"( up to 11 - 12 with no problems, except the wife :-) ). I presume the "problem" you have is in both channels? ;-)
I would not be afraid to lower the value of R361 a little bit, the amps usually are more stable with deeper feedback.

DF96 · 7th October 2011, 08:38 PM

At present one LTP base sees 51k, the other sees 100k in parallel with 100.62k. Near enough the same.

Adding 10dB to the feedback, as you propose, is quite likely to cause oscillation. Maybe not all the time, but with certain signals and certain loads.

jean-paul · 09:06 PM

Quote:

Originally Posted by tornislv

With rail voltage around 40V it seems to be non-ES series SONY amp. It may seem a little strange you have problems with over-sensitivity. I have a bunch of SONY in my possession currently (630 esD, 650esD, F470, F519R), and even with SACD player with 2Vpp output the volume is rather "normal"( up to 11 - 12 with no problems, except the wife :-) ). I presume the "problem" you have is in both channels? ;-)
I would not be afraid to lower the value of R361 a little bit, the amps usually are more stable with deeper feedback.

Yes, it is with both channels. It is not a problem but a feature

I can not set it at 12 o clock or my speakers will catch fire. This type is always like this so it is not abnormal behavior. I just wish I could change this.

It is a semi ES amp as it has ES parts (well there is ES written on the power transformer) and relay controlled inputs but is was not part of the ES series, I will let you guess which one it is.

jean-paul · 09:23 PM

Quote:

Originally Posted by DF96

At present one LTP base sees 51k, the other sees 100k in parallel with 100.62k. Near enough the same.

Adding 10dB to the feedback, as you propose, is quite likely to cause oscillation. Maybe not all the time, but with certain signals and certain loads.

Of course, you are right. the LTP bases both see nearly the same value. That is with R382 and with DC but what exactly is the purpose of this resistor ? Obviously a DC path but for what ? I checked some other Sony amps of that time and they have similar schematics ( with higher rail voltages) and the same feedback networks with R361 (51k), C356 and R357 (620 Ohm) but no R382 installed.

sakis · 8th October 2011, 05:02 PM

Quote:

Originally Posted by Frank Berry

Why not just put an attenuator on the input of the amp?
Changing the feedback will have multiple effects. It may make the amplifier oscillate.

cause it will kill the dynamics and drawn the source completely

alternative to lower the gain of the amp will obviously benefit the amp for the best if done properly

DF96 · 8th October 2011, 05:42 PM

"Kill dynamics"? Do you have access to a special source of music which has never been near any resistors?

"Drown the source"? Only if you use the wrong values.

7th October 2011, 07:35 PM	#2
Frank Berry diyAudio Member Join Date: Sep 2003 Location: Midland, Michigan	Why not just put an attenuator on the input of the amp? Changing the feedback will have multiple effects. It may make the amplifier oscillate. __________________ Frank

7th October 2011, 07:50 PM	#4
jean-paul diyAudio Moderator Emeritus Join Date: Sep 2002 Location: The Netherlands	It has a volume potentiometer but if I barely touch it it is very loud. Gain is very high which made me think to change the feedback circuit. I also thought of possible oscillation but my knowledge in this area is lacking to say the least. It occurred to me that the resistors are very high in value too for a power amp feedback circuit. Long ago I was taught that the impedances at both inputs of a differential pair should be equal but I forgot the reason That would imply that 51 kOhm would be OK. What to do with R382 then ? I should have made more I amps I see now. Putting an attenuator before the volume potentiometer to have the signal later on amplified a lot again seems counterproductive IMO but I guess it is the safest approach. __________________ It's only audio Last edited by jean-paul; 7th October 2011 at 08:20 PM.

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7th October 2011, 07:37 PM	#3
DF96 diyAudio Member Join Date: May 2007	You could try increasing C357 a little too, but HF stability is hard to predict as it depends to some extent on parasitic components which don't appear in the circuit diagram. You could find you have made a power oscillator! An attenuator at the input would be simpler.

7th October 2011, 08:12 PM	#5
tornislv diyAudio Member Join Date: Sep 2010 Location: Riga	With rail voltage around 40V it seems to be non-ES series SONY amp. It may seem a little strange you have problems with over-sensitivity. I have a bunch of SONY in my possession currently (630 esD, 650esD, F470, F519R), and even with SACD player with 2Vpp output the volume is rather "normal"( up to 11 - 12 with no problems, except the wife :-) ). I presume the "problem" you have is in both channels? ;-) I would not be afraid to lower the value of R361 a little bit, the amps usually are more stable with deeper feedback.

7th October 2011, 08:38 PM	#6
DF96 diyAudio Member Join Date: May 2007	At present one LTP base sees 51k, the other sees 100k in parallel with 100.62k. Near enough the same. Adding 10dB to the feedback, as you propose, is quite likely to cause oscillation. Maybe not all the time, but with certain signals and certain loads.

8th October 2011, 05:42 PM	#10
DF96 diyAudio Member Join Date: May 2007	"Kill dynamics"? Do you have access to a special source of music which has never been near any resistors? "Drown the source"? Only if you use the wrong values.

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