What is the purpose of NE5534 in this amp

Gasho · 10th April 2005, 03:29 PM

Here is shematic.

peranders · 10th April 2005, 03:53 PM

The NE5534 is a DC servo but the output isn't drawn correctly. The output isn't connected to any load.

I have a similar servo in my QRV-03 amp.

boholm · 10th April 2005, 03:55 PM

It looks to me to be a DC servo.

Gasho · 10th April 2005, 04:16 PM

Hmmm, if it´s DC servo , why I must to put a trimpot i series with R2 (connecting input with ground) to trim the DC on output.

Netlist · 10th April 2005, 04:21 PM

Hmm...the image reads "Redrawn".
Could someone have made a mistake?

/Hugo

moamps · 10th April 2005, 04:22 PM

Quote:

Originally posted by Gasho
Here is shematic.

A kind of an unconnected overload detector (diff.amp).

Quote:

Originally posted by peranders
The NE5534 is a DC servo but the output isn't drawn correctly.

Where in your opinion should the output be connected?
Take a look at RC values.

Regards,
Milan

jacco vermeulen · 10th April 2005, 04:23 PM

Quote:

why I must to put a trimpot i series with R2

Because you can make life difficult or easy for an opamp.

AndrewT · 10th April 2005, 04:34 PM

Hi,
I second Moamps. Opamp measures diff volts across LTP with a gain of about 50. But the output circuit is missing. A LED or relay?

Gasho · 10th April 2005, 04:50 PM

So if I rise value of feedback resistor R65 (from 33K to 50K) to make amplifier with smaller NFB , and put trimpot(20k) in series with R2 to correct output DC offset , will I change the overload protection point?

ilimzn · 10th April 2005, 05:31 PM

The schematic appears to have many errors, the unconnected output of the NE5534 is only one - the connection of the 'clip detect' and output transistors does not make much sense, for instance.

The 5534 seems to be a clip detector, but the output is drawn unconnected. The reason DC offset voltage changes when you change the resistor is that the combined bias current of the NE5534 (which is quite high, BTW) and the amp itself develops different voltages at the differential pair inputs, and you get this difference as an output offset. It remains a mystery why anyone would use a NE5534 in this application, but I guess if every chip in the amp is a NE5534, economy of scale applies - it may just have been the least expensive to use.

As for your proposal to reduce NFB - this also increases closed loop gain, from roughly 34 to 51, which also reduces bandwidth in proportion, and increases distortion. Why would you want to do this?
If you want to reduce NFB (which is not something to be done unless you know exactly what you are doing and why), you need to reduce open loop gain and keep closed loop gain the same. There are a number of ways you could do this on this amp, and the best ones have nothing to do with R65. If you are competent enough to arrive at the decision that NFB should be reduced, you should easily be competent enough to know what these methods are, and which one is the best to use.

10th April 2005, 03:53 PM	#2
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	The NE5534 is a DC servo but the output isn't drawn correctly. The output isn't connected to any load. I have a similar servo in my QRV-03 amp. __________________ /Per-Anders (my first name) or P-A as my friends call me

10th April 2005, 03:55 PM	#3
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	It looks to me to be a DC servo. __________________ Best regards Bo

10th April 2005, 04:34 PM	#8
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, I second Moamps. Opamp measures diff volts across LTP with a gain of about 50. But the output circuit is missing. A LED or relay? __________________ regards Andrew T.

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10th April 2005, 04:16 PM	#4
Gasho diyAudio Member Join Date: Nov 2004 Location: Zagreb	Hmmm, if it´s DC servo , why I must to put a trimpot i series with R2 (connecting input with ground) to trim the DC on output.

10th April 2005, 04:21 PM	#5
Netlist diyAudio Moderator Join Date: Jan 2003	Hmm...the image reads "Redrawn". Could someone have made a mistake? /Hugo

10th April 2005, 04:50 PM	#9
Gasho diyAudio Member Join Date: Nov 2004 Location: Zagreb	So if I rise value of feedback resistor R65 (from 33K to 50K) to make amplifier with smaller NFB , and put trimpot(20k) in series with R2 to correct output DC offset , will I change the overload protection point?

10th April 2005, 05:31 PM	#10
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	The schematic appears to have many errors, the unconnected output of the NE5534 is only one - the connection of the 'clip detect' and output transistors does not make much sense, for instance. The 5534 seems to be a clip detector, but the output is drawn unconnected. The reason DC offset voltage changes when you change the resistor is that the combined bias current of the NE5534 (which is quite high, BTW) and the amp itself develops different voltages at the differential pair inputs, and you get this difference as an output offset. It remains a mystery why anyone would use a NE5534 in this application, but I guess if every chip in the amp is a NE5534, economy of scale applies - it may just have been the least expensive to use. As for your proposal to reduce NFB - this also increases closed loop gain, from roughly 34 to 51, which also reduces bandwidth in proportion, and increases distortion. Why would you want to do this? If you want to reduce NFB (which is not something to be done unless you know exactly what you are doing and why), you need to reduce open loop gain and keep closed loop gain the same. There are a number of ways you could do this on this amp, and the best ones have nothing to do with R65. If you are competent enough to arrive at the decision that NFB should be reduced, you should easily be competent enough to know what these methods are, and which one is the best to use.

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