N-Fdbk Question

Zero Cool · 2nd October 2004, 07:49 PM

Without getting into a major discussion of Negative feedback...

I have been studying the Soundcraftsmen Amplifier design for sometime now. I have owned and repaired quite a few of these little amps like the PCR-800 and Pro-Power 4.

Soundcraftmen uses this same design for at least 10 different amplifier models. it is a simple design that i was once told was striaght out of the RCA transistor handbook.

The only differences between the models is a change in 2 of the componets in the neg feedback path and either 2 pairs or 3 pairs of mos-fets between the models designated between hom and semi-pro use.

I dont have a way to post the schematic, but i can email it to anyone interested.

In the Negative feedback path for the home amps they use a 1,600 ohm resistor inline between the speaker output and the 2nd Transistor of the differential pair and a 16K ohm resistor and 220uf cap to ground.

the pro models use a 680 ohm and 22K ohm resistors respectivly.

I would imagine they would do this to make the pro amp more stable??? one having more neg feedback then the other???

Can anyone offer any insights as to what changes occur by changing these 2 resistors?

Thanks

Zero

PMA · 2nd October 2004, 07:57 PM

Quote:

Originally posted by Zero Cool
Without getting into a major discussion of Negative feedback...

In the Negative feedback path for the home amps they use a 1,600 ohm resistor inline between the speaker output and the 2nd Transistor of the differential pair and a 16K ohm resistor and 220uf cap to ground.

Probably opposite resistor manner. The voltage gain would be about 20dB.

the pro models use a 680 ohm and 22K ohm resistors respectivly.

The voltage gain is 30dB now.

Simple answers written in italics.

Zero Cool · 2nd October 2004, 08:01 PM

Oops, your exactly right i got the values backwards.

Home 16K inline and 1.6 to 220uf to ground
Pro 22K Inline and 680 to 220uf to ground

Zero

Zero Cool · 2nd October 2004, 08:03 PM

So its just sensitivity it changes! I see, does the bandwidth stay the same etc?

AndrewT · 3rd October 2004, 10:45 AM

Hi, the lo frequency time constant is different; 352mS and 150mS. The low end will start to roll off and the phase angle will start to become significant at about 5Hz and 12Hz. both these values are probably O.K. for bass speakers. For a very low frequency sub-base you could try changing the 150mS time constant. It may show some difference when you try other values. Worth experimenting.
Do any others have info/experience on setting this RC time constant?

PMA · 3rd October 2004, 11:08 AM

I am usually setting the low frequency -3dB corner at 2 - 3 Hz.

Zero Cool · 3rd October 2004, 05:29 PM

Thank You everyone!!!!!

Jocko Homo · 3rd October 2004, 07:31 PM

The HF will start to roll-off a bit faster in the higher gain one.

Jocko

2nd October 2004, 07:49 PM	#1
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	N-Fdbk Question Without getting into a major discussion of Negative feedback... I have been studying the Soundcraftsmen Amplifier design for sometime now. I have owned and repaired quite a few of these little amps like the PCR-800 and Pro-Power 4. Soundcraftmen uses this same design for at least 10 different amplifier models. it is a simple design that i was once told was striaght out of the RCA transistor handbook. The only differences between the models is a change in 2 of the componets in the neg feedback path and either 2 pairs or 3 pairs of mos-fets between the models designated between hom and semi-pro use. I dont have a way to post the schematic, but i can email it to anyone interested. In the Negative feedback path for the home amps they use a 1,600 ohm resistor inline between the speaker output and the 2nd Transistor of the differential pair and a 16K ohm resistor and 220uf cap to ground. the pro models use a 680 ohm and 22K ohm resistors respectivly. I would imagine they would do this to make the pro amp more stable??? one having more neg feedback then the other??? Can anyone offer any insights as to what changes occur by changing these 2 resistors? Thanks Zero

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2nd October 2004, 08:01 PM	#3
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	Oops, your exactly right i got the values backwards. Home 16K inline and 1.6 to 220uf to ground Pro 22K Inline and 680 to 220uf to ground Zero

2nd October 2004, 08:03 PM	#4
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	So its just sensitivity it changes! I see, does the bandwidth stay the same etc?

3rd October 2004, 10:45 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, the lo frequency time constant is different; 352mS and 150mS. The low end will start to roll off and the phase angle will start to become significant at about 5Hz and 12Hz. both these values are probably O.K. for bass speakers. For a very low frequency sub-base you could try changing the 150mS time constant. It may show some difference when you try other values. Worth experimenting. Do any others have info/experience on setting this RC time constant?

3rd October 2004, 11:08 AM	#6
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	I am usually setting the low frequency -3dB corner at 2 - 3 Hz.

3rd October 2004, 05:29 PM	#7
Zero Cool Audio Junkie diyAudio Member Join Date: Sep 2004 Location: MN	Thank You everyone!!!!!

3rd October 2004, 07:31 PM	#8
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	The HF will start to roll-off a bit faster in the higher gain one. Jocko

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