LM4780 preamp Suggestions?

Wynand · 13th January 2006, 06:17 AM

Can anyone suggest a good preamp/schematic for audiosectors LM4780 amplifier?

Active or passive.

I'll be running stereo/bridged.

Maybe Peter can suggest something??

sangram · 13th January 2006, 06:45 AM

I'm using a 50K pot in front of a stereo 4780 and it works fine, the sound is a little mushy but that's due to the pot (a normal $0.25 philips carbon track) I think and the fact that the amp is AC-coupled with a 2.2 uF film cap. An attenuator will sound better.

My BPA runs off my soundcard transformers, DC-coupled, and it sounds simply amazing. A tad bright but nice and airy as well.

I am going to try a DRV134 and an opamp-based inverter/buffer for the BPAs next, as well as a ladder attenuator. These things are inexpensive to whip together and can be tweaked for months together, so feel free to make both and try...

Wynand · 13th January 2006, 08:02 AM

Whats the difference between a pot and an attenuator?

sangram · 13th January 2006, 11:08 AM

Pot would be the one with a wiper and track.

Attenuator would be a multi-position switch with fixed resistors in a ladder arrangement that selectively attenuate the signal.

Wynand · 13th January 2006, 01:04 PM

How many switching positions are there and is it dual for stereo?

sangram · 16th January 2006, 04:44 AM

You'll have to do a search, they range from 11 steps to 48, increasing in complexity and cost.

Yes, most of them are dual pole for stereo switching.

The one in the image is a 24-step attenuator from www.digitalanaloguediy.com there are many more available.

Wynand · 16th January 2006, 10:59 AM

What about switching noise?

Nordic · 16th January 2006, 11:25 AM

You see Wynand, the with the attenuator you can select two well matched resistors, one for each colume setting, for each channel, so that, as you turn the attenuator, both channels will see an equal resistance, with regular wafer pots, the top and bottom ones are never perfectly aligned, and they do not track as uniformly...

As for the switching noise... you get 2 kinds of attenuators... one is Make before break, and the other is brake before...
in other words, the switch makes a connection to the next setting before disconnecting the previous resistor.

For volume controls the make before break ones are prefferable as far as I know.

Wynand · 16th January 2006, 11:39 AM

Nordic, have you used them before?

1. Are they better than pots?
2. What do they cost?
3. Where did you get them?

Nordic · 16th January 2006, 11:43 AM

Nooo Wynand, way out of my price bracket for volume control... R700+ for a quality 24 step...

The one Sangram mentions is about R300, not familiar with it...
I think what makes the good ones so expensive is that they firstly use very good mechanical parts and secondly use over $40 worth of $1 a pop resistors...

13th January 2006, 06:17 AM	#1
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	LM4780 preamp Suggestions? Can anyone suggest a good preamp/schematic for audiosectors LM4780 amplifier? Active or passive. I'll be running stereo/bridged. Maybe Peter can suggest something??

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13th January 2006, 06:45 AM	#2
sangram diyAudio Member Join Date: Sep 2002 Location: Calcutta, India	I'm using a 50K pot in front of a stereo 4780 and it works fine, the sound is a little mushy but that's due to the pot (a normal $0.25 philips carbon track) I think and the fact that the amp is AC-coupled with a 2.2 uF film cap. An attenuator will sound better. My BPA runs off my soundcard transformers, DC-coupled, and it sounds simply amazing. A tad bright but nice and airy as well. I am going to try a DRV134 and an opamp-based inverter/buffer for the BPAs next, as well as a ladder attenuator. These things are inexpensive to whip together and can be tweaked for months together, so feel free to make both and try...

13th January 2006, 08:02 AM	#3
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	Whats the difference between a pot and an attenuator?

13th January 2006, 01:04 PM	#5
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	How many switching positions are there and is it dual for stereo?

16th January 2006, 04:44 AM	#6
sangram diyAudio Member Join Date: Sep 2002 Location: Calcutta, India	You'll have to do a search, they range from 11 steps to 48, increasing in complexity and cost. Yes, most of them are dual pole for stereo switching. The one in the image is a 24-step attenuator from www.digitalanaloguediy.com there are many more available.

16th January 2006, 10:59 AM	#7
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	What about switching noise?

16th January 2006, 11:25 AM	#8
Nordic diyAudio Member Join Date: Sep 2005	You see Wynand, the with the attenuator you can select two well matched resistors, one for each colume setting, for each channel, so that, as you turn the attenuator, both channels will see an equal resistance, with regular wafer pots, the top and bottom ones are never perfectly aligned, and they do not track as uniformly... As for the switching noise... you get 2 kinds of attenuators... one is Make before break, and the other is brake before... in other words, the switch makes a connection to the next setting before disconnecting the previous resistor. For volume controls the make before break ones are prefferable as far as I know.

16th January 2006, 11:39 AM	#9
Wynand diyAudio Member Join Date: Mar 2005 Location: Cape Town	Nordic, have you used them before? 1. Are they better than pots? 2. What do they cost? 3. Where did you get them?

16th January 2006, 11:43 AM	#10
Nordic diyAudio Member Join Date: Sep 2005	Nooo Wynand, way out of my price bracket for volume control... R700+ for a quality 24 step... The one Sangram mentions is about R300, not familiar with it... I think what makes the good ones so expensive is that they firstly use very good mechanical parts and secondly use over $40 worth of $1 a pop resistors...

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