Adjusting input sensitivity on Hafler DH-500 - diyAudio
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Old 10th July 2010, 01:33 PM   #1
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Default 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

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Old 10th July 2010, 05:04 PM   #2
djoffe is offline djoffe  United States
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Default 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...
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Old 10th July 2010, 05:11 PM   #3
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Quote:
Originally Posted by djoffe View Post
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
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Old 11th July 2010, 02:45 PM   #4
djk is offline djk
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I changed my DH500 feedback resistors from 150R to 47R, they worked fine.
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Old 11th July 2010, 02:58 PM   #5
sreten is offline sreten  United Kingdom
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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.
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Old 11th July 2010, 05:42 PM   #6
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So, it looks like increasing the Hafler's sensitivity will not cause any problem.

Thanks to all.
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Old 19th September 2013, 12:08 AM   #7
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Smile I need help with my Halfler DH500

I'm running my Hafler DH-500 with a newer digital readout Kenwood Preamp.

Is there anyway to adjust the amps sensitivity with this Kenwood preamp?

As it stands the volume control on the Kenwood needs to be pushed up very high to obtain a decent volume.

It will play very loud but I need to almost max out the volume control on the Kenwood Preamp. I get to 20 D.B. on the Kenwood's volume readout with only 20 D.B. to go to max out the preamp.

It's just bugging me that the amp needs to be pushed so hard to attain a nice loud volume.

When it hits 20 D.B. on the volume readout the amp will play much louder per click or single D.B. on the Kenwood. I was wondering if there was a way to change the amps sensitivity to have the volume control on the Kenwood preamp be more even?

I'm also looking for a Aluminum faceplate for my DH-500. Either a rack mountable face plate or just a normal size aluminum faceplate with the cut out for the power switch and the handle holes. I have looked every where on Ebay and can't seem to locate one.

Thanks, Steven Level
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Old 19th September 2013, 01:51 AM   #8
djk is offline djk
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I changed my DH500 feedback resistors from 150R to 47R, they worked fine.
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