Adjusting input sensitivity on Hafler DH-500

Lee in Montreal · 10th July 2010, 01:33 PM

I made a search on the net and specifically on this forum with no result.

Basically I run three amps in my system. A moded Quad 405, a moded Quad 606 and this Hafler. While the Quads have the input sensiivity set at 0.75v, the Hafler is set at 2.35v and therefore the volume is quite off.

I have the diagram but cannot locate the resistance that sets the input sensitivity. Anyone can help? And while I am there, perhaps I would even fit a potentiometer to make it adjustable.

Thanks in advance

Lee

djoffe · 10th July 2010, 05:04 PM

Looking up on the web:
==> the Quad 606 has voltage gain of 66.9, based on 140 Watts across 8 Ohms for an input of 0.5 VRMS.
==>the DH-500 has a voltage gain of 19.22, base on 255 Watts across 8 ohms for an input of 2.35 Volts RMS. ( I assume both quoted in RMS volts).

So the Quad has about 3.48 times the sensitivity of the Hafler.

It could be safer to put an attenuator in front of the Quad.

If you are really adventurous, you could change R28 of the Hafler from its current value 150 Ohms, to 41.5 Ohms. It should probably remain stable, as you are dropping the amount of feedback. The distortion will rise by a factor of 3.5x, but it was a pretty good amp to start with. Of course...funny things could happen...they shouldn't, but caveat emptor.

You could also do this by leaving existing resistor in place, and paralleling it with 57 Ohms.

Be careful...and good luck...

Lee in Montreal · 10th July 2010, 05:11 PM

Quote:

Originally Posted by djoffe

You could also do this by leaving existing resistor in place, and paralleling it with 57 Ohms.

Be careful...and good luck...

Exactly the type of suggestion I was looking for. The add-on resistance will be in parallel. Thanks.

Lee

djk · 11th July 2010, 02:45 PM

I changed my DH500 feedback resistors from 150R to 47R, they worked fine.

sreten · 11th July 2010, 02:58 PM

Hi,

FWIW increasing gain by reducing feedback improves the stability margins
and it will not cause any stability issues, they only occur when reducing
gain by increasing feedback.

/Sreten.

Lee in Montreal · 11th July 2010, 05:42 PM

So, it looks like increasing the Hafler's sensitivity will not cause any problem.

Thanks to all.

10th July 2010, 01:33 PM	#1
Lee in Montreal diyAudio Member Join Date: Oct 2009	Adjusting input sensitivity on Hafler DH-500 I made a search on the net and specifically on this forum with no result. Basically I run three amps in my system. A moded Quad 405, a moded Quad 606 and this Hafler. While the Quads have the input sensiivity set at 0.75v, the Hafler is set at 2.35v and therefore the volume is quite off. I have the diagram but cannot locate the resistance that sets the input sensitivity. Anyone can help? And while I am there, perhaps I would even fit a potentiometer to make it adjustable. Thanks in advance Lee

10th July 2010, 05:04 PM	#2
djoffe diyAudio Member Join Date: Oct 2008	Some observations and help Looking up on the web: ==> the Quad 606 has voltage gain of 66.9, based on 140 Watts across 8 Ohms for an input of 0.5 VRMS. ==>the DH-500 has a voltage gain of 19.22, base on 255 Watts across 8 ohms for an input of 2.35 Volts RMS. ( I assume both quoted in RMS volts). So the Quad has about 3.48 times the sensitivity of the Hafler. It could be safer to put an attenuator in front of the Quad. If you are really adventurous, you could change R28 of the Hafler from its current value 150 Ohms, to 41.5 Ohms. It should probably remain stable, as you are dropping the amount of feedback. The distortion will rise by a factor of 3.5x, but it was a pretty good amp to start with. Of course...funny things could happen...they shouldn't, but caveat emptor. You could also do this by leaving existing resistor in place, and paralleling it with 57 Ohms. Be careful...and good luck...

11th July 2010, 02:45 PM	#4
djk diyAudio Member Join Date: Feb 2001 Location: USA	I changed my DH500 feedback resistors from 150R to 47R, they worked fine. __________________ Candidates for the Darwin Award should not read this author.

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11th July 2010, 02:58 PM	#5
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Hi, FWIW increasing gain by reducing feedback improves the stability margins and it will not cause any stability issues, they only occur when reducing gain by increasing feedback. /Sreten.

11th July 2010, 05:42 PM	#6
Lee in Montreal diyAudio Member Join Date: Oct 2009	So, it looks like increasing the Hafler's sensitivity will not cause any problem. Thanks to all.

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